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detection of sacroiliac abnormalities, Coudry et al. (2006) showed a good correlation (77% - 51/66) between the two techniques. A marked IRU in nuclear scintigraphy without abnormal ultrasonographic findings was observed in 20% (13/


66) of sacroiliac joints, and abnormal ultrasonographic findings without any IRU detected in the sacroiliac area in 3% (2/66) of sacroiliac joints. These results show that ultrasonographically abnormal sacroiliac joints are highly correlated with active bone remodelling in the area and related to clinical features. Nevertheless, nuclear scintigraphy can be limited for diagnosing chronic diseases. Uptake from sacroiliac joint is also reduced by the thick gluteus medius muscle which causes attenuation. In the future, computed tomography performed with large diameter machines will probably complement soft tissue and physiological imaging.


Aetiopathogenesis and pathological findings Sacroiliac joint lesions can have traumatic or more often degenerative origins (Jeffcott et al. 1985; Haussler and Stover 1998). Osteoarthritis is the most common pathological finding in horses affected by sacroiliac pain although its clinical significance remains unclear (Haussler 2004). In 1986, Dalin and Jeffcott reviewed the gross morphology


and morphometric features of sacroiliac joints of 41 horses without any history of low back pain (Dalin 1984; Dalin and Jeffcott 1986a,b). They showed that pathological findings are due to cartilaginous degeneration progressing with advancing age. Age-related changes were previously suspected as mean age of horses with abnormal sacroiliac joints is higher than horses with normal sacroiliac joints (Rooney 1977). However, another study did not confirm this tendency (Haussler et al. 1999). The iliac surface is more prone to degenerative changes than the sacral surface, and at a lower age. This observation is consistent with findings in human subjects for which sacral hyaline cartilage has a better capacity to resist degenerative changes than iliac fibrocartilage (Dalin and Jeffcott 1986a). A suggested mechanism of degenerative changes is that shearing forces are higher than compressive forces on the sacroiliac joint, leading to a decreased turnover of the synovial fluid and thereby nutritional dysfunction (Dalin and Jeffcott 1986a). Moreover, the presence of higher contact forces on low-motion joints than high-motion joints may predispose sacroiliac joints to osteoarthritis (Haussler 2004). Pooling of biomechanical and anatomical data revealed that osteoarthritis lesions are mainly located at the caudomedial joint margin (Jeffcott et al. 1985; Dalin and Jeffcott 1986a,b; Haussler et al. 1999; Haussler 2004; Denoix 2016). Correlation between sacroiliac joint degenerative injuries and chronic sacroiliac ligament desmopathy with lack of stability of the joint has been discussed in the past (Rooney 1977; Jeffcott et al. 1985; Haussler et al. 1999). Ultrasonographic findings indicative of degenerative


lesions include modification of the bone surface shape (bone remodelling) of the sacrum and/or ilium, periarticular osteophytes of the auricular surfaces, and narrowing of the joint space (Denoix 1998; Denoix et al. 2006). The ventral sacroiliac ligament size and/or echogenicity can be modified (desmopathy, enthesopathy or even rupture). The grading system described in this paper is helpful to classify the severity of bone remodelling lesions, based on the severity of periarticular proliferation and modelling of the sacrum and/or ilium (Denoix 2016). Severity of lesions and clinical manifestations are not necessarily correlated. A horse’s


tolerance of lesions depends greatly on the discipline. For example, trotters have low tolerance of sacroiliac injuries compared with sport horses and Thoroughbreds because of the very specific biomechanical stresses applied to the pelvis at high-speed trot. Rooney also showed that Standardbreds are more affected and earlier in life by sacroiliac joint arthropathy than Thoroughbreds because of differences in biomechanics of gaits (Rooney 1977). In a necropsy study conducted on 36 Thoroughbred


racehorses which were subjected to euthanasia for other reasons than pelvis problems, Haussler et al. showed that 25% presented sacroiliac joint injuries at different stages (Haussler et al. 1999; Haussler 2004). Moreover, Dalin and Jeffcott observed obvious degenerative cartilage lesions on sacroiliac joints although horses included in the study did not present any signs of back pain (Dalin and Jeffcott 1986a). Therefore, prevalence of sacroiliac disease is probably high in the performance horses population and the clinical significance of these changes remains doubtful (Dalin 1984; Jeffcott et al. 1985; Haussler and Stover 1998; Haussler 2004). Degenerative osseous changes of the sacroiliac joints are


usually found on the left and right side at post-mortem examination (Dalin and Jeffcott 1986a; Haussler et al. 1999; Haussler 2004). In the same way, in clinical cases, ultrasonographic findings indicative of osteoarthritic lesions are commonly found bilaterally but the grade is often different on left vs. right side (Denoix 2016). In our daily practice, we observe in several hundreds of horses every year that ultrasonographic changes and grades of the sacroiliac joints lesions are more common and more severe in males and geldings than in females without having more clinical or athletic incidence.


Authors’ declaration of interests No conflicts of interest have been declared.


Ethical animal research Not applicable.


Source of funding None.


Authorship All authors contributed to the manuscript preparation.


References


Bergman, E.H.J., Puchalski, S.M. and Denoix, J.M. (2013) How to perform a transrectal ultrasound examination of the lumbosacral and sacroiliac Joints. Proc. Am. Ass. Equine Practnrs. 59, 229-237.


Coudry, V., Audigi e, F., Didierlaurent, D. and Denoix, J.M. (2006)


Comparison between nuclear scintigraphy and transrectal ultrasonography in the detection of sacroiliac abnormalities. In: International Radiology Association and American College of Veterinary Radiology 2006 Joint Conference, Vancouver, Canada. p 76.


Dalin, G. (1984) On the back of the horse. A morphological, clinicopathological and experimental study with special reference to the sacroiliac joint. PhD thesis, Faculty of Veterinary Medicine, Swedish University of Agricultural Sciences, Uppsala, Sweden.


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