Role of Minimally Invasive Surgery in Back Pain for Degenerative Spine
Roberto Bassani and Elena Serchi
The goals of surgery for degenerative diseases are to reduce the pain and disability and to improve the health-related quality of life. Selection of patients is crucial for treatment success. Data from prospective and randomized studies indicate that surgery is effective for the treatment of specific degenerative spine pathologies, such as disk herniation, lumbar radiculopathy, degenerative spinal stenosis, and lumbar spondylolisthesis. Surgery is also appropriate after conservative treatment has failed. More complex spinal disorders are suitable for surgery, although there is a higher risk of adverse events.1,2
The recent literature has addressed the issue of morbidity in spine surgery for degenerative pathologies, which increases the risk of an unsuccessful outcome and reduces the cost-effectiveness of treatment. Patients who are smokers and patients who have a large body habitus or diabetes mellitus have an increased risk of morbidity.3 Another factor that may increase the morbidity of degenerative spine surgery is the approach; a traditional open approach entails soft tissue damage, such as subperiosteal stripping of multiple spinal segments, leading to blood loss, devitalization of paravertebral muscles, and postoperative pain. From this first trigger event, an ongoing pathological chain is generated: postoperative pain leads to prolonged immobility and use of narcotics, increasing the risk of pneumonia, abdominal ileus, and deep venous thrombosis. Biologically, the large devitalized surface caused by retraction and electrocautery predisposes the patient to a deep wound infection, which can result in sepsis, risk of reoperation, and additional prolonged immobility.4 The degree of iatrogenic soft tissue injury negatively correlates with long-term clinical outcome, and can have mechanical effects (a 30% decrease in lumbar isokinetic strength on flexion testing), electrophysiological effects (15 to 20% of patients experience chronic denervation on electrophysiological testing of paraspinal muscles after open surgery), and biological effects (10 to 15% of patients experience histological and radiological alteration in muscles).2
This background explains the current trend in favor of using minimally invasive surgery (MIS) techniques to achieve the same goals as those of an open procedure while limiting surgery-related morbidity.2 MIS techniques are designed to minimize muscle and soft tissue injury, thus reducing postoperative pain and narcotics consumption, decreasing blood loss, and leading to more rapid mobilization.4 MIS approaches have been developed to reach most sites of the spine (cervical, thoracic, and lumbar spine), and in the lumbar spine include posterior, lateral, and anterior approaches. With the development of new technology, such as new endoscopic tools and autostatic retractors, MIS techniques are improving rapidly.
Another advantage of MIS techniques is that they enable treating high-risk populations of patients with pain or degenerative pathologies, such as the elderly and the obese, who are not candidates for the traditional open approaches.
■ High-Risk Patient Populations
The overall mortality from spine surgery doubles between the 65- to 69-year age group and the 80-year and older age group, and morbidity increases in parallel with age and is associated with longer hospitalizations and greater complications. In addition, the aging population has numerous comorbidities, such as cardiovascular and renal disease, poor nutritional status, immobility, obesity, and diabetes mellitus, that are associated with higher complication rates particularly due to cardiac and infectious complications. But the aging population has increased disability related to degenerative pathologies of the spine that lead to back pain and radicular symptoms and cause abnormal posture, which in turn can lead to easy fatigability and predispose to falls (due to alterations in vision and weight distribution). Globally, all these conditions lead to reduced mobility, which ultimately compromises the overall medical health status (in particular the cardiopulmonary status) and decondition the patient. The recent literature supports surgical intervention for these conditions among elderly patients, especially with less invasive techniques that are safer than the traditional open procedures with comparable biomechanical patterns.
