Unilateral Transforaminal Posterior Lumbar Interbody Fusion

Abstract

A prospective analysis of consecutive patients who had lumbar fusion using the unilateral transforaminal posterior lumbar interbody fusion with pedicle screw fixation is presented to assess the clinical and radiographic outcomes of the transforaminal posterior lumbar interbody fusion procedure and describe the technique and indication in the treatment of degenerative disease of the lumbar spine. Forty patients treated with transforaminal posterior lumbar interbody fusion for degenerative diseases of the lumbar spine (with anterior column deficiency) were followed up for a minimum of 3 years (mean, 3.4 years; range, 3–3.9 years). Radiographic assessment included plain and flexion and extension radiographs. Clinical outcome was based on pain relief, ability to do activities of daily living, and return to work. Thirty-six patients (90%) had solid fusions and at latest followup, segmental lordosis has increased in all patients. Eighty-five percent of patients had excellent or good clinical outcome(s). The unilateral transforaminal posterior lumbar interbody fusion provides bilateral anterior column support through a unilateral approach. The patients had high fusion rates and patient satisfaction as reported with similar complications found in other methods commonly used for spinal decompression and stabilization.

In 1981, Blume and Rojas ¹ described a unilateral approach for posterior lumbar interbody fusion to address some of the complications associated with the standard posterior lumbar interbody fusion procedure. This technique has been popularized by Harms et al. ⁷ The unilateral transforaminal posterior lumbar interbody fusion is a surgical technique in which bilateral anterior column support can be achieved through a unilateral posterolateral approach. Pedicle screw instrumentation is added to provide posterior column stabilization in addition to the anterior column support provided by the use of Ti mesh as an interbody implant.

To the authors’ knowledge, there have been no published reports assessing the efficacy of patients treated by transforaminal posterior lumbar interbody fusion for degenerative disorders of the lumbar spine. The current study will provide the authors’ indications for surgery, the surgical technique, complications, and early results with this procedure.

INDICATIONS

The key to successful outcomes for degenerative disorders of the lumbar spine is patient selection. Appropriate patients would include those with a healthy psychologic profile with realistic surgical goals who have been through a 3- to 6-month comprehensive nonoperative program for chronic low back pain. Radiographic studies including plain films with flexion and extension lateral views, magnetic resonance imaging, and occasionally discography should closely correlate with clinical findings and should document a surgically treatable disorder limited to one or two levels only. ⁸ Given that lumbar spine fusion for degenerative disorders is associated with failure rates as high as 40%, the key to success is the accurate identification of the pain generator in any given patient. ^16,17

Current operations for degenerative disc disease are intended primarily to decrease pain by decompression of stenotic segments and eliminating motion by achieving a solid arthrodesis. Secondary benefits of fusion in selected patients include restoration of segmental lordosis and reduction of isthmic or degenerative spondylolisthesis. ¹⁹ Although the exact etiology of adjacent segment syndrome currently is not well understood, it is thought that loss of lordosis likely is an important factor involved in its pathogenesis. ^6,13

The transforaminal posterior lumbar interbody fusion procedure provides for an anterior interbody arthrodesis and a posterior arthrodesis through one posterior surgical approach. It is indicated for anterior column deficiency associated with chronic mechanical low back pain. Although posterior and posterolateral instrumented fusions provide rigid posterior fixation, they still do not provide complete anterior column stabilization. Transpedicle fixation allows three-column fixation; however, several investigators have shown the potential for continued micromotion within the disc space despite a solid posterior or posterolateral fusion and discogenic back pain may continue in these patients. ^4,20 For that reason, the transforaminal posterior lumbar interbody fusion technique provides a higher likelihood of pain relief in patients with discogenic back pain than standard posterior or posterolateral instrumented techniques. Also, because instrumented posterolateral fusion does not restore biomechanical anterior column support in patients with instability, there is a greater risk of hardware breakage and pseudarthrosis. The transforaminal posterior lumbar interbody fusion procedure provides a more conducive environment for successful fusion because of the increased surface area and the increased vascularity of the interbody space. Patients who are at a higher risk for pseudarthrosis are better treated with this technique as opposed to instrumented posterior or anterior stand alone techniques because it provides for a combined anterior and posterior arthrodesis.

