LONG-TERM PLANTAR FLEXOR STIMULATION AFTER SPINAL CORD INJURY PREFERENTIALLY PRESERVES BONE MINERAL DENSITY IN THE POSTERIOR ASPECT OF THE DISTAL TIBIA

Abstract

Purpose/Hypothesis: We have recently demonstrated that long-term electrical stimulation of the paralyzed plantar flexors yields a significant trabecular bone mineral density (BMD)-sparing effect (∼31%) at the distal tibia, a common fracture site after spinal cord injury (SCI) (Shields and Dudley-Javoroski, J Neurophys 2006). Because the plantar flexors lie posterior to the tibia, plantar flexor muscle activation may most adequately load the posterior aspect of the tibia, yielding superior BMD-sparing effects in this region. The purpose of the study is to determine whether long-term plantar flexor electrical stimulation yields a differential bone-sparing effect across the distal tibia cross-section. Number of Subjects: 3 men with motor/sensory complete SCI. Materials/Methods: Subjects completed >3 years of unilateral isometric plantar flexor electrical stimulation training. They subsequently underwent magnetic resonance imaging (MRI) and peripheral quantitative computed tomography (pQCT) of both lower limbs (average 4.7 years post-SCI/4.4 years of training). We devised a method to partition the tibia image into anterior and posterior regions for sub-sectional analysis, based on bony landmarks. Results: Distal tibia trabecular BMD was 27% higher in trained limbs than in untrained limbs. BMD differed only by 6.9% between the anterior and posterior regions of the untrained tibia. Conversely, in the trained tibia, BMD of the posterior region was 57.8% higher than the anterior region. BMD of the posterior region of the trained tibia was 235.7 mg/ccm, only 6% lower than previously-published non-SCI reference values (Eser et al, Bone 2004). One subject was unable to undergo MRI due to metal fragments in his torso. In the remaining 2 subjects, the trained soleus was slightly (5%) or considerably (28%) larger than the untrained soleus. However, hypertrophy of the trained limb deep flexors was more uniform (34% and 37% larger than the untrained side). The gastrocnemius showed no hypertrophy in the trained limbs. Tibialis anterior was 19% larger in the trained limbs for both subjects. Conclusions: The BMD-sparing effect of long-term plantar flexor electrical stimulation was most evident in the posterior aspect of the distal tibia, suggesting the possibility of greater bone compressive loads in the posterior than the anterior region. A large training effect (hypertrophy) of the deep plantar flexor muscles supports this observation. Hypertrophy of trained-side tibialis anterior muscles (likely due to co-activation) was insufficient to offset the robust posterior tibia BMD-sparing effect of long-term plantar flexor training. Clinical Relevance: These findings support that long-term neuromuscular electrical stimulation may help preserve trabecular BMD in individuals with SCI. The BMD-sparing effect may be sensitive to the orientation of the compressive loads delivered by activated muscles. Physical therapists should consider using neuromuscular electrical stimulation of certain muscle groups to prevent the severe loss of bone following SCI.