Caused by Mycobacterium tuberculosis, tuberculosis (TB) most commonly affects the lungs, lymph nodes and serous membranes, the bones and joints being the last to be involved with the incidence as low as about 10%.
The major sites of osseous and articular tuberculous lesions include the epiphyses of the long bone shafts, bodies of the short bones: f.e. vertebrae and diaphyses of the digital phalanges. Anatomically, this type of tuberculosis falls into three major forms:
• tuberculous spondylitis, or spinal TB (40%);
• tuberculous coxitis, or hip joint TB (20%);
• tuberculous gonitis, or knee joint TB (15-20%).
Tuberculosis of the bones and joints is usually due to a secondary TB infection, which implies its lymphohaematogenous spread from the primary pulmonary focus or that of other organs. M. tuberculosis bacteria induce specific inflammatory reaction within the bone marrow of epiphyses, the site known to have a slower blood flow. This results in primary ostiitis, or tuberculous osteomyelitis. The initial tuberculous focus undergoes caseous («cheese-like») necrosis around which further tubercles form to merge with the initial focus. Subsequently, granulation occurs which, in turn, is subject to necrosis. This leads to sequestration: the bony tissues involved in the necrosis become completely separated from the underlying bone.
The progression of tuberculous inflammation into the joint forms an articular sac with subsequent caseous necrosis of the joint cartilage and contiguous osseous tissues.
Primary synovial tuberculosis (TB synoviitis) only occurs in about 5% of cases, with a favourable outcome as it is unlikely to cause the damage to the joint ends of the bones.