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: 5

Hip Joint Pathology

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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).

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.

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.

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.

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.

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

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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).

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.

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.

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.

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.

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.

End of Part - 5