Exercise-induced inhibition of remodelling is focally offset with fatigue fracture in racehorses

Bone remodelling is inhibited by high repetitive loading. However, in subchondral bone of racehorses in training, eroded surface doubled in association with fatigue fracture and there was greater surrounding trabecular bone volume suggesting trabecular modelling unloads the bone focally, allowing damage repair by remodelling.

INTRODUCTION

Remodelling replaces damaged bone with new bone but is suppressed during high magnitude repetitive loading when damage is most likely. However, in cortical bone of racehorses, at sites of fatigue fracture, focal porosity, consistent with remodelling, is observed in proportion to the extent of surrounding callus. Focal areas of porosity are also observed at sites of fatigue damage in subchondral bone. We hypothesised that fatigued subchondral bone, like damaged cortical bone, is remodelled focally in proportion to the modelling of surrounding trabecular bone.

METHODS

Eroded and mineralizing surfaces and bone area were measured using backscattered scanning electron microscopy of post-mortem specimens of the distal third metacarpal bone in 11 racehorses with condylar fractures (cases) and eight racehorses in training without fractures (controls).

RESULTS

Cases had a two-fold greater eroded surface per unit area at the fracture site than controls (0.81 ± 0.10 vs. 0.40 ± 0.12 mm(-1), P = 0.021) but not at an adjacent site (0.22 ± 0.09 vs. 0.30 ± 0.11 mm(-1), P = 0.59). Area fraction of surrounding trabecular bone was higher in cases than controls (81 ± 2 vs. 72 ± 2 %, P = 0.0020) and the eroded surface at the fracture site correlated with the surrounding trabecular area (adjusted R (2) = 0.63, P = 0.0010).

CONCLUSION

In conclusion, exercise-induced inhibition of remodelling is offset at sites of fatigue fracture. Modelling of trabecular bone may contribute to unloading these regions, allowing repair by remodelling.

Osteonal lamellae elementary units: lamellar microstructure, curvature and mechanical properties

The mechanical and structural properties of the sublayers of osteonal lamellae were studied. Young's modulus (E) of adjacent individual lamellae was measured by nanoindentation of parallel slices every 1-3 μm, in planes parallel and perpendicular to the osteon axis (OA). In planes parallel to the OA, the modulus of a lamella could vary significantly between sequential slices. Significant modulus variations were also sometimes found on opposing sides of the osteonal canal for the same lamella. These results are rationalized by considerations involving the microstructural organization of the collagen fibrils in the lamellae. Scanning electron microscope imaging of freeze fractured surfaces revealed that the substructure of a single lamella can vary significantly on the opposing sides of the osteonal axis. Using a serial surface view method, parallel planes were exposed every 8-10 nm using a dual-beam microscope. Analysis of the orientations of fibrils revealed that the structure is rotated plywood like, consisting of unidirectional sublayers of fibrils of several orientations, with occasional randomly oriented sublayers. The dependence of the measured mechanical properties of the lamellae on the indentation location may be explained by the observed structure, as well as by the curvature of the osteonal lamellae through simple geometrical-structural considerations. Mechanical advantages arising from the curved laminate structure are discussed.