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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.

Medial proximal sesamoid bones (PSBs) from Thoroughbred racehorses that did (Case) or did not (Control) experience unilateral biaxial PSB fracture were evaluated for bone volume fraction (BVF), apparent mineral density (AMD), tissue mineral density (TMD), and microdamage in Case fractured, Case contralateral limb intact, and Control bones. A majority of Case bones had a subchondral lesion with high microdamage density, and low BVF, AMD, and TMD. Lesion microdamage and densitometric measures were associated with training history by robust linear regression. Exercise intensity was negatively related to BVF (0.07 ≤ R2 ≤ 0.12) and positively related to microcrack areal density (0.21 ≤ R2 ≤ 0.29) in the lesion; however, in an undamaged site, the relationships were opposite in direction. Regardless of location, TMD decreased with event frequency for both Case and Control, suggesting increased bone remodeling with exercise. Measures of how often animals were removed from active training (layups) predicted a decrease in TMD, AMD, BVF, and microdamage at regions away from the lesion site. A steady-state compartment model was used to organize the differences in the correlations between variables within the data set. The overall conclusions are that at the osteopenic lesion site, repair of microdamage by remodeling was not successful (e.g., lower bone mass, increased damage, and lower mineralization) but that in regions away from the lesion remodeling successfully controlled damage (e.g., higher bone mass, less microdamage, and lower mineralization).

The most prevalent causes of death in racehorses are musculoskeletal injuries, causing ~83% of deaths within the racing industry in California and elsewhere. The vast majority of these injuries have preexisting lesions that predispose to fatal injury. A 4-y-old Thoroughbred colt suffered an acute suspensory apparatus failure, including biaxial proximal sesamoid bone fractures of the right front fetlock, causing loss of support of the fetlock joint and consequent fall with fractures of the cervical and sacral spine. Cervical fracture caused spinal cord damage that resulted in sudden death. A preexisting lesion in the medial proximal sesamoid bone likely predisposed to complete fracture of this bone and fetlock breakdown. Interestingly, a comparable osteopenic lesion was present in the intact medial proximal sesamoid bone of the left forelimb, which is consistent with bilateral repetitive overuse injury in racehorses. The morphologic features of the cervical and sacral spine fractures were compatible with acute injury; no evidence of preexisting lesions was seen. Most likely, these acute vertebral fractures occurred as a result of the horse falling. This case emphasizes the importance of performing a detailed autopsy in horses that suffer an appendicular musculoskeletal injury, particularly in fatal cases when the horse dies following a leg injury.

Many pathologic lesions and several types of fractures in the equine forelimbs involved the bony elements of the metacarpophalangeal joints (fetlock joints) especially the third metacarpal bones and the proximal sesamoid bones. Although the bones of the fetlock joint have been described in a general manner, the variations between equine breeds are still insufficient and differences within the breeds are not completely covered. Therefore, the aim of the current study was to identify the morphometrical variations of these bones between and within some different equine breeds. Right and left metacarpal, proximal phalangeal and proximal sesamoid bones were collected from 23 horses (10 Thoroughbreds, 5 Standardbreds and 8 Ponies) euthanized for non-orthopaedic reasons. The bones were boiled between 95°C and 99°C for 30 hours, then cleaned and left for 24 hours at room temperature for air drying. Finally, the bones were dried in the drying cabinet at 49.5°C for 8 hours. A number of dimensional parameters were measured using a custom-built apparatus, calliper and plastic tape. These measurements included proximodistal height, lateromedial width, and dorsopalmar depth. All the statistical data were analysed using the ANOVA test and SPSS statistic software except the repeatability data which were analysed by calculating the repeatability coefficient. This study showed that there were significant variations between the medial and lateral sides of all bones in all breeds. The length to the lateral condyle of Mc3 was greater than the length to the medial condyle of Mc3 in all breeds. The width and depth of the medial condyle was greater than the lateral condyle in all breeds. The lengths of the lateral sides of the P1 bones were greater than the lengths of the medial sides in the right forelimbs of Thoroughbreds and Standardbreds. The width of the medial articular cavity of P1 was greater than the width of the lateral articular cavity except in the left limb of Ponies. The dept

Stability of the thumb metacarpophalangeal joint is provided by the collateral ligament proper, the accessory collateral ligament, the proximal and distal palmar ligaments, and the palmar plate. The adductor pollicis and flexor pollicis brevis muscles also provide soft tissue support and insert on the proximal phalanx of the thumb by way of the sesamoid bones. Fractures of the metacarpophalangeal joint sesamoid bones are uncommon but are often associated with sporting injuries during which the joint is acutely hyperextended. Routine anteroposterior and lateral radiographs may be unremarkable, and oblique views are often necessary to document the fracture. We report eight cases of fracture of the thumb metacarpophalangeal joint sesamoid. Seven were isolated sesamoid fractures, and one was associated with a metacarpophalangeal joint dislocation. None of the fractures were evident on anteroposterior radiographs, but all were seen in the oblique projection. Six fractures were treated with splint or cast immobilization for 2 to 3 weeks, and two were taped. Clinical followup of the seven patients with isolated sesamoid fracture at 6 to 8 weeks revealed pain-free normal function. One patient could not be reached for followup.

Stability of the thumb metacarpophalangeal joint is provided by the collateral ligament proper, the accessory collateral ligament, the proximal and distal palmar ligaments, and the palmar plate. The adductor pollicis and flexor pollicis brevis muscles also provide soft tissue support and insert on the proximal phalanx of the thumb by way of the sesamoid bones. Fractures of the metacarpophalangeal joint sesamoid bones are uncommon but are often associated with sporting injuries during which the joint is acutely hyperextended. Routine anteroposterior and lateral radiographs may be unremarkable, and oblique views are often necessary to document the fracture. We report eight cases of fracture of the thumb metacarpophalangeal joint sesamoid. Seven were isolated sesamoid fractures, and one was associated with a metacarpophalangeal joint dislocation. None of the fractures were evident on anteroposterior radiographs, but all were seen in the oblique projection. Six fractures were treated with splint or cast immobilization for 2 to 3 weeks, and two were taped. Clinical followup of the seven patients with isolated sesamoid fracture at 6 to 8 weeks revealed pain-free normal function. One patient could not be reached for followup.