Surgery For Degenerative Disc Disease

Howard P. Hogshead, M.D.
Howard P. Hogshead, M.D. is an Orthopedic Spine Surgeon
with the Jacksonville Orthopaedic Institute.

About Degenerative Disc Disease

Degenerative disease of the spine is one of the inescapable consequences of aging. Osteoporosis or Osteopenia is more common in older Caucasian women and may result in degenerative disc deformity and compression fractures of the spine. However, Degenerative Disc Disease (DDD) is a separate and distinct entity, occurring in either sex and at an earlier age. Occasionally, symptomatic DDD occurs in adolescence and the incidence rises steadily through the decades. At age 70, nearly all lumbar discs are involved. Obviously osteoporosis and DDD may coexist in the same patient, but one or the other is nearly always more pronounced.

DDD begins as fragmentation of the longchain proteoglycan molecules of the nuclear material of the intervertebral disc. As a result, the physiochemical capacity of the disc to adsorb water is lost. Dehydration of the disc also leads to stiffness and loss of elasticity and the familiar loss of signal of the center of the disc seen on the MRI.

Premature DDD is noted to occur in families, reinforcing the idea of a biochemical defect in the makeup of the ground substance of the nucleus of the disc. As a clinical observation, DDD may also begin with a split or crack in the concentric layers of the annulus, which permits herniation of the nuclear material — the familiar herniated disc.

A normal disc cannot be herniated in laboratory testing of human specimens. Herniation of the disc occurs because the disc is in process of degenerating. It is unlikely that herniation of a normal disc or DDD can be caused by a single traumatic event. The evidence linking DDD to repetitive trauma is stronger; the incidence of DDD is greater in heavy laborers. Pain in a previously asymptomatic, but degenerated disc can be initiated as a result of trauma which causes an annular tear.

Many, perhaps the majority, of degenerated discs are asymptomatic. The reasons for this are not understood at present. Perhaps they remain asymptomatic unless or until a disc herniation, spondylolisthesis, or annular tear occurs. The natural history of DDD is not well understood. Many degenerated discs remain stable for decades while a few progress to a disastrous degenerative scoliosis with severe collapse at several adjacent levels.

Remarkable improvements in spinal surgery have occurred during the past decade. Contributions have come from many different sources with the technology of modern spinal implants being an obvious example. Many other events have contributed to the overall refinements of delivery of care. In many instances, the rapid pace of these developments means that more time must be allowed for full evaluation. The following is a current assessment of the state of surgical management of degenerative disc disease (DDD). Opinions expressed are those of the author.

Who Needs Surgery?

It should be self evident that surgery for degenerative disc disease (DDD) is not appropriate unless other reasonable measures have been exhausted. Surgery is driven by functional impairment, not by imaging studies, not by pain complaints alone, and not by physical findings alone.¹In most cases, patients choose surgery because of quality of life issues. This concept is fundamental and accounts for the fact that only a very small percent of patients who consult a spine specialist require surgery.

Evaluation Of The Patient

Clinical guidelines and algorithms for evaluation and management of adult patients with low back pain have been developed by the US Public Health Service, Agency for Health Care Policy and Research,² by the State of Florida Agency for Health Care Administration,³ and by the American Academy of Orthopaedic Surgeons.⁴ Although this approach may not be popular, a methodical and logical stepwise progression is proven to be most effective in clinical results and in the use of resources.

For the majority of patients presenting to the physician with acute back pain, the appropriate diagnosis is simply idiopathic back pain. Of those patients with persisting pain, a structural basis for their back pain should be considered. DDD is by far the most common structural basis for chronic or recurring back pain.

Most patients with DDD present with back pain, leg pain or varying degrees of both. Some elements of their pain may be due to nerve compression and some elements may be due to structural instability. It is important to differentiate exactly where the pain originates, i.e. it is important to identify the pain generator(s).

Nerve compression may be expressed as a single radicular pain, following a dermatomal distribution. It may also be expressed as a more diffuse type of pain pattern characterized by pseudoclaudication, defined as distance-limited walking due to pain, numbness, and weakness of the legs but without vascular insufficiency. The source of this nerve compression may be evident from the plain radiograph from the MRI or from a myelogram/CT scan. In some instances, where doubt remains, a diagnostic nerve block may be helpful. Again, correlation with the patient's history and the physical examination is essential.

