Fig. 73.—Dislocation on to Dorsum Ilii. Note relation of neck of femur to tendons of obturator internus and gemelli (diagrammatic).
Dislocation into the Vicinity of the Great Sciatic Notch, or “dislocation below the tendon.”—This variety of backward dislocation is less common than that on to the dorsum, although produced in the same way. The head of the femur passes beneath the obturator internus, and this tendon, catching on its neck, checks its upward movement ([Fig. 74]).
The clinical features are the same as those of the dorsal variety, but, on the whole, are less marked.
Differential Diagnosis.—Backward dislocation of the hip is usually easily recognised. When dislocation below the tendon occurs in a stout person, however, it is liable to be overlooked on account of the difficulty of feeling the displaced bone, and of the comparatively slight amount of deformity present. The nature of the accident, the absence of broadening of the trochanter, and the adduction and inversion of the limb are usually sufficient to prevent a dislocation being mistaken for an impacted extra-capsular fracture.
Dislocation into the Obturator Foramen ([Fig. 71]).—This dislocation is produced by great force applied from behind while the thigh is flexed and abducted, as when a weight falls on the back of a man stooping forward with the legs wide apart. It may also result from violent abduction by wide separation of the thighs.
The capsule gives way at its medial and lower part, and the head of the femur comes to rest on the surface of the external obturator muscle, its articular surface looking forward, while the trochanter looks backward.
Clinical Features.—In the standing position the thigh is slightly flexed and abducted, with the foot pointing directly forward or a little outward. The body is bent forward to relax the ilio-psoas muscle and the [inverted Y]-ligament, the foot is advanced and the heel drawn up. It is not uncommon for the patient to be able to walk after the accident, and only to seek advice some time later on account of inability to adduct and extend the limb. There is apparent lengthening of the limb due to tilting of the pelvis downward on the affected side. The hip is flattened, the trochanter less prominent than usual, and the head of the bone may sometimes be felt in its abnormal position.
Fig. 74.—Dislocation into the vicinity of the Ischiatic Notch. Note relation of neck of femur to tendons of obturator and gemelli, “Dislocation below the tendon” (diagrammatic).