Wear and fatigue damage to polyethylene components remain major factors leading to complications after total knee and unicompartmental arthroplasty. A number of wear simulations have been reported using mechanical test equipment as well as computer models. Computational models of knee wear have generally not replicated experimental wear under diverse conditions. This is partly because of the complexity of quantifying the effect of cross-shear at the articular interface and partly because the results of pin-on-disk experiments cannot be extrapolated to total knee arthroplasty wear. Our premise is that diverse experimental knee wear simulation studies are needed to generate validated computational models. We combined five experimental wear simulation studies to develop and validate a finite-element model that accurately predicted polyethylene wear in high and low crosslinked polyethylene, mobile and fixed bearing, and unicompartmental (UKA) and tricompartmental knee arthroplasty (TKA). Low crosslinked polyethylene (PE). A finite element analysis (FEA) of two different experimental wear simulations involving TKA components of low crosslinked polyethylene inserts, with two different loading patterns and knee kinematics conducted in an AMTI knee wear simulator: a low intensity and a high intensity. Wear coefficients incorporating contact pressure, sliding distance, and cross-shear were generated by inverse FEA using the experimentally measured volume of wear loss as the target outcome measure. The FE models and wear coefficients were validated by predicting wear in a mobile bearing UKA design. Highly crosslinked polyethylene (XLPE). Two FEA models were constructed involving TKA and UKA XLPE inserts with different loading patterns and knee kinematics conducted in an AMTI knee wear simulator. Wear coefficients were generated by inverse FEA.Background
Methods
Proper tibial rotation has been cited as an important prerequisite to optimal total knee replacement. The most commonly recognized rotational landmark is the medial 1/3rd of the tibial tubercle. The purpose of this study was to quantify the amount of variability this structure has from a common reference as well as to understand the effects of component design when referencing this structure. Subjects were prospectively scanned into a Virtual Bone Database (Stryker Orthopaedics, Mahwah, NJ), which is a collection of body CT scans from subjects collected globally. All CT scans displayed cropped bones were excluded. SOMA™ (Stryker) is a unique tool with the ability to take automated measurements of quantities such as distances and angles on a large number of pre-segmented bone samples which was then to perform calculations represented in this study. Demographic information for each subject was recorded were known. For the analysis, the mechanical axis of the tibia (MAT) was established by connecting the center of the proximal tibia to the center of the ankle. From the MAT, a perpendicular resection plane was made at a distance of 9 mm from the most proximal portion of the lateral condyle. This plane was then used as a virtual resection plane to establish the points for the remaining structures which was the medial 1/3rd of the tibial tubercle and the posterior notch of the PCL insertion. The following axes were identified: 3TT (line between the medial 1/3rd of the tibial tubercle and the posterior notch of the tibia); 3CTT (line between the medial 1/3rd of the tibial tubercle and the center of the tibia); and the posterior axis of the tibia (line connecting the two most posterior points of the tibia at the virtual resection plane). Measurements made were the angle of the 3TT Line to the posterior axis and the angle of the 3CTT Line to the posterior axis.INTRODUCTION:
METHODS:
Computational modeling has been used to simulate the natural and prosthetic kinematic and kinetic function in an attempt to compare designs and/or predict a desired motion path from a design. The levels of soft tissue can range from basic ligaments (MCL, LCL, and ACL & PCL) to more complex models. The goal of this study was to evaluate the sensitivity of the Posterior Cruciate ligament in a virtual model and its effects on the kinematic outcome in a commercially available and validated kinematics package (KneeSim, LifeModeler San Clemente, CA). KneeSIM is a musculoskeletal modeling environment that is built on the foundation of the ADAMS (MSC Software, Santa Ana CA), a rigid body dynamics solver to compute knee kinematics and forces during a deep knee bend. All parameters are customizable and can be altered by the user. Generic three dimensional