BPT

1. DR. DIBYENDUNARAYAN BID [PT]
T H E S A R V A 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 A T
Biomechanics
of the
Hip Complex

2. 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.

4.  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.

5.  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.

6.  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.

7.  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.

9. Structure of Hip Joint
Proximal 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).

10.  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).

11.  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.

12.  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.

13. Center Edge Angle of the Acetabulum
 Each 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.

14.  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°).

16.  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.

17.  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.

18. 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.

19. Acetabular Labrum
 Given 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.

20.  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.

21.  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.

22.  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.

23.  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.

24. 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.

25.  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.

27.  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.

28. 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.

29.  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.

30.  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.

31.  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.

32. 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.

34.  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).

36. 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.

37.  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.

38.  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°.

39.  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.

42.  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.

43.  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.

44.  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.

45.  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.

47.  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.

48. 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.

49.  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.

50.  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.

51.  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.

52.  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.

54. Hip Joint Capsule and Ligaments

55. 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.

56.  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.

57.  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).

58.  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.

59. 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).

60.  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.

61.  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.

63.  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.

64.  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.

65.  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.

68.  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.

69.  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.

70.  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).

71.  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.

72.  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.

73. 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.

74.  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.

75.  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.

76.  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.

77. Structural Adaptations to Weight-Bearing

78. 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.

80.  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.

81.  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).

83.  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).

84.  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.

85.  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.

86.  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.

88.  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.

89.  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.

90.  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.

91.  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.

92.  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.

93.  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.

94.  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.

95.  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.

96.  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.

97.  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).

98. References
1. PK Levangie & CC Norkin: Joint Structure &
Function - A Comprehensive Analysis; 4th Edition,
FA Davis Company, Philadelphia.

99. END OF PART - 1