Xenografts for tendon and ligament repair

Collagenous materials, usually of bovine or equine origin, have been popular starting points for the development of xenograft prostheses for tendon and ligament repair. Xenografts are highly attractive as they carry small risk of infectious disease, do not compromise the patient's remaining tissues and may have the 'correct' structure as the component being replaced. Animal studies, on dog, rabbit and chicken, have shown tremendous potential for this use of xenograft material as a tendon replacement. Why, therefore, have xenografts been almost universally a total failure in clinical application? The reasons would appear to be two-fold: the animal models have not been appropriate to the intended clinical use and the cross-linking of xenograft materials has not been optimized. Our work on xenograft, heterograft and autograft tissues indicates that both aspects deserve more attention. Quantitative histology indicates that the extent and type of response to xenograft materials differs widely with degree of cross-linking (glutaraldehyde). Attention must also be given to the join of the graft to the host. For both tendon and ligament the join is a site of particular fragility. Even with adequate strength in the mid-substance, tendon and ligament grafts can, and do, fail at the join. We have investigated a variety of mechanisms for joining tendon to tendon and ligaments to bone. The failures of these methods present some insight into the biology of the repair process involved and into how failure may be avoided in future.