When bone bordering on an articular surface becomes necrotic in osteomyelitis, it separates as a sequestrum which has the following characteristics:
1. It is denser than the adjacent living bone which undergoes atrophy.
2. Its articular cartilage disappears, being digested by the proteolytic ferments liberated in the joint.
3. The articular cortex of the sequestrum is preserved because of its inaccessibility to granulation tissues.
4. The articular cartilage and cortex of the rest of the joint, which is living, is usually lost. This gives the joint sequestrum a characteristic appearance which is easily recognizable in the roentgenogram as well as in the gross specimen.