models of cruciate retaining components of the femoral, tibial, and patellar are available with the software and were used to provide a common reference for the study. The following parameters were modified for each simulation to evaluate the sensitivity of the PCL in the model: 1) Model without PCL, 2) PCL with default properties, 3) PCL Shifted at femoral origin, 7 mm anterior, 7 mm inferior; tibial origin maintained; 4) PCL with increased stiffness properties (2x default), 5) position in the femur and tibia remained default position and 6) PCL with default properties and location, joint line shifted 4 mm superior. The standard output of tracking the flexion facet center (FFC) motion of the medial and lateral condyles was utilized (Figure 1). Figure 2 and 3 displays the output of the six conditions tested above. Comparing the curves for the medial and lateral motion show different patterns with the lateral point having more posterior translation than the medial. After approximately 95° of flexion, all cases exhibit an anterior translation in the model. This motion was consistent for all test cases. The model showed no difference with motion either with or without the PCL and with changing the stiffness. Altering the location of the PCL on the femoral insertion had the greatest effect on motion, while shifting the joint line superior was second. The shift of the ligament insertion and changing of the joint line results in the ligament being more parallel to the tibial surface which provides resistance to anterior motion or posterior translation.Methods:
Results:
Demand for TKR surgery is rising, including a more diverse patient demographic with increasing expectations [1]. Therefore, greater efforts are being devoted to laboratory testing. As a result, laboratory testing may set a clinical performance presumption for surgeons and patients. For example, oxidized ZrNB (Oxinium) femoral components have been projected to show 85% less wear than CoCr femoral components in bench-top testing [2]. However, recent clinical data show no difference in outcomes between Oxinium® and CoCr for the same design [3]. While it does not show lagging peformance for the Oxinium components, it does call into question the predictive ability of simulation. To better understand the performance of these two materials, a non standardized simulator evaluation was conducted. One commercially available design (Legion PS) was evaluated with two variations of femoral component material (n = 3/material) Oxinium® and Cobalt Chromium. All testing was conducted using a 7.5 kGy moderately crosslinked UHMWPE (XLPE). A 6-station knee simulator was utilized to simulate stair-climbing kinematics. The lubricant used was Alpha Calf Fraction serum which was replaced every 0.5 million cycles for a total of 5 million cycles. Soak controls were used to correct for fluid absorption and statistical analysis was performed using the Student's t-test. Total wear rate results for the tibial inserts are shown in Figure 1. There was no statistical difference in volume loss (p = 0.8) or wear rate (p = 0.9) for the Oxinium® system when compared to the CoCrsystem under stair-climbing kinematics. Visual examination revealed typical wear scars and features on the condylar surfaces, including burnishing. These results corroborate the recent clinical data showing no difference between Oxinium® components and their CoCr analogs [3]. The kinematics used here are not a combination of normal level walking with stair-climbing conditions as was published originally for the Oxinium® material [2], but stair-climbing kinematics only. Even though the stair-climbing profile utilized here does not represent standardized kinematics, it provided results that are in line with clinical observations for these femoral materials. Logic suggests that a combined duty cycle is more representative of patient behavior so there must be additional test factors contributing to the prediction previously reported. The goal of bench top testing is to simulate actual clinical performance so test models must be validated as clinicaly relevant in order to be predictive. Furthermore, the results of this test indicate that the different femoral materials evaluated in this study do not alter the wear characteristics of this TKR. This is further supported by a similar previous study showing the relative contribution of design versus materials in terms of wear behavior [4]. The main determination comes from clinical evidence, and as it has been demonstrated by Kim, et al [3], there is no significant difference in the clinical results of the two TKR devices analyzed.
