Initial Assessments of a Handheld Indentation Probe's Correlation With Cancellous Bone Density, Stiffness, and Strength: An Objective Alternative to “Thumb Testing”

Abstract

During shoulder arthroplasty, surgeons must select the optimal implant for each patient. The metaphyseal bone properties affect this decision; however, the typical resection 'thumb test' lacks objectivity. The purposes of this investigation were: to determine the correlation strength between the indentation depth of a handheld mechanism and the density, compressive strength and modulus of a bone surrogate; as well as to assess how changing the indenter tip shape and impact energy may affect the correlation strengths. A spring-loaded indenter was developed. Four tip shapes (needle, tapered, flat and radiused cylinders) and four spring energies (0.13J-0.76J) were assessed by indenting five cellular foam bone surrogates of varying density. The indentation depth was measured and correlated with apparent density, compressive strength and modulus. Indentation depth plateaued as the bone surrogate's material properties increased, particularly for indentation tips with larger footprints and the 0.13J spring. All tip shapes produced strong (R²≥0.7) power-law relationships between the indentation depth metric and the bone surrogate's material properties (density: 0.70 ≤ R² ≤ 0.95, strength: 0.75 ≤ R² ≤ 0.97, modulus: 0.70 ≤ R² ≤ 0.93); though use of the needle tip yielded the widest indentation depth scale. These strong correlations suggest that a handheld indenter may provide objective intraoperative evidence of cancellous material properties. Further investigations are warranted to study indenter tip shape and spring energy in human tissue; though the needle tip with spring energy between 0.30J and 0.76J seems the most promising.