The hip is the most typical example of a ball-and-socket joint in the body, the round liead of the femur being received into the cup-shaped cavity of the acetabulum.

Class. - Diarthrosis. Subdivision. - Enarthrodia.

Both articular surfaces are coated with cartilage, that covering the head of the femur being thicker above where it has to bear the weight of the body, and thinning out to a mere edge below; the pit for the ligamentum teres is the only part uncoated, but the cartilage is somewhat heaped up around its margin.

Covering the acetabulum, the cartilage is horseshoe-shaped, and thicker above than below, being deficient over the depression at the bottom of the acetabulum, where a mass of fatty tissue - the so-called synovial or Haversian gland is lodged. 

The ligaments of the joint are:

  1. Articular capsule.
  2. Ligamentum teres.
  3. Transverse.
  4. Glenoid lip.

Anterior View of the Articitlar Capsule op the Hip-joint. 

The articular capsule is one of the strongest ligaments in the body. It is large and somewhat loose, so that in every position of the body some portion of it is relaxed. At the pelvis it is attached, superiorly, to the base of the anterior inferior iliac spine; curving backward, it becomes blended with the deep surface of the reflected tendon of the rectus Jemoris; posteriorly, it is attached a few millimetres from the acetabular rim; and below, to the upper edge of the groove between the acetabulum and tuberosity of the ischium. Thus it reaches the transverse ligament, being firmly blended with its outer surface, and frequently sends fibers beyond the notch to blend with the obturator membrane. Anteriorly it is attached to the pubis near the obturator notch, to the ilio-pectineal eminence and thence backward to the base of the inferior iliac spine.

A thin strong stratum is given off from its superficial aspect behind; this extends beneath the gluteus minimus and small rotators, to be attached above to the dorsum of the ihum higher than the reflected tendon of the rectus, and posteriorly to the ilium and ischium nearly as far as the sciatic notch. As this expansion passes over the long tendon of the rectus, the tendon may be described as being in part contained within the substance of the capsule.

At the femur, the capsule is fixed to the anterior portion of the upper border of the great trochanter and to the cervical tubercle. Thence it runs down, the intertrochanteric line as far as the medial border of the femur, where it is on a level with the lower part of the lesser trochanter. It then runs upward and backward along an oblique line about 1.6 cm. (f in.) in front of the lesser trochanter, and continues its ascent along the back of the neck nearly parallel to the intertrochanteric crest, and from 12 to 16 mm. (| to f in.) above it; finally, it passes along the medial side of the trochanteric fossa to reach the anterior superior angle of the great trochanter.

On laying open the capsule, some of the deeper fibers are seen reflected upward along the neck of the femur, to be attached much nearer the head: these are the retinacula. One corresponds to the upper, and another to the lower, part of the intertrochanteric line; a third is seen at the upper and back part of the neck. They form flat bands,which lie on the femoral neck.

Superadded to the capsule, and considerably strengthening it, are three auxiliary bands, whose fibers are intimately blended with, and in fact form part of, the capsule, viz., the ilio-femoral, ischio-capsular, and pubo-capsular ligaments. 

Upper Extremity of the Femur (Anterior View), to show the Relation OP THE Articular Capsule op the Hip-joint (in red) to the Epiphysial Lines.

The ilio-femoral ligament is the longest, widest, and strongest of the bands. It is of triangular shape, with the apex attached above to a curved line on the iUum immediately below and behind the anterior inferior spine, and its base below to the anterior edge of the greater trochanter and to the spiral line as far as the medial border of the shaft. The highest or most lateral fibers are coarse, almost straight, and shorter than the rest; the most medial fibers are also thick and strong, but obhque. This varying obliquity of the fibers, and their accumulation at the borders, explain why this band has been described as the Y-shaped ligament; but it should be noted that the Y is inverted. About the center of its base, near the femoral attachment, is an aperture transmitting an articular twig from the ascending branch of the external circumflex artery.

