Fatigue damage in the distal metacarpal subchondral bone is common in Thoroughbred racehorses undergoing post-mortem and appears to accumulate throughout a racing career. Reduced intensity or duration of training and racing and/or increased duration of rest periods may limit microdamage accumulation. Focal subchondral bone sclerosis indicates the presence of microdamage.

During the early stages of articular osteochondrosis, cartilage is retained in subchondral bone, but the pathophysiology of this condition of growing humans and domestic animals is poorly understood. A subtractive hybridization study was undertaken to compare gene expression between the cartilage of early experimentally induced equine osteochondrosis lesions and control cartilage. Of the many putative differentially expressed genes identified, eight were confirmed by quantitative PCR analysis as differentially expressed, in addition to those already known to be associated with early lesions. Genes encoding vacuolar H þ -ATPase V 0 subunit d 2 (ATP6V0D2), cathepsin K, integrin-binding sialoprotein, integrin aV, low density lipoprotein receptor-related protein 4, lumican, osteopontin, and thymosin b4 (TMSB4) were expressed at higher levels in lesions than in control cartilage. These genes included 34 genes not previously identified in cartilage. Some genes identified as associated with early lesions are known chondrocyte hypertrophyassociated genes, and in transmission electron microscopy studies normal hypertrophic chondrocytes were observed in lesions. Differential expression of ATP6V0D2 and TMSB4 in the cartilage of early naturally occurring osteochondrosis lesions was confirmed by immunohistochemistry. These results identify novel osteochondrosis-associated genes and provide evidence that articular osteochondrosis does not necessarily result from failure of chondrocytes to undergo hypertrophy. Most bones grow through the process of endochondral ossification, in which chondrocytes play a central role. Growth cartilage is comprised of chondrocytes arranged in zones that correspond to the stages of an organized program of sequential biological events. A zone of resting chondrocytes blends into a zone of proliferative chondrocytes and then a zone of hypertrophic chondrocytes, which ultimately undergo physiological death. Within the zone of hypertrophy, the cartilage matrix surrounding individual chondrocytes is partially degraded, leaving behind cartilage remnants that form vertical struts onto which bone matrix is deposited by invading osteoblasts (reviewed by Mackie et al., 2011 1 ). The processes of chondrocyte proliferation, differentiation, and hypertrophy are tightly regulated by a variety of growth factors, hormones, transcription factors, and components of the cartilage matrix. Many of these factors have been characterized, but it is likely that more remain to be identified. The process of physiological death undergone by chondrocytes is less well characterized, and there has been a debate in the literature as to whether it occurs through apoptosis or a non-apoptotic mode of death.2,3 Moreover, the chondrocytes in growth cartilage express factors capable of regulating the behavior of cells in the invading ossification front, including vascular endothelial cells, osteoclasts, and osteoblasts.

Background: Proximal sesamoid bone fractures are common catastrophic injuries in racehorses. Understanding the response of proximal sesamoid bones to race training can inform fracture prevention strategies.Objectives: To describe proximal sesamoid bone microstructure of racehorses and to investigate the associations between microstructure and racing histories. Study design: Cross-sectional.Methods: Proximal sesamoid bones from 63 Thoroughbred racehorses were imaged using micro-computed tomography. Bone volume fraction (BVTV) and bone material density (BMD) of the whole bone and four regions (apical, midbody dorsal, midbody palmar and basilar) were determined. Generalised linear regression models were used to identify the associations between bone parameters and race histories of the horses. Results:The mean sesamoid BVTV was 0.79 ± 0.08 and BMD was 806.02 ± 24.66 mg HA/ccm. BVTV was greater in medial sesamoids compared with lateral sesamoids (0.80 ± 0.07 vs 0.79 ± 0.08; P < .001) predominantly due to differences in the apical region (medial-0.76 ± 0.08 vs lateral-0.72 ± 0.07; P < .001). BVTV in the midbody dorsal region (0.86 ± 0.06) was greater than other regions (midbody palmar-0.79 ± 0.

