Modified posterior lumbar interbody fusion using a single cage with unilateral pedicle screws: a retrospective clinical study

Posterior lumbar interbody fusion (PLIF) is widely performed for patients suffering from degenerative lumbar spine diseases, such as spinal stenosis, lumber spondylolisthesis and lumber disc herniation with instability. PLIF has its advantages for restoring disc height, disc stabilisation, nerve root decompression and reinforcement of the anterior spinal column [1]. However, the traditional PLIF technique always involves implantation of two cages through a bilateral approach and bilateral laminectomy and requires bilateral transpedicle screw fixation for the initial stability [2, 3]. A study has reported that bilateral interbody cages and pedicle screw fixation can increase the successful fusion rate [4]. However, when inserting two cages, destroying posterior elements of the spine, such as the bilateral lumbar facet joint and lamina, is necessary, which can lead to iatrogenic instability of the posterior elements and can cause postoperative back pain syndrome [5]. Bilateral pedicle screw fixation can cause unnecessary trauma to the lumbar musculoligamentous complex and can consequently increase infection rates and lumbar musculoligamentous complex injury, which can result in poor clinical outcomes [5]. Additionally, a low medical cost-effectiveness is expected.

This study has retrospectively analysed the clinical and radiographic outcomes of performing unilateral PLIFs by inserting a single cage filled with an autogenously morselised bone via the symptomatic side and performing unilateral pedicle screw fixation.

Twelve months postoperatively, all patients had achieved an Excellent or Good outcome (Excellent in 28 patients and Good in 3). The mean pain score was 6.8 prior to surgery and had decreased to 2.3 at the 3-month postoperative examination. The decrease remained 12 months postoperatively. No significant complication or neurological deterioration occurred during the 12-month follow-up.

The radiological outcomes were listed as follows. At the 12-month postoperative examination, none of the 31 patients appeared to have fusion failure, as defined by the presence of a definite radiolucent line around the cage or pedicle screws or greater than 5° of motion on dynamic flexion-extension views. CT scan showed bony trabeculae bridging the fusion level. No broken screws, screw loosening, significant cage migration or subsidence was observed in any of the cases. The intervertebral disc height significantly improved from 9.11 mm preoperatively to 13.1 mm immediately postoperatively and was 12.25 mm at the latest follow-up. The mean increase in the intervertebral disc height of 3.14 mm from the preoperative measurement to the latest follow-up examination was statistically significant (p <0.05) (Table 1).

The patients all had a single-level fusion, and the duration of the surgeries was 94 min (range, 80 to 120 min). The mean haemorrhage volume was 250 ml, and no blood transfusion was required for any of the cases. The mean hospitalisation period was 7 days (range, 5 to 10 days) (Table 2).

PLIF not only relieves the pain resulting from nerve compression by neural decompression of the symptomatic side but also restores disc height, maintains vertebral alignment, restores weight bearing and reconstructs stability of the segment. PLIF has been reported to obtain a higher rate of fusion of the intervertebral segments and more satisfactory clinical outcomes than posterolateral bone grafting [6, 7].

The traditional PLIF technique is usually performed by inserting two cages via a bilateral approach with extensive laminectomy or posterior facetectomy and combining bilateral pedicle screws to provide spinal stability. The procedure itself has some disadvantages. Primarily, wide laminectomy, bilateral facetectomy and extensive intraoperative paraspinal muscle exposure around the posterior segments in the procedure increase the trauma and blood loss, causing denervation and atrophy of the paraspinal muscle, which results in a failed back syndrome [8]. Destruction of the bilateral facet joints and posterior ligament intraoperation can decrease spinal stability and hence increase the risk of perioperative or postoperative complications, such as cage migration [9]. In addition, the nerve root and dural sac always need to be retracted substantially to create two cages, which can cause bilateral nerve root injury or dural tearing. Eventually, additional bilateral pedicle screw fixation also requires contralateral extensive muscle release, which again increases trauma, blood loss and medical cost.

To solve the shortcomings of traditional PLIF, the technique has been modified in this study. Unilateral PLIF was performed by inserting a single cage filled with an autogenously morselised bone via the symptomatic side and unilateral pedicle screw fixation. The technique has several clear advantages over traditional two-cage PLIF supplemented with bilateral pedicle screws. First, one cage can be placed from the symptomatic side to avoid excessive retraction of the nerve root and dural sac of the asymptomatic side, which decreases the risks of epidural fibrosis and injury to neural structures related to retraction. Following the point mentioned above, in our technique, we only performed a partial laminectomy and facetectomy on the symptomatic side rather than a total laminectomy and bilateral facetectomy. The spinous process, supraspinous and interspinous ligaments, and the contralateral vertebral plate and facet joints are preserved in the procedure, which is very significant for maintaining stability of the lumbar spine [10, 11]. The destruction of the above structures always causes postoperative complications, such as cage migration and iatrogenic instability. Another important point is that the contralateral paraspinal muscle can be kept intact, decreasing blood loss, trauma and operative time. According to the report, excessive intraoperative paraspinal muscle exposure can also lead to denervation and atrophy, which results in a failed back syndrome. Finally, our procedure decreases medical cost by reducing the requirements for cages, transpedicle screws and transfusions.

The biomechanical tests on a calf lumber specimen after PLIF were conducted. The tests showed that the stability of the specimens with unilateral PLIF by inserting a single cage and unilateral pedicle screw fixation was weaker, but there were no significant differences than intact specimens. This finding suggests that this technology can provide adequate initial stability. Many studies have reported that one cage is enough in PLIF or TLIF. Oxland and Lund [12] also advised that single cage PLIF provides high stability in flexion; the supplementary use of pedicle screws improved stabilisation in all directions, and two-cage PLIF might increase the risk of damage to the bilateral nerve roots. Fogel [13] and Chang [14] reportedly suggested good results with unilateral cages and showed that patients with a unilateral cage had equal fusion and clinical success compared to those with bilateral cages. Zhao [15] noted that PLIF using a single threaded cage with a supplementary transpedicular screw and rod enables sufficient decompression and solid interbody fusion.

In our cases, satisfying clinical results and solid fusion union were both achieved. There were no complications, such as infection or neurological deterioration. No broken screw, screw loosening, significant cage migration or subsidence was observed in any of the cases.

All of our patients are ethnic Chinese who were lighter in weight and whose vertebral bodies were relatively smaller in size than those of Caucasians. The study does not provide sufficient evidence of whether this technique can provide adequate initial stability in Caucasian patients whose vertebrae are relatively bigger. During the interval, our patients only had unilateral radiculopathy caused by foraminal stenosis or degenerative disc disease. The patients with conditions requiring bilateral nerve root decompression and cage insertion, such as bilateral radiculopathy or spondylolisthesis and those with spinal osteoporosis, should choose traditional standard PLIF, and the unilateral PLIF techniques may not apply. Eventually, obese patients were not chosen for this surgery because of the expected risk that unilateral pedicle screws may not provide sufficient initial stability and hence lead to failure of the internal fixation.

Unilateral PLIF with a single cage via a unilateral approach with supplementary unilateral transpedicular screws enables sufficient decompression and solid interbody fusion while maintaining intact posterior elements. This technique is ideal for the patients with severe symptomatic axial LBP with radiculopathy from disc herniation or stenosis. However, identifying applicable patients is crucial for the success of the surgery.