Analysis of retrieved polymer fiber based replacements for the ACL.

The present retrospective analysis of 117 surgically excised anterior cruciate ligament (ACL) prostheses was designed to elucidate the etiology and mechanisms of failure of synthetic ligamentous prostheses. They were harvested from young and active patients (26 +/- 7 yrs) at various orthopaedic centers in France between 1983 and 1993. The average duration of implantation of augmentation and replacement prostheses were 21.5 +/- 12.6 and 33.2 +/- 25.3 months, respectively. The principal causes for their excision were ruptures and synovitis. Each ACL prosthesis was examined macroscopically, histologically, and, after tissue removal, by scanning electron microscopy (SEM) to determine the model, manufacturer, surgical technique used at implantation, the extent of healing, the site of rupture, and the morphology of the damaged fibers. Fourteen types of ACL prostheses were analysed, each fabricated using a different combination of polymers, fibers and textile constructions. Consequently, they generated a variety of healing characteristics and mechanical responses in vivo. SEM observations revealed that abrasion of the textile fibers as a result of yarn-on-yarn and/or yarn-on-bone contact was a common phenomenon to almost all models, and was the primary cause of prosthetic failure. Healing inside the synthetic ACL was poorly organized, incomplete and unpredictable as the extent of collagenous infiltration into the textile structure did not increase with the duration of implantation. In fact, the collagenous infiltration into certain models appeared to be more detrimental than beneficial since it caused deterioration and fraying of the textile structure rather than serving as a reinforcing matrix around the prosthesis. In conclusion, the present study shows that three mechanisms may be involved in the failure of ACL prostheses: (1) inadequate fiber abrasion resistance against osseous surfaces; (2) flexural and rotational fatigue of the fibers, and (3) loss of integrity of the textile structure due to unpredictable tissue infiltration during healing.

In vivo time course studies of the tissue responses to resorbable polylactic acid implants by means of MRI.

Magnetic resonance (MR) imaging and relaxation time measurements of bioresorbable implants made of polylactic acid (PLA), as well as the surrounding tissues, were carried out over a period of 6 months to monitor the implant state and the body's responses, and to determine how these processes are reflected in MR data. Twelve rabbits each received two subcutaneous PLA implants (45 x 10 x 2 mm). Changes in tissue relaxation rates demonstrated inflammation and tissue healing time courses but were not simply linear functions of the tissue water content and so provide new insight into MR characterization of inflammatory processes.

Use of supercritical fluid extraction as a method of cleaning anterior cruciate ligament prostheses: in vitro and in vivo validation.

The process of supercritical fluid extraction (SFE) using carbon dioxide as the mobile phase is finding increasing numbers of applications in a wide variety of industries for the extraction, separation, and cleaning of materials. This study assessed the usefulness of this approach in removing surface contaminants from a knitted polyester anterior cruciate ligament (ACL) prosthesis before packaging and sterilizing the product during manufacture. The physical, dimensional, and chemical properties of SFE treated compared with commercially scoured control samples were characterized using a number of textile test methods: electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry, and solvent extraction analysis. The biocompatibility of the samples was measured in terms of their ability to generate CD18 integrin expression on activated human polymorphonuclear cells, and their inflammatory response when implanted for up to 30 days in the knee joint of rats. SFE treatment was successful in removing most of the nonpolar contaminants from the ACL prosthesis and reducing the amount of residuals to a commercially acceptable level. However, some nitrogen containing compounds and polar salts were not removed by the SFE process. The results from the biocompatibility tests demonstrated that the cleaner SFE treated prosthesis induced significantly lower CD18 expression than the scoured control fabric, and was also associated with a milder inflammatory response and a more rapid rate of healing during the 30 day animal trial. Another effect of SFE processing was to cause the polyester device to shrink and lose porosity because of yarn contraction and modification of the polymer's microcrystalline structure.

Failure mechanisms of anterior cruciate ligament prostheses: in vitro wear study.

Previous retrieval studies analyzing the cause of failure of anterior cruciate ligament (ACL) prostheses identified a wear mechanism. However, the relative importance of yarn on bone compared to yarn on yarn wear has not been clearly understood. Therefore, the objective of this study was to elucidate which type of wear is the dominant cause of clinical failure. A variety of ACL prosthetic structures were exposed to two in vitro tests: one for yarn on yarn and the other for a novel yarn on bone wear test system. The latter included the use of both smooth (uncut) and rough (cut) bone surfaces to simulate the conditions around the condyle and at the exit of the tibial tunnel, respectively. The damaged textile structures were viewed by SEM. The various fiber fracture morphologies were identified and classified for the two types of wear tests; for the smooth and rough bone surfaces; for the braided, knitted, woven, and twisted textile structures; and for the three types of fibers that were included: polyethylene terephthalate, polypropylene, and ultrahigh molecular weight polyethylene. The results confirmed that yarn on bone and yarn on yarn wear phenomena are associated with significantly different failure mechanisms. While the more aggressive rough (or cut) bone causes more rapid and intense fiber damage and faster ACL failure than the smooth (uncut) osseous surface, both abradants cause the same type of abrasive wear phenomenon. Differences in failure mechanisms were identified between the different textile structures and the different fiber types. By interpreting the damaged fiber images from clinically failed and retrieved ACL prostheses, we are now able to confirm that the predominant cause of synthetic ACL failure is yarn on bone abrasion. Improvements in future ACL prosthesis designs will only be possible by eliminating or minimizing the effect of this type of abrasive wear.

[Macroscopic, histologic and ultrastructural study of 89 prostheses of anterior cruciate ligament excised because of prosthesis failure]. / Etude macroscopique, histologique et ultrastructurale de 89 prothèses du ligament croisé antérieur explantées pour échec.

PURPOSE OF THE STUDY: This study concerns the etiology of failed synthetic anterior cruciate ligament (ACL) prostheses, and attempts to identify the primary mechanisms that lead to their premature rupture. MATERIAL AND METHODS: A total of 89 failed and surgically excised ACL prostheses were retrieved from young and active patients (27 +/- 7 years) at various orthopaedic centres in France. Their average duration of implantation was 34 +/- 24 months. They were examined macroscopically, histologically and by scanning electron microscopy (SEM) to determine the model, the manufacturer, the surgical technique used at implantation, the extent of healing, the site of rupture, as well as the morphology of the damage fibers. RESULTS: Seventy two of these explants represented 6 different models. While all 6 were fabricated from polyester fibres, each had a different textile construction, and each were associated with a unique healing and mechanical response in vivo. SEM observations confirmed that abrasion of the textile fibres were a phenomenon common to all models, and were the primary cause of prosthesis failure. Such wear zones were particularly prevalent at the exit of the tibial tunnel and around the femoral condyle. Collagenic infiltration into the synthetic ACL was poorly organized and unpredictable. It did not increase with the duration of implantation. In fact in certain models, it appeared to have caused deterioration and fraying of the textile structure rather than serving as a reinforcing matrix around the prosthesis. DISCUSSION: A synthetic ACL prosthesis is to be preferred for patients who do not have tissue available for autologous ligamentoplasty. Yet none of the synthetic devices examined in the present study were capable of stabilizing the knee over the long term. Among the factors that influenced their failure we found that the three most common mechanisms were flat abrasion against an osseous surface, flexural and rotational fatigue of the fibres, and loss of integrity of the textile structure due to unpredictable tissue infiltration during healing. CONCLUSION: The results of the present study show that none of the current models succeed in replacing the natural ACL. Future improvements may be achieved by developing surgical procedures for implantation combined with a prosthesis made from fibres and textile structures which are more abrasion resistant and promote predictable and controlled tissue infiltration.