Synthetic Ligaments in Hip Surgery

Ligament replacement or augmentation with synthetic materials is an accepted part of orthopaedic practice. Synthetic materials have a long history of use, particularly around the knee, where they have been used to repair or replace both intra- and extra-articular ligaments. They are also used extensively around the shoulder to repair rotator cuff tears and acromio-clavicular joint dislocations. Other joints now routinely reconstructed with synthetic ligaments include the ankle, elbow and, recently, the hip.

Ligament reconstruction surgery dates from the early 1900s, when Corner¹ described replacing a ruptured anterior cruciate ligament (ACL) with silver wire. It was not until the 1980s that it became a popular technique, following the approval of Dacron®, Gortex® (polytetrafluoroethylene) and the Kennedy Ligament Augmentation Device (polypropylene) for general orthopaedic use. The lack of donor-site morbidity, earlier return to function and limitless supplies were cited as advantages over autograft techniques. These initial materials acted as prostheses designed to permanently replace the ligament. Unfortunately, these materials were stiff with low ultimate tensile strengths and poor abrasion properties. Not only did grafts fail after cyclical loading, but also marked synovitis ² and cartilage destruction from the wear particles was noted when used intraarticularly. These significant side effects markedly limited the popularity of synthetic ligaments in the late 1980s and 1990s.

The second generation of synthetic ligaments that followed were designed to act as scaffolds, providing initial strength following reconstruction but relying on biological in-growth of native tissues in order to provide longevity. Although they encourage biological integration, they are not joint-specific and concerns have been raised about the quality of the collagen that grows onto these scaffolds on account of the stiffness of the polyethylene. ³

The current generation of synthetic ligaments are highly engineered, being both ligament- and joint-specific while also incorporating biological function. The current generation of ACL ligaments are sided, with woven ends to enable fixation into bone tunnels and unwoven intra-articular portions to prevent abrasion of the articular cartilage and third-body wear. Initial results of the use of these third-generation ligaments have been encouraging in terms of both restoration of joint function and fewer and less severe side effects.⁴

Basic Science
First-generation synthetic ligaments were found not to be biocompatible, and in fact produced quite severe inflammatory reactions. Both second- and third-generation materials have addressed these concerns by using bio-compatible materials such as polyethylene washed in collagen. This encourages fibroblastic adhesion and later differentiation into native tissue. These newer materials have stress–strain characteristics that more closely resemble the natural ligament, and have often been tested up to 10 million cycles. The breaking strength of these materials is such that it is usual for the fixation method to be the rate-limiting step in terms of graft survival.

Applications in Hip Surgery
Dislocation
The first description of a synthetic ligament being used around the hip was to prevent recurrent dislocation following total hip replacement (THR).⁵ The ligament was used as a static leash, anchored to the edge of the acetabulum with a screw and secured around the prosthesis neck like a noose. This restricted the arc of movement of the prosthesis, hence preventing dislocation. The obvious drawback was reduced movement; however, it was mooted as a less radical treatment than complete revision of both components. A wide variety of revision implants and techniques have limited this indication to a select group of patients.