Summary

We assess the fixation strength of anterior cruciate ligament supplementary fixation comparing suture anchors to staples.

Abstract

Introduction

Suture anchors are increasingly used in supplementary tibia fixation for anterior cruciate ligament (ACL) reconstruction. They have the advantage of being low profile with less risk of soft tissue irritation and implant prominence compared to conventional staples. Our study aims to assess the fixation strength and cyclic stiffness of the two modalities.

Methods

Porcine grafts were implanted in a square box artificial bone block (Sawbones) using two fixation systems: Spiked Ligament Staples (11 x 20mm) and PEEK Swivelock Anchor (4.75 x 19.1mm) by Arthrex (Naples, FL). Eight assemblies were performed with four samples per fixation modality. The proximal end of the graft was sutured to itself to form a loop using high-resistance sutures FiberWire #2 (Arthrex). This was passed around the hook of the traction machine (MTS Bionix Model 370.02). The distal end was secured to the bone block which was held at the machine base with a clamp. Axial traction was applied on the graft using the following protocol: 10N to 50N over 250 cycles (at rate of 1Hz) after preconditioning, followed by pull to failure (at a rate of 20mm/minute). Study parameters comprised of pull-out strength (ultimate failure load), mean cyclic stiffness (N/mm) and pull-out stiffness (N/mm).

Results

The mean pull-out strength for the staple was 459.92 +/- 7.87N (range: 451.24 to 458.01N) and for the suture anchor, 154.93 +/- 26.19N (range: 130.34 to 179.25N). The staple had a significantly higher pull-out strength (p = 0.02).

The mean cyclic stiffness for the staple was 124.71 +/- 25.44N/mm (range 103.61 to 160.18N/mm) and for the suture anchor, 95.15 +/- 12.69N/mm (range 76.76 to 104.69N/mm). The staple had a significantly higher mean cyclic stiffness (p = 0.04).

The mean pull-out stiffness for the staple was 70.34 +/- 11.3N/mm (range 59.95 to 81.48N/mm) and for the suture anchor, 22.12 +/- 2.42N/mm (range 19.12 to 24.62N/mm). The staple had a significantly higher mean pull-out stiffness (p = 0.02).

Conclusion

In this in-vitro study, the staple showed better pull-out strength, mean cyclic stiffness and pull-out stiffness than the suture anchor. Granted the suture anchor is a supplementary fixation, but surgeons would be prudent to know that its low-profile advantage comes at the price of a reduced fixation strength and stiffness.