Instability is a common reason for revision after total knee arthroplasty. A balanced flexion gap is likely to enhance stability throughout the arc of motion. This is achieved differently by the gap balancing and measured resection techniques. Given similar clinical results with the two techniques, one would expect similar rotation of the femoral component in the axial plane. We assessed posterior-stabilized femoral component axial rotation placed with computer navigation and a modified gap balancing technique. We hypothesized that there would be little variation in rotation. 90 surgeons from 8 countries used a modified gap-balancing technique and the same posterior-stabilized implant for this retrospective study. Axial rotation of the femoral component was collected from a navigation system and reported relative to the posterior condylar line. Patients were stratified by their preoperative coronal mechanical alignment (≥ 3° varus, < 3° varus to < 3° valgus, and ≥ 3° valgus).Introduction
Methods
As previous meta-analyses on the alignment outcomes of Computer-assisted orthopaedic surgery (CAOS) did not differentiate between CAOS systems, limited information is available on the accuracy of a specific CAOS system based on clinical cases. This study assessed the accuracy and precision of achieving surgical goals in approximately 7000 cases using a specific contemporary CAOS system. Alignment parameters were extracted from the technical logs of 6888 TKA surgeries performed between October 2012 and January 2017 using a contemporary CAOS system. The following surgical parameters were investigated: 1) planned resection defined by the surgeon prior to the bone cuts; 2) Checked resection defined as digitalisation of the bony cuts. Deviations in alignment between planned and checked resections were evaluated, with acceptable resections defined as no more than 3° of resection deviations. For the tibial resection, deviations in tibial varus/valgus angle and posterior tibial slope were 0.06 ± 0.94° and −0.09 ± 1.73°, respectively. For the femoral resection, deviations in femoral varus/valgus angle amd femoral flexion were 0.00 ± 0.97° and −0.17 ± 1.44°, respectively. High percentages of the resections were found to be acceptable (>94% of the cases). The CAOS system investigated was shown to provide accurate and precise intra-operative assistance to the surgeon in achieving targeted resections. The study summarised a large number of cases spanning the application history of the specific CAOS system, including both experienced users and new adopters of the technology. The data provided a complete clinical relevant evaluation demonstrating its high accuracy and precision in resection alignment.
Computer-assisted orthopaedic surgery (CAOS) has been demonstrated to increase accuracy to component alignment of total knee arthroplasty compared to conventional techniques. The purpose of this study was to assess if learning affects resection alignment using a specific CAOS system. Nine surgeons, each with >80 TKA experience using a contemporary CAOS system were selected. Prior to the study, six surgeons had already experienced with CAOS TKA (experienced), while the rest three were new to the technology (novice). The following surgical parameters were investigated: 1) planned resection, resection parameters defined by the surgeon prior to the bone cuts; 2) checked resection, digitalisation of the realised resection surfaces. Deviations in the alignment between planned and checked resections were compared between the first 20 cases (in learning curve) and the last 20 cases (well past learning curve) within each surgeon. Any significance detected (p < 0.05) with >1° difference in means indicated clinically meaningful impact on alignment by the learning phase. Both pooled and surgeon-specific analysis exhibited no clinically meaningful significant difference between the first 20 and the last 20 cases from both experienced and novice surgeon groups. The resections in both the first 20 and the last 20 cases demonstrated acceptable rates of over 95% in alignment (<3° deviation) for both experienced and novice surgeons. This study demonstrated that independent of the surgeon's prior CAOS experiences, the CAOS system investigated can provide an accurate and precise solution to assist in achieving surgical resection goals with no clinically meaningful compromise in alignment accuracy and outliers during the learning phase.
Despite that computer-assisted orthopaedic surgery (CAOS) has been shown to offer increased accuracy to the bony resections compared to the conventional techniques [1], previous studies of CAOS have mostly focused on alignment outcomes based on a small number of patients [1]. Although several recent meta-analyses on the CAOS outcomes have been reported [2], these analyses did not differentiate between systems, while system-dependency has been reported to influence alignment parameters [3]. To date, no study has benchmarked a specific CAOS system based on a large number of clinical cases. The purpose of this study is to assess the accuracy and precision of bony resection in more than 4000 cases using a specific contemporary CAOS system. Technical logs of 4292 TKAs performed between October 2012 and January 2016 using a contemporary CAOS system (ExactechGPS, Blue-Ortho, Grenoble, FR) were analyzed. The analyses were performed on: 1) planned resection, defined by the surgeon prior to the bone cuts. These parameters serve as inputs for the CAOS guidance; and 2) Checked resection, defined as digitalization of the actual resection surfaces by manually pressing an instrumented checker onto the bony cuts. Deviations in alignment and resection depths (on the referenced side) between planned and checked resections were calculated in coronal and sagittal planes for both tibia and femur (planned vs checked).INTRODUCTION
Materials and Methods
Although several meta-analyses have been performed on total knee arthroplasty (TKA) using computer-assisted orthopaedic surgery (CAOS) [1], understanding the inter-site variations of the surgical profiles may improve the interpretation of the results. Moreover, information on the global variations of how TKA is performed may benefit the development of CAOS systems that can better address geographic-specific operative needs. With increased application of CAOS [2], surgeon preferences collected globally offers unprecedented opportunity to advance geographic-specific knowledge in TKA. The purpose of this study was to investigate geographic variations in the application of a contemporary CAOS system in TKA. Technical records on more than 4000 CAOS TKAs (ExactechGPS, Blue-Ortho, Grenoble, FR) between October 2012 and January 2016 were retrospectively reviewed. A total of 682 personalized surgical profiles, set up based on surgeon's preferences, were reviewed. These profiles encompass an extensive set of surgical parameters including the number of steps to be navigated, the sequence of the surgical steps, the definition of the anatomical references, and the parameters associated with the targeted cuts. The profiles were compared between four geographic regions: United States (US), Europe (EU), Asia (AS), and Australia (AU) for cruciate-retaining (CR) and posterior-stabilized (PS) designs. Clinically relevant statistical differences (CRSD, defined as significant differences in means ≥1°/mm) were identified (significance defined as p<0.05).INTRODUCTION
Materials and Methods
