Posterior-stabilized Total Knee Prosthesis Research Articles

Effectiveness of Periarticular Pin Tracker Placement Through a Single Main Incision in Robotic-Assisted Total Knee Arthroplasty: Technical Note and Short-Term Results.

Background and Objectives: Robotic-assisted total knee arthroplasty (TKA) is gaining popularity worldwide, leading to a potential increase in the number of pin tracker-related complications. This study determined the effectiveness of periarticular pin tracker placement in the distal femur and proximal tibia through a single main incision during robotic-assisted TKA over a minimum follow-up period of 6 months. Materials and Methods: A consecutive series of 149 TKAs was performed in 108 patients using the triathlon posterior-stabilized total knee prosthesis with a robotic-assisted system at our hospital from December 2023 to February 2024. Clinical outcomes and complications associated with pin tracker sites, including pin-site infection, neurovascular injury, hematoma, soft-tissue morbidity, and pin-site fracture, were assessed. Results: The mean Knee Society knee score improved from 42.5 preoperatively to 76.3 points at the final follow-up, whereas the mean Knee Society function score improved from 43.1 preoperatively to 78.1 points at the final follow-up (both p < 0.05). No patient experienced any minor or major complications related to the use of pin trackers in the distal femur and proximal tibia. Conclusions: This periarticular technique that uses pin trackers in the distal femur and proximal tibia through a single main incision could be a useful option for orthopedic surgeons while performing robotic-assisted TKA.

Surgical Technique for Complementing Robotic-Assisted Total Knee Arthroplasty in Middle-Aged Patients with Rigid Knee Bones.

This study reports 12 cases of inaccurate bone cutting from a single-surgeon series of 509 consecutively performed robotic-assisted total knee arthroplasty (TKA) for 1 year. In addition, a complementary technique with the combined benefits of robotic-assisted and manual techniques that address this issue is described. From June 2020 to May 2021, a consecutive series of 509 TKAs was performed on 338 patients using a posterior-stabilized total knee prosthesis with a robotic-assisted system at our hospital. The surgical records were reviewed to identify the causative bone locations and bone re-cutting events correlated with improper trial instrument positioning. The re-cutting rate was 2.4% (12/509). All re-cutting attempts occurred because of improper cutting of the femoral posterior chamfer. Re-cutting was attempted mostly on middle-aged male patients. This complementary technique can facilitate manual bone cutting while retaining the advantages of robotic surgery during robotic-assisted TKA. Additionally, the combined technique of manual bone cutting and robotic-assisted surgery can be a useful alternative for middle-aged male patients with rigid knee bones.

No higher tibial post fracture rate with highly cross-linked polyethylene inserts in total knee arthroplasty at a minimum 10-year follow-up.

The purpose of this study was to assess tibial post fracture between highly cross-linked polyethylene (HXPE) and conventional polyethylene inserts in consecutive posterior-stabilized (PS) total knee arthroplasty (TKA) over a minimum 10-years follow-up period. Between July 2008 and December 2011, a consecutive series of 3264 primary TKAs was performed in 2098 patients using a PS total knee prosthesis at a single institution. The final cohort was classified into the HXPE group (792 cases) and the conventional group (2113 cases) according to insert material used during the procedure. The mean follow-up period was 11.4years in the HXPE group and 11.8years in the conventional group. Both cases of tibial post fracture occurred in the HXPE group, a complication rate of 0.25%. These two cases required replacement with a thicker tibial insert with retention of the femoral and tibial components. There were no cases of tibial post fracture in the conventional group. There was no difference in tibial post fracture rate between the HXPE group and the conventional group in PS TKA on long-term follow-up. IV.

Mid-term clinical outcomes of a posterior stabilized total knee prosthesis for Japanese patients: A minimum follow-up of 5 years