Obesity, defined as a body mass index (BMI) ≥ 30, is an increasing worldwide problem. Obese patients often suffer from pathologies of the spine, but spine surgery entails higher risks for complications due to medical comorbidities, higher infection rates (the risk is doubled due to the wider skin incision, elevated blood glucose, and poor antibiotic penetration in fat), and more difficult access to the surgical site, often requiring a longer surgical incision, leading to additional tissue injury. To reduce the complications related to tissue dissection, new transmuscular MIS techniques can be applied. These techniques entail an overall complication rate of 21.8%, with a postoperative infection rate of 0.7% (with an open procedure it is 29 to 33%), a higher incidence of intraoperative durotomies (9.4% vs 3 to 5% of microsurgical diskectomies, apparently related to the greater working distance in this particular population), and a reoperation rate of 9.4%.5
■ Anatomic and Biological Factors in Preserving Lumbar Musculature7
The paraspinal cutaneous tissue is vascularized by a double arterial network, a median and a lateral one, both arising from the lumbar arteries. A part of the lateral lumbar cutaneous territory is also vascularized by the arterial system that emerges through the thoracolumbar aponeurosis at between 5 and 9 cm lateral to the midline.
Two posterior paraspinal muscle groups are present, both of them attaching caudally and running along the thoracolumbar spine:
- The deep paramedian transversospinalis muscle group, including the multifidus (MF), intertransversarii (IT), and quadratum lumborum (QL)
- The more superficial and lateral erector spinae longissimus (Lo) and iliocostalis (IC) (Fig. 13.1)
Globally, posterior paraspinal muscles provide motion and dynamic stability of the multisegmented spinal column, among which the MF muscle plays a key role.
Numerous studies have investigated the anatomy, histochemical properties, and radiological imaging of many of these muscles, with the goal of improving clinical and surgical results and reducing muscular iatrogenic trauma.
Fig. 13.1 Axial T2-weighted magnetic resonance imaging (MRI) of the L4-L5 disk. MF, multifidus; Lo, longissimus; IC, iliocostalis; IT, intertransversalis; QL, quadratus lumborum; PS, psoas muscle.
The structure, function, and design of the upper and lower extremity muscles are well described, but only a few studies have investigated paraspinal muscles. Studies have evaluated the number and orientation of muscle fibers within a muscle, defined as the “skeletal muscle architecture,” in order to predict muscle function. Lumbar spinal muscles were found to have relatively short fibers (~ 10 cm) with moderate-sized physiological cross-sectional areas (~ 10 cm2), which confirms that their global function is to provide stabilization. Among all paraspinal muscles, the MF muscle stands out as the most extreme example of a muscle designed to stabilize the lumbar spine against flexion; the MF showed a greater cross-sectional area than the other lumbar muscles, This design enables the MF to produce vary large forces over a narrow range of lengths. Another peculiar aspect of the MF among the other paraspinal muscles is the uniqueness of its fiber arrangement: an MF slack sarcomere length of 2.2 μm and an elastic moduli of 35 kPa (kilopascals), indicating biomechanical properties comparable with those of the extremities and the quadriceps muscles. Indirectly, assuming that mammalian muscle generates ~ 250 kPa of stress under conditions of optimal sarcomere length and maximum activation, the MF muscle could direct ~ 60 N of extension force to the spine, which is more than twice the amount that could be generated by any other lumbar extensor muscle.
Morphologically, the MF consists of several bundles that originate from the spinous process, that spread caudally for two to five segments, and then insert into the mammillary process of the facet joints and the iliac crest. Functionally, the MF is divided into two layers: deep (dMF) and superficial (sMF).6 The dMF is formed by short muscle bundles and a high percentage of type I fibers (i.e., slow oxidative muscle fibers with high mitochondrial content, which differ from type IIa fibers, which are fast glycolytic fibers with low mitochondrial content, and from type IIb fibers, which are fast oxidative fibers with high mitochondrial content), and it seems to provide compressive force and proprioception. The sMF generates an extension force. Electromyography (EMG) studies documented that the dMF is activated to stabilize the spine, regardless of the direction of stress, whereas the sMF is activated in accordance with the direction of the external load.
The MF muscle receives innervation from only the medial branch of the dorsal ramus, with no intersegmental supply (Fig. 13.2). This nervous branch is relatively fixed as it runs beneath the fibro-osseous mamilloaccessory ligament, exits the intertransversalis fascia, and finally enters the MF muscle from its cranial side.
