INTRODUCTION
In parallel with the fibrosis of the multifidus muscle that is induced by intervertebral disc (IVD) degeneration, recent work has demonstrated a similar fibrosis of the muscle spindles (sensory organs) of these muscles. This is likely to limit transmission of muscle stretch to the sensory receptor and explain the proprioceptive deficits common in back pain. The overall objective of this study was to investigate whether these spindle adaptations could be addressed with exercise. Whole-body exercise reduces fibrosis in multifidus, but not completely. Targeted multifidus muscle exercise reduces atrophy and restores activation in humans. In an animal model, neurostimulation reduces structural and functional changes in multifidus, including reduced fibrosis throughout the muscle. This antifibrotic effect might also reduce muscle spindle changes. We tested the hypothesis that targeted multifidus activation in a model of IVD degeneration would attenuate the increased muscle spindle capsule thickness and reduce presence of Collagen-I.
METHODS
In eighteen sheep, lumbar (L)1-2 and L3-4 IVD degeneration was induced by partial thickness anulus fibrosis incision and a neurostimulator was implanted. After IVD-degeneration developed for 3 months, neurostimulation was activated in nine randomly selected animals. Multifidus muscle (L2, L4) was harvested 3 months after activation. Muscle spindles were identified in Van Giessen’s-stained sections. Connective tissue spindle capsule thickness, and cross-sectional area (CSA) of the spindle, its periaxial fluid and sensory elements were measured. Immunofluorescence assays evaluated Collagen-I and -III.
RESULTS
Multifidus muscle spindle capsule thickness and Collagen-1 (but not Collagen-III) were significantly less in the neurostimulation than IVD-injury animals across L4 multifidus muscle (stimulated muscle, but across the medial and lateral fascicles) (P<0.05), but not L2 (non-stimulated muscle). Spindle capsule thickness was less in lateral than medial regions. CSA of periaxial fluid and sensory elements of the muscle spindle was less in neurostimulated animals at L4 (medial region only).
DISCUSSION
Targeted activation of the multifidus muscle by neurostimulation reduced the structural changes to the multifidus muscle spindles induced by IVD injury, and this was observed within both the stimulated lateral muscle fascicles and those adjacent. Animals that received neurostimulation had less muscle spindle capsule thickening, less accumulation of Collagen-1, smaller total and fluid CSA, and smaller nuclear and bag fibres. These findings suggest that targeted muscle activation either reversed or prevented the structural changes in the muscle spindles and might have clinical value as an effective solution to resolve the commonly identified proprioceptive deficits in back pain and maintain the sensorimotor function of the spine.