COMPRESSION STAPLES – COMPRESSION OR DISTRACTION?

Abstract

Introduction: Compression staples are indicated for use in forefoot osteotomies and midfoot and forefoot fusions. The staple design can be divided into “Mechanical Compression” or “Shape Memory”. Although they are becoming increasingly popular because of their ease of use, there is little data published on the effectiveness of the true compression achieved across bony surfaces. There is no data on the optimal limb-length to staple width ratio required for compression. We aimed to compare four commercially available types of compression staple and measure the compression force achieved.

Methods and Materials: Compression in porcine tibia, cancellous bone substitute and perspex was compared using a load cell mounted within a simulated fusion site between two test blocks. The amplified output was continuously recorded using a datalogger and the data analysed. Two designs of “mechanical” compression staple and two designs of “shape memory” staple were tested. The effect of altering limb length on compression was also noted.

Results: The “mechanical” compression staples splayed open with the limbs of both designs causing either no compression or even distraction at simulated fusion site. Distractive forces of up to 23N were recorded. By contrast, the “shape memory” staples all achieved compression at the fusion site of between 5–25N. Limb length did not appear to alter the compression force achieved.

Discussion: “Mechanical” compression staples act in a similar way to basic AO principles of a 2-hole compression plate used without a lag screw technique or pre-bending. Although there is compression of the cis-cortex, the limbs of the staple splay open with a fulcrum around the bridge-limb intersection resulting in distraction of the trans-cortex. “Shape memory” staples compress both the cis- and trans-cortices along the length of the limb leading to adequate stability and compression forces across the fusion site.

Conclusion: This study demonstrates that “mechanical” compression staples cause a distractive force rather than a compressive force and manufacturers should be aware that further design modifications are required to prevent this. We recommend that “shape memory” staples, standard staples or lag screws are used instead. The limb-length to staple width ratio does not appear to be important.