Bony island within the articular cartilage of the knee in a child: a rare condition for early osteoarthritis

Abstract

Articular cartilage is a specific type of connective tissue composed of hydrated proteoglycans within a matrix of collagen fibrils. In the elderly population, it shows degenerative changes that may results in osteoarthritis. The more severe form of osteoarthritis occasionally demonstrates bone formation within the cartilage, which is designated as a bony protuberance, however, such lesions are rare in children. This report presents the case of a 10-year-old boy with a bony protuberance within the articular cartilage of the knee. The patient initially complained of knee pain and he subsequently developed flexion contracture. Radiological and arthroscopic examinations revealed a bony protuberance in the articular cartilage and degenerative changes of the cartilage above it. He was successfully treated by the removal of the bony protuberance and osteochondral grafting. The bony protuberance may have caused cartilage degradation since the thickness of the cartilage above it was thinner than that around the lesion. The bony protuberance within the articular cartilage formed in the younger population may be a possible cause of osteoarthritis. This case is a noteworthy with regard to the pathogenesis of osteoarthritis.

Keywords: articular cartilage; bone protuberance; child.; osteoarthritis.

Figure 1

Plain radiographs of the left knee. (A) an anteroposterior view, (B) a lateral view. (A,B) At the 2nd visit to our outpatient clinic a free-body like bony fragment is detected in the medial compartment of the left knee (arrowheads). (C,D) 18 months after Figure 1A, the bony fragment detected is united with medial femoral condyle just like a bone protuberance (arrowheads).

Figure 2

Coronal plain T2-weighted magnetic resonance imaging of the left knee.(A) At the 2^nd visit to our outpatient clinic, a low signal intensity spot is detected within the articular cartilage (white arrowhead). (B) 18 months after Figure 2-A, the low signal intensity spot is united with medial femoral condyle (white arrowhead).

Figure 3

An arthroscopic examination reveals that the surface of the articular cartilage above the bony protuberance is fibrillated and slightly depressed.

Figure 4

Coronal plain T2-weighted magnetic resonance imaging of the left knee at 10 years after the osteochondral plug autograft demonstrates the surface of the medial condyle is covered with well-reconstructed articular cartilage (arrowhead).

Figure 5

Histological findings of the removed osteochondral plug. (A) Low magnification photograph around the bone protuberance. Bony protuberance protrudes into the articular cartilage. The articular cartilage over the protuberance is thinned and there is a cleft in the articular cartilage filled with fibrous tissue. There are also regions with hypertrophic subchondral trabeculae. (Hematoxylin-eosin staining. The original magnification is 15×). (B) High magnification photograph at the osteochondral junction. There are many cluster formations of the articular chondrocytes (arrowheads). (Hemato xylin-eosin staining. The original magnification is 100×). (C) High magnification photograph at the trabecule bone beneath the cleft within the articular cartilage. Osteoblasts (arrows) and osteoclasts (arrowheads) are detected around the trabelulae suggesting that active bone remodelling beneath the cleft. (Hematoxylin-eosin staining. The original magnification is 100×).