5.1.1 Hip Subluxation in Cerebral Palsy

The incidence of hip abnormalities in patients with CP ranges from 25 to 30%. ^{1 ,​ 2} It is generally agreed that the incidence is higher with increasing degree of neurological impairment. Overactivity and muscle imbalance around the hip joint (most commonly flexors, adductors, and medial hamstrings) result in fixed musculotendinous contractures that will ultimately become fixed joint contractures. Muscle imbalances lead to typical posturing of the lower extremities with the hip in a flexed, adducted, and internally rotated position.

Normally the acetabulum and the femoral head develop congruently, which is essential for proper development of both structures. The hip in children with CP is considered normal at birth. Abnormalities in weight-bearing, muscle imbalance, and spasticity cause alterations of the femoral head and acetabular relationship leading to progressive subluxation of the joint. Patients with CP tend to have increase muscular imbalances and tone in the adductors, iliopsoas, and hamstrings, resulting in the proximal femur being directed away from the acetabulum. Excessive pressure on the outer acetabular margin caused by the position of the femoral head may prevent or distort normal acetabular development. The acetabulum becomes dysplastic as subluxation progresses. The process of subluxation includes structural bone deformation in both the acetabulum and the femoral head. The deformation of the acetabulum and femoral head can lead to dislocation of the hip. Posterior dislocation is most common because of the direction of muscle pull in the typical position of hip adduction, flexion, and internal rotation.

Migration percentage or index (MI) is the most commonly used measurement of the hip status in individuals with CP (Fig. 5‑1). The measure indicates the amount of ossified femoral head uncovered by the ossified acetabular roof. It is closely associated with acetabular index, so that as one measure increases, the other does as well. Miller et al ³ as well as Reimers ⁴ developed a classification based on the MI as to whether the hip is subluxed (MI > 30%) or is at risk of dislocation (MI > 50%). In the CP population, the MI can increase rapidly at a mean rate of 5.5% per year. ⁴ The tendency to dislocate is directly related to the degree of impairment. Children who are in a persistent posture of hip adduction with little voluntary movement, unable to weight bear, and have difficulty maintaining head and trunk stability are at greatest risk.

5.1.2 Pelvic Obliquity

Pelvic obliquity refers to a deviation of the pelvis from the horizontal in the frontal plane. Fixed pelvic obliquities can be attributed to contractures either above (suprapelvic) or below (infrapelvic) the pelvis.

Suprapelvic obliquity is secondary to significant scoliosis in which the pelvis acts as an end vertebra. Most authors have shown that as the severity of scoliosis increases, so does the degree of pelvic obliquity.

Infrapelvic obliquity develops because of abnormalities in the position of the hip and imbalances in muscle pull on the pelvis. Both pelvic rotation in the transverse plane and pelvic tilt in the sagittal plane occur with obliquity, and the contribution of each is variable. Hip adduction contractures, weak abductors, iliotibial band contracture, and medial hamstring tightness are all implicated in the development of infrapelvic obliquity.

5.1.3 The Windblown Deformity

Letts et al ⁵ popularized the concept of the “windblown hip syndrome,” which is the triad of hip dislocation, pelvic obliquity, and scoliosis, noting an incidence of 13.3% in their series. The clinical appearance is one in which one hip and femur are pointing toward the midline (adducted), whereas the opposite hip and femur are directed away from the midline (abducted). Letts et al ⁵ reviewed 22 patients with windblown hip syndrome and noted that in 15 patients the first pathology was subluxation followed by dislocation of the hip. Dislocation of the hip was then followed by the development of pelvic obliquity in 16 patients, and then by scoliosis in 12 patients. The authors concluded that their analysis of the “temporal sequence” was most consistent with hip subluxation, pelvic obliquity, and finally progressive scoliosis. They theorized that spasticity of the iliopsoas muscle led to hip subluxation, then pelvic obliquity, and finally spinal curvature (Fig. 5‑2). Although the authors noted that in 12 children scoliosis developed after hip subluxation and pelvic obliquity, in 6 children they found scoliosis on the initial radiographic finding prior to hip subluxation and pelvic obliquity. It is also worth noting that the pelvic obliquity and convexity of the scoliosis were on the opposite side of the hip dislocation in 17 patients; however, in 5 patients these were on the same side as the hip dislocation, challenging their conclusions of the temporal relationship between hip subluxation, pelvic obliquity, and scoliosis. Nonetheless, the authors strongly recommend that hip stability be maintained to prevent subluxation so that the development of pelvic obliquity and scoliosis can be avoided.

