Abstract
, we carried out a prospective study of 100 patients with a low-grade isthmic spondylolisthesis (Meyerding grade II or below), who were randomised to receive a single-level and instrumented posterior lumbar interbody fusion with either one or two cages. The minimum follow-up was for two years. At this stage 91 patients were available for review. A total of 47 patients received one cage (group 1) and 44 two cages (group 2). The clinical and radiological outcomes of the two groups were compared.
Abstract
There were no significant differences between the two groups in terms of post-operative pain, Oswestry Disability Score, clinical results, complication rate, percentage of post-operative slip, anterior fusion rate or posterior fusion rate. On the other hand, the mean operating time was 144 minutes (100 to 240) for patients in group 1 and 167 minutes (110 to 270) for those in group 2 (p = 0.0002). The mean blood loss up to the end of the first post-operative day was 756 ml (510 to 1440) in group 1 and 817 ml (620 to 1730) in group 2 (p < 0.0001).
Abstract
Our results suggest that an instrumented posterior lumbar interbody fusion performed with either one or two cages in addition to a bone graft around the cage has a low rate of complications and a high fusion rate. The clinical outcomes were good in most cases, regardless of whether one or two cages had been used.

The results of fusion for instability resulting from isthmic spondylolisthesis have been improved by the use of pedicled screw devices, which provide rigid fixation. [1] [2] [3] However, it is difficult to achieve a satisfactory fusion from the posterior approach because of the lack of graft bed, as only the transverse processes and the lateral aspects of the facet joints are available. In addition, there is a large gap between the fusion elements, which results from anterior vertebral displacement, and a lack of stability from the incompetent disc. [1] [2] [3] Posterior lumbar interbody fusion allows reconstruction of the anterior support. Moreover, a circumferential fusion may be achieved through a single posterior approach if a posterior lumbar interbody fusion is combined with posterolateral fusion, thereby stabilising both columns. 3, 4 The theoretical advantages of this include support of the anterior column, the removal of the degenerative disc, indirect foraminal decompression, restoration of the normal sagittal contour, a reduction in the degree of slip by ligamentotaxis, and an improvement in spinal biomechanics. [5] [6] [7] [8] Consequently, interbody fusion is considered by many to be the treatment of choice for degenerative disorders of the lumbar spine. 3, [9] [10] [11] [12] [13] [14] Posterior lumbar interbody fusion using bilateral interbody cages has been the standard treatment. However, occasionally only a single cage can be placed at the time of surgery for technical reasons, such as large epidural vessels which cause bleeding, a narrow disc space, nerve root anomalies that block access to the disc space, or difficulty in retracting the nerve roots because of epidural scarring from previous surgery. It is unclear whether a single cage provides acceptable balance, strength and rate of fusion.

Despite a large number of publications on the subject, there is little scientific support for posterior lumbar interbody fusion as a method of fusion. In a randomised study, Zhao et al 12 reported a similar outcome from posterior lumbar interbody fusion with one and two Bagby and Kuslich cages in patients with a degenerative spondylolisthesis of the lumbar spine. Fogel et al 15 assessed a group of patients with recurrent disc disease, symptomatic spondylolisthesis and pseudarthroses, and showed a similar outcome from posterior lumbar interbody fusion whether using one or two cages. Molinari, Sloboda and Johnstone 16 had similar results in a study of patients with degenerative lumbar disc disease and low-grade isthmic spondylolisthesis. There are, however, few studies comparing the outcome of a posterior lumbar interbody fusion using one or two cages in a well-defined population of patients with an isthmic spondylolisthesis. We have attempted to do so in this study.
Patients and Methods
Between March 2000 and February 2006, we carried out a prospective study of 100 patients with a low-grade isthmic spondylolisthesis (Meyerding grade II or below 10 ), who were treated by decompression and single-level posterior lumbar interbody fusion using a pedicled screw system and one or two cages. The inclusion criteria were disabling back and/or leg pain with or without neurological symptoms which failed to respond to at least six weeks of conservative treatment, and a moderate to severe degree of neural canal stenosis proven by MRI at the affected level. The exclusion criteria were spinal fracture, infection, tumour, previous surgery, the possibility of secondary gain from surgical fusion, and more than one level of spinal stenosis. Patients involved in litigation as a result of traffic accidents or industrial injury, were also excluded.
