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Biomechanics of the Human Hip

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ERICL. RADIN, M.D.

The hip joint is the pivot upon which the fly and rarely sit. Thus the hip joint in man
human body is balanced in gait. True. has certain unique design requirements.
bipedalism is limited to birds and man, and This article explores what these require-
in both the stability of this joint is dependent ments are and how they are met.
upon the bony configuration of the joint.
Lack of adequate bony configuration of the FUNCTIONAL ANATOMY
hip joint socket will lead to dislocation. A
basic ball-and-socket design has evolved. A The anatomy of the hip joint has been
simple hinge hip joint would not permit the discussed in a previous article in this
rotation needed for gait. In birds the center symposium. The acetabulum and femoral
of gravity of the body mass is below that head are composed mainly of spongy
of the hip joints so that the body of the trabecular bone which provides some elastic-
bird acts as a pendulum (Fig. 1). In birds ity ( i . e . , an ability to be deformed without
little force is required to balance the body sustaining structural damage). The socket
in stance.’ The center of gravity in human is not a fully formed cup but is in shape
beings is above the hip joints; thus, mech- like a horseshoe and surrounds the femoral
anisms must exist to balance the body’s head much as a bicycle pants clip surrounds
mass on the hip. The only forces which can a pants cuff (Fig. 2). The presence of
act in this fashion are muscular. The con- large quantities of relatively deformable
bone in this configuration suggests spread-
stant use of muscles takes energy; for man
ing under load and, indeed, spreading
to stand all day is energy-consuming. While
standing, man tries to shift his weight from occurs7 and is essential, if the stress
one leg to another, to lean back on the (force per unit area) on the articular cartilage
ligaments of Bigelow to lock the hips in is to be kept within tolerable limits. One can
hyperextension, or to lean against some- hypothesize that, given very high loads
thing, or else he tries to sit down. The under which the hip joint must function
reason the military “at ease” position can (up to five times body weight in running13),
be sustained for longer periods of time than deformation of the epiphyseal subchondral
the “at attention” position is that in the bone must occur.2 Since the bony portions
former, body weight can be shifted from of the hip deform under load, the design
one limb to another. Birds do not have specifications must call for maximum con-
this problem and are capable of standing all tact area and congruence in the deformed
their lives; in fact, many species cannot position (Fig. 3). Such congruity should
occur only under full load. A congruous
fit under no load would lead to an incon-
Department of Orthopedic Surgery, WVU Medical gruous fit under high load when the femoral
Center, Morgantown, WV 26506.
Reprint requests to Eric L. Radin, M.D. head tends to flatten (Fig. 4). Under such
Received: October 24. 1979. circumstances the femoral head would
0009-921X/80/1000/028$00.85 0 J. B. Lippincott Co.

28
 Number 152
October, 1980 Biomechanics of Human Hip 29

articulate only with part of the acetabulum, / '-\

decreasing the available potential contact
area and increasing the force on whatever
contact areas remain. Thus it is of great
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functional advantage to have the hip joint \

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slightly incongruous under low loads, so \

that with the flattening of the subchondral \
bone the joint achieves maximum surface \
contact under high load, diminishing the
force per unit area and maintaining it within
tolerable limits. It has been suggested that
deformation under load has a significant
effect on protecting the overlying articular
cartilage from impulsive load but one sus-
pects that the true sparing effects of
trabecular bone on the overlying cartilage
involve an increase in the available potential
contact area. l1 Excessive deformation of

li FIG. 2. The horseshoe shape of the aceta-

bulum allows spreading of this structure under
load.

trabecular bone can lead to microfracture. l4

A certain level of trabecular fracture is
clearly physiologic, but high levels, sus-
tained repetitively, can lead to bone re-
modeling and actual stiffening of the un-
derlying trabecular network.l0 It has been
suggested that stiffening and loss of congru-
ence can lead to deterioration of the
articular surfaces and to osteoarthrosis.2

