Abstract
Introduction: By characterising ACL strain behaviour in intact and posteromedial deficient knees under a variety of external loading conditions the aim of this work was to demonstrate whether posteromedial corner insufficiency could increase strain in an ACL reconstruction graft.
Materials and Methods: 15 fresh cadaveric knees were mounted on a materials testing machine. A miniature extensometer was implanted onto the anteromedial bundle (AMB) of the ACL. The knees were loaded in: Anterior draw (150N), varus/valgus rotation (5Nm) and internal/external rotation (5Nm) at 0°, 15°, 30°, 60° & 90° flexion. The posteromedial corner structures – posteromedial capsule, superficial MCL and deep MCL – were cut sequentially and the effect AMB strain measured.
Results: Strain data for analysis was available for 11 intact knees: Tibial internal rotation produced increased strain in the AMB at all angles of knee flexion (p< 0.05). Tibial external rotation reduced ACL strain at 0° to 30° (p< 0.05) and 60° to 90° knee flexion (p> 0.05).
Anterior loading of the tibia increased AMB strain. With the tibia free to rotate, strain was highest at 90 degrees knee flexion (5.3%) and lowest at 0 degrees (1.6%). Fixed internal and external tibial rotation reduced AMB strain produced by a 150 N anterior drawer force at all knee flexion angles.
Strain data for analysis was available for 6 Posteromedial Corner deficient knees:
With the tibia free to rotate or when locked in internal rotation, cutting the posteromedial structures had no effect on AMB strain with a 150 N anterior drawer force applied to the tibia. However, with the tibia locked in external rotation, cutting the posteromedial structures increased AMB strain at 60 and 90 degrees flexion. This difference however did not reach statistical significance.
Conclusions: The findings that division of the posteromedial structures may cause increased AMB strain and that there is significant load sharing by the peripheral ligamentous structures, suggests that valgus and rotational stresses to the knee in a patient with posteromedial corner insufficiency could lead to increased strain in the ACL graft, that would otherwise have been restrained by the posteromedial corner complex. It would also therefore seem to be appropriate to recommend the use of a collateral ligament brace in the post-operative period when combining a repair of the posteromedial structures and the ACL, to again prevent excessive graft strains.
Honorary Secretary – Mr Roger Smith. Correspondence should be addressed to BASK at the Royal College of Surgeons, 35 - 43 Lincoln’s Inn Fields, London WC2A 3PN
