Lateral Tibial Posterior Slope Induces Anterior Tibial Translation While Lateral-Medial Slope Difference Induces Internal Rotation: A Cadaveric Biomechanical Study.

BACKGROUND: Posterior tibial slope (PTS) influences tibiofemoral kinematics and anterior cruciate ligament (ACL) function. While previous studies have evaluated symmetrical slope modifications, the isolated influences of the medial or lateral PTS remain unclear. This study investigated the influence of medial and lateral PTS changes on anterior tibial translation (ATT) and internal rotation (IR) under axial loading, using unicondylar slope-changing osteotomies.

HYPOTHESIS: Increased lateral PTS increases ATT, while a greater delta PTS (lateral minus medial PTS) increases IR.

METHODS: Eight fresh-frozen knee specimens were mounted in a custom-built fixture in a compression/torsion loading machine. Tibiofemoral translation/rotation kinematics were measured under a 500-N axial compression load at 0° and 20° of knee flexion using optical motion tracking. Unicondylar tibial slope-changing osteotomies were performed on the medial and lateral tibial plateaus with a custom cutting guide, preserving key ligament and meniscus root attachments. Medial and lateral PTS were independently adjusted to -5°, 0°, +5°, and +10°, resulting in medial and lateral PTS ranging from -5° to +10°, using 3-dimensionally printed wedges. Repeated-measures analyses of variance, post hoc t tests, and Pearson correlations were used for analysis.

RESULTS: An increase of +10° in lateral PTS significantly increased ATT (P = .012), while increasing medial PTS did not (P > .99). IR was most influenced by delta PTS. At 0° of flexion, altering lateral or medial PTS alone was insufficient to significantly change rotation, but changing delta PTS +10° produced 7.6° more rotation (P = .015). At 20° of flexion, changing individual slopes ±10° significantly increased/decreased rotation (P < .036). The largest rotational differences correlated with opposing delta PTS values (r = 0.960; P = .0006).

CONCLUSION: Lateral PTS was the principal determinant of ATT, whereas delta PTS was the principal determinant of IR under axial loading.

CLINICAL RELEVANCE: Understanding the independent biomechanical roles of the medial and lateral PTS provides insight into mechanisms of anterior and rotational instability in ACL injury and reconstruction. These findings suggest the potential utility of differential correction of PTS as a novel surgical approach for optimizing tibiofemoral rotational kinematics.

STUDY DESIGN: Controlled laboratory study.