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Posterior lumbar interbody fusion with metal cages: Current techniques
POSTERIOR LUMBAR INTERBODY FUSION WITH METAL CAGES: CURRENT TECHNIQUES ATUL PATEL, MD*, and WILLIAM C. WELCH-t, MD, FACS, FICS
Posterior lumbar interbody fusion (PLIF) is a technically rigorous surgical fusion procedure. The procedure has had a resurgence in application and acceptance due to recent technological advances in cage design. We review the history of this procedure, indications, and patient evaluation. We also describe the surgical technique of freestanding PLIF and a new technique utilizing a single cage with pedicle screw supplementation. KEY WORDS: lumbar fusion, interbody fusion, posterior lumbar interbody fusion
INTRODUCTION Recently reintroduced and popularized, posterior lumbar interbody fusion (PLIF) can be found mentioned in the literature as far back as 1936 when Mercer, writing in the Edinburgh Journal of Neurosurgery, suggested that "the ideal operation for fusing the spine would be an interbody fusion." He never was to perform this procedure, however, saying that "the surgical difficulties encountered...would make the operation technically impossible. 'q Other early proponents of PLIF include Briggs and Mulligan, who packed the intervertebral space with bone chips and even recommended driving a round bone peg into the interspace, 2 and Ovens and Williams, who performed an intervertebral spine fusion after removal of a herniated lumbar disc. 3 It is Ralph B. Cloward, however, who is commonly credited with popularizing PLIE Dr. Cloward's first PLIF was performed in 1940 after a lumbar discectomy for sciatica in a schoolteacher. This patient died of a pulmonary embolus, and 3 years were to pass before World War II compelled Cloward to again undertake PLIE In caring for many of those who sustained back injuries in the line of duty, Cloward noted that although discectomy alone relieved the radicular pain these patients experienced, the patients were left with significant low back pain. When he added PLIF to discectomy, Cloward noted a significant improvement in his patient's residual low back pain. In fact, in presentations describing his early results, first to the Hawaii Territorial Medical Association (1945), then to the Harvey Cushing Society (1947), and eventually to the American Medical Association (1951), Cloward estimated a 30% to 50% higher cure rate when PLIF was performed with discectomy over From the *[Departments of Neurologic Surgery and 1"Orthopaedic Surgery, and the 1-School of Rehabilitative Sciences and Technology, University of Pittsburgh School of Medicine, Pittsburgh, PA. Address reprint requests to William C. Welch, MD, FACS, FICS, University of Pittsburgh School of Medicine, UPMC-Health System, Department of Neurosurgery, 200 Lothrop St, Suite B-400, Pittsburgh, PA 15213. Copyright ~ 2000 by W.B. Saunders Company 1048-6666/00/1004-0008510.00/0 doi: 10.1053/otor.2000.16705
discectomy alone, asserting that 82 of 100 such patients were able to return to manual labor. Cloward remained so confident of the utility of PLIF that he recommended that the simple discectomy, the decompressive laminectomy and chemonucleolyis be eliminated, asserting that "the PLIF technique is the answer to the treatment of diseases of the lumbar spine and may be the operation of the future. ''4 From these early beginnings, PLIF has undergone many variations, both procedural and technological, and the certitude of Cloward's statement has been supplanted by controversy. Indeed, opposing viewpoints can be found in the literature concerning the various indications for PLIE its biomechanical stability, the surgical technique in its many modifications (including the necessity of posterior fixation), and perhaps most importantly, clinical results after PLIE We briefly illustrate these disputes and describe the current technique of posterior lumbar interbody fusion with metal cages.
INDICATIONS The list of indications for PLIF has grown steadily since its initial use as the primary treatment of herniated discs. It now includes recurrent disc herniation, spinal stenosis with associated instability, degenerative spondylosis with resultant mechanical back pain, spondylolisthesis, spinal instability, and discogenic back pain. Interbody fusion cages have been used during the correction of scoliosis and in cases of failed prior lumbar fusion attempts. Although none of these indications have been proven by randomized, controlled clinical trial and outcome data based on specific indications is rare, supportive studies are encountered in the literature for each of these categories. As previously mentioned, Cloward is perhaps the strongest proponent of PLIF in lumbar disc disease. He believes that, after simple discectomy, "up to 70% [of patients] return" and 20% require a second operation with fusion. 1 Others confirm these poor results. Rish assigns a 12% failure rate to discectomy alone with up to 30% of patients requiring additional surgery. 5 In his 12-year experience with 250 patients, he notes that "posterior lumbar interbody fusion
Operative Techniques in Orthopaedics, Vol 10, No 4 (October), 2000: pp 311-319
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significantly reduces the rate of recurrent problems requiring subsequent surgery (8.4% v 18%).''~ Hurter surveys literature summarizing results in over 10,000 patients and recounts a 17% to 20% failure rate after discectomy alone, with up to 50% of patients requiring a second operation. He additionally notes that, after simple discectomy for radiculopathy, low back pain can develop in up to 30% of patients at 4 years and 80% of patients at 10 years. 7 Similar to results in surgery for herniated discs, surgery to relieve spinal stenosis has a short-term failure rate of 15% to 20% and up to a 50% failure rate at 10 years. Hutter reports an 80% excellent or good outcome in a subgroup of 142 patients undergoing PLIF for spinal stenosis followed-up for a minimum of 3 years. 7 Even better results have been seen in subgroups of patients treated with PLIF for spondylolisthesis, both degenerative and postlaminectomy, with Cloward reporting a remarkable 95% excellent clinical result. 4 Perhaps the most interesting role of PLIF has been in treating patients with pain thought to derive from internal disc derangements, termed discogenic pain. This population, which usually suffers from low back pain without associated radiculopathy, has classically been believed to receive no benefit from lumbar disc surgery. Again, however, use of PLIF to treat this group is not a new idea. Cloward reports diagnosing and treating discogenic back pain by cortisone injection since 1959.4 A recent study by Schechter demonstrates an 89% good or excellent clinical result with a 95% fusion rate in PLIF for discogenic back pain diagnosed by provocative discography. ~ Lee notes an identical 89% satisfactory clinical result, with 89% patient satisfaction, a 93% return to work rate, and a 94% fusion rate in 62 patients with chronic disabling low back pain attributed to internal disc derangements treated with PLIF?
A standard physical examination is performed with an emphasis placed on examination of the mechanical properties of the lumbar spine. Gower's maneuver, when spontaneously exhibited by the patient upon arising from a flexed position, may be a strong indicator of instability. Straight leg raise and crossed straight leg raise tests are performed to determine the amount of active nerve root irritation. 11 All patients undergo standardized radiographic studies, including plain spine radiographs with flexion-extension views if instability is suspected. Generally, lumbar magnetic resonance imaging (MRI) is the next test of choice. The discs are evaluated for desiccation, loss of height, and end plate changes as described by Modic (Fig 1). 12,13The study should be evaluated for the presence of conjoined nerve roots because it may be dangerous to perform posterior interbody fusion in the presence of conjoined roots. A computed tomography (CT) myelogram may be a necessary adjunct to MRI to assess structural abnormalities of the lumbar spine. The CT scans provide increased bony detail compared with MRI alone. The postmyelographic CT scan shows the relationship between the structural bone abnormalities and the cauda equina in tremendous detail. This may help to determine the amount of neuroforaminal decompression required. Sagittal reconstructions are useful to determine disc space height, measure the amount of listhesis, and plan on interbody fusion cage size. Discography may be considered next. There is a large body of literature reviewing the advantages and disadvantages of discography, which will not be reviewed here. M-I~ We frequently perform discography to help diagnose discogenic back pain, using the information to determine how many levels require interbody fusion. We do not perform this study on all patients, however. Specifically, patients with the more typical history of mechanical lower
PREOPERATIVE EVALUATION The most common indication for PLIF is lumbar instability and discogenic lower back pain. The decision to operate on patients with these diagnoses is based on patient history, physical examination, radiographic studies, provocative tests, and response to previous treatments. A careful history is taken to assess the chronicity, quality, and location of pain. Efforts are made to determine the mechanical quality of pain (ie, if the pain is worse with movement). We specifically ask the patient to determine during which part of the day the pain is at a minimum. Generally, the pain is reduced when the patient is in a recumbent position. Another important question is to determine the ratio between the patient's lower back pain and their leg pain. Patients with much greater leg pain than back pain may respond to therapies directed at nerve decompression alone and may not require interbody fusion. Those with an equal distribution of leg and back pain or a preponderance of back pain are more commonly regarded as interbody fusion candidates. The remainder of the history is taken with suspicion as to the other myriad of potential causes of lower back pain. m Patients are strongly encouraged to stop using tobacco products, and any medicines with anticoagulant effects are discontinued.
