|dc.description.abstract||Background: Persons who sustain a spinal cord injury (SCI) experience a dramatic loss of muscle and bone, and a dramatic increase in adipose tissue. It has been suggested that the muscle atrophy, obesity, and sublesional osteoporosis (SLOP) that occurs after SCI is due in part to the loss of voluntary control of the skeletal muscles in the lower extremities, impaired energy metabolism below the level of the lesion, and cessation of sufficient mechanical strain on bone. The prevalence of obesity and SLOP after SCI leads to increased cardiovascular disease and fracture risk, respectively. Current body composition screening procedures for the general population fail to identify individuals with SCI who are obese or have SLOP.
Muscle contractions provide physiological loads on bone; thereby a muscle-bone relationship is proposed with proportional declines in muscle and bone after SCI. In addition, both positive and negative relationships have been proposed between adipose tissue and bone; increased skeletal load bearing from excess adipose tissue mass may account for the positive associations reported to date. Due to a lack of load bearing activity after SCI, there should be a negative association between adipose tissue and bone.
Objectives: The primary objective is to characterize body composition among adults with chronic SCI using valid, reliable, and interpretable measures, and to suggest screening procedures for the detection of obesity and SLOP in this population. The secondary objectives are to explore the associations between: 1) muscle and bone, and 2) adipose tissue and bone.
Design and Setting: Cross sectional observational.
Population: A sample of 16 individuals (13 men, 3 women) with chronic SCI participated in this study. The neurological level of lesion ranged from C3-T12, with 9 motor complete and 7 incomplete SCI. Average±standard deviation for age was 51.12±12.37 years, and duration of injury 16.5±7.87 years. An additional 29 individuals with chronic SCI were included when exploring the relationship between muscle and bone. Forty-one individuals (31 men, 9 women) were included in this analysis; the neurological level of lesion ranged from C2-T12, with 13 motor complete and 28 incomplete SCI. Average±standard deviation for age was 48.7±13.36 years, and duration of injury 114.22±10.4 years.
Methods: Lean tissue, adipose tissue, and bone tissue were measured via surrogates of body adiposity, as well as two different scanning technologies. Lean tissue was assessed via muscle cross sectional area (CSA) (mm2) and muscle density (mg/cm3), and measured using peripheral quantitative computed tomography (pQCT). Adipose tissue was assessed via body mass index (BMI) (kg/m2), waist circumference (WC) (cm), and % body fat, and measured using a floor scale, tape measure, and dual energy x-ray absorptiometry (DXA), respectively. Bone tissue was assessed via hip, distal femur, and proximal tibia areal bone mineral density (aBMD) (g/cm2) using DXA, as well as cortical thickness (mm) and total volumetric bone mineral density (vBMD) (mg/cm3) at the 1/3 proximal tibia, and trabecular vBMD (mg/cm3) and total vBMD (mg/cm3) at the distal tibia using pQCT. The relationships between muscle and bone, and adipose tissue and bone, were determined by correlating muscle CSA with indices of bone strength, and indices of obesity with indices of SLOP, respectively.
Results: The majority of participants had lean tissue values below able-bodied norms (67-100%). When using the able-bodied definition of BMI >30 kg/m2, 19% of individuals were obese, whereas 63% and 81% were obese when using SCI-specific definitions of BMI >25 kg/m2 or >22 kg/m2, respectively. One hundred percent of individuals had SLOP using distal femur Z-score, and over 50% were at risk of fracture using distal femur fracture threshold of <0.78 g/cm2. Weak (r=0.42) to moderate (r=0.57) correlations were found between muscle CSA and indices of bone strength, supporting the theory of a muscle-bone unit. No correlations were found between adipose tissue and bone.
Conclusions: Based on the cohort data, we propose that individuals with ≥2 risk factors (female, ≥60 years of age, duration of injury (DOI) ≥10, tetraplegia, motor complete) should be screened for obesity using % body fat from DXA as well as a combination of carefully interpreted SCI-specific BMI and WC. In addition, these same individuals should be screened for SLOP using a distal femur Z-score and fracture threshold from DXA. It is clear that due to the prevalence of obesity and SLOP in this population, intervention for prevention or treatment is essential. The presence of a muscle-bone unit indicates that muscle atrophy contributes to a reduction in bone strength; this is clinically important, as muscle strength is potentially amenable to rehabilitation intervention. No correlation was found between adipose tissue and bone. Future work should continue to explore these relationships using appropriate technology.||en