Anterior Discectomy and Fusion Using Simmons Keystone Technique: The Problem

The Problem

Although low back pain is the most common cause of disability in the adult, Hult recognized that neck shoulder and arm pain affected 51% of the adult population.1 This is most commonly caused by cervical spondylosis with degeneration of the intervertebral disc, hypertrophy of the uncovertebral joints, and narrowing of the neuroforamen (fig. 1). It commonly produces neck pain and referred pain to the shoulder and medial border of the scapula. It is commonly responsible for headaches, and produces pain radiating below the elbow, numbness and tingling in the fingers, weakness and loss of coordination in the upper extremities, and in advanced cases, may result in compressive lesions of the spinal cord resulting in spastic paraplegia.

(fig1) Drawing of severly arthritic cervical motion segment
due to collapse of the disc, with growth of marginal
osteopophytes, enlargement of unconvetebral joints,
narrowing of the nerve root canals and overiding of
the facet joints which can result in a variety of cervical
pain syndromes.

Usually, this is a self-limited process that can be treated with conservative measures, such as home traction, exercise, anti inflammatory medication, or a soft cervical collar. For more recalcitrant cases chiropractic or physical therapy are valuable adjuncts. Occasionally, however, the symptoms do not respond to conservative measures. The Simmons Keystone anterior cervical discectomy and fusion has evolved as an effective surgical procedure in the treatment of these more difficult cases.
The diagnosis and treatment of neck disorders is very easy once we remember that the spine has two basic functions. One, it serves as a series of articulated joints that allows us to position our eyes and ears in space, and allows optimal locomotor function of our legs and prehensile function of our arms. Each motor segment consists of a disk and two facet joints which are subject to the same degenerative changes that affect our hips, knees, and other joints. In some circumstances, when the disc collapses movements of the affected segment are altered and initiate an inflammatory process in the spinal joint which can cause headache, shoulder pain, pain referred to the medial border of the scapula, and neck pain. This commonly occurs in the absence of a herniated disc. We have a difficult time as clinicians accepting this basic fact but if we accept it occurring in other joints, it should be intuitively obvious that it should affect the spine.

The second basic function is that the spine serves as a scaffold for the nervous system. With advanced degenerative conditions, herniated discs, tumors, trauma, or infections, the scaffolding can compress or destroy the spinal cord or spinal segmental nerve, producing radicular pain, spastic quadriplegia, or paralysis. An essential point to remember is that neck, shoulder, upper arm and scapular symptoms usually occur as a result of referred pain from compromised spinal joints, not from herniated discs and compressed nerves. Conversely, pain which commonly travels below the elbow and produces numbness and weakness in the forearm and hand is usually caused by a compressed spinal nerve and a herniated disc. Removal of a compromised disc causing one of the referred pain syndromes will commonly make the condition worse since further disc space collapse and joint dysfunction pursues. Furthermore denying the patient with a severely degenerative spinal joint treatment leads to a very unhappy patient and a failure as well.

Why the Keystone?

In 1901, Taylor was one of the first to recommend laminectomy and a transdural approach for excision of a "ventral extradural chondroma". In 1960 Scoville modified this approach for removal of herniated discs, however such procedures were unsatisfactory in bringing about permanent and lasting relief of pain. Adequate exposure of the midline was impossible and there was a risk of damaging the spinal cord. Postoperative morbidity was prolonged and with the exception of the patient with a lateral disc extrusion, few patients improved. Furthermore, narrowing of the disc, compromise of the neuroforamen, and pain emanating from the arthritic spinal joint which worsened as a result of the surgery, continued.

In the 1950's, Smith and Robinson popularized the fact that "disc degeneration with or without osteophyte formation, subluxations, instability of one cervical vertebra on another, or intervertebral disc protrusions are the pathologic changes usually associated with neck, shoulder arm, and hand pain." They published their results of anterior discectomy and fusion in 1958. Nine of fourteen patients had complete relief of symptoms which correlated with solid fusion of the spine. Problems occurred when the small horseshoe shaped graft collapsed and failed to heal (fig. 2).

(fig. 2) Smith-Robinson anterior discectomy and fusion with
horseshoe shaped graft as originally described.

In 1958 Ralph Cloward obtained excellent results in 42 of 47 patients with anterior discectomy and fusion using the drill and dowel technique. Problems included paralysis in several patients from drilling into the spinal cord. Symmetrical drilling of adjacent vertebrae was technically difficult making fusion less predictable. The dowel was not an inherently stable structure and could not maintain fixed distraction. Spontaneous extrusion of the graft was also common making reoperation necessary (fig. 3).

(fig. 3) Cloward cylindrical dowel cannot maintain fixed
distraction and is prone to spontaneous extrusion.
Asymmetrical drilling can also lead to non-union.

In 1969 Simmons and Bhalla published results on 84 patients undergoing anterior cervical discectomy and fusion using the Keystone technique. Good results were obtained in 80.8% of patients. No patient had a non-union, and only one graft was ejected. When compared with a cylindrical graft, a one-level Keystone has 30% and a two-level Keystone has a 70% more surface area available for fusion. The Keystone itself is an inherently stable structure and resists extrusion and lateral bend. Biomechanical tests have shown that the dowel extrudes at 20 to 25 degrees of extension, whereas the keystone graft does not extrude until the posterior elements either are fractured or disrupted (fig. 4).

The Keystone allows excellent exposure of the spine for excision of disc, tumor, osteophytes, or fracture fragments if necessary. It allows the surgeon to treat nerve root or spinal cord compression under direct vision. It provides instant rigid immobilization of the painful arthritic spinal segment in distraction, which effectively enlarges the nerve root canal and eliminates pain. It results in an extremely high rate of fusion even with prior surgery and is associated with minimal postoperative morbidity and few complications. Healing is usually complete in 12 weeks and return to normal activity occurs a short time after.