Neuroimaging of the lumbar MF muscle with magnetic resonance imaging (MRI, Fig. 13.3), computed tomography (CT), and ultrasound assesses its morphology and function, and their possible correlation with pain and disability. Morphology and quantitative measures of the MF muscle are best obtained with MRI. T1-weighted acquisitions are used to measure the cross-sectional area of the muscle, and T2-weighted scans measure the intramuscular adipose tissue. Function assessment is best obtained with ultrasound imaging by measuring changes in MF thickness during submaximal contraction task.
In healthy subjects, the lumbar MF is symmetrical bilaterally and increases its size caudally. The cross-sectional area is larger in males, unrelated to age. The mean adipose infiltration in MF muscle is reported to be between 15% and 29%.
Fig. 13.2 (a) Anatomic model of the innervation and vascularization of the paravertebral musculature. (b) The medial branch (arrow) of the nerve supply to the multifidus.
Pathological Changes in the Multifidus Muscle2,6,7
Preservation of normal anatomy and functioning of paraspinal muscles, particularly of the MF, is a crucial factor in treating low back pain and preventing the postoperative failed back syndrome. Because the adverse effects caused by iatrogenic muscle damage may potentially persist for several years postoperatively, techniques that cause less damage to the paraspinal muscle should be considered when performing lumbar surgery.
During back pain, both acute and chronic, the size and consistency of the lumbar MF change; specific and localized patterns of atrophy are documented in the MF muscle in chronic back pain, greatest at the L5 disk level, with parallel alteration of the neurocontrol.
Fig. 13.3 Wiltse’s approach. (a) Postoperative coronal T2 MRI. (b) Postoperative axial T2 MRI: no muscular trauma is visible.
Physiological and morphological alterations have been observed in dMF and sMF in patients with recurrent/chronic spontaneous low back pain; the changes in the control of the lumbar MF include decreased activation of the sMF, lack of anticipatory contraction of the dMF, and changes in the composition of muscle fibers. Ultrasonography shows an unclear image of the MF in patients with chronic neck pain. The same applies to the lumbar MF: the boundaries of the different layers are less clear and the fat content is higher. Fatty degeneration of the MF muscle has been studied to determine if there is a correlation between the risk of developing chronic low back pain and failed back surgery syndrome.
The traditional midline open procedure causes the detachment of the MF from the spinous process, which compromises its neurovascular supply (Fig. 13.2) and compresses the muscle with prolonged retraction. These factors cause adverse histological and biomechanical changes, resulting in muscle atrophy and consequent decreased force-production capacity of the muscle. Kim et al2 compared trunk muscle strength in patients treated with open midline posterior spinal instrumentation and in those treated with the paraspinal approach, and found that those treated with a midline paraspinal approach had > 50% improvement in lumbar extension strength, whereas those treated with a midline open procedure had no improvement.
Muscle biopsy specimens from patients undergoing revision spine surgery have revealed a change in the type of fibers that form the paraspinal muscle (selective type II fiber atrophy, as well as widespread fiber-type grouping, a sign of reinnervation), and a higher glycerol concentration in the paraspinal muscles than in the deltoid muscles of the same patients; glycerol is an important component of glycerophospholipid, the basic structure of the cell membrane; when the integrity of a cell membrane is destroyed, glycerol is released into the interstitial fluid. Muscle denervation has been proposed to be the main pathological mechanism leading to muscle atrophy (the nerve supply to the MF is monosegmental, making it especially vulnerable to injury), which often worsens with prolonged retraction.8 But other authors proposed that injury is induced by a crush mechanism similar to that caused by a pneumatic tourniquet during surgery on the extremities.9 During the application of self-retaining retractors, elevated pressure leads to decreased intramuscular perfusion. The severity of the muscle injury is affected by the degree of the intramuscular pressure and the length of the retraction time. Muscle biopsies in patients with failed back surgery syndrome showed signs of advanced chronic denervation.2
Indirect evaluation of paraspinal muscle damage has been done with neuroimaging. The iatrogenic damage to paraspinal muscles is demonstrable as T2-weighted hyperintensity on MRI that corresponds to muscle edema, denervation, and fatty infiltration that leads to muscle atrophy (Fig. 13.3).
