Multi-ligament knee injuries require complex surgery. Hinged external fixators propose to control the tibio-femoral relationship, protect reconstructions and allow early mobilisation. However, a uniaxial hinge may be too simplistic for such a complex joint. We investigated the influence of an external fixation device on ligament strains and joint contact forces.

Six fresh frozen cadaveric lower limbs (41–56 years old) were obtained. Displacement transducers (Microstrain, USA) were attached to mid-substance lateral (LCL) and medial collateral (MCL) ligaments, and the anterior and posterior cruciate (PCL) ligaments through minimal soft tissue incisions. Joint pressures were measured by transducers (Tekscan) introduced in the medial and lateral compartments through small sub-meniscal arthrotomies. Flouroscopic imaging was used to construct the hinged fixator centred over the epicondylar axis. Ligament tensile strains and joint contact forces were determined through a passive arc of 20 to 110 degrees of flexion and extension, with and without the external fixator (ExFix^™, EBI Biomet Australia).

The application of the external fixation device resulted in minimal change in the mean peak percentage strain of the PCL, MCL and ACL ligaments, while the LCL peak percentage strain decreased. Generally the peak percentage strain for each ligament occurred at or near the same flexion angle in both the un-instrumented and instrumented case within each limb, but the peak percentage strain flexion angles varied significantly across limbs. Peak joint contact forces increased significantly (p < 0.05) in the lateral compartment after attachment of the external fixation device. There was no difference seen in the medial compartment joint contact forces.

This study shows that a uniaxial hinged external fixator can be used in a multi-ligament reconstructed knee to maintain joint congruence and allow early postoperative rang of motion without compromising the results of reconstructions or repairs.