The synovial membrane lines the inner surface of the capsule, from which it is reflected inwards on to the neck along the capsular insertion, being separated from the neck by the cervical ligaments. It is inserted at the head round the margin of the articular cartilage, which ends close to the epiphyseal plate for the head. At its acetabular extremity, where it also is inserted at the margin of the articular cartilage, it is reflected over the cotyloid ligament, overlies the fat in the non-articular part, and covers the ligamentum teres. It sometimes communicates with the bursa lying under the psoas on its anterior aspect.
The vessels of the joint are chiefly derived from the internal and external circumflex and obturator arteries.
The nerve-supply of the hip-joint is (a) from the anterior crural at the front, (b) from the obturator at the lower and inner part, and (c) from the sacral plexus and sciatic nerve posteriorly. In hip-joint disease pain is frequently referred to the knee, so much so that the condition of the hip is not infrequently overlooked and the knee alone complained of. It is therefore interesting to note that hip and knee have an almost identical nerve-supply, the knee receiving an anterior crural branch in front, an obturator branch posteriorly, and sciatic branches laterally and posteriorly. In so-called ' hysterical hip,' the patient frequently simulates some forms of hip disease, and it is supposed that the origin of the sciatic nerve from sacral segments of the cord, which also supply the pelvic viscera, may so far account for this.
Fractures of the neck of the femur may be either intracapsular, or so-called extracapsular. The former generally occurs at the junction of head and neck, as a result of slight indirect violence in elderly persons in whom (a) the angle between neck and shaft has diminished from 160 in the child to 125 in the adult, or even less ; and (b) there is absorption or fatty degeneration of osseous trabecule, and notably of the calcar femorale. These trabecular run in two series-the first from the lower part of the neck near the small trochanter to the upper part of the head ; and the second from below the great trochanter to the lower part of the head, thus forming together a bracket-shaped arrangement to distribute the weight. The calcar femorale runs from near the small trochanter to the under surface of the head. This fracture is rarely ^impacted, but when it is, the narrow compact neck is driven into the broad cancellous head. The head is supplied with ˛lood from the neck, cervical ligaments, and the ligamen-turn teres, but if the two first sources of blood-supply are cut off, the last is not sufficient to promote osseous union.
An extracapsular fracture of the neck of the femur is anatomically impossible in front, where the capsule is inserted into the intertrochanteric line, but is possible behind. Generally such fractures are extracapsular behind and intracapsular in front, or even, owing to the thickness of the capsule at that part, intracapsular. Such fractures are generally caused by considerable violence, occur usually in males about middle life, and impaction is common, the contracted neck being driven into the upper end of the shaft and trochanter, which may be split by it.
Certain symptoms are associated with both forms of fracture of the neck of the femur : Shortening is due to the glutei, hamstrings, adductors, rectus, etc. Eversiˇn is due to the weight of the limb, the centre of gravity of which lies to the outer side ; to the action of the ilio-psoas, adductors, pectineus, and small rotators ; and to the fracture being generally more extensive posteriorly than in front, owing to the more fragile nature of the bone posteriorly. Shortening is evidenced by direct measurement, by the rising of the great trochanter above Nelaton's line, and by relaxation of the fascia lata stretching between the trochanter and the iliac crest. Shortening is generally greater in extra- than in intracapsular fractures. A fulness is sometimes produced just under Poupart's ligament, either by effused blood or the broken fragments pushing the capsule forwards.
Dislocation of the hip is generally due to violence, but may rarely be due to muscular action, while it is a frequent sequela of hip disease, and is not infrequently congenital. The only portion of the acetabulum which is shallow, and where the rim is deficient, is below, in the region of the cotyloid notch, and here the capsule also is rather weak. When the limb is markedly abducted, the head of the bone tends to glide out of the socket, and come against this weak portion of acetabulum and capsule ; and if the abduction be increased, the great trochanter hitches on the summit of the acetabulum, acts as a fulcrum, and so enables the head to burst through the capsule toward the thyroid foramen. It is believed that all dislocations of the hip are produced when the limb is in this abducted position, and are primarily downwards ; the lower part of the capsule is torn, extending from about the cotyloid notch to near the small trochanter, and thence along the back of the neck ; the ligamentum teres is torn, but the Y-shaped ligament remains intact. Four typical forms of dislocation of the hip are described, which, resulting from the original downward displacement, depend largely upon the character of the dislocating force and upon the intact Y-shaped ligament.
The most common dislocation of the hip is that backwards on to the dorsum, and is produced by a position of flexion and internal rotation of the thigh on the pelvis, assisted perhaps by the glutei, hamstring, and adductor muscles. The head of the bone lies on the dorsum above the tendon of the obturator internus. The limb is shortened ; it is flexed, adducted, and rotated inwards, abduction and rotation outwards being impossible ; the ilio-psoas is much stretched, and the quadratus femoris, pyriformis, obturator, and gemelli lacerated, even the pectineus and glutei frequently suffering, while the great sciatic nerve may be compressed. The flexion is due to tension upon the Y-shaped ligament and ilio-psoas, the adduction and inversion to the altered position of the head, the Y-shaped ligament remaining intact. The gluteal fold is raised, the depression behind the great trochanter is obliterated, and the head may appear as a fulness of the buttock. Dislocation backwards on to the ischium is similar in mode of production and symptoms to the former, the flexion and inward rotation at the time of production being more marked, while the head ultimately lies on the ischium near the spine, and below the tendon of the obturator internus. This form is sometimes referred to as dislocation into the sciatic notch ; it is doubtful if such a displacement ever occurs.