Because the MF receives innervation from the medial branch of the dorsal ramus in its cranial part, increased T2 signal due to denervation is usually seen in the caudal part of the MF.7 The degree of MF atrophy significantly correlates with the level of low back discomfort as measured with Visual Analogue Scale (VAS) scores.10 Mori et al reported that detection of high T2 signal on MRI after 1 year is valuable as an indicator of paraspinal muscular damage.11 Progressive reduction of edema up to 1 year after surgery and reinnervation of muscles that were denervated during surgery is associated with recovery of signal intensity on T2 MRI. After 3 years, the T2 signal is reported to be almost at the preoperative level, and its evaluation at this time point after the procedure may be less valuable.
Soft tissue trauma can have widespread regional and systemic effects that laboratory tests can demonstrate. Serum creatine phosphokinase (CPK) peaks on postoperative day 1 and subsequently declines, reaching preoperative values 1 week after surgery. Because an increased CPK level is associated with gender and individual muscle volume, the CPK ratio is usually compared instead of its absolute value. Kim et al found that levels of creatine CPK, aldolase, proinflammatory cytokines (interleukin [IL]-6 and IL-8), and anti-inflammatory cytokines (IL-10 and IL-1 receptor antagonist) in patients treated with open surgery were increased severalfold compared with the levels in patients treated with the MIS. Some studies found the CPK ratio to be lower in the paraspinal approaches versus the open midline surgeries, whereas other studies found no difference in postoperative CPK ratio levels between MIS and open surgery, and the authors thus concluded that muscle injury was directly related to the muscle retraction time during surgery.12
■ Posterior Surgical Approaches to the Lumbar Spine
Minimally Invasive Diskectomy3,12
The first pain generator in the spine is the intervertebral disk, which is where degenerative failure begins. The first description of the pathophysiology of sciatica was made in 1934 by Mixter, and many refinements in diagnosis and treatment techniques of the herniated disk have been made subsequently. Numerous MIS techniques have been developed to treat a degenerated disk, preserving notable structures, such as the musculature and soft tissue, from iatrogenic injury. Among these techniques are percutaneous chemonucleolysis, nucleotomy, percutaneous laser disk decompression, and percutaneous endoscopic/microscopic diskectomy.
Chymopapain chemonucleolysis, which enzymatically dissolves the nucleus, resulted in satisfactory treatment in 72% of patients, but the technique has progressively fallen into disfavor because of the reported complications of back pain and stiffness in 20 to 40% of cases, which is sometimes intractable for months, anaphylaxis in 1% of patients, and cauda equine syndrome and acute transverse myelitis in one case each.
Nucleotomy is indicated only for patients with a contained disk. The technique is based on a direct puncture of the annulus to the retroperitoneum to let the nucleus pulposus extrude on the opposite side from the spinal canal. The reported success rate was 72%, but the technique has not been widely accepted, because of potential vascular and nerve damage and the risk of infection in the disk space. The technique is contraindicated in previously operated patients in obese patients and in L5-S1 disk for its specific anatomy.
Laser diskectomy is indicated to obtain reduction in intradiskal pressure due to tissue absorption activity and ablation delivery of the laser, and in contained disks (i.e., prolapsed, not herniated). But few controlled studies are reported in the literature, although there are anecdotal reports of satisfactory results in 60 to 85% of patients. The technique has not been widely accepted.
Microsurgical diskectomy is the first MIS procedure of spine surgery for degenerative pathology. Although more refined techniques have been developed subsequently, microsurgical diskectomy is still the gold standard of care.
Microscopic and endoscopic diskectomy are basically the same procedure, differing only in the type of magnification used; under direct visualization of the herniated disk (increasing the safety and efficacy of the procedure) the sequestered fragment is removed and decompression of the annulus is possible.
Soft tissue retraction during the surgical procedure is widely reported in the literature to cause a regional ischemia, even after short microdiskectomy in young patients. Among recent technical refinements, the transmuscular tubular diskectomy has reduced muscle trauma. Despite a clear perception of better clinical results, a prospective randomized multicenter study failed to demonstrate that tubular diskectomy compared with conventional microdiskectomy results in a statistically significant improvement in the clinical outcome scores. A possible explanation is that the standard microsurgical diskectomy is defined as an open approach, but the surgical incision and muscles dissection are very small (usually 1 to 3 cm), and can be performed in an MIS technique.
