Aims

It is unknown whether gap laxities measured in robotic arm-assisted total knee arthroplasty (TKA) correlate to load sensor measurements. The aim of this study was to determine whether symmetry of the maximum medial and lateral gaps in extension and flexion was predictive of knee balance in extension and flexion respectively using different maximum thresholds of intercompartmental load difference (ICLD) to define balance.

Introduction. The patella experiences large forces and variable kinematic patterns throughout flexion which could influence function and patient satisfaction after a total knee arthroplasty (TKA). Therefore, the objective of this study is to analyze in vivo patellar mechanism forces and kinematics throughout flexion to determine influencing factors that may lead to patient dissatisfaction. Methods. Fifty subjects were evaluated in this study, 40 having a Journey II bi-cruciate stabilized (BCS) TKA and 10 having normal, healthy knees. Similar demographics were controlled for each group. Each subject performed a deep knee bend. Kinematics were evaluated using a validated 3D-to-2D fluoroscopic technique while forces were determined using a validated inverse mathematical knee model. A two-tailed t-test was used to evaluate statistical significance. Results. Subjects averaged 2.96 ± 0.30 xBW and 2.82 ± 0.27 xBW of maximum femorotibial contact forces, 4.07 ± 0.39 xBW and 3.30 ± 0.55 xBW of maximum quadriceps forces, and 4.20 ± 0.94 xBW and 3.94 ± 0.67 xBW of maximum patellofemoral forces for the BCS and normal groups, respectively. Therefore, the TKA subjects in this study experienced larger, but similar forces compared to the normal subjects. For both groups, lateral condylar rollback was correlated with both femorotibial contact forces (p<0.0001) and quadriceps forces (p<0.0002). Higher patellotibial tilt was correlated with lower patellofemoral contact forces (p=0.0294). Conclusion. Most TKAs resect the ACL and only substitute for the PCL in PS TKAs, but BCS subjects may receive an advantage for ACL substitution as they experienced normal-like kinematic and kinetic patterns. The lone exception seems to be the quadriceps forces which were higher in the BCS TKA group. This could be influenced by femoral condylar shape, leading to earlier quadriceps wrapping

Background. While posterior cruciate retaining (PCR) implants are a more common total knee arthroplasty (TKA) design, newer bi-cruciate retaining (BCR) TKAs are now being considered as an option for many patients, especially those that are younger. While PCR TKAs remove the ACL, the BCR TKA designs keep both cruciate ligaments intact, as it is believed that the resection of the ACL greatly affects the overall kinematic patterns of TKA designs. Various fluoroscopic studies have focused on determination of kinematics but haven't defined differentiators that affect motion patterns. This research study assesses the importance of the cruciate ligaments and femoral geometry for Bi-Cruciate Retaining (BCR) and Posterior Cruciate Retaining (PCR) TKAs having the same femoral component, compared to the normal knee. Methods. The in vivo 3D kinematics were determined for 40 subjects having a PCR TKA, 10 having a BCR TKA, and 10 having a normal knee, in a retrospective study. All TKA subjects had the same femoral component. All subjects performed a deep knee bend under fluoroscopic surveillance. The kinematics were determined during early flexion (ACL dominant), mid flexion (ACL/PCL transition) and deep flexion (PCL dominant). Results. During the first 30 degrees of flexion, the ACL played an important role, as subjects having a BCR TKA experienced kinematic patterns more similar to the normal knee. During mid flexion, both TKAs experienced random kinematic patterns, which could be due to the ACL and PCL being less active or resected in PCR TKA. In deeper flexion, both TKAs experienced kinematic patterns similar to the normal knee, thus supporting the assumption that the PCL played a dominant role [Fig. 1, Fig. 2]. All three groups generally experienced progressive axial rotation throughout flexion [Fig. 3]. On average, subjects having a PCR TKA experienced 112.3° of flexion, which was greater than the BCR subjects. Conclusions. Both the BCR TKA and normal groups experienced similar kinematic patterns, but the femoral geometrical differences from the anatomical condition may have influenced decreased motion compared to the normal knee. Both TKAs experienced similar kinematic patterns in deeper flexion, with the PCR TKA experiencing excellent weight-bearing flexion. Results from this study suggest that the cruciate ligaments can play a role in kinematics, but femoral geometry working with the ligaments may be an option to consider

