Rotational positioning of the femoral component during the realisation of a total knee arthroplasty is an important part of the surgical technique and remains a topic of discussion in the literature. The challenge of this positioning is important because it determines the anatomical result and its effect on the flexion gap and clinical outcome mainly through its impact on patellofemoral alignment. The intraoperative identification of the axis transepicondylar visually or by navigation is not reliable or reproducible. The empirical setting to 3 ° of external rotation, the procedure used to cut or dependent or independent is not adapted to the individual variability of knee surgery. Indeed, the angle formed by the posterior condylar axis and trans-epicondylar axis is subject to large individual variations. The authors propose a novel technique, using the navigation of the trochlea to determine the rotation of the femoral component. The principle is to consider the rotation of the femoral implant as “ideal” when it makes a perfect superposition of the prosthetic trochlea with the native bony trochlea on patellofemoral view at 60° when planning the femur. The bottom of the prosthetic trochlea is well aligned with the trochlea groove, identified during the trochlear morphing, itself perpendicular to the trans-epicondylar axis. The authors hope to encourage centering patellofemoral joint prosthesis, thus favoring the original kinematics of the extensor apparatus. The purpose of this study is to demonstrate firstly, that the navigation of the trochlea is a reliable and reproducible method to adjust the rotation of the femoral component relative to the trans-epicondylar axis taken as reference and the other, the rotation control by this method is not done at the expense of the balance gap in flexion. It is a bi-centric study prospective, nonrandomised, including continuously recruited 145 patients in two French centers. All patients were included in the year 2010 and have all been revised three months and one year of surgery. The average age of patients was 71 years [53, 88]. It was made no selection of patients who have all been included consecutively in the study and in the two centres. In all cases, the rotation of the femoral component was determined by intraoperative navigation of the trochlea. The authors compared the alpha angle (angular divergence between the plane and the posterior bicondylar plane and trans-epicondylar axis) obtained by this method and that calculated on a pre-or postoperative scan. The authors also measured the space between femur and tibia internal and external side in flexion (90°) to assess the impact on the balance in flexion. There is excellent agreement between the results obtained by the method of CT scan and the trochlear navigation technique. In addition, this technique allows us to achieve a quadrilateral space gap in flexion. The authors found large individual variation in the distal femoral epiphyseal torsion (angle alpha). They demonstrate that the navigation of the trochlea is a reliable and reproducible method to adjust the rotation of the femoral component relative to the trans-epicondylar axis taken as reference and provides, concomitantly, a quadrilateral space gap in flexion

Rotational positioning of the femoral component during the realisation of a total knee arthroplasty is an important part of the surgical technique and remains a topic of discussion in the literature. The challenge of this positioning is important because it determines the anatomical result and its effect on the flexion gap and clinical outcome mainly through its impact on patellofemoral alignment. The intraoperative identification of the axis transepicondylar visually or by navigation is not reliable or reproducible. The empirical setting to 3 ° of external rotation, the procedure used to cut or dependent or independent is not adapted to the individual variability of knee surgery. Indeed, the angle formed by the posterior condylar axis and trans-epicondylar axis is subject to large individual variations. The authors propose a novel technique, using the navigation of the trochlea to determine the rotation of the femoral component. The principle is to consider the rotation of the femoral implant as “ideal” when it makes a perfect superposition of the prosthetic trochlea with the native bony trochlea on patellofemoral view at 60 ° when planning the femur. The bottom of the prosthetic trochlea is well aligned with the trochlea groove, identified during the trochlear morphing, itself perpendicular to the trans-epicondylar axis. The authors hope to encourage centering patellofemoral joint prosthesis, thus favouring the original kinematics of the extensor apparatus. The purpose of this study is to demonstrate firstly, that the navigation of the trochlea is a reliable and reproducible method to adjust the rotation of the femoral component relative to the trans-epicondylar axis taken as reference and the other, the rotation control by this method is not done at the expense of the balance gap in flexion. It is a bi-centric study prospective, nonrandomised, including continuously recruited 145 patients in two French centres. All patients were included in the year 2010 and have all been revised three months and one year of surgery. The average age of patients was 71 years [53, 88]. It was made no selection of patients who have all been included consecutively in the study and in the two centres. In all cases, the rotation of the femoral component was determined by intraoperative navigation of the trochlea. The authors compared the alpha angle (angular divergence between the plane and the posterior bicondylar plane and trans-epicondylar axis) obtained by this method and that calculated on a pre-or postoperative scan. The authors also measured the space between femur and tibia internal and external side in flexion (90°) to assess the impact on the balance in flexion. There is excellent agreement between the results obtained by the method of CT scan and the trochlear navigation technique. In addition, this technique allows to achieve a quadrilateral space gap in flexion. The authors found large individual variation in the distal femoral epiphyseal torsion (angle alpha). They demonstrate that the navigation of the trochlea is a reliable and reproducible method to adjust the rotation of the femoral component relative to the trans-epicondylar axis taken as reference and provides, concomitantly, a quadrilateral space gap in flexion

