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Contains
Hip complex
Describing and articulating components

Articulating
components The hip is the articulation between large spherical head of
Angulations the femur and the deep socket provided by the acetabulum
Acetabular labrum of the pelvis. The hip plays a dominant kinesiologic role
Joint Capsule in movements across a large part of the body. The hip
Functions – joint has many anatomic features that are well suited for
osteokinematics stability during standing, walking, and running. The
Pelvifemoral motion femoral head is stabilized by a deep socket that is
Arthrokinematics surrounded and sealed by an extensive set of connective
Muscular involvement tissues. Pathology or trauma affecting the hips typically
Stability in erect causes a wide range of functional limitations.
bilateral and unilateral
stance
Attitude of anatomical
structures during usage
of assistive devices
Biomechanical aspects
of coxa valga and coxa
vara

Articulating aspects
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Angulations

A, The center-edge angle

measures the fixed orientation of
the acetabulum within the frontal
plane, relative to the pelvis. This
measurement defines the extent
to which the acetabulum covers
the top of the femoral head. The
center-edge angle is measured
as the intersection of a vertical, Acetabular anteversion
fixed reference line (stippled)
with the acetabular reference
line (bold solid line) that
connects the upper lateral edge
of the acetabulum with the
center of the femoral head. A
more vertical acetabular
reference line results in a
smaller center-edge angle,
providing lesssuperior coverage
of the femoral head.

B, The acetabular anteversion

angle measures the fixed
orientation of theacetabulum
within the horizontal plane,
relative to the pelvis. This
measurement indicates the extent
to which the acetabulum covers Angle of inclination
the front of the femoral head.
The angle is formed by the
intersection of a fixed
anteriorposterior reference line
(stippled) with an acetabular
reference line (bold solid line)
that connects the anterior and
posterior rim of the acetabulum.
A larger acetabular anteversion
angle creates less acetabular
containment of the anterior side
of the femoral head. (A normal
femoral anteversion of 15
degrees is also shown.)
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Angle of torsion /
Femoral Torsion
Femoral torsion
describes the relative
rotation (twist) between
the bone’s shaft and
neck. Normally, as
viewed from above, the
femoral neck projects
about 15 degrees
anterior to a medial-
lateral axis through the
femoral condyles. This
degree of torsion is
called normal
anteversion. In
conjunction with the
normal angle of
inclination, an
approximate 15-degree
angle of anteversion Angle of torsion / Femoral torsion
affords optimal
alignment and joint
congruence. Femoral
torsion that is markedly
different from 15
degrees is considered
abnormal. Torsion
significantly greater
than 15 degrees is
called excessive
anteversion. In
contrast, torsion
significantly less than
15 degrees (i.e.,
approaching 0 degrees)
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Acetabular labrum
The acetabular labrum is a flexible ring of primary fibrocartilage
that surrounds the outer circumference (rim) of the acetabulum. It
provides significant stability to the hip by “gripping” the femoral
head and by deepening the volume of the socket by approximately
30%.

The seal formed around the joint by the labrum helps maintain a
negative intra-articular pressure, thereby creating a modes suction
that resists distraction of the joint surfaces. The circumferential seal
also holds the synovial fluid within the joint; therefore the labrum
indirectly enhances the lubrication and load dissipation functions of
the articular cartilage.

The labrum directly protects the articular cartilage by reducing

contact stress (force/area) by increasing the surface area of the
acetabulum. Consisting primarily of fibrocartilage, the labrum is
poorly vascularized, receiving only modest blood supply to its outer
one third. For this reason, a torn labrum has a very limited ability to
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heal. In contrast to its poor vascularization, the labrum is well

supplied by afferent nerves capable of providing proprioceptive
feedback and, when the labrum is acutely injured, the sensation of
pain will be carried by the proprioceptors to the higher center.