Introduction. A common goal of total knee arthroplasty (TKA) is to restore normal knee kinematics. While substantial data is available on TKA kinematics, information regarding non-implanted knee kinematics is less well studied especially in larger patient populations. The objectives of this study were to determine normal femorotibial kinematics in a large number of non-implanted knees and to investigate parameters that yield higher knee flexion with weight-bearing activities. Methods. Femorotibial kinematics of 104 non-implanted healthy subjects performing a deep knee bend (DKB) activity were analyzed using 3D to 2D fluoroscopy. The average age and BMI were 38.1±18.2 years and 25.2±4.6, respectively. Pearson correlation analysis was used to determine statistical correlations. Results. On average, subjects experienced 21.5±7.2 mm, 13.8±8.9 mm, and 27.1°±12.1° of lateral rollback, medial rollback, and external femorotibial axial rotation, respectively (Figure 1). Most rollback occurred in early flexion, with 10.2±6.4 mm and 5.3±6.3 mm of rollback for the lateral and medial condyles, respectively. While the lateral condyle consistently moved posteriorly, the medial condyle experienced 1.8±4.8 mm of anterior sliding between 90° to 120° of flexion. There was a positive correlation between higher weight-bearing flexion and lateral condylar rollback (r=0.5480, p<.0001) (Figure 2), medial condylar rollback (r=0.3188, p=0.001) (Figure 3), and external axial rotation (r=0.5505, p<.0001) (Figure 4). There was an inverse correlation between advancing age and knee flexion (r=-0.7358, p<.0001) as well as higher BMI and flexion (r=-0.3332, p=0.0007), indicating that multiple factors contribute to postoperative range-of-motion. Conclusion. This represents one of the largest studies on normal knee femorotibial kinematics in non-implanted healthy subjects. These results indicate that increased condylar rollback and external axial rotation correlate with increased weight-bearing knee flexion, while increased age and BMI yield decreased flexion. Therefore, in order to achieve higher weight-bearing flexion following TKA, normal-like kinematics such as high rollback and external axial rotation should be incorporated into TKA design. For any figures or tables, please contact the authors directly

Aims

This study aims to investigate the effects of posterior tibial slope (PTS) on knee kinematics involved in the post-cam mechanism in bi-cruciate stabilized (BCS) total knee arthroplasty (TKA) using computer simulation.

Aims. Mobile-bearing unicompartmental knee arthroplasty (UKA) with a flat tibial plateau has not performed well in the lateral compartment, leading to a high rate of dislocation. For this reason, the Domed Lateral UKA with a biconcave bearing was developed. However, medial and lateral tibial plateaus have asymmetric anatomical geometries, with a slightly dished medial and a convex lateral plateau. Therefore, the aim of this study was to evaluate the extent at which the normal knee kinematics were restored with different tibial insert designs using computational simulation. Methods. We developed three different tibial inserts having flat, conforming, and anatomy-mimetic superior surfaces, whereas the inferior surface in all was designed to be concave to prevent dislocation. Kinematics from four male subjects and one female subject were compared under deep knee bend activity. Results. The conforming design showed significantly different kinematics in femoral rollback and internal rotation compared to that of the intact knee. The flat design showed significantly different kinematics in femoral rotation during high flexion. The anatomy-mimetic design preserved normal knee kinematics in femoral rollback and internal rotation. Conclusion. The anatomy-mimetic design in lateral mobile UKA demonstrated restoration of normal knee kinematics. Such design may allow achievement of the long sought normal knee characteristics post-lateral mobile UKA. However, further in vivo and clinical studies are required to determine whether this design can truly achieve a more normal feeling of the knee and improved patient satisfaction. Cite this article: Bone Joint Res 2020;9(7):421–428

Introduction. Forward solution joint models (FSMs) can be powerful tools, leading to fast and cost-efficient simulation revealing in vivo mechanics that can be used to predict implant longevity. Unlike most joint analysis methods, mathematical modeling allows for nearly instantaneous evaluations, yielding more rapid surgical technique and implant design iterations as well as earlier insight into the follow-up outcomes used to better assess potential success. The current knee FSM has been developed to analyze both the kinematics and kinetics of commercial TKA designs as well as novel implant designs. Objective. The objective of this study was to use the knee FSM to predict the condylar translations and axial rotation of both fixed- and mobile-bearing TKA designs during a deep knee bend activity and to compare these kinematics to known fluoroscopy evaluations. Methods. The knee joint is modeled mathematically using Kane's dynamics, incorporating muscle controllers to predict the muscle forces, contact detection algorithms to compute the knee joint forces, and nonlinear ligaments at the knee joint. The tibiofemoral kinematics data for 20 subjects implanted with fixed-bearing (FB) PS TKA and 20 subjects implanted with mobile-bearing (MB) PS TKA were collected using fluoroscopy data during a deep knee bend (DKB) activity from full extension to 120° of flexion. All subjects were implanted by the same surgeon. The same CAD models for these implanted were incorporated in the FSM to predict the tibiofemoral kinematics. The average component placement from fluoroscopy data were used as an initial condition for the placement of the component in the mathematical model. Results. Overall, fluoroscopy results showed patients experienced 6.8 mm and 6.4 mm posterior rollback of the lateral femoral condyle for FB and MB PS TKA groups, respectively. The FSM predicted 5.9 mm and 6.3 mm of lateral posterior rollback for FB and MB PS TKA models, respectively (Figure 1). On average, media condyle translated posteriorly −2.9 mm and −2.5 mm, for FB and MB subjects, respectively. The mathematical model prediction for FB and MB models was −1.4 mm and −2.4 mm, respectively (Figure 2). The overall axial rotation was 5.1° and 4.5°, for FB and MB subjects from fluoroscopy, respectively. The axial rotation prediction using the FSM was 6.0° and 4.2°, for FB and MB models, respectively (Figure 3). Conclusion. Overall, it is clear that the FSM can accurately predict both the patterns and magnitudes of fixed- and mobile-bearing TKA condylar translations and axial rotations, showing consistent rollback of the lateral condyle, less translation of the medial condyle, and consistent axial rotation throughout flexion, all of which were also observed in the fluoroscopy data. The correlation between the theoretically predicted and experimentally confirmed kinematic patterns demonstrates the viability of forward solution modeling as a valuable and accurate method to evaluate total joint replacement mechanics. For any figures or tables, please contact the authors directly

