DR. DIBYENDUNARAYAN BID [PT]T H E S A R VA J A N I K C O L L E G E O F P H Y S I O T H E R A P Y,

R A M P U R A , S U R AT

Biomechanics of the

Hip Complex

Introduction

The hip joint, or coxofemoral joint, is the articulation of the acetabulum of the pelvis and the head of the femur (Fig. 10-1).

These two segments form a diarthrodial ball-and-socket joint with three degrees of freedom: flexion/extension in the sagittal plane, abduction/adduction in the frontal plane, and Medial/lateral rotation in the transverse plane.

Although the hip joint and the shoulder complex have a number of common features, the functional and structural adaptations of each to its respective roles have been so extensive that such comparisons are more of general interest than of functional relevance.

The role of the shoulder complex is to provide a stable base on which a wide range of mobility for the hand can be superimposed.

Shoulder complex structure gives precedence to open-chain function.

The primary function of the hip joint is to support the weight of the head, arms, and trunk (HAT) both in static erect posture and in dynamic postures such as ambulation, running, and stair climbing.

The hip joint, like the other joints of the lower extremity that we will examine, is structured primarily to serve its weight-bearing functions.

Although we examine hip joint structure and function as if the joint were designed to move the foot through space in an open chain, hip joint structure is more influenced by the demands placed on the joint when the limb is bearing weight.

As we shall see later in this chapter, weight-bearing function of the hip joint and its related weight-bearing responses are basic to understanding the hip joint and the interactions that occur between the hip joint and the other joints of the spine and lower extremities.

Structure of Hip JointProximal Articular Surfaces

The cuplike concave socket of the hip joint is called the acetabulum and is located on the lateral aspect of the pelvic bone (innominate or os coxa).

Three bones form the pelvis: the ilium, the ischium, and the pubis. Each of the three bones contributes to the structure of the acetabulum (Fig. 10-2A).

The pubis forms one fifth of the acetabulum, the ischium forms two fifths, and the ilium forms the remainder.

Until full ossification of the pelvis occurs between 20 and 25 years of age, the separate segments of the acetabulum may remain visible on radiograph (see Fig. 10-2B).

The acetabulum appears to be a hemisphere, but only its upper margin has a true circular contour, and the roundness of the acetabulum as a whole decreases with age.

In actuality, only a horseshoe-shaped portion of the periphery of the acetabulum (the lunate surface) is covered with hyaline cartilage and articulates with the head of the femur (see Fig. 10-2A).

The inferior aspect of the lunate surface (the base of the horseshoe) is interrupted by a deep notch called the acetabular notch. The acetabular notch is spanned by a fibrous band, the transverse acetabular ligament, that connects the two ends of the horseshoe.

The transverse acetabular ligament also spans the acetabular notch to create a fibro-osseous tunnel, called the acetabular fossa, beneath the ligament, through which blood vessels may pass into the central or deepest portion of the acetabulum.

The acetabulum is deepened by the fibrocartilaginous acetabular labrum, which surrounds the periphery.

The acetabular fossa is nonarticular; the femoral head does not contact this surface (Fig. 10-3).

The acetabular fossa contains fibroelastic fat covered with synovial membrane.

Center Edge Angle of the AcetabulumEach acetabulum, in addition to its obvious lateral

orientation, is oriented on each innominate bone some-what inferiorly and anteriorly.

The magnitude of inferior orientation is assessed on radiograph by using a line connecting the lateral rim of the acetabulum and the center of the femoral head. This line forms an angle with the vertical known as the center edge (CE) angle or the angle of Wiberg (see Fig. 10-3) and is the amount of inferior tilt of the acetabulum.

The inferior tilt is essentially a measure of the amount of coverage or “roof” there is over the femoral head.

Using computed tomography (CT), Adna and associates found CE angles in adults to average 38° in men and 35 in women (with ranges in both sexes to be about 22° to 42°).

Other investigators, using radiographs, have found the CE angles to be similar between men and women, and across ages groups in women (25 to 65 years old).

The similarity of the CE angle between men and women is somewhat surprising, given the increased diameter and more vertical orientation of the sides of the female pelvis.

There is also evidence that the CE angle increases from childhood to skeletal maturity.

The implication is that young children have relatively less coverage over the head of the femur and, therefore, relatively decreased joint stability than do adults.

Genda and colleagues used radiographs and modeling to conclude that there was a significant positive correlation (r = 0.678) between CE angle and joint contact area.