Mechanical alignment (MA) techniques for total knee arthroplasty (TKA) may introduce significant anatomic modifications, as it is known that few patients have neutral femoral, tibial or overall lower limb mechanical axes. A total of 1000 knee CT-Scans were analyzed from a database of patients undergoing TKA. MA tibial and femoral bone resections were simulated. Femoral rotation was aligned with either the trans-epicondylar axis (TEA) or with 3° of external rotation to the posterior condyles (PC). Medial-lateral (DML) and flexion-extension (DFE) gap differences were calculated. Extension space ML imbalances (3mm) occurred in 25% of varus and 54% of valgus knees and significant imbalances (5mm) were present in up to 8% of varus and 19% of valgus knees. For the flexion space DML, higher imbalance rates were created by the TEA technique (p < 0 .001). In valgus knees, TEA resulted in a DML in flexion of 5 mm in 42%, compared to 7% for PC. In varus knees both techniques performed better. When all the differences between DML and DFE are considered together, using TEA there were 18% of valgus knees and 49% of varus knees with < 3 mm imbalances throughout, and using PC 32% of valgus knees and 64% of varus knees. Significant anatomic modifications with related ML or FE gap imbalances are created using MA for TKA. Using MA techniques, PC creates less imbalances than TEA. Some of these imbalances may not be correctable by the surgeon and may explain post-operative TKA instability. Current imaging technology could predict preoperatively these intrinsic imitations of MA. Other alignment techniques that better reproduce knee anatomies should be explored

Mechanical alignment (MA) techniques for total knee arthroplasty (TKA) introduces significant anatomic modifications and secondary ligament imbalances. A restricted kinematic alignment (rKA) protocol was proposed to minimise these issues and improve TKA clinical results. A total of 1000 knee CT-Scans were analyzed from a database of patients undergoing TKA. rKA tibial and femoral bone resections were simulated. rKA is defined by the following criteria: Independent tibial and femoral cuts within ± 5° of the bone neutral mechanical axis and, a resulting HKA within ± 3° of neutral. Medial-lateral (ΔML) and flexion-extension (ΔFE) gap differences were calculated and compared with MA results. With the MA technique, femoral rotation was aligned with either the trans-epicondylar axis (TEA) or with 3° of external rotation to the posterior condyles (PC). Extension space ML imbalances (>/=3mm) occurred in 33% of TKA with MA technique versus 8% of the knees with rKA (p /=5mm) were present in up to 11% of MA knees versus 1% rKA (p < 0 .001). Using the MA technique, for the flexion space ΔML, higher imbalance rates were created by the TEA technique (p < 0 .001). rKA again performed better than both MA techniques using TEA of 3 degrees PC techniques (p < 0 .001). When all the differences between ΔML and ΔFE are considered together: using TEA there were 40.8% of the knees with < 3 mm imbalances throughout, using PC this was 55.3% and using rKA it was 91.5% of the knees (p < 0 .001). Significantly less anatomic modifications with related ML or FE gap imbalances are created using rKA versus MA for TKA. Using rKA may help the surgeon to balance a TKA, whilst keeping the alignment within a safe range

