The biomechanical strength of a hardware-free femoral press-fit method for ACL bone-tendon-bone graft fixation.

Abstract

The purpose was to investigate graft slippage and ultimate load to failure of a femoral press-fit fixation technique for anterior cruciate ligament (ACL) reconstruction. Nine fresh-frozen knees were used. Standardized harvesting of the B-PT-B graft was performed. The femora were cemented into steel rods, and a tunnel was drilled outside-in into the native ACL footprint and expanded using a manual mill bit. The femoral bone block was fixed press-fit. To pull the free end of the graft, it was fixed to a mechanical testing machine using a deep-freezing technique. A motion capture system was used to assess three-dimensional micro-motion. After preconditioning of the graft, 1000 cycles of tensile loading were applied. Finally, an ultimate load to failure test was performed. Graft slippage in mm ultimate load to failure as well as type of failure was noted. In six of the nine measured specimens, a typical pattern of graft slippage was observed during cyclic loading. For technical reasons, the results of three knees had to be discarded. 78.6% of total graft slippage occurred in the first 100 cycles. Once the block had settled, graft slippage converged to zero, highlighting the importance of initial preconditioning of the graft in the clinical setting. Graft slippage after 1000 cycles varied around 3.4±3.2mm (R=1.3-9.8mm) between the specimens. Ultimate loading (n=9) revealed two characteristic patterns of failure. In four knees, the tendon ruptured, while in five knees the bone block was pulled out of the femoral tunnel. The median ultimate load to failure was 852N (R=448-1349N). The implant-free femoral press-fit fixation provided adequate primary stability with ultimate load to failure pull forces at least equal to published results for interference screws; hence, its clinical application is shown to be safe.