Acetabular Anteversion

The acetabulum faces not only somewhat inferiorly but also anteriorly. The magnitude of anterior orientation of the acetabulum may be referred to as the angle of acetabular anteversion.

Adna and associates found the average value to be 18.5° for men and 21.5° for women, although Kapandji cited larger values of 30° to 40°.

Pathologic increases in the angle of acetabular anteversion are associated with decreased joint stability and increased tendency for anterior dislocation of the head of the femur.

Acetabular LabrumGiven the need for stability at the hip joint, it is not

surprising to find an accessory joint structure. The entire periphery of the acetabulum is rimmed by a ring of wedge-shaped fibrocartilage called the acetabular labrum (see labrum cross-section in Fig. 10-3).

The labrum is attached to the periphery of the acetabulum by a zone of calcified cartilage with a well-defined tide-mark.

The acetabular labrum not only deepens the socket but also increases the concavity of the acetabulum through its triangular shape and grasps the head of the femur to maintain contact with the acetabulum.

Although the labrum appears to broaden the articular surface of the acetabulum, experimental evidence suggests that load distribution in the acetabulum is not affected by removal of the labrum.

Histological examination demonstrated free nerve endings and sensory receptors in the superficial layer of the labrum, as well as vascularization from the adjacent joint capsule only in the superficial third of the labrum.

The evidence suggests that the labrum is not load-bearing but serves a role in proprioception and pain sensitivity that may help protect the rim of the acetabulum.

Ferguson and colleagues found that hydrostatic fluid pressure within the intra-articular space was greater within the labrum than without, which suggests that the labrum may also enhance joint lubrication if the labrum adequately fits the femoral head.

The transverse acetabular ligament is considered to be part of the acetabular labrum, although, unlike the labrum, it contains no cartilage cells.

Although it is positioned to protect the blood vessels traveling beneath it to reach the head of the femur, experimental data do not support the notion of the transverse acetabular ligament as a load-bearing structure.

Konrath and colleagues supported the hypothesis of others that the ligament served as a tension band between the anteroinferior and posteroinferior aspects of the acetabulum (the “feet” of the horseshoe-shaped articular surface) but were not able to corroborate this from their data.

Distal Articular Surface

The head of the femur is a fairly rounded hyaline cartilage-covered surface that may be slightly larger than a true hemisphere or as much as two thirds of a sphere, depending on body type.

The head of the femur is considered to be circular, unlike the more irregularly shaped acetabulum.

The radius of curvature of the femoral head is smaller in women than in men in comparison with the dimensions of the pelvis.

Just inferior to the most medial point on the femoral head is a small roughened pit called the fovea or fovea capitis (Fig. 10-4).

The fovea is not covered with articular cartilage and is the point at which the ligament of the head of the femur is attached.

The femoral head is attached to the femoral neck; the femoral neck is attached to the shaft of the femur between the greater trochanter and the lesser trochanter. The femoral neck is, in general, only about 5 cm long.

The femoral neck is angulated so that the femoral head most commonly faces medially, superiorly, and anteriorly.

Although the angulation of the femoral head and neck on the shaft is more consistent across the population than is angulation of the humeral head and neck on its shaft, there are still substantial individual differences and differences from side to side in the same individual.

Angulation of the Femur

There are two angulations made by the head and neck of the femur in relation to the shaft.

One angulation (angle of inclination) occurs in the frontal plane between an axis through the femoral head and neck and the longitudinal axis of the femoral shaft.

The other angulation (angle of torsion) occurs in the transverse plane between an axis through the femoral head and neck and an axis through the distal femoral condyles.

The origin and variability of these angulations can be understood in the context of the embryonic development of the lower limb.

In the early stages of fetal development, both upper extremity and lower extremity limb buds project laterally from the body as if in full abduction.

During the seventh and eighth weeks of gestational age and before full definition of the joints, adduction of the buds begins.

At the end of the eighth week, the “fetal position” has been achieved, but the upper and lower limbs are no longer positioned similarly.

Although the upper limb buds have undergone torsion somewhat laterally (so that the ventral surface of the limb bud faces anteriorly), the lower limb buds have undergone torsion medially, so that the ventral surface faces posteriorly.

The result for the lower limb is critical to understanding function.

The knee flexes in the opposite direction from the elbow, and the extensor (dorsal) surface of the lower limb is anteriorly rather than posteriorly located.

Although the head and neck of the femur retain the original position of the limb bud, the femoral shaft is inclined medially and undergoes medial torsion with regard to the head and neck.