Patients with isthmic or degenerative spondylolisthesis with translational instability often are candidates for the transforaminal posterior lumbar interbody fusion procedure because the discectomy and anterior column support combined with application of compression to the posterior pedicle instrumentation facilitates reduction of spondylolisthesis and creates a load-sharing construct.

In patients with degenerative lumbar scoliosis who require fusions to the sacrum, the transforaminal posterior lumbar interbody fusion is a means of providing anterior interbody support without the need for an anterior surgical approach.

Finally, because the transforaminal posterior lumbar interbody fusion procedure incorporates the combination of interbody structural support with pedicle instrumentation, the application of compression to the posterior instrumentation allows for restoration of segmental lordosis in patients with segmental kyphosis related to degenerative disc disease. Restoration of lordosis may help prevent adjacent segment syndrome although stiffness of the fused levels may be an additional risk factor. ^6,13

The advantages of the transforaminal posterior lumbar interbody fusion over the combined anterior and posterior fusion are the avoidance of a second anterior operation with the risk of other complications. Although rare, neurovascular injuries do occur. Although rare when single-level fusion is done with careful exposure, impotence in males has been reported after anterior lower lumbar approaches. ^2,11,12 The advantages over the standard posterior lumbar interbody fusion procedure include the ability to provide bilateral anterior column support through a unilateral transforaminal approach with less risk of adjacent segment instability, epidural fibrosis, and injury to neural structures related to excessive retraction. ^10,18,22

PROCEDURE

Figures 1 through 4 show representative steps in the transforaminal posterior lumbar interbody fusion procedure. The patient is placed on a suitable spine frame in the prone position with the hips in maximum extension, helping to maintain lumbar lordosis, and when present, affording partial reduction of an isthmic or degenerative spondylolisthesis. A standard midline approach is used. Careful dissection is done out to the tips of the transverse processes of the levels included in the fusion. Through the same incision, one iliac crest is exposed but muscles are left attached. A small window is made in the posterior iliac crest and an adequate amount of cancellous bone is removed from between the cortical tables of the iliac crest to use for bone graft. The fascia over the iliac crest is closed with interrupted sutures. The authors have observed very little donor site pain postoperatively since using this technique.

Fig 1.: This drawing shows packing of iliac bone graft into the interior ½ of disc space after decortication. The posterior ½ of the end plates is left intact to provide support for Ti mesh cages. (Reprinted with permission from Medtronic Sofamor Danek, Memphis, TN.)

Fig 2.: This drawing shows how cages are rolled to each side using the packed bone graft as a surface to prevent cages from being placed too far anteriorly. (Reprinted with permission from Medtronic Sofamor Danek, Memphis, TN.)

Fig 3.: This drawing shows that the construct is completed by application of compression to screws to create segmental lordosis. (Reprinted with permission from Medtronic Sofamor Danek, Memphis, TN.)

Fig 4.: This drawing shows the completed transforaminal posterior lumbar interbody fusion including unilateral facet fusion and posterolateral fusion. (Reprinted with permission from Medtronic Sofamor Danek, Memphis, TN.)

A localizing radiograph is taken for level identification purposes. Decompression, if indicated, then is done based on preoperative symptoms and imaging studies. Multiaxial pedicle screws are inserted at the appropriate levels and proper placement is confirmed with biplanar fluoroscopy and direct stimulation of the screws while recording electromyographic responses in the adjacent nerve roots (evoked electromyograms). ³

The side of the spine selected for the transforaminal posterior lumbar interbody fusion is chosen arbitrarily or based on preoperative symptoms. If a disc herniation or foraminal stenosis is present and predominantly one-sided, then that side is chosen. After bilateral pedicle screw insertions, the rods are contoured in lordosis and cut approximately 1 cm longer than usually would be used to allow for disc space distraction. The rod and locking screws are inserted into the multiaxial screw heads and as bilateral distraction is applied, the locking screws are tightened. A ¼-inch osteotome and Kerrison rongeurs (Zimmer, Warsaw, IN) are used to remove the inferior articular process and superior portion of the superior articular process on the side chosen for the transforaminal posterior lumbar interbody fusion.

