Abstract
Background: The healthy, adult human disc is innervated but the nerves are restricted to the outer few millimetres of the annulus fibrosus. In degenerate discs with associated back pain, however, the nerves are more numerous and penetrate further in.
We have used a sheep model of intervertebral disc degeneration to monitor the presence and organisation of nerves in the disc as degeneration progresses. This model has been used to study morphological and bio-chemical changes of the disc as it degenerates, in addition to associated alterations in end-plate vascularity and vertebral bone remodelling. One aspect of this model which has not been studied to date is how the innervation of the disc may change with the onset of degeneration. This is the object of the present study.
Materials and Methods: Four-year old, skeletally mature Merino wether sheep (n=64) were divided randomly into lesion and control groups. A surgical incision was created in the anterolateral annulus in the L1–L2 and L3–L4 discs of the lesion group. The control group received the same retroperitoneal surgical approach but the annulus was not incised. Intact lumbar discs encompassed by adjacent vertebral bodies were removed at 3,6,12 and 26 months post operation. Specimens were fixed, decalcified and paraffin embedded before sectioning (7μ thick, vertical sagittal sections) and stained immunohistochemically with the neuronal marker, PGP9.5, together with standard histological stains.
Results: The incised region of the outer annulus underwent collagenous re-organisation, consistent with an active repair process as early as three months post-operatively. However, the inner annular lesion had a poor repair response and propagated with time, sometimes through to the nucleus. In contrast, remodelling of the outer annular lamellae occurred across the cut region. For example, in one sample at two years post injury there were up to six lamellae “bridging the gap”. Nerves were present in all samples but in the sham animals they were very few and confined to the very outer annulus or longitudinal ligament. In the operated animals, nerves were more extensive, occurring in the matrix adjacent to the fissure where there was often blood vessel ingrowth. The maximum number of nerves was seen at 12 months post-operatively, before diminishing in number at 24 months post-op. This paralleled the presence and extent of blood vessel penetration in this experimental model.
Conclusions: We have used an animal model to follow longitudinally the penetration of nerves into the ovine intervertebral disc in association with disc degeneration. Whilst we obviously cannot assess back pain in these animals, and not all nerves are nociceptive, nerves nevertheless are a pre-requisite for the perception of pain. Hence the greater numbers, size and penetration of nerves into degenerate discs demonstrated here has important implications not only for the aetiopathogenesis of degenerative disc disease but also for the treatment of its associated symptoms. Further characterisation of this innervation, i.e. whether autonomic or sensory, may provide an indication as to its nociceptive potential.
The abstracts were prepared by Dr P Dolan. Correspondence should be addressed to him at the British Orthopaedic Association, Royal College of Surgeons, 35-43 Lincoln’s Inn Fields, London WC2A 3PN.
