gms | German Medical Science

Objective: Previous experimental studies have documented a loss of graft tension with dynamic motion of an ACL reconstructed knee joint, correlating well with an increase in postoperative A-P knee laxity. When considering the excessive and unphysiologic changes in substitute loading angle at the femoral tunnel entrance with knee motion, it is hypothesized that both graft source and graft fixation technique determine the amount of loss of tension during dynamic flexion-extension loading.

Methods: In an experimental laboratory study, the course of graft tension during dynamic flexion-extension loading was investigated in 48 porcine press-fit fixated patellar tendon bone and 48 porcine interference screw fixated superficial digital flexor tendon (soft tissue) grafts. Specimens were mounted to a dynamic loading rig and were submitted to a cyclic loading protocol consisting of three loading ranges of 1000 cycles (Group I: 0° to 45°, Group II: 45° to 90°, Group III: 0° to 90°) with an initial tension of 13.3 % and 10.7 % strain applied to patellar tendon and soft tissue grafts, respectively. The force-time history was continuously derived and evaluated with respect to graft tension and force amplitude. After cyclic loading, grafts were loaded to failure and compared to a reference group, which was loaded to failure without cyclic loading. Yield loads, force amplitudes and the course of graft tension were compared using a one-way analysis of variance. A Pearson’s correlation analysis was conducted to evaluate whether the force amplitude and the initial graft tension determined the amount of loss of tension. Significance was set at p<0.05.

Results: Dynamic flexion-extension loading induced force amplitudes, the course of which determined the amount of loss of graft tension. Press-fit fixated patellar tendon bone grafts showed a significantly lower course of force amplitude within the first ten cycles when compared to interference screw fixated soft tissue grafts. The mean loss of graft tension was significantly higher in soft tissue grafts when compared to patellar tendon grafts in all groups, averaging 69.5 to 84.1 % and 25 to 47.9 % in soft tissue and patellar tendon grafts, respectively. Post dynamic single cycle load to failure testing revealed mean yield loads not to significantly differ from the reference testing, averaging 211.3 to 305.6 N and 302 to 305.3 N in soft tissue and patellar tendon grafts, respectively.

Conclusion: The loss of graft tension is determined by the eccentric point of force application with respect to the center of the femoral tunnel, resulting in force amplitudes during graft loading. In press-fit fixated patellar tendon bone grafts, the oblique geometry of the ligamentous insertion to the patellar bone plug results in a non-homogenous stress distribution within the substitute during unaxial loading. In soft tissue grafts, interference screw insertion results in a deviation of the center of the ligament fibers from the center of the bone tunnel, generating unisometrical loading patterns with knee motion. As the magnitude of the resultant force amplitude was found to determine the amount of loss of graft tension, the fixation-specific geometry should be considered with femoral bone tunnel placement in ACL reconstruction.