BackgroundA high–flexion posterior-stabilized total knee prosthesis has been developed for the Asian population. The component design was based on computed tomography images of Japanese osteoarthritic knees. The femoral component is composed of zirconia ceramics, which exhibit low friction and high durability. The present study aimed to evaluate the mid-term clinical outcomes of this implant. MethodsThis study included 334 knees of 210 patients who underwent primary total knee arthroplasty with this implant at our hospital between October 2010 and December 2014. The patients comprised 28 men and 172 women with an average age of 73 years. The average follow-up period was 5.9 years, and the follow-up rate was 71.1%. Clinical outcomes were assessed using the Knee Society scoring system, 2011 Knee Society questionnaire, and Knee Injury and Osteoarthritis Outcome Score. Kaplan–Meier survivorship analysis was performed to determine the cumulative prosthesis survival rate. ResultsIn terms of clinical outcomes at the final follow-up, the average ranges of motion were −2.0 in extension and 126.7 in flexion. The Knee Society knee and function scores were 94.2% and 72.6%, respectively. With revision surgery or radiographic failure for any reason as the endpoint, the survival rates at 5 and 9 years were 98.2% and 95.5%, respectively. The most common reason for revision surgery or radiological failure was aseptic loosening. ConclusionsDespite several revision cases mainly due to aseptic loosening, the present study found that this new high-flexion posterior-stabilized total knee arthroplasty prosthesis design showed comparable results for Asian populations with other PS prosthesis. Levels of evidenceLevel Ⅱ (Prospective cohort study).

Musculoskeletal multibody dynamics investigation of posterior-stabilized total knee prosthesis

Posterior-stabilized total knee prostheses have been widely used in orthopedic clinical treatment of knee osteoarthritis, but the patients and surgeons are still troubled by the complications, for example severe wear and fracture of the post, as well as prosthetic loosening. Understanding the in vivo biomechanics of knee prostheses will aid in the decrease of postoperative prosthetic revision and patient dissatisfaction. Therefore, six different designs of posterior-stabilized total knee prostheses were used to establish the musculoskeletal multibody dynamics models of total knee arthroplasty respectively, and the biomechanical differences of six posterior-stabilized total knee prostheses were investigated under three simulated physiological activities: walking, right turn and squatting. The results showed that the post contact forces of PFC Sigma and Scorpio NGR prostheses were larger during walking, turning right, and squatting, which may increase the risk of the fracture and wear as well as the early loosening. The post design of Gemini SL prosthesis was more conductive to the knee internal-external rotation and avoided the edge contact and wear. The lower conformity design in sagittal plane and the later post-cam engagement resulted in the larger anterior-posterior translation. This study provides a theoretical support for guiding surgeon selection, improving posterior-stabilized prosthetic design and reducing the prosthetic failure.

Distal Femoral Pin Tracker Placement Prevents Pin Tract-induced Fracture in Robotic-Assisted Total Knee Arthroplasty.

The purpose of this study was to determine the effectiveness of the placement of pin trackers in the medial sagittal plane of the distal femur in robotic-assisted total knee arthroplasty (TKA) over a minimum follow-up period of 3 months. From August 2020 to October 2020, a consecutive series of 81 TKAs were performed in 59 patients using the Triathlon posterior-stabilized total knee prosthesis with a robotic-assisted system (Mako) at our hospital. Patient charts were reviewed for complications associated with the pin sites, including fracture, infection, thigh pain, and the need for reoperation. No patients had any minor or major complications associated with distal femoral pins. This technique, which used pin trackers in the medial sagittal plane of the distal femur, could be a useful option for surgeons performing robotic-assisted TKA. This is a Level IV study.

Two-Year Results of a Posterior-Stabilized Total Knee Prosthesis Using Zirconia Ceramic Femoral Component Designed for Japanese Comparing with Another Posterior-Stabilized Model

Two-Year Results of a Posterior-Stabilized Total Knee Prosthesis Using Zirconia Ceramic Femoral Component Designed for Japanese Comparing with Another Posterior-Stabilized Model

10-Year patient satisfaction compared between computer-assisted navigation and conventional techniques in minimally invasive surgery total knee arthroplasty

Background: Both minimally invasive surgery (MIS) and computer-assisted surgery (CAS) in total knee arthroplasty have been scientifically linked with surgical benefits. However, the long-term results of these techniques are still controversial. Most surgeons assessed the surgical outcomes with regard to knee alignment and range of motion, but these factors may not reflect subjective variables, namely patient satisfaction.Purpose: To compare satisfaction and functional outcomes between two technical procedures in MIS total knee arthroplasty, namely computer-assisted MIS and conventional MIS procedure, operated on a sample group of patients after 10 years.Methods: Seventy cases of posterior-stabilized total knee prostheses were implanted using a computer-assisted system and were compared to 74 cases of matched total knee prostheses of the same implant using conventional technique. Both groups underwent arthrotomy by 2 cm limited quadriceps exposure minimally invasive surgery (2 cm Quad MIS). At an average of 10 years after surgery, self-administered patient satisfaction and WOMAC scales were administered and analyzed.Results: Demographic data of both groups including sex, age, preoperative WOMAC and post-operative duration were not statistically different. Post-operative WOMAC for the computer-assisted group was 38.94 ± 5.68, while the conventional one stood at 37.89 ± 6.22. The median of self-administered patient satisfaction scales of the computer-assisted group was 100 (min37.5–max100), while the conventional one was 100 (min25–max100). p Value was 0.889. There was one re-operative case in the conventional MIS group due to peri-prosthetic infection which was treated with debridement, polyethylene exchanged and intravenous antibiotics.Conclusions: The 10-year outcomes of computer-assisted MIS total knee arthroplasty are not superior to that of the conventional MIS technique in function and patient satisfaction. 10 years may not be enough to show the difference between these two techniques.

