Abstract
Objectives: To measure the abductor moment at the hip joint in internal and external rotation and neutral position. To study the relationship between femoral ante-version and the abductor moment generated.
Design: A controlled prospective study comparing a group of children with cerebral palsy with an age-matched control group.
Setting: Gait Analysis Laboratory.
Subjects: The study group of 15 children with cerebral palsy was selected from new referrals with internal rotation sent to the gait lab and our existing database, aged between 6 and 8 years. The control group was recruited from siblings of patients and children of staff.
Methods: The child is positioned supine on a table with their legs hanging over the edge. The knee is bent and the shank placed in a frame at a given position of either 30° internal, neutral or 30° external rotation. An abduction wedge of 15° is inserted between the thighs to give a starting point. The table height is adjusted so that the hip is in 0° flexion and the knee remains in 90° flexion. The position for the dynamometer is marked on the leg, a known distance from the Anterior Superior Iliac Spine. The pelvis is stabilised by an assistant. The child is asked to push the dynamometer away as hard as possible. The maximum force generated is recorded. 3 consecutive readings are taken with a 30 second recovery period between each trial. The test is repeated for each leg position.
An MRI scan of the pelvis and femur is performed. Femoral anteversion and abductor cross sectional area are measured.
Results: Wilcoxon Signed Ranks Tests and paired t-tests were performed.
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The maximum moment generated increased with internal rotation – p< 0.002.
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Children with cerebral palsy generated less moments than the control group – p< 0.05.
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No significant difference in femoral anteversion (hence lever arm) between groups – p< 0.12.
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Cross sectional muscle area (CSA) was reduced in the study group, st dev 327mm2, p< 0.037.
Conclusion: Moments are a product of lever arm length x muscle strength. Differences between groups in abductor moments cannot be attributed to changes in lever arm length. In children with cerebral palsy there is a clear reduction in muscle CSA and therefore strength. These findings suggest that the internal rotation is a compensation for muscle weakness. Initial treatment should therefore entail extensive strengthening exercises, not derotation osteotomy.
The abstracts were prepared by Mr Ray Moran. Correspondence should be addressed to him at Irish Orthopaedic Associaton, Secretariat, c/o Cappagh National Orthopaedic Hospital, Finglas, Dublin 11.