Technique (Transmuscular Approach)
The patient is placed in the prone position with the abdomen free and the spine flexed to aid exposure of the interlaminar space. Under fluoroscopic control, a needle is used to localize the disk level. The entry point is 1.5 to 2 cm off the midline and can be checked under fluoroscopy (on the medial border of the anteroposterior [AP] projection of the pedicles). A small linear longitudinal incision is made, and a guidewire is directed toward the inferior edge of the superior lamina. Through the skin incision, sequential cannulated dilators are inserted over the wire, with the initial dilator directed to the region between the spinous process and the facet complex, just above the inferior edge of the lamina. Magnification is obtained with the microscope or endoscope. The laminar edge is identified, and a medial facetectomy is accomplished. Once the nerve root has been identified, it is retracted medially, and the herniated disk is then removed.
Far Lateral Disk Diskectomy
The protrusion of the herniated disk is most commonly located in the preforaminal space, but it can occur in different areas along the perimeter of the annulus. The extrusion of the herniated disk in the extraforaminal space occurs in 7 to 12% of cases, but it leads to the same symptom of radiculopathy as does any other nerve root conflict situation. Conservative treatment is commonly tried first, along with injection of steroids in the extraforaminal area.
Surgical treatment is achieved via a more obliterative approach (monolateral laminectomy and arterectomy) or via an MIS transmuscular approach, described by several authors, that is better known as the far lateral or Wiltse approach.13 This approach is reported to be safe and effective, and it avoids the risk of secondary spinal instability. But it may be especially tricky to perform it at the L5-S1 disk if the L5 vertebra is impacted; at higher levels this is not a problem. This surgical approach is very anatomic but it is technically demanding and is not recommended for surgeons who lack expertise in using it for more typical disk herniations.
The patient is positioned prone. The affected disk level is checked under fluoroscopy, and a linear longitudinal skin incision is made 4.5 to 5 cm off the midline (the exact position is determined by the AP X-ray projection, at the lateral border of the pedicles). Once the muscular aponeurosis is exposed, the fascia is incised to locate the passage between the MF and the longissimus, which is then dissect with a finger to palpate the transverse process of the vertebra. An autostatic retractor is positioned (Beckman, Williams, or Taylor models) and fixed to the operating table. Bony structures are detached and used as anatomic landmarks (upper vertebra isthmus, upper and lower vertebrae joint bone, and lower vertebra transverse process), and the intertransverse fascia is exposed. It is important to search for the perforating branch of the dorsal branch of the lumbar artery and coagulate it by bipolar forceps, because it is a landmark for the nerve root under the fascia. The safer area is the lower part of the fascia near the lower vertebra transverse process; this is where the fascia is coagulated and cut and then removed. It is not necessary to remove all the intertransversalis fascia but at least remove its medial half. The nerve root is located transversally going from the upper medial aspect of this area, downward and laterally. It may be flattened by the herniated fragment, and mistaken for the hernia. It is essential not to attempt to remove the hernia before having properly visualized the nerve root. Once the root is identified, it may be protected with a nerve retractor and the herniated disk can be safely removed (preferably proceeding from the shoulder of the nerve root). The disk need not be removed. At end of the procedure, it is mandatory to check along the canal and along the root, using an oblique hook.
Minimally Invasive Treatment of Synovial Cysts4
Among degenerative lumbar pathologies causing pain, synovial cysts are a rare condition (incidence 0.8%) referred to as degenerative arthropathy of the facets joints. The first-line treatment is percutaneous aspiration or steroid injection (may require a multiple injection, with 75% response rate), but surgery might be recommended in cases of recurrence or intractable pain (immediate symptomatic improvement in > 90%). The laminectomy and partial medial facetectomy can be achieved through a transmuscular MIS tubular retractor system aided with a microscope or endoscope.
