The sacroiliac joints: evaluation using nuclear scintigraphy. Part 1: The normal horse

Sacroiliac (SI) disease is recognised as a cause of poor hindlimb action but differential diagnosis is often difficult.

That in clinically normal horses there would be a significant difference in the ratio of radiopharmaceutical uptake (RU) between the fifth lumbar vertebra (L5) and each tuber sacrale (TS) and between L5 and each SI joint; and that these ratios would alter with age, but ratios would be bilaterally symmetrical.

Dorsal scintigraphic images of the SI region of 15 horses, selected randomly from the clinic database, were analysed by 2 of the authors, comparing noncorrected and motion-corrected images. To determine scintigraphic anatomy, the scintigraphic images of 10 Thoroughbred horses were superimposed over a ventrodorsal radiographic image of an isolated pelvis. Dorsal scintigraphic images of 40 clinically normal horses age 3-16 years were evaluated using subjective examination, profile analysis and quantification using regions of interest.

The tubera sacrale were seen as 2 well-defined oval regions immediately to the left and right of the midline, abaxial to which were larger, approximately oval areas with less RU, representing uptake in the SI joints. The definition between the SI region and the TS was more obvious in younger horses. Nonmotion-corrected images were often not of diagnostic quality or could be misinterpreted as abnormal. There were significant differences in RU between the TS and SI joints compared to L5, and decreased RU in the tubera sacrale with increasing age, but no change of uptake in the SI joint region and no effect of gender on RU. There was a high degree of left-right symmetry of the TS and SI joint regions.

The scintigraphic images of horses with suspected sacroiliac joint disease should be compared with images of normal horses of comparable age. In normal horses, there was a high degree of symmetry; therefore, marked left-right asymmetry is likely to be abnormal.