A new noninvasive mechanical bending test accurately predicts ulna bending strength in cadaveric human arms

Abstract

Background: High error rates in the prediction of fragility fractures by bone mineral density have motivated searches for better clinical indicators of bone strength, and the high incidence of non-hip, non-spine fractures has raised interest in cortical bone. The aim of this study was to assess the accuracy of Cortical Bone Mechanics Technology™. CBMT is a new non-invasive 3-point bending technique for measuring the mechanical properties of cortical bone in the ulnas of living humans.

Methods: 35 cadaveric human arms were obtained from small women and large men ranging widely in age (17 < Age < 99 years) and body size (14 < BMI < 40 kg/m²). Noninvasive CBMT measurements of the flexural rigidity of the ulna bones within these arms (EI_CBMT) were compared to measurements of EI by Quasistatic Mechanical Testing in the ulnas excised from those arms (EI_QMT). Ulna bending strength was also measured by QMT as the peak moment before fracture (M_peak). The open source BoneJ plugin to ImageJ image processing software was used to calculate cortical porosity (CP) in micro-computed tomography images of a 2 mm length of the mid-shaft of each fractured ulna, and the interosseous diameter (IOD) of each ulna was also measured in those images.

Results: EI_CBMT measurements (13 < EI_CBMT < 97 Nm²) explained 99% of the variance in QMT measurements of ulna bending strength (11 < M_peak < 90 Nm), but EI_CBMT was biased high by 30% (p < 0.0001) relative to EI_QMT (11 < EI_QMT < 69 Nm²). After correcting this bias, EI_CBMT and EI_QMT measurements lay along the identity line (y = 1.00x, R² = 0.99, SEE = 3.1 Nm²). Predictions of M_peak by EI_CBMT were less accurate than predictions by EI_QMT (both R² = 0.99; SEE_CBMT = 5.9 Nm vs SEE_QMT = 4.5 Nm, F = 2.92, p = 0.001), but EI_CBMT predictions were substantially more accurate than those by IOD (R² = 0.79; SEE_IOD = 10.6 Nm, F = 3.30, p < 0.001) and CP (R² = 0.35; SEE_CP = 18.9 Nm, F = 10.45, p < 10^-9). Predictions by EI_CBMT were also more accurate than predictions by arm donor height (R² = 0.63; SEE = 14.3 Nm, F = 5.87, p < 10^-6), body weight (R² = 0.77; SEE = 11.1 Nm, F = 3.54, p < 0.001) and BMI (R² = 0.64; SEE = 14.1 Nm, F = 2.39, p < 0.01). In forward stepwise multiple regression beginning with EI_CBMT, only age explained any additional variance in ulna bending strength (ΔR² = 0.3%, F = 8.03, p = 0.008).

Conclusion: Noninvasive CBMT measurements of ulna EI explain 99% of individual differences in QMT measurements of ulna bending strength in cadaveric human arms.

Keywords: Bone fragility; Bone mechanics; Bone strength; Cortical bone; Cortical porosity; Ulna.