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Fig 1. Sagittal MRI scan showing disc desiccation and degeneration. PATEL AND WELCH
back pain, single level disease on MRI, and no excluding factors are not sent for discography. Discography is performed on patients with prior surgery, multilevel disease, or atypical histories. Patients with concordant pain and radiographic evidence of disc derangement on discography are offered PLIF with discectomy. Other studies are performed as necessary. Abdominal and pelvic CT or MR] scans are performed if retroperitoneal mass or plexus lesions are considered likely. Electromyograph (EMG) testing may be required to help determine which level of nerve root dysfunction is present. Rarely, bone scanning is required to evaluate for the possibility of metastatic disease. Complete blood count and erythrocytic sedimentation rate blood studies are obtained if infection is deemed possible. Patients suffering from chronic low back pain secondary to degenerative disease must complete an aggressive physical therapy regimen and must fail 6 months of nonoperative conservative treatment before PLIF is considered. Other potential treatments include long-term nonsteroidal anti-inflammatory medications, intradiscal electrothermy, 17 facet blocks, and epidural steroid injections. These treatments must be weighed against their temporal effects and the patient's desire for a long-term solution. We generally consider PLIF techniques for patients with posterior pathology, such as pars interarticularis fractures, spinal stenosis with instability, and discogenic disc disease with significant radicular complaints and findings. The surgery must include thorough decompression of the involved nerve root(s). Patients with 90% or more back pain and no contraindications generally undergo anterior lumbar interbody fusion for treatment of the underlying degenerative disc disease. There is a long history of successful PLIF performed as a stand-alone procedure described in the literature and reviewed previously. Over the past few years, however, there has been increasing interest in supplementing posteriorly inserted cages with pedicle screw fixation. The rationale for this combined approach is straightforward. During the insertion of posterior cages, a significant amount of stabilizing structures, such as the facet joints, may need to be removed. This has the potential to destabilizes the spine. TM The addition of pedicle screw fixation creates an extremely strong construct, which may lead to higher fusion rates. This 360 ° fusion may also prevent or reduce hardware fracture and obviate the need for postoperative bracing. The PLIF technique we initially used is described below. We routinely add pedicle screw fixation to a modified PLIF technique and describe this as the POLAr (posterior oblique lumbar arthrodesis) technique. Longterm follow-up studies are pending, but our initial review yields very successful outcomes. ~"
OPERATIVE TECHNIQUE S t a n d - a l o n e PLIF
Cloward first described PLIF as a 3-step operation: removal of the bone grafts, laminectomy and removal of the intervertebral disc and the spinal fusion. 1 When possible, Cloward advocated the use of a vertebral spreader to PLIF WITH METALCAGES
perform bilateral laminotomy rather than a complete laminectomy, with preservation of the ligamentum flavum hinged medially. In all cases, generous medial facetectomies were performed to provide a wide transverse opening, and the cortical surfaces of the vertebral bodies removed before fusion by using tricortical bone plugs harvested from the patient's iliac crest. Lin recommended leaving the facets as well as the vertebral end plates intact, multiply perforating the cortical surfaces instead, which, in theory, will prevent settling. 2° Cloward strongly disagreed with these changes. Today, the operative technique of PLIF has remained fundamentally unchanged from Cloward's original version, but advances in technology and materials have led to some variations. Threaded titanium cages are only one of many choices available to the surgeon performing PLIF today. Bone graft remains a viable option and has been explored by many investigators. 21 The rectangular tricortical grafts used by Cloward have been altered to become cylindrical bone dowels ~-24 or loose chips. 2s With the recent explosion in fusion implant technology, synthetic graft substitutes abound. These derive in principle from the Bagby threaded steel basket (Sulzar Spine Tech, Minneapolis, MN) used to treat wobbler's syndrome in horses. As originally described, an empty steel basket was packed with autologous bone and inserted into the intervertebral space in horses to achieve cervical fusion. 2" All subsequent synthetic implants share this hollow construction design. Various materials are used to construct a cage with openings such that bone packed into the frame can contact the vertebral bodies above and below. Cages employed for PLIF include Brantigan's carbon fiber reinforced polymer rectangular implant (DePuy Acromed Inc, Cleveland, OH), 27 Ray's threaded titanium fusion cage (Sulzer Spine Tech), 28 and Kuslich's BAK (Bagby and Kuslich) titanium cage (Sulzer Spine Tech). 2. At our institution, posterior lumbar interbody fusion is performed under general anesthesia. Patients are placed in a prone position, with care taken to ensure that the abdomen is not compressed. This prevents increased intraabdominal pressure, in turn allowing for easier ventilation as well as decreased venous engorgement and epidural bleeding. Optimal positioning is achieved either by use of bilateral chest rolls extending from the clavicle and shoulder to the anterior superior iliac spine or by use of radiolucent spine table, such as the Modular Table System (OSI Orthopaedic Systems, Union City, CA). As the patient will be fused in this position, care must be taken to place the patient in a physiologic position. Anti-embolism stockings, sequential compression devices, and Foley catheters are used on all patients, both to prevent increased intra-abdominal pressure from a distended bladder and to monitor the patient's vascular status. All patients routinely receive a dose of the antibiotic, cefazolin sodium (or vancomycin HC1, if allergic to penicillin), 30 minutes before skin incision. Preoperative steroids (10 mg of dexamethasone) are administered if imaging studies show significant compression of neural structures from tight stenosis. They can alternatively be administered intraoperatively if excessive manipulation of the nerve root becomes necessary. We also use intraoperative neurophysiologic monitoring 31 3
during these procedures. The technique includes somatosensory evoked potentials (SSEP) monitoring, spontaneous EMG activity during interbody cage insertion, and evoked EMG activity during pedicle screw insertion. These adjuncts may help to reduce the incidence of intraoperative nerve root and cauda equina complications.3°-32 Before or in conjunction with exposure of the spine, the iliac crest is exposed to obtain cortical and cancellous bone. A standard exposure may be obtained for this purpose. 33 Alternatively, a more limited exposure may be performed to expose the cortical surface of the iliac crest. The cortical surface may be removed, and cancellous bone may be removed with curettes or other instruments. The bone is used to pack the cages before and after insertion. Exposure of the lumbar spine is accomplished through a midline incision just rostral to the disc space of interest. Intraoperative fluoroscopy is used throughout the procedure to confirm the level(s), which increases safety during the surgery. Dissection is carried down to the lumbodorsal fascia, which is incised in the midline such that a subperiosteal exposure of the posterior elements of the vertebral bodies of interest can be completed. This is extended rostrocaudally to expose the vertebral bodies above and below the level of pathology and laterally to expose the origin of the transverse processes. Next, we perform sufficient laminectomy and bilateral medial facetectomies to permit ample access to the disc space (Fig 2). Whereas much of this bone should be taken by rongeur and saved to pack into the interbody fusion cage, a high-speed air drill can be used to complete the bony decompression. Often, the superior part of the pedicle needs to be removed to permit safe placement of the interbody fusion cage. Ultimately, the size of the cage needed and the patient's anatomy will dictate the amount of bone removal.
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Fig 2. Bilateral laminectomies and medial facetectomies that permit access to the disc space.
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Fig 3. Interbody distraction plugs in place.
We try to preserve the pars interarticularis when performing free-standing PLIE The ligamentum flavum is now removed, and meticulous hemostasis of the epidural venous complex is obtained. The thecal sac and nerve roots must be mobilized to allow for adequate medial displacement from both sides without undue traction on these structures. All attempts are made to limit retraction of the thecal sac so that it is not retracted past the midline. The superior nerve root is identified and retracted superiolaterally. The exiting nerve root and thecal sac is retracted medially by specialized root retractors. (Sulzer Medica, Minneapolis, MN) Once exposure is satisfactory, standard instruments are used to perform extensive, bilateral discectomy. The nerve roots are protected throughout the procedure as described previously. The disc space is sequentially distracted with interbody plugs or dilators (Fig 3). Alternating sides while inserting dilators of progressively increasing diameter size then restores disc space height and reopens narrowed foramina. This further facilitates discectomy in cases where degenerative changes have led to collapse of the interspace. The advantage of using distraction plugs over intervertebral distractors is that the plugs distract the vertebral bodies in a parallel fashion and avoid creating nonphysiologic angulation. The distraction plugs also allow the surgeon to determine the size of the cage needed. This is based on the ease by which the plugs are pulled out of the disc space once they are placed. The plugs cause the annular ligaments to be placed in tension; the correct size plug should be difficult to remove from the disc space once it is inserted. Once distraction is performed, a retractor is placed over one of the distracting plugs. A rotating cutter (reamer) is then used to create a parallel, partial decortication of the end plates. The rotating cutter is placed into the disc space PATEL AND WELCH
and turned to ensure full extrication of soft tissue material. This is completed by vigorously abrading the end plates to remove their cartilaginous covering. After reaming is completed, the cylindrical space is tapped as necessary. A combination of depth markings, positive instrument stops, and fluoroscopy is used to prevent penetration of the instruments past the anterior longitudinal ligament with potential subsequent injury to the vascular structures ventral to the vertebral body. Generally, instruments are not placed into the disc space to a depth greater than 30 mm. After bilateral preparation, the disc space is ready for placement of an interbody graft. Generally, the metal cages are packed with morselized bone, either that removed from the iliac crest, during the lumbar exposure, obtained from the reamer, or with allograft bone chips. We prefer to use autologous iliac crest bone whenever possible. The cages use various means to prevent graft migration, from the reverse teeth of Brantigan's carbon polymer implant to the threaded construction of the titanium cages. Cages are placed under fluoroscopic guidance and are countersunk to prevent protrusion into the spinal canal. Bone is packed within the remaining space of the cage and dorsal to the cage as well. A similar procedure is performed on the opposite side for stand-alone PLIF procedures (Fig 4).
