Training drives turnover rates in racehorse proximal sesamoid bones

Abstract

Focal bone lesions are often found prior to clinically relevant stress-fractures. Lesions are characterized by low bone volume fraction, low mineral density, and high levels of microdamage and are hypothesized to develop when bone tissue cannot sufficiently respond to damaging loading. It is difficult to determine how exercise drives the formation of these lesions because bone responds to mechanical loading and repairs damage. In this study, we derive steady-state rate constants for a compartment model of bone turnover using morphometric data from fractured and non-fractured racehorse proximal sesamoid bones (PSBs) and relate rate constants to racing-speed exercise data. Fractured PSBs had a subchondral focus of bone turnover and microdamage typical of lesions that develop prior to fracture. We determined steady-state model rate constants at the lesion site and an internal region without microdamage using bone volume fraction, tissue mineral density, and microdamage area fraction measurements. The derived undamaged bone resorption rate, damage formation rate, and osteoid formation rate had significant robust regression relationships to exercise intensity (rate) variables, layup (time out of exercise), and exercise 2-10 months before death. However, the direction of these relationships varied between the damaged (lesion) and non-damaged regions, reflecting that the biological response to damaging-loading differs from the response to non-damaging loading.

Figure 1

Compartment model of the bone “tissue turnover cycle”. There are four tissue volumes: damaged mineralized bone, undamaged mineralized bone, marrow, and osteoid. Each tissue type is a portion of the total tissue volume (TV) and is defined using histological features. The summation of the four volume compartments is constant, as this compartment model is closed. Tissue volumes can transform following the paths indicated by the arrows at the specified rates (k_i). Tissue mineral density (TMD) is defined for the undamaged and damaged mineralized bone volumes.

Figure 2

Flowchart of study methods. Study participants: Data were collected from 30 racehorse proximal sesamoid bones (PSBs). Specifically, data were measured within ten fractured medial PSBs (FX-PSB) from Case racehorses euthanized due to unilateral biaxial PSB fracture, ten contralateral limb intact medial PSBs (CLI-PSBs) from the same Case racehorses, and ten medial PSBs from Control racehorses (CTRL-PSBs) euthanized for reasons unrelated to PSB fracture. All horses were in race-training at the time of death. Sample analysis: The whole PSB was used to determine the peak tissue mineral density of the sample. Data was collected in two sagittal plane regions of interest (ROIs): A Non-Damaged ROI (blue) and Damaged ROI (red). These regions are described in detail in Shaffer et al., 2022. Data analysis: Steady-state rate constants were first determined in the Non-Damaged ROI (1). Second, steady-state rate constants were determined within the Damaged ROI (2), using the rate constants for the Non-Damaged ROI (1) as initial conditions. Finally, the rate constants were compared among groups (3). CLI-PSB and Control PSB rate constants were related to exercise variables (4). Note that FX rate constants were excluded from this final analysis (4) because Case FX and Case CLI PSBs are from the same horses and, therefore, have the same exercise history.

Figure 3

Non-damaged ROI (open symbols) and damaged ROI (filled symbols) solutions. (A) Undamaged resorption rate (k₁) shown against measured bone volume fraction, (B) k₁ shown against measured tissue mineral density, (C) osteoid formation rate (k₂) shown against measured bone volume fraction, (D) k₂ shown against measured tissue mineral density. In all panels, data from FX PSB (triangle) CLI PSBs (square) and CTRL PSBs (circles) are shown.

Figure 4

Damaged ROI solutions for the damage formation rate (k₄) and damage resorption rate (k₅) shown for the FX PSBs (triangle), CLI PSBs (square) and CTRL PSBs (circle). (A) k₄ plotted against the stained microcrack area fraction (Cr.Af); note, in the model, BV_D/TV ≡ Cr.Af. (B) k₄ plotted against k₅. In both panels, solutions for the non-damaged ROI are not shown, as Cr.Af, k_4, and k₅ were zero in the non-damaged ROI.

Figure 5

Significant robust linear regressions between k1, k2, k4 and exercise history variables in the damaged ROI (A,B) and non-damaged ROI (C,D); the 95% confidence intervals (dashed lines) are shown. All regressions were made using CTRL PSB (circle) and CLI PSB (square) data. The direction of increasing exercise intensity is shown by the arrow along the exercise axis. Data from FX PSB (triangle) PSBs were not used to construct the robust regressions; however, when CTRL & FX data were used to construct the regressions, the directions of the relationships shown stayed the same.