Exposure of the underlying disc space is facilitated by removal of the lateral margin of the ligamentum flavum. Identifying the exiting nerve root inferior and medial to the upper pedicle to be instrumented helps orient the surgeon, because the remainder of the anatomy is consistent in relation to this structure. Epidural bleeding frequently is encountered at this point during separation of the nerve root from epidural fat and venous plexus. Irrigating bipolar electrocautery is useful in controlling epidural bleeding and thrombin-soaked gelfoam and cottonoids also can be used if needed. Once hemostasis is achieved, the underlying disc space (lateral ⅓), dural sac, and exiting nerve root should be readily visible. The exiting nerve root rarely needs retraction except at the L5–S1 level. A 45° nerve root retractor placed around the dural sac improves exposure and access to the underlying disc space.

A 15-blade scalpel is used to create an annular window. The medial border of the window is the lateral margin of the dural sac, and the lateral border is the lateral edge of the visible annulus. The incised annulus is removed with a pituitary rongeur. A ¼-inch osteotome is used to enlarge the window and remove posterior osteophytes, allowing easy access to the disc space. Specialized straight and angled osteotomes, pituitary rongeurs, rasps, and curettes are used to elevate and remove disc material and cartilaginous end plate. Additional distraction of the pedicle instrumentation is applied relying on ligamentotaxis. The disc space is irrigated with Bacitracincontaining saline and then reinspected to confirm complete removal of disc material. A disc spanner is inserted to determine the appropriate size cage, which should be 1 mm smaller in height than the spanner measurement to allow for lordosis when compression is applied to the posterior instrumentation, 12-to-15 mm diameter cages with heights ranging from 7 to 9 mm are most commonly used. A ¼-inch angled osteotome is used to decorticate only the anterior ⅓ of the adjacent end plates. The posterior ⅔ of the adjacent end plates are left intact to provide support for the cages. This decortication provides an excellent graft bed adjacent to the anterior annulus. Previously harvested iliac crest bone graft then is packed tightly into the anterior ⅓ of the disc space with a bone tamp as shown in Figure 1. Two Ti mesh cages then are packed with autograft and inserted into the disc space. The first cage is inserted into the posterior interbody interspace and maneuvered across the disc space to the contralateral side using straight and angled impactors. The second cage is inserted into the ipsilateral posterior disc space (Fig 2). Distraction then is released and biplanar fluoroscopy is used to confirm proper placement of the cages. Ideally, the cages should be placed in the posterior ⅓ of the disc space. This provides structural support close to the center of rotation for the motion segment and allows later radiographic assessment of fusion anterior to the cages. Compression then is applied, locking the cages in place and maximizing segmental lordosis as shown in Figure 3. The contralateral facet and transverse processes bilaterally are decorticated and packed with iliac bone graft completing the procedure (Fig 4). The wound is closed in layers over closed suction drainage completing the procedure.

Patients are mobilized on postoperative Day 1, and no external orthosis is required. For the first 6 weeks postoperatively, patients are encouraged to walk as much as possible. Progressive range of motion (ROM), strengthening exercises, and light aerobic exercises then are initiated progressively. By 6 months, patients are allowed full activities as tolerated. A solid fusion usually is confirmed radiographically at 12 months postoperatively. Case 1 (Fig 5) shows radiographs obtained before and after surgery of a patient with degenerative spondylolisthesis who had an excellent radiographic and clinical outcome after a transforaminal posterior lumbar interbody fusion procedure.

