Once the anterior cruciate ligament has been diagnosed as being torn, surgery is often required to return the individual to an active lifestyle. Bone-patellar tendon-bone grafts have the highest initial graft strength relative to the normal ACL (Irrgang, 1997). Weaker graft materials include the semitendinosis, gracilis and Achilles tendon. Surgical procedures that use doubled, tripled, or quadrupled strands of semitendinosis and gracilis have been developed to overcome this limitation (Irrgang, 1997). The use of strong graft materials with solid fixation, such as that provided by an interference screw, has allowed for more aggressive rehabilitation in the immediate postoperative period (Irrgang, 1997).

The graft is strongest at the time of reconstruction. Over time the graft undergoes necrosis and remodeling. Initially, the graft is avascular. By six weeks the graft is enveloped in a synovial sheath. However, the graft shows signs of avascular necrosis at this time. Revascularization begins at approximately 8 to 10 weeks postoperatively and is nearly complete by 16 weeks. One year after reconstruction, the graft takes on the appearance of a ligament with densely organized collagen bundles. Graft strength decreases during the time of necrosis and then increases as the graft remodels and matures, but it does not reach the strength of the normal ACL. Biomechanical studies have revealed that the strength of the graft is 30 to 60% of the normal ACL at 6 to 12 months after surgery (Drez et al, 1991).

Graft elongation behavior at the time of reconstruction is a critical factor that influences the long-term success of anterior cruciate ligament reconstruction (Tohyama, 1996).

Autografts

The use of autograft bone-patellar tendon-bone has been associated with a higher incidence of loss of motion and development of patellofemoral pain. Rehabilitation following ACL reconstruction with this material should ensure restoration of full active and passive extension. This may require judicious use of patellar mobilization, active and passive stretching, biofeedback, and/or electrical stimulation.

In terms of force magnitude, internal torque applied to an extended knee is likely to be the most dangerous loading state for an athlete who has a patellar tendon graft (Markolf, 1996).

With appropriate fixation, both patellar tendon and gracilis-semitendinosus grafts approximated the intact anterior cruciate ligament in strength, but only patellar tendon grafts secured with interference screws were comparable in stiffness (Steiner et al, 1994).

In a study conducted by Rowden et al (1997), an autologous patellar tendon graft with proximal and distal interference screws technique was compared to the use of quadruple-stranded semitendinosus tendons fixed proximally by a titanium button and braided tape and distally by tibial post screw. The results showed that the semitendinosus reconstruction is approximately 50% stronger at the time of surgery and is comparable to the patellar tendon in stiffness.

Allografts

Evidence suggests that at 6 months post-operatively, allografts, compared to autografts, demonstrate a greater decrease in structural properties from the time of implantation, a slower rate of biological incorporation, and prolonged presence of an inflammatory response (Jackson, 1993).

A study conducted by Shino et al (1995) revealed that anterior cruciate ligament allografts had collagen fibril profiles that did not resemble autografts or normal anterior cruciate ligaments, even several years after surgery.