Patients and Methods
The patients were randomly assigned to two treatment groups, using sealed envelopes containing serial numbers ranging from one to 100. The envelopes were not opened until the operation had begun. Patients with odd numbers were placed in group 1 and those with even numbers in group 2. Nine patients were lost to follow-up and were excluded. In total, 91 patients with a minimum follow-up of two years were included in the study. There were 47 patients in group 1 (1 cage) and 44 in group 2 (2 cages). The major symptoms of the patients in group 1 was back pain in 36 and leg pain in 40 patients. In this group, the spondylolisthesis was located at the L5-S1 and L4-5 level in 30 and 17 patients, respectively. In group 2, 38 and 41 patients had symptoms of back pain and leg pain, respectively. Spondylolisthesis was at the L5-S1 and L4-5 level in 30 and 14 patients, respectively. There were eight smokers in group 1 and seven in group 2. Associated chronic medical illness was found in 18 patients in each group. The patients' age, gender distribution, surgery level, smoking status and associated medical illness were similar in both groups (Table I) . Surgical technique. One surgeon (JSL) performed each operation using the same operative technique. A full posterior decompression, including laminectomy, total facetectomy and foraminotomy, was performed. Fusion was carried out using one or two non-threaded interbody cages filled with local bone and stabilised with four pedicled screws and rods. Bone graft was packed around the cages and into the paravertebral gutters to achieve a posterolateral fusion. In each group, bone was harvested from the iliac crest and combined with that obtained from the resected lamina and spinous process. After three days, patients were mobilised in a low-profile thoracic lumbosacral orthosis which they wore for three months. Clinical assessment. Clinical outcome was assessed on improvement in back and leg pain measured using a tenpoint visual analogue scale, disability by the Oswestry Disability Questionnaire, 17 and overall clinical outcome using the Kirkaldy-Willis criteria. 18 These were assessed pre-operatively and after one and two years. In addition, the operating time, the volume of blood lost during surgery and on the first post-operative day, and the incidence of complications were recorded. Radiological assessment. Spondylolisthetic slips were expressed as the percentage of forward displacement in millimetres of the anteroposterior diameter of the slipped ver- There were no significant differences between the groups, as calculated using t-and chisquared tests tebral body. Anterior fusion was defined as the presence of a bony bridge across the anterior part of the cage, or less than 5° movement on lateral flexion and extension views and no radiolucency around the cage or cage migration. 19 Posterior fusion was graded A to D (with A noting solid bilateral fusion and D noting the absence of fusion) on the latest anteroposterior radiograph, as described by Lenke et al. 20 Posterior pseudarthrosis was defined as the presence of a grade C or D posterior fusion mass. Statistics. All data were examined using SPSS version 11.0 (SPSS Inc., Chicago, Illinois). A chi-squared test was used to calculate statistical significance for non-continuous variables. A repeated-measures analysis of variance was used to examine the statistical significance within each group; t-and chi-squared tests were used to determine the statistical significance between the groups. A p-value < 0.05 was considered significant.
Results
The mean operating time was 144 minutes (100 to 240) for patients in group 1 and 167 minutes (110 to 270) for those in group 2. The difference in mean operating time was significant (p = 0.0002). The mean total blood loss to the end of the first post-operative day was 756 ml (510 to 1440) in group 1 and 817 ml (620 to 1730) in group 2. The difference between the two groups was significant (p < 0.0001). Post-operative complications in group 1 included one transient nerve palsy and one deep infection. In group 2 there was one transient and one permanent nerve palsy and one deep infection. The postoperative infections were treated by incision and drainage, and with the appropriate antibiotics. No patient needed a revision procedure. Both groups showed a significant reduction in low back and leg pain (p < 0.001). There was no significant difference between the two groups (p > 0.05). Both showed a similar improvement in the mean Oswestry Disability score (p > 0.05) (Table II) . Using the Kirkcaldy-Willis criteria, 89.4% and 85.1% of group 1 had a good or excellent result after one and two years, respectively. The corresponding figures for group 2 were 90.9% and 84.1% (Table III) . The change in the mean slip angle post-operatively and after one and two years are shown for each group in Table IV , and the fusion rates for each group at each stage are shown in Table V .
Discussion
The aims of lumbar interbody fusion are to relieve the pain of nerve root compression and to achieve a stable surgical The level of pain was measured using a ten-point visual analogue scale (VAS), and functional disability was assessed with the Oswestry Disability Questionnaire. The VAS score ranged from 0 to 10 (maximum pain), and the Oswestry score ranged from 0 to 100 (maximum severity). Repeated analysis of variance was used to calculate the differences wtihin each group during the follow-up. Significant differences between the pre-and post-operative scores were found in each group (p < 0.05). A t-test was used to calculate the differences between the groups. No significant differences were found between the groups. There were no significant differences between the groups, as calculated using the chi-squared test construct. Posterior lumbar interbody fusion allows bone to be placed close to the axis of vertebral movement; theoretically, this results in greater stiffness when fusion occurs. An increase in intervertebral height with consequent widening of the neural foramina can be achieved using fusion cages. Although solid fusion does not guarantee a good clinical outcome, the primary aim should be to achieve this with a single surgical procedure.
