Mechanisms of Failure for Anterior Cruciate Ligament Prostheses Implanted in Humans

Abstract

This study was a retrospective analysis of 79 surgically excised anterior cruciate ligament (ACL) prostheses. These devices were constructed from a variety of textile structures using different types of polymeric and carbon fibres. They were harvested at a number of different hospitals in France between 1985 and 1992, and were excised following periods of implantation varying from 1 month to 107 months. Since the reason for removal was invariably failure of a ligamentoplasty, the objective of the study was to understand more fully the nature of the articulation experienced by the ACL prostheses, and to identify the main causes of failure. The study was divided into three parts. First all 79 prostheses were examined macroscopically to determine the type, manufacturer, surgical technique and site of damage. The most common types were examined histologically, and then, following tissue removal, specimens were viewed by scanning electron microscopy (SEM) to identify the morphology of the damaged fibres. These second and third stages involved 60 explants representing the following five types of devices: Stryker® (n=23), Proflex® (n=17), Lygeron® (n=9), ABC Surgicraft® PET/PET (n=6) and SEM® (n=5). The clinical data confirmed that the patients who receive ACL prostheses are young (average age at implantation: 27 +/- 7 years) and active; 88% of patients ruptured their natural ACL during a sporting accident. As a result an ACL prosthesis needs to have high strength and dimensional stability and maintain these properties over an extended life span of many years. All five types of prostheses examined at the second and third stage were made from polyethylene terephthalate (PET) fibres, but their textile structures were different. The results from microscopic and histological examinations, as well as SEM observations of the damaged fibre morphology, have identified a unique healing and mechanical response for each of the five types of ligaments studied. This suggests that the textile structure plays an important role both in influencing the biological response to the prosthesis and the type of movement in the rehabilitated knee joint, as well as the long term success of the ACL surgery. In spite of these five different responses, some general observations were also made about this type of device. Scanning electron micrographs confirmed that abrasion of the textile fibres was a common phenomenon invariably experienced by all the prostheses, especially at the exit of the tibial tunnel and around the femoral condyle. Zones of wear and fibre fracture were usually observed at both these locations. In addition there was always evidence of some collagenous infiltration, but the structure of the collagen was poorly organised and the extent of infiltration and encapsulation was unpredictable. For example, the amount of tissue infiltration was not influenced directly by the duration of implantation. In conclusion, it appears that inadequate fibre abrasion resistance is the main cause of failure of ACL prostheses. However, with such a large standard deviation for the average duration of implantation for each type of prosthesis, it is clear that the cause of failure cannot be explained by a single mechanism. A number of other factors is believed to influence this wear phenomenon, particularly the extent of collagenous infiltration, which is usually not only unpredictable but also leads to a separation and breakdown of the textile structure and hence a loss in mechanical integrity.