Chapter 161 Lumbar Microdiscectomy
Indications and Techniques
Sciatica and back pain are two of the most common reasons for referral to spine specialists today. Although the overwhelming majority of these patients will not require surgical intervention, surgery may be necessary in some patients. The lumbar discectomy is the most common surgical procedure performed in the United States today for back and leg pain,1 and the outcomes are considered excellent for patients who are good surgical candidates. Despite its widespread use, however, the lumbar microdiscectomy is not without its challenges and potential complications. Prudent patient selection, appropriate preoperative work-up, and attention to operative detail can help maximize good outcomes for this bedrock of spine surgery.
History
Sciatica has been described as far back as ancient Roman and Greek texts. In 1764, Cotugno wrote that the origin of sciatic nerve irritation was due to “acrid humors” and was the first to link symptomatology to the sciatic nerve.2 Vesalius had first identified and described the intervertebral disc in 1555, and in 1779, Pott identified the link between sciatica and deformities of the spine.3 The degenerative processes of the spine and intervertebral disc were described by Luschka in 1858, but the nature of the disc itself was unclear. Early descriptions identified the herniated disc as an “enchondroma,” which was thought to be neoplastic. The true association between the herniated disc and sciatica proved elusive until the early 20th century, when many physicians began to describe relief of sciatica with surgical interventions of the spine. In 1929, Dandy described a patient who had cauda equina due to extruded disc material resulting in nerve root impingement.4 Finally, in 1934, Mixter and Barr published their landmark paper describing their findings in 34 patients with herniated discs that were amenable to surgical intervention.5 Their treatment at that time consisted of a hemilaminectomy for decompression. While the concept was not met with universal acceptance, their work paved the way for improving surgical treatment of herniated discs over the years.6
Pathophysiology
The intervertebral disc is composed of three parts. The first is the cartilaginous end plate. These plates abut the adjacent vertebral bodies above and below and serve as a firm barrier between the disc and the surrounding vertebrae. The second component is the nucleus pulposus, a semigelatinous structure that forms the core of the disc. It serves to absorb the biomechanical strain from axial loading associated with an upright posture. The nucleus is composed of a high concentration of proteoglycans, which serve to increase the osmotic gradient between the nucleus and adjacent plasma allowing for the influx of water into the disc during recumbency. This keeps the disc hydrated and increases its ability to withstand the mechanical forces of an upright posture. To counteract this influx of water due to osmotic forces, there is an efflux of water from the disc when upright due to the mechanical compressive force of the spine. Finally, the third component of the disc is the annulus fibrosus. This collagen-laden structure surrounds the nucleus and serves to limit the lateral expansion due to compressive forces placed on the nucleus by axial loading.7,8
The intervertebral disc loses its structural integrity with time. This gradual, cumulative process is due to changes in both the biochemical and the structural composition of the disc.9 Within the nucleus itself, the concentration of proteoglycans decreases with age, resulting in a decrease in water influx and a constant efflux due to compressive forces and in a net loss of water and elasticity of the disc. Meanwhile, over the years, the annulus wears down because of the strain of mechanical forces. These two changes do not necessarily happen at the same rate. It has been proposed that herniated discs may result if there is a preferential loss of annulus, resulting in a disproportionate strain on the nucleus. Over time, the weakened annulus gives way, with extrusion of disc material.10 Interestingly, despite the fact that disc degeneration is progressive, the peak incidence of disc herniation is in the fourth decade of life (most commonly in the third through fifth decades). This is thought to be because a hydrated, expansive disc is necessary for herniation. As a person gets older, with continued disc desiccation, the nucleus is not flexible enough to extrude past the annulus. During the third to fifth decades, the situation of a hydrated, expansive disc exerting strain against a weakening annulus results in herniation.9
Although disc herniation can present with back pain alone, it classically presents as radiculopathy. The radicular pain seen is due to the location of the disc herniation. Herniations between the midline and neural foramen (posterolateral herniations) are by far the most common. By contrast, far lateral disc herniations comprise only about 10% to 15% of herniated discs.11 Central disc herniations also occur but are rare. They are clinically relevant, however, because of their propensity for causing cauda equina syndrome (Fig. 161-1).
The classic, posterolateral herniation12 is most common because of the anatomy of the posterior longitudinal ligament (PLL). The PLL is thickest near the midline and weakens out near the lateral margin. When a disc herniates, it extrudes through the weaker, lateral margin of the PLL. This results in pain from compression of the nerve root, and sciatica may ensue.