The magnitude of medial inclination and torsion of the distal femur (with regard to the head and neck) is dependent on embryonic growth and, presumably, fetal positioning during the remaining months of uterine life.

The development of the angulations of the femur appear to continue after birth and through the early years of development.

Angle of Inclination of the Femur

The angle of inclination of the femur averages 126° (referencing the medial angle formed by the axes of the head/neck and the shaft), ranging from 115° to 140° in the unimpaired adult (Fig. 10-5).

As with the angle of inclination of the humerus, there are variations not only among individuals but also from side to side. In women, the angle of inclination is somewhat smaller than it is in men, owing to the greater width of the female pelvis.

With a normal angle of inclination, the greater trochanter lies at the level of the center of the femoral head.

The angle of inclination of the femur changes across the life span, being substantially greater in infancy and childhood (see Fig. 10-2B) and gradually declining to about 120° in the normal elderly person.

A pathologic increase in the medial angulation between the neck and shaft is called coxa valga (Fig. 10-6A), and a pathologic decrease is called coxa vara (see Fig. 10-6B).

Angle of Torsion of the Femur

The angle of torsion of the femur can best be viewed by looking down the length of the femur from top to bottom.

An axis through the femoral head and neck in the transverse plane will lie at an angle to an axis through the femoral condyles, with the head and neck torsioned anteriorly (laterally) with regard to an angle through the femoral condyles (Fig. 10-7).

This angulation reflects the medial rotatory migration of the lower limb bud that occurred during fetal development. The apparent contradiction between medial torsion of the embryonic limb bud and lateral torsion of the femoral head and neck simply reflects a shift in reference.

In medial torsion of the limb bud, the proximal end is fixed and the distal end migrates medially.

When torsion of the femur is assessed in a child or adult, the reference is an axis through the femoral condyles (the knee joint axis) that is generally presumed to lie in the frontal plane.

If the axis through the femoral condyles lies in the frontal plane (as it functionally should), then the head and neck of the femur are torsioned anteriorly, relatively speaking, on the femoral condyles.

The angle of torsion decreases with age. In the newborn, the angle of torsion has been estimated to be 40°, decreasing substantially in the first 2 years.

Svenningsen and associates found a decrease of approximately 1.5° per year until cessation of growth among children with both normal and exaggerated angles of anteversion.

In the adult, the normal angle of torsion is considered to be 10° to 20°.

Noble and colleagues used three-dimensional analysis of CT scans on 54 women without impairments whose ages ranged from 18 to 82 years. These investigators found an average anterior torsion of 35.6° (13.7°), which indicated a greater variation than is ordinarily appreciated.

A pathologic increase in the angle of torsion is called anteversion (Fig. 10-8A and 10-8B), and a pathologic decrease in the angle or reversal of torsion is known as retroversion (see Fig. 10-8 C).

There may not be one angulation at which pathologic femoral torsion may be diagnosed, given the substantial normal variability.

Heller and colleagues used an angle of 30° to model effects of anteversion, acknowledging that children with cerebral palsy have demonstrated angles of 60° or more.

Noble and colleagues found an average angle of 42.3° (16°) among 154 women diagnosed with developmental hip dysplasia who had not had surgical intervention.

It should be recognized that both normal and abnormal angles of inclination and torsion of the femur are properties of the femur alone (i.e., both can be measured or assessed independently of the continuous bones, as in Fig. 10-8).

However, abnormalities in the angulations of the femur can cause compensatory hip changes and can substantially alter hip joint stability, the weight-bearing biomechanics of the hip joint, and muscle biomechanics.

Although some structural deviations such as femoral anteversion and coxa valga are commonly found together, each may occur independently of the other.

Each structural deviation warrants careful consideration as to the impact on hip joint function and function of the joints both proximal and distal to the hip joint.

As shall be evident when the knee and foot are discussed in subsequent chapters, femoral anteversion is often implicated in dysfunction at both the knee and at the foot.

The other pathologic angulations of the femur (retroversion, coxa vara, and coxa valga) similarly affect the hip joint and other joints proximally and distally.

The impact of abnormal angulations of the femur on hip joint function will continue to be discussed in this chapter.

Articular Congruence

The hip joint is considered to be a congruent joint.

However, there is substantially more articular surface on the head of the femur than on the acetabulum.

In the neutral or standing position, the articular surface of the femoral head remains exposed anteriorly and somewhat superiorly (Fig. 10-9A).

The acetabulum does not fully cover the head superiorly, and the anterior torsion of the femoral head (angle of torsion) exposes a substantial amount of the femoral head’s articular surface anteriorly.