Cooperman et al ⁶ also felt that the deformities of the hip, spine, and pelvis are interrelated. Although the others did note a unilateral hip dislocation with a level pelvis and no scoliosis in 6 patients in their series, they found more commonly unilateral hip dislocations in concert with pelvic obliquity and scoliosis. The authors concluded that successful reduction of a unilateral dislocation increased the likelihood of a level pelvis at maturity and hence decreased the incidence of developing scoliosis.

More recent literature echoes similar findings with authors reporting a positive relationship between hip and spine abnormalities in this patient population. ^{7 ,​ 8 ,​ 9} Porter et al ¹⁰ reported a significant relationship between hip dislocation and the emergence of scoliosis. A study by Kalen et al ¹¹ also noted a relationship between hip dislocation and the development of scoliosis.

Others, however, have challenged the temporal sequence of events of the triad of hip subluxation, and pelvic obliquity, followed by the development of scoliosis. ^{12 ,​ 13 ,​ 14 ,​ 15 ,​ 16 ,​ 17} Lonstein et al’s ¹⁸ cross-sectional study of 464 patients with CP found that hip abnormalities, pelvic obliquity, and scoliosis were most prevalent in severely involved, wheelchair-dependent patients. Although they found a 57% incidence of hip subluxation, a 58% incidence of pelvic obliquity, and an 82% incidence of scoliosis in their cohort, they found no relationship between hip dislocation, windswept direction, and scoliosis. Similarly, Pritchett and colleagues ¹² studied 80 institutionalized CP patients with unstable hips. They found 35 had a level pelvis despite unilateral or bilateral unstable hips and none of these patients had severe scoliosis. Of the 45 patients with pelvic obliquity, 32 had severe scoliosis. In 38 of the 45 patients with pelvic obliquity, the dislocated hip was on the high side of the pelvis. The authors felt that the unstable hip was associated with pelvic obliquity and scoliosis but not causal to their development. They concluded that scoliosis and pelvic obliquity were correlated with the severity of neurological involvement rather than with the mechanics of a dislocated hip.

Young et al ¹⁹ found evidence of a relationship between tonal asymmetry and direction of windblown deformity in a subgroup of 33 patients, with the hips tending to wind blow toward the side of the lower tone; they found no relationship between direction of tone and direction of scoliosis in another subgroup of 22 patients. In 26 patients who demonstrated both deformities, no relationship was found between direction of windblown deformity and direction of scoliosis.

Abel and colleagues, ²⁰ although hypothesizing that the subluxed hip would be opposite the scoliosis apex and ipsilateral to the high side of the pelvis, were not able to substantiate this in their study of 37 patients with total body involvement CP. They found that hip subluxation strongly correlated with the degree of femoral adduction and weakly with the magnitude of suprapelvic obliquity. They found that at a young age, infrapelvic deformity predominates with asymmetric hip adduction; however, later the windblown position seems to result largely from progressive pelvic obliquity and rotation associated with scoliosis.

Senaran et al’s ¹³ most recent prospective study found that in most patients with unilateral hip dislocation, subluxation occurred on the high side with a significant increase in pelvic obliquity; however, no significant relationship between hip dislocation and the emergence and progression of scoliosis was reported. Others have also shown a poor correlation between the triad of hip subluxation, pelvic obliquity, and scoliosis. ¹⁴