A number of cases have been recorded in which arthritis deformans has followed upon antecedent disease of the joint, such as pyogenic or gonorrhoeal synovitis, upon repeated haemorrhages into the knee-joint in bleeders, and in unreduced dislocations in which a new joint has been established.

[Illustration:  FIG. 157.—­Arthritis Deformans of Elbow, showing destruction of articular surfaces and masses of new bone around the articular margins.

(Anatomical Museum, University of Edinburgh.)]

Lastly, Poncet and other members of the Lyons school regard arthritis deformans as due to an attenuated form of tuberculous infection, and draw attention to the fact that a tuberculous family history is often met with in the subjects of the disease.

[Illustration:  FIG. 158.—­Arthritis Deformans of Knee, showing eburnation and grooving of articular surfaces.

(Anatomical Museum, University of Edinburgh.)]

Morbid Anatomy.—­The commonest type is that in which the articular surfaces undergo degenerative changes.  The primary change involves the articular cartilage, which becomes softened and fibrillated and is worn away until the subjacent bone is exposed.  If the bone is rarefied, the enlarged cancellous spaces are opened into and an eroded and worm-eaten appearance is brought about; with further use of the joint, the bone is worn away, so that in a ball-and-socket joint like the hip, the head of the femur and the acetabulum are markedly altered in size and shape.  More commonly, the bone exposed as a result of disappearance of the cartilage is denser than normal, and under the influence of the movements of the joint, becomes smooth and polished—­a change described as eburnation of the articular surfaces (Fig. 158).  In hinge-joints such as the knee and elbow, the influence of movement is shown by a series of parallel grooves corresponding to the lines of friction (Fig. 158).

[Illustration:  FIG. 159.—­Hypertrophied Fringes of Synovial Membrane in Arthritis Deformans of Knee.

(Museum of Royal College of Surgeons, Edinburgh.)]

While these degenerative changes are gradually causing destruction of the articular surfaces, reparative and hypertrophic changes are taking place at the periphery.  Along the line of the junction between the cartilage and synovial membrane, the proliferation of tissue leads to the formation of nodules or masses of cartilage—­ecchondroses—­which are subsequently converted into bone (Fig. 157).  Gross alterations in the ends of the bone are thus brought about which can be recognised clinically and in skiagrams, and which tend to restrict the normal range of movement.  The extension of the ossification into the synovial reflection and capsular ligament adds a collar or “lip” of new bone, known as “lipping” of the articular margins, and also into other ligaments, insertions of tendons and intermuscular septa giving rise to bony outgrowths or osteophytes not unlike those met with in the neuro-arthropathies.