Articular contact between the femur and the acetabulum can be increased in the normal non-weight-bearing hip joint by a combination of flexion, abduction, and slight lateral rotation (see Fig. 10-9B).

This position (also known as the frog-leg position) corresponds to that assumed by the hip joint in a quadruped position and, according to Kapandji, is the true physiologic position of the hip joint.

Konrath and colleagues concluded both from their work and from evidence in the literature that the hip joint actually functions as an incongruent joint in non–weight-bearing, given the larger femoral head.

In weight-bearing, the elastic deformation of the acetabulum increases contact with the femoral head, with primary contact at the anterior, superior, and posterior articular surfaces of the acetabulum.

An additional contribution to articular congruence and coaptation of joint surfaces may be made by the nonarticular and non-weight-bearing acetabular fossa.

The acetabular fossa may be important in setting up a partial vacuum in the joint so that atmospheric pressure contributes to stability by helping maintain contact between the femoral head and the acetabulum.

Wingstrand and colleagues concluded that atmospheric pressure in hip flexion activities played a stronger role in stabilization than capsuloligamentous structures.

It is also true that the head and acetabulum will remain together in an anesthetized patient even after the joint capsule has been opened. The pressure within the joint must be broken before the hip can be dislocated.

Hip Joint Capsule and Ligaments

Hip Joint Capsule

Unlike the relatively weak articular capsule of the shoulder, the hip joint capsule is a substantial contributor to joint stability.

The articular capsule of the hip joint is an irregular, dense fibrous structure with longitudinal and oblique fibers and with three thickened regions that constitute the capsular ligaments.

The capsule is attached proximally to the entire periphery of the acetabulum beyond the acetabular labrum.

Fibers near the proximal attachment are aligned in a somewhat circumferential manner.

The capsule itself is thickened anterosuperiorly, where the predominant stresses occur; it is relatively thin and loosely attached posteroinferiorly, with some areas of the capsule thin enough to be nearly translucent.

The capsule covers the femoral head and neck like a cylindrical sleeve and attaches to the base of the femoral neck.

The femoral neck is intracapsular, whereas both the greater and lesser trochanters are extracapsular.

The synovial membrane lines the inside of the capsule.

Anteriorly, there are longitudinal retinacular fibers deep in the capsule that travel along the neck toward the femoral head.

The retinacular fibers carry blood vessels that are the major source of nutrition to the femoral head and neck.

The retinacular blood vessels arise from a vascular ring located at the base of the neck and formed by the medial and lateral circumflex arteries (branches of the deep femoral artery).

As with the other joints already described, there are numerous bursae associated with the hip joint.

Although as many as 20 bursae have been described, there are commonly recognized to be three primary or important bursae.

Because the bursae are more strongly associated with the hip joint muscles rather than its capsule, the bursae will be described with their corresponding musculature.

Hip Joint Ligaments

The ligamentum teres is an intra-articular but extrasynovial accessory joint structure. The ligament is a triangular band attached at one end to both sides of the peripheral edge of the acetabular notch.

The ligament then passes under the transverse acetabular ligament (with which it blends) to attach at its other end to the fovea of the femur; thus, it is also called the ligament of the head of the femur (Fig. 10-11).

The ligamentum teres is encased in a flattened sleeve of synovial membrane so that it does not communicate with the synovial cavity of the joint. The material properties of the ligament of the head are similar to those of other ligaments, and it is tensed in semiflexion and adduction.

However, it does not appear to play a significant role in joint stabilization regardless of joint position.

Rather, the ligamentum teres appears to function primarily as a conduit for the secondary blood supply from the obturator artery and for the nerves that travel along the ligament to reach the head of the femur through the fovea.

The hip joint capsule is typically considered to have three reinforcing capsular ligaments (two anteriorly and one posteriorly), although some investigators have further divided or otherwise renamed the ligaments.

For purposes of understanding hip joint function, the following three traditional descriptions appear to suffice.

The two anterior ligaments are the iliofemoral ligament and the pubofemoral ligament.

The iliofemoral ligament is a fan-shaped ligament that resembles an inverted letter Y (Fig. 10-12).

It often is referred to as the Y ligament of Bigelow. The apex of the ligament is attached to the anterior inferior iliac spine, and the two arms of the Y fan out to attach along the intertrochanteric line of the femur. The superior band of the iliofemoral ligament is the strongest and thickest of the hip joint ligaments.