The pathophysiology of radicular pain is not exactly known but does require a compressive component. There is often incongruence between foraminal stenosis and clinical symptoms. As many as 20% of asymptomatic individuals will have a compressive disc herniation.13 In addition, most patients with radiculopathy due to a herniated disc will clinically improve with time despite the fact that their radiographic findings persist. The mechanism by which a compressed nerve root generates pain is incompletely understood. Proposed theories, which have all been demonstrated in animal models, include nerve edema, alterations of nutritional transport, and axonal conduction inhibition.14 Local inflammatory markers have been proposed to play a role. This is supported by the success of anti-inflammatory medications for symptomatic relief. Phospholipase A2 has been identified in surgically excised herniated discs.15
Patient Evaluation and Surgical Indications
In evaluating someone with a herniated lumbar disc, great care must be taken to accurately correlate imaging findings with clinical picture. The incidence of a herniated disc is quite high in the general population, and radiographic presence alone is not a sufficient indication for surgical intervention. Proper patient selection (and not necessarily surgical technique) is believed to be the best predictor of a good outcome.10 Radiographic findings must support the clinical decision to operate, and this cannot be compromised to justify surgical intervention.
Presentation
In most centers, magnetic resonance imaging (MRI) is the standard imaging modality for evaluating back pain; however, modern imaging has failed to accurately correlate findings with the true cause of back pain. The common scenario is the identification of abnormalities without the known significance of these findings.16 For instance, the presence of a “dark nucleus” has been shown to predict the likelihood of back pain, and thus, patients with known pathologic imaging findings are more likely to experience poor outcome,17,18 although a dark nucleus has not been definitively related to any pathologic condition.
While mechanical back pain alone is not generally accepted as an indication for discectomy, exceptions may exist. One such exception is the disc herniation resulting in central spinal stenosis. While patients with disc herniation with central spinal stenosis most often present with claudication symptoms, they often also develop a stooped posture in an attempt to open the spinal canal. This chronically flexed positioning can result in lumbar fatigue and subsequent back pain. These patients may benefit from surgical decompression to alleviate the stenosis. With the resolution of stenosis, posture restores to a more physiologic stance and back pain may improve.19 Another scenario in which back pain alone may indicate a discectomy is the large central disc herniation with avulsion of the PLL. This has been proposed to cause tension on the PLL, which can be a significant pain generator. It has been demonstrated that surgical discectomy in these patients will relieve this tension, resulting in improved back pain.20 Although we acknowledge that exceptions exist, as a general rule, radicular symptoms, not isolated back pain, should be the deciding factor in whether to perform a lumbar discectomy.
As time progresses, back pain may yield to radicular symptoms, and, in fact, there is usually an inverse temporal relationship between the two. These symptoms may include pain, numbness, or paresthesias, which may initially be attributed to other causes, such as osteoarthritis of the hip or peripheral neuropathy. Once correctly identified as radiculopathy, the clinical observer can often (up to 70% to 80% of cases) predict the location of the pathologic disc based on clinical symptomatology alone.21–24 For instance, a standard posterolateral disc herniation should compress the root exiting from the next lower neural foramen (e.g., an L5–S1 posterolateral disc herniation will compress the S1 root) (Fig. 161-2). This is not what is expected from a far lateral disc herniation, where the herniated disc compresses the exiting root from the same level (e.g., an L5–S1 far lateral disc herniation should compress the L5 root). True radiculopathy should follow a dermatomal distribution and may by exacerbated by standing, walking, or Valsalva maneuvers. Severity of pain, on the other hand, can be quite difficult to interpret. For instance, the location of the disc has been shown to affect the severity of symptoms. Extraforaminal disc herniations have been found to be more painful, perhaps because of their direct compression of the dorsal root ganglion.25 In addition, pain tolerance varies significantly between individuals, and thus the overall effect of the pain on the patient’s quality of life is most important. Generally, in the scenario in which a patient has a true disc herniation but symptoms are inconsistent with what is expected, good outcomes cannot be expected from surgical decompression.