While multiple factors contribute to the variability of prosthesis placement during total knee arthroplasty (TKA), the accuracy of the surgeon's resection planning (positioning of the cutting block) is arguably the most critical. One may postulate that proper training, including enabling the surgeon to passively receive quantitative feedback on the cutting block position, may help him/her improve resection accuracy. The purpose of this study was to test the hypothesis that passive reception of feedback on cutting block position improves the accuracy of the successive TKA resection planning. Five cadaveric knees (tibia and foot only) were studied. After arthrotomy, the tracker of an imageless navigation system (ExactechGPS®, Blue-Ortho, Grenoble, FR) was attached to the tibia. A navigated TKA procedure was initiated starting with registration of anatomical landmarks. Four surgeons then positioned the tibial cutting block (without pinning) on each knee using standard extramedullary mechanical instruments. The planned target resection was neutral varus/valgus, 3° posterior slope, and 10mm resection depth referencing the lateral plateau. Each surgeon performed 3 planning trials on each of the 5 knees, removing the cutting block between attempts. The planned resections were measured using an instrumented checker provided with the navigation system, referencing the cutting block. Surgeons were informed of the resection parameters measured by the navigation system after each planning trial. The deviations in resection parameters between the resection target and the cutting block position were calculated for each planning trial. The effect of receiving passive feedback on the accuracy of successive placement of the cutting block was assessed by comparing the deviations between each surgeon's 3 trials on the same cadaver (paired-t test). Statistical significance was defined as p<0.05.INTRODUCTION
Materials and Methods
The alignment of components in total knee arthroplasty (TKA) is perceived to be one of the most influential factors in determining the long-term outcomes. A contemporary debate exists regarding the choice of the alignment method. As a vast majority of the surgeons support the basis of the mechanical alignment philosophy (MA), others believe in the concept of anatomical alignment theory (AA) to closely match the anatomy of the femur and the tibia of the native knee [1]. This study was intended to evaluate the accuracy of achieving a planned tibial resection target using either the MA or AA methods. Five healthy cadaveric knees (tibia and foot only) were studied. Four surgeons were independently asked to position a tibial cutting block (without pinning) using conventional extramedullary mechanical instrumentation (Exactech LPI instrumentation, Gainesville, FL, USA). Surgeons were asked to target a predefined proximal tibial cut according to MA (Varus= 0°, posterior slope= 3°, resection level= 10 mm) or to AA (Varus= 3°, posterior slope= 6°, resection level= 9 mm). Once the surgeon expressed satisfaction with the achieved position of the tibial cutting block, the planned resection was recorded using an imageless guidance system (ExactechGPS®, Blue-Ortho, Grenoble, FR). Surgeons completed at least three positioning trial for each alignment method on each cadaver. The accuracy and outliers (deviated more than 2°/mm from the target [2]) of resection planning were compared between the MA and AA methods. Statistical significance was defined as p< 0.05.INTRODUCTION
Materials and Methods
Studies have reported that only 70–80% of the total knee arthroplasty (TKA) cases using conventional instruments can achieve satisfactory alignment (within ±3° of the mechanical axis). Computer-assisted orthopaedic surgery (CAOS) has been shown to offer increased accuracy and precision to the bony resections compared to conventional techniques [1]. As the early adopters champion the technology, reservation may exist among new CAOS users regarding the ability of achieving the same results. The purpose of this study was to investigate if there are immediate benefits in the accuracy and precision of achieving surgical goals for the novice surgeons, as compared to the experienced surgeons, by using a contemporary CAOS system. Two groups of surgeons were randomly selected from TKAs between October 2012 and January 2016 using a CAOS system (ExactechGPS, Blue-Ortho, Grenoble, FR), including:
All the surgeries from the INTRODUCTION
Materials and Methods
One main perceived drawback for the adoption of computer assisted orthopedic surgery (CAOS) during total knee arthroplasty (TKA) relates to the increased surgical time compared to the use of standard mechanical instrumentation [1]. This study compared the time efficiency between a next generation CAOS system (ExactechGPS®, Blue-Ortho, Grenoble, FR) and conventional mechanical instrumentation, and assessed the impact of surgeon experience level on the efficiency. Surgical time was retrospectively reviewed on 63 primary TKAs performed by a board-certified orthopedic surgeon (PP) using a cemented postero-stabilized knee system (Optetrak Logic PS, Exactech, Gainesville, FL), grouped as 1) Group I (control): 21 TKAs using conventional mechanical instruments; 2) Group II: 21 TKAs performed using the CAOS system with an early experience level (first 21 cases); and 3) Group III: 21 TKAs using the CAOS system with an advanced experience level (beyond 30 cases). Surgical time was compared across the three groups (with significance defined as p<0.05).Introduction
Materials and methods
Total knee arthroplasty (TKA) can effectively treat end-stage knee osteoarthritis. For cruciate-retaining (CR) TKA, the posterior tibial slope (PTS) of the reconstructed proximal tibia plays a significant role in restoring normal knee kinematics as it directly affects the tension of the posterior cruciate ligament (PCL) [1]. However, conventional cadaveric testing of the impact of PTS on knee kinematics may damage/stretch the PCL, therefore impact the test reproducibility. The purpose of this study was to assess the reproducibility of a novel method for the evaluation of the effects of PTS on knee kinematics. Cemented CR TKAs (Logic CR, Exactech, Gainesville, FL, USA) were performed using a computer-assisted surgical guidance system (ExactechGPS®, Blue-Ortho, Grenoble, FR) on six fresh frozen non-arthritic knees (PCL presumably intact). The tibial baseplate was specially designed (Fig. 1) with a mechanism to modify the PTS in-situ. Knee kinematics, including anteroposterior (AP) translation, internal/external (IE) rotation, and hip-knee-ankle angles, were evaluated by performing a passive range of motion from extension up to ∼110° of flexion, three separate times at 5 PTSs: 10°, 7°, 4°, 1°, and then 10° again. The repeatability of the test was investigated by comparing the kinematics between the first and the last 10° tests. Any clinically relevant deviation (1.5° for the hip knee ankle angle, 1.5mm for anterior-posterior translation and 3° for internal-external rotation) would reflect damage to the soft-tissue envelope or the PCL during the evaluation. Potential damage of PCL was investigated by comparing the kinematic parameters from the first and last 10° slope tests at selected flexion angles (Table 1) by paired t-test, with statistical significance defined as p<0.05.Introduction
Materials and Methods
Evaluations of Computer-assisted orthopaedic surgery (CAOS) systems generally overlooked the intrinsic accuracy of the systems themselves, and have been largely focused on the final implant position and alignment in the reconstructed knee [1]. Although accuracy at the system-level has been assessed [2], the study method was system-specific, required a custom test bench, and the results were clinically irrelevant. As such, clinical interpolation/comparison of the results across CAOS systems or multiple studies is challenging. This study quantified and compared the system-level accuracy in the intraoperative measurements of resection alignment between two CAOS systems. Computer-assisted TKAs were performed on 10 neutral leg assemblies (MITA knee insert and trainer leg, Medial Models, Bristol, UK) using System I (5 legs, ExactechGPS®, Blue-Ortho, Grenoble, FR) and System II (5 legs, globally established manufacturer). The surgeries referenced a set of pre-defined anatomical landmarks on the inserts (small dimples). Post bone cut, the alignment parameters were collected by the CAOS systems (Introduction
