Bone health in children with cerebral palsy

Abstract

Background. Children with cerebral palsy (CP) encounter a number of orthopedic complications as a result of abnormalities in motor function. One of the most significant complications is fragility fractures, occurring in up to 23% of children in prior reports. Despite a growing literature on how to best interpret bone densitometry in children, little research has determined optimal utilization of dual x-ray absorptiometry (DXA) in children with CP in order to characterize the patients' bone health status. This study outlines the use of the mechanostat theory of bone physiology to classify osteopenia and interpret bone complications in this population. The mechanostat theory posits that muscle forces have the greatest impact on bone strength and that low bone mass will result from one of two pathologic circumstances: a primary disorder of abnormally low bone mass despite normal muscle forces, and a secondary disorder of bone mass due to abnormally low muscle forces on bone. The later category, secondary osteopenia, is hypothesized to be the bone health state of most children with CP, due to the motor dysfunction resulting from brain injury in these children. Bone morbidity is expected to be greater in those with osteopenia. Methods. Single, community-based, rehabilitation centre, cross sectional study of 53 subjects with CP age 2-15 years of age. Subjects underwent a baseline interview, examination, x-ray, laboratory and DXA bone densitometry. Calculations of z-score values for total body bone mineral content and muscle mass were made based on published normal children. The z-scores determined the classification of osteopenia with -2 defined as abnormally low bone mineral content or muscle mass. Results. The subjects (51% females) had a mean age of 9 years (s.d.=3.8, range=2.5-15.8). All types and severity of CP were represented in the sample. Normal DXA bone parameters were seen in 24 children, with 11 children classified as having primary osteopenia, five having secondary osteopenia and three with both primary and secondary (mixed). Three children had fragility fractures. Using the classification proposed herein, the fractures occurred only in children defined as having osteopenia. Having at least one bone complication and joint subluxation were more prevalent in the osteopenic subjects compared to non-osteopenic subjects (Chi square, p<0.05). Using z-scores for bone mineral content as an outcome variable, only one CP specific factor, the Gross Motor Function Classification System, was an important independent variable (beta=-0.48, R2=0.18, p<0.05). The final model also included age (beta=0.52, R2=0.34, p<0.05) and gender (beta=-0.36, R2=0.12, p<0.05), showing lower z-scores in males and those of younger age. Use of anti-convulsants, type of CP, family history, calcium and vitamin D intake did not contribute to the model. Measures of pain or quality of life, although worse in osteopenic subjects, were not significantly related to reductions in bone mineral content, when severity of CP was controlled. Conclusions. Using the mechanostat theory to interpret bone density DXA measurements is a more physiologic way to interpret bone health in children and appears foundationally sound in this sample of children with CP. In the reported subjects orthopedic complications were more common in those with osteopenia, and fragility fractures were accurately classified in functional terms according to whether the osteopenia resulted from a primary or secondary bone defects.