Factors affecting the achievement of Japanese-style deep knee flexion after total knee arthroplasty using posterior-stabilized prosthesis with high-flex knee design

BackgroundAchievement of very deep knee flexion after total knee arthroplasty (TKA) can play a critical role in the satisfaction of patients who demand a floor-sitting lifestyle and engage in high-flexion daily activities (e.g., seiza-sitting). Seiza-sitting is characterized by the knees flexed >145º and feet turned sole upwards underneath the buttocks with the tibia internally rotated. The present study investigated factors affecting the achievement of seiza-sitting after TKA using posterior-stabilized total knee prosthesis with high-flex knee design. MethodsSubjects comprised 32 patients who underwent TKA with high-flex knee prosthesis and achieved seiza-sitting (knee flexion >145º) postoperatively. Another 32 patients served as controls who were capable of knee flexion >145º preoperatively, but failed to achieve seiza-sitting postoperatively. Accuracy of femoral and tibial component positions was assessed in terms of deviation from the ideal position using a two-dimensional to three-dimensional matching technique. Accuracies of the component position, posterior condylar offset ratio and intraoperative gap length were compared between the two groups. ResultsThe proportion of patients with >3º internally rotated tibial component was significantly higher in patients who failed at seiza-sitting (41%) than among patients who achieved it (13%, p=0.021). Comparison of intraoperative gap length between patient groups revealed that gap length at 135º flexion was significantly larger in patients who achieved seiza-sitting (4.2±0.4mm) than in patients who failed at it (2.7±0.4mm, p=0.007). Conversely, no significant differences in gap inclination were seen between the groups. ConclusionsFrom the perspective of surgical factors, accurate implant positioning, particularly rotational alignment of the tibial component, and maintenance of a sufficient joint gap at 135º flexion appear to represent critical factors for achieving >145º of deep knee flexion after TKA.

Advantage of minimal anterior knee pain and long-term survivorship of cemented single radius posterior-stabilized total knee arthroplasty without patella resurfacing.

BackgroundThe single radius total knee prosthesis was introduced with the advantage of reduced patellar symptoms; however, there is no long-term follow-up study of the same. The purpose of this study was to determine the survival rate of single radius posterior-stabilized total knee arthroplasty and patellofemoral complication rates in a consecutive series.MethodsSeventy-one patients (103 knees) who underwent arthroplasty without patellar resurfacing using a single radius posterior-stabilized total knee prosthesis were followed up for a minimum 10 years. Clinical evaluation using Knee Society knee and function scores and radiologic evaluation were performed at regular intervals. Anterior knee pain as well as patellofemoral complications were evaluated with a simple questionnaire. The Kaplan-Meier product-limit method was used to estimate survival.ResultsSeventeen patients (23 knees) were excluded due to death (12 knees) or lost to follow-up (11 knees). Of the 80 knees enrolled, all femoral components and 78 tibial components were well fixed without loosening at final follow-up. Two revisions were performed because of tibial component loosening and periprosthetic joint infection. One patient with tibial component loosening refused to have revision surgery. No obvious tibial insert polyethylene wear was observed. The survivorships at 132 months were 96.7% using revision or pending revision as end points. Anterior knee pain was present in 6 patients (6 knees, 7.5%) at the latest follow-up. No patellofemoral complication requiring revision was encountered.ConclusionsThe single radius posterior-stabilized total knee prosthesis demonstrated an excellent minimum 10-year survivorship. The low rates of implant loosening and 7.5% of anterior knee pain as a patellofemoral complication are comparable with those reported for other modern total knee prosthesis.

