Bone Mineral Density After Spinal Cord Injury: A Reliable Method for Knee Measurement

Abstract

Objectives-To test the interrater reliability of a standardized method to analyze knee bone mineral density (BMD) using dual-energy x-ray absorptiometry (DXA); to compare spine, hip, and knee BMD of people with spinal cord injury (SCI) with able-bodied controls; and to determine the relation between hip BMD and knee BMD in SCI and able-bodied subjects. Design-Criterion standard and masked comparison. Setting-Primary care university hospital.Participants-A convenience sample of 11 subjects with complete SCI was age and sex matched with 11 able-bodied control subjects. Interventions-Not applicable.Main Outcome Measures-Four raters analyzed regions of interest according to operational definitions recently developed to standardize the analysis of BMD of the knee. Subjects with chronic SCI and matched controls underwent conventional DXA scans of the spine and hips and "less conventional" scans of the distal femurs and proximal tibias. The relation between hip and knee BMD was analyzed.Results-The knee measurements were highly reliable (femur intraclass correlation coefficient model 2,1 [ICC 2,1 ]=.98; tibia ICC 2,1 =.89). Subjects with SCI had lower BMD values than controls at all hip and knee sites (P<.05). Lumbar spine BMD did not differ between groups. Hip BMD was moderately predictive of distal femur BMD (R 2 =.67), but less correlated with the proximal tibia (R 2 =.38).Conclusions-Knee BMD can be reliably analyzed using DXA with this protocol. Subjects with SCI have diminished knee and hip BMD. Low hip BMD is associated with low distal femur BMD. KeywordsFractures; Osteoporosis; Paralysis; Rehabilitation; X-ray absorptiometry; dual energy No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Osteoporosis is a severe and debilitating secondary complication of complete spinal cord injury (SCI). Bone mineral density (BMD) of the paralyzed extremities declines precipitously in the first 2 years after SCI, eventually reaching a level below fracture threshold. 1-4 Between 1% and 6% of people with SCI will sustain fractures in their paralyzed extremities. [5][6][7] Fractures are often caused by trivial insults to the limbs 5,7,8 that people with SCI frequently encounter during everyday mobility. NIH Public AccessThe main instrument used to measure BMD is dual-energy x-ray absorptiometry (DXA). In postmenopausal women, DXA-based BMD T scores are used to diagnose osteoporosis and the technique is valuable for monitoring longitudinal changes in bone density. The progression of osteoporosis after SCI has been described for the hip and lumbar spine. In general, hip BMD declines rapidly for the first several months, then declines more slowly until reaching equilibrium at 12 to 16 months postinjury. 1,2,4 Lumbar spine BMD generally does not decline and may even increase after SCI. 4,9 By far the most common sites for fracture after SCI are the distal femu...

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