Materials and Methods
Computer-assisted orthopaedic surgery (CAOS) has been shown to assist in achieving accurate and reproducible prosthesis position and alignment during total knee arthroplasty (TKA) [1]. The most prevalent modality of navigator tracking is optical tacking, which relies on clear line-of-sight (visibility) between the localizer and the instrumented trackers attached to the patient. During surgery, the trackers may not always be optimally positioned and orientated, sometimes forcing the surgeon to move the patient's leg or adjust the camera in order to maintain tracker visibility. Limited information is known about tracker visibility under clinical settings. This study quantified the rotational limits of the trackers in a contemporary CAOS system for maintaining visibility across the surgical field. A CAOS system (ExactechGPS®, Blue-Ortho, Grenoble, FR) was set up in an operating room by a standard surgical table according to the manufacture's recommendation. A grid with 10×10 cm sized cells was placed at the quadrant of the surgical table associated with the TKA surgical field [Fig. 1A,B]. The localizer was set up to aim at the center of the grid. A TKA surgical procedure was then initiated using the CAOS system. Once the trackers-localizer connection was established, the CAOS system constantly monitored the root mean square error (RMS) of each tracker. The connection was immediately aborted if the measured RMS was above the defined threshold. Therefore, “visibility” was defined as the tracker-localizer connection with proper accuracy level. An F tracker from the tracker set (3 trackers with similar characteristics) was placed at the center of each cell by a custom fixture, facing along the +Y axis [Fig. 1]. The minimum and maximum angles of rotation around the Z axis (RAZ_MIN and RAZ_MAX) and X axis (RAX_MIN and RAX_MAX) for maintaining tracker visibility were identified. For each cell, the rotational limit of the tracker was calculated for each axis of rotation as the difference between the maximum and minimum angles (RLX and RLZ).Introduction
Materials and Methods
While total knee arthroplasty (TKA) improves postoperative function and relieves pain in the majority of patients with end stage osteoarthritis, its ability to restore normal knee kinematics is debated. Cadaveric studies using computer-assisted orthopaedic surgery (CAOS) system [1] are one of the most commonly used methods in the assessment of post-TKA knee kinematics. Commonly, these studies are performed with an open arthrotomy; which may impact the knee kinematics. The purpose of this cadaveric study was to compare the knee kinematics before and after (open or closed) arthrotomy. Kinematics of seven non-arthritic, fresh-frozen cadaveric knees (PCL presumably intact) was evaluated using a custom software application in an image-free CAOS system (ExactechGPS, Blue-Ortho, Grenoble, FR). Prior to the surgical incision, one tracker was attached to the diaphysis of each tibia and femur. Native intact knee kinematics was then assessed by performing passive range of motion (ROM) three separate times, from full extension to at least 110 degrees of flexion, with the CAOS system measuring and recording anatomical values, including flexion angle, internal-external (IE) rotation and anterior-posterior (AP) translation of the tibia relatively to the femur, and the hip-knee-ankle (HKA) angle. Next, an anterior incision with a medial parapatellar arthrotomy was performed, followed by acquisition of the anatomical landmarks used for establishing an anatomical coordinate system in which all the anatomical values were evaluated [2]. The passive ROM test was then repeated with closed and then open arthrotomy (patella manually maintained in the trochlea groove). The anatomical values before and after knee arthrotomy were compared over the range of knee flexion using the native knee values as the baseline.Introduction
Materials and Methods
Computer-assisted orthopaedic surgery (CAOS) has been shown to help achieve accurate, reliable and reproducible prosthesis position and alignment during total knee arthroplasty (TKA) [1]. A typical procedure involves inputting target resection parameters at the beginning of the surgery and measuring the achieved resection after bone cuts. Across CAOS systems, software/hardware design, mechanical instrumentation, and system-dependent work flow may vary, potentially affecting the intraoperative measurement of the achieved resection. This study assessed the cumulative effect of system-dependent differences between two CAOS systems by comparing the alignment deviation between the measurement of the achieved resection and the targeted parameters. TKA resections were performed on 10 neutral whole leg assemblies (MITA knee insert and trainer leg, Medial Models, Bristol, UK) by a board-certified orthopaedic surgeon (BH) using System I (5 legs, ExactechGPS®, Blue-Ortho, Grenoble, FR) and System II (5 legs, globally established manufacturer). The surgeon was deemed as “experienced” user (>30 surgeries) with both systems. The target parameters for the TKA resections, as well as major differences between the two systems are summarized in Table 1A. The deviations of the intraoperative alignment measurements on the achieved distal femoral and proximal tibial resection from the target were calculated and compared between the two systems with significance defined as p<0.05.Introduction
Materials and Methods
Cemented total knee arthroplasty (TKA) is a widely accepted treatment for end-stage knee osteoarthritis. During this procedure, the surgeon targets proper alignment of the leg and balanced flexion/extension gaps. However, the cement layer may impact the placement of the component, leading to changes in the mechanical alignment and gap size. The goal of the study was to assess the impact of cement layer on the tibial mechanical alignment and joint gap during cemented TKA. Computer-assisted TKAs (ExactechGPS®, Blue-Ortho, Grenoble, FR) were performed by two fellowship trained orthorpaedic surgeons on five fresh-frozen non-arthritic pelvis-to-ankle cadaver legs. All the surgeries used a cemented cruciate retaining system (Optetrak Logic CR, Exactech, Gainesville, FL). After the bony resection, the proximal tibial resection plane was acquired by manually pressing an instrumented checker onto the resected tibial surface (resection plane). Once the prosthesis was implanted through standard cementing techniques, the top surface of the implanted tibial component was probed and recorded using an instrumented probe. A best fit plane was then calculated from the probed points and offset by the thickness of the prosthesis, representing the bottom plane of the component (component plane). The deviation of component alignment caused by the cement layer was calculated as the coronal and sagittal projection of the three-dimensional angle between the resection plane and the component plane. The deviation of the component height, reflecting a change in the joint gap, was assessed as the distance between the two planes calculated at the lowest points on the medial and lateral compartments of the proximal tibial surface. Statistical significance was defined as p≤0.05.INTRODUCTION
MATERIAL
