This study examines how well computed tomography (CT) imaging performed during spine surgery - known as intraoperative CT - can detect instability in the lower (lumbar) part of the spine. Spinal instability means that the bones of the spine move more than they normally should, which can contribute to pain and may affect the outcome of surgery.

Before surgery, doctors usually check for this instability using X-rays taken while the patient bends forward and backward. These X-rays do not always show instability clearly. During surgery, the patient lies face down and the muscles are fully relaxed, which can change how the spine appears on imaging.

The purpose of this study is to learn whether CT images taken during surgery can identify signs of lumbar spinal instability as reliably as standard imaging performed before surgery, and whether they can reveal signs of instability that were not visible beforehand. To do this, the researchers will review imaging and clinical information from adults who are undergoing surgery for degenerative (wear-related) conditions of the lower spine.

This study does not change a patient's surgery or treatment. It only analyzes imaging and clinical information that are already part of standard care. The findings may help guide imaging and surgical decisions in lumbar spine surgery in the future.

This single-center observational cohort study evaluates the diagnostic performance of intraoperative computed tomography (O-arm) in characterizing radiologic features of lumbar segmental instability in patients treated surgically for degenerative spinal disease. Both retrospective and prospectively enrolled participants are included; in the latter, acquisition is embedded in the routine operative workflow, with no procedure added beyond established care.

Image assessment is carried out by independent, blinded reviewers applying a predefined, standardized evaluation framework. Eligibility, endpoints, and the analytic approach are specified in the corresponding sections of this record, while operational parameter definitions are retained in the study protocol.

Inclusion Criteria:

• Adults aged 18 years or older
• Degenerative lumbar spine pathology requiring surgical treatment (e.g., degenerative spondylolisthesis, lumbar spinal stenosis, isthmic spondylolisthesis)
• Planned or performed lumbar decompression with instrumented fusion
• Available intraoperative computed tomography (CT) with comparative preoperative imaging (retrospective cohort), or intraoperative CT performed during the index surgery (prospective cohort)
• No prior surgery at the spinal level under evaluation; previous surgery at other levels permitted
• Stable general medical condition allowing planned clinical evaluation
• Written informed consent prior to participation (prospective cohort only)

Exclusion Criteria:

• Non-degenerative spinal pathology, including acute traumatic spinal injury, primary or metastatic spinal tumor, spinal infection (e.g., osteomyelitis, discitis), or inflammatory spinal disease (e.g., ankylosing spondylitis)
• Inadequate or incomplete imaging data, including poor-quality intraoperative CT or absence of comparative preoperative imaging
• Emergency or non-elective surgery
• Revision surgery involving the same spinal level(s)
• Inability to provide informed consent (prospective cohort only)
• Neuromuscular or congenital spinal deformity significantly altering spinal biomechanics (e.g., scoliosis >20°, congenital segmentation anomalies)

Adults treated surgically for degenerative lumbar spine disease with instrumented fusion at a single spine surgery center. The population comprises two cohorts: a retrospective cohort of previously operated patients for whom intraoperative computed tomography (CT) and corresponding preoperative imaging are available in medical records, and a prospectively, consecutively enrolled cohort in whom intraoperative CT is acquired as part of the standard operative workflow. Both adult sexes are included; no upper age limit applies.