The ischio-capsular ligament is formed of very strong fibers attached all along the upper border of the groove for the external obturator, and to the ischial margin of the acetabulum above the groove. The highest of these incline a little upward as they pass laterally to be fixed to the greater trochanter in front of the insertion of the piriformis tendon, while the other fibers curve more and more upward as they pass laterally to their insertion at the inner side of the trochanteric fossa, blending with the insertion of the external rotator tendons. When the joint is in flexion, these fibers pass in nearly straight lines to their femoral attachment, and spread out uniformly over the head of the femur; but in extension they wind over the back of the femur in a zonular manner [zona orbicularis], embracing the posterior aspect of the neck of the femur.

The pubo-capsular (pectineo-femoral) band is a distinct but narrow set of fibers which are individually less marked than the fibers of the other two bands; they are fixed above to the obturator crest and to the anterior border of the iUo-pectineal eminence, reaching as far down as the pubic end of the acetabular notch. Below, they reach the neck of the femur, and are fixed above and behind the lowermost fibers of the iho-femoral band, with which they blend.

Posterior View of the Articular Capsule of the Hip-joint.

 

Section through the Hip-joint, showing the Glenoid Lip, Ligamentum Teres, and Retinacula 

In thickness and strength the capsule varies greatly; thus, if two lines be drawn, one from the anterior inferior spine to the medial border of the femur near the lesser trochanter, and the other from the anterior part of the groove for the external obturator to the trochanteric fossa, all the ligament between these lines on the lateral and upper aspects of the joint is very thick and strong, while that below and to the medial side, except at the narrow pubo-capsular ligament, is thin and weak, so that the head of the bone can be seen through it. The capsule is thickest in the course of the iho-femoral ligament, toward the lateral part of which it measures over 6 mm. Between the ilio-femoral and ischio-capsular ligaments the capsule is very strong, and with it here, near the acetabulum, is incorporated the reflected tendon of the rectus, and here also a triangular band of fibers runs downward and forward to be attached by a narrow insertion to the ridge on the front border of the greater trochanter near the gluteus minimus (the ilio-trochanteric band).

The capsule is strengthened also at this point by a strong band from the under surface of the gluteus minimus, and by the tendino-trochanteric band which passes down from the reflected tendon of the rectus to the vastus lateralis (externus). This is closely blended with the capsule near the lateral edge of the ilio-femoral Ugament.

The thinnest part of the capsule is between the pubo-capsular and ilio-femoral ligaments; this is sometimes perforated, allowing the bursa under the psoas to communicate with the joint. The capsule is also very thin at its attachment to the back of the femoral neck, and again opposite the acetabular notch.

 

Hip-joint after Dividing the Aeticulab Capsule and Disarticulating the Femur.

The ligamentum teres is an interarticular flat band which extends from the acetabular fossa to the head of the femur, and is usually about 3.7 cm. (1| in.) long. It has two bony attachments, one on either side of the acetabular notch immediately below the articular cartilage, while intermediate fibers spring from the lower surface of the transverse ligament. The ischial portion is the stronger, and has several of its fibers arising outside the cavity, below and in connection with the origin of the transverse ligament, where it is also continuous with the capsule and periosteum of the ischium. At the femur it is fixed to the front part of the depression on the head, and to the cartilage round the margin of the depression.

It is covered by a prolongation of synovial membrane, which also covers the cushion of fat in the recess of the acetabulum; the portion of the membrane reflected over the fatty tissue does not cling closely to the round hgament, but forms a triangular fold, the apex of which is at the femur.

The transverse ligament passes across the acetabular notch and converts it into a foramen; it supports part of the glenoid fibro-cartilage, and is connected with the ligamentum teres and the capsule. It is composed of decussating fibers, which arise from the margin of the acetabulum on either side of the notch, those coming from the pubis being more superficial, and passing to form the deep part of the ligament at the ischium, while those superficial at the ischium are deep at the pubis. It thus completes the rim of the acetabulum. 

Portions Of Ischium And Pubis, Showing The Acetabular Notch And The Ligamentum Teres Attached Outside The Acetabulum. 