Computer-assisted orthopaedic surgery (CAOS) provides great value in ensuring accurate, reliable and reproducible total knee arthroplasty (TKA) outcomes [1,2]. Depending on surgeon preferences or patient factors (e.g. BMI, ligament condition, and individual joint anatomy), resection planning (guided adjustment of cutting blocks) is performed with different knee flexion, abduction/adduction (ABD/ADD) and internal/external (I/E) rotation angles, potentially leading to measurement errors in the planned resections due to a modified tracker/localizer spatial relationship. This study assessed the variation in the intraoperative measurement of the planned resection due to leg manipulation during TKA, and identified the leg position variables (flexion, ABD/ADD, and I/E rotation) contributing to the variability. Computer-assisted TKA (ExactechGPS®, Blue-Ortho, Grenoble, FR) was performed on a neutral whole leg assembly (MITA knee insert and trainer leg, Medial Models, Bristol, UK) by a board-certified orthopaedic surgeon (BH) at his preferred leg flexion, ABD/ADD, and I/E rotation angles. A cutting block was adjusted and fixed to the tibia, targeting the resection parameters listed in Table 1A. An instrumented resection checker was then attached to the cutting block to measure the planned resection at the same leg position (baseline). Next, the surgeon moved the leg to 9 sampled positions, representing typical leg position/orientation associated with different steps during TKA [Table 1B]. The planned resection was tracked by the CAOS system at each leg position. Tibial resection parameters at each sampled position were compared to the baseline. Regression was performed to identify the variables (flexion, ABD/ADD, I/E rotation) that significantly contribute to the measured variation (p<0.05).Introduction
Materials and Methods
Patellofemoral joint is an important aspect of the tri-compartmental knee joint complex. Total knee arthroplasty (TKA) replaces the articulating surfaces of distal femur and proximal tibia, and often times the patella as well. Understanding the size relationship between the femur and patella bones can provide valuable information for new prosthesis design and biomechanical analysis. However, taking anthropometric measurements on a large population of patients or even cadaveric specimens could be a challenge. As a result, there are currently little quantitative data existing in the literature regarding the size relationship between TKA patient's femur and patella. This study attempted to attack this question using a novel statistical approach and a large TKA patient database. A multi-site clinical database operated by Exactech was used in this study. The database contains patient information of Optetrak TKA implant recipients from over 30 physicians in US, UK, and Colombia since 1995. Nine femoral implant sizes (0, 1, 2, 2.5, 3, 3.5, 4, 5 and 6) and six patellar implant sizes (26, 29, 32, 35, 38, 41 mm) were seen in these patients. Due to the low usage, femoral sizes 0 and 6 were excluded from this analysis. Taking primary TKA only, a total of 2,698 cases were included in this study. The size relationship between femoral implant and patellar implant was analyzed in this patient population. Gender effect was also examined.Introduction
Methods
Total knee arthroplasty (TKA) is an effective technique to treat end-stage osteoarthritis of the knee. One important goal of the procedure is to restore physiological knee kinematics. However, fluoroscopy studies have consistently shown abnormal knee kinematics after TKA, which may lead to suboptimal clinical outcomes. Posterior slope of the tibial component may significantly impact the knee kinematics after TKA. There is currently no consensus about the most appropriate slope. The goal of the present study was to analyze the impact of different prosthetic slopes on the kinematics of a PCL-preserving TKA. The tested hypothesis was that the knee kinematics will be different for all tested tibial slopes. PCL-retaining TKAs (Optetrak Logic CR, Exactech, Gainesville, FL) were performed by fellowship trained orthopedic surgeons on six fresh frozen cadaver with healthy knees and intact PCL. The TKA was implanted using a computer-assisted surgical navigation system (ExactechGPS®, Blue-Ortho, Grenoble, FR). The implanted tibial baseplate was specially designed (figure 1) to allow modifying the posterior slope without repeatedly removing/assembling the tibial insert with varying posterior slopes, avoiding potential damages to the soft-tissue envelope.INTRODUCTION
MATERIAL
When evaluating the biomechanical performance of a total knee arthroplasty (TKA) implant design, device companies are usually required to select the “worst case scenario” for testing by the regulatory bodies. However, most test standards (e.g., ASTM, ISO) do not explicitly specify how the “worst case” should be determined. It is quite often that an extreme size (the smallest or the largest) in a system is taken as the “worst case” size. The smallest size is sometimes selected under the rationale that it has the smallest geometry thus the weakest mechanical structure. While the largest size is sometimes selected under the rationale that it is used on the biggest patients associated with the highest loads. However, implant geometry and in vivo load are two compounding factors that together determine the implant's biomechanical challenge. As the result, the true “worst case” must be determined considering both factors, and the choice could be design-specific. This study evaluated the femorotibial contact stress of a TKA implant system, and demonstrated that the extreme sizes may not simply be the “worst case”. The femorotibial contact stress of a posterior-stabilizing TKA implant system was assessed using finite element analysis. Multiple sizes ranging from size 0 to 6 were analyzed. For each size, the CAD models were assembled at knee extension. A load equivalent to 4 times of patient body weight was applied. Average patient body weights were calculated based on the company's clinical database: 72.5, 76.0, 80.0, 87.4, 95.2, 103.4, and 111.0 kg for sizes 0, 1, 2, 3, 4, 5 and 6, respectively. Von Mises stresses in the polyethylene tibial insert were examined and compared among different sizes.Introduction
Methods
One main perceived drawback for the adoption of computer assisted orthopaedic surgery (CAOS) during total knee arthroplasty (TKA) relates to the increased surgical time compared to the use of standard mechanical instrumentation. This study compared the time efficiency between a next generation CAOS system (ExactechGPS®, Blue-Ortho, Grenoble, FR) and conventional mechanical instrumentation, and assessed the impact of surgeon experience level on the efficiency. Surgical time was retrospectively reviewed on 63 primary TKAs performed by a board-certified orthopaedic surgeon (PP) using the Optetrak Logic® PS knee system (Exactech, Gainesville, FL), grouped as 1) Group I (control): 21 TKAs using conventional mechanical instruments; 2) Group II: 21 KAs performed using the CAOS system with an early experience level (first 21 cases); and 3) Group III: 21 TKAs using the CAOS system with an advanced experience level (beyond 30 cases). Patient condition (age, BMI, gender, etc.), surgical technique, and post-operative guidelines were similar across the three groups. No cases were lost and no patient had any intra-operative complications. Surgical time was compared across the three groups (with significance defined as p<0.05). Compared to the TKAs using conventional mechanical instrumentation, the average surgical time for the navigated TKAs performed with an early experience was 7 minutes longer. However, with an advanced experience level, the average surgical time was 2 minutes less than the time required using conventional mechanical instrument. Further, navigated TKAs with an advanced experience level exhibited the least variability among the three groups. None of the time differences were significant (p>0.20). No significant difference in TKA surgical time was found between the evaluated CAOS system (both within or pass the learning curve) and the conventional instrumentation. Nevertheless, once the learning curve was reached, the system decreased the time variability compared to conventional mechanical instrumentation. The comparable efficiency reported in this study to the conventional mechanical instrumentation may be attributed to the unique features of the ExactechGPS system, such as indication for use inside the sterile field, blood occlusion-resistant tracker design, customisable operative technique tailored to the surgeon's preference, and compact and reduced number of instruments.