Chronic periodontitis is characterised by gingival inflammation and alveolar bone loss. A major aetiological agent is Porphyromonas gingivalis, which secretes proteases that activate protease-activated receptor 2 (PAR ). PAR expressed on oral keratinocytes is activated by proteases released by P. gingivalis, inducing secretion of interleukin 6 (IL-6), and global knockout of PAR prevents bone loss and inflammation in a periodontal disease model in mice. To test the hypothesis that PAR expressed on gingival keratinocytes is required for periodontal disease pathology, keratinocyte-specific PAR -null mice were generated using K14-Cre targeted deletion of the PAR gene (F2rl1). These mice were subjected to a model of periodontitis involving placement of a ligature around a tooth, combined with P. gingivalis infection ("Lig + Inf"). The intervention caused a significant 44% decrease in alveolar bone volume (assessed by microcomputed tomography) in wildtype (K14-Cre:F2rl1 ), but not littermate keratinocyte-specific PAR -null (K14-Cre:F2rl1 ) mice. Keratinocyte-specific ablation of PAR prevented the significant Lig + Inf-induced increase (2.8-fold) in the number of osteoclasts in alveolar bone and the significant up-regulation (2.4-4-fold) of the inflammatory markers IL-6, IL-1β, interferon-γ, myeloperoxidase, and CD11b in gingival tissue. These data suggest that PAR expressed on oral epithelial cells is a critical regulator of periodontitis-induced bone loss and will help in designing novel therapies with which to treat the disease.

Reasons for performing the study: 'Sesamoiditis' is classically defined by equine practitioners as the presence of and abnormalities in vascular channel appearance within the proximal sesamoid bones (PSB). It is the most common finding in Thoroughbred yearling presale radiographs, as well as often being evaluated on radiographs of adult racehorses with lameness and poor performance. Despite this, the pathogenesis and clinical significance of changes in vascular channel morphology are poorly understood, and the association of 'sesamoiditis' with racing performance is inconsistently reported. Objectives:To determine the microstructural characteristics of the PSB associated with the radiographic appearance of vascular channels in Thoroughbred racehorses using micro-computed tomography (µCT), and to determine whether the presence, number and size of vascular channels has an association with past racing performance.Methods: Study design was cross-sectional. One pair of PSB were isolated from a randomly selected forelimb of 59 Thoroughbred racehorses presenting for post-mortem examination over the study period.Each PSB (n=118) was radiographed, assigned a vascular channel grade using previously published and novel radiographic grading systems, then imaged using µCT and similarly assessed. Racing history for each horse was collected. Uni-and multi-variable generalized linear models accounting for clustering at the horse level were generated to investigate associations between radiographic, µCT and performance variables.Results: All PSB had vascular channels observed on µCT originating from the abaxial border (mean 3.6, s.d 0.89), yet in only 63.6% (75/118) were channels observed radiographically. PSB with a higher bone volume fraction (OR 1.08; P=0.031) and wider channel diameter on µCT (OR 20.67; P=0.001) were more likely to have vascular channels identified on radiographs. Radiographic channel number (OR 0.96; P=0.043) and channel size (OR 0.96; P=0.049) were negatively associated with career placings. Main Limitations:Only the forelimb proximal sesamoid bones were collected, radiographs of isolated bones avoided normal superimposition of soft tissue encountered in the live horse, and a cross-sectional study design meant changes in sesamoid vasculature over time and work load could not be assessed. Conclusions:The ability to identify vascular channels radiographically indicates widening of channels and densification of the bone. Increased radiographic channel number and size is associated with poorer measures of past performance suggesting that these changes are not desirable. DECLARATIONS This is to certify that:▪ This thesis compromises only my original work towards the Masters of Veterinary Science.▪ Due acknowledgement has been made in text to all other material used. ▪ This thesis is fewer than the maximum word limit in length, exclusive of tables, maps, list of references and appendices.