Fig 4. Lumbar myelogram of stand-alone PLIF.
PLIF WITH METALCAGES
Posterolateral fusion, if planned, is now completed• Remaining bone from the iliac crest is combined with bone from the reaming procedure and bone obtained during the canal exposure. This bone is placed in an onlay fashion over the decorticated transverse processes and in the interfacet line for bilateral lateral fusion. Gelfoam may be placed on top of the exposed dura and nerve roots, but should not be packed into the lateral gutter. Closure is performed over a drain as for any lumbar surgery, with a watertight fascial layer, followed by closure of the subcutaneous layer and skin. Patients may be placed in a rigid brace if instability is encountered intraoperatively, but often do not require any external support after fusion using metal cages. 2s,2" We encourage patients to ambulate early during recovery to prevent adverse sequelae, such as pneumonia and deep venous thrombosis leading to pulmonary embolism. Average patient length of stay is 5 days. P O L A r Technique
The combination of pedicle screw fixation and interbody fusion is an evolutionary procedure made popular by Steffee. 21 Dr. Steffee's concerns were that transpedicular screw fixation with bilateral lateral fusion would provide inadequate anterior column support and would lead to implant failure. He obtained anterior column support with bone implants in an effort to bolster the posterior fixation. Since metal implants for interbody fusion have been available, surgeons have become concerned that standalone PLIF causes a destabilization of the posterior spinal column and reduces the likelihood of successful fusion. The addition of transpedicular screw fixation provides immediate restoration of stability after decompression of the neural elements. Although often believed to provide higher fusion rates, there is debate with respect to the biomechanical necessity of such a construct• In fact, Hoshijima found that "the PLIF construct is more stable than the intact [spine] in flexion-extension and lateral bending •..which may support the practice of PLIF without posterior instrumentation. ''z2 Brodke, on the other hand, assigns clear benefit to PLIF augmented by posterior pedicular screw instrumentation over PLIF alone in both flexionextension and torsional stiffness.23 Interestingly, he agrees with Hoshijima that PLIF alone is sufficient to provide greater flexion-extension stiffness than the intact spine (2.5 N m / m m v 1.3 Nm/mm), but only when the BAK titanium cage is used. Recently, biomechanical evidence has been presented which suggests that a single, obliquely directed BAK interbody fusion cage provides biomechanical stability equal to that of 2 anterior-posteriorly directed cages. 4 In light of the biomechanical data, it has been our practice to insert a single cage obliquely within the interspace and supplement this with bilateral pedicle screw fixation (POLAr). Overall, the technique is similar to PLIF except that a single cage is placed on the side with the more symptomatic radiculopathic findings• The construct is then supplemented with pedicle screw fixation. The autograft bone is harvested in the usual manner, and the lumbar spine is exposed. The lamina is resected on the symptomatic side in all cases and is resected on the less, or nonsymptomatic, side as is believed necessary to obtain
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adequate disc and nerve root decompression. The transverse processes are exposed bilaterally in the usual fashion. A wide decompression is performed on the symptomatic side, and an extensive facetectomy is performed (Fig 5). The pars interarticularis may be removed as necessary because the approach is unilateral and stability will be restored immediately with the pedicle screw construct. The thecal sac and nerve roots on the symptomatic side are mobilized as necessary, and the nerves are protected. The surgeon may now place pedicle screws and use them to distract the interspace. Alternatively, pedicle screws may be placed after the PLIF. An extensive discectomy is performed, bilaterally if necessary. The next step is to distract the disc space sequentially with unilaterally placed interbody plugs (Fig 6). The difference between the unilateral and bilateral cage placement is that all efforts are made to direct the unilateral cage 12 ° to 20 ° medially. Custom-made retractors with 22.5 ° of angulation cut into them can be ordered (SulzerMedica). This retractor is placed over the distracting plug. Reaming is performed in the standard fashion with emphasis on reaming more deeply than one would using the bilateral PLIF technique (Fig 7). This enables the surgeon to countersink the cage at an oblique angle without an edge of the cage protruding into the canal (Fig 8). The hole is tapped to depth, and a single BAK interbody fusion cage (usually 20 m m in length) is packed with morselized bone. The cage is then inserted so that the most dorsal edge of the cage does not protrude into the canal (Fig 9). Bone is packed into the dorsal aspect of the cage and within the remaining disc space. Once the cage is inserted, the posterolateral fusion is completed. Pedicle screws may be placed, if not done so earlier. The entire construct may be placed under compression or allowed to remain in a neutral state. The pedicle
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Fig 6. Oblique placement of distraction plug. screw system may be cross-connected. Onlay bone is placed over the decorticated transverse processes and in the interfacet line. Routine closure is performed.
RESULTS Just as there are a multitude of indications for which PLIF is performed and a n u m b e r of variations in the operative
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Fig 5. Unilateral facectomy for POLAr technique.
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Fig 7. Unilateral, oblique reaming. PATEI_AND WEIGH
Fig 8. Placement of interbody fusion cage for POLAr technique.
technique, there is no uniformity of outcome measures in the series encountered in the literature. Even after Prolo's 1986 attempt to standardize outcome assessment by introducing an objective scale of the patient's postoperative economic and functional status, 34 studies continued to use an ill-defined conventional rating system of poor, fair, good, or excellent results. These distinctions were loosely based on a m o u n t of residual pain, a m o u n t of pain medication used, limitation on activity, and postoperative employment status (Table 1). When a 3-tiered system was used, good and fair results were not distinguished. With these
Fig 9. Postoperative CT scan showing interbody fusion cage placement with pedicle screw augmentation (POLAr technique). PLIF WITH METAL CAGES
deficiencies in mind, the variation in outcomes reported in large series is not surprising. The earliest results found in the literature do not even use the conventional system mentioned previously. In his historic article in the 1953 issue of Neurosurgery, Cloward reports on 321 patients with 0.5 to 8 years of follow-up. He claims a 98% rate of fusion with an 85% long-term cure rate. 1 In contrast, Rish rates outcomes in PLIF versus simple discectomy as satisfactory (61% v 64%), poor (21% v 24%), or failure (18% v 12%), demonstrating no significant advantage to PLIF. 5 He was later to confirm these results in his 12-year experience with 250 patients, but he notes that the incidence of subsequent surgery diminishes with PLIF." Other early series have results as impressive as Cloward's. In a 6-year follow-up, Collis evaluates 50 consecutive patients of his 750 total patients, achieving 92% satisfactory results (some or total pain relief) with 94% fusion by 2 years. 3~ Hutter's subgroup of 142 spinal stenosis patients evaluated at 3 or more years shows a 91% fusion rate and an 80% excellent or good outcome, 7 whereas Prolo, in converting his scale to the conventional 4-tiered system, demonstrates a 94% fusion rate and an 85% satisfactory (excellent or good) outcome at 1 year in 34 patients. ~ Schecter tries to control for patient variance and sets strict entrance criteria (back pain for more than 3 months; no disc herniation, stenosis, or instability; no prior operation; single level pathology) in 25 patients with a 2- to 4-year follow-up. He reports a 95% fusion rate with 89% excellent or good outcomes and no poor outcomes, s Lee used similar entrance criteria, but also required discogram concordant pain, in 62 patients observed for 2 years. He achieved results almost identical to Schecter's with a 94% fusion rate and 89% excellent or good clinical status. Moreover, 93% of Lee's patients returned to work. ~ Of course, data from these 2 studies can only be extrapolated to persons similar to the homogeneous patient population selected for by their entrance criteria. More recent studies have involved larger patient numbers. Gill performed PLIF with pedicle screw fixation in 238 patients followed-up for 2 years, attaining a 92% fusion rate with 87% excellent or good outcome by using the Prolo equivalent. 36 Ray's multicenter, prospective randomized trial of his titanium threaded cage in 236 patients at 32 months found a 96% fusion rate by 2 years and a 65% excellent/good Prolo rating with an additional 25% rating results as fair. 2~ Kuslich also evaluated his BAK cage in a multicenter, randomized trial with 2-year follow-up. Of the 356 patients who u n d e r w e n t PLIF, 100% achieved fusion in single level surgery, and 90% achieved fusion in 2-level surgery. Clinical pain relief was reported in 84% and functional improvement in 91%, with 91% of patients also returning to work at 3 years. 2" The impressive results claimed by these investigators have recently been called into question by an independent evaluation of a carbon fiber cage with posterior fixation performed by Agazzi et al. 37 In this study, 71 patients were followed-up for 28 months. A 90% fusion rate was noted, but outcomes based on the Prolo scale were excellent/good in only 39°/,, and fair in 25%, with 67% of patients reported being satisfied with the outcome. One possible bias of this study concerns patients involved in compensation claims. Only 47% of these patients improved versus 84% of noncompensation
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T A B L E 1. Survey of Characteristics Used by I n v e s t i g a t o r s to E v a l u a t e Clinical O u t c o m e After PLIF
Excellent Good Fair Poor
Residual Pain
Medications
Activity Level
Work Status
No pain Improved but minor pain Improved but present pain Minimal improvement or no pain relief
None Minimal (NSAIDs) Frequent but not daily (narcotic) Daily medication (narcotics)
No limitations on activity level Strenuous activity may cause pain Limited to nonstrenuous activity Marked limitations of all activities
Return to original work Return to original work Return to lighter work Unable to work
NOTE. A survey of characteristics used by investigators to evaluate clinical outcome after PLIF is summarized in this 4-tiered rating system. These conventional ratings categorize patients based on success of pain relief, frequency of medication use, general level of activity, and postoperative work status. Prolo 34 suggested an objective scale by assigning 1 to 5 points in each of 2 categories. Economic potential was graded from complete invalid (score of 1) to return to previous occupation with no restrictions (score of 5). Functional capacity was graded from total incapacity (score of 1) to complete recovery allowing participation in all activities and sports (score of 5). The ratings from each of the 2 categories were summated for a total of 2 to 1 0 points. Excellent outcome was correlated with a score of 9 or 10, good outcome with a score of 7 or 8, fair outcome with a 5 or 6, and poor outcome with a score of 2, 3, or4.