Although total knee arthroplasty (TKA) is a largely successful procedure to treat end-stage knee osteoarthritis (OA), some studies have shown postoperative abnormal knee kinematics. Computer assisted orthopaedic surgery (CAOS) technology has been used to understand preoperative knee kinematics with an open joint (arthrotomy). However, limited information is available on the impact of arthrotomy on the knee kinematics. This study compared knee kinematics before and after arthrotomy to the native knee using a CAOS system. Kinematics of a healthy knee from a fresh frozen cadaver with presumably intact PCL were evaluated using a custom software application in an image-free CAOS system (ExactechGPS, Blue-Ortho, Grenoble, FR). At the beginning of the test, four metal hooks were inserted into the knee away from the joint line (one on each side of the proximal tibia and the distal femur) for the application of 50N compressive load to simulate natural knee joint. Prior to incision, one tracker was attached to each tibia and femur on the diaphysis. Intact knee kinematics were recorded using the CAOS system by performing passive range of motion 3 times. Next, a computer-assisted TKA procedure was initiated with acquisition of the anatomical landmarks. The system calculated the previously recorded kinematics within the coordinate system defined by the landmarks. The test was then repeated with closed arthrotomy, and again with open arthrotomy with patella maintained in the trochlea groove. The average femorotibial AP displacement and rotation, and HKA angle before and after knee arthrotomy were compared over the range of knee flexion. Statistical analysis (ANOVA) was performed on the data at ∼0° (5°), 30°, 60°, 90° and 120° flexion. The intact knee kinematics were found to be similar to the kinematics with closed and open arthrotomy. Differences between the three situations were found, in average, as less than 0.25° (±0.2) in HKA, 0.7mm (±0.4) in femorotibial AP displacement and 2.3° (±1.4) in femorotibial rotation. Although some statistically significant differences were found, especially in the rotation of the tibia for low and high knee flexion angles, the majority is less than 1°/mm, and therefore clinically irrelevant. This study suggested that open and closed arthrotomy do not significantly alter the kinematics compared to the native intact knee (low RMS). Maintaining the patella in the trochlea groove with an open arthrotomy allows accurate assessment of the intact knee kinematics.
Total knee arthroplasty (TKA) is an effective technique to treat end-stage knee osteoarthritis, targeting the restore a physiological knee kinematics. However, studies have shown abnormal knee kinematics after TKA which may lead to suboptimal clinical outcomes. Posterior slope of the tibial component may significantly impact the knee kinematics. There is currently no consensus about the most appropriate slope. The goal of the present study was to analyse the impact of different prosthetic slopes on the kinematics of a PCL-preserving TKA, with the hypothesis that posterior slopes can alter the knee kinematics. A PCL-retaining TKA (Optetrak CR, Exactech, Gainesville, FL) was performed by a board-certified orthopaedic surgeon on one fresh frozen cadaver that had a non arthritic knee with an intact PCL. Intact knee kinematic was assessed using a computer-assisted orthopaedic surgery (CAOS) system (ExactechGPS®, Blue-Ortho, Grenoble, FR) Then, TKA components were implanted using the guidance of the CAOS system. The implanted tibial baseplate was specially designed to allow modifying the posterior slope without repeatedly removing/assembling the tibial insert with varying posterior slopes, avoiding potential damages to the soft-tissue envelope. Knee kinematic was evaluated by performing a passive range of motion 3 separate times at each of the 4 posterior slopes: 10°, 7°, 4° and 1°, and recorded by the navigation system. Femorotibial rotation, antero-posterior (AP) translation and hip-knee-ankle (HKA) angle were plotted with regard to the knee flexion angle. Tibial slopes of 1° and 4° significantly altered the normal rotational kinematics. Tibial slopes of 7° and 10° led to a kinematics close to the original native knee. All tibial slopes significantly altered the changes in HKA before 90° of knee flexion, without significant difference between the different slopes tested. The magnitude of change was small. There was no significant change in the AP kinematics between native knee and all tested tibial slopes. Changing the tibial slope significantly impacted the TKA kinematics. However, in the implant studied, only the rotational kinematics were significantly impacted by the change in tibial slope. Tibial slopes of 7° and 10° led rotational kinematics that were closest to that of a normal knee. Alterations in knee kinematics related to changing tibial slope may be related to a change in the PCL strain. However, these results must be confirmed by other tests involving more specimens.
Total knee arthroplasty (TKA) implant systems offer a range of sizes for orthopaedic surgeons to best mimic the patient's anatomy and restore joint function. From a biomechanical perspective, the challenge on the TKA implants is affected by two factors: design geometry and in vivo load. Larger geometry typically means more robust mechanical structure, while higher in vivo load means greater burden on the artificial joint. For an implant system, prosthesis geometry is largely correlated with implant size, while in vivo load is affected by the patient's demographics such as weight and height. Understanding the relationships between implant size and patients' demographics can provide useful information for new prosthesis design, implant test planning, and clinical data interpretation. Utilizing a manufacturer supported clinical database, this study examined the relationships between TKA patient's body weight, height, and body mass index (BMI) and the received implant size of a well-established implant system. A multi-site clinical database operated by Exactech, Inc. (Gainesville, FL, USA) was utilized for this study. The database contains patient information of Optetrak TKA implant recipients from over 30 physicians in US, UK, and Colombia since 1995. Nine implant sizes (0, 1, 2, 2.5, 3, 3.5, 4, 5 and 6) are seen in the database, while size 0 was excluded due to very low usage. Taking primary TKA only, a total of 2,713 cases were examined for patient's body weight, height, BMI, and their relationships with the implant size.Introduction
Methods
Cementless total knee arthroplasty (TKA) has several advantages compared to the cemented approach, including elimination of bone cement, a quicker and easier surgical technique, and potentially a stronger long-term fixation. However, to ensure the successful long-term biological fixation between the porous implant and the bone, initial press-fit stability is of great importance. Undesired motion at the bone-implant interface may inhibit osseointegration and cause failure of biological fixation. Initial stability of a cementless femoral implant is affected by implant geometry, bone press-fit dimension, and characteristics of the porous coating. The purpose of this study was to compare the initial fixation stability of two types of porous femoral implants by quantifying the pull-out force using a paired cadaveric study design. The two types of cementless TKA femoral implants evaluated in this study had identical implant geometry but different porous coatings (Figure 1). The first type had a conventional spherical-bead coating (Type A), while the second type had an innovative irregularly-shaped-powder coating (Type B). The porous coating thickness was equivalent for both types of implants, thus the dimensional press-fit with bone was also equivalent. Three pairs of cadaveric femurs were prepared using standard TKA surgical technique, with each pair of the femurs receiving one of each porous implant type. An Instron 3366 load frame (Norwood, MA, USA) was used to pull the femoral implant out from the distal femur bone (Figure 2). The testing fixture was designed to allow free rotation between the implant and the actuator. The pullout was performed under a displacement control scheme (5 mm/min). Peak pull-out force was recorded and compared between the two implant groups.Introduction
Methods