FEMORAL NECK VARUS AND

FIG.1 . The hip joint of the bird, unlike man, ANTEVERSION
is above the center of gravity of the body mass. The anatomic varus position of the femoral
Thus, little muscle action is required for a bird
to balance itself in the upright position. Com- neck vis-h-vis the femoral shaft allows
pare this with the forces which must be expended the abductor muscles considerable func-
for man to stand. tional advantage as they counterbalance the
 Clinical Orthopaedics
30 Radin and Related Research
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FIG.3. The hip deforms under load to allow

for maximum contact area. FIG.5. A varus attitude of the femoral neck
provides a lever arm for the abductors (A). The
natural varus angle of the femoral neck performs
body weight in the frontal plane during one- the same function. (B).
legged stance (Fig. 5 ) . Ideally, the abductors
should be as far lateral from the hip joint
as possible in order to achieve muscle
stability in bipedal stance; however, a com-
promise must be made, because too extreme
a lateral placement will limit abduction.
Compared with the lever arm of the body
Acetabulum weight, the abductors’ lever arm is relatively
small. The lever-arm length obviously mul-
tiplies the torque produced by the con-
traction of the muscles. Increase in the
varus position, accidentally or from a
growth disturbance, fracture or malunion,
or intentionally by osteotomy , will increase
the lever arm through which the abductors
act and provide these muscles with an
increased mechanical advantage. Since the
total load across the hip joint is to a
large extent generated by the muscles,
diminishing the muscle force required by
increasing the muscle’s leverage will con-
siderably decrease the load on the hip
joint.
In the lateral or sagittal plane increased
leverage results from the anteverted attitude
of the femoral neck. This anteversion pro-
vides the gluteus maximus with a lever arm
FIG.4. What happens if the hip joint is con- and so multiplies the muscle’s effectiveness.
gruous in the unloaded state (A) and is then The longer this lever arm, the less force the
loaded (B). Note the significant diminution in gluteus maximus has to exert to maintain us
contact area. (Reproduced with permission from in the upright posture over the hip joint
Radin, E. L. and Paul, I. L.: The biomechanics of
congenital dislocated hips and their treatment, (Fig. 6). It should be remembered however,
Clin. Orthop. 98:33, 1974). that excessive anteversion can have a
 Number 152
October, 1980 Biomechanics of Human Hip 31

deleterious effect on the range of external

rotation of the joint.
Muscle forces balance the body over the
hip joint. Paraplegic patients need long-leg
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braces to stand; they also tend to lean back

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against their ligaments of Bigelow (Fig. 7).

Since the hip is a ball-and-socket joint, ca-
pable of motion in all directions, it must be
stabilized from all directions-thus the
need for large masses of muscles from every
direction. The pelvis can be considered a
substantial broad base from which to origi-
nate this multiplicity of muscles required
for hip stability. This also functionally ex-
plains why ball-and-socket joints are not
suitable for the distal articulations of the
appendages. Ball-and-socket joints require
a substantial surface area from which to
originate the muscles required for stability.

FIG. 7. Passive stability at the hip may be

obtained by leaning back, tightening the anteriorly
placed Y-shaped ligaments of Bigelow.

Such broad expanses of bony surface are

available only at the base of the appendages.
The flair of a broad-based pelvis became
a functional necessity with bipedalism. The
great apes’ gluteus medius and minimus
act as hip extensors, more useful in climbing
than is abduction. These primates walk with
a significant Trendelenburg lurch when bi-
pedal. In fact, chimpanzees are well known
for using their arms as canes, actually
weight-bearing on their knuckles. Note that
human iliac wings are flaired laterally in
contrast to those of the quadripeds, who
have no need for strong abductors (Fig. 8).
A lateral flair moves the abductors further
away from the center of rotation of the hip
FIG. 6. Anteversion of the femoral neck joint, increasing the lever arm through
creates a lever arm through which the gluteus which they act. The tuberous excrescence
maximus can act with greater efficiency. on which these muscles are inserted, the
 Clinical Orthopaedics
32 Radin and Related Research
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FIG. 8. A quadruped
pelvis (left). Note that the
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human pelvis is flaked and

its iliac crests are larger,
providing large origins
for the hip abductors.

