Tendon Healing in a Bone Tunnel With Biodegradable Interference Screw Fixation

Abstracts & Commentary

Synopsis: In this 2-part study presented as back-to-back articles, Weiler and colleagues showed that biodegradable interference screws provided good fixation for soft tissue grafts in tunnels. Biomechanically, the weak link during the early healing period was the graft at its tunnel entrance site. Weiler et al also demonstrated that screw degradation did not compromise graft fixation. Histologically, biodegradable interference screws allowed direct bony insertion without the development of a fibrous interzone, which has been shown to occur with nonaperture fixation.

Sources: Weiler A, et al. Arthroscopy. 2002;18:113-123; Weiler A, et al. Arthroscopy. 2002;18:124-135.

The debate rages on regarding the best fixation method for soft tissue grafts. Some authors have implicated nonaperture fixation as a cause of laxity in hamstring ACL reconstructions. Other authors have suggested that aperture fixation with biodegradable fixation is not as strong as fixation with other devices. Weiler et al studied the biomechanical and histological results of aperture fixation of soft tissue grafts in a sheep model.

Thirty-five skeletally mature sheep were assigned to 5 groups of 7 animals each. ACL reconstructions using ipsilateral half split Achilles tendon grafts were accomplished at time zero. All grafts were fixed with a poly-(D,L-lactide) biodegradable interference screw. The animals were sacrificed at 6, 9, 12, 24, and 52 weeks based on the group they were randomly assigned to. At the time of sacrifice, the grafts were tested mechanically with Drawer and Failure tests after cross-sectional areas were measured. Histologic analysis was performed using conventional stains and fluorescence microscopy to look at bone ingrowth. Five animals had to be excluded because of surgical failure in 2 and unrelated death in 3.

Drawer testing demonstrated that the grafts were most lax at 6 weeks, but this laxity gradually returned to normal at 52 weeks. Cross-sectional measurements demonstrated graft atrophy at 6 weeks and hypertrophy at 12 weeks. Failure testing demonstrated graft pull-out from the tunnels at time zero (done on contralateral control limbs), and midsubstance failure at 6 and 9 weeks. Two-thirds of the 6 week group specimens and all of the 9- week group specimens failed near the insertion sites. In the 12-, 24-, and 52-week groups, failure occurred by osteocartilaginous avulsion at the tibial or femoral insertion sites. The tensile strength of the graft was less than 10% of its original strength at 6 and 9 weeks, but gradually increased to approximately two thirds of its original strength at 52 weeks.

Histologic findings demonstrated that a partial fibrous interzone was present at 6 weeks with a mature, intratunnel, tendon-bone interface at 9-12 weeks. A normal ligamentous insertion was seen at 24 and 52 weeks. Using fluorescence microscopy, Weiler et al showed that direct healing to both the tunnel and the surface could occur without tunnel enlargement.

Comment by Mark D. Miller, MD

This is an interesting and well-designed study on biodegradable interference screws for soft tissue fixation. It would have been an even more valuable study if Weiler et al compared aperture and nonaperture fixation. It is important to point out that Weiler et al used poly-(D,L-lactide) screws. This is a different screw than is commonly used in the United States—the Poly-L-lactide screw—with absorption characteristically occurring at approximately 24 weeks vs. several years for the poly-L-lactide. The fact that the grafts were less than 10% of their original strengths at 6 and 9 weeks is concerning. Was this caused by the screws? Unfortunately, the study design precludes us from making this conclusion, but it is still a major consideration. It is not clear whether the screw itself or its insertion causes this weakness, but it did not result in graft rupture, at least in the animal model. Nevertheless, reduction of graft strength to less than 10% of its original strength within the first 2 months postoperatively certainly forces us to reconsider our rehabilitation protocols! Screw degradation occurred between 12 and 24 weeks and did not result in failure mode or loss of mechanical strength. We have little experience with this particular screw, but it appears to have an advantage over conventional PLLA screws.

The histologic findings demonstrating that there was direct healing both within the tunnel and at the tunnel surface was also interesting. This is likely a beneficial effect of interference screws. Fluorescence microscopy demonstrated bone formation much earlier than other studies have demonstrated. Weiler et al suggest that this is a result of anatomic interference fit fixation and propose that this is because of neutralization of graft motion. They also propose that this can reduce tunnel enlargement. Again, a comparative study design would help to substantiate this conclusion.

A final concern that I have regarding these studies is that the tibial interference screw was placed from inside out. Weiler et al state that cadaver pilot work and outside-in screw placement resulted in a weak fixation. It is unclear how much this modification affected their results. Nevertheless, this paper provides interesting information and more room for further research. Perhaps a future animal study comparing "anatomic" aperature fixation with extracortical fixation may provide more answers. 

Dr. Miller, Associate Professor, UVA Health System, Department of Orthopaedic Surgery, Charlottesville, VA, is Associate Editor of Sports Medicine Reports.