The document discusses various pathologies that can affect the hip joint due to alterations in biomechanics. Small changes in forces or joint structure can lead to increased stress and injury over time. Common issues include arthritis/arthrosis from wear and tear, and fractures of the femoral neck which become more likely with age-related bone loss. Conditions like coxa valga/vara and torsion abnormalities of the femur can further impact forces on the joint and predispose to problems. Understanding hip biomechanics and how dysfunctions can influence other areas is important for evaluation and treatment.

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: 5

2. Hip Joint
Pathology

3.  The very large active and passive forces crossing the
hip joint make the joint’s structures susceptible to:
 wear and tear of normal components and
 to failure of weakened components.
 Small changes in the biomechanics of the femur or
the acetabulum can result:
 in increases in passive forces above normal levels, or
 in weakness of the dynamic joint stabilizers.

4. Arthrosis
 The most common painful condition of the hip is due
to deterioration of the articular cartilage and to
subsequent related changes in articular tissues.
 It is known as osteoarthritis, degenerative arthritis,
or perhaps most appropriately as hip joint
arthrosis, and
 its prevalence rates are about 10% to 15% in those
older than 55 years, with approximately equal
distribution among men and women.

5.  Although trauma or malalignment such as femoral
anteversion may be associated with occurrence,
50% of the cases are considered to be idiopathic;
that is, half of the cases of hip joint arthrosis have no
evident underlying pathology.

6.  Changes may be due to:
 subtle deviations present from birth,
 to tissue changes inherent in aging,
 to the repetitive mechanical stress of loading the body
weight on the hip joint over a prolonged period,
to impingement between the femur and labrum or
adjacent acetabulum, or to interactions of each of
these factors.

7.  The factors most closely associated with idiopathic
hip joint arthrosis are increased age and increased
weight/height ratio.
 Lane and colleagues found no association between
osteoarthritic changes and running status among
older subjects.

8.  The mechanism for cartilaginous degeneration in the
hip joint is not clear-cut.
 When a biomechanical problem is not evident,
degenerative changes may not be due to excessive
forces at the hip joint but due to inadequate forces.
 This would explain why there is little or no
association between increased activity level with
sports or recreational activities and hip arthrosis.

9.  It may be that forces in excess of half the body
weight are needed to fully compress the femoral
head into congruent contact with the dome of the
acetabulum.

10.  Using a number of other studies as a base, Bullough and
associates hypothesized that –
we typically spend no more than 5% to 25% of our time
in unilateral lower extremity weight-bearing activities
in which the load may be sufficient to compress the
articular cartilage of the dome of the acetabulum.
 Lower loads and infrequent high joint forces may be
inadequate to maintain flow of nutrients and wastes
through the avascular cartilage.

11.  The theory of inadequate compression as a
contributing factor to hip joint degeneration is
supported by the fact that the more common
degenerative changes in the femur are at the
periphery of the head and the perifoveal area, rather
than at the superior primary weight-bearing area.

12.  The periphery of the head receives only about one
third the compressive force of the superior portion of
the head, whereas the superior portion of the head is
compressed not only in standing but is also in
contact with the posterior acetabulum during sitting
activities.
 The area of the femoral head around the fovea is
most commonly in the non-weight-bearing
acetabular notch and would undergo compression
relatively infrequently.

13.  Wingstrand and colleagues proposed that :
excessive intra-articular fluid from relatively benign
synovitis or trauma may reduce articular congruence
and the stabilizing effect of atmospheric pressure,
resulting in microinstability and unfavorable
cartilage loads.

14. Fracture
 Although the weight-bearing forces coming through
the hip joint may cause deterioration of the articular
cartilage, the bony components must also be of
sufficient strength to withstand the forces that are
acting around and through the hip joint.

15.  As noted in the section on the weight-bearing
structure of the hip joint, the vertical weight-bearing
forces that pass down through the superior margin of
the acetabulum in both unilateral and bilateral
stance act at some distance from GRF up the shaft of
the femur.
 The result is a bending force across the femoral neck
(see Fig. 10-15).

17.  Normally the trabecular systems are capable of
resisting the bending forces, but abnormal increases
in the magnitude of the force or weakening of the
bone can lead to bony failure. The site of failure is
likely to be in areas of thinner trabecular distribution
such as the zone of weakness (see Fig. 10-16).

