Fig. 119.—Arthritis Deformans, showing erosion of cartilage and lipping of articular edge of head of femur.
The changes in the joint are characteristic of the dry form of the disease, and affect chiefly the cartilage and bone. The atrophy and wearing away of the articular surfaces are accompanied by new formation of cartilage and bone around their margins. The head of the femur may acquire the shape of a helmet, a mushroom, or a limpet shell, and from absorption of the neck the head may come to be sessile at the base of the neck, and to occupy a level considerably below that of the great trochanter ([Fig. 120]). These changes sometimes extend to the upper part of the shaft, and result in curving of the shaft and neck, suggesting a resemblance to a point of interrogation ([Fig. 121]). The acetabulum may “wander” backwards and upwards, as in tuberculous disease. It is usually deepened, and its floor projects on the pelvic aspect; its margins may form a projecting collar which overhangs the neck of the femur, or grasps it, so that even in the macerated condition the head is imprisoned in the socket and the joint locked. There is eburnation of the articular surfaces in those areas most exposed to friction and pressure.
Fig. 120.—Upper End of Femur in advanced Arthritis Deformans of Hip. The shaft is curved and the head of the bone is at a lower level than the great trochanter.
Fig. 121.—Femur in advanced Arthritis Deformans of Hip and Knee Joints. The upper end of the bone shows the condition of coxa vara; the lower end shows enlargement of the medial condyle and alteration in the axis of the articular surface.
These changes are necessarily associated with restriction of movement, and in advanced cases with striking deformity, which consists in shortening of the limb, usually with eversion and displacement of the trochanter upwards and backwards in relation to Nélaton's line.