A special sequela of disc herniation is the cauda equina syndrome. The typical scenario is an acute disc herniation in the setting of pre-existing central canal stenosis (Fig. 161-3). Presentation is one of perineal numbness, loss of bowel or bladder function, and possibly leg weakness. Back pain may also be present, but it is often disproportionate to the severity of neurologic symptoms. Cauda equina syndrome is a surgical emergency and must be immediately decompressed to prevent permanent loss of function. In a case series by Shapiro,26 those patients receiving decompression within 48 hours of onset were more likely to recover from motor and bladder deficits. This is certainly not a concrete rule, and even with emergent discectomy, recovery is not universal and bowel or bladder disturbances can be life-long deficits.19,27,28 Poor prognostic factors include acute onset, bilateral radicular pain, and saddle hypesthesia.27
On clinical examination, certain findings can also help to localize the level of the herniated disc. A positive straight leg raise is a classic finding with herniated lumbar discs and is most often seen with herniation of the L4–L5 and L5–S1 discs. Compression of the axilla of the nerve root is more likely at these levels and can be elicited with this maneuver. In contrast, the femoral stretch test is more likely to be positive with herniations at higher levels.29 Motor deficit is another finding in some cases. While a useful examination finding when present, because of significant overlap in innervation, true weakness to confrontational testing may be masked from the examiner. Of all historical and examination findings, monoradiculopathic leg pain is the best clinical correlate, superior to straight leg raise and the presence of sensorimotor deficits.21
Imaging
With advances in medical technology and concerns over medical liability, there has been a drastic increase in the use of medical imaging over the years. More than ever, patients often present requesting or even demanding that imaging be performed. It should be remembered that the decision to undertake a lumbar discectomy should be first and foremost a clinical one. Pursuing a diagnostic test should be done only if clinical suspicion is such that the practitioner is considering surgical intervention. If the clinical history is not consistent with a pathologic condition requiring surgery, then the decision to obtain imaging should be questioned. It is important to note that the radiographic size of a herniated disc does not correlate with symptom severity. Those patients with congenitally narrow foramina should experience earlier symptoms than those without. In addition, as stated above, variations in pain tolerance can also contribute to inconsistency.
Timing
Multiple studies have examined the natural history of herniated discs to determine the appropriate timing of intervention. Most data support pursuing conservative therapies for at least 4 weeks initially before offering surgery for radicular pain, even in patients in whom the clinical and radiographic data support a good surgical outcome. This is because, although quicker relief has been demonstrated with surgery,30 the long-term outcomes are similar for conservative treatment and surgery.31–34 It should also be reiterated that the rules for clinical correlation of clinical and radiographic findings should not be broken simply because conservative therapies fail. A patient who does not respond to conservative measures should not become a surgical candidate unless he or she has a correlating disc on MRI. If the imaging does not support the clinical picture, surgery is not a viable option.
Although the data suggest that long-term outcomes are similar for surgery and nonoperative management for radicular pain, in reality, at least in our practice, selected patients who are surgical candidates may be offered surgical discectomy before completion of a full course of conservative therapies for one of two reasons. The first is that patients are often not interested in pursuing conservative therapies and would rather proceed with urgent decompression because they cannot tolerate the pain. The second is the widely held belief that longer duration of symptoms before decompression will result in longer time until postoperative resolution. There is concern that permanent deafferentation can occur from prolonged compression.24,35 Histopathologic specimens have shown changes within the nerve root with prolonged compression that may result in irreversible damage.20,31,35 Although the exact timing is unknown, it is thought that this process takes at least 3 to 6 months to occur. The counterargument is that spontaneous disc resorption does occur and has been documented. While counterintuitive, this seems to be more likely with larger extruded discs than with smaller contained discs. Disc herniation recruits inflammatory cells and subsequent phagocytosis of extruded disc material.31,36–39
Another factor that may play a role in the decision of surgical timing is the issue of financial concern. While some argue that surgery is more costly, others contend that the cost of conservative therapies is higher because of the longer duration of treatment and the potentially longer period during which the patient may be out of work. Patients often wish to pursue early surgery to return to work more quickly. Indeed, studies have demonstrated that increased time off of work before surgery is associated with a poorer outcome.40 Although the cost of surgery can be high, the long-term cost of continued conservative therapies may actually prove more expensive than surgery in select cases.41
Although the patient with a herniated disc most often presents with radicular pain, those patients with motor weakness at presentation should be approached differently. It is generally believed that a motor deficit due to nerve root compression is a more urgent issue than pain alone. Few would argue against the claim that a severe, acute deficit is a clear-cut surgical indication. In addition, a stable deficit that has been managed conservatively but fails to improve is also a surgical indication.20,35,42 Patients who present with a minor, stable deficit represent a more ambiguous situation. It has been our experience that these patients can be treated and followed conservatively. Typically, the acute disc herniation will manifest with an acute deficit that is stable or improves with time. A progressive deficit, on the other hand, should raise suspicion of another pathologic condition. It is often observed that motor deficits improve along with pain over time. If radicular pain is improving, it may be prudent to monitor a mild focal deficit in anticipation that it will subside with time. Surgery for severe deficit with concordant radiculopathy, in contrast, should not be delayed, although patients may not recover completely even with urgent surgical intervention.
Preparation
Principles
Once the decision has been made to perform a surgical decompression for a herniated disc, certain principles should be followed. For instance, the goal of surgery is nerve root decompression, not necessarily to perform an aggressive discectomy. In those patients with foraminal stenosis due to facet arthropathy and a disc bulge, a laminoforaminotomy may be a better option than to open the annulus unnecessarily. The presence of a herniated disc does not mean a discectomy is necessary. In carrying out the decompression, the surgeon should do everything possible to minimize tissue trauma and bone removal. Reduced tissue trauma results in less postoperative scar formation. Overly aggressive bony decompression can lead to pars interarticularis fractures and future instability.