The pubofemoral ligament (see Fig. 10-12) is also anteriorly located, arising from the anterior aspect of the pubic ramus and passing to the anterior surface of the intertrochanteric fossa.

The bands of the iliofemoral and the pubofemoral ligaments form a Z on the anterior capsule, similar to that of the glenohumeral ligaments. The ischiofemoral ligament is the posterior capsular ligament.

The ischiofemoral ligament (Fig. 10-13) attaches to the posterior surface of the acetabular rim and the acetabulum labrum.

Some of its fibers spiral around the femoral neck and blend with the fibers of the circumferential fibers of the capsule.

Other fibers are arranged horizontally and attach to the inner surface of the greater trochanter.

There is at the hip joint, as at other joints, some dis-agreement as to the roles of the joint ligaments.

Fuss and Bacher provided an excellent summary of the similarities and discrepancies to be found among of a number of investigators.

It may be sufficient to conclude, however, that each of the hip joint motions will be checked by at least one portion of one of the hip joint ligaments and that the forces transmitted by the ligaments (and capsule) are dependent on orientation of the femur in relation to the acetabulum.

There is consensus that the hip joint capsule and the majority of its ligaments are quite strong and that each tightens with full hip extension (hyperextension).

However, there is also evidence that the anterior ligaments are stronger (stiffer and withstanding greater force at failure) than the ischiofemoral ligament.

The capsule and ligaments permit little or no joint distraction even under strong traction forces. When a dysplastic hip is completely dislocated, the capsule and ligaments are strong enough to support the femoral head in weight-bearing.

In these unusual conditions, the stresses on the capsule imposed by the femoral head may lead to impregnation of the capsule with cartilage cells that contribute to a sliding surface for the head.

Under normal circumstances, the hip joint, its capsule, and ligaments routinely support two thirds of the body weight (the weight of head, arms, and trunk, or HAT).

In bilateral stance, the hip joint is typically in neutral position or slight extension. In this position, the capsule and ligaments are under some tension.

The normal line of gravity (LoG) in bilateral stance falls behind the hip joint axis, creating a gravitational extension moment.

Further hip joint extension creates additional passive tension in the capsuloligamentous complex that is sufficient to offset the gravitation extension moment.

As long as the LoG falls behind the hip joint axis, the capsuloligamentous structures are adequate to support the superimposed body weight in symmetrical bilateral stance without active or passive assistance from the muscles crossing the hip.

Capsuloligamentous Tension

Hip joint extension, with slight abduction and medial rotation, is the close-packed position for the hip joint.

With increased extension, the ligaments twist around the femoral head and neck, drawing the head into the acetabulum. In contrast to most other joints in the body, the close-packed and stable position for the hip joint is not the position of optimal articular contact (congruence).

As already noted, optimal articular contact occurs with combined flexion, abduction, and lateral rotation. Under circumstances in which the joint surfaces are neither maximally congruent nor close-packed, the hip joint is at greatest risk for traumatic dislocation.

A position of particular vulnerability occurs when the hip joint is flexed and adducted (as it is when sitting with the thighs crossed).

In this position, a strong force up the femoral shaft toward the hip joint (as when the knee hits the dashboard in a car accident) may push the femoral head out of the acetabulum.

The capsuloligamentous tension at the hip joint is least when the hip is in moderate flexion, slight abduction, and midrotation.

In this position, the normal intra-articular pressure is minimized, and the capacity of the synovial capsule to accommodate abnormal amounts of fluid is greatest.

This is the position assumed by the hip when there is pain arising from capsuloligamentous problems or from excessive intra-articular pressure caused by extra fluid (blood or synovial fluid) in the joint. Extra fluid in the joint may be a result of such conditions as synovitis of the hip joint or bleeding in the joint from tearing of blood vessels with femoral neck fracture.

Wingstrand and colleagues proposed that minimizing intra-articular pressure not only decreases pain in the joint but also prevents the excessive pressure from compressing the intra-articular blood vessels and interfering with the blood supply to the femoral head.

Structural Adaptations to Weight-Bearing

Structural Adaptations to Weight-Bearing

The internal architecture of the pelvis and femur reveal the remarkable interaction between mechanical stresses and structural adaptation created by the transmission of forces between the femur and the pelvis.

The trabeculae (calcified plates of tissue within the cancellous bone) line up along lines of stress and form systems that normally adapt to stress requirements.

The trabeculae are quite evident on bony cross-section, as seen in Figure 10-14, along with some of the other structural elements of the hip joint.

