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
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