Introduction. Aseptic loosening is one of the highest causes for revision in total knee arthroplasty (TKA). With growing interest in anatomically aligned (AA) TKA, it is important to understand if this surgical technique affects cemented tibial fixation any differently than mechanical alignment (MA). Previous studies have shown that lipid/marrow infiltration (LMI) during implantation may significantly reduce fixation of tibial implants to bone analogs [1]. This study aims to investigate the effect of surgical alignment on fixation failure load after physiological loading. Methods. Alignment specific physiological loading was determined using telemetric tibial implant data from Orthoload [2] and applying it to a validated finite element lower limb model developed by the University of Denver [3]. Two high demand activities were selected for the loading section of this study: step down (SD) and deep knee bend (DKB). Using the lower limb model, hip and ankle external boundary conditions were applied to the ATTUNE. ®. knee system for both MA and AA techniques. The 6 degree of freedom kinetics and kinematics for each activity were then extracted from the model for each alignment type. Mechanical alignment (MA) was considered to be neutral alignment (0° Hip Knee Ankle Angle (HKA), 0° Joint Line (JL)) and AA was chosen to be 3° varus HKA, 5° JL. It is important not to exceed the limits of safety when using AA as such it is noted that DePuy Synthes recommends staying within 3º varus HKA and 3º JL. The use of 5º JL was used in this study to account for surgical variation [Depuy-Synthes surgical technique DSUS/JRC/0617/2179]. Following a similar method described by Maag et al [1] ATTUNE tibial implants were cemented into a bone analog with 2 mL of bone marrow in the distal cavity and an additional reservoir of lipid adjacent to the posterior edge of the implant. Tibial implant constructs were then subjected to intra-operative ROM/stability evaluation, followed by a hyperextension activity until 15 minutes of cement curing time, and finally 3 additional ROM/stability evaluations were performed using an AMTI VIVO simulator. The alignment specific loading parameters were then applied to the tibial implants using an AMTI VIVO simulator. Each sample was subjected to 50,000 DKB cycles and 120,000 SD cycles at 0.8 Hz in series; approximating 2 years of physiological activity. After physiological loading the samples were tested for fixation failure load by axial pull off. Results. Following alignment specific physiological loading the average fixation pull-off load for MA was 3289 ± 400 N and for AA was 3378 ± 133 N (Figure 1). There was no statistically significant difference fixation failure load by axial pull-off between the two alignment types (p=0.740). Conclusion. This study indicated that anatomic alignment, as defined with the alignment limits of this study, does not adversely affect the fixation failure load of ATTUNE tibial implants. For any figures or tables, please contact the authors directly

INTRODUCTION. Implant wear testing is traditionally undertaken using standardized inputs set out by ISO or ASTM. These inputs are based on a single individual performing a single activity with a specific implant. Standardization helps ensure that implants are tested to a known set of parameters from which comparisons may be drawn but it has limitations as patients perform varied activities, with different implant sizes and designs that produce different kinematics/kinetics. In this study, wear performance has been evaluated using gait implant specific loading/kinematics and comparing to a combination deep knee bend (DKB), step down (SD) and gait implant specific loading on cruciate retaining (CR) rotating platform (RP) total knee replacements (TKR). This combination activity profile better replicates patient activities of daily living (ADL). METHODS. Two sets of three ATTUNE. ®. size 5 right leg CR RP TKRs (DePuy Synthes, Warsaw, IN) were used in a study to evaluate ADL implant wear. Implant specific loading profiles were produced via a validated finite element lower limb model [1] that uses activity data such as gait (K1L_110108_1_86p), SD (K1L_240309_2_144p), and DKB (K9P_2239_0_9_I1) from the Orthoload database [2] to produce external boundary conditions. Each set of components were tested using a VIVO joint simulator (AMTI, Watertown, MA, Figure 1) for a total of 4.5 million cycles (Mcyc). All cycles were conducted at 0.8Hz in force-control with flexion driven in displacement control. Bovine calf serum lubricant was prepared to a total protein concentration of 18g/L and maintained at 37°±2°C. Wear of the tibial inserts was quantified via gravimetric methods per ISO14243–2:2009(E). Polyethylene tibial insert weights were taken prior to testing and every 0.5Mcyc there after which corresponded to serum exchange intervals. The multi-activity test intervals were split into10 loops of 1,250 DKB, 3,000 SD, and 45,750 gait cycles in series. Based on activity data presented by Wimmer et al. the number of cycles per activity and activities used is sufficient for a person that is considered active [3]. A loaded soak control was used to compensate for fluid absorption in wear rate calculations. Wear rates were calculated using linear regression. RESULTS SECTION. The wear rate for the gait-only activity test was calculated to be 0.20±0.04mg/Mcyc conversely the wear rate for the multi-activity test was 2.65±0.67mg/Mcyc (Figure 2). Wear scars can be found in Figure 3. Using a two-sided t-test of unequal variance, it was found that there was a significant difference between the two wear rates (p=0.004). DISCUSSION. Adding activities to the wear simulation test significantly increased the average wear rate of the test samples, confirming that changes in cross shear from different activities will tend to increase the wear of an implant. The results of this study prove that single activity wear testing may not be the most clinically severe wear testing that can be used for pre-clinical wear assessment. For any figures or tables, please contact the authors directly

