Doctoral Dissertation Improvement: A Histomorphometric Analysis of Muscular Insertion Regions: Understanding Enthesis Etiology

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

An enthesis is the interface where tendon meets bone, providing both muscle anchorage and stress dissipation. Previous anthropological research suggests size and complexity of entheses observable in skeletal material, are indicative of the strain magnitude resulting from repetitive muscle contractions during the performance of daily routines. However, enthesial expression has never explicitly been linked to activity intensity. This study investigates the relationship between enthesis location and mechanical loading (via muscle contraction) in non-weight bearing human radii, quantifying histological evidence of remodeled bone resulting from strain-induced fatigue damage. Methods employed in this study follow the principle that fatigue damage accumulated over time weakens bone, initiating cellular units to remove damaged matrix and replace it with new bone bundles referred to as osteons. Newly formed osteons are considered intact, while those partially removed over time following additional remodeling events are considered fragments. With this theoretical understanding, thin sections from three regions of the radial shaft associated with seven muscle bodies responsible for forearm movement are harvested from 30 human cadavers and prepared for histologic analysis. Osteon population densities (OPD), or number of intact and fragmentary osteons per unit area, reflect the visible remodeling history of bone compacta. Osteon area reflects strain level. Both variables are quantified in eight zones derived from the anterioposterior and mediolateral axes, and their intersecting planes; encompassing all potential regions of tendon insertion. Thus, zones of high strain are reflected by an elevation in mechanically-induced remodeling (OPD), and a presence of smaller osteons for increased fatigue resistance. Presence of considerable remodeling histories along axes associated with entheses will suggest stress concentrated in these regions potentially results from direct tendon pull, bolstering assertions that mechanical strain primarily influences enthesial development. Intellectually, this study potentially furthers our biomechanical understanding of entheses, while offering a preliminary investigation into the utility of enthesial measures in anthropologically-based behavioral reconstructions. In broader terms, understanding the unique intricacies and variances of human radial bone remodeling in response to strain, may help clarify understanding of skeletal responses to habitual activity in the locomotive-free human upper limb. Additionally, results will expand the theoretical core of mechanically-induced bone remodeling; which is paramount to paleoanthropologists, skeletal biologists, primatologists, and orthopaedic surgeons alike. This doctoral dissertation research project will contribute to the academic training of a graduate student.