Peripheral Quantitative Computed Tomography: Measurement Sensitivity in Persons With and Without Spinal Cord Injury

After spinal cord injury (SCI), paralyzed muscles and atrophybecome highly fatigable.1-4 The loss of regular weight bearing and muscle loading causes rapid bone demineralization (2%–4% per month),5-7 yielding profound disuse osteoporosis in paralyzed limbs.8,9 The lifetime fracture risk for people with SCI is approximately 2 times higher than in able-bodied individuals.10 Fractures readily occur because of low-magnitude forces,10,11 particularly in the distal and proximal epiphyses of long bones.12 These regions contain extensive trabecular bone surrounded by a thin cortical shell. Quantification of osteoporotic change in these regions has been hindered by technical limitations of dual-energy x-ray absorptiometry (DXA). DXA, as a 2-dimensional scan modality, cannot differentiate between cortical and trabecular bone. Because trabecular bone is more metabolically active and is more responsive to a range of stimuli than cortical bone,13 it may be particularly responsive to changes (increases or decreases) in osteogenic stimuli. A previous study revealed that DXA, but not peripheral quantitative computed tomography (pQCT), failed to detect the large increases in trabecular bone mineral density (BMD) in sheep tibiae that underwent a mechanical loading intervention.14 pQCT is being used to measure the effects of disuse osteoporosis in long bones, particularly at the epiphyses.15-17 pQCT has revealed that after SCI BMD rapidly declines in trabecular bone but not in cortical bone.15,17 Cortical bone appears to respond to long-term unloading by endosteal absorption, which decreases the thickness15,16,18 (and hence, the strength) of the cortical shell. Numerous studies19-22 have investigated ways to mitigate the deterioration of bone strength after SCI. All have reported only minimal or transient effects. Eser et al,23 by using conventional computed tomography scans, detected no changes in tibia diaphyseal cortical BMD in response to electrically stimulated cycle ergometry in people with SCI.23 Cortical thickness was not examined. De Bruin et al24 used pQCT to detect trends suggesting that early loading may attenuate the decline in distal tibia trabecular BMD. In studies that measure BMD response to loading, the scan site must be accurately located if between-subject comparisons are to be valid. A primary source of error is that an investigator must first identify external bony landmarks and measure total-limb segment length. In the case of the tibia, by using a scout view (rapid scan) of the ankle joint, the investigator places a reference line at the distal tibial endplate. By using this reference line and the externally measured tibia length, the pQCT scanner gantry advances the requisite distance up the tibia to obtain a scan at 4% of tibia length. The magnitude of BMD error attributable to limb-length measurement is currently unknown. If trabecular BMD is highly variable along the length of the distal tibial epiphysis, then errors in slice placement (because of mismeasurement of tibia length) may lead to false conclusions regarding BMD values across limbs. Accordingly, the purposes of this study are (1) to determine the error attributable to external measurements of tibia length and (2) to establish the difference in BMD along the distal tibia that may occur as a result of tibia length–measurement errors. We explore the difference in error estimates for people with and without spinal cord injury. We hypothesize that limb-length–measurement error could impact the distal tibia BMD measurements.