Joint capsule

A synovial membrane lines the internal surface of the hip joint

capsule. The iliofemoral, pubofemoral, and ischiofemoral ligaments
reinforce the external surface of the capsule.
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Close-Packed Position of the Hip

Full extension of the hip (i.e., about 20 degrees beyond the neutral
position) in conjunction with slight internal rotation and slight
abduction twists or “spirals” most of the fibers within the capsular
ligaments to their most taut position.
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FUNCTIONS –
OSTEOKINEMATICS
FEMUR ON PELVIS
Two terms are used to
describe the kinematics
at the hip. Femoral-on-
pelvic hip
osteokinematics
describes the rotation Connective Tissues and Selected Muscles That Become Taut at the
End-Ranges of Passive Hip Motion
of the femur about a
relatively fixed pelvis.
Pelvic-on-femoral hip
osteokinematics, in
contrast, describes the
rotation of the pelvis,
and often the
superimposed trunk,
over relatively fixed
femurs. Regardless of
whether the femur or
the pelvis is the moving
segment, the
osteokinematics are
described from the
anatomic position. The
names of the
movements are as
follows: flexion and
extension in the
sagittal plane,
abduction and
adduction in the frontal
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PELVIC-ON-
FEMORAL
OSTEOKINEMATICS
LUMBOPELVIC
RHYTHM

The caudal end of the

axial skeleton is firmly
attached to the pelvis by
way of the sacroiliac
joints. As a
consequence, rotation of
the pelvis over the
femoral heads typically
changes the
configuration of the
lumbar spine. This
important kinematic
relationship is known as
lumbopelvic rhythm,
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The near-maximal
range of pelvic-on-
femoral (hip) motion is
shown in the sagittal
plane (A), frontal plane
(B), and horizontal
plane (C). The motion
assumes that the
supralumbar trunk
remains nearly
stationary during the
hip motion (i.e.,
kinematics based on a
contradirectional
lumbopelvic rhythm).
The large colored and
black arrows depict
pelvic rotation and the
associated “offsetting”
lumbar motion

Tissues that are

elongated or pulled taut
are indicated by thin,
straight black arrows;
tissues slackened are
indicated by thin wavy
black arrows. The range
of motions depicted in
each figure has been
estimated by observing
photographs of healthy
young adults.
Note – see the explained
terminology follows.
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Pelvic Rotation in the Sagittal Plane: Anterior and Posterior Pelvic

Tilting

Hip flexion can occur through an anterior pelvic tilt (see Figure
A). A “pelvic tilt” is a shortarc, sagittal plane rotation of the pelvis
relative to stationary femurs. The direction of the tilt—either
anterior or posterior— is based on the direction of rotation of a
point on the iliac
crest.
The anterior tilt of the pelvis occurs about a mediallateral axis of
rotation through both femoral heads. The associated increased
lumbar lordosis offsets most of the tendency of the supralumbar
trunk to follow the forward rotation of the pelvis. While sitting with
90 degrees of hip flexion, the normal adult can perform about 30
degrees of additional pelvic-on-femoral hip flexion before being
restricted by a completely extended lumbar spine.
Full anterior tilt of the pelvis slackens most of the ligaments of the
hip, most notably the iliofemoral ligament. Marked tightness in any
hip extensor muscle—such as the hamstrings—could theoretically
limit
the extremes of an anterior pelvic tilt.
As depicted in Figure A, however, because the knees are flexed, the
partially slackened hamstring muscles would not normally produce
any noticeable resistance to an anterior pelvic rotation. During
standing (and with knees fully extended), however, the more
elongated hamstrings are more likely to resist an anterior pelvic tilt,
but the amount of resistance is usually insignificant unless the
muscle is physiologically impaired and
generating extreme resistance to elongation.

As depicted in Figure A, the hips can be extended about 10 to 20

degrees from the 90-degree sitting posture via a posterior tilt of the
pelvis. During sitting, this short-arc pelvic rotation would increase
the length (and therefore tension) only minimally in the iliofemoral
ligament and rectus femoris muscle. As depicted in the figure A, the
lumbar spine flexes, or flattens, as the pelvis posteriorly tilts.

Pelvic Rotation in the Frontal Plane

Pelvic-on-femoral rotation in the frontal and horizontal planes is
best described assuming a person is standing on one limb. The
weight-bearing extremity is referred to as the support hip.
Abduction of the support hip occurs by raising or “hiking” the iliac
crest on the side of the nonsupport hip (see Figure B). Assuming
that the supralumbar trunk remains nearly stationary, the lumbar
spine must bend in the direction opposite the rotating pelvis. A
slight lateral convexity occurs within the lumbar region toward the
side of the abducting hip. Pelvic-on-femoral hip abduction is
restricted to about 30 degrees, primarily because of the natural
limits of lateral bending in the lumbar spine. Marked tightness in
hip adductor muscles or the pubofemoral ligament can limit pelvic-
onfemoral hip abduction. In the event of a marked adductor
contracture, the iliac crest on the side of the nonsupport hip remains
lower than the iliac crest of the support hip, which may interfering
with walking.
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Adduction of the support hip occurs by a lowering of the iliac crest

on the side of the nonsupport hip. This motion causes a slight lateral
concavity within the lumbar region on the side of the adducted hip.