Most of the weight-bearing stresses in the pelvis pass from the sacroiliac joints to the acetabulum.

In standing or upright weight-bearing activities, at least half the weight of the HAT (the gravitational force) passes down through the pelvis to the femoral head, whereas the ground reaction force (GRF) travels up the shaft.

These two forces, nearly parallel and in opposite directions, create a force couple with a moment arm (MA) equal to the distance between the superimposed body weight on the femoral head and the GRF up the shaft.

These forces create a bending moment (or set of shear forces) across the femoral neck (Fig. 10-15).

The bending stress creates a tensile force on the superior aspect of the femoral neck and a compressive stress on the inferior aspect.

A complex set of forces prevents the rotation and resists the shear forces that the force couple causes; among these forces are the structural resistance of two major and three minor trabecular systems (Fig. 10-16).

The medial (or principal compressive) trabecular system arises from the medial cortex of the upper femoral shaft and radiates through the cancellous bone to the cortical bone of the superior aspect of the femoral head.

The medial system of trabeculae is oriented along the vertical compressive forces passing through the hip joint.

The lateral (or principal tensile) trabecular system of the femur arises from the lateral cortex of the upper femoral shaft and, after crossing the medial system, terminates in the cortical bone on the inferior aspect of the head of the femur.

The lateral trabecular system is oblique and may develop in response to parallel (shear) forces of the weight of HAT and the GRF.

There are two accessory (or secondary) trabecular systems, of which one is considered compressive and the other is considered tensile.

Another secondary trabecular system is confined to the trochanteric area femur.

Heller and colleagues used data from instrumented in vivo hip prostheses and mathematical modeling to conclude that the loading environment in the femur during activity was largely compressive, with relatively small shear forces.

The areas in which the trabecular systems cross each other at right angles are areas that offer the greatest resistance to stress and strain.

There is an area in the femoral neck in which the trabeculae are relatively thin and do not cross each other. This zone of weakness has less reinforcement and thus more potential for failure.

The zone of weakness of the femoral neck is particularly susceptible to the bending forces across the area and can fracture either when forces are excessive or when compromised bony composition reduces the tissue’s ability to resist typical forces.

The primary weight-bearing surface of the acetabulum, or dome of the acetabulum, is located on the superior portion of the lunate surface (see Fig. 10-14).

In the normal hip, the dome lies directly over the center of rotation of the femoral head.

Genda and colleagues, using radiographs and modeling, found peak contact pressures during unilateral stance to be located near the dome but with some variation that was positively correlated with CE angle.

They also found that the contact area was significantly smaller in women than in men and that the peak contact forces were higher in women. The dome shows the greatest prevalence of degenerative changes in the acetabulum.

The primary weight-bearing area of the femoral head is, correspondingly, its superior portion.

Although the primary weight-bearing area of the acetabulum is subject to the most degenerative changes, degenerative changes in the femoral head are most common around or immediately below the fovea or around the peripheral edges of the head’s articular surface.

Athanasiou and colleagues proposed that the variations in material properties, creep characteristics, and thickness may explain the differences in response of articular cartilage in the acetabular and femoral primary weight-bearing areas.

If loading of the hip joint is necessary to achieve congruence and optimize load distribution between the larger femoral head and the acetabulum, persisting incongruence in the dome of the acetabulum in the moderately loaded hip (especially in young adults) could result in incomplete compression of the dome cartilage and, therefore, inadequate fluid exchange to maintain cartilage nutrition.

The superior femoral head receives compression not only from the dome in standing but also from the posterior acetabulum in sitting and the anterior acetabulum in extension.

The more frequent and complete compression of the cartilage of the superior femoral head, according to this premise, accounts for better nutrition within the cartilage.

It must be remembered, however, that avascular articular cartilage is dependent on both compression and release to move nutrients through the tissue; both too little compression and excessive compression can lead to compromise of the cartilage structure.

The forces of HAT and GRF that act on the articular surfaces of the hip joint and on the femoral head and neck also act on the femoral shaft.

The shaft of the femur is not vertical but lies at an angle that varies considerably among individuals. However, the vertical loading on the oblique femur results in bending stresses in the shaft.

The medial cortical bone in the shaft (diaphysis) must resist compressive stresses, whereas the lateral cortical bone must resist tensile stresses (Fig. 10-17).

References

1. PK Levangie & CC Norkin: Joint Structure & Function - A Comprehensive Analysis; 4th

Edition, FA Davis Company, Philadelphia.

END OF PART - 1