greater trochanter, further acts to increase than would appear to be required for the
the abductor lever arms. Former attempts activities of daily living. However, since
to move the greater trochanter distally down man sits for a considerable period of the
the shaft, although perhaps achieving some time, hyperflexion is a most useful range
form of short-lived tenodesis, tended to for sitting on chairs which are lower than
increase the overall load on the hip joint the knees, or cross-legged on the ground, as
because the muscles, robbed of the mechan- well as to assure that the lumbosacraljunction
ical advantage of their leverage, had to is in hyperflexion while sitting. The clinical
work harder (Fig. 9). More recent attempts syndrome of degeneration of the lumbosa-
to move the greater trochanter laterally have cral joints after loss of motion of the hip
met with significant success in cases in has been well described.
which hyperpressure had existed on the The necessity for rotation in bipedal gait
articulating surfaces. involves the required pivoting on the stance
phase leg in order to allow for efficient bi-
MOTIONS OF THE HIP pedal forward gait3 (Fig. 10). Quadrupeds
Because it is a ball-and-socket joint, the
hip allows a wide range of motion, greater

FIG. 9. Transfer of the greater trochanter FIG.10. External rotation of the hip is essential
laterally increases the lever arm and diminishes for smooth forward progression in bipedal gait.
the overall force these muscles contribute to the (Reproduced with permission from Dec, J. B., et
load across the joint (A). Moving the greater al.: Major determinants in normal and patho-
trochanter distally has the opposite effect (B). logical gait, J. Bone Joint Surg. 35A:549, 1953.)
 Number 152
October, 1980 Biomechanics of Human Hip 33

do not require significant external rotation

of the hips in gait. A quadruped pivots
the pelvis by rotating the spine relative to
the pelvis. Watch a dog walk.
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FORCES ON THE HIP

We have already acknowledged that the

forces on the hip joint are multiples of body
weight created by muscles which act across
the joint.4 As discussed previously, lowering
the stress on the hip can be accomplished
by increasing the joint contact area by ap-
propriate osteotomy or by diminishing the
overall force by lateral displacement of the
greater trochanter. Selective tenotomy about
the hip, although it will create a limp
because of diminution of stabilizing forces,
FIG. 1 1 . The femoral neck is stressed in
is another effective method by which to bending, creating tensile stress in the superior
lower the overall force. It is stress which neck and compressive strains in the inferior
is the critical mechanical determinant in neck.
joint deterioration. It has been shown that,
when appropriate, reducing the stress on
the hip joint can have a significant thera- remaining load-bearing cartilage is “con-
peutic effect and allow the tissues a degree centrated” and one can with certainty
of functional healing.g As attested to by predict late degenerative change. Opera-
any orthopedist who has operated on a tions which successfully “improve cover-
patient with pseudarthrosis, connective tis- age” actually increase the contact area of
sue is capable of cartilage formation and the joint and minimize the stress acting
joint fluid secretion. The secret of functional on the articulating tissues. They may also
healing of the hip joint depends upon mo- increase joint stability.
tion, a source of cells, and the surgeon’s The varus position of the femoral neck
ability to create the proper mechanical causes it to bend (Fig. 11). The muscles
situation ( i . e . , lowered stress) so that which pull up on the greater trochanter
the tissues have a chance to differentiate further accentuate this bending of the
and mature. Attention to mechanical factors femoral neck, creating compression medially
is, therefore, an important consideration in and tension laterally. The trajectoral pattern
the treatment of osteoarthrosis of the hip. of the trabeculae of the femoral head and the
Since the acetabulum cannot significantly condensation of cortical bone medially in
change in volume during hip rotation, the the calcar region have been well d e ~ c r i b e d . ~
socket always “covers” an identical quantity Failure to appreciate the significant bending
of femoral head. What is implied by the to which the femoral neck is subject has led
term “hip coverage” is the attitude of the to significant problems in attempts to
femoral head with the acetabulum in the internally fix fractures of this region, a
neutral position. “Lack of coverage” is subject discussed in a subsequent article in
better stated as lack of containment. this symposium. Muscles not only act to
Clearly, in situations where the load-bearing stabilize joints and, of course, provide the
area of the femoral head is diminished forces required to move the bones around
because of deformity, the stress on the the joints, but they also can act to diminish
 Clinical Orthopaedics
34 Radin and Related Research

