Bone loss and mechanical properties of tibia in spinal cord injured men

The effects of Spinal Cord Injury (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 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 the neurological level of injury.

Fifty men were included in this study: 39 had complete SCI in chronic stage. As chronic stage, we considered paraplegia >1.5 years (yrs). Men were separated as follows: Group A (18 men, high paraplegia: Thoracic (T)4-T7 level, mean age: 33 yrs, duration of paralysis: 5.9 yrs) and group B (21 men, low paraplegia: T8-T12 level, mean age: 39 yrs, duration of paralysis: 5.6 yrs) in comparison with 11 healthy men as a control group (C) of similar age, height, and weight. None of the subjects was given bone acting drugs. The neurological profile of each patient was assessed according to the American Spinal Injury Association (ASIA). All subjects were measured by peripheral quantitative computed tomography (pQCT). 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 bone parameters, bone mass density (BMD) trabecular, BMD total, BMD cortical, and cortical thickness have been measured at 4% and 38%, respectively, of the tibia length proximal to this point, and the periosteal and endocortical was 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.

In both groups A and B most bone mass parameters were statistically decreased in comparison with controls. In each group we calculated the median deltaSSI(3-2) (SSIPol3 - SSIPol2). In the paraplegic groups Spearman correlation coefficient between duration of paralysis and deltaSSI(3-2) was in group A: r=-0.178, p=N.S. and group B: r=0.534, p=0.027, respectively.

Despite the similar paralytic effect on bone in all paraplegic patients in our study and because of the non-significant duration of paralysis between paraplegic groups (p=0.87), the two paraplegic groups act differently in mechanical properties of the tibia. In addition, group A patients in respect to the level of injury, are susceptible to autonomic dysreflexia as a result of the disruption of the autonomic nervous system pathways. These results suggest that neurogenic factors are influencing geometric bone parameters.