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IMAGING


activities of the osteoblasts and osteoclasts,26 but local concentrations of excessive microcracks likely recruit activated osteoclasts causing focal mineral resorption. These focal areas of resorp- tion have a local strain-enhancing effect27 that could reinforce the activation of osteoclasts, bone removal, and void enlargement. We hypothesize when overload is persistent, remodeling becomes maladaptive, the void reaches a critical size, and the articular cartilage collapses. Cartilage col- lapse exposes bone to synovial fluid. Bone has been shown to be a primary source and reservoir for inflammatory mediators in osteoarthritis,28,29 and is also probably a factor in the formation of large lucencies in the MFC. Up-regulation of in- flammatory mediators in the fibrous tissue in equine SCL has been demonstrated, and tissue media from the cyst lining increases osteoclast activity.12,13 Inflammation contributes to further bone removal. Eventually, a pathologic steady state is achieved in the MFC and there is little net change in the size of the lucency. However, joint inflammation persists, and in concert with the roughened cartilage surface, can cause further cartilage and meniscal deterioration.30 We believe that extensive focal damage to MFC trabecular bone, the subsequent cartilage col- lapse, and local osteosynovial inflammation is suf- ficient for the formation of MFC RAs in young horses. It also explains the relatively small amount of articular cartilage damage (as com- pared with bone) in young horses with large lu- cencies. However, little is known about the timing of the disappearance of growth cartilage of the articular surface of the MFC and the forma- tion of the subchondral plate, so it is not possible to rule out a contribution from injury to growth cartilage (osteochondrosis) as a cause for MFC RA as has been postulated.11 The prevalence of MFC RA in this study is similar


to the 40% (yearlings) or 44% (2 year olds) reported for Quarter Horses.6 Comparison of Groups 2 and 3 suggest that healing of lucencies is possible in some horses, but that development of new lucen- cies can also occur. Horses that were younger when a lucency was discovered were more likely to heal, suggesting that early diagnosis might allow implementation of strategies such as restriction of exercise to promote healing. However, we have no information on the exercise allowed subject horses. Besides youth, it was not possible to de- termine other factors that contributed to healing. Studies are ongoing investigating whether exer- cise restriction will promote MFC RA healing. The right-sided predilection for MFC RA was first described by our group5 and similar results have been reported subsequently.6,15,30 The reasons for the asymmetry are unknown, but given that this study reports a right-sided predilection for MFC RAs in untrained horses, trained activity is unlikely to be the cause.


412 2016  Vol. 62  AAEP PROCEEDINGS Acknowledgments


Declaration of Ethics The Authors have adhered to the Principles of the Veterinary Medical Ethics of the AVMA.


Conflict of Interest The Authors declare no conflicts of interest.


References


1. Kane AJ, Park RD, McIlwraith CW, et al. Radiographic changes in Thoroughbred yearlings. Part 1: Prevalence at the time of the yearling sales. Equine Vet J 2003;35: 354–365.


2. Jeffcott LB, Kold SE. Clinical and radiological aspects of stifle bone cysts in the horse. Equine Vet J 1982;14:40–46.


3. Stewart B, Reid CF. Osseous cyst-like lesions of the medial femoral condyle in the horse. J Am Vet Med Assoc 1982;180: 254–257.


4. Jackson M, Vizard A, Anderson G, et al. A prospective study of presale radiographs of Thoroughbred yearlings. Australian Government Rural Industries Research and De- velopment Corporation. RIRDC Publication No. 09/082. 2009. Available from: https://rirdc.infoservices.com.au/ downloads/09-082.pdf.


5. Whitman JA, Morehead JP, Prichard MA, et al. Radio- graphic lucencies in the medial femoral condyle of Thor- oughbred yearlings; the effect on race records, in Proceedings. Am Assoc Equine Pract 2006;52:416–419.


6. Contino EK, Park RD, McIlwraith CW. Prevalence of radio- graphic changes in yearling and 2-year-old Quarter Horses intended for cutting. Equine Vet J 2012;44:185–95.


7. McIlwraith CW. Subchondral cystic lesions (osteochondro- sis) in the horse. Compend Contin Educ Pract Vet 1982;4: s282–s294.


8. Bramlage LR. Osteochondrosis related bone cysts, in Pro- ceedings. Am Assoc Equine Pract 1993;39:83–85.


9. Jeffcott LB, Kold SE, Melsen F. Aspects of the pathology of stifle bone cysts in the horse. Equine Vet J 1983;15:304–311.


10. White NA, McIlwraith CW, Allen D. Curettage of subchon- dral bone cysts in medial femoral condyles of the horse. Equine Vet J 1988;20(suppl 6):120–124.


11. Olstad K, Østevik L, Carlson CS, et al. Osteochondrosis can lead to formation of pseudocysts and true cysts in the sub- chondral bone of horses. Vet Pathol 2015;52:862–872.


12. von Rechenberg B, Guenther H, McIlwraith CW, et al. Fibrous tissue of subchondral cystic lesions in horses produce local mediators and neutral metalloproteinases and cause bone resorption in vitro. Vet Surg 2000;29:420–429.


13. von Rechenberg B, Leutenegger C, Zlinsky K, et al. Upregu- lation of mRNA of interleukin-1 and -6 in subchondral cystic lesions of four horses. Equine Vet J 2001;33:143–149.


14. Cohen ND, Carter GK, Watkins JP, O’Conor MS. Associa- tion of racing performance with specific abnormal radio- graphic findings in Thoroughbred yearlings sold in Texas. J Equine Vet Sci 2006;26:462–474.


15. Santschi EM, Williams JM, Morgan JW, et al. Preliminary investigation of the treatment of equine medial femoral con- dylar subchondral cystic lesions with a transcondylar screw. Vet Surg 2015:44;281–288.


16. Smith MA, Walmsley JP, Phillips TJ, et al. Effect of age at presentation on outcome following arthroscopic debride- ment of subchondral cystic lesions of the medial femoral condyle: 85 horses (1993–2003). Equine Vet J 2005;37: 175–180.


17. Dik KJ, Enzerink E, van Weeren PR. Radiographic devel- opment of osteochondral abnoramalities in the hock and stifle of Dutch Warmblood foal, from age 1 to 11 months. Equine Vet J 1999:suppl 31;9–15.


18. Frost HM. Vital biomechanics. Proposed general concepts for skeletal adaptation to mechanical usage. Calcif Tissue Int 1987;45:145–156.


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