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Pullout strength higher for tapered vs. nontapered ACL screws
The next step is validating performance of the tapered interference screws in humans.
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CARDIFF, Wales — Tapered stainless steel screws used for anterior cruciate ligament reconstruction had higher pullout strengths than nearly identical screws with nontapered shafts, according to the results of a randomized animal study.
--- Researchers recorded the insertion torque of tapered and nontapered screws using an instrumented torque screwdriver.
COURTESY OF CHARLES J. MANN
These findings suggest that reconstructions fail primarily by the type of interference screws used rather than the graft type applied.
Previous research in this area looked at screw length and diameter as factors affecting the outcome of anterior cruciate ligament (ACL) reconstructions. “The mechanism of failure of an interference screw is usually the tendon pulling past the screw rather than failure of the graft itself,” said Charles J. Mann, FRCS (Orth), of Wakefield Orthopaedic Centre and Repatriation General Hospital in Adelaide, South Australia.
During the British Orthopaedic Association Annual Congress, here, Mann said investigators studied the performance of two styles of stainless steel screws — a tapered and a parallel or untapered one — in identical graft constructs.
Researchers manufactured special 30-mm long screws for the study. The tapered ones had a 2-mm pitch, a 1-mm thread depth and a 5° combined taper angle. Nontapered screws had identical pitch and thread depth, but a small lead-in.
7-mm grafts studied
For the grafts, superficial flexor tendons were harvested from the hind limbs of sheep. They were whipstitched and frozen. Prior to testing, investigators paired the tendons and then quadrupled them to form 7-mm diameter grafts, which were measured using a commercial sizer.
Pairs of cadaveric 18-month-old cow knees were used to test the screws’ performance. Investigators stripped the knees of soft tissue, leaving the proximal tibias and distal femurs. They then placed the proximal tibias in a vice, inserted a guide wire and drilled a 7-mm tunnel. For those knees assigned to receive the nontapered screws, surgeons dilated the tunnel to 9 mm. Tunnels were dilated to 8 mm in knees receiving tapered screws. All the tapered tunnels were subsequently punched, creating a clearance of 2 mm between the tunnel sides and the size of the screw used.
Researchers performed bone scans on five pairs of cow knees to ensure that there was no difference between the right and left knee specimens. They also evaluated laxity. “Results showed that there was no significant difference between right and left knees,” Mann said.
Random screw selection
Screws used were randomly selected; however, if a tapered screw was placed in the right knee of a specimen, an untapered screw was used in the left, and vice versa. Grafts were pretensioned with a handheld tensioner attached to the distal end of the graft. This enabled graft tensioning during screw insertion and helped position the tendons at 90° to each other, which prevented them from bunching during insertion. Grafts were failed to destruction on an Instron machine. “Results of the pull-past tests showed significantly higher pull-past strengths for the tapered rather than the nontapered screw,” he said.
For your information:
- Mann CJ, Costi JJ, Stanley RM, et al. The effect of tapered vs. parallel interference screws on pullout strength of ACL reconstruction. Presented at the British Orthopaedic Association Annual Congress. Sept. 18-20, 2002. Cardiff, Wales.