32 Maturation of proximal sesamoid bones in Thoroughbred horses

Abstract

Proximal sesamoid bone (PSB) injuries account for approximately 50% of fatal catastrophic musculoskeletal injuries in Thoroughbred racehorses. In addition, PSB fractures are not uncommon in Thoroughbred foals and yearlings as a result of energetic or intense free exercise. The ability to interpret some PSB pathology assessments, however, is more difficult given the limited information published on the normal development and maturation of these paired sesamoid bones. Further, information on the level of normal variation in PSB morphological and structural parameters within the Thoroughbred population as a function of age, body size, gender, and inter-animal differences generally are not well documented. This project aims to address these knowledge gaps through analyses of PSB anatomy from 18 Thoroughbreds obtained on postmortem examination representing an age range from birth to skeletal maturity using micro–CT Bone Morphometry Evaluation (μCT BME, n = 8 horses) and clinical CT evaluation (n = 10 horses). The μCT BME was performed on the left front medial PSB in 8 horses ranging from 4 d to 5 years of age. When graphed, the following were observed: 2nd order polynomic relationships with a positive trend between age and bone volume fraction (R 2 = 0.8125), trabecular thickness (R 2 = 0.8084), and degree of anisotropy (R 2 = 0.914); a 2nd order polynomic relationship with a negative trend between age and trabecular number (R 2 = 0.9242); a negative logarithmic relationship between age and connectivity density (R 2 = 0.9439); and a positive exponential relationship between age and trabecular spacing (R 2 = 0.7818). Clinical CT evaluations were performed on medial and lateral PSBs from the respective right and left forelimbs in 10 horses ranging in age from 2 to 5 years of age utilizing Mindways CT Bone Investigational Toolkit for bone mineral density (BMD) analysis. A one-way ANOVA indicates no significant differences ( P = 0.755) in BMD between the 20 PSB in the left forelimb and the contralateral 20 PSB in the right forelimb. However, significant differences, measured though one-way ANOVA, existed between the 20 lateral PSB (10 from the left forelimb and 10 from the right forelimb) and the 20 medial PSB ( P = 0.006). These findings provide a first of its type reference on the development and maturation timeline of equine PSBs, as well as confirming structural differences between lateral and medial PSBs. The data also document rapid PSB changes between birth and 18 mo of age. The long-term objective of this project is to define normal structural and morphological characteristics of Thoroughbred PSBs as a basis for comparison to PSB pathologies.