Cruciate Retaining (CR) and Posterior Stabilizing (PS) are two common types of total knee arthroplasty (TKA) surgeries. The CR approach preserves the posterior cruciate ligament (PCL) while the PS approach sacrifices it. Implant size selection during a TKA surgery is primarily driven by the patient's bone size, but could also be affected by surgery types due to the influence of the PCL. The objective of this study was to investigate the effect of TKA surgery type on implant size selection, based on the clinical database of a well-established commercial implant system. A clinical database operated by Exactech, Inc. (Gainesville, FL, USA) was utilized for this study. The database contains TKA patient information of Optetrak® implant recipients from over 30 physicians in the US, UK, and Colombia since 1995. Patient height was used as a control factor for comparison of surgery types, and categorized by every 10 cm (e.g., the “170 cm” category contains patients from 170 to 179 cm). Taking primary TKA only and body heights from 130 cm to 199 cm, a total of 2,677 cases were examined. No statistical difference exists on patients' gender, body weight, or BMI within every height category between the CR and PS groups. The femoral implant size and tibial insert thickness were compared between the two groups.Introduction
Methods
Biological fixation through bone ingrowth and ongrowth to implants can be achieved with a variety of surface treatments and technologies. This study evaluated the effect of two different three dimensional surface coatings for CoCr where porosity was controlled through the use of different geometry of CoCr beads in the sintering process. Test specimens in Group A were coated with conventional spherical porous-bead technology. The porous coating technology used on Group B was a variation of the conventional porous-bead technology. Instead of spherical beads, cobalt-chromium particles in irregular shapes were sieved for a particular size range, and were sintered onto the specimen substrate using similar process as Group A. The geometry and the size variation of the particles resulted in a unique 3D porous structure with widely interconnected pores. Three implants were placed bicortically in the tibia. Two implants were placed in the cancellous bone of the medial distal femur and proximal tibia bilaterally with 4 implantation conditions (2 mm gap, 1 mm gap line-to-line, and press fit). Animals were euthanized at 4 or 12 weeks for standard mechanical, histological and histomorphometric endpoints.Introduction
Methods
From pre-operative planning to final implant cementation, total knee arthroplasty (TKA) preparation is a succession of many individual steps, each presenting potential sources of error that can result in devices being implanted outside the targeted range of alignment. This study assessed alignment discrepancy occurring during different TKA steps using an image-free computer-assisted orthopaedic surgery (CAOS) guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) in normal and abnormal mechanical axis. We used a commercially available artificial leg (MITA trainer leg M-00058, Medical Models, Bristol, UK) able to receive (neutral / varus / valgus) knee inserts simulating the proximal tibia and distal femur. A pre-surgical profile was established to define resection parameters for the proximal tibial and distal femoral cuts (Figure 1A). Data from the guidance system were collected at three separate steps: (1) cutting block adjusted but not pinned to the bone (Figure 1B), (2) cutting block adjusted and pinned to the bone (Figure 1C), and (3) after the cuts were checked (Figure 1D). These data were then compared to the resection target parameters to track potential dispersions occurring during the process. Due to the amount of data (i.e., four studied resection parameters per bone, three operative steps, and three knee model types), the authors introduced an “error index”, which was a unitless indication of overall error magnitude obtained by averaging the absolute values of all linear and angular measurement errors. Due to knee model dimensions (∼55 mm), the authors equally considered linear and angular measurement values (i.e., 1 mm equivalent to 1°).Introduction
Materials and methods
An emerging consensus in the surgical specialties is that skill acquisition should be more emphasized during surgical training.1 This study was an attempt to evaluate the effects of repetitive practices using an image-free computer-assisted orthopaedic surgery (CAOS) guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) on both technical and cognitive skills. A senior knee replacement surgeon with limited previous experience with the CAOS system performed a series of consecutive simulated knee surgeries using a commercially available artificial leg (MITA trainer leg M-00058, Medical Models, Bristol, UK). In order to assess the effects repetitive practice has on technical skills, we evaluated two indexes: Error index: A unitless indication of overall error magnitude obtained by averaging the absolute values of all linear and angular measurement differences between targeted and checked cuts. Time index: An indication of the time required to acquire landmarks, adjust the custom blocks, and make cuts. In order to assess the effect repetitive practice has on cognitive skills, we evaluated the number of times the surgeon elected to deviate from pre-surgical planning or re-acquire landmarks. We evaluated these parameters for three chronological and consecutive groups of simulated surgeries: Group A (knee models #1 to #10), Group B (knee models #11 to #20), and Group C (knee models #21 to #28).Introduction
Materials and methods
Clinical outcomes for total knee arthroplasty (TKA) are especially sensitive to lower extremity alignment and implant positioning.1 The use of computer-assisted orthopaedic surgery (CAOS) can improve overall TKA accuracy.2 This study assessed the accuracy of an image-free CAOS guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) in both a synthetic leg with a normal mechanical axis and legs with abnormal mechanical axis. A high-resolution 3D scanner (Comet L3D, Steinbichler, Plymouth, MI) was used to scan varus-deformed (n=12), neutral (n=12), and valgus-deformed (n=4) knee inserts (Mita M-00566, M-00598, M-00567; respectively, Medical Models, Bristol, UK) and collect pre-identified anatomical landmarks prior to using the models to simulate knee surgery. The image-free CAOS guidance system was then used to acquire the same landmarks. After adjusting the position and orientation of the cutting block to match the targets, bone resections were performed, and the knee models were re-scanned. The 3D scans made before and after the cuts were overlaid and the resection parameters calculated using the pre-identified anatomical landmark data and advanced software (UG NX, Siemens PLM, Plano, TX). Data sets obtained from the 3D scanner (see Figure 1A) were compared with data sets from the guidance system (see Figure 1B). Given the accuracy of the 3D scanner (<50μm), its measurements were used as the baseline for assessing CAOS system error.Introduction
Materials and methods
Femorotibial constraint is a key property of a total knee arthroplasty (TKA) prosthesis and should reflect the intended function of the device. With a validated simulation methodology, this study evaluated the constraint of two TKA prostheses designed for different intentions. TKA prostheses are semi-constrained artificial joints. Femorotibial constraint level is a major property of a prosthesis and should be designed to match the device's intended function. Cruciate Retaining (CR) prostheses are usually indicated for patients with a functioning posterior cruciate ligament (PCL). For patients without a fully functioning PCL, CR-Constrained (CRC) prostheses with additional built-in constraint may be indicated. A CRC prosthesis usually consists of a CR femoral component and a tibial insert which has a more conforming sagittal profile to offer an increased femorotibial constraint. This study evaluated the anterior-posterior (AP) constraint behavior of two lines of prostheses (CR and CRC) from a same TKA product family. Using a validated computer simulation approach, multiple sizes of each product line were evaluated.Summary Statement
Introduction
The constraint behavior of total knee arthroplasty (TKA) prosthesis usually has to be physically tested. This study presents a computer simulation model using finite element analysis (FEA) and demonstrates its effectiveness in predicting the femorotibial constraint behavior of TKA implants. TKA prostheses are semi-constrained artificial joints. A well-functioning TKA prosthesis should be designed with a good balance between stability and mobility, meaning the femorotibial constraint of the artificial joint cannot be excessive or too lax. To assess the constraint behavior of a TKA prosthesis, physical testing is usually required, and an industrial test standard has been developed for this purpose. Benefiting from technological advancement, computer simulation has become increasingly useful in many industries, including medical device research and development. FEA has been extensively used in stress analysis and structural evaluation of various orthopaedic implants. This study presented an FEA-based simulation to evaluate the femorotibial constraint behavior of TKA prosthesis, and demonstrated the effectiveness of the method by validating it through physical testing.Summary Statement