SUMMARY
The hip joint is one of the body’s most
amenable joints to a lowering of its inter-
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articular pressure by surgery. It has been

shown that such a lowering of pressure
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can predictably have a significant amelio-

ratory effect on osteoarthrotic degenera-
tion. A workable understanding of the bio-
mechanics of the hip is essential in the
planning of any meaningful orthopedic
intervention to attempt to delay or reverse
hip joint degeneration.

REFERENCES
1 . Bechtol, C. 0.: Camp. Lecture, Massachusetts
General Hospital, 1%5.
2. Bullough, P., Goodfellow, J., and O’Connor, J.:
The relationship between degenerative changes
and load bearing in the human hip, J. Bone Joint
Surg 55B:746, 1973.
3. Dec, J. B., Saunders, M., Inman, V. T., and Eber-
hart, H. D.: Major determinants in normal and
pathological gait, J. Bone Joint Surg. 35A:543,
1953.
4. Inman, V. T.: Functional aspects of the abductor
muscles of the hip, J. Bone Joint Surg. 29:607,
1947.
5. Koch, J. C.: The laws of bone architecture, Am
FIG. 12. The fascia lata (A) acts as a guy wire
J. Anat. 21:177, 1917.
reducing the tensile stress on the lateral femoral 6. Maquet, P.: Reduction de la pression articulaire de
shaft when the tensor fascia lata muscle is la hanche par lateralisation chirurgical du grand
tightened (B). trochanter, Acta Orthop Belg. 42:266, 1976.
7. Mital, M. A.: Biomechanical characteristics of the
human hip joint, M.S.C. thesis, Univ. of Strath-
the bending stress on the bones. Consider Clyde, Glasgow, 1970.
8. Pauwels F.: Biomechanics of the Normal and
the femoral neck, stressed in bending, Diseased Hip, New York, Springer Verlag, 1976.
transmitting this bending stress down onto 9. Radin, E. L., Maquet, P., and Parker, H.: Ra-
the femoral shaft which is angled inward tionale and indications for the “hanging hip”
procedure: A clinical and experimental study, Clin.
toward the midline (Fig. 12). A guy wire Orthop. 112:221, 1975.
would be useful on the lateral side of the 10. Radin, E. L., Parker, G. H., Pugh, J. W.,
femur in order to pull it straighter and Steinberg, R. S., Paul, I. L., and Rose, R. M.:
Response ofjoints to impact loading 111. Relation-
diminish the potentially harmful tensile ship between trabecular microfracture and carti-
strains (deformations)8 in the bone. This is lage degeneration, J. Biomech. 6 5 1 , 1973.
accomplished by the fascia lata, which has a 1 1 . Radin, E. L., Paul, I. L., Rose, R. M., and Simon,
S. R.: The physiology ofjoints as it relates to their
muscle at its proximal end that maintains it degeneration. AAOS Symposium on Osteoarthritis,
at the appropriate level of tightness. The St. Louis, C.V. Mosby, 1976, p. 34.
fascia lata and its associated muscle should 12. Radin, E. L., Simon, S. R., Rose, R. M., and Paul,
I. L.: Biomechanics for the Practicing Orthopedic
be considered as a lateral tension band.12 Surgeon. New York, John Wiley, 1979.
Care should be taken at operation to 13. Rydell, N.: Biomechanics of the hip joint, Clin.
maintain the integrity of this important Orthop. 92:6, 1973.
14. Townsend, P. R., Rose, R. M., and Radin, E. L.:
muscle; without it the strains within the Buckling studies of single human trabeculae. J.
femoral shaft will be increased. Biomech. 8:199, 1975.