Degenerative spondylolisthesis is a common spinal degenerative disease. It is defined as the slippage of one vertebrae on the vertebrae bellow. In the process of spinal ageing and spinal joint degeneration, the spine becomes subjected to degenerative development that results in joint instability, shifting of vertebrae and can be responsible for a progressive kyphosis of the lumbar spine and sagittal imbalance with forward inclination of the trunk and chronic low back pain development. To address these changes and restore stability, lumbar spinal fusion has been developed and is nowadays a common procedure for unstable degenerative spine disorders.

In the past several years, studies that highlight the importance of sagittal balance analysis with the restoration of adequate lumbar lordosis, have emerged. However, it remains a challenge to determine the correct amount of lumbar lordosis that is required for each patient to maintain optimal post-fusion sagittal balance. Additionally, the relationship between pelvic incidence (PI) and impact of LL correction has been highlighted in literature. The position of fused vertebrae is of paramount importance, as sagittal alignment should be done with minimizing muscle work during posture. Failure to reach proper sagittal balance can result in compensatory mechanisms such as increased pelvic tilt (PT), cervical and thoracic segment hyperextension, and knee flexion. These compensatory mechanisms have adverse effects such as chronic pain, disability and muscle fatigue.

With this study the investigators aim to analyze long-term clinical and spinopelvic radiographic parameter outcomes of patients who underwent a one-level spinal fusion procedure for single level degenerative spondylolisthesis disease at a single institution.

TLIF (transforaminal interbody fusion) is a safe and effective treatment option for single-level lumbar degenerative disease. It is less invasive, yields good outcomes and has fewer complications after long-term follow up compared to other fusion techniques. In literature, many studies have compared sagittal parameters and functional outcomes in single-level TLIF surgery. In most literature reports, however, the fusion was performed at different lumbar levels, depending on the characteristics of the studied cohort. As demonstrated by Roussouly et. al. in the asymptomatic population, the L4 vertebra represents the apex of lumbar lordosis and is the most suitable site for lordosis correction. There are several reported surgical techniques and methods to obtain adequate lordosis correction during TLIF.

At Department of Orthopaedic surgery of Ljubljana University medical centre, TLIF is performed using a consistent brand of instrumentation. In knee-chest position, polyaxial pedicle screws (Xia®, Stryker Spine, Allendale, NJ, USA), combined with a crescent-shaped interbody cage (T-Plus®, Pioneer Surgical Technology, Marquette, MI, USA), bilateral facet joint removal and laminectomy on both sides are used. Compression over the rods is performed to obtain adequate segmental lordosis. The angle of lordosis correction is calculated according Schwab's formula (ideal lumbar lordosis (LL) = pelvic incidence (PI) + 9).

There is little published information regarding cohorts with long-term evaluation. As many authors suggested, changes in lordotic parameters can become clinically evident several years after the procedure. With this study's cohort analysis, a rarely reported link between spinopelvic parameters and long-term clinical evaluation will be demonstrated. Selected participants will be analyzed using the same protocol as preoperatively. A clinical examination will be made, ODI questionnaires will be administered and radiographic parameters on full-standing lateral radiographs, will be analyzed. Changes between preoperative and long-term postoperative parameters will be calculated and compared. For statistical analysis paired t-test and Pearson correlation will be used. All statistical tests will be two tailed and significance will be set at p<0.05.

Inclusion Criteria:

• Patients with one-level degenerative spondylolysthesis treated with single-level TLIF at Department of Orthopaedic surgery of Ljubljana University medical centre between 2011 and 2013.
• TLIF performed at L3/4 or L4/5

Exclusion Criteria:

• Patients who refuse to participate in study or do not respond to our invitation
• Patients with additional instrumental spinal surgery or spinal trauma
• Patients with additional types of adult spinal deformity
• Patients with inflammatory spinal conditions
• Patients with flexion contractures of the hips or knees

Participants will be selected from patients who underwent a single-level TLIF procedure at Department of Orthopaedic surgery of Ljubljana University medical centre between 2011 and 2013.

All IPD that underlie results in a publication and are in accordance with ethical standards of the National ethics committee of Slovenia on human experimentation and with the Helsinki Declaration of 1975.

Starting within 1 year after publication.

IPD and any additional supporting information will be shared by request with the Principal investigator.