Abbreviation: NSAIDs, nonsteroidal anti-inflammatory drugs.
patients. Further, only 6% of compensation patients returned to w o r k versus 80% of n o n c o m p e n s a t i o n patients. Although there are u n d o u b t e d l y patients in w h o m PLIF is indicated, better stratification is required before meaningful conclusions can be d r a w n as to w h o benefits m o s t from this procedure.
COMPLICATIONS PLIF shares m a n y of the complications found in posterior laminectomy and discectomy, p e r h a p s with s o m e higher risk of occurrence because of the more involved nature of PLIE Of these, skin infections are reported to occur in 0% to 7°/,, of cases. 5,s,~,27-3~.384° Prone positioning resulting in anterolateral femoral cutaneous neuritis has been reported by one investigator to occur in 6% of cases, although always transiently. 3s The more aggressive retraction of the thecal sac during PLIF leads to a cerebrospinal fluid leak in 0% to 10% of cases. 2s-3"~.37.40Likewise, the increased m a n i p u lation of the nerve root required by PLIF results in not infrequent neurologic deficit. These disturbances are sensory in 2% to 5% of patients and are almost always temporary. Motor deficits, encountered transiently in 0.8% to 10% of patients, are p e r m a n e n t in less than 0.8% of patients. 2s-4° Urinary disturbances are very rare (<0.5%) and are always transient. ~-37 It should be r e m e m b e r e d that the neurologic deficits complicating PLIF are not always related to surgical manipulation. They m a y be a direct result of fusion, arising from pedicle screw placement, for example. Additional complications ascribed to the fusion portion of this procedure include graft migration in 0% to 6% of patients, 1,27-3~,-~sq° with catastrophic consequences if the graft is extruded into the spinal canal. This was generally a problem in earlier studies; Ray reports no graft retropulsion in his large series using threaded cage fusion technique. 2s Perhaps the most difficult complication observed in PLIF is failure of fusion. Although encountered in 3°/. to 15% of cases, 1.4~,2741 failure of fusion is not s y n o n y m o u s with clinical failure. In fact, Blumenthal and Gill suggest that correlation of radiographic evidence of fusion with actual intraoperative findings is only 69%, with a false positive rate of 42% and a false negative rate of 29°/,,. In his reexamination of 49 patients, successful arthrodesis was confirmed in 90% of the patients. The investigators theorize that the premineralized osteoid tissue that was functionally fused probably appeared radiolucent on radiographs. 41 Although radiographic fusion is not seen in all
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patients with satisfactory outcomes, s o m e investigators suggest that every patient with excellent pain relief will d e m o n s t r a t e a solid fusion mass. 7
CONCLUSIONS N o n u n i f o r m i t y of indications in existing studies and lack of a standard m e t h o d of evaluation m a k e definitive conclusions difficult at best. PLIF is certain to h a v e a role in the treatment of l u m b a r spinal disorders, especially discogenic lower back pain. Although excellent results are seen with stand-alone PLIF procedures, there is a growing trend towards reinforcement of the PLIF procedure with posterior fixation. Pedicle screw augmentation, such as with the POLAr technique, seems particularly well suited for use in failed back s y n d r o m e , especially w h e n p o s t l a m i n e c t o m y spondylolisthesis is present. POLAr m a y also be of use in patients with chronic low back pain attributable to internal disc d e r a n g e m e n t s w h e n confirmed by the presence of concordant pain on provocative discography. Additionally, it is useful in the treatment of patients with gross l u m b a r instability and microinstability, especially w h e n evidence of degenerative collapse of the disc space is present on MRI. As with all cases, conservative therapy should be exhausted before interbody fusion techniques are considered.
REFERENCES 1. Cloward RB: The treatment of ruptured lumbar intervertebral discs by vertebral body fusion - I. [ndications, operative technique, after care. J Neurosurg 10:154-168, 1953 2. Briggs H, Mulligan PR: Chip fusion of the low back following exploration of the spinal canal. J Bone Joint Surg 26:125-130, 1944 3. Ovens JM, Williams HG: [ntervertebral spine fusion with removal of herniated intervertebral disk. Am J Surg 70:24-26, 1945 4. CIoward RB: Posterior lumbar interbody fusion updated. Clin Orthop 193:16-19, 1985 5. Rish BL: A comparative evaluation of posterior lumbar interbody fusion for disc disease. Spine 10:855-857, 1985 6. Rish BL: A critique of posterior lumbar interbody fusion - 12 years' experience with 250 patients. Surg Neurol 31:281-289, 1989 7. Hutter CG: Spinal stenosis and posterior lumbar interbody fusion. Clin Orthop 193:103-114, 1985 8. Schechter NA, France MP, Lee CK: Painful internal disc derangements of the lumbosacral spine - Discographic diagnosis and treatment by posterior lumbar interbody fusion. Orthopedics 14:447-451, 1991 9. Lee CK, Vessa P, Lee JK: Chronic disabling low back pain syndrome caused by internal disc derangements - The results of disc excision and posterior lumbar interbody fusion. Spine 20:356-361, 1995 10. McDonald JV, Welch WC: Patient history, general physical examinaPATEL AND WELCH
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tion and net, rologic examination with studies, in Welch WC, Jacobs GB, Jackson RP (eds): Operative Spinal Surgery: Stamford, CT, Appleton & Lange, pp 13-26, 1999 Deville WLJM, van der Windt DAWM, Dzaferagic A, et al: The test of Leaegue: Systematic review of the accuracy in diagnosing herniated discs. Spine 25:1140-1147, 2000 Modic IVIT,Masaryk TJ, Ross JS, eds. Magnetic Resonance Imaging of the Spine. Chicago, IL, Yearbook Medical, 1989, pp 75-119 Bell GR: Perioperative diagnostic imaging studies, in Welch WC, Jacobs GB, Jackson RP (eds): Operative Spinal Surgery. Stamford, CT, Appleton & Lange, pp 27-77, 1999 Holt EP: The question of lumbar discography. J Bone Joint Surg Am 50:720-726,1968 Simmons JW, April CN, Dwyer AP, et al: A reassessment of Holt's data on "the question of lumbar discography." Clin Orthop 1988:237:120124 Bogduk N, Modic MT: Controversy: Lumbar discography. Spine 21:402-404,1996 Saal JS, Saal JA. Management of chronic discogenic low back pain with a thermal intradiscal catheter-A preliminary report. Spine 25:382-388, 2000 Zdeblick T, David S, Cheng B: A prospective comparison of surgical approach for anterior L4-5 fusion: Laparoscopic vs mini-PLIE Presented at the 14th Annual Meeting of the North American Spine Society, October 20-23, 1999. Welch WC, Gerszten PC, Pratt RL: Safety and effectiveness of a new posterior It, mbar fusion technique, POLAr. Congress of Neurologic Surgeons, San Antonio, TX, September 23-28, 2000 Lin PIVI: A technical modification of Cloward's posterior lumbar interbodv fusion. Neurosurgery 1:118-123, 1977 Steffee AD, Sitkowski DJ: Posterior lumbar interbodv fusion and plates. Clin Orthop 227:99-102, 1988 Hoshijima K, Nightingale RW, Yu JR, et al: Strength and stabilib,, of posterior lumbar interbody fusion - Comparison of titanium fiber mesh implant and tricortical bone graft. Spine 22:1181-1188, 1997 Brodke DS, Dick JC, Kunz DN, et al: Posterior lumbar interbody fusion - A biomechanical comparison, incfuding a new threaded cage. Spine 22:26-31, 1997 Wiltberger BR: The prefit dowel intervertebral body fusions used in lumbar disc therapy. Am J Surg 86:723-734, 1953 Simmons JW: Posterior lumbar interbody fusion with posterior elements as chip grafts. Clin Orthop 193:85-89, 1985 DeBowes RIVl, Grant BD, Bagby GW, et al: Cervical vertebral interbodv fusion in the horse - A comparative study of bovine xenografts and autografts supported bv stainless steel baskets. Am J Vet Res 45:191-199, 1984
PLIF WITH METAL CAGES
