This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Dislocations of the hip are either anterior or posterior (Allis)1. If the innominate bone is held horizontally it will be seen that the Roser-Nelaton line from the tuberosity to the anterior superior spine passes through the acetabulum. It forms the apex of a wedge the two sides of which pass down, one anteriorly and the other posteriorly (Fig. 512). Therefore when the head of the femur leaves its socket it passes down either anteriorly or posteriorly and we have either an anterior or a posterior luxation.
1 Dr. Oscar H. Allis has given the clearest exposition of dislocations of the hip with which we are acquainted in his Gross Prize Essay entitled," An Inquiry into the Difficulties Encountered in the Reduction of Dislocations of the Hip," Philadelphia, 1896.
The attachment of the iliofemoral ligament immediately above the acetabulum and of the ischiofemoral directly below also tend to prevent the head's emerging at these places and favor its going anteriorly or posteriorly. Anterior luxations may be either low or high. The primary luxation is a low one into the thyroid foramen.
Fig. 512. - Innominate bone, resting on its inner side, to show the wedge-shaped formation of its outer surface. The apex of the wedge is Nelaton's line, running from the anterior superior spine to the tuberosity of the ischium; the anterior plane inclines downward and forward toward the pubis and the posterior plane inclines downward and backward on the ilium.
If then the thigh is rotated outward the head rises, and it becomes a pubic luxation. Posterior luxations may also be either high or low. The primary luxation is a low one either on the spine of the ischium or in the sciatic notch, and by rotation of the thigh inward it becomes a high one on the dorsum of the ilium (Fig. 513). In certain very rare cases in which there has been an excessive amount of twisting the rotation is extreme and a form of dislocation called inverted is produced; it will be explained later. Mechanism of the Production of Luxations. - The following should be borne in mind:
1. The neck of the femur makes with the shaft an angle of approximately 128 degrees.
2. In speaking of inward and outward rotation is meant inward and outward rotation of the shaft of the femur. Thus if the head (and neck) is pointing inward and we rotate the shaft inward, the head rotates outward posteriorly. If, however, we rotate the shaft inward while the head is pointing outward then the head moves inward anteriorly. Thus it is seen that in rotating the shaft inward the head is moved inward or outward according to its original position.
3. That while actually the axis of the head and neck does not coincide with a line drawn transversely through the condyles, but inclines forward at an angle of 10 or 12 degrees, nevertheless for practical purposes we may consider that it does so coincide and normally points directly inward.
4. The position of the greater trochanter can be recognized by its being directly above the external condyle, and the position of the head by its being directly above the internal condyle.
5. The muscles may be disregarded in the production of luxations, and the action of only the bones and ligaments considered.
6. A luxation results from the capsule being made tense or even ruptured by a leverage action of the bones, and the head then being thrust out on the anterior or posterior plane.
Fig. 513. - Diagram illustrating the position of the head in high and low dislocations on the anterior and posterior planes.
7. The primary luxation is a low anterior or posterior one. This may be changed by subsequent rotation of the, thigh.
8. Luxations may occur either when the thigh is in abduction or adduction. Luxation by Abduction. - If the thigh is forcibly abducted the adductor muscles rupture and. the abduction increasing, the head is raised out of the socket by the lever action of the femur as its neck strikes the rim of the acetabulum and its greater trochanter the ilium above. The head and neck are the short arm of the lever, the rim of the acetabulum or ilium is the fulcrum, and the shaft and distal extremity of the femur are the long arm. The head rises from the socket, ruptures a part at least of the capsular ligament, and then a thrusting force is added which pushes the head forward, producing a thyroid luxation (Fig. 514).
Fig. 514. - Luxation of the hip by indirect or leverage action. The shaft of the femur, from the greater trochanter out. is the long arm of the lever, the head and neck form the short arm and the upper edge of the acetabulum and ilium immediately above is the fulcrum. When the femur is abducted the head is lifted out of its socket rupturing the capsular ligament.
If while the limb is hyperabducted the shaft of the femur is rotated out and the limb brought straight down, parallel with that of the opposite side, then likewise the head may pass forward into the thyroid or pubic position. If while the head is on the anterior plane the thigh is flexed and the shaft rotated inward, then the head follows around the outer edge of the acetabulum and passes from a thyroid to a dorsal position, forming a posterior luxation.
Fig. 515. - Posterior luxation of the hip produced by rotation and direct thrust. The femur is seen to be flexed on the pelvis, adducted and rotated inward; a thrust in the direction of the arrow then sends the head out of the acetabulum onto the posterior plane.