The effect of weightbearing and external loading on anterior cruciate ligament strain

Abstract

A force balance between the ligaments, articular contact, muscles and body weight maintains knee joint stability. Thus, it is important to study anterior cruciate ligament (ACL) biomechanics, in vivo, under weightbearing conditions. Our objective was to compare the ACL strain response under weightbearing and non-weightbearing conditions and in combination with three externally applied loadings: (1) anterior–posterior shear forces, (2) internal–external torques, and (3) varus–valgus moments. A strain transducer was implanted on the ACL of 11 subjects. All joint loadings were performed with the knee at 20° of flexion. A significant increase in ACL strain was observed as the knee made the transition from non-weightbearing to weightbearing. During anterior shear loading, the strain values produced during weightbearing were greater than those of the non-weightbearing knee (shear loads <40 N). At higher shear loads, the strain values became equal. During axial torsion, an internal torque of 10 Nm strained the ACL when the knee was non-weightbearing while an equivalent external torque did not. Weightbearing significantly increased ACL strain values in comparison to non-weightbearing with the application of external torques and low internal torques (<3 Nm). The strains became equal for higher internal torques. For V–V loading, the ACL was not strained in the non-weightbearing knee. However, weightbearing increased the ACL strain values over the range of moments tested. These data have important clinical ramifications in the development of rehabilitation protocols following ACL reconstruction since weightbearing has been previously thought to provide a protective mechanism to the healing graft.

Introduction

One of the most important ligaments to knee joint stability, and yet one of the most frequently ruptured, is the anterior cruciate ligament (ACL). It is well known that the ACL is a primary restraint to anterior translation of the tibia relative to the femur and a secondary restraint to internal rotation in the non-weightbearing knee (Ahmed et al., 1987; Bach and Hull, 1998; Beynnon et al (1992), Beynnon et al (1997); Butler et al., 1980; Hsieh and Walker, 1976; Markolf et al (1976), Markolf et al (1990); Sakane et al., 1997). However, there is some disagreement on whether or not the ACL is a secondary restraint to external rotation, varus and/or valgus angulation (Berns et al., 1992; Beynnon et al., 1997; Markolf et al (1976), Markolf et al (1990); Seering et al., 1980). Since the tibiofemoral contact force and musculature play an active role in maintaining joint stability, it is important that investigations of ACL function be performed, in vivo, where physiological loading conditions may be examined.

Compressive loading of the knee, such as occurs during weightbearing, have been shown to reduce A–P laxity and stiffen the tibiofemoral joint in comparison to the non-weightbearing condition (Bargar et al., 1980; Fleming et al., 1999; Hsieh and Walker, 1976; Markolf et al., 1981; Torzilli et al., 1994). Several of these studies have been used to support the popular belief that compressive loading stabilizes the knee by forcing the tibiofemoral surfaces together, serving a protective function for the injured ACL or healing ACL graft during rehabilitation. However, a recent study by Torzilli et al. (1994) suggested that the relationship between joint compressive load and anteroposterior knee laxity might be more complicated. They performed a cadaver study and noted an “anterior neutral shift” of the tibia with respect to the femur when a compressive load in combination with a quadriceps force was applied to the knee in ACL-deficient knees. The anterior neutral shift could potentially strain the ACL intact or ACL reconstructed knee.

The objective of this study was to compare the ACL strain response under non-weightbearing and weightbearing conditions, and in combination with three externally applied loadings: (1) anterior–posterior (A–P) shear forces, (2) internal–external (I–E) torques, and (3) varus–valgus (V–V) moments. The following hypotheses were addressed: (1) ACL strain increases as the knee transitions from non-weightbearing to weightbearing. (2) Application of a joint compressive force will increase ACL strains in comparison to the non-weightbearing knee when applied in combination with A–P shear loading. (3) Application of a joint compressive force will increase ACL strains in comparison to the non-weightbearing knee when applied in combination with I–E torques. (4) Application of a joint compressive force will increase ACL strains in comparison to the non-weightbearing knee when applied in combination with V–V moments. These data will be evaluated to demonstrate that weightbearing does not always strain shield the ACL when rehabilitating an ACL, or ACL graft.