Introduction. Robotics have been applied to total knee arthroplasty (TKA) to improve surgical precision in components’ placement, providing a physiologic ligament tensioning throughout knee range of motion. The purpose of the present study is to evaluate femoral and tibial components’ positioning in robotic-assisted TKA after fine-tuning according to soft tissue tensioning, aiming symmetric and balanced medial and lateral gaps in flexion/extension. Materials and Methods. Forty-three consecutive patients undergoing robotic-assisted TKA between November 2017 and November 2018 were included. Pre-operative radiographs were performed and measured according to Paley's. The tibial and femoral cuts were performed based on the individual intra-operative fine-tuning, checking for components’ size and placement, aiming symmetric medial and lateral gaps in flexion/extension. Cuts were adapted to radiographic epiphyseal anatomy and respecting ±2° boundaries from neutral coronal alignment. Robotic data were recorded, collecting information relative to medial and lateral gaps in flexion and extension. Results. Patients were divided based on the pre-operative coronal mechanical femoro-tibial angle (mFTA). Only knees with varus deformity (mFTA<178°), 29 cases, were taken into account. On average, the tibial component was placed at 1.2°±0.5 varus. Femoral component fine-tuning based on soft-tissues tensioning in extension and flexion determined the following alignments: 0.2°±1.2 varus on the coronal plane and 1.2°±2.2° external rotation with respect to the trans-epicondylar axis (TEA) as measured on the CT scan in the horizontal plane. The average gaps after femoral and tibial resections, resulted as follows: 19.5±0.8 mm on the medial side in extension, 20.0±0.9 mm on the lateral side in extension, 19.1±0.7 mm on the medial side in flexion and 19.5±0.7 mm on the lateral side in flexion. On average, the post-implant coronal alignment as reported by the robotic system resulted 2.0°±1.5 varus. Discussion. The proposed robotic-arm assisted TKA technique, aiming to preserve the integrity of the ligaments, provides balanced and symmetric gaps in flexion and extension and an anatomic femoral and tibial component's placement with post-implant coronal alignment within ±2° from neutral alignment

INTRODUCTION. Mechanical alignment in TKA introduces significant anatomic modifications for many individuals, which may result in unequal medial-lateral or flexion-extension bone resections. The objective of this study was to calculate bone resection thicknesses and resulting gap sizes, simulating a measured resection mechanical alignment technique for TKA. METHODS. Measured resection mechanical alignment bone resections were simulated on 1000 consecutive lower limb CT-Scans from patients undergoing TKA. Bone resections were simulated to reproduce the following measured resection mechanical alignment surgical technique. The distal femoral and proximal tibial cuts were perpendicular to the mechanical axis, setting the resection depth at 8mm from the most distal femoral condyle and from the most proximal tibial plateau (Figure 1). If the resection of the contralateral side was <0mm, the resection level was increased such that the minimum resection was 0mm. An 8mm resection thickness was based on an implant size of 10mm (bone +2mm of cartilage). Femoral rotation was aligned with either the trans-epicondylar axis or with 3 degrees of external rotation to the posterior condyles. After simulation of the bone cuts, media-lateral gap difference and flexion-extension gaps difference were calculated. The gap sizes were calculated as the sum of the femoral and tibial bone resections, with a target bone resection of 16mm (+ cartilage corresponding to the implant thickness). RESULTS. For both the varus and valgus knees, the created gaps in the medial and lateral compartments were reduced in the vast majority of cases (<16mm). The insufficient lateral condyle resection distalises the lateral joint surface by a mean of 2.1mm for the varus and 4.4mm for the valgus knees. The insufficient medial tibial plateau resection proximalises the medial joint surface by 3.3mm for the varus and 1.2mm for the valgus knees. Medio-lateral gap imbalances in the extension space of more than 2mm) occurred in 25% of varus and 54% of valgus knees and significant imbalances of more than 5mm were present in up to 8% of varus and 19% of valgus knees. Higher medio-lateral gap imbalances in the flexion space were created with trans epicondylar axis versus 3 degrees to the posterior condyles (p<0.001). Using trans epicondylar axis, only 49% of varus and 18% of valgus knees had less than 3mm of imbalance in both media-lateral and flexion-extension gaps together. DISCUSSION AND CONCLUSION. A systematic use of the tested measured resection mechanical alignment technique for TKA leads to many cases with medio-lateral or flexion-extension gap asymmetries. Some medio-lateral imbalances may not be correctable surgically and may results in TKA instability. Other versions of the mechanical alignment technique or other alignment methods that better reproduce knee anatomies should be explored. For any figures or tables, please contact the authors directly

