Kinematic knee replacement fails to outperform mechanical alignment in cadaver study

Kinematic alignment has been promoted as a way to make total knee replacement feel more natural by preserving each patient’s individual joint anatomy, but a new laboratory study suggests that this philosophy does not automatically translate into more natural knee mechanics. In a dynamic knee rig experiment on cadaveric varus knees, researchers found that kinematic alignment did not restore native tibiofemoral or patellofemoral kinematics more closely than conventional mechanical alignment. The findings, published in Archives of Orthopaedic and Trauma Surgery, challenge the assumption that the favorable clinical outcomes reported after kinematically aligned knee replacement stem from biomechanics that better replicate the healthy knee.

The study was motivated by a long-standing tension in knee replacement surgery. Biomechanical work dating back to the 1970s and 1980s framed postoperative varus malalignment as a major risk factor for persistent pain and premature polyethylene wear driven by medial overload, a doctrine echoed in later literature. Yet anatomical surveys show that roughly 32 percent of men and 17 percent of women have a constitutional varus knee exceeding 3 degrees, determined largely by the medial proximal tibial angle. Forcing such anatomy into the neutral target of mechanical alignment can substantially alter the native joint line position. Clinical studies of kinematic alignment, which respects that individual anatomy, have reported higher patient satisfaction, less early postoperative pain on visual analog scales, and significantly better Forgotten Joint Score and Knee Society Score results, with the advantage most pronounced in CPAK type I phenotypes while CPAK type IV patients fare similarly under both techniques.

From a biomechanical standpoint, it had been hypothesized that these clinical gains correlate directly with optimized functional kinematics. Gait analyses indicated that kinematically aligned replacement replicates natural gait patterns and native knee kinematics more closely than mechanical alignment when compared with healthy control cohorts, and a reduced knee adduction moment has been observed despite a residual varus axis, possibly reflecting preserved joint line obliquity. Intraoperative force measurements confirmed that residual anatomical varus does not overload the medial compartment, and mid-term studies found no adverse effect on reoperation rates or implant survival. However, the evidence was not settled: a preliminary knee rig study using specimens with straight limb axes reported improved tibiofemoral kinematics under kinematic alignment, while in vivo gait analyses have failed to show substantial kinematic differences between the two strategies. The Munich research team therefore set out to test the direct influence of the two alignment philosophies in the varus knee, hypothesizing that kinematic alignment would produce kinematics closer to the native condition.

To answer the question, the group tested seven fresh-frozen human cadaveric knee specimens with constitutional varus ranging from 1.4 to 8.3 degrees, obtained from four donors with a mean age of 87.8 years. The specimens were mounted in a previously validated dynamic knee rig with six degrees of freedom, which drove an active, muscle-controlled squat from 30 to 130 degrees of flexion while maintaining a constant ground reaction force of 50 newtons. A sensor-controlled actuator regulated the rectus femoris muscle, and additional weights loaded the vastus medialis, vastus lateralis, semitendinosus, and biceps femoris tendons. An optoelectronic motion tracking system with reflective markers on the femur, tibia, and patella recorded three-dimensional kinematics continuously, while a thin pressure-mapping sensor fixed to the retropatellar surface captured contact area and peak pressure. Each knee was tested in three successive states within the same specimen: native, after kinematically aligned total knee arthroplasty, and after conversion to mechanical alignment using a medial-pivot implant design.

The surgical protocol preserved the posterior cruciate ligament and left the patella unsurfaced. Kinematic alignment followed the unrestricted calipered technique, with symmetrical 6-millimeter distal femoral resections and a balanced 10-millimeter tibial resection respecting the native joint line and tibial slope. Cementation was deliberately omitted so the kinematic components could be exchanged within each specimen, and the conversion to mechanical alignment used custom 3D-printed femoral components whose outer articulation geometry was identical to the standard implant, with the inner fitting surface compensating for the resection differences. Collateral ligament stability was assessed manually in both configurations, accepting no mediolateral laxity in extension and slight lateral laxity in flexion, in line with the medial-pivot design rationale. Statistical analysis used one-dimensional statistical non-parametric mapping, with non-parametric ANOVA and pairwise tests for kinematics and paired-samples tests for contact mechanics, all corrected for three pairwise comparisons using the Holm-Bonferroni method.

The central result was a pattern of descriptive differences that dissolved under statistical correction. Between the two alignment techniques, no statistically significant differences emerged in kinematics or patellofemoral biomechanics after adjustment, with all adjusted p-values above 0.05. Descriptively, kinematic alignment showed a trend toward unphysiological anterior femoral translation in deep flexion between 100 and 120 degrees, which reached an uncorrected p-value of 0.028 but fell to an adjusted value of 0.084, losing significance. The native knee translated the femur progressively posterior by a total of 5 millimeters up to 130 degrees of flexion, a trajectory closely mirrored by mechanical alignment, whereas kinematic alignment drove 5 millimeters of anterior femoral translation up to 90 degrees before a secondary posterior shift that still left the femur more anterior at full flexion, deviating more than 5 millimeters from the native baseline.