27. Brantigan JW, Steffee AD: A carbon fiber implant to aid interbody lumbar fusion - Two-year clinical results in the first 26 patients. Spine 18:2106-2107, 1993 28. Ray CD: Threaded titanium cages for lumbar interbody fusions. Spine 22:667-680, 1997 29. Kuslich SD, Ulstrom CL, Griffith SL, et al: The Bagby and Kuslich method of lumbar interbody fusion - History, techniques, and 2-year follow-up results of a United States prospective, multicenter trial. Spine 23:1267-1279, 1998 30. Rose RD, Welch WC, Balzer JR, et al: Persistently electrified pedicle stimulation instruments (PEPSI) in spinal instrumentation: Technique and protocol development. Spine 3:334-343, 1997 31. Welch WC, Rose RD, Balzer JR, et al: Evoked EMG evalt, ation during lumbar instrumentation: A prospective study J Neurosurg 87:397-402, 1997 32. Balzer JR, Rose RD, Welch WC, et al: Techniques and technical applications: Simultaneous somatosensorv evoked potential and electromyographic recordings during lumbosacral decompression and instrumentation. Neurosurgery 42:1318-1325, 1998 33. Kang JD: Posterior lumbar spine fusion without instrumentation, in Welch WC, Jacobs GB, Jackson RP (eds): Operative Spinal Surgery. Stamford, CT, Appleton & Lange, pp 188-193, 1999 34. Prolo DJ, Oklund SA, Butcher M: Toward uniformity in evafuating results of lumbar spine operations - A paradigm applied to posterior lumbar interbodv fusions. Spine 11:601-606, 1986 35. Collis JS: Total disc replacement - A modified posterior It,mbar interbody fusion. Report of 750 cases. Clin Orthop 193:64-67, 1985 36. Gill K, Blumenthal SL: Posterior lumbar interbodv fusion - A 2-year follow-up of 238 patients. Acta Orthop Scand 64:108-110, 1993 (suppl 251 ) 37. Agazzi S, Reverdin A, May D: Posterior lumbar interbody fusion with cages: An independent review of 71 cases. J Neurosurg 91:186-192, 1999 38. Zhao J, Hal Y Ordway NR, et al: Posterior lumbar interbody fusion using posterolateral placement of a single cylinderical threaded cage. Spine 25:425-430, 2000 39. Lin PM, Cautilli RA, Joyce MF: Posterior lumbar interbody fusion. Clin Orthop 180:154-168, 1983 40. Ma GWC: Posterior lumbar interbody fusion with specialized instruments. Clin Orthop 193:57-63, 1985 41. Blumenthal SL, Gill K: Can lumbar spine radiographs accurately determine fusion in postoperative patients? Correlation of routine radiographs with a second surgical looks at lumbar fusions. Spine 18:1186-1189, 1993
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Posterior lumbar interbody fusion (PLIF) is a technically rigorous surgical fusion procedure. The procedure has had a resurgence in application and acceptance due to recent technological advances in cage design. We review the history of this procedure, indications, and patient evaluation. We also describe the surgical technique of freestanding PLIF and a new technique utilizing a single cage with pedicle screw supplementation. KEY WORDS: lumbar fusion, interbody fusion, posterior lumbar interbody fusion
INTRODUCTION Recently reintroduced and popularized, posterior lumbar interbody fusion (PLIF) can be found mentioned in the literature as far back as 1936 when Mercer, writing in the Edinburgh Journal of Neurosurgery, suggested that "the ideal operation for fusing the spine would be an interbody fusion." He never was to perform this procedure, however, saying that "the surgical difficulties encountered...would make the operation technically impossible. 'q Other early proponents of PLIF include Briggs and Mulligan, who packed the intervertebral space with bone chips and even recommended driving a round bone peg into the interspace, 2 and Ovens and Williams, who performed an intervertebral spine fusion after removal of a herniated lumbar disc. 3 It is Ralph B. Cloward, however, who is commonly credited with popularizing PLIE Dr. Cloward's first PLIF was performed in 1940 after a lumbar discectomy for sciatica in a schoolteacher. This patient died of a pulmonary embolus, and 3 years were to pass before World War II compelled Cloward to again undertake PLIE In caring for many of those who sustained back injuries in the line of duty, Cloward noted that although discectomy alone relieved the radicular pain these patients experienced, the patients were left with significant low back pain. When he added PLIF to discectomy, Cloward noted a significant improvement in his patient's residual low back pain. In fact, in presentations describing his early results, first to the Hawaii Territorial Medical Association (1945), then to the Harvey Cushing Society (1947), and eventually to the American Medical Association (1951), Cloward estimated a 30% to 50% higher cure rate when PLIF was performed with discectomy over From the *[Departments of Neurologic Surgery and 1"Orthopaedic Surgery, and the 1-School of Rehabilitative Sciences and Technology, University of Pittsburgh School of Medicine, Pittsburgh, PA. Address reprint requests to William C. Welch, MD, FACS, FICS, University of Pittsburgh School of Medicine, UPMC-Health System, Department of Neurosurgery, 200 Lothrop St, Suite B-400, Pittsburgh, PA 15213. Copyright ~ 2000 by W.B. Saunders Company 1048-6666/00/1004-0008510.00/0 doi: 10.1053/otor.2000.16705
discectomy alone, asserting that 82 of 100 such patients were able to return to manual labor. Cloward remained so confident of the utility of PLIF that he recommended that the simple discectomy, the decompressive laminectomy and chemonucleolyis be eliminated, asserting that "the PLIF technique is the answer to the treatment of diseases of the lumbar spine and may be the operation of the future. ''4 From these early beginnings, PLIF has undergone many variations, both procedural and technological, and the certitude of Cloward's statement has been supplanted by controversy. Indeed, opposing viewpoints can be found in the literature concerning the various indications for PLIE its biomechanical stability, the surgical technique in its many modifications (including the necessity of posterior fixation), and perhaps most importantly, clinical results after PLIE We briefly illustrate these disputes and describe the current technique of posterior lumbar interbody fusion with metal cages.
INDICATIONS The list of indications for PLIF has grown steadily since its initial use as the primary treatment of herniated discs. It now includes recurrent disc herniation, spinal stenosis with associated instability, degenerative spondylosis with resultant mechanical back pain, spondylolisthesis, spinal instability, and discogenic back pain. Interbody fusion cages have been used during the correction of scoliosis and in cases of failed prior lumbar fusion attempts. Although none of these indications have been proven by randomized, controlled clinical trial and outcome data based on specific indications is rare, supportive studies are encountered in the literature for each of these categories. As previously mentioned, Cloward is perhaps the strongest proponent of PLIF in lumbar disc disease. He believes that, after simple discectomy, "up to 70% [of patients] return" and 20% require a second operation with fusion. 1 Others confirm these poor results. Rish assigns a 12% failure rate to discectomy alone with up to 30% of patients requiring additional surgery. 5 In his 12-year experience with 250 patients, he notes that "posterior lumbar interbody fusion
Operative Techniques in Orthopaedics, Vol 10, No 4 (October), 2000: pp 311-319
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significantly reduces the rate of recurrent problems requiring subsequent surgery (8.4% v 18%).''~ Hurter surveys literature summarizing results in over 10,000 patients and recounts a 17% to 20% failure rate after discectomy alone, with up to 50% of patients requiring a second operation. He additionally notes that, after simple discectomy for radiculopathy, low back pain can develop in up to 30% of patients at 4 years and 80% of patients at 10 years. 7 Similar to results in surgery for herniated discs, surgery to relieve spinal stenosis has a short-term failure rate of 15% to 20% and up to a 50% failure rate at 10 years. Hutter reports an 80% excellent or good outcome in a subgroup of 142 patients undergoing PLIF for spinal stenosis followed-up for a minimum of 3 years. 7 Even better results have been seen in subgroups of patients treated with PLIF for spondylolisthesis, both degenerative and postlaminectomy, with Cloward reporting a remarkable 95% excellent clinical result. 4 Perhaps the most interesting role of PLIF has been in treating patients with pain thought to derive from internal disc derangements, termed discogenic pain. This population, which usually suffers from low back pain without associated radiculopathy, has classically been believed to receive no benefit from lumbar disc surgery. Again, however, use of PLIF to treat this group is not a new idea. Cloward reports diagnosing and treating discogenic back pain by cortisone injection since 1959.4 A recent study by Schechter demonstrates an 89% good or excellent clinical result with a 95% fusion rate in PLIF for discogenic back pain diagnosed by provocative discography. ~ Lee notes an identical 89% satisfactory clinical result, with 89% patient satisfaction, a 93% return to work rate, and a 94% fusion rate in 62 patients with chronic disabling low back pain attributed to internal disc derangements treated with PLIF?