A hypomobile lumbar spine and/or reduced extensibility in the

iliotibial band or hip abductor muscles, such as the gluteus medius,
piriformis, or
tensor fasciae latae, may restrict the extremes of this motion.

Pelvic Rotation in the Horizontal Plane

Pelvic-on-femoral rotation occurs in the horizontal plane about a
longitudinal axis of rotation (see green circle at femoral head in
Figure C). Internal rotation of the support hip occurs as the iliac
crest on the side of the nonsupport hip rotates forward in the
horizontal plane.

During external rotation, in contrast, the iliac crest on the side of the
nonsupport hip rotates backward in the horizontal plane.
If the pelvis is rotating beneath a relatively stationary trunk, the
lumbar spine must rotate (or twist) in the opposite direction as the
rotating pelvis.

The small amount of axial rotation normally permitted in the lumbar

spine significantly limits the full expression of horizontal plane
rotation of the support hip. The full potential of pelvic-on-femoral
rotation requires that the lumbar spine and trunk follow the rotation
of the pelvis—a movement strategy more consistent with an
ipsidirectional lumbopelvic rhythm.

Arthrokinematics

During hip motion, the nearly spherical femoral head normally

remains snugly seated within the confines of the acetabulum. The
steep walls of the acetabulum, in conjunction with the tightly fitting
acetabular labrum, limit significanttranslation between the joint
surfaces.
Hip arthrokinematics are based on the traditional convex-on-
concave or concaveon-convex principles. Figure shows a highly
mechanical illustration of a hip opened to enable visualization of the
paths of articular motion. Abduction and adduction occur across the
longitudinal diameter of the joint surfaces. With the hip extended,
internal and external rotation occur across the transverse diameter of
the joint surfaces. Flexion and extension occur as a spin between the
femoral head and the lunate surfaces of the acetabulum. The
axis of rotation for this spin passes through the femoral head.
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There are many

similarities between the
shoulder and hip joints.
Like the shoulder, the
hip has a group of one-
joint muscles that
provide most of the
control and a group of
longer, two-joint
muscles that provide the
range of motion.
These muscles can also
be grouped according
to their location and
somewhat by their
function. For example,
the anterior muscles
tend to beflexors, lateral
muscles tend to be
abductors, posterior
muscles tend to be
extensors, and medial
muscles tend to be
adductors.The figure
shows muscles of the
anterior hip region. The
right side of the body
shows flexors and
adductor muscles. Many
muscles on the left side
are cut to expose the
adductor brevis and
adductor magnus.
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The posterior muscles

of the hip. The left
side highlights the
gluteus maximus and
hamstring muscles
(long head of the
biceps femoris,
semitendinosus, and
semimembranosus).

The right side shows

the hamstring muscles
cut to expose the
adductor magnus and
short head of the
biceps femoris. The
right side shows the
gluteus medius and
five of the six short
external rotators (i.e.,
piriformis, gemellus
superior and inferior,
obturator internus, Stability during an erect bilateral Stance
and quadratus
In erect bilateral stance, both hips are in neutral or slight
femoris). hyperextension, and weight is evenly distributed between both legs.
The LoG falls just posterior to axis for flexion/extension of the hip
joint. The posterior
location of the LoG creates an extension moment of force around
the hip that tends to posteriorly tilt the pelvis on the femoral heads.
The gravitational extension moment is largely checked by passive
tension in
the hip joint capsuloligamentous structures, although slight or
intermittent activity in the iliopsoas muscles in relaxed standing
may assist the passive structures.
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In right unilateral stance,