The rotation point analysis reinforced this picture. In the native knee, a minor anterior medial femoral translation from 30 to 90 degrees of flexion accompanied a typical lateral femoral rollback pivoting around the medial side, with relatively symmetrical posterior translation above 90 degrees totaling 7.5 millimeters laterally and 2.0 millimeters medially. Mechanical alignment produced a similar overall pattern, though total lateral rollback exceeded 14 millimeters. Kinematic alignment, by contrast, did not display a classic medial-pivot pattern: it began with a more centrally located rotation, followed by parallel anterior translation up to 100 degrees and parallel posterior translation thereafter, leaving the mean tibiofemoral profile visibly altered relative to the native state. Tibial rotation and varus-valgus deviation told a more favorable story for kinematic alignment, whose mean trajectories tracked closer to the native baseline, while mechanical alignment showed internal rotation exceeding 8 degrees and valgus deviation terminating at 7 degrees, about 2 degrees above both native and kinematic conditions, though neither contrast reached significance.

Patellofemoral findings were equally nuanced. After kinematic alignment the patella was markedly medialized at the initial 30 degrees of flexion, a delta of up to 5 millimeters relative to native, and remained a constant 2 millimeters medial to the native baseline beyond 100 degrees, whereas mechanical alignment converged with the native curve from 90 degrees onward. The authors note that this descriptive medialization may reflect a medial displacement of the prosthetic trochlea caused by preserved joint line obliquity and the absence of femoral component external rotation, raising the possibility that standard femoral components designed for mechanical alignment could alter patellofemoral tracking when used in a kinematic setting. On contact mechanics, both replacement techniques significantly reduced patellar contact area compared with the native knee, with adjusted p-values below 0.01, an effect the authors attribute to total knee arthroplasty itself rather than to either alignment philosophy. Peak pressure trends under kinematic alignment remained elevated up to 90 degrees of flexion without reaching significance, and quadriceps force showed no relevant or significant differences between the strategies.

The authors are careful about what these results do and do not prove. Their initial hypothesis could not be confirmed, and they emphasize that the findings remained inconclusive at a sample size of seven. Visually apparent descriptive differences did not survive adjustment for multiple comparisons, and the team explicitly warns that the non-significant results should not be construed as definitive evidence of biomechanical equivalence between the two techniques, since a Type II error, in which true differences go undetected because of limited statistical power and inter-specimen variability, cannot be excluded. Other limitations compound the caution: the knee rig cannot replicate all muscle groups relevant to joint motion, transferability of an active squat to gait or stair climbing is limited, bilateral specimens may introduce side-to-side correlations, the fixed testing sequence could have introduced time-dependent effects such as soft-tissue creep that preferentially affected the mechanical alignment condition tested last, the 3D-printed components have higher surface friction than polished cobalt-chromium implants despite petroleum jelly lubrication, and the medial-pivot design narrows generalizability to cruciate-retaining or posterior-stabilized implants.

Even with those caveats, the study delivers a pointed message for a field increasingly drawn to individualized alignment. Kinematic alignment was not shown to restore native knee kinematics or patellofemoral mechanics more closely than mechanical alignment in this dynamic knee rig model of the varus knee, and descriptive kinematic variations suggest that it does not automatically optimize functional joint mechanics when standard implant geometries are used. The authors conclude that the data do not provide evidence that restoration of native-like kinematics explains the favorable clinical outcomes associated with kinematic alignment reported in previous studies. Whether the observed biomechanical trends translate into clinical differences, or whether alternative, alignment-specific component geometries are needed to better restore native patellofemoral tracking, remains an open question for future research, particularly as early clinical studies of kinematic-specific designs have reported promising results including a superior Forgotten Joint Score.

Subject of Research: Biomechanical comparison of kinematic versus mechanical alignment in total knee arthroplasty of the varus knee using a dynamic knee rig model

Article Title: Tibiofemoral and patellofemoral kinematics after kinematic versus mechanical alignment in varus TKA—a dynamic knee rig study

Article References: Simon, J.-M., Siedow, T. M., Thorwächter, C., Petri, T. E., Bauer, L., Holzapfel, B. M., Müller, P. E., & Niethammer, T. R. (2026). Tibiofemoral and patellofemoral kinematics after kinematic versus mechanical alignment in varus TKA—a dynamic knee rig study. Archives of Orthopaedic and Trauma Surgery, 146(1), Article 395. https://doi.org/10.1007/s00402-026-06559-y

Image Credits: AI Generated

DOI: 10.1007/s00402-026-06559-y

Keywords: total knee arthroplasty, kinematic alignment, mechanical alignment, varus knee, tibiofemoral kinematics, patellofemoral kinematics, knee rig, cadaveric study, medial-pivot implant, joint line obliquity, patellar tracking, biomechanics