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Redesigning Metal Interference Screws Can Improve Ease of Insertion While Maintaining Fixation of Soft-Tissue
Kiron K Athwal1, Breck R Lord1, Piers E Milner1
1Biomechanics Group, Department of Mechanical Engineering, Imperial College London, London, England.
Purpose:
To compare the fixation strength and loads on insertion of a titanium alloy interference screw with a modified tip against a conventional titanium interference screw.
Methods:
Slippage of bovine digital extensor tendons (as substitutes for human tendon grafts) under cyclic loading and interference fixation strength under a pullout test were recorded in 10 cadaveric knees, with 2 tunnels drilled in each femur and tibia to provide pair-wise comparisons between the modified-tip screw (MS) and conventional screw (CS). To analyze screw insertion, 10 surgeons blindly inserted pairs of the MS and CS into bone-substitute blocks (with polyester shoelaces as graft substitutes), with insertion loads measured using a force/torque sensor.
Results:
No differences were found between the MS and CS either in graft slippage from the femur (P = .661) or tibia (P = .950) or in ultimate load to failure from the femur (P = .952) or tibia (P = .126). On insertion, the MS required less axial force application (78 ± 38 N, P = .001) and fewer attempted turns (2 ± 1, P < .001) to engage with the bone tunnel than the CS (99 ± 43 N and 4 ± 4, respectively). In 90% of the paired insertion tests, the screw identified by the surgeon as being easier to initially insert was the MS.
Conclusions:
The MS was found to be easier to engage with the bone tunnel and initially insert than the CS while still achieving similar immediate postsurgical fixation strength.
Clinical Relevance:
The study shows that screw designs can be improved to ease insertion into a bone tunnel, which should reduce any likelihood of ligament reconstruction graft damage.