Background. Accurate implant positioning is of supreme importance in total knee replacement (TKR). The rotational profile of the femoral and tibial components can affect outcomes, and the aim is to achieve coronal conformity with parallelism between the medio-lateral axes of the femur and tibia. Aims. The aim of this study is to determine the accuracy of implant rotation in total knee replacement. Methods. Intra-operatively, the trans-epicondylar axis of the femur (TEA) and Whiteside's line were used as the reference points, aiming to externally rotate the femoral component by 1 degree. The medial third of the tibial tuberosity was used as the anatomical reference point, aiming to reproduce the rotation of the native tibia. Pre-and post-operative CT scans were reviewed. The difference in femoral rotation was calculated by determining the femoral posterior condylar axis (PCA) of the native femur pre-operatively and the implant post-operatively. Tibial rotational difference was calculated between the native tibial posterior condylar axis and tibial baseplate. Results. Pre and post-operative CT scans of 41 knees in 31 patients were analysed. All surgeries were carried out by a single surgeon using the same implant. The mean difference in rotation of the femur post-operatively was 1.2 degrees external rotation (ER), range −4.7 to 6.9 degrees ER. 83% of femoral components were within 3 degrees of the target rotation. Mean difference in tibial rotation was −3.8 degrees ER, range −11.1 to 12.4 ER. Only 39% of tibial components were within 3 degrees of the target rotation. A line perpendicular to the midpoint of the tibial PCA was actually medial to the tibial tubercle in 33 knees, and only corresponded to the medial 1/3 of the tibial tubercle in 8 of 41 knees. Conclusions. Femoral component rotation is seen to be more accurate than tibial in this group. It may be that the anatomical landmarks used intra-operatively to judge tibial rotation are more difficult to accurately identify. Posterior landmarks are difficult to locate in vivo. This study would suggest that using the anterior anatomical landmark of the medial 1/3 of the tibial tubercle does not allow accurate reproduction of tibial rotation in total knee replacement

Valgus deformity is less common than varus. There is an associated bone deformity in many cases – dysplasia of the lateral femoral condyle. There are also soft tissue deformities, including tightness of the lateral soft tissues, and stretching of those on the medial side. Unlike varus, where the bone deformity is primarily tibial, in valgus knees it is most often femoral. There is both a distal and a posterior hypoplasia of the lateral femoral condyle. This results in a sloping joint line, and failure to correct this results in valgus malalignment. Posterior lateral bone loss also results in accidental internal rotation of the femoral component, which affects patellar tracking. Using the trans-epicondylar axis and Whiteside's line helps to position the femoral component in the correct rotation. Soft tissue balancing is more complex in the valgus knee. Releases are performed sequentially, depending on the particular combination of deformities. It is important to note whether the knee is tight in flexion, in extension, or both. Tightness in extension is the most common, and is corrected by release of the iliotibial band. Tightness in flexion as well as extension requires that the lateral collateral ligament +/− the popliteus tendon be released. Cruciate substituting designs are helpful in many cases, and in extreme deformity with medial stretching, a constrained or “total stabilised” design is needed. Patellar maltracking is common, and a lateral retinacular release may be needed. Beware of over-releasing the posterolateral corner, as excessive release may cause marked instability. Use the pie-crust technique of Insall

Background. Body Mass Index (BMI) is used to quantify generalised obesity, but does not account for variations in soft tissue distribution. Aims. To define an index quantifying the knee soft tissue depth, utilising underlying bony anatomy, and compare with BMI as a measure of individual patient's knee soft tissue envelopes. We performed a practicality and reproducibility study to validate the Bristol Knee Index for future prospective use. Method. Femoral trans-epicondylar axis, and the proximal tibial plateau width were measured on 225 antero-posterior pre-operative knee radiographs. Corresponding measurements of soft tissue were performed at both levels. These were expressed as a ratio: Soft tissue width (mm)/Bone width (mm) = BKI. Time taken performing each measurement was recorded, and inter- and intra-observer variability was assessed. Results. Average BMI was 32 (18-54). Measuring femoral and tibial BKI averaged 35 seconds. Inter-observer interclass correlation coefficient (ICC) for femoral and tibial BKI was 0.994 and 0.997 respectively. Intra-observer ICC was 0.996 for both. Correlation of BKI to BMI was 0.64 (for both femoral and tibial BKI). When divided into BMI subgroups (normal, overweight, obese, morbidly obese), the correlation was poor. BMI cannot be used to predict the amount of knee soft tissue in the individual patient. Tibial measurement was the most reproducible method. Conclusions. BKI is a fast, reproducible measurement to assess knee soft tissue depth. BMI cannot be used to assess individual patient's knee soft tissue. We plan to correlate BKI to ‘surgical’ orthopaedic complications such as malalignment, wound complications and infection

