Vertical facetal instability: Is it the point of genesis of spinal spondylotic disease?

Standing human posture presents unique challenges to the human spine and an added strain to the muscles to maintain the erect spinal posture. Over the years, several researchers have evaluated the biomechanics of spine in general and the overall role of facets in particular. Telescoping of the vertebral spinal segments or vertical instability appears to be the price that humans pay to enjoy the luxury of standing on two legs.

A number of studies in the literature have evaluated the role of disc in the weight bearing, movements, and stability of the spine. Although the contribution of the facets in the stability and movements of the spine has been evaluated, its prime role and seminal contribution in both stability and movements is relatively undervalued. The facets are the only true joints in the spine. The facets are not only the principal site of weight-bearing but also of movements. Odontoid process and discs are the brains and facets are the brawn of spinal movements and stability. Intervertebral discs in the spine and odontoid process in the craniovertebral junction provide a purpose and direction to the movements that are initiated and completed in the facets. We recently likened odontoid process to a rickshaw puller and facets to the large wheels of the rickshaw.[1] The movements of the rickshaw are directed by the rickshaw puller, but are essentially executed and produced by the wheels. In the spine, odontoid process and discs have a role similar to rickshaw puller and the role of facets is similar to the large wheels of the rickshaw.

The facets are located laterally and have an oblique profile. Before the era of computer-based diagnosis, identification of the facets on plain radiographs was less than optimal. The entire diagnostic focus was on evaluation of the intervertebral disc space and on the vertebral bodies. Even with the modern, high-end, computer-based imaging; the obliqueness of the facets does not permit a circumferential evaluation and assessment.

The extensor muscles of the spine have a lifelong role of keep the human form erect and to stand on two feet. The short and long paravertebral muscles not only keep the spine erect but also keep each spinal segment apart. Muscle abuse, disuse, and neglect can lead to weakness of these groups of muscles and can lead to ‘vertical’ spinal instability. The rostral facet slips downwards over the inferior facet. This slippage of the facets is subtle but defining. Although not clearly identifiable, the interfacet articular space reduces due to facet override. The facetal event is probably the beginning of pathogenesis of spondylotic degenerative phenomenon. The rest of the musculoskeletal and discal events in spondylotic degeneration appear to be secondary to primary ‘vertical’ facetal instability. It may also be that the secondary changes like ligamental hypertrophy, disc space reduction, and osteophyte formation are all secondary events that are reactionary and protective in their role rather than pathological or harmful.

Facetal instability has been labeled as retrolisthesis in the cervical spine and facetal overriding in the lumbar spine. The articular capsule that covers the facets becomes loose and dehiscent in some cases, particularly in the cervical spine. The facetal joint space reduces and the articular cartilage progressively erodes. The perifacetal ligaments and articular cartilage separate from the bone resulting in osteophyte formation. The facetal osteophytes are relatively small and circumferential around the facets. In the lumbar spine, the facets become thick and bulbous due to overriding and osteophyte formation and sometimes encroach on to the spinal canal leading to spinal stenosis. Multilevel reduction in joint space due to vertical instability leads to reduction in the height of an individual, as he grows ‘older’.

Whilst the facets of the subaxial spine are oblique or vertical in their transposition, the facets of craniovertebral junction that include occipital condyles and atlantoaxial facets are transversely placed. The facets of atlas and axis are firm rectangular blocks that are placed one over the other. Facetal incompetence can lead to ‘vertical’ atlantoaxial instability or basilar invagination. The vertical instability can be demonstrated to be mobile and reducible.[2] In cases with chronic vertical instability that leads to basilar invagination; dynamic imaging may not demonstrate mobility or instability. Atlantoaxial joint is the most mobile joint of the body and is most susceptible to instability. On the other hand, occipitoatlantal instability is an extremely rare clinical event. On lateral profile imaging, the facet of atlas lies parallel and over the facet of axis. We identified three types of atlantoaxial facets instability.[34] In type 1 instability, the facet of atlas dislocates anterior to the facet of axis. We likened such instability to atlantoaxial listhesis and mimics lumbosacral spondylolisthesis. Anterior dislocation of atlas over the axis leads to basilar invagination and atlantoaxial dislocation. We identified type 2 atlantoaxial facetal dislocation as a situation wherein the facet of atlas is dislocated posterior to the facet of axis. We also identified situations (type 3 atlantoaxial facetal dislocation) wherein the facets were in alignment, but the joint was unstable. Such instability is identified on the basis of clinical and radiological parameters and was labeled as central or axial instability. Despite the presence of instability, the atlantodental interval in types 2 and 3 dislocation remains within normal limits.

Although facetal instability in the craniovertebral can be relatively easily identified due to the large size and flat profile of the facets, subaxial facetal instability is difficult to demonstrate radiologically. However, facetal instability is an underestimated fact and deserves attention. Facetal instability is the primary and sole site of instability in the spine. The instability in the facets may not produce any direct radiological alteration, but may be the cause of back pain. The obliqueness of the facets leads to vertical instability. All the other phenomenon observed in spondylotic spinal disease like reduction of the foraminal height, spinal canal dimensions; ligamentous buckling, disc space reduction, and osteophyte formation seem to be secondary phenomenon to primary facetal instability. When the instability in the facets is marked, it may be manifested by listhesis of the vertebral bodies.

In the year 2010, we proposed an alternative method of treatment of single- and multilevel cervical radiculopathy and myelopathy and lumbar canal stenosis.[56789] The treatment involved distraction of the facets and introduction of intra-articular spacers. The surgery was aimed at distraction arthrodesis of the spinal segment. Distraction of the facets led to reversal of all the alterations generally noted in degenerative spinal disease. There was an immediate postoperative increase in the foraminal height and spinal canal dimensions, stretch unbuckling of the ligaments, and an increase in the disc space height. We proposed that the spinal degenerative issue could be treated by distraction of facets without removal of any part of the bone, ligaments, or discs. It was also observed that there was a potential of reformation of disc fluid and reduction of the size of osteophyte and even bone fusions could be reversed. Similar treatment of distraction and arthrodesis of facets of atlas and axis and attempts at craniovertebral realignment forms a rational method of treatment of basilar invagination that is a form of vertical instability that is more easily visualized radiologically.[1011121314] Essentially, basilar invagination and irreducible atlantoaxial instability in the craniovertebral junction and facetal instability in the subaxial spine are similar in their pathogenesis and effects. Spondylotic spinal disease is secondary to facetal instability and is similar to facetal events that lead to instability at the craniovertebral junction.

As our understanding of mechanics of spine matured, we realized that arthrodesis and fixation of the unstable spinal segments are more crucial in the treatment than attempts at realignment. We proposed only fixation as the form of treatment of degenerative spinal disease.[15] Fixation also seems to be the more important component in the treatment of craniovertebral junction instability in basilar invagination. Bone or ligamentous decompression as the form of treatment was not recommended both at the craniovertebral junction and in the subaxial spine.