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without changing the echogenicity of the fibres. Using a nonweight-bearing technique for visualising the SL is known to improve visualisation of the relationship between the SL and the splint bones due to decreased palmar soft tissue depth and ease of manipulation of the vessels. However, this technique can falsely show contact between the SL and the splint bones due to laxity of the soft tissue structures and extrusion abaxially of the SL with the pressure of the ultrasound probe. Increased osseous reaction of the affected splint bone
and mineralisation of the soft tissue structures or SL in response to chronic inflammation further hinder the ability to adequately assess the abaxial margins of the SL with ultrasound. This makes ultrasonography of the metacarpal region less sensitive and less specific for diagnosing adhesions or impingement between the exostosis and SL. Radiographs of the affected region are beneficial in
visualising the abaxial margin of the affected splint bone and diagnosing additional pathologies of the splint bone such as fractures. Due to summation of the axial border of the splint bone with the cannon bone, it is often not possible to determine the extent of exostosis axially, and thus potential impingement of the suspensory body, with radiography alone. Reports in the literature of nuclear scintigraphy show evidence of increased bone activity even after the bone is clinically quiescent for long periods of time. This makes it difficult to differentiate between active and inactive areas of exostosis and therefore does not provide a definitive diagnosis. Additionally, the lack of detail on scintigraphy makes it impossible to associate the increased signal with specific pathologies such as impingement or adhesions. Recent advances in computed tomography with contrast enhancement may provide beneficial information in diagnosing cases of suspensory desmitis associated with exostosis and potential associated adhesions, although no specific case studies have been reported using this imaging modality for this specific disease process. Magnetic resonance imaging (MRI) is the current gold
standard for the definitive diagnosis of musculoskeletal injury of the distal limb in horses. MRI allows detailed visualisation of the osseous and soft tissue structures. This modality has become increasingly available to the equine practice over the last 20 years and is commonly utilised in diagnosis of lameness. Two types of systems are available: low-field MRI and high-field MRI. The difference clinically between the two systems has been reported (Werpy 2007). Low-field MRI systems are defined as having a field strength of less than 0.3 tesla (T). There are a number of commercially available low-field MRI systems marketed for horses. Advantages to this system include decreased purchase cost and the ability to image some horses without general anaesthesia. Disadvantages include increased slice thickness, increased artefact, decreased detail, less fat suppression, increased scan length and increased movement artefact. A high-field magnet has field strength of greater than 1.0 T.
The majority of high-field MRI units used in equine practice are human units that have been modified to facilitate imaging of the horse. Typically, this limits imaging to the distal limbs, head and neck of adult equine cases. The cost of a high-field MRI system is significantly higher than a low-field system although the client cost of imaging a region of the case is similar. The image quality is superior with a high-field system and this directly correlates with diagnostic accuracy.
Specifically, high-field systems can more effectively decrease the amount of partial volume averaging by allowing for thinner slices and accurate imaging of smaller anatomical structures and pathologies. High-field MRI is 97% successful in identifying the cause of lameness in horses with proximal metacarpal injuries (Brokken et al. 2007). In the current study, the adhesion between the area of exostosis and the SL was not adequately visualised with low-field MRI, and thus was unable to be diagnosed prior to surgery. In our experience, subtle changes in the SL architecture secondary to impingement and the adhesions between the SL and the splint bone can be definitively diagnosed using a high-field MRI. Differentiating between acute and chronic injuries of the SL
is important to direct treatment and rehabilitation and can be achieved with the high-field MRI system. Acute lesions have local enlargement of the ligament and increased signal intensity in thedamaged areas, likely asaresult of inflammation, increased cellularity and vascularity to the injury (Murray et al. 2006). Chronic lesions have focal or diffuse enlargement of the SL, with intermediate signal intensity mixed with the normal lattice of high signal intensity within the low signal of the SL, due to the muscle and fascial tissues, indicating the presence of scar tissue in the area (Crass et al. 1992). Due to the normal variation of muscular fibres in the SL, it is essential to image the same region in the contralateral limb for comparison. High-field MRI is able to definitively visualise the axial
surface of the splint bones and their relationship with the SL (Zubrod et al. 2004). There is normally a distinct high signal intensity separation between the suspensory ligament and the axial surface of the splint bone. Additionally, they described adhesions in the space between the splint bones and SL as a focal low signal intensity that traverses the high signal area between the SL and splint bone. The SL often had increased signal intensity on the same side and just distal to the area of adhesion extending to the periphery, indicating associated desmitis. Definitive diagnosis of the pathologies allows for better preparation for surgical therapy and post- operative treatment and outweighs the risk of general anaesthesia.
Treatment
Conservative medical therapy is the mainstay of treatment of acute cases of exostosis. The majority of horses respond well to rest, systemic and local anti-inflammatories and compression bandaging. In cases where the exostosis impinges axially on the suspensory or where adhesions have formed, surgical therapy is required to resolve the lameness. Two surgical techniques are commonly reported for correction of splint bone exostosis with associated SL desmitis or adhesion. The first technique involves ostectomy of the affected splint bone proximal to the level of the exostosis and removing the entire splint distal to the exostosis (Milne and Turner 1979). This technique was performed in the current study. However, based on the included radiograph, the entire region of exostosis was not removed and no attempt was made to bevel and smooth the distal portion of the remaining splint bone. Leaving a portion of the exostosis could provide a nidus for adhesion reformation and proliferation of exostosis in the area. Albeit reports show leaving less than one-third of the splint can cause carpal instability, the metacarpal bone in question has adequate
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