Percutaneous Pedicle Screw Fixation
Degeneration of the spine not only often affects the disk but also determines the extent of the deformity and instability, and the extent of bone and ligaments causing stenosis. In most of these conditions, pain and disability are present and after an attempt at conservative treatment (including percutaneous injections) surgery is often recommended (decompression and fusion).
Fusion is achieved with an internal fixation, usually using pedicle screws with or without interbody cages. There have been numerous reports in the literature on this subject.
Fixation of the lumbar spine can be performed in an MIS fashion with a percutaneous approach, eliminating the need for a large midline incision and significant paraspinous muscle dissection.5 (The advantages of muscle preservation were earlier in the chapter.) The procedure was first described by Magerl,14 and it has continued to evolve. But all versions of the procedure use a small paramedian skin incision and position the screws under fluoroscopy. The risk of malpositioning a pedicle screw without direct anatomic vision is low (despite an obvious learning curve) and is reported to be as low as that with open surgical procedures (6.6%, with none requiring surgical revision).15
The rod is then inserted and connected to the screws as in open procedures, but the technique of rod insertion differs, as does the instrumentation used.
In degenerative pathologies, is often recommended to add an internal fusion with an interbody cage to the posterior screw and rod fixation, as this improves stability and provides long-lasting fusion. As in open procedures, interbody fusion is achieved with two cages, positioned through a preforaminal working corridor, such as with posterior lumbar interbody fusion (PLIF), or by using one longer cage and one transverse cage, positioned through a transforaminal working corridor, such as with transforaminal lumbar interbody fusion (TLIF).
Both the PLIF and TLIF techniques can be realized in an MIS fashion (Fig. 13.4). In both cases the patient is positioned prone, with the operating table accessible to X-ray for the latero-lateral (LL) and AP projections.
Minimally Invasive Posterior Lumbar Interbody Fusion16
Open PLIF has the following disadvantages: a 0.3 to 2.4% risk of posterior extrusion of the graft, a 0.5 to 4% risk of retraction injury to the nerve roots, as well as epidural fibrosis and chronic radiculitis, dural tears, and simultaneous destabilization of anterior and posterior columns because PLIF requires the most extensive destabilization of posterior elements (soft tissues, ligaments such as the interspinous ligament complex, leading to the loss of flexion strength and delayed spinal stability, loss of strength in the facets, and extensive laminectomy). Overall clinical failure of open PLIF is reported to be 16%; most authors believe that this failure is directly correlated with the degree of iatrogenic paraspinal muscle injury and with an increased incidence of postoperative failed back syndrome.
Both open and MIS PLIF are reported to achieve 25% restoration of intervertebral disk height, and some degree of retrolisthesis correction is obtained via ligamentopexis; segmental lordosis improved by 29% on average, and foramina volume increased of 20%.
The skin incision is made 1 cm lateral to the exact projection of the pedicle to achieve the correct angulation for accurate screw positioning. The position and trajectory of each pair of pedicles is checked with LL and AP fluoroscopy. Two small paramedial transmuscular incisions (similar to the Wiltse approach described for the far lateral disk diskectomy, above) are made; they are used for both screw positioning and a minimally invasive PLIF-type decompression (hemilaminotomy, flavectomy, facetectomy, or foraminotomy) on each side. With the aid of the microscope or endoscope, the root and dural sac is recognized and retract medially. The external aspect of the annulus is identified on both sides, and an aggressive diskectomy is performed to prepare the intervertebral disk space for placement of the interbody grafts. The appropriate size (especially height and lordosis) are calculated on preoperative lateral radiographs and checked on LL projection during the procedure (disk height is suggested to be as similar as possible to that of the upper and lower level disk); in the L5-S1 segment, extra care should be taken with addressing the lordosis. The correct positioning of the cages is checked with both lateral and AP projections. It is of particular importance to assess the height of the selected cage (cage height should be similar to that of the closer normal disk) and the position in the ventrodorsal direction (the cage must be anterior to the spinal canal; the more anterior it is, the more lordotic an angle is obtained). After the insertion of the cages, the rods are inserted and the system is closed in compression, according to the instrumentation system used. A posterolateral bone fusion also can be added in this MIS approach.