A standard physical examination is performed with an emphasis placed on examination of the mechanical properties of the lumbar spine. Gower's maneuver, when spontaneously exhibited by the patient upon arising from a flexed position, may be a strong indicator of instability. Straight leg raise and crossed straight leg raise tests are performed to determine the amount of active nerve root irritation. 11 All patients undergo standardized radiographic studies, including plain spine radiographs with flexion-extension views if instability is suspected. Generally, lumbar magnetic resonance imaging (MRI) is the next test of choice. The discs are evaluated for desiccation, loss of height, and end plate changes as described by Modic (Fig 1). 12,13The study should be evaluated for the presence of conjoined nerve roots because it may be dangerous to perform posterior interbody fusion in the presence of conjoined roots. A computed tomography (CT) myelogram may be a necessary adjunct to MRI to assess structural abnormalities of the lumbar spine. The CT scans provide increased bony detail compared with MRI alone. The postmyelographic CT scan shows the relationship between the structural bone abnormalities and the cauda equina in tremendous detail. This may help to determine the amount of neuroforaminal decompression required. Sagittal reconstructions are useful to determine disc space height, measure the amount of listhesis, and plan on interbody fusion cage size. Discography may be considered next. There is a large body of literature reviewing the advantages and disadvantages of discography, which will not be reviewed here. M-I~ We frequently perform discography to help diagnose discogenic back pain, using the information to determine how many levels require interbody fusion. We do not perform this study on all patients, however. Specifically, patients with the more typical history of mechanical lower
PREOPERATIVE EVALUATION The most common indication for PLIF is lumbar instability and discogenic lower back pain. The decision to operate on patients with these diagnoses is based on patient history, physical examination, radiographic studies, provocative tests, and response to previous treatments. A careful history is taken to assess the chronicity, quality, and location of pain. Efforts are made to determine the mechanical quality of pain (ie, if the pain is worse with movement). We specifically ask the patient to determine during which part of the day the pain is at a minimum. Generally, the pain is reduced when the patient is in a recumbent position. Another important question is to determine the ratio between the patient's lower back pain and their leg pain. Patients with much greater leg pain than back pain may respond to therapies directed at nerve decompression alone and may not require interbody fusion. Those with an equal distribution of leg and back pain or a preponderance of back pain are more commonly regarded as interbody fusion candidates. The remainder of the history is taken with suspicion as to the other myriad of potential causes of lower back pain. m Patients are strongly encouraged to stop using tobacco products, and any medicines with anticoagulant effects are discontinued.
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Fig 1. Sagittal MRI scan showing disc desiccation and degeneration. PATEL AND WELCH
back pain, single level disease on MRI, and no excluding factors are not sent for discography. Discography is performed on patients with prior surgery, multilevel disease, or atypical histories. Patients with concordant pain and radiographic evidence of disc derangement on discography are offered PLIF with discectomy. Other studies are performed as necessary. Abdominal and pelvic CT or MR] scans are performed if retroperitoneal mass or plexus lesions are considered likely. Electromyograph (EMG) testing may be required to help determine which level of nerve root dysfunction is present. Rarely, bone scanning is required to evaluate for the possibility of metastatic disease. Complete blood count and erythrocytic sedimentation rate blood studies are obtained if infection is deemed possible. Patients suffering from chronic low back pain secondary to degenerative disease must complete an aggressive physical therapy regimen and must fail 6 months of nonoperative conservative treatment before PLIF is considered. Other potential treatments include long-term nonsteroidal anti-inflammatory medications, intradiscal electrothermy, 17 facet blocks, and epidural steroid injections. These treatments must be weighed against their temporal effects and the patient's desire for a long-term solution. We generally consider PLIF techniques for patients with posterior pathology, such as pars interarticularis fractures, spinal stenosis with instability, and discogenic disc disease with significant radicular complaints and findings. The surgery must include thorough decompression of the involved nerve root(s). Patients with 90% or more back pain and no contraindications generally undergo anterior lumbar interbody fusion for treatment of the underlying degenerative disc disease. There is a long history of successful PLIF performed as a stand-alone procedure described in the literature and reviewed previously. Over the past few years, however, there has been increasing interest in supplementing posteriorly inserted cages with pedicle screw fixation. The rationale for this combined approach is straightforward. During the insertion of posterior cages, a significant amount of stabilizing structures, such as the facet joints, may need to be removed. This has the potential to destabilizes the spine. TM The addition of pedicle screw fixation creates an extremely strong construct, which may lead to higher fusion rates. This 360 ° fusion may also prevent or reduce hardware fracture and obviate the need for postoperative bracing. The PLIF technique we initially used is described below. We routinely add pedicle screw fixation to a modified PLIF technique and describe this as the POLAr (posterior oblique lumbar arthrodesis) technique. Longterm follow-up studies are pending, but our initial review yields very successful outcomes. ~"
OPERATIVE TECHNIQUE S t a n d - a l o n e PLIF
Cloward first described PLIF as a 3-step operation: removal of the bone grafts, laminectomy and removal of the intervertebral disc and the spinal fusion. 1 When possible, Cloward advocated the use of a vertebral spreader to PLIF WITH METALCAGES
perform bilateral laminotomy rather than a complete laminectomy, with preservation of the ligamentum flavum hinged medially. In all cases, generous medial facetectomies were performed to provide a wide transverse opening, and the cortical surfaces of the vertebral bodies removed before fusion by using tricortical bone plugs harvested from the patient's iliac crest. Lin recommended leaving the facets as well as the vertebral end plates intact, multiply perforating the cortical surfaces instead, which, in theory, will prevent settling. 2° Cloward strongly disagreed with these changes. Today, the operative technique of PLIF has remained fundamentally unchanged from Cloward's original version, but advances in technology and materials have led to some variations. Threaded titanium cages are only one of many choices available to the surgeon performing PLIF today. Bone graft remains a viable option and has been explored by many investigators. 21 The rectangular tricortical grafts used by Cloward have been altered to become cylindrical bone dowels ~-24 or loose chips. 2s With the recent explosion in fusion implant technology, synthetic graft substitutes abound. These derive in principle from the Bagby threaded steel basket (Sulzar Spine Tech, Minneapolis, MN) used to treat wobbler's syndrome in horses. As originally described, an empty steel basket was packed with autologous bone and inserted into the intervertebral space in horses to achieve cervical fusion. 2" All subsequent synthetic implants share this hollow construction design. Various materials are used to construct a cage with openings such that bone packed into the frame can contact the vertebral bodies above and below. Cages employed for PLIF include Brantigan's carbon fiber reinforced polymer rectangular implant (DePuy Acromed Inc, Cleveland, OH), 27 Ray's threaded titanium fusion cage (Sulzer Spine Tech), 28 and Kuslich's BAK (Bagby and Kuslich) titanium cage (Sulzer Spine Tech). 2. At our institution, posterior lumbar interbody fusion is performed under general anesthesia. Patients are placed in a prone position, with care taken to ensure that the abdomen is not compressed. This prevents increased intraabdominal pressure, in turn allowing for easier ventilation as well as decreased venous engorgement and epidural bleeding. Optimal positioning is achieved either by use of bilateral chest rolls extending from the clavicle and shoulder to the anterior superior iliac spine or by use of radiolucent spine table, such as the Modular Table System (OSI Orthopaedic Systems, Union City, CA). As the patient will be fused in this position, care must be taken to place the patient in a physiologic position. Anti-embolism stockings, sequential compression devices, and Foley catheters are used on all patients, both to prevent increased intra-abdominal pressure from a distended bladder and to monitor the patient's vascular status. All patients routinely receive a dose of the antibiotic, cefazolin sodium (or vancomycin HC1, if allergic to penicillin), 30 minutes before skin incision. Preoperative steroids (10 mg of dexamethasone) are administered if imaging studies show significant compression of neural structures from tight stenosis. They can alternatively be administered intraoperatively if excessive manipulation of the nerve root becomes necessary. We also use intraoperative neurophysiologic monitoring 31 3
during these procedures. The technique includes somatosensory evoked potentials (SSEP) monitoring, spontaneous EMG activity during interbody cage insertion, and evoked EMG activity during pedicle screw insertion. These adjuncts may help to reduce the incidence of intraoperative nerve root and cauda equina complications.3°-32 Before or in conjunction with exposure of the spine, the iliac crest is exposed to obtain cortical and cancellous bone. A standard exposure may be obtained for this purpose. 33 Alternatively, a more limited exposure may be performed to expose the cortical surface of the iliac crest. The cortical surface may be removed, and cancellous bone may be removed with curettes or other instruments. The bone is used to pack the cages before and after insertion. Exposure of the lumbar spine is accomplished through a midline incision just rostral to the disc space of interest. Intraoperative fluoroscopy is used throughout the procedure to confirm the level(s), which increases safety during the surgery. Dissection is carried down to the lumbodorsal fascia, which is incised in the midline such that a subperiosteal exposure of the posterior elements of the vertebral bodies of interest can be completed. This is extended rostrocaudally to expose the vertebral bodies above and below the level of pathology and laterally to expose the origin of the transverse processes. Next, we perform sufficient laminectomy and bilateral medial facetectomies to permit ample access to the disc space (Fig 2). Whereas much of this bone should be taken by rongeur and saved to pack into the interbody fusion cage, a high-speed air drill can be used to complete the bony decompression. Often, the superior part of the pedicle needs to be removed to permit safe placement of the interbody fusion cage. Ultimately, the size of the cage needed and the patient's anatomy will dictate the amount of bone removal.
