The Etiology of Neuromuscular Hip Dysplasia and Implications for Management: A Narrative Review

Abstract

This study summarizes the current knowledge of the etiology of hip dysplasia in children with neuromuscular disease and the implications for management. This article is based on a review of development of the hip joint from embryology through childhood growth. This knowledge is then applied to selective case reviews to show how the understanding of these developmental principles can be used to plan specific treatments. The development of the hip joint is controlled by genetic shape determination, but the final adult shape is heavily dependent on the mechanical environment experienced by the hip joint during growth and development. Children with neuromuscular conditions show a high incidence of coxa valga, hip dysplasia, and subluxation. The etiology of hip pathology is influenced by factors including functional status, muscular tone, motor control, child's age, and muscle strength. These factors in combination influence the development of high neck-shaft angle and acetabular dysplasia in many children. The hip joint reaction force (HJRF) direction and magnitude determine the location of the femoral head in the acetabulum, the acetabular development, and the shape of the femoral neck. The full range of motion is required to develop a round femoral head. Persistent abnormal direction and/or magnitude of HJRF related to the muscular tone can lead to a deformed femoral head and a dysplastic acetabulum. Predominating thigh position is the primary cause defining the direction of the HJRF, leading to subluxation in nonambulatory children. The magnitude and direction of the HJRF determine the acetabular shape. The age of the child when these pathomechanics occur acts as a factor increasing the risk of hip subluxation. Understanding the risk factors leading to hip pathology can help to define principles for the management of neurologic hip impairment. The type of neurologic impairment as defined by functional severity assessed by Gross Motor Function Classification System and muscle tone can help to predict the risk of hip joint deformity. A good understanding of the biomechanical mechanisms can be valuable for treatment planning.

Keywords: cerebral palsy; hip development; hypotonic hip; neuromuscular hip dysplasia; spastic hip.

Figure 1

The growth plate of the proximal femur is divided into three sections. The lateral aspect (yellow) is an apophyseal growth plate stimulated by tension and grows laterally and longitudinally. The intermediate growth plate has longitudinal femoral growth (green), and capital epiphyseal growth plate (red) aligns to the hip joint reaction force responding to compression loading. This whole proximal growth plate remains active for longitudinal growth until skeletal maturity, although it contributes to a lower percentage of longitudinal growth as the child ages. Initially up to the age of 5 years, 50% of longitudinal growth comes from the proximal femur, but this gradually decreases to 10% at maturity.

Figure 2

This is a 10-year-old child (A) with spastic cerebral palsy, GMFCS V, whose hip demonstrates the common changes in the lateral and posterior openings of the acetabulum, flattening of the medial side of the femoral head, and lateral femoral head overgrowth. The lateral side of the femoral head also shows lower bone density than the medial side. The changes are due to the limited range of motion and a femoral head that is not contained in the opened acetabulum, which is the result of hip joint reaction force focused on the lateral acetabulum and medial femoral head. This 11-year-old child (B) is GMFCS III with severe weakness and hypotonia but is able to cruise along furniture and use a walker. His acetabulum is shallow and small with poorly defined edges, and the femoral head is round, has a completely horizontal epiphyseal plate, and is larger than the acetabulum. The femoral neck is in severe valgus and is very thin and long. GMFCS, Gross Motor Function Classification System.

Figure 3

The proportion of longitudinal growth contributed by the proximal femoral growth plate versus the distal is not documented from birth to 4 years of age. We have one case where the growth is documented by pamidronate lines; from age 3–4 years, there was an equal amount of proximal growth compared with distal growth. This graph was independently created using data presented by Pritchett [20] from age 6 to maturity showing that the proximal femur progressively contributes a smaller percentage of total femoral growth with increasing age. This likely accounts for the more rapid proximal femoral shape changes in early childhood.

Figure 4

The normal growth plate of the developing femur in a 2-year-old child (A), a 7-year-old child (B), and an adolescent (C) has apophyseal and epiphyseal segments that are in continuity with the intermediate epiphysis. The triangular-shaped epiphyseal plate has differential growth with a relatively slower growth of the apophyseal side compared with the epiphyseal side, creating a more sharply angled growth plate. This difference in apophyseal versus epiphyseal growth in children with cerebral palsy compared with the typically developing child explains the observed difference in proximal femoral shapes (D,E). The intermediate growth plate contributes to longitudinal femoral growth and width of the femoral neck as shown in this girl, Gross Motor Function Classification System level V, who was treated with pamidronate at age 8 years, leaving a dense line; this radiograph was taken at the completion of growth at age 14 years (F). The shape of the growth plate is unchanged until the end of growth. The acetabulum, however, has increased lateral ossific growth as the ring apophysis is ossified. This lateral growth, combined with the femoral head having symmetric medial and lateral growths, has reduced the migration percentage from 28% at age 8 years to 20% at age 14 years (F).