The glenoid lip (cotyloid fibro-cartilage) is a yellowish-white structure, which deepens the acetabulum by surmounting its margin. It varies in strength and thickness, but is stronger at its iliac and ischial portions than elsewhere. Its base is broad and fixed to the bony rim as well as to the articular cartilage of the acetabulum on the inner, and the periosteum on the outer, side of it, and blends inseparably with the transverse ligament which supports it over the acetabular notch.

 

The Upper Extremity op the Femur (Posterior View), to show the Relation of the Articular Capsule op the Hip-joint (in red) to the Epiphysial Lines.

Its free margin is thin; on section it is somewhat lunated, having its outer surface convex and its articular face concave and very smooth in adaptation to the head of the bone, which it tightly embraces a little beyond its greatest circumference. It somewhat contracts the aperture of the acetabulum, and retains the head of the femur within its grasp after division of the muscles and capsular hgament. It is covered on both aspects by synovial membrane.

The synovial membrane lines the capsule and both surfaces of the glenoid lip, and passes over the border of the acetabulum to reach and cover the fatty cushion it contains. The part covering the fatty cushion is unusually thick, and is attached round the edges of the rough bony surface on which the cushion rests.

The membrane is loosely reflected off this on to the ligamentum teres, along which it is prolonged to the head of the femur; thus the fibers of the round ligament are shut out from the joint cavity. From the capsule the synovial membrane is also reflected below on to the neck of the femur, whence it passes over the retinacula to the margin of the articular cartilage. A fold of synovial membrane on the under aspect of the neck often conveys to the head of the femur a branch of an artery - generally a branch of the medial circumflex.

Blood supply

The arterial supply comes from - (a) the transverse branches of the medial and lateral circumflex arteries; (6) the lateral branch of the obturator sends a branch through the acetabular notch beneath the transverse ligament, which ramifies in the fat at the bottom of the acetabulum, and travels down the round ligament to the head of the femur; (c) the inferior branch of the deep division of the superior gluteal; and (d) the inferior gluteal (sciatic) arteries. The branch from the obturator to the ligamentum teres is sometimes very large when the branch from the medial circumflex does not also supply the ligament.

Ligamentum Teres, lax in Flexion.

The superior and inferior gluteal send several branches through the innominate attachment of the articular capsule: these anastomose freely beneath the capsule around the outer aspect of the acetabulum, and supply some branches to enter the bone, and others which enter the substance of the glenoid lip. There is quite an arterial crescent upon the posterior and postero-superior portions of the acetabulum; but no vessels are to be seen on the inner aspect of the glenoid lip. 

The nerve-supply comes from - (a) femoral (anterior crural), (6) anterior division of the obturator, (c) the accessory obturator, and (d) the sacral plexus, by a twig from the nerve to the quadratus femoris, or from the upper part of the great sciatic, or from the lower part of the sacral plexus.

Relations. - In front and in contact with the capsule are the psoas bursa, the tendinous part of the psoas magnus, and the Uiacus. StiU more anteriorly and not in contact are the femoral artery, the femoral (anterior crural) nerve, the rectus femoris, the sartorius, and the tensor fasciae latse. 

Above and in close relation with the capsule are the piriformis, the obturator internus and the gemelli, and the reflected head of the rectus femoris, whilst more superficially lie the gluteus minimus and medius.

Behind and in close relation with the capsule are the obturator externus, the gemelli and obturator internus, and the piriformis. More superficially he the quadratus femoris, the sciatic nerves, and the gluteus maximus. 

Below the obturator externus, the pectineus, and the medial circumflex artery are in close relation with the capsule.

The movements

The hip-joint, like the shoulder, is a ball-and-socket joint, but with a much more complete socket and a corresponding limitation of movement. Each variety of movement is permitted, viz., flexion, extension, abduction, adduction, circumduction, and rotation; and any two or more of these movements not being antagonistic can be combined, i. e., flexion or extension associated with abduction or adduction can be combined with rotation in or out.