Structural instability of the spine is more difficult to evaluate, as several definitions of instability are in current use.⁵ All definitions recognize abnormal movement of the disc or motion segment when subject to physiologic loads. A discussion of this aspect of instability is found in the AMA Guide to the Evaluation of Permanent Impairment, Fourth Edition.⁶ Many definitions also recognize the inability of the disc to bear physiologic loads without producing pain. Pain produced under load from a degenerating disc is known as discogenic pain. An MRI of the lumbar spine in a middleaged person frequently reveals two or more of the five discs in various stages of disrepair. Which, if any, is responsible for disabling back pain? Although an annular tear may be identified in the MRI, it is essential to obtain confirmation of the suspect disc as the pain generator. Injecting a contrast material directly into the center of the disc under fluoroscopic control provides information about the internal architecture of the disc as well as the leakage of dye out of the disc. More importantly, the painful disc, when injected with pressure, will reproduce the patient's pain pattern. Originally developed along empiric lines, the research documenting the specificity and sensitivity of discography has lagged. It is not to be used as a screening test for persons who might desire surgery. It is useful when used selectively in the context of the other indications for surgery.

Spinal Imaging

The single greatest refinement in spinal surgery is the result of improved imaging, particularly the Magnetic Resonance Imaging.⁷ Prior to the MRI, the surgeon was forced to rely upon the myelogram/CT together with the history and physical examination. Myelograms and CT scans are still needed under certain circumstances. A combination of the MRI and the myelogram/CT in certain situations allows precise visualization of the pathologic lesions and therefore enhancing preoperative planning.

Unfortunately, the advantages of precise visualization also bring with it a significant disadvantage: precise visualization itself. We are now able to see and ponder many abnormalities of which we were previously blissfully unaware. More than 50% of asymptomatic individuals will have abnormalities such as disc protrusions, spinal stenosis or other described abnormalities.^8,9 In other words, the MRI of the lumbar spine provides an extreme sensitivity, but a very low order of specificity. Ironically, the extreme sensitivity of the MRI creates a greater burden for the clinician because findings of the history and physical examination must absolutely correlate with the MRI and vice versa.

Surgical Planning

The surgical procedure must be planned in advance and in detail. Nerve root or cauda equina decompression is usually addressed first. If previous surgery has been performed, there certainly will be dense, adherent scar tissue. It may be necessary to sacrifice a facet joint in order to decompress the nerve root. The use of the surgical microscope or loupes is essential for this part of the procedure. The recent release of a new scarpreventing gel, placed on the dura prior to closure, may help prevent future scar problems. A preoperative blueprint drawn from the patient's own radiographs are very helpful.

The blueprint should also describe the spinal fusion procedure and internal fixation devices. The spinal fusion procedure is basically a joining together of two or more adjacent vertebra to form a solid unit. The spinal fusion requires bone graft (preferably from the patient) to be placed upon a prepared surface of bone and immobilized long enough to permit solid bony healing.

Biology Of The Fusion Process

Fusion is a biological process requiring adequate blood supply, immobilization, and other factors similar to the biological process of fracture healing. Usually 3090 ml. of autologous bone graft containing both marrow cell elements and osteogenic cells can be harvested from the adjacent iliac crest through the same midline skin incision. (Contrary to conventional wisdom, the bone graft donor site is not the major contributing factor to post operative back pain. Most patients cannot identify the side from which the graft was removed.) The autologous bone graft may be augmented with commercially available allograft bone paste or putty. Also available to fill large spaces is a synthetic bone graft manufactured as a porous calcium hydroxyapatite complex. The combined graft mixture is carefully packed into the prepared fusion bed and immobilized. The young osteogenic cells upon which the success of this venture depends are extremely sensitive to electrical fields. They respond to an electrical bone growth stimulator, which may be internally implanted at the time of surgery or may be worn externally as a belt for a few months after the operation. Currently, surgeons are awaiting the FDA release of Bone Morphogenetic Protein (BMP) which is a product of recombinant gene technology. In experimental animal models, the percentage of successful fusion with BMP is vastly improved compared to the control group. Such a tool would greatly simplify the surgical procedure by possibly eliminating the need for autologous bone graft harvesting. Elimination of nonunion problem as a complication of the fusion would greatly improve the success of spinal fusion surgery.

At least three to six months are usually required for solid union in the adult. Internal fixation devices are a means to achieve that end with a higher percentage of success. The instability or abnormal motion responsible for the discogenic pain is eliminated and the load of body weight and movement is carried by the bony mass of the fusion. Once the fusion is accomplished, the internal fixation device is no longer necessary and theoretically could be removed at the discretion of the patient and the surgeon. Retained hardware is seldom responsible for persistent back pain unless there is a nonunion or other structural problem.

