Subject: The dislocation of the graft and fixation material within the femoral tunnel is a concern in ACL-reconstruction but is not directly accessible to biomechanical investigation. The current study was carried out to address particularly the intra-tunnel-movement of the graft under repetitive load.

Material and methods: Three graft/fixation combinations were biomechanically investigated: a human BPTB-Graft fixed with a 7x23mm interference screw and a double tendon loop (porcine foot extensor) fixed with either a TransFix post or a continuous loop Endobutton. The grafts and the fixation material were fitted with multiple tantalum markers (0.5 mm). Then the grafts were anchored in human femora according to clinical standards. A servo-hydraulic materials testing machine was used to repetitively load the test specimen with a force between 50 and 250 N (max. 1000 cycles). Each fixation type was tested seven times. After definite intervals the position of the markers was recorded using fluoroscopy. The dislocation of the grafts was determined from the recorded images using appropriate software. Standard geometry could be employed due to the uni-dimensional nature of the intra-tunnel movement.

Results: Premature failure occurred neither with BTB-grafts nor with TransFix anchored tendon loops but was observed with the Endobutton fixation, where the Endobutton was pulled through the lateral cortex in two cases. The dislocation of the grafts after 200 and 500 cycles was significantly higher with the Endobutton fixation (1.3±0.9 and 1.9±1.2 mm resp.). In the TransFix group the dislocation was 0.14±0.10 and 0.40±0.27 mm, which was not statistically different from the BTB group with 0.13±0.13 mm and 0.24±0.16 mm respectively.

Conclusions: The stability of TransFix anchored tendon loops under repetitive submaximal loads reaches that of BPTB grafts fixed with interference screws. In the model employed here the extra-cortical fixation showed less resistance against dislocation.

Clinical relevance: The current results can aid the surgeon in the choice of the graft/fixation combination. Factors other than the biomechanical stability have to be considered, particularly the ingrowth behavior of different grafts and fixation types.