Understanding Stifle Joint Radiolucency

This study examined tibial tuberosity radiolucency (TTR) on radiographs to better understand its causes and significance. Radiographs (n = 675) of canine stifle joints were reviewed; 21.5% were found to have TTR proximal and caudal to the tibial tuberosity. Radiolucency (ie, dark areas) ranged from small and faint to larger and well-defined. For dogs that had bilateral stifle radiographs available (n = 52), radiolucency was bilateral in 96.2% of cases. Radiolucency size did not correlate with stifle disease. Breed size was significantly associated with TTR; toy-, small-, and medium-breed dogs were more commonly affected. Young age and medial patellar luxation (MPL) were also significantly associated with TTR. TTR may be caused by a retained cartilage core and may be associated with MPL. A cause-and-effect relationship could not be established, and further studies to identify the cause for this radiographic lesion are warranted.

When contralateral limb radiographs were available, 96.2% had bilateral lucency. Dogs with lucency were 10 times as likely to have medial patellar luxation and much less likely to have a cranial cruciate ligament tear. No correlation was noted between radiolucency size and MPL grade. Histologically, these areas were comprised of hyaline cartilage and were consistent with retained cartilaginous cores near the growth plate.

It was not clear whether the retained cartilage was present because of other skeletal abnormalities usually found in dogs with MPL (eg, tibial torsion, femoral varus). If noted on a radiograph, this radiolucency should not be confused with neoplastic disease, but patellar disease should be investigated.—Jonathan Miller, DVM, MS, DACVS

Prevalence, association with stifle conditions, and histopathologic characteristics of tibial tuberosity radiolucencies in dogs. Paek M, Engiles JB, Mai W. VET RADIOL ULTRASOUN 54:453-458, 2013.