Pedicle Screw Fixation

The introduction of the first practical internal fixation for the lumbar spine in this country was the pedicle screw device in 1986. The Harrington rod technique for scoliosis is not suited for the lumbar spine because it obliterates the normal lumbar lordosis. Pedicle Screws had been tried on a limited scale in the 1960's by Paul Harrington and were then used in France on a larger scale in the 1970's. In this procedure, threaded screws -- 57 millimeters in diameter and 3060 millimeters in length -- are drilled into the body of the vertebra via the pedicle. The pedicle is the structurally strongest part of the vertebra, providing very secure purchase of the segment. The screws in adjacent vertebra are then linked together with a plate or a rod, forming a solid framework for immobilization. Placement of the screws is not without some risk and use of pedicle screws has been the subject of numerous lawsuits. Similar to the silicone breast implant controversy, a class action suit by a group of allegedly injured plaintiffs has been filed against the manufacturers of these devices. While these cases are being litigated, surgeons are free to continue to implant these devices, assuming informed consent and reasonable judgement. Continued use of the pedicle screw fixation is important because many complex spinal cases done today could not be done without them. It is hoped that a successful resolution will permit surgeons who demonstrate competence in this technology to use it in situations where it is shown to be advantageous.

Interbody Cage Fixation

An alternative, less complex procedure has become available within the past two years. Two threaded, hollow, 12 18-millimeter titanium cages filled with autogenous bone graft are drilled snuggly into the disc and adjacent vertebral end plates. The cages wedge open or expand the disc space, placing all the surrounding ligaments on stretch. A tight fit assures solid immobilization. The cancellous bone from the vertebra above and below provides the cellular elements and vascular supply to achieve fusion with the bone graft inside the cage in about three months. The surgery can be performed from either the anterior or the posterior approach, but it does not require both. The procedure requires two or three hours and patients usually go home after three or four hospital days. Convalescence is about three months, aided in part by not having disturbed the large pampinal muscles. A 92% successful fusion rate has been reported by the multicenter trials.¹⁰

Degenerative Spondylolistimsis

For discussion, the management of two common forms of DDD is presented. Degenerative spondylolisthesis (see Figures 1A and 1B) of the spine is a not uncommon finding in patients presenting to a spine specialist with back and leg pain.¹¹ It occurs most frequently between the fourth and fifth vertebra in middle aged and elderly women. It is not the result of an injury and is not to be confused with isthmic spondylolisthesis found in both sexes and at a younger age. Degenerative spondylolisthesis occurs because of DDD and instability at this level and as the disc disintegrates, the body weight causes the fourth vertebra to slip or slide forward on the fifth. A kink in the spinal canal is thereby created. The slip is seldom more than 1.5 centimeters. As the disc is not capable of carrying the physiologic load, the facet joints become load bearing and they rapidly enlarge to fill this responsibility. As a result of the combination of the slippage with the inevitable bulging disc and the gross thickening of the facet joints, the spinal canal is narrowed. Spinal stenosis with restricted walking distance due to pseudoclaudication is the result.

Successful surgical relief must accomplish a decompression of the cauda equina by opening up the spinal canal and the foramina, and also restore stability of this segment by a fusion (see Figures 1C and 1D). Success is greatly improved with the use of pedicle screws in this procedure, which on average should require two to three hours operating time and five days hospital stay. Return to full activities requires 3 months for this older group of patients.


Figures 1A and 1B. Degenerative spondylolisthesis with severe spinal stenosis AP and lateral radiograph of 68 year old retiree with severe bilateral leg pain which prevented her from walking more than 50 feet. Note the anterior displacement of L4 on L5.	Figures 1C and 1D. Laminectomy for decompression of the cauda equina and fifth lumbar roots followed by bilateral lateral fusion with pedicle screw fixation from L4 to the sacrum. Note the developing bone, graft fusion mass. Forward slippage of L4 on L5 has been corrected.

Degenerative Disc With Annular Tear

DDD involves breakdown of both the nucleus (see sidebar) and the annulus of the disc. The annulus is the tough, outer casing of the disc and is constructed much like a belted radial tire. It is avascular and consequently heals poorly. Only the outer layer contains sensory nerve fibers. A tear in the annulus from degenerative processes over time can allow degradation products from the nucleus to leak out into the epidural space. Such leaks in the viscinity of the nerve root, seen on discogram, are thought to be the explanation of leg pain without true nerve root compression. Although the patient frequently relates the onset to trauma or other activity, it is not a necessary event. In the experimental laboratory, it is impossible to rupture a normal disc, short of crushing the vertebral bone itself. Annular tears of the disc may occur in healthy young adults and are more frequent in heavy laborers. Deep, persistent, disabling back pain that responds poorly to conservative management is a characteristic of this syndrome.

Successful surgical relief requires fusion of the motion segment, but may also require ablation of the leaking disc. This may require both a front and back surgical approach to the spine often referred to as a "360". In addition to the bone graft and pedicle screw technique described above, a second procedure under the same anesthesia is performed from the anterior, retroperitoneal approach. The major portion of the disc is removed and replaced with a femoral ring allograft or with a cage packed with autogenous bone. Such procedures may require six hours or more and five days in the hospital. Return to work may be from two to six months depending upon job requirements. Return to heavy labor is not encouraged.