In the operating room, standard surgical steps must be used to prevent wrong patient, level, or side surgery. Treatment with preoperative antibiotics is the standard of care as these have clearly been demonstrated to reduce postoperative infections.43 The use of elastic stockings and compressive boots on patients is recommended to reduce venous thromboembolism, although these have not been proven definitively to be beneficial. A urinary catheter is generally not necessary because of the expected short operative time.
The decision to use microscopic visualization is made according to the surgeon’s preference. Loupe magnification with a headlight and the operating microscope are both good options, and results are equal with either.20 Use of the operating microscope is preferred at our institution because it offers the benefit of ideal visualization for both surgeon and first assistant. It does have the drawbacks of increased cost, lower availability, and unwieldiness when the surgeon is not familiar with its use. Regardless of which device is used, microscopic technique should be employed, as it has been shown to improve accuracy while reducing pain, tissue trauma, blood loss, and hospital stay. That being said, long-term patient satisfaction is no different between standard open discectomy and microdiscectomy.44,45
Positioning
While each of these positions has its indications and advantages, the most widely used is the prone position (Fig. 161-4). The patient is placed on a Wilson frame, a Kambin frame, or gel rolls, with the surgical region centered at the apex of the frame. Pressure on the abdomen should be minimized to reduce airway pressures, epidural venous congestion, and intrathecal pressure. The patient’s arms should be placed on arm boards slightly abducted and extended in a cephalad direction in the “Superman” position. By having the arms in a cephalad direction as opposed to down along the patient’s side, the surgeon is afforded more working room against the patient. This also provides more room for x-ray or fluoroscopy when the time comes to obtain a localizing film. Shoulder extension and abduction should be kept less than 90 degrees to avoid brachial plexus injury at the thoracic outlet. All pressure points should be liberally padded to prevent pressure-related ischemia. Ocular compression from spine surgery, especially in conjunction with hypotension can result in blindness.46 The ulnar nerve can be protected by padding the medial condyle of the elbow. Palpation of peripheral pulses is recommended to evaluate for vascular compression.
An alternative to placing the patient prone on a frame or gel rolls is the knee-chest position. The knee-chest position has been used for obese patients in whom mechanical ventilation would be impossible. This seemingly has the added advantage of reducing epidural venous engorgement and also allows for the abdominal great vessels to fall away from the spine, theoretically reducing their risk of injury.10 While theoretically beneficial in reducing abdominal and thoracic compression, the knee-chest position has not been demonstrated to have significantly different intra-abdominal and thoracic pressures than the prone position.47
A third option is the lateral decubitus position. This is another viable option for those patients whose obesity makes mechanical ventilation in the prone position an impossibility. The patient is placed symptomatic-side-up in maximal flexion with knees and shoulders curled to chest. The region of interest is centered at the operating table joint, and the table is flexed to further widen the interlaminar space. This allows for easier mechanical ventilation at the expense of comfortable working conditions. It also has the advantages of greater possible flexion and a cleaner surgical field as any bleeding runs out of the field as opposed to pooling.
Technique
Posterolateral Disc Herniation
Approximately 70% to 90% of symptomatic disc herniations are in the posterolateral location.10,48 To access these herniations, either a midline or paramedian vertical incision centered above the disc space is planned. This region is prepped in a standard, sterile fashion, and the skin is incised sharply. The decision to use local anesthetic is left to the surgeon. The dissection is then carried down through the subcutaneous tissues with monopolar cautery to the dorsal lumbar fascia. The fascia is incised with cautery on the ipsilateral side of the spinous processes, and the paraspinous musculature is dissected off of the spinous processes in a subperiosteal fashion. Care should be taken to preserve the interspinous ligaments in an attempt to minimize destabilization. By staying in the subperiosteal plane, muscular bleeding can be kept to a minimum. This technique is performed with the monopolar cautery in the surgeon’s dominant hand, and gentle traction of the muscle provided by either suction or a periosteal elevator in the other hand. The dissection is carried down to the lamina, and the muscles are further swept laterally out to the medial border of the facet joint. This lateral exposure can be continued with the cautery or can be done bluntly with a Cobb periosteal elevator. Dissection does not need to be carried out to the transverse process, although exposure of the facet joint aids in placing the retractor. The facet capsule itself must not be violated to avoid destabilizing the joint.
For further visualization, a deeper self-retaining retractor should now be used. Many options exist, including the Taylor, Williams, and speculum-style retractors (Fig. 161-5
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