Introduction
Ability to accommodate increased range of motion is a design objective of many modern TKA prostheses. One challenge that any “high-flex friendly” prosthesis has to overcome is to manage the femorotibial contact stress at higher flexion angle, especially in the polyethylene tibial insert. When knee flexion angle increases, the femorotibial contact area tends to decrease thus the contact stress increases. For a high-flex design, considerations should be taken to control the contact stress to reduce the risk of early damage or failure on the tibial insert. This study evaluated the effect of femoral implant design on high flexion contact stress. Two prostheses from a same TKA family were compared – one as a conventional design and the other as a high-flex design. Two cruciate retaining (CR) prostheses from a same TKA product family were included in this study. The first is a conventional design for up to 125° of flexion (Optetrak CR, Exactech, USA). The second is a high-flex design for up to 145° of flexion (Logic CR, Exactech, USA). The high-flex design has a femoral component which has modified posterior condyle geometry (Figure 1), with the intent to increase femorotibial contact area and decrease contact stress at high flexion. Three sizes (sizes 1, 3, and 5) from each prosthesis line were included to represent the commonly used size spectrum. Contact stress was evaluated at 135° of flexion using finite element analysis (FEA). The CAD models were simplified and finite element models were created assuming all materials as linear elastic (Figure 2). For comparison purpose, a compressive force of 20% body weight was applied to the femoral component. The average body masses of sizes 1, 3 and 5 patients are 69.6 kg, 89.9 kg, and 106.3 kg based on the manufacture's clinical database. A nonlinear FEA solver was used to solve the simulation. Von Mises stress in the tibial insert was examined and compared between the two prostheses.Introduction
Methods
Ideally, a patient receiving a unicondylar knee replacement will have fully functional anterior and posterior cruciate ligaments. When at least one of the cruciate ligaments is not fully functional, femoral and tibial implant contact position can potentially increase along the anterior-posterior (AP) axis. Where unicondylar implant wear testing typically uses AP resistance assuming fully functional cruciate ligaments, the authors used reduced AP resistance intended to simulate deficient cruciate ligaments. Optetrak Logic® Uni (Exactech Inc, Gainesville, FL USA) unicondylar test specimens featuring an all-UHMWPE tibial component and a cobalt chromium femoral component were used in this study. The system has a semi-constrained articular geometry. Testing was conducted at an independent testing facility (EndoLab GMBH, Thansau, Rosenheim, Germany). A four-station knee simulator was used (EndoLab knee simulator) with two unicondylar knee implants per station, giving a total of eight test specimens. Two different tibial fixation designs (keeled and peg) with identical articulating surfaces were tested. Tibial test specimens were 6 mm in thickness. Unloaded soak controls were stored in distilled water at 37°C. The test was conducted according to ISO 14243–1: 2009 [1]. Test specimens were immersed in calf serum (PAA GmBH, Cölbe, LOT B00111-5126) with a protein content of 20 g/l. Custom polyurethane molds allowed for individual component measurement. Per the ISO 14243-1, a 7% medial offset was incorporated into the set-up. The unicondylar knee implants were set at neutral position in extension. Tibial rotational restraint was 0.36 Nm/° and zero when the test specimen was within ± 6° of the reference position. This test was conducted with an AP resistance of 9.3N/mm to maximize AP displacement and simulate deficient cruciate ligaments. Typical unicondylar knee wear testing is conducted with an AP resistance of 44N/mm, which assumes functional cruciate ligaments.Introduction
Methods
Total knee arthroplasty (TKA) prostheses are semi-constrained artificial joints. A well-functioning TKA prosthesis should be designed with a good balance between stability and mobility, meaning the femorotibial constraint of the artificial joint should be appropriate for the device's function. To assess the constraint behavior of a TKA prosthesis, physical testing is typically required, and an industrial testing standard has been developed for this purpose [1]. Computer simulation has become increasingly useful in many industries, including medical device research and development where finite element analysis (FEA) has been extensively used in stress analysis and structural evaluation. This study presents an FEA-based simulation to evaluate the femorotibial constraint behavior of TKA prosthesis, and demonstrated the effectiveness of the method by validating through physical testing. A Cruciate Retaining (CR) TKA prosthesis design (Optetrak Logic CR, Exactech, USA) was used in this study. CAD models of the implants assembled at 0° of flexion were used for the simulation. Finite element models were generated using with all materials assumed linear elastic. Boundary conditions were set up according to the ASTM F1223 standard (Figure 1). The tibial baseplate was fixed distally. A constant compressive force (710 N) was applied on the femoral component. Nonlinear Surface-Surface-Contact was defined at the femorotibial articulating surfaces. Coefficient of friction was determined from physical test. The femoral component was driven under a displacement-controlled scheme to slide along the anterior-posterior (AP) direction on the tibial insert. At each time step, constraint force occurring at the articulating surface was derived from the reaction force at the distal fixation of the tibial baseplate. A nonlinear FEA solver (NX Nastran SOL601, Siemens, USA) was used to solve the simulation. In addition, five samples of the prostheses were physically tested, and the results were compared with the simulation.Introduction
Methods
Clinical outcomes for total knee arthroplasty (TKA) are especially sensitive to lower extremity alignment and implant positioning.1 The use of computer-assisted orthopedic surgery (CAOS) can improve overall TKA accuracy.2 This study assessed the accuracy of an image-free CAOS guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) used in TKA. A high-resolution 3D scanner (Comet L3D, Steinbichler, Plymouth, MI) was used to scan seven knee models (MITA, Medical Models, Bristol, UK) and collect pre-identified anatomical landmarks (see Figure 1) prior to using the models to simulate knee surgery. The image-free CAOS guidance system was then used to acquire the same landmarks. After adjusting the position and orientation of the cutting block to match the targets, bone resections were performed, and the knee models were re-scanned. The 3D scans made before and after the cuts were overlaid (see Figure 2) and the resection parameters calculated using the pre-identified anatomical landmark data and advanced software (XOV & XOR, RapidForm, Lakewood, CO and UG NX, Siemens PLM, Plano, TX). Data sets obtained from the 3D scanner were compared with data sets from the guidance system. Given the accuracy of the 3D scanner, its measurements were used as the baseline for assessing CAOS system error.Introduction
Materials and methods:
From pre-operative planning to final implant cementation, total knee arthroplasty (TKA) can be defined by a succession of individual steps, each presenting potential errors that can result in devices being implanted outside the desired range of alignment. Our study used an image-free computer-assisted orthopedic surgery (CAOS) guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) to evaluate alignment discrepancies occurring during different steps of a typical TKA procedure. A surgical profile was established to define resection parameters and steps for proximal tibial and distal femoral cuts (see Figure 1A) to be made on seven synthetic knee models (MITA, Medical Models, Bristol, UK). First, the guidance system was used to acquire pre-identified landmarks. Next, a cutting block was adjusted to match the resection targets and then fixed to the bone using locking pins. Bone cuts were performed and then checked. Data was collected from the guidance system at three steps: (1) cutting block adjusted but not pinned to bone (see Figure 1B), (2) cutting block adjusted and pinned to bone (see Figure 1C), and (3) after checking cuts (see Figure 1D). These data were then compared to the resection target parameters to assess potential discrepancies.Introduction
