A simple pathological classification of arthritis is:
• inflammatory: rheumatoid arthritis (Fig. 22.10), ankylos-ing spondylitis;
• degenerative: osteoarthritis;
• infective: tuberculosis, postseptic arthritis (Fig. 22.11);
• post-traumatic: an arthropathy may be secondary to trauma causing premature degenerative changes within the joint because of loss of bone stock, soft-tissue support or alignment;
• idiopathic: avascular necrosis (Fig. 22.12);
• metabolic: gout and pseudo-gout secondary to inflammation resulting from crystal deposits in the synovium (crystalline arthropathies);
• haematological: haemophilia (Fig. 22.13).
The pathological changes within the joint will vary depending on the aetiology; however, the final common pathway generally involves destruction of the articular cartilage to the
subchondral bone, loss of the normal elasticity of the capsular tissues and acute or chronic inflammation of the synovium. Capsular contractures result in stiffness and deformity of the joint, together with restriction of the range of movement. Erosion of the articular cartilage together with destruction of the subchondral bone due to abnormal load through the
joint or chronic inflammation also results in loss of alignment (deformity) and normal smooth gliding of the articular surfaces over one another (pain and stiffness) (Fig. 22.14).
Acute or chronic inflammation involving the synovium produces either haemorrhagic or seropurulent effusions within the joint that result in distension, together with loss of mobility and associated pain and stiffness. The secondary effects of the pathological process involving the joint result in loss of use with subsequent atrophy of the muscles involved in movement of the joint. Osteopaenia from a degree of disuse further aggravates loss of function, not only within the joint itself but also in the involved limb.
Appropriate investigations of an arthropathy include plain X-rays of the joint and long films of the entire limb to assess alignment. Routine blood investigations, particularly looking at the white cell count and various acute phase react-ants (erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP)) are performed, as well as appropriate serological tests if a postviral arthropathy is suspected.
Inflammatory arthropathies include rheumatoid arthritis (Fig. 22.10), arthropathy associated with inflammatory bowel disease and the spondyloarthropathies, particularly ankylosing spondylitis. Appropriate serological investigations include latex glutination tests, HLAB 27 serology and antinuclear antibody screens.
Diagnosis of a purely idiopathic degenerative condition essentially relies on clinical assessment with the assistance of plain films (Fig. 22.15). Arthropathies secondary to metabolic conditions or coagulopathies, in particular haemophilia, rely on the appropriate biochemical assays, haematological screening for clotting factor deficiencies radiological assessment of changes specific to these conditions (Fig. 22.13).
If all the above investigations fail to confirm an exact diagnosis, then it may be appropriate to obtain a specimen of joint fluid by aspirating the synovial effusions. This must be performed under strict aseptic conditions so as not to cause an iatrogenic infective arthritis. This specimen is sent to the laboratory for appropriate investigation, particularly cyto-logical assessment, polarizing microscopy to look for crystals and routine culture of anaerobic or aerobic organisms and tuberculosis. The latter is necessary even if skin testing for tuberculosis has been performed.
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