Discussion
The results of traditional fusion with two cages have been extensively reported and are similar to those of posterolateral fusion. 14, 21 In 2005, McAfee et al 11 reported a 97.5% fusion rate in 120 patients with spondylolisthesis treated by transforaminal lumbar interbody fusion with a unilateral cage. Molinari et al, 16 Fogel et al, 15 and Zhao et al 12 have also reported generally good results, and noted that patients with a single cage did as well as those with two cages.
Discussion
The disadvantages of the two-cage procedure are the increased risk of excessive epidural bleeding and prolonged or excessive dural retraction. Several studies have shown a higher complication rate with bilateral cases. Elias et al 22 found a 15% incidence of dural tear and post-operative radiculopathy in 67 patients who had undergone an uninstrumented fusion using bilateral titanium threaded cages. Of the ten patients with a post-operative radiculopathy, six had epidural fibrosis, one arachnoiditis and one a recurrent disc herniation on MRI. In addition, one patient suffered a permanent motor deficit with sexual dysfunction. Okuyama et al 23 observed an 8% incidence of neurological impairment after instrumented posterior lumbar interbody fusion and concluded that the bilateral procedure is technically demanding, with a high rate of complications. In our study, however, the complication rates were similar at 13% (6 of 47) for group 1 and 14% (6 of 44) for group 2.
Discussion
The dura was not torn in any case. This may be due to the wider exposure gained by bilateral facetectomy, which was made possible by the use of posterior instrumentation. Neurological problems were few: 1 of 47 (2.1%) in group 1 and 2 of 44 (4.5%) in group 2. Post-operative neurological deficit is directly related to the degree of medial retraction of the dural sac, which is needed to insert a larger interbody cage.
Discussion
Recent mechanical data suggests that there is no significant difference in initial stability and stiffness between a transforaminal lumbar interbody fusion with a single cage and a posterior lumbar interbody fusion with two cages. [24] [25] [26] Chen et al 25 found that a unilateral cage and unilateral pedicled screw produced similar stability to a unilateral cage with bilateral pedicled screws in a porcine model. Kettler et al 26 reported that a unilateral cage is as stable as bilateral cages in the human cadaver. The position of the cage did not adversely affect the stability of the construct. 24, [27] [28] [29] The stability of the constructs were improved by bilateral pedicled screw fixation. In our study, a single oblique cage was used in group 1 cases and bilateral cages in group 2. In both groups the mean pre-operative slip was reduced to 10% by the second post-operative year. The improvement was similar in each group, which suggests that a single cage and pedicled screws are as stable as two cages and pedicled screws.
Discussion
Bone grafting of the maximum available surface area of the disc space is essential for a successful fusion. Prolo, Oklund and Butcher 30 reported that successful fusion filled 77% of the available disc space with bone. A clinical study using computed tomography showed that more than 56% of the cross-sectional area of the endplate could be cleared using a unilateral transforaminal lumbar interbody There were no significant differences between the groups, as calculated using the t-test There were no significant differences between the groups, as calculated using the chi-squared test.
Discussion
approach. 31 A single cage occupies only a small percentage of the endplate, therefore additional bone graft should be packed around the cage to fill as much of the available space as possible. Our results showed similar anterior fusion rates in the patients whether one or two cages had been used.
Discussion
Lower rates of posterior fusion have been reported by Freeman et al 13 in patients undergoing instrumented posterior lumbar interbody fusion procedures when a solid anterior fusion is present. The patients in our series achieved anterior fusion more frequently than posterior fusion. Although the mean rate of anterior fusion was 87% for all patients, lower posterior fusion rates were observed in both groups (mean 66%). These are consistent with reports in the literature of instrumented posterior fusions using one or two cages. Molinari et al 16 reported an 88% rate of anterior fusion and 72% of posterior fusion in patients who had undergone a one or two cage posterior lumbar interbody fusion in combination with pedicled screws.
Discussion
Our results show that an instrumented posterior lumbar interbody fusion procedure performed with either one or two interbody cages has a low rate of complications and a high rate of fusion. The clinical outcomes were good in most cases, regardless of whether one or two cages had been used.
Discussion
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

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