Anterior Cruciate and Posterolateral Ligament Injuries Result in Different Degrees of the External Rotation Recurvatum Test: A Robotic Analysis in Male Cadaveric Knees.

PURPOSE: To investigate the abnormal knee hyperextension and coupled external rotation, varus, and anterior translation that occurs using sequential ligament and capsular sectioning in multiligamentous injuries.

METHODS: A six-degree-of-freedom robotic system measured knee hyperextension and coupled external rotation, varus, and anterior-posterior translation with selective tissue cutting in 23 cadaveric knees. Tests were performed up to 25 Nm hyperextension load before and after cutting 6 structures: oblique popliteal ligament (OPL), posterolateral capsule with the fabellofibular ligament (PLC+FFL), posteromedial capsule with posterior oblique ligament (PMC+POL), lateral collateral ligament and popliteus tendon (LCL+POP), and the anterior cruciate ligament (ACL). Sequence 1 sectioned the posterior capsular structures (OPL, PLC+FFL, and PMC+POL) followed by the LCL+POP and ACL. Sequence 2 simulated a posterolateral injury (LCL+POP) and ACL injury followed by the posterior capsular injuries (OPL, PLC+FFL, and PMC+POL).

RESULTS: In sequence 1, sectioning the posterior capsule caused statistically significant increases to hyperextension (2.5° ± 0.9°; P = .02). Additional injury of the LCL+POP and ACL significantly increased hyperextension to 7.6°± 4.4° (P < .01), external rotation to 2.7° ± 2.8° (P < .01), and varus rotation to 3.2°± 3.9° (P < .01). In sequence 2, sectioning the LCL+POP significantly increased hyperextension (1.4° ± 0.6°; P < .01) and additional ACL sectioning significantly increased hyperextension (2.1° ± 1.0°; P < .01). Further sectioning the posterior capsule significantly increased hyperextension to 6.9° ± 1.7° (P < .01), external rotation to 2.9° ± 2.3° (P < .01), and varus rotation to 2.5° ± 1.4° (P < .01).

CONCLUSIONS: Posterolateral injuries (LCL+POP and PLC+FFL) resulted in small increases in knee hyperextension, external tibial rotation, and varus rotation. Sectioning the entire posterior capsular structures (PLC+FFL, PMC+POL, and OPL) is necessary for major increases in knee hyperextension.

CLINICAL RELEVANCE: Posterolateral ligament injuries may not result in a positive external rotation recurvatum test as only modest increases in external rotation and varus occur with knee hyperextension. Other ligament tests including the dial test for external tibial rotation and varus loading for lateral joint opening in knee flexion are recommended.