18.  Crabtree and colleagues used both patients and
cadavers with a fracture to conclude that a loss in
cortical bone, not cancellous bone (trabeculae), may
be the source of the problem.
 Although cancellous bone mass was similar for cases
and controls in similar age groups, there was a 25%
reduction in cortical bone mass in the fracture cases.

20.  Bony failure in the femoral neck is uncommon in the
child or young adult, even with large applied loads.
 However, femoral neck fractures occur at the rate of
about 98/100,000 people in the United States, with
the average age at occurrence being in the seventies.

21.  There is a predominance of fractures in women,
although this is certainly influenced by their greater
longevity.
 Of middle-aged people, women actually suffer fewer
hip fractures than do men, although the fractures in
this age group are usually attributable to substantial
trauma.

22.  In 87% of cases of hip fracture among the elderly
population, the precipitating factor appears to be
moderate trauma such as that caused by a fall from
standing, from a chair, or from a bed.
 There is consensus that hip fracture is associated
with, but not exclusively due to, diminished bone
density.

23.  Bone density decreases about 2% per year after age
50 and trabeculae clearly thin and disappear with
aging.

24.  Cummings and Nevitt believed the exponential
increase in hip fractures with age could not be
accounted for by decreased bone density alone and
proposed that the slowed gait characteristic of the
elderly may play an important part.

25.  They contended that the slowing of gait makes it less
likely that momentum will carry the body forward in
a fall (generally onto an outstretched hand) and
more likely that the fall will occur backward on to the
hip area weakened by bone loss and no longer
padded by the fat and muscle bulk of youth.

26.  Hip fracture will continue to receive considerable
attention because of the high health care costs of
both conservative and operative treatment.
 Of all fall-related fractures, hip fractures cause the
greatest number of deaths and lead to the most
severe health problems and reduced quality of life.

27.  Not only is the condition painful, but malunion of
the fracture can lead to joint instability or
cartilaginous deterioration (or both) as a result of
poorly aligned bony segments.
 Although the femoral head may receive some blood
supply via the ligament of the femoral head, an
absent or diminished supply through the ligament of
the head (as occurs with aging) means reliance on
anastomoses from the circumflex arteries.

28.  This circumflex arterial supply may be disrupted by
femoral neck fracture, which leaves the femoral head
susceptible to avascular necrosis and necessitates
replacement of the head of the femur with an
artificial implant.
 Femoral neck fracture also has an associated
mortality rate that may be as high as 20%.

29. Bony Abnormalities of the Femur
 When the bony structure of the femur is altered
through abnormal angles of torsion or inclination,
subsequent changes in the direction and magnitude
of the forces acting around the hip can lead to other
pathologic conditions such as increased likelihood of
joint arthrosis, increased likelihood of femoral neck
fracture, or muscular weakness.

30.  The normal angles of inclination and torsion appear
to represent optimal balance of stresses and muscle
alignment.
 Alterations may actually appear to result in
advantages in relation to some functions but are
always accompanied by concomitant disadvantages
in relation to others.

31. Coxa Valga/Coxa Vara
 In coxa valga (see Fig. 10-6A), the angle of
inclination in the femur is greater than the normal
adult angle of 125°.
 The increased angle brings the vertical weight-
bearing line closer to the shaft of the femur,
diminishing the shear, or bending, force across the
femoral neck.
 The reduction in force is actually reflected in a
reduction in density of the lateral trabecular system.

33.  However, the decreased distance between the
femoral head and the greater trochanter also
decreases the length of the MA of the hip abductor
muscles.
 The decreased muscular MA results in an increased
demand for muscular force generation to maintain
sufficient abduction torque to counterbalance the
gravitational adduction moment acting around the
supporting hip joint during single-limb support.

34.  Either the additional muscular force requirement
will increase the total joint reaction force within the
hip joint or the abductor muscles will be unable to
meet the increased demand and will be functionally
weakened.
 Although the abductors may be otherwise normal,
the reduction in biomechanical effectiveness may
produce the compensations typical of primary
abductor muscle weakness.

35.  Coxa valga also decreases the amount of femoral
articular surface in contact with the dome of the
acetabulum.
 As the femoral head points more superiorly, there is
a decreasing amount of coverage from the
acetabulum superiorly.
 Consequently, coxa valga decreases the stability of
the hip and predisposes the hip to dislocation.