Component and limb alignment (especially varus >3°) have been associated with soft-tissue imbalance, increased polyethylene wear, and tibial tray subsidence. However, not all clinical outcome studies have found significant correlation between tibial varus and revision surgery. While the link between limb alignment and failure has been attributed to increased medial compartmental loading and generation of shear stress, quantitative biomechanical evidence to directly support this mechanism is incomplete. In this study, we analyzed the effect of limb alignment and tibial tray alignment on the risk for bone damage and subsequent risk for tray loosening. A finite element model of knee arthroplasty previously validated with in vitro cadaver testing was used. Models of four subjects were constructed with tibial resections simulating a 0°, 3°, 5°, and 7° varus alignment with respect to the mechanical axis of the tibia and the tray implanted at the corresponding angles. Tibial tray orientation was simulated without change in limb alignment (i.e. maintaining the mechanical axis of the knee at 0°) and with limb alignment ranging from 3° valgus to 7° varus (Fig 1). A static load equivalent to three times the bodyweight of the subject was applied in line with the mechanical knee axis. Relative motion between the tibial tray and tibial bone was calculated. Elements with an equivalent von Mises strain >0.4% were selected and assigned an elastic modulus of 5 MPa to reflect damaged bone. Simulation was repeated and after-damage micromotion recorded.Introduction
Methods
It is difficult for surgeons to make the decision on which design or material to use given multiple available options for total knee arthroplasty. Due to the complex interaction of soft tissue, implant position, patient anatomy, and kinematic demands of the patient, the prosthetic design of a knee device has traditionally been more important than materials. The purpose of this study was to examine the overall influence of both implant design and materials on volumetric wear rates in an Two different designs (single radius and J-curve) with two highly crosslinked materials (Sequentially crosslinked and annealed PE (X3®, Stryker Orthopaedics, Mahwah, NJ) (7.5 kGy moderately crosslinked UHMWPE (XLPE, Smith and Nephew, Memphis, TN)) were evaluated. The two designs tested were the Triathlon® CR knee system (single radius design)(Stryker Orthopaedics, Mahwah, NJ) and the Legion™ Oxinium® CR knee system (J-curve design) (Verilast™, Smith and Nephew, Memphis, TN). Three inserts per condition were tested in this study. This comparison incorporates the effects of both materials and designs: different femoral component materials, different tibial bearing materials, and implant geometry (J-curve vs. single radius saggital profile). All devices were tested under ISO 14243-3 normal walking using an MTS knee simulator for a total of 5 million cycles. Standard test protocols were used for cleaning, weighing and assessing the wear loss of the tibial inserts (ASTM F2025). Soak control specimens were used to correct for fluid absorption with weight loss data converted to volumetric data (by material density). Statistical analysis was performed using the Student's t-test. Total volume loss results are shown in Figure 1. Test results show a 36% reduction (p<0.05) in volume loss and a 30% reduction (p<0.05) in wear rate for the single radius design compared to the J-curve design, respectively. All comparisons are statistically significant by the t-test method (p<0.05). Visual examination of all worn inserts revealed typical wear scars and features on the condylar surfaces, including burnishing. Results indicate superior wear resistance for the single radius system. This finding indicates that a combination of implant design and prosthesis material plays a significant role in knee wear rates. The
Wear and polyethylene damage have been implicated in up to 22% of revision surgeries after unicompartmental knee replacement. Two major design rationales to reduce this rate involve either geometry and/or material strategies. Geometric options involve highly congruent mobile bearings with large contact areas; or moderately conforming fixed bearings to prevent bearing dislocation and reduce back-side wear, while material changes involve use of highly crosslinked polyethylene. This study was designed to determine if a highly crosslinked fixed-bearing design would increase wear resistance. Gravimetric wear rates were measured for two unicompartmental implant designs: Oxford unicompartmental (Biomet) and Triathlon X3 PKR (Stryker) on a knee wear simulator (AMTI) using the ISO-recommended standard. The Oxford design had a highly conforming mobile bearing of compression molded Polyethylene (Arcom). The Triathlon PKR had a moderately conforming fixed bearing of sequentially crosslinked Polyethylene (X3). A finite element model of the AMTI wear simulation was constructed to replicate experimental conditions and to compute wear. This approach was validated using experimental results from previous studies. The wear coefficient obtained previously for radiation-sterilized low crosslinked polyethylene was used to predict wear in Oxford components. The wear coefficient obtained for highly crosslinked polyethylene was used to predict wear in Triathlon X3 PKR components. To study the effect design and polyethylene crosslinking, wear rates were computed for each design using both wear coefficients.INTRODUCTION