Fig 5A–D.: (A) A preoperative anteroposterior radiograph of a 41-year-old woman shows degenerative lateral listhesis at L4–L5. (B) The preoperative lateral radiograph shows degenerative spondylolisthesis L4–L5 with loss of lordosis at that level. (C) The anteroposterior radiograph obtained 2 years postoperative shows posterolateral fusion. (D) A lateral radiograph obtained 2 years postoperative shows an increased disc space height and increased segmental lordosis.

RESULTS

Clinical and radiographic results of 40 patients who had a transforaminal posterior lumbar interbody fusion procedure with a minimum of 2 years followup are presented. From January 1996 to December 1997, 23 women and 17 men with a mean age of 44.9 years (range, 24.1–69.2 years) had a transforaminal posterior lumbar interbody fusion. The followup averaged 3.4 years (range, 3–3.9 years). Thirty-four patients had a single-level transforaminal posterior lumbar interbody fusion and six patients had two levels fused. All 40 patients had degenerative disc disease and in addition, 11 had Grade 1 to 3 isthmic or degenerative spondylolisthesis and six had a recurrent disc herniation at the L4–L5 level. Radiographic fusion was thought to be present in 36 of 40 (91%) patients based on the presence of obliteration of the disc space anterior to the cages and continuous trabecular bone throughout the intertransverse fusion mass and no demonstrable motion on flexion and extension radiographs. One patient had a confirmed pseudarthrosis requiring reoperation and three had radiographic evidence suggestive of pseudarthrosis based on radiographs obtained 2 years postoperative. These patients have remained asymptomatic. Segmental lordosis for one-level and two-level transforaminal posterior lumbar interbody fusion improved 29.6% and 13.6%, respectively. These values were statistically significant (p < 0.05). A comparison of preoperative and 2-year postoperative radiographs (Case 1) show radiographic evidence of arthrodesis and increased segmental lordosis.

Pain level on a 10-point visual analog scale improved from a mean preoperative value of 8.3 +/−1.97 to 3.2 +/−2.06 (paired t test, p < 0.0001) at latest followup. Less than 10% (three of 40) of the patients were able to do their activities of daily living with no or little difficulty preoperatively and ⅔ (29 of 40) were able to do activities of daily living postoperatively. When asked whether they would have the surgery again based on their outcome 33 patients (82.5%) said they would have the surgery again. A questionnaire was developed for the study that focused on pain and activities of daily living. Patients filled out a questionnaire preoperatively and again 2 years postoperatively. Similarly, 85% (34 of 40) of the patients were rated as having excellent or good results based on questionnaire scores before and after surgery. There were six fair and no poor results based on questionnaire scores. Preoperatively, there were 23 patients working and 10 were not working because of back pain; seven were retired. Postoperatively, all 23 patients who were working returned to work and eight of 10 who were not working because of back pain returned to work. All of the patients who had retired preoperatively remained retired postoperatively.

There were no statistically significant differences in fusion rates between smokers and nonsmokers. Likewise, there were no statistically significant differences in clinical outcome between patients who were receiving worker’s compensation and patients who were not receiving worker’s compensation in this series, although only six patients sustained work-related injuries.

Complications other than pseudarthrosis included two dural tears repaired intraoperatively, one transient neurapraxia related to nerve root retraction, and one late infection (> 2 years) necessitating removal of pedicle instrumentation. The fusion was observed to be solid during hardware removal. There was no evidence of clinical arachnoiditis or cage-related complications in any of the patients.

DISCUSSION

Surgical techniques that include interbody fusion have shown high fusion rates with distinct advantages including anterior column load sharing large surface area for fusions, and the ability to restore normal sagittal profile while achieving passive foraminal decompression. ^{2,9,10,12,14,18} Interbody fusion techniques seem to be the most effective treatment for patients with discogenic back pain who do not respond to conservative treatment. Weatherley et al ²⁰ reported persistent low back pain in patients who had solid posterolateral fusion. All patients had positive discograms and had pain relief after anterior interbody fusions. Similar results were reported by Derby et al. ⁴

