CAN THE ENVELOPE OF MOTION OF THE KNEE EXPLAIN SEEMINGLY CONTRADICTING PIVOT LOCATIONS

Introduction Advancements in knee surgery require a profound understanding of knee mechanics. However, there are seemingly contradicting reports regarding certain aspects of normal knee function, such as the location of the pivot of internal-external rotation in the transverse plane. Among others, it has been suggested to be located close to the knee center or in the medial compartment. We hypothesized that this apparent contradiction is a result of different studied knee motions and that it can be explained by the underlying envelopes of motion. The study objective was to characterize normal knee behavior in-vitro with an emphasis on pivot location. Methods Thirty-four cadaveric human knee specimens (Age: 61±8 years, BMI: 25±7) underwent CT and MR imaging and load controlled in-vitro testing using an industrial robot (KUKA, Augsburg, Germany). The robot simulated passive knee flexion and assessed the envelopes of motion through anterior-posterior (AP, ±100 N), medial-lateral (ML, ±100 N) and internal-external (IE, ±6 Nm) laxity testing at five flexion angles. Kinematics were expressed by the femoral flexion facet centers (FFC). The pivot location was determined for IE laxity testing and passive flexion by computing the center of transverse femoral rotation in a least squares sense. Groups were compared by one-way ANOVA (α = 0.05). Results are stated as average ± standard deviation. Results During IE laxity testing the pivot was located centrally, exhibiting a small medial offset from the tibia center (Fig. 1). The medial offsets were 4.1±3.0 mm, 3.6±1.9 mm, 4.4±1.9 mm, 5.3±2.0 mm, and 5.4±2.2 mm at 0°, 30°, 60°, 90° and 120° of flexion. In contrast, the passive flexion pivot location was close to the medial plateau border (Fig 2.). Its medial offset from the center amounted to 36.0±11.7 mm and was significantly larger than any offset detected during IE rotation at a given flexion angle (p « 0.001). The resulting envelopes of motion corresponded to these findings (Fig. 3). The average AP laxities of the medial and lateral FFCs were 14.9±2.9 mm and 17.1±3.0 mm whereas laxity at the knee center was only 6.0±2.8 mm. The average IE laxity was 37.8±6.1°. Over the arc of flexion, the envelope centers shifted posteriorly by −0.3±3.1 mm, 14.5±3.9 mm and 10.3±2.9 mm for the medial FFC, lateral FFC and the knee center respectively. Discussion and Conclusion Our results confirm that the pivot location can vary and is influenced by the type of knee motion. Furthermore, fundamental characteristics of knee biomechanics such as AP stability, IE laxity as well as femoral rollback and external rotation with flexion help explain what could be construed as contradictions in the literature. AP stability and rollback are controlled centrally by the cruciate ligaments. A central pivot during IE laxity testing is a direct consequence of the central AP stabilization. However, a medial pivot during passive flexion results from the superposition of the rollback guided by the cruciates and external rotation with flexion. This current study provides a comprehensive evaluation of the intact knee that when examined as a whole begins to explain contradictory data in the literature and provides a broader picture of passive knee kinematics.