Preoperative templating of femoral and tibial components can assist in choosing the appropriate implant size prior to TKA. While weight bearing long limb roentograms have been shown to provide benefit to the surgeon in assessing alignment, disease state, and previous pathology or trauma, their accuracy in size prediction is continually debated due to scaling factors and rotated views. Further, they represent a static time point, accounting for boney anatomy only. A perceived benefit of robotic-assisted surgery is the ability to pre-operatively select component sizes with greater accuracy based on 3D information, however, to allow for flexibility in refining based on additional data only available at the time of surgery. The purpose of this study was to determine the difference of pre-operative plans in size prediction of the tibia, femur, and polyethylene insert. Eighty four cases were enrolled at three centers as part of an Investigational Device Exemption to evaluate a robotic-assisted TKA. All patients had a CT scan as part of a pre-operative planning protocol. Scans were segmented and implant sizes predicted based on the patients boney morphology and an estimated 2mm cartilage presence. Additional information such as actual cartilage presence and soft tissue effects on balance and kinematics were recorded intra-operatively. Utilizing this additional information, surgical plans were fine tuned if necessary to achieve minimal insert thickness and balance. Data from the Preoperative CT plan sizing and final size were compared to determine the percentage of size and within one size accuracy.Introduction
Methods
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:
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
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
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