Correlation Between Knee Kinematics and Patellofemoral Contact Pressure in Total Knee Arthroplasty

The aim of this study is to evaluate the relationship between patellofemoral contact stress and intraoperative knee kinematic patterns after mobile bearing total knee arthroplasty (TKA). Medial osteoarthritic knees of forty-six posterior-stabilized total knee prostheses were evaluated using a computed tomography-guided navigation system. Subjects were divided into two groups based on intraoperative knee kinematic patterns: the medial pivot group (n=19) and the non-medial pivot group (n=27). Mean intraoperative patello-femoral contact stress was significantly lower in the medial pivot group than in the non-medial pivot group (1.7MPa vs. 3.2MPa, P<0.05). An intraoperative medial pivot pattern results in reduced patello-femoral contact stress.

Intraoperative assessment of midflexion laxity in total knee prosthesis

BackgroundSoft-tissue balancing of the knee is fundamental to the success of a total knee arthroplasty (TKA). In posterior-stabilized TKA, there is no stabilizer of the anterior–posterior translation in the midflexion range in which the cam-post mechanism does not engage yet. Therefore, instability in the midflexion range is suspected to occur in posterior-stabilized TKA. The purpose of this study was to measure the joint gap throughout a full range of motion and to analyze the joint gap laxity in the midflexion range after implantation of a mobile-bearing posterior-stabilized total knee prosthesis. MethodsJoint gap kinematics in 259 knees with varus osteoarthritis were measured during TKAs using a tensor device with the same shape of a total knee prosthesis of the same design was used. After the implantation of a mobile-bearing posterior-stabilized prosthesis and the reduction of the patellofemoral joint, the joint gap was measured at 0°, 30°, 60°, 90°, 120°, and 145° of flexion. ResultsThe center size of the joint gap was tight in extension and deep flexion and loose at midflexion ranges, especially at 30° of flexion (p<0.001). The symmetry of the joint gap was varus at 0° and 145° of flexion (p<0.001). ConclusionsOur results showed the joint gap laxity in the midflexion range after the implantation of a mobile-bearing posterior-stabilized prosthesis. Our new tensor device, which can attach the polyethylene insert trial, will provide the important information about the joint gap kinematics after implantation of total knee prostheses. Level of evidenceIV.

Mobile-Bearing Total Knee Arthroplasty: A Full Traumatic Rotation of 180°

From February 2008 to September 2012 we implanted 204 mobile-bearing knee prostheses in 192 patients. All the prostheses were cemented (both femoral and tibial components), and the patella was not replaced. Only one early complication of the implants (1/204 = 0.004%) occurred after a traumatic event as a full 180° rotation of the mobile-bearing polyethylene insert. A 78-year-old woman presented with swelling and severe pain at her right knee. This traumatic event was the only case among our mobile-bearing insert patients. The failed polyethylene inserts were retrieved and studied using a scanning electron microscope (SEM, ZEISS EVO 50 EP, Cambridge, UK) operating at 20 kV. Scratching and pitting were found on the UHMWPE insert perpendicular to the machining tracks for the concave surface. SEM micrographs of the insert showed burnishing on the concave surfaces and longitudinal scratches were clearly detectable and well-marked on the analyzed surfaces. A traumatic, fully rotating, polyethylene insert is rare and our case is the first report describing a traumatic event with a complete 180 degree rotation mobile-bearing in a total knee prosthesis. In the literature few reports discuss clinical outcomes after total knee arthroplasty in patients with Parkinson's disease and they cite mixed results. However, some authors suggest that posterior-stabilized and cruciate-retaining TKA should work well while others prefer cruciate-retaining, condylar constrained kinetics, or hinged devices. Although we did not implant a posterior-stabilized mobile-bearing total knee prosthesis or a constrained prosthesis, we obtained good clinical and radiological results at the 2-year followup.

