Injuries to Multiple Ligamentous and Capsular Structures Cause Abnormal Knee Hyperextension: A Robotic Study in Human Knees

PURPOSE: To determine the relationship of multiple ligamentous and capsular structures that may be injured in multidirectional knee injuries with abnormal knee hyperextension. METHODS: A 6-degree-of-freedom robotic system measured intact motion limits and increases in knee hyperextension under extension loading up to 25 Nm and 25 N compression with selective tissue sectioning in 23 cadaveric knees. Two cutting sequences for sectioning ligamentous and capsular structures were used to simulate knee injuries with combined anterior cruciate ligament (ACL) disruption, posterolateral injury (lateral collateral ligament + popliteus tendon [LCL + POP]), and posterolateral and posteromedial capsular structures (PLC, PMC). The first cutting sequence sectioned the posterior capsular structures (OPL, PLC + FFL, PMC + POL) followed by the LCL + POP and ACL. The second cutting sequence simulated a common combined posterolateral and ACL injury followed by the posterior capsule (OPL, PLC + FFL, and PMC + POL). RESULTS: Posterior capsule (OPL, PLC + FFL, and PMC + POL) disruption in sequence 1 is not associated with major increases in knee hyperextension (2.6° ± 1.0°, P < .01). Additional injury to the LCL + POP produces major hyperextension (4.5° ± 2.1°, P < .01). In sequence 2, disruption of the posterolateral complex (LCL + POP) and ACL is not associated with major increases in knee hyperextension (2.2° ± 1.0°, P < .01). Major increases in hyperextension occur after injury to the LCL + POP, ACL, and OPL (3.1° ± 1.6°, P < .01). CONCLUSIONS: Multidirectional ligament injury requires injury to 3 major capsular and ligamentous structures for a major abnormal increase in knee hyperextension. The results show the importance of the posteromedial and posterolateral capsules in limiting abnormal knee hyperextension, important for diagnosis and treatment considerations. Major increases in knee hyperextension occur with an intact, uninjured ACL when there is a disruption of the posteromedial and posterolateral capsule and posterolateral damage (LCL + POP). CLINICAL RELEVANCE: This study shows the role of multiple ligamentous and capsular structures providing stability to the knee joint and resisting abnormal knee hyperextension.