Validating Internal Density Calibration in The Proximal Humerus to Estimate Bone Stiffness Using Finite Element Analysis for Stemless Shoulder Arthroplasty

Abstract

Stemless humeral head components are a popular choice for patients undergoing shoulder arthroplasty for end-stage osteoarthritis (OA). OA is known to alter bone density in the humeral head, which can compromise implant stability and increase the need for surgical revisions. Current pre-operative clinical assessments are limited in evaluating bone mineral density (BMD) and fail to consider the mechanical properties of bone in the region directly supporting the stemless component, leaving a critical gap in understanding the structural integrity of the bone supporting the stemless component. Traditionally, in-scan phantom calibration determines volumetric bone mineral density (vBMD) from greyscale intensity in computed tomography (CT) images, but this method is rarely used in clinical practice due to limited time and resources. As a result, alternative density measures for determining accurate vBMD from clinical CT images are needed. Internal density calibration using internal tissues as references has been validated in the spine and hip, however, it has yet to be validated in the proximal humerus. Additionally, vBMD derived from internal density calibrated images has yet to be linked to finite element model (FEM) apparent stiffness in the context of stemless shoulder arthroplasty. Establishing stiffness as a measure of bone mechanical properties is a first step in accurately predicting bone strength in clinical CT images.

The purpose of this thesis was to 1) determine the correlation between phantom and internal density calibration in the proximal humerus using three different tissue combinations, 2) compare vBMD in an end-stage OA patient group to a non-pathologic group, and 3) determine the correlation between vBMD and apparent stiffness. Non-pathologic cadaveric single-energy CT images containing a dipotassium phosphate (K2HPO4) phantom were used to analyze a 10 mm thick volume of interest (VOI) directly below the anatomic neck. Phantom and internal density calibration was performed on each phantom-containing cadaveric specimen. vBMD was extracted and FEMs were generated from the VOI. The internal calibration with the lowest bias was used to calibrate images for all end-stage OA patient specimens. VOIs were created for cortical, trabecular, and combined (integral) bone compartments across specimens and vBMD was extracted for each compartment. FEMs were generated using the integral VOI to estimate apparent stiffness.

Statistical analysis revealed a strong correlation between internal and phantom density calibration, establishing internal calibration as a valid metric for determining vBMD (AAdC R2 = 0.80; AAdCM R2 = 0.88; ACM R2 = 0.90). The ACM (Air, Cortical Bone, Skeletal Muscle) tissue combination had the lowest error (Mean: 13.08 mgK2HPO4/cm3). The end-stage OA patient group had significantly lower integral (Patient: 119 mg K₂HPO₄/cm³; Cadaver: 159 mg K₂HPO₄/cm³), cortical (Patient: 518 mg K₂HPO₄/cm³; Cadaver: 643 mg K₂HPO₄/cm³), and trabecular (Patient: 79.8 mg K₂HPO₄/cm³; Cadaver: 110 mg K₂HPO₄/cm³) vBMD than the non-pathologic cadaveric group (p<0.001), highlighting the biological relevance of vBMD. Mean apparent stiffness was found to be significantly lower in the end-stage OA group (672 MPa) relative to the non-pathologic cadaveric group (1261 MPa) (p < 0.001), however stiffness was not correlated with cortical vBMD in either group (Patient: R² = -0.018, p = 0.73; Cadaver: R² = -0.018, p = 0.71), suggesting the need for a multi-factorial approach when quantifying mechanical properties using FEMs.