In Vitro Evaluation of Tibiofemoral Contact Stress and Graft Loading After Modified MacIntosh Anterior Cruciate Ligament Reconstruction.

BACKGROUND

The modified MacIntosh anterior cruciate ligament reconstruction (ACLR) consists of a combined intra- and extra-articular construct using iliotibial band autograft. It is used in skeletally immature patients to avoid physeal injury. However, its effect on articular cartilage contact mechanics, rotational laxity, and graft loading is poorly understood.

HYPOTHESIS: Relative to the intact knee, a modified MacIntosh ACLR would (1) increase and anteriorize lateral tibiofemoral contact stress, (2) decrease internal rotation (IR) and varus laxity, and (3) offload the intra-articular graft relative to the native ACL in response to simulated pivot-shift loads.

STUDY

DESIGN

Controlled laboratory study.

METHODS

Eight cadaveric knees (mean age 56 ± 7 years; 50% male) were tested on a robotic manipulator in 3 states: intact, ACL-sectioned, and modified MacIntosh ACLR. Peak contact stress and the anterior-posterior position of the center of contact stress (CCS) in the lateral compartment were calculated during simulated pivot shifts. Laxity was assessed for 3 uniplanar loading conditions (134 N anterior, 8 N·m varus, and 5 N·m IR) at knee flexion angles from 0° to 90°. Tissue forces were determined via the principle of superposition.

RESULTS

After modified MacIntosh ACLR, peak stress increased by 0.31 to 0.35 MPa (P < .03) and the CCS shifted anteriorly by 1.3 to 3.3 mm (P < .05) relative to the intact knee. ACLR decreased IR and varus laxity by 18% to 24% (P≤ .01) and 13% to 24% (P < .02), respectively, compared with the intact state. During simulated pivot shift, in situ force in the intra-articular graft component (27 to 36 N) was 51% to 65% less than native ACL force, whereas the extra-articular component carried 25 to 30 N.

CONCLUSION

In a cadaveric model of a simulated pivot shift, a modified MacIntosh ACLR anteriorized and mildly increased lateral compartment contact stress compared to the native knee, while decreasing lateral compartment contact stress relative to the ACL-deficient knee. The extraarticular limb also increased constraint to rotational loads, partially offloading the intra-articular graft.

CLINICAL RELEVANCE: Partial intraarticular graft offloading with clinical pivoting maneuvers provides a biomechanical rationale for the low number of graft failures after modified MacIntosh ACLR. The ability of a modified MacIntosh ACLR to decrease and anteriorize contact stress in the posterior aspect of the lateral compartment offloads the region most affected by ACL injury and the pivot shift contact. Long-term clinical studies are needed to determine whether altered contact mechanics after modified MacIntosh ACLR influence long-term cartilage health.