METHODS
Wear and fracture of patellar components has been frequently reported as a failure mode for cemented and press-fit patellar components. Malalignment of the patellar components may cause higher contact stresses, which may lead to excessive wear, delamination, and/or component fracture. A 6 station MTS (Eden Prairie, MN) knee joint wear simulator and Alpha Calf Fraction serum (Hyclone Labs, Logan, UT) diluted to 50% with a pH-balanced 20-mMole solution of deionized water and EDTA was used (protein level = 20 g/l) for testing. Asymmetric, all-polyethylene, patellar components with an overall construct thickness of 11 mm (Duracon®, Stryker Orthopaedics, Mahwah, NJ) were used. Appropriately sized cobalt-chrome femoral components articulated against the patellae. The patellae were cemented (Simplex, Stryker Orthopaedics, Mahwah, NJ) to delrin fixtures, which placed the patella in 10° of lateral tilt (Figure 1). This angle was chosen based off the work of Huang et al, which was one of the larger average tilt angles reported The loading and kinematic profiles used for testing were published previously (maximum axial load: 2450N and maximum patellofemoral angle: 54°. Variations of the loading profile were studied by evaluating the effects of heavier patients, which increased the maximum axial load to 3100N(250lb patient) and 3750N(300lb patient) (Figure 2). Lateral offset was tested to evaluate the effect of malalignment. Increments of 1mm were analyzed starting from the neutral position, eventually reaching a maximum lateral offset of 5mm. A 6-dof load cell was placed beneath the patella fixturing to capture dynamic loads (ATI, Apex, NC). The axial and medial/lateral shear loads where used to calculate the resultant medial/lateral shear force being applied to the patellar pegs.INTRODUCTION
MATERIALS AND METHODS
Recent studies indicated that the knee has a single flexion/extension axis but debated the location of this axis. The relationship of the flexion/extension axis in the coronal plane to the mechanical axis has received little attention. The purpose of this study was to investigate the relationship of the various axes and references with respect to the mechanical axis in the coronal plane Subjects were prospectively scanned into a Virtual Bone Database (Stryker Orthopaedics, Mahwah, NJ). Database is a collection of body CT scans from subjects collected globally. Only CT Scans that met the following qualifications were accepted: ≤1 mm voxels and had slice thickness that was equal to the spacing between the slices (≤ 1.0mm). For each CT Scan, a frontal plane was created through the 2 most posterior points of the medial/lateral condyles and the most posterior point of the trochanter. Then, a transverse plane was created perpendicular to the frontal plane and bisects the 2 most distal points on the medial/lateral condyles. Finally, a saggital plane was created that was perpendicular to the frontal and transversal planes. The following axes were identified: Mechanical Axis of the Femur (MAF) (line between the center of the femoral head and the center of the knee sulcus); Transepicondylar Axis Posterior Cylindrical Axis (PCA) (line between the Medial/Lateral Condylar Circle – best fit circle to three points identified on surface). Measurements made: Angle of MAF and the Joint-Line (Femoral Joint Angle), Angle of the MAF and the Transepicondylar Axis (Femoral TE Angle), and Angle of the MAF and the Posterior Cylindrical Axis (Femoral PC angle). Angles measuring 90° were neutral or perpendicular to the MAF. Angles measured <90° were valgus and >90° were varus.INTRODUCTION
MATERIALS AND METHODS
Many studies have looked at the effects of titanium tibial baseplates compared to cobalt chrome baseplates on backside wear. However, the surface finish of the materials is usually different (polished/unpolished) [1,2]. Backside wear may be a function not only of tray material but also of the locking mechanism. The purpose of this study was to evaluate the wear performance of conventional polyethylene inserts when mated with titanium tibial trays or cobalt chrome tibial trays that both have non-polished topside surfaces. Three titanium (Ti) trays were used along with three cobalt chrome (CoCr) trays. The Ti trays underwent Type II anodization prior to testing. All trays were Triathlon® design (Stryker Orthopaedics, Mahwah, NJ). Tibial inserts were manufactured from GUR 1020 conventional polyethylene then vacuum/flush packaged and sterilized in nitrogen (30 kGy). Appropriate sized CoCr femoral components articulated against the tibial inserts (Triathlon®, Stryker Orthopaedics, Mahwah, NJ). Surface roughness of the tibial trays was taken prior to testing using white light interferometry (Zygo Corp, Middlefield, CT). A 6-station knee simulator (MTS, Eden Prairie, MN) was used for testing. Two phases were conducted. The first phase used a normal walking profile, as dictated by ISO 14243-3 [3]. The second phase used waveforms created specifically for stair climbing kinematics. Testing was conducted at a frequency of 1 Hz for 2 million cycles for each test with a lubricant of Alpha Calf Fraction serum (Hyclone Labs, Logan, UT) diluted to 50% with a pH-balanced 20-mMole solution of deionized water and EDTA (protein level = 20 g/l) [4]. The serum solution was replaced and inserts were weighed for gravimetric wear at least every 0.5 million cycles. Standard test protocols were used for cleaning, weighing and assessing the wear loss of the tibial inserts [5]. Soak control specimens were used to correct for fluid absorption with weight loss data converted to volumetric data (by material density). Statistical analysis was performed using the Student's t-test (p<0.05).INTRODUCTION