Materials and methods:
Total knee arthroplasty (TKA) prostheses are semi-constrained artificial joints. Femorotibial constraint is a key property of a TKA prosthesis and should be designed to match the device's intended function. Cruciate Retaining (CR) prostheses are usually used for patients with a functioning posterior cruciate ligament (PCL). For patients without a fully functioning PCL, CR-Constrained (CRC) prostheses may be used. A CRC tibial insert usually has a more conforming sagittal profile especially in the anterior aspect to provide increased constraint to prevent paradoxical femoral translation during knee flexion. A quantitative understanding of the constraint behavior of a prosthesis design is critical to ensure its functional outcome. Using a validated computer simulation, this study evaluated the anterior-posterior (AP) constraint of two types of tibial inserts (CR and CRC) from a same TKA product family. Both the CR and CRC prostheses are from the same TKA product family (Optetrak Logic, Exactech, USA). Three sizes (sizes 1, 3, and 5) from each product line were included in this study. Computer simulations using finite element analysis (FEA) were performed at 0° flexion per ASTM F1223 standard [1] (Figure 1). The simulation has been validated with physical testing (more details submitted in a separate abstract to ISTA 2013). Briefly, FEA models were created with all materials considered linear elastic. The tibial baseplate was distally fixed and a constant compressive force (710 N) was applied to the femoral component. Nonlinear Surface-Surface-Contact was established at the articulating surfaces. A coefficient of friction of 0.1 was assumed for all articulations [2]. The femoral component was driven under a displacement-controlled scheme to slide along AP direction on the tibial insert. Constraint force occurring at the articulation was derived from the reaction force at the distal fixation. A nonlinear FEA solver was used to solve the simulations.Introduction
Methods
Clinical outcomes for total knee arthroplasty (TKA) are especially sensitive to lower extremity alignment and implant positioning. The use of computer-assisted orthopedic surgery (CAOS) can improve overall TKA accuracy. This study assessed the accuracy of an image-free CAOS guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) used in TKA. A high-precision 3D scanner (Comet L3D, Steinbichler, Plymouth, MI) was used to scan seven knee models (MITA, Medical Models, Bristol, UK) and collect pre-identified anatomical landmarks prior to using the models to simulate knee surgery. The Exactech GPS was then used to acquire the same landmarks. After adjusting the Exactech GPS cutting block to match the targets, bone resections were performed, and the knee models were re-scanned. The 3D scans made before and after the cuts were overlaid and the resection parameters calculated using the pre-identified anatomical landmark data and advanced software (XOV & XOR, RapidForm, Lakewood, CO and UG NX, Siemens PLM, Plano, TX). Data sets obtained from the 3D scanner were compared with data sets from the guidance system. Given the accuracy of the 3D scanner, its measurements were used as the baseline for assessing CAOS system error. The CAOS system bone resection measurement errors had an overall mean of less than 0.35 mm. The mean errors for joint angle measurement was less than 0.6°. Even considering the ranges, errors were no more than 1 mm for all bone resection measurements and no more than 1° for all joint angle measurements. The low variability is also supported by small SD values. To our knowledge, this is the first study to use a high-resolution 3D scanner to assess the accuracy of surgical cuts made with image-free CAOS system assistance. Determining precise landmarks using CAOS for TKA has been shown to be of critical importance. For this reason, the anatomical landmarks used by the scanner and guidance system were carefully identified and prepared to ensure consistency. The study demonstrated that the evaluated image-free CAOS system was able to achieve a high level of
A tibial insert with choices in size, thickness, and posterior slope is proposed to improve ligament balancing in total knee arthroplasty. However, increasing posterior slope, or the angle between the distal and proximal insert surfaces, will redistribute ultra-high molecular weight polyethylene (UHMWPE) thickness in the sagittal plane, potentially affecting wear. This study used in-vitro testing to compare wear for a standard cruciate-retaining tibial insert (STD) and a corresponding 6° sloped insert (SLP), both manufactured from direct-compression molded (DCM) UHMWPE. Our hypothesis was slope variation would have no significant effect on wear. Two of each insert (STD and SLP) were tested on an Instron-Stanmore knee simulator with a force-control regime. The gait cycle and other settings followed ISO 14243-1 and -2, except for reference positions. The STD insert was tilted 6° more than the SLP insert to level the articular surfaces. Wear was gravimetrically measured at intervals according to strict protocol.Introduction
Methods
While clinically successful for decades, CR TKA is persistently compromised by inconsistent PCL function. Problems of mid-flexion instability, incomplete knee flexion, erratic kinematic behavior and posterior instability, not seen with PS devices, raise concerns about the consistency of the technique, and the devices used. Most TKA systems offer at least 2 different geometries of tibial inserts to address this clinical problem. We hypothesize these problems are a result of compromise of PCL anatomy. To avoid compromise to the PCL 3 steps are required: 1) The slope of tibial resection must be less than 5°; 2) the depth of tibial resection must be based off the insertion footprint of the PCL, not the deficiencies of the tibial articular surface; and 3) the tibial insert must be modified to allow intraoperative balancing of the PCL. The CR Slope ™ implants and technique (Exactech) (“Posterior Cruciate Referencing Technique (PCRT)”) reflect this philosophy and have allowed consistent surgical intervention without PCL release and without multiple inserts. We present data identifying, the footprint, and the instrument and technique modifications that allow for predictable identification of the depth and angle of resection. At 2 years post implantation in the first 100 patients implanted, the study group has demonstrated similar operative time, LOS and Oxford knee scores (OKS), while ROM averaged 5° greater, and time to achieved flexion was decreased.Introduction
Results
A tibial insert with choices in posterior slope, size, and thickness is proposed to improve ligament balancing in total knee arthroplasty. However, increasing slope, or the angle between the distal and proximal insert surfaces, will redistribute ultra-high molecular weight poly-ethylene (UHMWPE) thickness in the sagittal plane, potentially affecting wear. This study used in-vitro testing to compare UHMWPE wear for a standard cruciate-retaining (CR) tibial insert (STD) and a corresponding 6° sloped insert (SLP). Our hypothe sis was that slope variation would have little effect on wear. Two of each style inserts were tested on an Instron-Stanmore knee simulator with a force-control regime. The gait cycle and other settings followed ISO 14243-1 &
2, except for the reference position, which was posteriorly shifted 6 mm to simulate the worst-case scenario. The STD insert was tilted 6° more than the SLP to level the articular surfaces. Wear was gravimetrically measured at intervals according to strict protocol. No statistical difference (p=0.36) was found between wear for the STD (9.5 ±1.8 mg/Mc) and SLP (11.4 ±0.5 mg/Mc) inserts. The overall wear rate measured was higher than previously published rates using implants similar to the STD inserts. This may relate to the shift in the reference position and the 6° slope, leading to increased shear loads. This is the first time the effect of tibial insert slope on wear has been evaluated in-vitro. When limited to 6°, wear testing suggests that al tering the tibial insert slope will have a minor effect on UHMWPE wear.