\
/
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J
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Fig 2. Bilateral laminectomies and medial facetectomies that permit access to the disc space.
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Fig 3. Interbody distraction plugs in place.
We try to preserve the pars interarticularis when performing free-standing PLIE The ligamentum flavum is now removed, and meticulous hemostasis of the epidural venous complex is obtained. The thecal sac and nerve roots must be mobilized to allow for adequate medial displacement from both sides without undue traction on these structures. All attempts are made to limit retraction of the thecal sac so that it is not retracted past the midline. The superior nerve root is identified and retracted superiolaterally. The exiting nerve root and thecal sac is retracted medially by specialized root retractors. (Sulzer Medica, Minneapolis, MN) Once exposure is satisfactory, standard instruments are used to perform extensive, bilateral discectomy. The nerve roots are protected throughout the procedure as described previously. The disc space is sequentially distracted with interbody plugs or dilators (Fig 3). Alternating sides while inserting dilators of progressively increasing diameter size then restores disc space height and reopens narrowed foramina. This further facilitates discectomy in cases where degenerative changes have led to collapse of the interspace. The advantage of using distraction plugs over intervertebral distractors is that the plugs distract the vertebral bodies in a parallel fashion and avoid creating nonphysiologic angulation. The distraction plugs also allow the surgeon to determine the size of the cage needed. This is based on the ease by which the plugs are pulled out of the disc space once they are placed. The plugs cause the annular ligaments to be placed in tension; the correct size plug should be difficult to remove from the disc space once it is inserted. Once distraction is performed, a retractor is placed over one of the distracting plugs. A rotating cutter (reamer) is then used to create a parallel, partial decortication of the end plates. The rotating cutter is placed into the disc space PATEL AND WELCH
and turned to ensure full extrication of soft tissue material. This is completed by vigorously abrading the end plates to remove their cartilaginous covering. After reaming is completed, the cylindrical space is tapped as necessary. A combination of depth markings, positive instrument stops, and fluoroscopy is used to prevent penetration of the instruments past the anterior longitudinal ligament with potential subsequent injury to the vascular structures ventral to the vertebral body. Generally, instruments are not placed into the disc space to a depth greater than 30 mm. After bilateral preparation, the disc space is ready for placement of an interbody graft. Generally, the metal cages are packed with morselized bone, either that removed from the iliac crest, during the lumbar exposure, obtained from the reamer, or with allograft bone chips. We prefer to use autologous iliac crest bone whenever possible. The cages use various means to prevent graft migration, from the reverse teeth of Brantigan's carbon polymer implant to the threaded construction of the titanium cages. Cages are placed under fluoroscopic guidance and are countersunk to prevent protrusion into the spinal canal. Bone is packed within the remaining space of the cage and dorsal to the cage as well. A similar procedure is performed on the opposite side for stand-alone PLIF procedures (Fig 4).
Fig 4. Lumbar myelogram of stand-alone PLIF.
PLIF WITH METALCAGES
Posterolateral fusion, if planned, is now completed• Remaining bone from the iliac crest is combined with bone from the reaming procedure and bone obtained during the canal exposure. This bone is placed in an onlay fashion over the decorticated transverse processes and in the interfacet line for bilateral lateral fusion. Gelfoam may be placed on top of the exposed dura and nerve roots, but should not be packed into the lateral gutter. Closure is performed over a drain as for any lumbar surgery, with a watertight fascial layer, followed by closure of the subcutaneous layer and skin. Patients may be placed in a rigid brace if instability is encountered intraoperatively, but often do not require any external support after fusion using metal cages. 2s,2" We encourage patients to ambulate early during recovery to prevent adverse sequelae, such as pneumonia and deep venous thrombosis leading to pulmonary embolism. Average patient length of stay is 5 days. P O L A r Technique
The combination of pedicle screw fixation and interbody fusion is an evolutionary procedure made popular by Steffee. 21 Dr. Steffee's concerns were that transpedicular screw fixation with bilateral lateral fusion would provide inadequate anterior column support and would lead to implant failure. He obtained anterior column support with bone implants in an effort to bolster the posterior fixation. Since metal implants for interbody fusion have been available, surgeons have become concerned that standalone PLIF causes a destabilization of the posterior spinal column and reduces the likelihood of successful fusion. The addition of transpedicular screw fixation provides immediate restoration of stability after decompression of the neural elements. Although often believed to provide higher fusion rates, there is debate with respect to the biomechanical necessity of such a construct• In fact, Hoshijima found that "the PLIF construct is more stable than the intact [spine] in flexion-extension and lateral bending •..which may support the practice of PLIF without posterior instrumentation. ''z2 Brodke, on the other hand, assigns clear benefit to PLIF augmented by posterior pedicular screw instrumentation over PLIF alone in both flexionextension and torsional stiffness.23 Interestingly, he agrees with Hoshijima that PLIF alone is sufficient to provide greater flexion-extension stiffness than the intact spine (2.5 N m / m m v 1.3 Nm/mm), but only when the BAK titanium cage is used. Recently, biomechanical evidence has been presented which suggests that a single, obliquely directed BAK interbody fusion cage provides biomechanical stability equal to that of 2 anterior-posteriorly directed cages. 4 In light of the biomechanical data, it has been our practice to insert a single cage obliquely within the interspace and supplement this with bilateral pedicle screw fixation (POLAr). Overall, the technique is similar to PLIF except that a single cage is placed on the side with the more symptomatic radiculopathic findings• The construct is then supplemented with pedicle screw fixation. The autograft bone is harvested in the usual manner, and the lumbar spine is exposed. The lamina is resected on the symptomatic side in all cases and is resected on the less, or nonsymptomatic, side as is believed necessary to obtain
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adequate disc and nerve root decompression. The transverse processes are exposed bilaterally in the usual fashion. A wide decompression is performed on the symptomatic side, and an extensive facetectomy is performed (Fig 5). The pars interarticularis may be removed as necessary because the approach is unilateral and stability will be restored immediately with the pedicle screw construct. The thecal sac and nerve roots on the symptomatic side are mobilized as necessary, and the nerves are protected. The surgeon may now place pedicle screws and use them to distract the interspace. Alternatively, pedicle screws may be placed after the PLIF. An extensive discectomy is performed, bilaterally if necessary. The next step is to distract the disc space sequentially with unilaterally placed interbody plugs (Fig 6). The difference between the unilateral and bilateral cage placement is that all efforts are made to direct the unilateral cage 12 ° to 20 ° medially. Custom-made retractors with 22.5 ° of angulation cut into them can be ordered (SulzerMedica). This retractor is placed over the distracting plug. Reaming is performed in the standard fashion with emphasis on reaming more deeply than one would using the bilateral PLIF technique (Fig 7). This enables the surgeon to countersink the cage at an oblique angle without an edge of the cage protruding into the canal (Fig 8). The hole is tapped to depth, and a single BAK interbody fusion cage (usually 20 m m in length) is packed with morselized bone. The cage is then inserted so that the most dorsal edge of the cage does not protrude into the canal (Fig 9). Bone is packed into the dorsal aspect of the cage and within the remaining disc space. Once the cage is inserted, the posterolateral fusion is completed. Pedicle screws may be placed, if not done so earlier. The entire construct may be placed under compression or allowed to remain in a neutral state. The pedicle
\
Fig 6. Oblique placement of distraction plug. screw system may be cross-connected. Onlay bone is placed over the decorticated transverse processes and in the interfacet line. Routine closure is performed.
RESULTS Just as there are a multitude of indications for which PLIF is performed and a n u m b e r of variations in the operative
,
f
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Fig 5. Unilateral facectomy for POLAr technique.