Introduction. Numerous fluoroscopic studies have been conducted to investigate kinematic variabilities of total knee arthroplasty (TKA). In those studies, subjects having a posterior stabilized (PS) TKA experience greater weightbearing knee flexion and posterior femoral rollback of the lateral condyle. In those same studies, subjects did experience a high incidence of variable medial condyle motion and reverse axial rotation, especially occurring when the cam engaged the post. More recently, a PS TKA was designed to accommodate both gender and ethnicity. Therefore, the objective of this study was to assess in vivo kinematics for subjects having this TKA type to determine if subjects having this PS TKA experienced more optimal knee kinematics. Methods. Twenty-five subjects in this study were asked to perform a deep knee bend to maximum knee flexion and a step-up maneuver while under fluoroscopic surveillance. All subjects were patients of one experienced surgeon and received the same PS TKA. Using a 3D-2D registration technique, the CAD models, supplied by the sponsoring company, were superimposed over x-ray images at specified increments throughout the fluoroscopic footage. The kinematics were then analyzed to evaluate lateral anterior/posterior (LAP) and medial anterior/posterior (MAP) condyle translation as well as axial rotation of the femur with respect to the tibia. Results. During the DKB activity, the average flexion for the PS TKA subjects was 1108°. On average subjects experienced a lateral condyle motion in the posterior direction of 7.3mm, with the maximum amount of posterior rollback being 12.8 mm. These same subjects experienced an average medial condyle motion in the posterior direction of 4.8 mm with the maximum amount of posterior motion being 7.8 mm. Therefore, with the lateral condyle rolling more posterior than the medial condyle, these subjects experienced an average amount of 7.1° of axial rotation, with a maximum of 12.0°. Only one subject in this study experienced a reverse axial rotation from full extension to maximum knee flexion. During the step-up maneuver, subjects consistently experienced a roll forward motion of both their condyles. Discussion. Subjects in this study experienced a high incidence and magnitude of lateral condyle posterior femoral rollback, leading a normal-like axial rotation pattern, although less in magnitude compared to the normal knee. There was variability occurring with the medial condyle as some experience experienced an anterior slide while others rolled in the posterior direction. As seen in previous studies, during mid flexion both condyles experienced a more variable motion pattern. Twenty-five subjects having a posterior cruciate retaining TKA are being added to this study to determine if retention of the PCL in a similarly designed TKA leads to more normal-like kinematic patterns

Introduction. Patella implant research is often overlooked despite its importance as the third compartment in a total knee replacement. Wear and fracture of resurfaced patellae can lead to implant failure and revision surgeries. New simulation techniques have been developed to analyze the performance of patella designs as they interact with the trochlear groove in total knee components, and clinical validation is sought to ensure that these simulations are appropriate. The objective of this work was to subject several patellar designs to patient-derived deep knee bend (DKB) inputs on a 6 degree of freedom (DOF) simulator and compare the resultant wear scars to clinical retrievals. Materials and Methods. Previously reported DKB profiles were developed based on in vivo patellofemoral data and include a wide range of patient variability. The profiles chosen for this body of work were based on the stress in the patellar lateral facet; maximizing this stress whilst maintaining the ability to run the profile stably on the simulator. Load/kinematic profiles were run on three patellar designs (n=3 per group) for 220,000 cycles at 0.8Hz on an AMTI VIVO joint simulator. A comparison cohort of clinically retrieved devices of the same design was identified in an IRB-approved database. Exclusion criteria included gross delamination, cracking secondary to oxidation, and surgeon-reported evidence of malalignment leading to mal-tracking. 29 Patellae were included for analysis: PFC. ®. All Poly (n=14), ATTUNE. ®. Anatomic (n=6), and ATTUNE. ®. Medialized Dome (n=9). Mean in vivo duration was 70.1 months. Patellae were analyzed under optical microscope in large-depth-of-field mode to map the surface damage profile. Burnishing ‘heat-maps’ were generated for retrievals and simulated patellae by normalizing the patellar size and overlaying silhouettes from each component of the same type using a custom-developed MatLAB code. Results. Burnishing heat-map comparisons between retrievals and simulator specimens for each of the three designs were compared. Retrievals show more variation than simulator devices, however the general loci and relative area of burnished regions is closely aligned for each of the three designs. The retrieved and simulated burnishing scar heat-maps on all-poly PFC. ®. patellae are centered medio-laterally with a wider profile on the lateral aspect. The burnishing marks are continuous. A similar observation may be made of the ATTUNE. ®. medialized dome, retrievals and simulator specimens, though the contact areas appear to be more concentrated away from the apex. The anatomic patellae show two primary regions of contact, and minimal burnishing at the apex. The simulator specimens likewise show two principal regions of contact. Discussion. Wear scar analysis shows that joint simulation on AMTI VIVO yields clinically relevant wear patterns across a variety of device types. Clinically relevant damage provides insight that load and motion inputs to the simulator deliver results that may be used to interpret in vivo performance or analyze future designs and/or materials. This qualitative surface contact analysis will help to inform future quantitative mass loss and fatigue failure studies. For any figures or tables, please contact authors directly