Figure 5

This 16-year-old adolescent with Gross Motor Function Classification System level V cerebral palsy had a severe windblown deformity with fixed right hip abduction, external rotation, and extension contracture, causing the development of acetabular protrusio due to the high medial pressure of the femoral head in the acetabulum.

Figure 6

This 3-year-old child with Gross Motor Function Classification System level IV cerebral palsy demonstrates the typical coxa valga, eccentric femoral epiphysis formation, and lateral opening of the acetabulum due to hip joint reaction force focused on the lateral aspect of the acetabulum and the medial aspect of the femoral head.

Figure 7

Due to a malignancy in this young male child, at the age of 5 months, the whole abductor medius and minimus was resected along with partial tensor fascia lata and gluteus maximus. At age 15 months, femoral valgus was first seen, and gait was reported to be wide based. This image shows the severe coxa valga 6 years after tumor resection. At this age, this child has no active hip abduction. (Used with permission from Lippincott Williams & Wilkins [23]) “The Creative Commons license does not apply to this content. Use of the material in any format is prohibited without written permission from the publisher, Wolters Kluwer Health, Inc. Please contact permissions@lww.com for further information”.

Figure 8

In the normal child (A), the primary weight-bearing activity that impacts femoral development occurs during the stance phase of gait and creates a hip joint reaction vector (A [Fj]) directed into the acetabulum. This resultant summated force (A [Fj]) is due to body weight moment (A [W]), which must be balanced by the abductor force (A [Fm]). In the weak or hypotonic ambulatory child whose trunk lurches over the weight-bearing hip during gait (B), the abductor force (B [Fm]) is significantly reduced, thereby reducing the hip joint reaction force (B [Fj]), and the hip joint reaction force is also more vertical (B [Fj]). In the child whose summated primary weight-bearing activity is static standing (C), the femoral joint reaction force (C [Fj]) is even smaller, approximately half the body weight (C [W]) and completely vertical (C [Fj]).

Figure 9

In the child with severe spasticity who is not ambulatory, the summated hip force acting on the proximal femoral shape formation comes from multiple muscles all acting in concert. The hip adductor flexor group (AB, AL, G) tends to have more spasticity, creating an adducted and flexed posture. This posture produces the hip joint reaction force vector in the posterosuperior direction, driving the femoral head out of the acetabulum, as well as acetabular dysplasia and valgus proximal femur due to the growth plate aligning at a right angle to this hip joint reaction force as a way to reduce shear stress. Some hips, however, tend to fall more into abduction, external rotation, and extension at which point the abductor, extensors, and external rotators of the hip develop a mechanical moment arm advantage, thereby propagating this posture. The abducted hip has a high hip joint reaction force vector directed at the central acetabulum aligned along the shaft of the femur, also causing increased coxa valga.

Figure 10

This 9-year-old girl with severe hypotonia, Gross Motor Function Classification System level II, developed anterior hip dislocation. The femoral head is enlarged due to not being contained in the acetabulum but round because she has a full range of motion. The acetabulum is shallow and open globally (A). A reconstruction to create anterior lateral coverage with a (Pemberton-type) pelvic procedure was not able to provide sufficient coverage (B). By age 13 years, she developed multidirectional instability that was repaired with a periacetabular osteotomy (Bernese-type) (C).

Figure 11

This is an 8-year-old boy with thoracic-level myelomeningocele who is completely flaccid in his lower extremities. He has an infantile-shaped femoral neck shaft angle that has not been affected by weight bearing or muscle force. The acetabulum, however, is very shallow due to the lack of force to stimulate deep socket formation.

Figure 12

A 2.5-year-old girl was presented at the stage where she was just able to stand in her walker. This is her first screening radiograph (A) showing coxa valga, eccentric femoral epiphysis, severe subluxation, and acetabular dysplasia. Infantile hip ultrasounds were reported as normal. The etiology of this hip was felt to be due to her spasticity. Hip adductor lengthening was performed with immediate return to therapy and no splinting or casting. At the age of 14 years, she is at a Gross Motor Function Classification System level III and is walking with crutches, and her hip is well covered but still has a mild superior placement of the acetabulum as evidenced by the break in Shenton’s line (B). The only treatment for this hip was force balancing by a muscle-lengthening procedure, which was then enhanced by her increased walking ability as her motor development improved.

Figure 13

In the acetabulum that has been opened and is then closed with a peri-ilial osteotomy (A), the overgrowth of the medial acetabular wall especially in the area of the triradiate growth plate becomes apparent ((A), arrow). The acetabular prominence is due to the lack of pressure against the medial wall, and afterward, reduction may prevent the femoral head from seating deep in the acetabulum. In a child such as this 10-year-old, under the influence of femoral head pressure and motion, this prominence can remodel into a congruent acetabulum; however, there is still a thickened medial wall 2 years after reconstruction (B).