An anterior cruciate ligament (ACL) tear is a severe knee injury, creating instant joint instability. The current state-of-the-art solution is surgical replacement of the torn ACL by an autograft formed by folding a hamstring tendon in 4 parallel strands. However, 10-30% of patients experience a retear and 14-92% develop knee instability caused by inferior mechanical properties. The underlying cause for this inadequate mechanical performance is poorly understood; however, differences in mechanical properties between the 4 graft strands may contribute. In particular, the myotendinous junction (MTJ) region, the part of the tendon that connects to muscle, is a potential graft weakness partially due to the coarse treatment during graft processing for removal of attached muscle tissue.
This leads to the hypothesis that the MTJ region is mechanically inferior to the other 3 strands.
Experimental work was conducted in order to comparatively characterize the mechanical behaviour of human hamstring tendons left over from ACL reconstructive surgeries. Particle tracking analysis during tensile loading and subsequent strain tracking was performed to investigate regional strain distributions in the tendons. Dynamic tensile straining was performed to investigate changes in resistance to tensile strain and viscoelastic timedependent tendon behaviour.
Results indicate comparable mechanical behaviour of all tendon regions suggesting that they contribute to the graft equally, refuting the hypothesis of the MTJ’s inferiority, and supporting the continued use of the complete tissue. The results also elucidate an increase in tissue slackness after graft straining to levels analogous to those expected in the ACL. This implies that handling and straining substantially affect the graft’s condition and slackness at the point it is placed in the knee, and that initial graft placement may considerably influence long-term stability.
In conclusion, mechanical analysis of hamstring tendons did not clearly indicate a “weak” region that can explain in vivo deterioration of graft mechanical properties.