INTRODUCTION. While computational models have been used for many years to contribute to pre-clinical, design phase iterations of total knee replacement implants, the analysis time required has limited the real-time use as required for other applications, such as in patient-specific surgical alignment in the operating room. In this environment, the impact of variation in ligament balance and implant alignment on estimated joint mechanics must be available instantaneously. As neural networks (NN) have shown the ability to appropriately represent dynamic systems, the objective of this preliminary study was to evaluate deep learning to represent the joint level kinetic and kinematic results from a validated finite element lower limb model with varied surgical alignment. METHODS. External hip and ankle boundary conditions were created for a previously-developed finite element lower limb model [1] for step down (SD), deep knee bend (DKB) and gait to best reproduce in-vivo loading conditions as measured on patients with the Innex knee (. orthoload.com. ) (Figure1). These boundary conditions were subsequently used as inputs for the model with a current fixed-bearing total knee replacement to estimate implant-specific kinetics and kinematics during activities of daily living. Implant alignments were varied, including variation of the hip-knee-ankle angle-±3°, the frontal plane joint line −7° to +5°, internal-external femoral rotation ±3°, and the tibial posterior slope 5° and 0°. Through varying these parameters a total of 2464 simulations were completed. A NN was created utilizing the NN toolbox in MATLAB. Sequence data inputs were produced from the alignment and the external boundary conditions for each activity cycle. Sequence outputs for the model were the 6 degree of freedom kinetics and kinematics, totaling 12 outputs. All data was normalized across the entire data set. Ten percent of the simulation runs were removed at random from the training set to be used for validation, leaving 2220 simulations for training and 244 for validation. A nine-layer bi-long short-term memory (LSTM) NN was created to take advantage of bi-LSTM layers ability to learn from past and future data. Training on the network was undertaken using an RMSprop solver until the root mean square error (RMSE) stopped reducing. Evaluation of NN quality was determined by the RMSE of the validation set. RESULTS. The trained NN was able to effectively estimate the validation data. Average RMSE over the kinetics of the validation data set was 140.7N/N∗m while the average RMSE over the kinematics of the validation data set was 4.47mm/deg (Figure 2,3–DKB, Gait shown). It is noted the error may be skewed by the larger magnitude kinetics and kinematics in the DKB activity as the average RMSE for just SD and gait was 85.9N/N∗m and 2.8mm/deg for the kinetics and kinematics, respectively. DISCUSSION. The accuracy of the generated NN indicates its potential for use in real-time modeling, and further work will explore additional changes in post-operative soft-tissue balance as well as scaling to patient-specific geometry

Introduction. Many fluoroscopic studies on total knee arthroplasty (TKA) have identified kinematic variabilities compared to the normal knee, with many subjects experiencing paradoxical motion patterns. The intent of this study was to investigate the results of a newly designed PCR TKA to determine kinematic variabilities and assess these kinematic patterns with those previously documented for the normal knee. Methods. The study involves determining the in vivo kinematics for 80 subjects compared to the normal knee. 10 subjects have a normal knee, 40 have a Journey II PCR TKA and 40 subjects with the Journey II XR TKA (BCR). Although all PCR subjects have been evaluated, we are continuing to evaluate subjects with a BCR TKA. All TKAs were performed by a single surgeon and deemed clinically successful. All subjects performed a deep knee bend from full extension to maximum flexion while under fluoroscopic surveillance. Kinematics were calculated via 3D-to-2D registration at 30° increments from full extension to maximum flexion. Anterior/posterior translation of the medial (MAP) and lateral (LAP) femoral condyles and femorotibial axial rotation were compared during ranges of motion in relation to the function of the cruciate ligaments. Results. Of the 40 PCR TKAs, the average overall flexion was 112.6°, while the average for normal subjects was 139.0°. Initial BCR subjects revealed a higher than expected 128.0°. From 0=30° knee flexion, PCR subjects demonstrated −4.74±4.94 mm of posterior LAP movement, −2.04±4.07 mm of MAP movement and 3.61±8.13° of external axial rotation. In the same range of motion, normal subjects exhibited −8.80±3.32 mm of LAP movement, −3.81±1.03 mm of MAP movement and an axial rotation of 11.34±3.78°. From 30=90° knee flexion, PCR subjects demonstrated 4.37±8.26 mm of LAP movement, 0.12±7.95 mm of MAP movement and 0.79±11.43° of axial rotation. In the same range of motion, normal subjects exhibited −4.28±3.13 mm of LAP movement, −1.11±2.76 mm of MAP movement and axial rotation of 6.54±4.33°. From 0°-maximum flexion, PCR subjects demonstrated −2.71±5.37 mm of LAP movement, 1.79±4.88 mm of MAP movement and 5.99±5.26° of axial rotation. In the same range of motion, normal subjects exhibited −17.83±6.04 mm of LAP movement, −9.11±4.93 mm of MAP movement and axial rotation of 23.66±7.81°. Overall, the BCR subject displayed kinematic patterns similar to those of a normal knee; more detailed numbers will be presented in the presentation. Discussion. Subjects having a PCR TKA experienced excellent weight-bearing flexion and kinematic patterns similar to the normal knee, but less in magnitude. These subjects experienced posterior femoral rollback in early and late flexion. During mid-flexion, subjects having a PCR TKA did experience some variable motion patterns, which may be due to the absence of the ACL. Subjects having a BCR TKA experienced more continuous rollback throughout flexion, more similar to the normal knee. Similar to the normal knee, subjects having a PCR TKA did experience progressive axial rotation throughout knee flexion (Figures). Significance. While they still experience normal-like rollback during early (0°–30°) and late flexion (90°-120°), subjects with a PCR TKA consistently demonstrated Anteriorization of the joint in mid-flexion

