Bone—patellar tendon—bone grafts for anterior cruciate ligament reconstruction: The effects of graft pretensioning

Abstract

Viscoelastic creep is a well-know phenomenon associated with collagenous soft tissues under sustained tensile load. Despite our understanding of this phenomenon and the potential for “loosening” of the graft over time, pretensioning of bone—patellar tendon—bone (B-PT-B) grafts for reconstruction of the anterior cruciate ligament (ACL) to eliminate this elastic deformation is not commonly practiced. This investigation quantified viscoelastic creep in B-PT-B grafts using both an in vivo and an in vitro model. In vivo, 10-mm B-PT-B grafts were procured and prepared in a standard manner for arthroscopically assisted ACL reconstruction. A total of 153 grafts were evaluated. During preparation, each graft was tensioned using a commercially available graft preparation board (Smith & Nephew DonJoy). An initial tensile load of 2.25 N (0.5 lb) was applied to the graft and a measurement was taken between bone-tendon junctions at either end. A sustained load of 89 N (20 lb) was then applied for a minimum of 4 minutes and the measurement repeated. In the in vitro model, grafts were harvested in a standard manner, then placed in a servohydraulic for tensile loading. A differential variable reluctance transducer was implanted in each graft to quantify net displacement during 15 minutes of sustained tensile loading at 89 N (20 lb). A total of 13 specimens were evaluated. In the in vivo model, mean pretension bone-tendon junction length was 43.6 mm (range, 29 to 64 mm; SD, ± 6.7). Mean post-tension bone-tendon length was 49.6 mm (range, 33 to 71 mm; SD, ± 7.1), representing a mean increase in length of 6.0 mm (range, 2 to 12; SD, ± 2.1) or 14.0% (range, 3.8 to 28.6; SD, ± 5.2). In the in vitro model, the mean pretension tendon length was 42.81 mm (range, 35.20 to 51.48; SD, 4.54). The mean posttension length was 47.11 mm (range, 38.05 to 56.23; SD, ± 5.04) representing a mean increase of 4.30 mm or 10.12%. These data would seem to support the hypothesis that without pretensioning, significant postimplantation graft creep will occur.