Alterations of bone density and geometrical properties of the tibia and soft tissue composition in chronic paraplegic men

Spinal cord injured (SCI) patients lοse lean tissue mass and bone but gain body fat. The effects of SCI on bone in paralyzed areas are well documented but there are few data for the importance of the level of injury in the decrease of mechanical strength in paralyzed legs. The aim of the present study was: 1) to describe bone loss of the separate compartments of trabecular and cortical bone in spinal cord injured men and to compare possible changes in mechanical properties of tibia with neurological level of injury and 2) to describe differences in total body, upper and lower limbs in lean mass (LM), fat mass (FM) and body mass index (BMI) between persons with paraplegia following SCI and able-bodied controls. Τhe influence of the neurological level of injury (NLoI) and the duration of paralysis in relation with total body and regional limb LM and FM, were also investigated. Sixty Greek men were included in the study: 30 complete (ASIA A) in chronic stage (>1.5 years in paraplegia) SCI patients were grouped according to the neurological level of injury (NLoI) in high paraplegia group and low paraplegia group in comparison with 30 able bodied subjects as control group of similar age, height, and weight. None of the subjects was given bone acting drugs. All subjects were measured by peripheral quantitative computed tomography (p QCT ). Measurements were performed at the tibia with a Stratec XCT 3000 (Stratec Medizintechnik, Pforzheim, Germany) scanner . The distal end of the tibia was used as an anatomical marker. The parameters of total content , total bone mineral density (BMD) and total area were measured at 4%, 14% and 38% of the tibia length, trabecular BMC, BMD and area at 4%, cortical BMC, BMD, area and cortical thickness at 38% of the tibia length proximal to this point. The periosteal and endocortical circumference were measured at 14% of the tibia. We calculated stress strain index (SSI) a bone strength estimator derived from the section modulus and the volumetric density of the cortical area at 14% (SSIPol2) and 38% (SSIPol3) of the tibia length proximal to the distal end of the tibia. At 66% we measured bone, muscle and fat area. Whole body dual X-ray absorptiometry (DEXA, Norland XR 36, Norland Corporation) was used in order to study subjects with SCI and controls and estimate the values of regional (arm and legs) and total body LM and FM (kg). In all measurements the region of the head was excluded. The results are as follows: 1) Spinal cord injury induce bone loss in paraplegics. A decline in bone mineral density, bone mineral content as well as geometric characteristics of bone is expected in paralyzed tibias. 2) The almost identical losses in BMD trab and BMD tot in the epiphyses show that bone loss was distributed homogenously over the whole epiphyseal cross-sectional area (central and peripheral area) especially in low paraplegics. In contrary in high paraplegics we observed a higher loss of trabecular bone.Because of the similar loss of BMD in both paraplegic groups, we suggest that the cortical shell is more affected in low paraplegics. No geometric adaptations to paraplegia were found in the tibia epiphyses. 3) Bone loss in the diaphyses was manifested in geometric changes of the shaft which suggest the continous catabolic action of paraplegia in this area. The endosteal circumference is increased in paraplegics while the periosteal circumference is not. This finding explains the decrease in cortical thickness in this area. ........................................................................................................