Background. Although early TKA designs were symmetrical, during the past two decades TKA have been designed to include asymmetry, pertaining to either the trochlear groove, femoral condylar shapes or the tibial component. More recently, a new TKA was designed to include symmetry in all areas of the design, in the hopes of reducing design and inventory costs. Objective. The objective of this study was to determine the in vivo kinematics for subjects implanted with this symmetrical TKA during a weight-bearing deep knee bend activity. Methods. In vivo deep knee bend (DKB) kinematics for 21 subjects implanted with symmetrical posterior cruciate sacrificing (PCS) fixed bearing TKA were obtained using fluoroscopy. A 3D-to-2D registration technique was used to determine each subjects anteroposterior translation of lateral (LAP) and medial (MAP) femoral condyles and tibiofemoral axial rotation and their weight-bearing knee flexion. Results. During the DKB, the average maximum weight-bearing flexion was 111.7° ± 13.3°. On average, from full extension to maximum knee flexion, subjects experienced 2.5 mm ± 2.0 mm femoral rollback on lateral condyle −2.5 mm ± 2.2 mm of medial condyle motion in the anterior direction (Figure 1). This medial condyle motion was consistent for the majority of the subjects with the lateral condyle exhibiting rollback from 0° to 60° of flexion and then an average anterior slide of 0.3 mm from 60° to 90° of flexion. On average, the subjects in this study experienced 6.6° ± 3.3° of axial rotation, with most of rotation occurring in early flexion, averaging 4.9° (Figure 2). Discussion. Although subjects in this study were implanted with a symmetrical TKA, they did experience femoral rollback of the lateral condyle and positive axial rotation. Both of these kinematic parameters were normal-like in pattern, compared to the normal knee in early flexion, but in deeper flexion the pattern of motion varied from the normal knee. Also, the magnitude of posterior femoral rollback and axial rotation revealed similarities to previous fluoroscopy studies on subjects implanted with an asymmetrical TKA design. This was only a single surgeon study, so it is unclear if the results are TKA or surgeon influenced. Therefore, it is proposed that more patients be analyzed having this TKA implanted by other surgeons. For any figures or tables, please contact the authors directly

Aims. Commonly performed unicompartmental knee arthroplasty (UKA) is not designed for the lateral compartment. Additionally, the anatomical medial and lateral tibial plateaus have asymmetrical geometries, with a slightly dished medial plateau and a convex lateral plateau. Therefore, this study aims to investigate the native knee kinematics with respect to the tibial insert design corresponding to the lateral femoral component. Methods. Subject-specific finite element models were developed with tibiofemoral (TF) and patellofemoral joints for one female and four male subjects. Three different TF conformity designs were applied. Flat, convex, and conforming tibial insert designs were applied to the identical femoral component. A deep knee bend was considered as the loading condition, and the kinematic preservation in the native knee was investigated. Results. The convex design, the femoral rollback, and internal rotation were similar to those of the native knee. However, the conforming design showed a significantly decreased femoral rollback and internal rotation compared with that of the native knee (p < 0.05). The flat design showed a significant difference in the femoral rollback; however, there was no difference in the tibial internal rotation compared with that of the native knee. Conclusion. The geometry of the surface of the lateral tibial plateau determined the ability to restore the rotational kinematics of the native knee. Surgeons and implant designers should consider the geometry of the anatomical lateral tibial plateau as an important factor in the restoration of native knee kinematics after lateral UKA. Cite this article: Bone Joint Res 2019;8:593–600

BACKGROUND. UKA is functionally superior to TKA, with kinematics similar to native knees, nevertheless, UKA implants are used in less than 10% of cases. While advantages of UKA are recognized, ACL-deficiency is generally considered a contraindication. The hypothesis of this study was that fix bearing UKA in ACL-deficient knees, with appropriate adaptation of implant placement, would result in similar kinematic trends to conventional UKA with an intact ACL. METHODS. Ten conventional UKA patients were compared to eight patients with the same implant but a deficient ACL. A 50% tibial slope reduction was applied to compensate for instability resulting from the deficient ACL. Knee kinematics were evaluated using a moving fluoroscope allowing to track the knee joint during deep knee bend, level walking, ramp descent and stair descent. The results were further compared to six TKA patients. RESULTS. During standing, a posterior shift of the femur was observed for the ACL-deficient UKA patients compared to conventional UKA patients. This posterior shift was also present during the first 25% of deep knee bend. Most parameters revealed no difference in range of motion across all activities between the two UKA groups. This is in contrast to TKA patients showing different motion trends and decreased range of motion. CONCLUSIONS. Despite the posterior femoral shift due to ACL-deficiency, both UKA groups showed similar kinematic trends, indicating that posterior tibial slope reduction can partially compensate for ACL function. This confirmed our hypothesis that fix bearing UKA can be a viable treatment option for selected ACL-deficient patients