Introduction. Proper rotational alignment of the tibial component is a critical factor affecting the outcome of TKA. Traditionally, the tibial component is oriented with respect to fixed landmarks on the tibia without reference to the plane of knee motion. In this study, we examined differences between rotational axes based on anatomic landmarks and the true axis of knee motion during a functional activity. Materials and Methods. 24 fresh-frozen lower limb specimens were mounted in a joint simulator which enable replication of lunging and squatting through application of muscle and body-weight forces. Kinematic data was collected using a 3D motion analysis system. Computer models of the femur and tibia were generated by CT reconstruction. The motion axis of each knee (TFA) was defined by the 3D path of the femur with respect to the tibia as the knee was flexed from 30 to 90 degrees. The orientation the TFA was compared to 5 different anatomic axes commonly proposed for alignment of the tibial component. Results. The average alignment error of the 5 different anatomic axes ranged from 0.1° ER to 10.7°IR from the true direction of knee flexion. The most accurate indicator of the direction of motion was derived by projecting the trans-epicondylar axis of the femur onto the tibial plateau. On average, this axis was externally rotated by 0.1±6.9°. However, values varied over 21.6°. In comparison, an axis passing through the medial-third of the tibial tubercle from the center of the plateau was internally rotated by 0.3°±6.0° (range: 23.9°). Conclusion. This study demonstrates that rotational axes derived from anatomic landmarks on the proximal tibia provide an estimate of the direction of movement of the femur that is highly variable. Constructions based on the epicondylar axis and the medial third of the tibial tubercle are accurate when averaged over large numbers of cases. However, these methods can lead to up to 10 degrees of internal rotation of the tibial component in individual cases

There is still want of evidence in the current literature of any significant improvement in clinical outcome when comparing computer-assisted total knee arthroplasty (CA-TKA) with conventional implantation. Analysis of alignment and of component orientation have shown both significant and non-significant differences between the two methods. Not much work has been reported on clinical evidence of stability of the joint. We compared computer-assisted and conventional surgery for TKA at 5.4 years follow-up for patients with varus osteoarthritic knees with deformity of more than 15∗. Our goal was to assess clinical outcome, stability and restoration of normal limb alignment. We used CT and Cine video X ray techniques to analysize our results in Computer navigated and conventional TKRs. A three dimentional CT scan of the whole extremity was performed and evaluation was done in three planes; saggital, coronal and transverse views. CT scan was done between 10 to 14 days postoperative. Mean deviations in the mechanical axis, femoral and tibial plateau angles, and in transverse view, the trans-epicondylar axis vs posterior condylar axis were measured. The prospective randomized study comprised of 98 patients with surgery done on knees, one side navigated and other side conventional. Mean deviation in the mechanical axis was 2.2∗ in conventional knees and 1.8∗ in navigated knees. In 5 % of cases retinacular release was needed and CT analysis showed TEA in deviation of more than 2 ∗ in these cases. We analysed intraoperative data (surgical time and intraoperative complications), postoperative complications, lower limb alignment, radiographic complication on X-ray imaging, and clinical outcome throughknee and function score, range of motion and joint stability. Our results showed that CAS had greater consistency and accuracy in implant placement and stability of joint in full extension and 90∗ flexion. In the coronal view, 93.3% in the CAS group had better outcomes compared with EM (73.4%). In the sagittal axis, 90.0% CAS also had better outcomes compared with EM (63.3%). Computer-navigated total knee arthroplasty helps increase accuracy and reduce “outliers” for implant placement