36.  Coxa vara is considered to give the advantage of
improved hip joint stability (if angle reduction is not
too extreme).
 The apparent improvement in congruence occurs
because the decreased angle between the neck and
shaft of the femur will turn the femoral head deeper
into the acetabulum, decreasing the amount of
articular surface exposed superiorly and increasing
coverage from the acetabulum.

37.  A varus femur, if not caused by trauma, may also
increase the length of the MA of the hip abductor
muscles by increasing the distance between the
femoral head and the greater trochanter.

38.  The increased MA decreases the amount of force that
must be generated by the abductor muscles in single-
limb support and reduces the joint reaction force.

39.  However, coxa vara has the disadvantage of
increasing the bending moment along the femoral
head and neck. This increase in bending force can
actually be seen by the increased density of
trabeculae laterally in the femur, caused by the
increase in tensile stresses.
 The increased shear force along the femoral neck will
increase the predisposition toward femoral neck
fracture.

40.  Coxa vara may increase the likelihood in the
adolescent child that the femoral head will slide on
the cartilaginous epiphysis of the head of the femur.
In childhood, the epiphysis is fairly horizontal.

41.  Consequently, the superimposed weight merely
compresses the head into the epiphyseal plate. In
adolescence, growth of the bone results in a more
oblique orientation of the epiphyseal plate.
 The epiphyseal obliquity makes the plate more
vulnerable to shear forces at a time when the plate is
already weakened by the rapid growth that occurs
during this period of life.

42.  Weight-bearing forces may slide the femoral head
inferiorly, resulting in a slipped capital femoral
epiphysis.
 As is true for a hip fracture, the altered biomechanics
and at-risk blood supply necessitate restoration of
normal alignment before secondary degenerative
changes can occur.

43. Anteversion/Retroversion
 Variations in the angle of torsion also affect hip
biomechanics and function. Anteversion of the
femoral head reduces hip joint stability because the
femoral articular surface is more exposed anteriorly.
 The line of the hip abductors may fall more posterior
to the joint, reducing the MA for abduction.

44.  As is true for coxa valga, the resulting need for additional
abductor muscle force may predispose the joint to
arthrosis or may function-ally weaken the joint,
producing energy-consuming and wearing gait
deviations.
 The effect of femoral anteversion may also be seen at the
knee joint.
 When the femoral head is anteverted, pressure from the
anterior capsuloligamentous structures and the anterior
musculature may push the femoral head back into the
acetabulum, causing the entire femur to rotate medially.

45.  Although the medial rotation of the femur improves the
congruence in the acetabulum, the knee joint axis
through the femoral condyles is now turned medially,
altering the plane of knee flexion/extension and
resulting, at least initially, in a toe-in gait.
 The toe-in position of the foot may appear to diminish
over time, because it is not uncommon to see a
compensatory lateral tibial torsion develop.
 Although the foot placement looks better, the underlying
hip problem generally remains (with some
developmental reduction).

46.  As noted earlier, the abnormal position of the knee
joint axis is commonly labeled medial femoral
torsion. Medial femoral torsion and femoral
anteversion are the same abnormal condition of the
femur.
 The label designates whether the exaggerated twist
in the femur is altering the mechanics at the hip joint
(femoral anteversion) or at the knee joint (medial
femoral torsion).

47.  As shall be seen in the next two chapters, an
anteverted femur will also affect the biomechanics of
the patellofemoral joint at the knee and of the
subtalar joint in the foot.
 Femoral retroversion is the opposite of anteversion
and creates opposite problems from femoral
anteversion.

48. Summary
 The normal hip joint is well designed to withstand
the forces that act through and around it, assisted by
the trabecular systems, cartilaginous coverings,
muscles, and ligaments.

49.  Alterations in the direction or magnitude of forces
acting around the hip create abnormal
concentrations of stress that predispose the joint
structures to injury and degenerative changes.

50.  The degenerative changes, in turn, can create
additional alterations in function that not only affect
the hip joint’s ability to support the body weight in
standing, in locomotor activities, and in other
activities of daily living but may also result in
adaptive changes at more proximal and distal joints.

51.  Consequently, the we must understand both
the dysfunction that might occur at the hip
and the associated dysfunctions that may
result in or from dysfunction elsewhere in
the lower extremity and spine.

52. End of Part - 5