It results from the obliquity of tne neck of the femur that the movements of the head in the acetabulum are always more or less of a rotatory character. This is more especially the case during flexion and extension, and two results follow from it. First, the bearing surfaces of the femur and acetabulum preserve their apposition to each other, so that the amount of articular surface of the head in the acetabulum does not sensibly diminish pari passu with the transit of the joint from the extended to the flexed position, as would necessarily be the case if the movement of the femoral head, like that of the thigh itself, was simply angular, instead of rotatory and angular. Secondly, as rotation of the head can continue until the ligaments are tight without being checked by contact of the neck of the thigh bone with the rim of the acetabulum, flexion of the thigh so far as the joint is concerned is practically unlimited. Flexion is the most important, most frequent, and most extensive movement, and in the dissected limb, before the ligaments are disturbed, can be carried to 160°, and is then checked by the lower fibers of the ischio-capsular ligament. In the living subject simple flexion can continue until checked by the contact of the soft parts at the groin, if the knee be bent; if the knee be straight, flexion of the hip is checked in most persons by the hamstring muscles at nearly a right angle. This is very evident on trying to touch the ground with the fingers without bending the knees, the chief strain being felt at the popliteal space. This is due to the shortness of the] hamstrings. Extension is limited by the ilio-femoral ligament.

Ligamentum Teres, very lax in Complete Extension.

 

Abduction and lateral rotation can be performed freely in every position of flexion and extension - abduction being limited by the pubo-capsular hgament; lateral rotation by the ilio-femoral Ugament, especially its medial portion, during extension; but by the lateral portion, as well as by the ligamentum teres, during flexion.

Adduction is very limited in the extended thigh on account of the contact with the opposite limb. In the slightly flexed position adduction is more free than in extension, and is then limited by the lateral fibers of the ilio-femoral band and the superior portion of the capsule. In flexion the range is still greater, and limited by the ischio-capsular hgament, the hgamentum teres being also rendered nearly tight. Medial rotation in the extended position is limited by the lower fibers of the ilio-femoral ligament; and in flexion by the ischio-capsular ligament and the portion of the capsule between it and the ilio-femoral band. 

The ilio-femoral band also prevents the tendency of the trunk to roU backward on the thigh bones in the erect posture, and so does away with the necessity for muscular power for this purpose; it is put on stretch in the stand-at-ease position.

The ligamentum teres is of little use in resisting violence or in imparting strength to the joint. It assists in checking lateral rotation, and adduction during flexion. A ligament can only be of use when it is tight, and it was found by trephining the bottom of the acetabulum, removing the fat, and threading a piece of whipcord round the ligament, that the ligament was slack in simple flexion, and very loose in complete extension, but that its most slack condition was in abduction. It is tightest in flexion combined with adduction and lateral rotation and almost as tight in flexion with lateral rotation alone, and in flexion with adduction alone.

Ligamentum Teres, drawn Tight in I'lexion Combined with Lateral Rotation and Adduction.

Muscles which act upon the hip-joint

Flexors

The psoas and iliacus, the rectus femoris, the pectineus, the adductors, the sartorius, the tensor fasciae latse, and the gluteus medius.

Extensors

The gluteus maximus, the posterior fibers of the glutei medius and miQimus, the biceps, the semitendinosus, the semimembranosus, and the ischial fibers of the adductor magnus; also (slightly) the piriformis, obturator internusand gemelli.

Abductors

Gluteus maximus (upper fibers), tensor fasciae latae, gluteus medius, gluteus minimus, and, when the joint is flexed, the pii-iformis, obturator internus, the gemelli, and the sartorius also become abductors.

Adductors

Adductores magnus, longus, brevis, and minimus, semitendinosus, biceps the gracilis, the peotineus, the quadratus femoris, and the lower fibers of the gluteus maxunus.

Medial rotators

Psoas (slightly), adductor magnus, semimembranosus, the anterior fibers of the gluteus medius and minimus, and the tensor fascise latae.

Lateral rotators

Gluteus maximus, posterior fibers of gluteus medius and minimus, the adductors, obturator extemus, quadratus femoris, obturator internus, the gemelli, and the piriformis when the joint is extended.

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