Strength of ACL reconstructions using semitendinosus tendon grafts.

PURPOSE: Patellar tendon autografts have been considered by many as the gold standard for intraarticular reconstruction of the anterior cruciate ligament (ACL). Hamstring tendon grafts are being used increasingly, however there are few studies focusing on their mechanical properties. The objectives of the present study are to determine the strength of a semitendinosus graft at various postoperative periods. METHODS: In Part I of the study, a looped semitendinosus graft was used to reconstruct the ACL in five frozen human cadaver knees with a simulated endoscopic technique. The immediate post-operative strength was determined by loading the knees of failure using a mechanical testing system. In Part II the effects of graft maturation and incorporation were considered. A similar graft was used to reconstruct the ACL in twelve ewe sheep. Their knees were harvested and tested at four, eight, and twelve weeks postoperatively. RESULTS: The mean maximum strength and stiffness of the cadaveric reconstructions were 352 N and 8.18 N/mm, respectively. All failures occurred at the fixation sites. The average strength of the sheep reconstructions was similar at four, eight, and twelve weeks (376 N. 415 N. and 323 N. respectively). The stiffness increased from 21.1 N/mm at four weeks to 46.7 N/mm at eight weeks and then remained the same. The failures occurred primarily by tendon pull-out of the femoral tunnel at four, eight and twelve weeks. CONCLUSIONS: Graft pull-out from the femoral tunnel implies incomplete tendon incorporation during the initial three months post-operatively. Activities which place high loads on the graft should be avoided during this period. Additionally, the mechanical properties of this hamstring graft reconstruction are similar to those reported for patellar tendon grafts in other animal models.

Biomechanical effects of gamma irradiation on fresh frozen allografts in vivo.

The effect of gamma irradiation on the mechanical properties of allograft in a rabbit model was examined. Tibial allografts were tested in torsion to failure to determine maximum torque and maximum stress. Statistically significant degradation in the torsional strength of irradiated tibias vs controls was demonstrated. Femoral allografts were orthotopically implanted into experimental rabbits and secured with an intramedullary pin. Rates of incorporation and failure patterns were significantly altered by irradiation. These data suggest that sterilization of fresh-frozen allografts with 2.5 Mrad or greater severely degrades the ability of the graft to resist torsion. The rate of incorporation of fresh-frozen allografts also may be dramatically decreased as a result of irradiation.

An analysis of autograft fixation after anterior cruciate ligament reconstruction in a rabbit model.

Fixation and incorporation of a tendon implanted within the bone pose theoretical as well as practical concerns for the surgeon who treats instability problems of the knee. Understanding the physiology involved in graft-bone incorporation is necessary for the appropriate rehabilitation of patients who undergo anterior cruciate ligament surgery. The purpose of the study was to examine the histologic and biomechanical changes of a semitendinosus autograft reconstruction of the anterior cruciate ligament in a rabbit model at the graft-bone tunnel interface in the femur. The results indicate that by 3 weeks, failure of the bone-graft-bone or construction is through the intraarticular portion of the graft, not as a result of pullout from the bone tunnel. Graft fixation of the bone tunnel occurs by an intertwining of graft and connective tissue and anchoring of connective tissue to bone by collagenous fibers and bone formation in the tunnels. The collagenous fibers have the appearance of the Sharpey's fibers seen in an indirect tendon insertion.