Background. Total knee arthroplasty (TKA) is an effective surgical procedure to alleviate excruciating pain and correct dysfunction due to severe knee deformity. The satisfaction rate with current TKA is 80%, While 20% of the patients report uncomfortable feeling during stair descending and deeply knee bending. Preserving the ligaments might allow a restoration close to the natural function, although sacrifice of the ACL is common with the conventional TKA technique. The current bicruciate-retaining (BCR) TKA would be a way to go concerning this issue. This study aimed at evaluating the intraoperative kinematics and joint laxity on BCR TKA if the native function would be replicated and thus assessing the range of motion (ROM) at final followup. Methods. BCR TKAs were performed in 22 knees (12 women, 10 men, average aged 67.2-year-old) with image-free navigation system (Kolibli. TM. ) under general anesthesia. The intraoperative kinematics was evaluated about flexion extension gap (FEG), anterior-posterior translation (APT, bi-condylar rollback) and axial rotation (AR, medial pivot) with passive motion. These kinematic patterns were assessed with the post-operative ROM. Results. There was no paradoxical anterior translation in any cases. The implant kinematics was regulated to the medial pivot motion at early flexion phase and the bi-condylar rollback motion to full flexion angle. The mean flexion was changed from 132 degrees at preoperation to 126 degrees at followup, and the mean flexion contracture improved from 4 degrees to 1 degree. Conclusion. BCR TKA were preserved the nature kinematics including the medial pivot motion and rollback mechanism. Postoperative ROM was quite similar when the preoperative knee flexion was not restricted

Posterior stabilized (PS) total knee arthroplasty (TKA), wherein mechanical engagement of the femoral cam and tibial post prevents abnormal anterior sliding of the knee, is a proven surgical technique. However, many patients complain about abnormal clicking sensation, and several reports of severe wear and catastrophic failure of the tibial post have been published. In addition to posterior cam-post engagement during flexion, anterior engagement with femoral intercondylar notch can also occur during extension. The goal of this study was to use dynamic simulations to explore sensitivity of tibial post loading to implant design and alignment, across different activities. LifeModeler KneeSIM software was used to calculate tibial post contact forces for four contemporary PS implants (Triathlon PS, Stryker; Journey BCS and Legion PS, Smith & Nephew; LPS Flex, Zimmer Biomet). An average model of the knee, including cartilage and soft tissue insertion locations, created from MRI data of 40 knees was used to mount and align the component. The Triathlon femoral component was mounted with posterior and distal condylar tangency at: a) both medial and lateral condylar cartilage (anatomic alignment), b) at the medial condylar cartilage and perpendicular to mechanical axis (mechanical alignment with medial tangency), and c) at lateral condylar cartilage and perpendicular to mechanical axis (mechanical alignment with lateral tangency). The influence of implant design was assessed via simulations for the other implant systems with the femoral components aligned perpendicular to mechanical axis with lateral tangency. Five different activities were simulated. The anterior contact force was significantly smaller than the posterior contact force, but it varied noticeably with tibial insert slope and implant design. For Triathlon PS, during most activities anatomic alignment of the femoral component resulted in greater anterior contact force compared to mechanical alignment, but absolute magnitude of forces remained small (<100N). Mechanical alignment with medial tangency resulted in greater posterior contact force for deep knee bend and greater anterior force for chair sit activity. For all implants, peak posterior contact forces were greater for activities with greater peak knee flexion. The magnitude of posterior contact forces for the various implants was comparable to other reports in literature. Overall activity type, implant design and slope had greater impact on post loading than alignment method. Tibial insert slope was shown to be important for anterior post loading, but not for posterior post loading. Anatomic alignment could increase post loading with contemporary TKA systems. In the case of the specific design for which effect of alignment was evaluated, the changes in force magnitude with alignment were modest (<200N). Nonetheless, results of this study highlight the importance of evaluating the effect of different alignment approaches on tibial post loading

