Facet distraction spacers for treatment of degenerative disease of the spine: Rationale and an alternative hypothesis of spinal degeneration

Degenerative diseases of the spine are common. The cervical and lumbar spine are more frequently affected and the dorsal spine is less commonly involved in the process. Degeneration of the disk or reduction in its ‘water’ content has been recognized as the principal initiating factor that starts off a cascade of secondary ‘degenerative’ effects on the spine. Numerous processes occur consecutively and the overall effect is reduction of the spinal canal and root canal dimensions, a phenomenon that is manifested by pain and symptoms of spinal cord, cauda equina, and spinal root compression. The disabling nature of the clinical symptoms leads the patient to a doctor. The accepted concept of spinal degeneration is that reduction of the disk space height results in posterior buckling of the posterior longitudinal ligament, and the ‘periosteal reaction’ thus initiates the formation of osteophytes. The osteophytes progressively increase in size resulting in an increasing indentation into the spinal canal. Simultaneous to the buckling of the posterior longitudinal ligaments, there occurs in-folding of the ligamentum flavum. Both these anterior and posterior indentations result in reduction of the spinal canal dimensions and ultimately cause symptoms of neural compression. The spinal degeneration is more common in the junctional zones of the spine at the C5-6 and C6-7 and the L4-5 and L5-S1 levels. Simultaneous to the events that occur in the midline, there occur relatively less apparent, but probably more significant, effects of degeneration on the facet joints. The facets are relatively small in size and the overall movements that occur at these joints are less obvious and get overwhelmed by the imposing presence of the intervertebral disk, which corners the entire focus. Although difficult to evaluate and quantify, reduction of the facet joint space can be an early sign that signals the initiation of the process of degeneration. In the cervical and dorsal spine, the facets being more horizontally (transversely) and obliquely inclined, there occurs a phenomenon of retrolisthesis, wherein, the superior facet slips on to the inferior facet. In the lumbar spine, the facets being more vertically aligned, the articular capsule becomes lax due to vertical facetal override (superior facet slips inferiorly in relationship to the inferior facet) and the joint appears bulkier. Degenerative osteophytes can also form in the facets. The degenerative effects on the facets ultimately result in the reduction in height of the spinal root canal or the intervertebral neural foramina. The symptom of local back pain could also originate from the facet joints. The symptom of claudication pain may also be related to muscle fatigue on walking and resultant exacerbation of facetal override. The entire process of degeneration results in spinal and root canal stenosis. Degeneration can occur at a single or more than one segments of the spine. More often the process of degeneration is generalized and multiple segments and regions of the spine are involved simultaneously. A number of theories and concepts of degeneration of the spine have been proposed and discussed over the last century. However, the basic premise of the hypothesis of origin of the disease process from the primary disk degeneration has been universally accepted and has not been questioned. In the year 2006, we proposed an alternative method of treatment for spinal degeneration, which involved distraction of the facets and forced introduction of ‘Goel facet spacers’.[1 2] Although the technique of introduction of the spacers into the facet joint varied in the lumbar spine, when compared to the cervical and dorsal spines, the basic concept and principle of its action was similar. The process of facetal distraction resulted in a remarkable reversal of almost the entire gamut of changes in the degeneration of the spine.[3] The increase in the height of the facets resulted in an increase in the spinal canal dimensions and in the height and diameter of the intervertebral foramina. The interlaminar distance increased. The intervertebral body height increased in its entirety and there was restoration of height of the disk space. The disk water content also seemed to increase and get restored. The posterior buckling of the posterior longitudinal ligament and the anterior bulge of the ligamentum flavum was simultaneously reduced. There was essentially a reversal of all the major known pathological events of spinal degeneration. Facetal instability could clearly be observed during surgery. Distraction of the facets by forced introduction of spacers resulted in restoration of the facetal height and fixation and alignment of the spinal segment. Over a period of five years, approximately a hundred patients underwent treatment by facetal distraction in our department. Specially designed instruments were used for the purpose. Goel facetal spacers were made from Titanium metal. [The implants are proprietary items of General Surgical Company (GESCO India) Pvt. Ltd. and patent has been filed by Dr. Goel. The implants are not yet commercially available.] The spacers were in the form of a disk, the height of which ranged from 2.5 to 4 mm and the diameter was 8 to 12 mm. The spacer impactor was fixed over the base of the spacer by a screw-type joint. The spacer impactor assisted in impacting the spacer within the facet joint and also directed and controlled its traverse. The procedure of impaction was remarkably safe with regard to the nerve root, spinal cord, and vertebral artery. Essentially, decompression of the spinal canal and root canal was obtained without removal of any part of the bone, ligaments or disk. It was observed that such a treatment was relatively straightforward and easy to perform when compared to the other available methods. The technique resulted in an immediate postoperative relief of symptoms of spinal cord and root compression. Apart from the preoperative clinical and radiological guides, direct intraoperative observation of the status of the facets also guided the decision regarding the levels of the spine that needed treatment. Superior and inferior extension of the level of surgery was significantly easier. Bone graft pieces harvested from the iliac crest were then placed over the laminae, between the spinous processes, and over the treated facets, after appropriately preparing the host area. The procedure ultimately resulted in fusion of the spinal segment. On the basis of our observations during the surgery that involved facetal distraction we present an alternative hypothesis regarding the pathogenesis and progress of spinal degeneration. It appears that spinal degeneration may not be initiated in the disk. Muscular laxity or weakness can lead to spinal instability that is manifested by facetal changes related to its incompetence. It appears that instability of the spinal segment may play a crucial role in the presenting clinical scenario and the observed radiological features. The entire phenomenon of spinal degeneration and resultant changes in the spine may be secondary to facetal incompetence. Such instability is rather easily observed on direct visualization of the joint during surgery, even when preoperative dynamic radiographs do not depict such an event. Reversal of all major known changes, generally associated with spinal degeneration, following a single process of insertion of distraction spacers within the joint, provides credence to this hypothesis. Spinal instability as a result of laxity of muscles of the spine can lead to retrolisthesis of facets of the cervical and dorsal spine and facetal overriding of the lumbar spine. The associated changes with reduction of the disk space height, osteophyte formation, buckling of the posterior longitudinal ligament and ligamentum flavum into the spinal canal, and reduction of the spinal canal and root canal dimensions, may all be secondary phenomena. The fact that traction of the cervical and lumbar spines has formed the principal and successful form of non-surgical treatment over several decades provides support to the concept. The effectiveness of traction as a treatment method can be gauged by its lasting popularity and clinical success. Advocacy of physiotherapy and muscle exercises and consequent relief from symptoms also point in the same direction. Our preliminary observations suggest that distraction of the facets by manual implantation of metal spacers within the articular cavity results in sustained traction and fixation of the spinal segment, and provide an opportunity for local arthrodesis in a distracted position. Although reduction of inter-facet space height has been discussed in the literature, it appears that this may be one important radiological guide that can determine the stability of the region. Decrease in height and retrolisthesis of the facets are indicators of instability and determine the need for distraction surgery. The oblique profile, relatively large size, firmness, and the biomechanical strength of the facets and of the pedicles can be used effectively and safely for distraction of the spinal segments and fixation. Distraction of the facets is done by surgically implanting specially designed spacers. Impaction of spacers in the facets results in several structural changes, all resulting in reversing the pathological effects of spondylosis.