the weight of HAT acts 10
cm from the right hip
joint. The 10 cm moment
arm slightly
An anterior view of the pelvis in normal erect bilateral stance. The
underestimates the
weight acting on the right hip joint (WR)multiplied by the distance
location of the LoG from the right hip joint axis to the body’s center of gravity (DR) is
because it does not equal to the weight acting at the left hip joint (WL) multiplied by the
distance from the left hip to the body’s center of gravity
account for the weight of (DL).Therefore, WR × DR = WL × DL.
the hanging left limb. The
hip abductors have a
moment arm of
approximately 5 cm. Stability during an erect unilateral Stance
Inset. The pull of the
abductors (Fms) on the
horizontally oriented
pelvis will resolve into a
parallel component (Fx)
that will pull the
acetabulum into the
center of the femoral
head and a perpendicular
component (Fy) that will
pull the pelvis down on
the superior aspect of the
femoral head, as well as
pull the ilium closer to
the femur, producing a
hip abduction torque.
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Synopsis
The hip joints function as basilar joints for both the axial skeleton
and the lower extremities. As such, the hips form the central pivot
point for common movements of the body as a whole, especially
those involving flexion and extension. Consider, for instance, lifting
the leg to ascend a steep stair, or bending down at the waist to pick
up an object from the floor. Both motions demand a significant
amount of movement and production of muscular force between the
proximal femurs and the pelvis.

Weakness, instability, or pain in the hips therefore typically causes

marked difficulty in performing a wide range of activities—from
getting in and out of a chair to engaging in even moderate aerobic
exercise.

The osteology and arthrology of the healthy hip joint are designed
more for ensuring stability than for providing excessive mobility, a
condition essentially the opposite of that for the glenohumeral joint
—the analogous joint of the upper extremity.

A deep-seated and well-contained femoral head, surrounded by

thick capsular ligaments and muscles, ensures stability especially in
the weight-bearing phase of walking—a phase that occupies 60% of
a given gait cycle.

A surprisingly small amount of muscle activity is required to

stabilize the hips while one stands in an upright relaxed posture,
assuming that one or both of the hips are fully extended. Such a
posture orients the body’s line of gravity just posterior to the
medial-lateral axis of rotation at the hips.
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The force of gravity therefore acts to maintain the hips passively

extended. Being pulled relatively taut in near or full hip extension,
the ligaments of the hip create useful tension that adds further to the
stability of the extended hips. Indeed, muscle forces are periodically
required to augment or readjust the stability of the hips while one
stands at ease; however, this active mechanism is normally used as a
reserve or secondary source.

This is not the case, however, with a hip flexion contracture;

stability while one stands in partial flexion demands significant and
constant activation from the hip extensor muscles. Such a condition
not only is metabolically “expensive” but places unnecessarily large
muscular-based forces across the hip joints. These forces, acting
over time, may be harmful in a malaligned joint that cannot properly
dissipate stress. The full extent to which the hip joints contribute to
full body movement requires an understanding of both femoralon-
pelvic and pelvic-on-femoral kinematics.
Femoral-onpelvic movements are often associated with a change in
location of the body as a whole relative to the surroundings, such as
while walking. Pelvic-on-femoral movements, on the other hand,
are often performed to change the position of the pelvis—and often
the entire superimposed trunk—relative to fixed lower extremities.
Pelvic-on-femoral movements are expressed in many forms, from
subtle oscillations of the pelvis during the stance phase of each gait
cycle to more obvious large-arc rotations of the pelvis (and trunk) as
a figure skater spins on the ice while bending forward at the waist.

Adding to the complexity of pelvic-on-femoral movements is the

strong association with the kinematics of the lumbar spine. Clinical
evaluation of the causes of reduced or abnormal motions at the hip
must therefore include an evaluation of the flexibility and prevailing
posture of the lumbar region. Limitations of movement at either the
lumbar spine or the hips alter the kinematic sequencing throughout
the trunk and proximal end of the lower kinematic chain.

Being able to localize the source of abnormal kinematics within this

broad area of the body certainly improves the likelihood of
successful clinical diagnosis and intervention.

Nearly one third of muscles that cross the hip joint attach
proximally to the pelvis and distally to either the tibia or the fibula.
An imbalance of force within any of these muscles— generated
actively or passively—can therefore influence the posture and range
of motion across multiple segments, including the lumbar spine, hip,
and knee.

Clinicians regularly evaluate and treat functional limitations that

may arise from impairments in and among these and other
synergistic muscles. Treatment requires a thorough understanding of
the muscular interactions that exist across a very mechanically
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interrelated region of the body.