The Outcome of Rotating-Platform Total Knee Arthroplasty with Cement at a Minimum of Ten Years of Follow-up

Durable, long-term results have been reported for patients managed with first-generation mobile-bearing total knee arthroplasty. Second-generation mobile-bearing total knee arthroplasty has been introduced to enhance instrumentation, to increase flexion, and to improve functional outcome, but, to our knowledge, no long-term results are available. From May 1999 to June 2000, 116 consecutive rotating-platform total knee arthroplasties were performed by the two senior authors in 112 patients with use of the LPS-Flex Mobile cemented prosthesis, which was implanted with a measured resection technique. The patella was resurfaced in every knee. The average age of the patients at the time of surgery was sixty-nine years (range, thirty-seven to eighty-five years), and seventy-seven patients (eighty knees) were women. The predominant diagnosis was osteoarthritis. The clinical and radiographic evaluation was performed with use of the Knee Society rating system. The level of activity and patient-reported functional outcome were evaluated with use of the University of California at Los Angeles (UCLA) score and the Knee injury and Osteoarthritis Outcome Score (KOOS), respectively. The average duration of follow-up was 10.6 years (range, ten to 11.8 years). Three patients were lost to follow-up, and five patients died of causes unrelated to knee arthroplasty. Two knees were revised, one because of infection and one because of failure of the medial collateral ligament. Kaplan-Meier survivorship analysis showed an implant survival rate of 98.3% at ten years. For the 104 patients (108 knees) who were evaluated at a minimum of ten years, the average Knee Society knee and function scores improved from 34 to 94 points and from 55 to 88 points, respectively, at the time of the latest follow-up. There was no periprosthetic osteolysis and no evidence of implant loosening on follow-up radiographs. The average knee flexion was 117° preoperatively and 128° at the time of the latest follow-up evaluation. At the time of the latest follow-up, the KOOS quality-of-life score was significantly better for patients with >125° of flexion (p = 0.00034). This study demonstrated durable clinical and radiographic results at a minimum of ten years after total knee replacement with a second-generation, cemented, rotating-platform, posterior-stabilized total knee prosthesis. According to the functional outcome results obtained in this study, we believe that this design is a valuable option for active patients undergoing total knee arthroplasty.

Effect of Femoral Component Design on Patellofemoral Crepitance and Patella Clunk Syndrome After Posterior-Stabilized Total Knee Arthroplasty

The purpose of this study was to determine if recent changes to the femoral component of a particular posterior-stabilized total knee prosthesis would affect the incidence of postoperative patellofemoral crepitance and patella clunk syndrome. One hundred eight total knee arthroplasties were performed with the conventional design; 136 were performed after the femoral component was changed. Complications were compared between the groups with an average follow-up of 17.7 months and 12.4 months, respectively. Thirteen knees with the conventional design (12%) were found to have patellofemoral complications; no complications were noted with the new design (P < .0001). Femoral components with a deep trochlear groove and smooth transition of the intercondylar box appear to better accommodate any peripatellar fibrous nodule that may form after total knee arthroplasty.

Differing Prosthetic Alignment and Femoral Component Sizing Between 2 Computer-assisted CT-free Navigation Systems in TKA

We adopted a new computed tomography (CT)-free navigation system and noticed that, compared with our previous experience using a different type of CT-free navigation system that led to oversized and extended femoral component implanted compared with the conventional implantation, the femoral components were similarly implanted to the conventional method. The purpose of our study was to compare alignment and sizing with these 2 image-free systems and determine whether the method to determine the knee center could explain these differences in femoral component size and flexion of the component. Thirty posterior-stabilized total knee prostheses were implanted using the OrthoPilot CT-free navigation system (B. Braun-Aesculap, Tuttlingen, Germany). Postoperative coronal and sagittal long leg radiographs were retrospectively compared with those of a control group of 30 matched-paired total knee prostheses previously implanted using the VectorVision CT-free navigation system (Depuy-BrainLAB, Heimstetten, Germany). Sagittal alignment of the femoral component in the VectorVision group showed a significantly extended position compared with that in the OrthoPilot group. In addition, the size of the selected femoral component in the Vector Vision group was significantly larger than that in the OrthoPilot group. This discrepancy may be explained by the differences in the determined knee center, which influenced the postoperative implantation. When surgeons select a navigation system, the characteristics of each system should be taken into consideration.

