Posterior Lumbar Interbody Fusion via a Unilateral Approach

A lumbar fusion through an available surgical technique, can reduce pain and decrease disability in patients with chronic lower back pain (LBP). Posterior lumbar interbody fusion (PLIF) has the advantages of restoring the disc height, immobilizing the unstable degenerated intervertebral disc area, decompressing the nerve roots, and restoring load bearing to anterior structures.1 The use of two cages has recently become routinely as a standard PLIF technique.2, 3 However, it is very difficult to insert two cages of an appropriate size without surgical destruction of the posterior element of the spine. Extensive laminectomy and bilateral facetectomy cause iatrogenic instability of the posterior elements, which may cause postoperative back pain syndrome.4 Therefore, most clinical reports recommend that PLIF should be combined with pedicle screw fixation for a good clinical and radiological outcome.5, 6 However, the wide exposure required for a circumferential fusion can cause unnecessary trauma to the lumbar musculoligamentous complex, which is one of the causes of a poor postoperative outcome.4, 7 Most patients with a chronic disc herniation of the lumbar spine suffer from long standing LBP with instability. This is sometimes combined with a unilateral radiating leg pain, relative to the location of the disc herniation. We performed a PLIF, using two PEEK™ (Poly-Ether-Ether-Ketone) O.I.C. cages, with a unilateral medial facetectomy and hemilaminectomy, to avoid extensive iatrogenic destabilization of the posterior elements of the lumbar spine. Forty-one patients with unilateral leg pain accompanied by instability were treated with this unilateral approach. In this report we present our surgical experience, results of an average follow-up of 26-months, and a review of the literature regarding PLIF.

All patients underwent a one-level fusion. The vertebral levels at which the implants were inserted were as follows: L2-3 (one patient), L4-5 (25 patients), and L5-S1 (15 patients). With regard to the complications, there was one case of transient weakness of the foot. A re-operation was required in 1 patient due to implant migration. No implant fractures or deformities occurred.

At the time of the last follow-up visit, 37 patients (90%) were satisfied with the results of the surgery. No patients, with the exception of one, felt they were worse after the surgery. Twenty-five patients (61%) experienced excellent results, 12 (29%) had good results, three (7%) had fair results, and one (2%) had a poor clinical result. The radicular symptoms were resolved in all patients, with the exception of three, who still suffer from mild discomfort of the leg.

One patient, with worsened LBP 1 year after surgery, underwent a percutaneous pedicle screw fixation using the Sextant system. A good outcome was achieved 6 months after the second operation. A successful radiographic fusion was achieved at the time of the last follow-up in 35 patients (85%). Fig. 2 demonstrates a case of solid fusion after PLIF via a unilateral approach.

Fig. 2
A 53-year-old woman presented with a 1-year history of LBP and radiating right leg pain. (A) Plain dynamogram revealing flexion instability at L4-5. (B-C) Spinal MRI showing a severe disc protrusion with degenerative disease at the same level. The patient underwent L4-5 PLIF using PEEK cage. (D) Plain radiograph obtained one and a half years after the surgery demonstrates a solid fusion. At the last follow-up visit, she has returned to daily activities without any symptoms.

• Click for larger image

The fusion status was not associated with the clinical outcome (p = 0.15). The majority of patients felt they had improved postoperatively and would opt for the same surgery again. The average disc height before surgery was 8.1 mm, which was restored to an average of 11.7 mm immediate after surgery. At the last follow-up, the average disc height had decrease to 10.1 mm. The disc height after the PLIF procedure was increased significantly. However, there was no corelation between the change of disc height and the Prolo scale (Fig. 3).

Fig. 3
This case showed retropulsion of the inserted cage at 6 months postoperative. (A, B) Preoperative MRI showed degenerative disc herniation at L5/S1 segments. (C) At 6 months after PLIF, the cage migrated posteriorly and compressed the dural sac.

• Click for larger image

Over the last few decades, surgical fusion of the lumbar spine has been increasingly performed on patients with chronic LBP. A variety of different surgical techniques can be used to achieve lumbar fusion.9 Posterolateral fusion has been one of the standards for surgical treatment of lumbar spinal instability, and with the use of spinal instrumentation, it has been widely used for lumbar degenerative pathology.1 However, posterolateral fusion may not restore the disc space height or sagittal segmental alignment, even when spinal instrumentation is used.

The origin of chronic LBP remains unclear and controversial. In degenerative disc disease with instability, the major source of the pain is thought to be a result of the degenerative disc itself and/or the facet joints. If the source of pain is believed to be a degenerative disc, it seems reasonable to remove this structure and replace it with a bone transplant, using either PLIF or anterior lumbar interbody fusion (ALIF).2, 3 PLIF, pioneered by Cloward, has the advantage of allowing the restoration of the disc space height, sagittal plane alignment, and weight bearing through the anterior column.3-5 Since the introduction of various cages for PLIF, the surgical technique and its outcomes have been greatly improved. An interbody fusion with a cage alone has been increasingly accepted for the treatment of a lumbar degenerative change.6, 7

However, PLIF with a stand-alone cage has been criticized by many spine surgeons due to the need for removing a significant amount of the posterior supporting spinal structures. This can be achieved by a bilateral facetectomy or a wide laminectomy, for the bilateral placement of the cage into the disc space. Therefore, most surgeons recommend that PLIF should be supplemented by posterior instrumentation.5, 8, 9 However, a greater exposure and disruption of the posterior elements would be needed to perform a circumferential fusion of the unstable segment.

