Lumbar spondylolisthesis is a common spinal disorder in adults affecting approximately 5% of the general population [12345]. Slipping of the cranial vertebra over the caudal vertebra leads to changes in the neural foramen morphology with subsequent entrapment of the nerve root in the flattened and narrowed neural foramen. Clinical presentation is variable, ranging from mild to severe symptoms of low back pain with or without radiculopathy [2356] most commonly at L5/S1 and L4/5. Segmental spinal fusion is widely accepted as the method of choice for treatment of symptomatic cases not responding to conservative measures (activity modification, physical therapy and medication) [124578910].

All surgical techniques aim for decompression and stabilization of the involved vertebrae. A single-level posterior instrumented interbody fusion is commonly performed [67]. Posterolateral fusion (PLF), posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) are widely accepted fusion techniques [11]. Theoretically, reduction of the slip in spondylolisthesis may restore the original neuroforaminal morphology, which indirectly decompresses the nerve root. However, there is no consensus in the literature concerning whether a true beneficial effect of reduction on outcome can be expected.

There are arguments for and against reduction. Reduction of slip is a more extensive and expensive surgery with a higher risk of neurologic complications due to increased tension on the nerve roots during the reduction maneuver, especially in high grade listhesis [121314]. Equally high rates of fusion and clinical improvement were described in studies with and without reduction [11315].

This study focuses on clinical outcome and evaluates a correlation between the radiographic extent of slip reduction and clinical improvement in patients with a single-level low-grade spondylolisthesis.

This prospective non randomized study involved 46 patients (30 females and 16 males; mean age; 37.5 years; range; 17-48 years) with a diagnosis of grade 1 or 2 isthmic type lumbar spondylolisthesis who failed to respond to conservative therapy. Admitted to our hospital between 2009 and 2014. Surgeries were carried out by a single team consisting of the senior authors (U.K.D. and J.R.M) as lead. Inclusion criteria were a low-grade lumbar isthmic (Meyerding grade I, II), instrumented single-level interbody fusion and radiographic and clinical examination at baseline and 1-year after surgery.

Patients who reported any prior spinal surgery for spondylolisthesis or patients with an inadequate disk space for performing TLIF were excluded. This study was approved by Institutional Ethical Committee, and all patients proved informed written consent.

On admission, a questionnaire containing visual analogue score (VAS) and Oswestry disability index (ODI) queries described below was completed by the patient under supervision of a resident.

In the operation room, all patients were positioned prone. After a midline incision and complete bony exposure, transverse processes were exposed. Decompressive procedure was done thorough laminectomy, medial facetectomy and extensive foraminotomy. Then after pedicle screw fixation all patients underwent interbody fusion using interbody cages (TLIF; Capstone/Crescent, Medtronics Sofomor Danek, Fukushima, Japan) introduced through a standard transforaminal route and identical pedicle screw and rod instrumentation (M8; Medtronics). Autologous bone graft from the spinous process was used for fusion.

In the 46 patients with low grade spondylolisthesis had isthmic instability, the most commonly affected segment at L5/S1 (84%) followed by L4/5 (16%). All patients underwent surgery an average 12 months (range, 6–21 months) following onset of symptoms. Almost all (95%) of the patients had predominant leg pain and radiculopathy.

Multiple options for surgical treatment of adult isthmic low grade spondylolisthesis (grade 1 and 2) are available to surgeons. Some authors recommend circumferential fusion alone with decompression and others suggest circumferential fusion with reduction of listhesis [123567111415222324]. The literature is divided regarding the surgical management with good to excellent clinical outcomes.

The surgical strategy for low grade listhesis was followed in all patients; i.e., reduction of the vertebral slip with decompression of the spinal canal and nerve roots followed by interbody fusion in the form of TLIF and postero-lateral fusion with pedicle instrumentation. Interbody fusion maintains the foraminal height and maintains structural integrity. Posterior reduction prior to the TLIF helps to restore the sagittal alignment, decompress the spinal canal effectively and maintain slip reduction [56710111724].

Reduction of the listhesis is commonly advocated but the relationship between reduction itself andimprovement of clinical outcome has only recently attracted clinical interest [2720]. Comparison of PLIF with reduction versus in-situ fusion in low grade spondylolisthesis showed no influence of reduction on clinical outcome [2]. Another prospective study compared patients with and without reduction of high and low grade spondylolisthesis with only postero-lateral fusion; no difference in the outcome was apparent [7]. A study of low grade isthmic spondylolisthesis could not establish any correlation of clinical outcome score improvement with the extent of radiographic reduction of spondylolisthesis in patients who underwent postero-lateral fusion [20]. In the study, there was no improvement in the neural foraminal dimensions although a wide neural decompression was achieved. Spondylolisthesis reduction was achieved through pedicle screws without extensive release of the disc and soft issues. In our series, in addition to a wide neural decompression, a discectomy and curettage of the end plate was performed for the TLIF cage insertion which maintained the reduction and improved the neural foraminal dimensions significantly.

Average anterior disc height, posterior disc height, C distance and F diameter improved significantly as compared to a similar study [20]. The maintenance of structural integrity of the interbody cages prompted the hypothesis that the clinical outcome may be related with the radiological outcome. This hypothesis failed after the results suggested that there was only moderate correlation between the VAS score and spondylolsithesis reduction distance (r=0.46) and C diameter (vertical foraminal dimension) (r=0.44). All other parameters had no correlation with VAS or ODI scores. All patients with reduction and solid fusion had good clinical outcome. This could mean that there may be moderate improvement in pain in patients who undergoes reduction of slip. Overall, the reduction of slip has no influence on the clinical outcomes scores of ODI.

There are several strengths of the study: only low grade spondylolisthesis were presented in this series and all patients had undergone the same surgery for a single level spondylolisthesis. This is the only study to date reporting a clinical and radiological correlation for Interbody fusion in low grade isthmic spondylolisthesis. The selected sample was homogeneous since all the patients were isthmic type of spondylolisthesis. The same surgeons performed the surgery. Finally, the radiographic measurements were made at only two intervals by two authors to minimize the measuring errors. Limitations of the study included being a mid-term analysis and lack of subjective outcome measures.

The results suggest that neural decompression and TLIF with postero-lateral instrumented fusion significantly improves pain and disability. Improvement in pain status after 1 year displayed a moderate correlation after radiographic reduction of slip and maintenance of the reduction with an interbody fusion. The overall clinical outcome was not related to the obtained radiographic reduction of the slipped vertebra.