An alternative, less complex procedure has become widely available within the past two years. Two-threaded hollow, 12-18 millimeter titanium cages filled with autogenous bone graft is drilled into the disc. The cages expand the disc space, placing all the surrounding ligaments on stretch. A tight fit assures good immobilization and the cancellous bone from the vertebra above and below bond firmly with the bone graft inside the cage in about three months. (See Figures 2A-2D.) The surgery can be performed from either the anterior or the posterior approach, but it does not require both. The procedure takes 2-3 hours and patients usually go home after 3-4 days of hospital stay. Convalescence is about 3 months, aided in part by not having disturbed the large paraspinal muscles. A 92% successful fusion rate is claimed by the sponsored research trials.¹¹ A return to heavy labor is a possibility, but depends primarily upon the health of adjacent discs.


Figures 2A and 2B. AP and lateral radiographs of a 42 year old secretarial worker with severe back pain which restricted sitting to no more than a few minutes. Note marked narrowing of the fifth lumbar disc, AU other discs am normal. Discogram reproduced typical pain.	Figures 2C and 2D. Two titanium cages packed with autologous bone graft are inserted via anterior retroperitoneal approach. Note restoration of the original disc height as well as the foramina.

A multicenter clinical trial comparing the results of the cage fusion procedure performed as open operations with results with cage fusion procedures performed through the endoscope. The endoscopic procedures did not decrease the blood loss, the operative time, or the hospital length of stay compared to the cohort open procedures performed in the conventional manner.

Complications Of Surgery

Nonunion of the bony healing process is still a concern, especially in individuals with diabetes and other metabolic diseases. Tobacco smokers are a special problem⁶and some surgeons are unwilling to operate upon those patients who cannot quit. Despite large dead space, prophylactic antibiotics, deep wound infections can occur because of devitalized tissue, implants, and prolonged operative time. Spinal fluid leaks because of dural tears occur more frequently in redo procedures but seldom have any lasting consequences. Paralysis in the usual sense is extremely rare, but failure to relieve pain due to scar tissue and compression damage to the nerve root or the dorsal root ganglion is unfortunately, still a serious problem. Thromboembolic disease for reasons unknown is very rare with spinal surgery, especially in comparison to procedures for total hip and total knee replacement. Altogether, the combined complication rate from all sources should be less than 20%.

Back pain problems may occur ten to twenty years after a successful fusion due to junctional stresses. Because the fused segment does not bend, additional physical stresses are placed upon the adjacent motion segment by the loads of daily body movements. Larger forces, e.g. laborers or very heavy individuals, magnify the effect. Premature degenerative breakdown of the next level can result in instability and spinal stenosis.

Fusion Class And Critical Pathway

Not all the improvements in spinal surgery involve new hardware or new biologic products. The spinal fusion process for a single patient from beginning to end requires hundreds of individual decisions and actions by a dozen individuals over several months. As a process, this vastly increases the possibility for error. Patients undergoing this experience may be confused or bewildered. Spinal fusion classes for patients and families about to undergo this type of surgery are deeply appreciated. Spinal fusion critical pathways for nursing, laboratory personnel, and social workers have streamlined the hospital phase of the treatment. Each day of the hospital stay is broken down to the individual components so that all health care personnel are aware of their responsibilities as part of the overall process. These are examples of the benefits of having such surgery performed in centers where surgical volume is sufficient to develop expertise.

The Future Of Surgery For DDD

In the future, less invasive surgery for discogenic back pain may become available. As an outpatient procedure, a 17-gauge needle is used to thread a wire circumferentially within the disc. As the wire is heated, the collagen fibers are shrunk and sensory nerve fibers coagulated. A success rate of 75% in avoiding an open spinal fusion procedure was reported in sixty patients in this preliminary report.¹²Despite the available resources and immense study, the clinical results of surgery for DDD are still well below that of surgery for fractures of the spine, scoliosis, and other types of spinal surgery. Why does spinal surgery for degenerative disease have a poor reputation among the general public?^13,14The public and their lawyers regard DDD as an injury rather than a disease process, and as with diseases in any other organ systems, it may be progressive. The surgeon may be able to reset the clock to take off a few years, but the clock keeps right on ticking thereafter. Despite efforts by the surgeon, patients frequently build up unrealistic expectations for the surgical result.

Psychosocial factors frequently obscure the picture. Results comparing the outcome of identical surgical procedures in the workers compensation setting and the private setting reveals a twofold increase in resources consumed with a corresponding 50% decrease in the clinical results. The same is true of patients who are involved in litigation. Better tools for evaluation of the end result of fusion surgery would be helpful. In too many instances at present, the only detectable difference between a good result and a poor result is whether the patient complains of pain. Reliance upon the patient's subjective assessment of the relief of pain is an imprecise endpoint upon which to base an evaluation of the success or failure of a procedure. It is much easier to cite percentage of successful fusion or return to work.

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