Combined nanoindentation testing and scanning electron microscopy of bone and articular calcified cartilage in an equine fracture predilection site.

The research investigates whether abnormalities in the articular calcified cartilage (ACC) and subchondral bone (SCB) found in the third metacarpal bone of young Thoroughbred horses could contribute to the prevalent occurrence of condylar fractures at older ages. The study employs combined nanoindentation testing and scanning electron microscopy techniques.

Background of the Research

• The study is motivated by the high incidence of condylar fractures in the third metacarpal bone (Mc3) among Thoroughbred racehorses – the leading factor of racetrack fatalities for the species.
• Previous observations have linked linear defects in the hyaline articular cartilage, the articular calcified cartilage (ACC), and the subchondral bone (SCB) to the initiation site of these fractures, which typically occur in the sagittal grooves of the Mc3 condyle.
• The researchers noted the presence of thickened and abnormally mineralised ACC in the sagittal grooves of several 18-month-old horses at the exact site where older Thoroughbred horses typically develop linear defects and condylar fractures.

Methods

• To investigate if these tissue abnormalities affected mechanical properties, the research team prepared bone slices embedded in Polymethyl methacrylate (PMMA), a type of inexpensive acrylic glass.
• These bone slices were studied using combined quantitative backscattered electron scanning electron microscopy (qBSE) and nanoindentation testing methods, allowing for the correlation of mineralisation density and tissue stiffness at the micron scale.
• The team conducted these studies on both normal and affected grooves, as well as normal condylar regions.

Findings

• The results demonstrated that the ACC was more highly mineralised but less stiff in early linear defects than in control regions.
• The subchondral bone (SCB) was found to be more highly mineralised and stiffer in specimens with early linear defects than those without.
• Through these findings, the research suggests that both ACC and SCB mineralisation may be abnormal in a class of early linear defect in 18-month-old Thoroughbred horses.
• These abnormalities could potentially be contributing factors to the later fracture of the Mc3 condyle, leading to the condylar fractures that are a common cause of racetrack fatalities.