MATERIALS AND METHODS
Single use instrumentation had a significant reduction on OR Turnover time and instrument setup/clean up time compared to traditional instrumentation. Recently, focus has shifted to improving OR efficiency by surgeons and hospital admin. The purpose of this study was to determine the effect of traditional instrumentation vs. single use instrumentation (SUI) on OR efficiency in navigated primary TKA.Summary
Introduction
For cementless TKA, highly crosslinked UHWMPE is traditionally used with modular components because of manufacturing and sterilization complexities of monoblock metal-backed components. However, it would be very useful to have a highly crosslinked UHMWPE monoblock metal-backed cementless component to address historical clinical issues. The purpose of this study was to evaluate the wear properties of a unique process for achieving a monoblock metal-backed cementless component featuring highly crosslinked polyethylene to standard highly crosslinked UHWMPE. The knee system used for testing consisted of cobalt chrome femoral components and tibial trays (Triathlon®, Stryker Orthopaedics, Mahwah, NJ). Modular tibial inserts were machined from GUR 1020 polyethylene that was irradiated to 30 kGy and annealed three times (Modular, n=5) (X3, Stryker Orthopaedics, Mahwah, NJ). Monoblock tibias were direct compression molded to a metal substrate and then irradiated to 30 kGy and annealed three times. For the purposes of this test, the polyethylene was removed from the monoblock component and machined into a standard tibial insert (Monoblock, n=5). A 6-station knee simulator was utilized for testing (MTS, Eden Prairie, MN). All motion and loading was computer controlled and waveforms followed ISO 14243-3 [1]. Testing was conducted at a frequency of 1 Hz for 3 million cycles. The lubricant used was Alpha Calf Fraction serum (Hyclone Labs, Logan, UT) diluted to 50% with a pH-balanced 20-mMole solution of deionized water and EDTA [2]. The serum solution was replaced and inserts were weighed for gravimetric wear at least every 0.5 million cycles. Standard test protocols were used for cleaning, weighing and assessing the wear loss of the tibial inserts [3]. Soak control specimens were used to correct for fluid absorption with weight loss data converted to volumetric data (by material density). Statistical analysis was performed using the Student's t-test with significance determined at the 95% confidence level (p < 0.05).INTRODUCTION
MATERIALS AND METHODS
Traditional instrumentation relies on rigid IM rods to determine the distal femoral resection which influences size and orientation of the femoral component. Anterior femoral bowing may unexpectedly affect implant sizing. The purpose of this study was to determine the sensitivity of a flexible rod to the femoral anterior bow versus a traditional rod. A database of 93 Asian bone models from CT images was utilized. The bones were subdivided into those having proximal third, distal third, or overall femoral bows. Only the latter group was selected for further analysis, which consisted of 54 with an average bow of 98cm (±20cm). The rigid and flexible rods were placed iteratively so that the proximal portion of the rod touched the anterior cortical-cancellous boundary and no portion of the rod protruded through that boundary. The flexible rod was allowed to flex, as a substantially thin central portion flexes exclusively in the sagittal plane. The relative angle difference between the position of the flexible and rigid rod were calculated. Three femura were chosen from the subset with bows of 123cm, 100cm and 78cm. The femura showed differences between the rigid and flexible rod of 7.5°, 4.5° while no significant angle measured for the smallest bow. Implants were virtually assembled onto the bones and the greatest bowed femur's component reduced one size from the rigid to the flexible rod orientation. The results of this study show that higher bowed femura yielded larger angular deviations between rigid and flexible rods. For higher bowed femura, the flexible rod allows smaller components to be implanted. The flexible rod serves the same purpose as a conventional rod by defining the distal valgus orientation but allows component orientation in the sagittal plane closer to the femoral bow.
Aligning the tibial tray is a critical step in total knee arthroplasty (TKA). Malalignment, (especially in varus) has been associated with failure and revision surgery. While the link between varus malalignment and failure has been attributed to increased medial compartmental loading and generation of shear stress, quantitative biomechanical evidence to directly support this mechanism is incomplete. We therefore constructed and validated a finite element model of knee arthroplasty to test the hypothesis that varus malalignment of the tibial tray would increase the risk of tray subsidence.Introduction
Methods
