Weakness in the hip abductor muscles and a lack of functional weight-bearing may contribute to hip displacement seen in patients with spinal muscular atrophy (SMA) and cerebral palsy (CP), according to a new study published in the Journal of Children’s Orthopaedics.

Persistent proximal femoral lateral physeal tilt by head shaft angle was observed in both groups of patients but the progression to hip displacement was earlier and more severe in patients with SMA (34 months vs 40 months; P =.003). The abnormal physeal alignment eventually resulted in coxa valga and secondary acetabular dysplasia.

Previous theories believed that the dysplasia seen in patients with CP was due to spastic hip flexors and adductors overpowering the extensors and abductors leading to a muscle imbalance that moves the hip out of the joint. The presence of similar hip issues between hypertonic patients with CP and hypotonic patients with SMA indicates more common underlying features between the 2 disorders, however.

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The authors hypothesized that the abnormal alignment may be due to weakness of the hip abductor muscles and a lack of functional weight-bearing.

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“Thus, rather than spasticity, the development of [hip displacement] may be more related to features that are common to both hypertonic and hypotonic disorders, with abductor muscle weakness and a lack of functional weight-bearing being probable causes of persistent proximal femoral lateral physeal tilt,” they said.

“As such, strategies aimed at early modulation of proximal femoral physeal growth may be warranted to help prevent or treat [hip displacement].”

During the study, the head shaft angle was high and did not change as children aged. A significant change in migration percentage was observed in both groups of patients between the age groups of 1-2.5 years (T1), 3-5 years (T2), and 6-8 years (T3). Patients with CP had T1, T2, and T3 percentages of 23%, 36%, and 45%, respectively, (P <.01) while patients with SMA went from 37% to 57% and finally 61%, respectively (P =.02).

Using regression analysis, migration percentage showed statistically significant increases by age for patients with CP (r =0.41; P <.001) but not for patients with SMA (r =0.18; P =.09). The acetabular index also increased with age for both SMA and CP groups (r =0.48; P <.001; and r =0.41; P <.001, respectively).

A total of 41 patients with CP (including 42 hips) and 19 patients with SMA (including 37 hips) met the inclusion criteria and were enrolled in the retrospective case-control study. The ages at the first radiograph were 22.6 months for CP and 24.1 months for SMA. At the time of the study audit, the mean age of all patients was 72.1 months.


Ulusaloglu AC, Asma A, Rogers KJ, Shrader MW, Graham HK, Howard JJ. The influence of tone on proximal femoral and acetabular geometry in neuromuscular hip displacement: a comparison of cerebral palsy and spinal muscular atrophy. J Child Orthop. 2022;16(2):121-127. doi:10.1177/18632521221084184