Interbody fusion can be achieved through anterior, posterior, or combined approaches. ^{2,9,10,12,14,18} Proponents of the anterior approach cite shorter hospital stays and the prevention of fusion disease, which relates to musculoligamentous pain and stiffness resulting from paraspinous muscle stripping. ^9,12,23 High fusion rates and patient satisfaction in properly selected patients have been reported with this approach ²³; however, threaded cage devices as stand alone implants for treatment of the degenerative lumbar spine probably are not indicated for other than a degenerative collapsed disc. ^9,23 Complications related to improper cage placement and exposure do occur (injury to iliac vessels, retrograde ejaculation). ^2,9,23

Posterior interbody techniques allow the surgeon to concomitantly address all of the disorders through one approach. When combined with pedicle screw fixation, anterior and posterior column fixation can be achieved. The addition of an interbody fusion to a posterolateral fusion provides a 360° circumferential fusion bed and may be associated with improved fusion rates, especially in patients with other medical comorbidities such as diabetes, obesity, and nicotine abuse. ¹⁸ A biomechanical study of posterior lumbar interbody fusion without additional posterior instrumentation has suggested that significant destabilization of the fused segment may occur. ¹⁵ Significant bilateral bony and ligamentous removal often is required to allow accurate placement of properly sized implants, and it is difficult to provide segmental lordosis. Perhaps the greatest concern with a standard posterior lumbar interbody fusion is the amount of neural retraction needed potentially leading to nerve root injury, dural laceration, and epidural fibrosis.

The transforaminal unilateral posterior lumbar interbody fusion was developed to address many of these problems. ^1,7 Advantages over the standard posterior lumbar interbody fusion include the ability to provide bilateral anterior column support through one posterolateral approach. Because of the transforaminal approach, this technique preserves the anterior and posterior longitudinal ligamentous complex, which provides a tension band for compression of the graft and prevents retropulsion of the graft. It avoids excessive soft tissue dissection, which may help prevent scarring and instability of adjacent segments, and injury to the exiting nerve root. The ability to provide additional lordosis in degenerative levels is advantageous in this group of patients.

The radiographic fusion rate of 91% and objective clinical excellent or good outcome of 85% compare favorably with previous reports using other fusion techniques. ^5,21 This may be partly attributable to an overall favorable patient population. The majority of the patients were not smokers, nor were they seeking disability, or were receiving workers’ compensation at the time of surgery. However, the results partly underscore the importance of proper patient selection when considering surgery to treat degenerative diseases of the lumbar spine.

The transforaminal posterior lumbar interbody fusion can be mastered easily but a learning curve exists. The exiting nerve root must be observed throughout the procedure although retraction of the existing nerve root rarely is needed except at the L5–S1 level. Meticulous attention to disc removal is essential. This allows the largest surface area possible for fusion and aids in proper cage placement. The authors have found that incomplete removal of disc material just ventral to the posterior annulus can compromise cage placement. Because a unilateral approach is used, the surgeon is relying on indirect observation and tactile feedback when working across the disc space. Specialized instruments greatly facilitate disc removal during this portion of the case. Pedicle fracture can occur during distraction, and this procedure is not recommended for patients with severe osteopenia (bone mineral density < 60% predicted). The authors have experienced no difficulties with implant failure related to the use of pedicle instrumentation as a means of distraction. Other contraindications include bilateral epidural fibrosis, previous failed anterior fusion, and a fusion of more than two levels.

The benefits of posterior lumbar interbody fusion through a unilateral transforaminal approach include less retraction of the thecal sac and exiting nerve root than with the traditional posterior lumbar interbody fusion. It is particularly useful in patients who have had a previous unilateral laminotomy and discectomy. It is a safe and reproducible technique that provides bilateral anterior column support through a unilateral posterior approach. A sound biomechanical construct is achieved with the ability to normalize segmental lordosis and improve alignment of spondylolisthesis. High fusion rates with good clinical outcomes can be achieved with few complications using this technique. Proper patient selection continues to be the most important factor in good clinical outcome.