There are several reports that an iatrogenic soft-tissue injury correlates with negative long-term clinical outcomes.10 Excessive intra-operative dissection and retraction of the paraspinal musculature can lead to denervation and atrophy. The damaged muscle tissue acts as a pain generator, which results in an increased risk of"failed back syndrome".11 Extensive bilateral facetectomy is also associated with postoperative pain and prolonged disability. Therefore, in patients with chronic LBP with a unilateral disc herniation, wide exposure and circumferential fusion may produce iatrogenic flat back syndrome or contralateral leg discomfort, despite the improvement in ipsilateral leg pain. Transforaminal lumbar interbody fusion is another surgical method to achieve successful fusion with no risk of nerve root injury.12, 13 With this approach, the intervertebral disc can be accessed, unilaterally, through the neural foramen for the introduction of the bone graft material or a fusion cage. Many surgeons favor this approach over PLIF for the treatment of spondylolisthesis.

However, this approach also has the disadvantage of a wide exposure of the lumbar spine in patients with a single level instability without spondylolisthesis. In 1985, Blume14 reported that unilateral PLIF with a bone dowel and cancellous bone chip had the advantage of preserving the posterior ligament structure. Conversely, a bone graft without cages may cause disc space collapse in the long-term. Zhao et al. recently reported on the use of a single threaded interbody fusion cage, placed obliquely, for degenerative spondylolisthesis.15 They found that a unilateral facetectomy enabled sufficient decompression for the safe placement of the cage device, while maintaining the important posterior supporting spinal structures. They reported that PLIF using a single long threaded cage inserted obliquely through a unilateral facetectomy and hemilaminectomy, was significantly stiffer than that achieved with two posterior cages inserted through a bilateral facetectomy and laminectomy.16 Abumi et al.17 found that a bilateral facetectomy produced a marked increases in the motions of flexion and axial rotation, whereas a unilateral facetectomy produced less increases in motion, especially in rotation. Therefore, by avoiding bilateral facet disruption, surgeons can achieve solid fusion with PLIF.

With our surgical approach, we can save most portions of the posterior elements, including the spinous process and all the structures on the contralateral side. A total facetectomy on the ipsilateral side is not required. In contrast to the single cage technique, we used two cages with a unilateral approach. It was possible to insert two appropriately sized cages using this unilateral approach with the decompression of unilateral stenosis, with no major complications. A unilateral partial hemi-laminectomy and medial facetectomy, which is the most common procedure for a simple disc herniation, is enough for a PLIF with two cages to be achieved. Since the remaining posterior elements can maintain their physiological range of motion in flexion and rotation, we think that supplemental posterior instrumentation is not necessary as long as the facet joints are preserved.

To increase the solid fusion rate, it is essential to increase the area of the bone fusion and the total amount of graft bone. Another advantage of our approach is that an additional bone graft could be implanted before the insertion of the cage into the contralateral intervertebral space, with no risk of bone graft retropulsion or collapse. Compared to the cylindrical cage, a rectangular PEEK cage is smaller, which eliminates the need for extensive retraction of the neural tissue or the total removal of the facet joints, for its insertion. We had one case of cage retropulsion that required re-operation. There is a risk of posterior migration when rounded, stand-alone cages are used. This can be prevented by the surgeon using an appropriate cage.

PLIF in revision surgery is a technically demanding procedure. Chitnavis et al., reported excellent results following a PLIF using a carbon cage in patients with recurrent disc herniations.18 They proposed that the cage offered structural support while biological fusion occurred. However, most patients with recurrent disc herniations usually have unilateral leg pain. A bilateral approach may cause pain or sensory disturbance to the contralateral leg. We performed our surgical method in 6 patients with recurrent disc herniations accompanied by segmental instability. Of these patients, 4 had excellent clinical outcomes, and the other 2 had good outcomes. No patient felt their condition was worse after the surgery. At the time of the last follow-up visit, 4 patients were not satisfied with the results of the surgery. Of these, two patients had a normal disc space but with flexion instability. A facet fracture occurred during the insertion of larger cages into the intervertebral space, and the cages and grafted bone were unable to fully fill the intervertebral space. We had difficulty moving the first cages to the contralateral side and inserting the two large cages through the unilateral route. One of them underwent percutaneous pedicle screw fixation with the Sextant system.10 This produced a good outcome 6 months after the second operation. Two other patients had a mild to moderate degree of LBP despite a radiological solid fusion at the time of the last follow-up.

In our series, the radiological fusion rate was slightly lower than that with other PLIF techniques. A sound fusion is thought to be a parameter for clinical success in patients with mechanical LBP. However, the fusion rates did not correlate with the clinical outcomes in our patients. In previous studies, the clinical outcome did not always parallel the radiographically solid fusion.19 Agazzi et al. have reported that a PLIF with a cage may restore and maintain the disc height and the sagittal balance. This may improve the clinical results despite the lack of solid fusion.7 The primary indication from our operative method was a chronic degenerative disc disease with unilateral leg pain combined with radiological instability, but without spondylolisthesis. This technique can manage both problems successfully, without the need for a circumferential operation. The limitations of this unilateral approach for bilateral PLIF include, instability with a normal disc height, spondylolisthesis, and bilateral foraminal stenosis, requiring bilateral decompression. Further study and long-term follow-up will be necessary. A posterior lumbar interbody fusion with two cages, via a unilateral approach, enables sufficient decompression and solid interbody fusion to be achieved, while the majority of the posterior elements are maintained. It is an ideal procedure in a patient with severe symptomatic axial LBP with radiculopathy from disc herniation or stenosis.