Myositis ossificans circumscripta in an adult male kangaroo

An adult male 10 years old red kangaroo was presented with an
11-month history of chronic progressive pelvic limb lameness and unwillingness
to jump. Clinical signs were partially responsive to meloxicam.
Indurations of lumbar epaxial muscles were noted on palpation. Magnetic
resonance imaging of the lumbar region showed two bilateral asymmetric,
well circumscribed mass lesions within both lumbar longissimus muscles.
The lesions have a patchy to multicystic appearance with hyper-, hypoand
isointense areas in T2 weighted (w) and T1w sequences without
contrast enhancement. The described areas had a hypodense center surrounded
by a hyperdense rim giving an “eggshell appearance” on computer
tomography. Histopathological analysis of a biopsy revealed
proliferation of fibroblasts and degeneration of myofibers with dystrophic
mineralization lined by well differentiated bony trabeculae with rare islets
of chondrocytes without evidence of inflammation or malignancy, which is
consistent with differentiated and organized metaplasia of muscle tissue
into bone. Based on the clinical and histopathological presentation and
diagnostic imaging findings, myositis ossificans circumscripta was
suspected. This disease is well known in athletes, where chronic-repeated
muscular microtrauma due to intensive exercise consequently leads to heterotopic
ossification caused by pathological repair mechanisms. Conservative
treatment with stall rest and non-steroidal anti-inflammatory drugs
(meloxicam) led to marked amelioration of clinical signs in the kangaroo
within weeks, and no deterioration was observed within a year after initial
examination.