Introduction. The marriage of rapid prototyping technologies with Arthroplasty has resulted in the fabrication and use of cutting jigs and guides which are tailored to a patients' individual anatomy. These disposable cutting blocks are designed based on input parameters obtained from pre-operative CT and MRI scans and manufactured using 3-D printers. Indirect benefits include a reduction in inventory and a decrease in the burden for central sterilising units. This approach is advantageous for the surgeon in the attainment of ideal mechanical alignment, which is known to be associated with an improved clinical outcome and implant longevity. This study evaluated the postoperative alignment parameters from a single surgeon series of patients following TKA with the Signature (Biomet) system. Methods and Materials. The postoperative alignment of a single surgeon series of 60 consecutive patients receiving a Vanguard cruciate retaining TKR (Biomet) using the Signature patient-specific surgical positioning guides was performed. Postoperative CT and preoperative templating MRI scans were imported into Mimics 14.0 (Materialise, Belgium) where specific bony landmarks were identified in both data sets. A subset of these points was used to transform the MRI data into the CT coordinate frame to enable the computation of femoral mechanical alignment in the absence of a full-length lower limb CT scan. CT and transformed MRI landmarks were then imported into ProEngineer (PTC, MA) where angular measurements were made by projecting axes onto anotomical planes. Flexion, rotation, valgus/varus of the femoral component and posterior slope, rotation and valgus/varus of the tibial component were computed. Femoral rotation was referenced to the trans-epicondylar axis as opposed to Whiteside's line. Overall limb alignment was determined based on individual component position. Results. Results are presented as mean ± standard deviation. Overall, the mean position of the femoral component was found to be 0.8° ± 2.1° in valgus, 4.2° ± 3.4° in flexion and 0.3° ± 2.0° in external rotation. Likewise, the mean position of the tibial component was found to be in 0.4° ± 1.4° in varus, 4.6° ± 2.9° in posterior slope and 1.9° ± 6.6° in external rotation. Overall mean limb alignment was found to be 0.4° ± 2.5° in varus. Discussion. With respect to varus/valgus alignment of the components, the number of outliers outside the ± 3° accepted range was small (8/60 for the femoral component and 1/60 for the tibial component). This data demonstrates that the early results for knee replacements performed using the Signature patient specific jigs delivers an improved level of prosthetic alignment when compared to published data for standard instrumented knees

There are basically 4 ways advocated to determine the proper femoral component rotation during TKA: (1) The Trans-epicondylar Axis, (2) Perpendicular to the “Whiteside Line,” (3) Three to five degrees of external rotation off the posterior condyles, and (4) Rotation of the component to a point where there is a balanced symmetric flexion gap. This last method is the most logical and functionally, the most appropriate. Of interest is the fact that the other 3 methods often yield flexion gap symmetry, but the surgeon should not be wed to any one of these individual methods at the expense of an unbalanced knee in flexion. In correcting a varus knee, the knee is balanced first in extension by the appropriate medial release and then balanced in flexion by the appropriate rotation of the femoral component. In correcting a valgus knee, the knee can be balanced first in flexion by the femoral component rotation since balancing in extension almost never involves release of the lateral collateral ligament (LCL) but rather release of the lateral retinaculum. If a rare LCL release is anticipated for extension balancing, then it would be performed prior to determining the femoral rotation since the release may open up the lateral flexion gap to a point where even more femoral component rotation is needed to close down that lateral gap. It is important to know and accept the fact that some knees will require internal rotation of the femoral component to yield flexion gap symmetry. The classic example of this is a knee that has previously undergone a valgus tibial osteotomy that has led to a valgus tibial joint line. In such a case, if any of the first 3 methods described above is utilised for femoral component rotation, it will lead to a knee that is very unbalanced in flexion being much tighter laterally than medially. A LCL release to open the lateral gap will be needed, increasing the complexity of the case. My experience has shown that intentional internal rotation of the femoral component when required is well-tolerated and rarely causes problems with patellar tracking. It is also of interest to note that mathematical calculations reveal that internally rotating a femoral component as much as 4 degrees will displace the trochlear groove no more that 2–3 mm (depending on the FC size), an amount easily compensated for by undersizing the patellar component and shifting it medially those few mm. There are basically 3 ways to determine the proper tibial component rotation during TKA: (1) Anatomically cap the tibial cut surface with an asymmetric tibial component, (2) Align the tibial rotation relative to a fixed anatomic tibial landmark (most surgeons use this method and align relative to the medial aspect of the tibial tubercle), (3) Rotate the tibial component to a point where there is rotational congruency in extension between the femoral and tibial articulating surfaces. This third method must be used with fixed bearing arthroplasties (especially with conforming articulations) to avoid rotational incongruency between the components during weight-bearing that can create abnormal and deleterious torsional forces on posterior stabilised posts, insert tray interfaces and bone-cement interfaces. Rotating platform articulations can tolerate rotational mismatch unless it is to a point where the polyethylene insert rotates excessively and causes symptomatic soft tissue impingement