Biomechanical Properties of Nitinol Staples: Effects of Troughing, Effective Leg Length, and 2-Staple Constructs

Abstract

Royalty: Zimmer Biomet (Gaston), Arthrex (Lee) Receipt of Intellectual Property Rights: Arthrex (Lee) Consulting Fee: BME, Endo Pharmaceuticals, Smith and Nephew (Gaston), Zimmer Biomet (Gaston and Lee), AxoGen, Synthes (Lee) Speaker’s Bureau: Endo Pharmaceuticals (Gaston), AxoGen, Synthes (Lee) Contracted Research: Toyoba (Lee) Other: JHS: Editorial Board (Gaston), ASSH Committee Member (Gaston and Lee), BME (McKnight) Nitinol memory compression staples are a recent addition to carpal fixation. Compared with traditional staples, they have been shown to have superior compression at the far cortex relative to standard and traditional compression staples.1 Their ability to apply constant compression between the staple legs has been attributed to their increased union rates compared with Kirschner wires and traditional compression staples.2–4 Troughing is defined as using a rongeur to place a small recess in the bone to allow for countersinking of the staple. The purpose of this study is to (1) determine the effective leg length of different nitinol staples, (2) confirm the effect of 1 versus 2 staples on biomechanical compression, and (3) determine the effect of troughing the bone on staple biomechanical properties. We hypothesized that there would be measurable compression distal to the tip of the staple leg, 2 staples would apply greater compression, and troughing the bone would not significantly change the construct's biomechanical properties. Three commonly used nitinol staples of various bridge and leg lengths were used in a bicortical sawbone block construct with density of 15 lb/ft3 (PCF) (0.24 g/cm2). There were 3 separate constructs tested which included single staple, double staple, and troughed. A Tekscan force sensor was placed between the 2 blocks once the staple was inserted. Compression force, stiffness, and bending strength was measured for each construct before and after 4-point bending cyclical loading for 100 cycles. Compression mapping was used to determine the effective leg length of each staple, calculated as the sum of the staple leg length and the distance that compression extended beyond the tip of the leg. Effective leg length for each staple construct extended 2 mm distal to the longest staple leg length (Figure 41-1). Two staple constructs more than doubled compressive force and increased bending strength by greater than 90% in all staple types. There was no loss of compressive force before or after loading for single, double, or troughed constructs with any staple type (Figure 41-2). •This study supports that nitinol staples do not have to be placed bicortically to achieve adequate compression; placing staples 2 mm short of the far cortex has the same compression as bicortical; and troughing of the bone will not significantly diminish the biomechanical properties of the construct.•This study supports troughing the bone to minimize implant prominence.Figure 41-2Compressive force with and without troughing the bone.View Large Image Figure ViewerDownload Hi-res image Download (PPT)