Introduction. Kinematics post-TKA are complex; component alignment, component geometry and the patient specific musculoskeletal environment contribute towards the kinematic and kinetic outcomes of TKA. Tibial rotation in particular is largely uncontrolled during TKA and affects both tibiofemoral and patellofemoral kinematics. Given the complex nature of post- TKA kinematics, this study sought to characterize the contribution of tibial tray rotation to kinematic outcome variability across three separate knee geometries in a simulated framework. Method. Five 50. th. percentile knees were selected from a database of planned TKAs produced as part of a pre-operative dynamic planning system. Virtual surgery was performed using Stryker (Kalamazoo, MI) Triathlon CR and PS and MatOrtho (Leatherhead, UK) SAIPH knee medially stabilised (MS) components. All components were initially planned in mechanical alignment, with the femoral component neutral to the surgical TEA. Each knee was simulated through a deep knee bend, and the kinematics extracted. The tibial tray rotational alignment was then rotated internally and externally by 5° & 10°. The computational model simulates a patient specific deep knee bend and has been validated against a cadaveric Oxford Knee Rig. Preoperative CT imaging was obtained, landmarking to identify all patient specific axes and ligament attachment sites was performed by pairs of trained biomedical engineers. Ethics for this study is covered by Bellberry Human Research Ethics Committee application number 2012-03-710. Results and Discussion. From the 360 Knee Systems database, 1847 knees were analysed, giving an average coronal alignment of 4.25°±5.66° varus. Five knees were selected with alignments between 4.1° and 4.3° varus. Kinematic outcomes were averaged over the 5 knees. The component geometries resulted in characteristically distinct kinematics, in which femoral rollback was most constrained by the PS components, whereas tibiofemoral axial rotation was most constrained in MS components. Patella lateral shift was comparable amongst all components in extension, medialising in flexion. Patella shift remained more lateral in MS components compared to PS and CR. Average patella lateral shift, medial and lateral facet rollback separated by tibial tray rotation are shown for all component systems in Figure 1. Medial and lateral facet rollback in the PS and CR components are symmetrical and opposite, indicating that with tibial tray rotation, the tibiofemoral articulation point balances between component rotation and neutral alignment, reflecting the restoring force exerted by the simulated collateral ligaments. As such, with higher internal tibial rotation and subsequent lateralisation of the tubercle, patella lateral shift increases. MS medial and lateral facet rollback however are not symmetrical nor opposite, reflecting the chirality of the tibiofemoral articulation. With internal tibial tray rotation, relatively high lateral facet rollback is observed, lateralising the femoral component centre, giving the patella component a relatively more medial position. Conclusions. Component geometry was found here to produce characteristically distinct tibiofemoral and patellofemoral kinematics. Medial stabilised components reported asymmetric kinematic changes, compared to either CR or PS components, in which a higher rate of change was observed for internal tray rotation, indicating that neutral or external rotation of medial stabilised components will result in more predictable kinematic outcomes

Introduction. Many fluoroscopic studies on total knee arthroplasty (TKA) have identified kinematic variabilities compared to the normal knee, with many subjects experiencing paradoxical motion patterns. The intent of this research study was to investigate the results of customized-individual-made (CIM) and off-the-shelf (OTS) PS and PCR TKA to determine kinematic variabilities and to assess these kinematic patterns with those previously documented for the normal knee. Methods. In vivo kinematics were assessed for 151 subjects – 44 with CIM-PCR, 75 with OTS-PCR, 14 with CIM-PS, and 18 with OTS-PS TKA – using a mobile fluoroscopic system and then evaluated using a 3D-2D registration technique. This was a multicenter evaluation so the group of implants were implanted by two surgeons and selected based on recruitment criteria. Each subject performed a deep knee bend activity (DKB) while under fluoroscopy. The kinematics assessed for each subject were condyle translation (LAP/MAP) and rotation (axial rotation). Results. During the DKB, the average LAP of the CIM-PCR was −2.0 mm (s = 4.0), the OTS-PCR was −2.1 mm (s = 3.0), the CIM-PS was −9.0 mm (s = 6.0), and the OTS-PS was −4.3 mm (s = 3.3) (Figure 1). The average MAP of the CIM-PCR was 2.0 mm (s = 2.9), the OTS-PCR was 2.4 mm (s = 3.3), the CIM-PS was −1.2 mm (s = 5.2), and the OTS-PS was 1.1 mm (s = 1.7) (Figure 2). The average axial rotation of the CIM-PCR was 4.6° (s = 5.8), the OTS-PCR was 5.7° (s = 4.8), the CIM-PS was 9.3° (s = 4.8), and the OTS-PS was 7.5° (s = 3.5) (Figure 3). Eleven of 44 (25%) subjects having a CIM-PCR TKA, 16/75 (21.3%) subjects having an OTS-PCR TKA experienced an anterior slide of their lateral condyle, while no subjects having a CIM-PS TKA and 3/18 (16.6%) of OTS-PS subjects experienced this slide. Nine of 44 (20.5%) CIM- PCR, 8/75 (10.6%) OTS-PCR experienced a reverse axial rotation pattern, while only one subject having a CIM-PS and not OTS-PS subjects experienced this non-normal rotation pattern. Discussion. Subjects having a CIM-PS TKA experienced the greatest amount of lateral condyle posterior femoral rollback and axial rotation, although less in magnitude to the normal knee seen in previous fluoroscopic studies. This was the only group to experience posterior motion of their medial condyle during flexion. More subjects having a PCR TKA experienced a paradoxical anterior lateral condyle sliding pattern and reverse axial rotation pattern, which was not commonly seen in the subjects having a PS TKA. Significance. Subjects that had a CIM-PS TKA demonstrated greater magnitudes of lateral condyle rollback and subjects having a PS TKA experienced more normal axial rotation patterns