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Fig 7. Unilateral, oblique reaming. PATEI_AND WEIGH
Fig 8. Placement of interbody fusion cage for POLAr technique.
technique, there is no uniformity of outcome measures in the series encountered in the literature. Even after Prolo's 1986 attempt to standardize outcome assessment by introducing an objective scale of the patient's postoperative economic and functional status, 34 studies continued to use an ill-defined conventional rating system of poor, fair, good, or excellent results. These distinctions were loosely based on a m o u n t of residual pain, a m o u n t of pain medication used, limitation on activity, and postoperative employment status (Table 1). When a 3-tiered system was used, good and fair results were not distinguished. With these
Fig 9. Postoperative CT scan showing interbody fusion cage placement with pedicle screw augmentation (POLAr technique). PLIF WITH METAL CAGES
deficiencies in mind, the variation in outcomes reported in large series is not surprising. The earliest results found in the literature do not even use the conventional system mentioned previously. In his historic article in the 1953 issue of Neurosurgery, Cloward reports on 321 patients with 0.5 to 8 years of follow-up. He claims a 98% rate of fusion with an 85% long-term cure rate. 1 In contrast, Rish rates outcomes in PLIF versus simple discectomy as satisfactory (61% v 64%), poor (21% v 24%), or failure (18% v 12%), demonstrating no significant advantage to PLIF. 5 He was later to confirm these results in his 12-year experience with 250 patients, but he notes that the incidence of subsequent surgery diminishes with PLIF." Other early series have results as impressive as Cloward's. In a 6-year follow-up, Collis evaluates 50 consecutive patients of his 750 total patients, achieving 92% satisfactory results (some or total pain relief) with 94% fusion by 2 years. 3~ Hutter's subgroup of 142 spinal stenosis patients evaluated at 3 or more years shows a 91% fusion rate and an 80% excellent or good outcome, 7 whereas Prolo, in converting his scale to the conventional 4-tiered system, demonstrates a 94% fusion rate and an 85% satisfactory (excellent or good) outcome at 1 year in 34 patients. ~ Schecter tries to control for patient variance and sets strict entrance criteria (back pain for more than 3 months; no disc herniation, stenosis, or instability; no prior operation; single level pathology) in 25 patients with a 2- to 4-year follow-up. He reports a 95% fusion rate with 89% excellent or good outcomes and no poor outcomes, s Lee used similar entrance criteria, but also required discogram concordant pain, in 62 patients observed for 2 years. He achieved results almost identical to Schecter's with a 94% fusion rate and 89% excellent or good clinical status. Moreover, 93% of Lee's patients returned to work. ~ Of course, data from these 2 studies can only be extrapolated to persons similar to the homogeneous patient population selected for by their entrance criteria. More recent studies have involved larger patient numbers. Gill performed PLIF with pedicle screw fixation in 238 patients followed-up for 2 years, attaining a 92% fusion rate with 87% excellent or good outcome by using the Prolo equivalent. 36 Ray's multicenter, prospective randomized trial of his titanium threaded cage in 236 patients at 32 months found a 96% fusion rate by 2 years and a 65% excellent/good Prolo rating with an additional 25% rating results as fair. 2~ Kuslich also evaluated his BAK cage in a multicenter, randomized trial with 2-year follow-up. Of the 356 patients who u n d e r w e n t PLIF, 100% achieved fusion in single level surgery, and 90% achieved fusion in 2-level surgery. Clinical pain relief was reported in 84% and functional improvement in 91%, with 91% of patients also returning to work at 3 years. 2" The impressive results claimed by these investigators have recently been called into question by an independent evaluation of a carbon fiber cage with posterior fixation performed by Agazzi et al. 37 In this study, 71 patients were followed-up for 28 months. A 90% fusion rate was noted, but outcomes based on the Prolo scale were excellent/good in only 39°/,, and fair in 25%, with 67% of patients reported being satisfied with the outcome. One possible bias of this study concerns patients involved in compensation claims. Only 47% of these patients improved versus 84% of noncompensation
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T A B L E 1. Survey of Characteristics Used by I n v e s t i g a t o r s to E v a l u a t e Clinical O u t c o m e After PLIF
Excellent Good Fair Poor
Residual Pain
Medications
Activity Level
Work Status
No pain Improved but minor pain Improved but present pain Minimal improvement or no pain relief
None Minimal (NSAIDs) Frequent but not daily (narcotic) Daily medication (narcotics)
No limitations on activity level Strenuous activity may cause pain Limited to nonstrenuous activity Marked limitations of all activities
Return to original work Return to original work Return to lighter work Unable to work
NOTE. A survey of characteristics used by investigators to evaluate clinical outcome after PLIF is summarized in this 4-tiered rating system. These conventional ratings categorize patients based on success of pain relief, frequency of medication use, general level of activity, and postoperative work status. Prolo 34 suggested an objective scale by assigning 1 to 5 points in each of 2 categories. Economic potential was graded from complete invalid (score of 1) to return to previous occupation with no restrictions (score of 5). Functional capacity was graded from total incapacity (score of 1) to complete recovery allowing participation in all activities and sports (score of 5). The ratings from each of the 2 categories were summated for a total of 2 to 1 0 points. Excellent outcome was correlated with a score of 9 or 10, good outcome with a score of 7 or 8, fair outcome with a 5 or 6, and poor outcome with a score of 2, 3, or4.
Abbreviation: NSAIDs, nonsteroidal anti-inflammatory drugs.
patients. Further, only 6% of compensation patients returned to w o r k versus 80% of n o n c o m p e n s a t i o n patients. Although there are u n d o u b t e d l y patients in w h o m PLIF is indicated, better stratification is required before meaningful conclusions can be d r a w n as to w h o benefits m o s t from this procedure.
COMPLICATIONS PLIF shares m a n y of the complications found in posterior laminectomy and discectomy, p e r h a p s with s o m e higher risk of occurrence because of the more involved nature of PLIE Of these, skin infections are reported to occur in 0% to 7°/,, of cases. 5,s,~,27-3~.384° Prone positioning resulting in anterolateral femoral cutaneous neuritis has been reported by one investigator to occur in 6% of cases, although always transiently. 3s The more aggressive retraction of the thecal sac during PLIF leads to a cerebrospinal fluid leak in 0% to 10% of cases. 2s-3"~.37.40Likewise, the increased m a n i p u lation of the nerve root required by PLIF results in not infrequent neurologic deficit. These disturbances are sensory in 2% to 5% of patients and are almost always temporary. Motor deficits, encountered transiently in 0.8% to 10% of patients, are p e r m a n e n t in less than 0.8% of patients. 2s-4° Urinary disturbances are very rare (<0.5%) and are always transient. ~-37 It should be r e m e m b e r e d that the neurologic deficits complicating PLIF are not always related to surgical manipulation. They m a y be a direct result of fusion, arising from pedicle screw placement, for example. Additional complications ascribed to the fusion portion of this procedure include graft migration in 0% to 6% of patients, 1,27-3~,-~sq° with catastrophic consequences if the graft is extruded into the spinal canal. This was generally a problem in earlier studies; Ray reports no graft retropulsion in his large series using threaded cage fusion technique. 2s Perhaps the most difficult complication observed in PLIF is failure of fusion. Although encountered in 3°/. to 15% of cases, 1.4~,2741 failure of fusion is not s y n o n y m o u s with clinical failure. In fact, Blumenthal and Gill suggest that correlation of radiographic evidence of fusion with actual intraoperative findings is only 69%, with a false positive rate of 42% and a false negative rate of 29°/,,. In his reexamination of 49 patients, successful arthrodesis was confirmed in 90% of the patients. The investigators theorize that the premineralized osteoid tissue that was functionally fused probably appeared radiolucent on radiographs. 41 Although radiographic fusion is not seen in all
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patients with satisfactory outcomes, s o m e investigators suggest that every patient with excellent pain relief will d e m o n s t r a t e a solid fusion mass. 7
CONCLUSIONS N o n u n i f o r m i t y of indications in existing studies and lack of a standard m e t h o d of evaluation m a k e definitive conclusions difficult at best. PLIF is certain to h a v e a role in the treatment of l u m b a r spinal disorders, especially discogenic lower back pain. Although excellent results are seen with stand-alone PLIF procedures, there is a growing trend towards reinforcement of the PLIF procedure with posterior fixation. Pedicle screw augmentation, such as with the POLAr technique, seems particularly well suited for use in failed back s y n d r o m e , especially w h e n p o s t l a m i n e c t o m y spondylolisthesis is present. POLAr m a y also be of use in patients with chronic low back pain attributable to internal disc d e r a n g e m e n t s w h e n confirmed by the presence of concordant pain on provocative discography. Additionally, it is useful in the treatment of patients with gross l u m b a r instability and microinstability, especially w h e n evidence of degenerative collapse of the disc space is present on MRI. As with all cases, conservative therapy should be exhausted before interbody fusion techniques are considered.
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