Retained ulnar cartilage cores (RCC)

Retained ulnar cartilage cores (RCC) is a developmental disorder in young, large and giant breed dogs characterized by a failure of endochondral ossification in the distal ulnar physis, leading to physeal calcification arrest, restricted growth of forelimb and consequently angular limb deformities. The most common site is the distal ulna, but RCC can also occur at the distal tibia and femur.

Retained cartilage core is considered a type of osteochondrosis where there is failure of enchondral ossification from physeal cartilage to normal bone. The chondrocytes remain present in the metaphyseal bone.The retained cartilage core is bilateral and occurs in the distal ulna. Histologically, large chondrocytes are present in the distal ulnar metaphysis. The exact etiology is unknown, but dietary factors and genetic predisposition are suspected.

Clinical signs:
Dogs might be asymptomatically in cases where the retained cartilage core is an incidental finding.
Clinical signs may include pain, lameness and angular limb deformities (the disruption of enchondral ossification causes shortening of the ulna and results in procurvatum of the distal radius (also called , rotation and valgus deviation of the forepaw), some may present subluxation of the carpal and elbow joints.

Diagnosis
The retained cartilage cores will present radiographically as a radiolucent, flame-shaped areas that extend 3-4 cm into the ulnar metaphysis, with the wide end of the cone confluent with the physis, also referred to as “"Candlestick ulna". There may be a rim of sclerotic bone separating it from the metaphysis. Secondary radiographic changes include procurvatum (“bowing”) of the radius due to shortening of the ulna, elbow and/or carpal subluxation, and valgus deviation of the forepaw.

The use of computed tomography show elongated focal radiolucent regions in the distal radial metaphysis.

Bone biopsy and histopathology of the regions will confirm a retained cartilaginous core characterised by bony trabeculae with frequently retained central cartilaginous cores. 

Treatment
Treatment can be conservative or surgical. Conservative treatment consists of interruption of dietary supplements and a balanced diet. Surgical treatment consists of corrective osteotomy or osteotomies to reduce limb deformation.

Prognosis
The prognosis depends on the severity of the condition. 

Sources:

Pollard R & Phillips K. Orthopedic Diseases of Young and Growing Dogs and Cats. Textbook of Veterinary Diagnostic Radiology. 2018;:348-65. 
Nelson JT, Monahan CF, Sledge DG, Kim S. Retained cartilaginous core in the distal radius of a small breed dog. J Small Anim Pract. 2023 May;64(5):359-362.