Knee Arthroplasty With a Medially Conforming Ball-and-Socket Tibiofemoral Articulation Provides Better Function

A knee design with a ball-and-socket articulation of the medial compartment has a femoral rollback profile similar to the native knee. Compared to a conventional, posterior-stabilized knee design, it provides AP stability throughout the entire ROM. However, it is unclear whether this design difference translates to clinical and functional improvement. We asked whether the medially conforming ball-and-socket design differences would be associated with (1) improved ROM; and (2) improved American Knee Society, WOMAC, Oxford Knee, SF-36, and Total Knee Function Questionnaire scores compared to a conventional, fixed-bearing posterior-stabilized TKA. We enrolled 82 patients in a single-center, single-blinded, randomized, controlled trial comparing the medially conforming ball-and-socket design knee prosthesis to a posterior-stabilized total knee prosthesis. Our primary end point was ROM. Our secondary end points were American Knee Society, WOMAC, Oxford Knee, SF-36, and Total Knee Function Questionnaire scores. All patients were followed at 1 and 2 years. The mean ROM was 100.1° and 114.9° in the posterior-stabilized and medially conforming ball-and-socket groups, respectively. The physical component scores of SF-36 and Total Knee Function Questionnaire were better in the medially conforming ball-and-socket group. We found no difference in American Knee Society, WOMAC, and Oxford Knee scores. Both implant designs similarly relieved pain and improved function. The medially conforming ball-and-socket articulation provided better high-end function as reflected by the Total Knee Function Questionnaire. Level I, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Mobile-Bearing Total Knee Arthroplasty Improves Patellar Tracking and Patellofemoral Contact Stress: In Vivo Measurements in the Same Patients

Controversies exist in clinical study concerning the effect of rotating platform on patellar tracking. The aim of this in vivo study was to compare tibial rotation, patellar tracking, and patellofemoral contact stress in mobile and fixed-bearing platform intraoperatively in the same knee. Sixty-six knees of posterior-stabilized total knee prostheses were evaluated using a computed tomography–guided navigation system. Medial shift and lateral tilt of patella were significantly smaller in mobile knee. Averaged maximum contact stress was significantly smaller in mobile knee than fixed knee. However, tibial rotation during flexion has no significant difference. This study showed that mobile platform total knee arthroplasty significantly improved patellar tracking and decreased patellofemoral contact stress.

5- to 9-year Survivorship of Single-radius, Posterior-stabilized TKA

We studied 1030 consecutive cemented primary TKAs performed by the primary author (OMM) using a single-radius, posterior-stabilized total knee prosthesis with 5 years' minimum followup to determine whether an accelerated early failure rate was associated with this design. At 5 to 9.5 postoperative years, 32 knees had been revised at an average of 2.4 postoperative years (range, 0.1-8.2 years) because of infection (11), periprosthetic fracture (10), aseptic loosening (eight), stiffness (two), and late hemarthrosis (one). Four had only the tibial insert revised. One-half of all failures occurred within 1.5 years. The cases of aseptic loosening involved the femoral component in one patient, tibial component in five, and both components in two. With only seven patients (0.7%) having unknown outcomes, the overall failure rate was 4.9 per 1000 person-years for the study period. The Kaplan-Meier survivorship using any part of the prosthesis removed or revised for any reason as the end point was 95.8% (95% confidence interval, 93.7%-95.5%), and with aseptic loosening as the end point, it was 98.6% (95% confidence interval, 96.5%-99.4%). The midterm survivorship rates were comparable to those of other posterior-stabilized total condylar designs and are not suggestive of excessive risk of early failure. Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Rotational constraint in posterior-stabilized total knee prostheses.

Rotational stresses from box-post impingement have been implicated in the loosening of posterior-stabilized total knee prostheses. A bench model was constructed to assess the forces generated by tibiofemoral rotation. Rotational torque under load was measured in two different posteriorstabilized total knee prostheses using an axial-torsion load cell at 0 degrees, 20 degrees, and 40 degrees flexion over 20 degrees internal and external rotation. The Sigma posterior-stabilized prosthesis generated little torque through 5 degrees internal and external rotation. An increase in torque then occurred because of box-post impingement, generating peak torques of 17 to 18 N-m at 12 degrees to 14 degrees rotation. The bench model produced the same deformation of the polyethylene post as seen on retrieved specimens. The Scorpio posterior-stabilized prosthesis had a relatively continuous rise in generated torque from tibiofemoral conformity. Box-post impingement did not occur resulting in 32% lower torque between 12 degrees and 14 degrees rotation. Peak rotational torques of 15 to 16 N-m were reached at 19 degrees to 20 degrees rotation. Tibiofemoral conformity is the primary source of rotational constraint. Box-post impingement can be a source of additional rotational constraint. Depending on specific design features, small changes in relative tibiofemoral component rotation can more than double the generated torque. Axial rotation of the knee in vivo can generate substantial torque. Relative tibiofemoral rotational position is an important factor influencing component function and fixation.