Development of a unified and comprehensive definition of successful spinal fusion: a systematic review

OBJECTIVE: Spinal fusion is a commonly performed surgical procedure used to relieve pain, deformity, and instability of various spinal pathologies. Although there have been attempts to standardize spinal fusion assessment radiologically, there is currently no unified definition that also considers clinical symptomology. This review attempts to create a more holistic and standardized definition of spinal fusion. METHODS: A systematic review of the current literature on cervical, thoracic, and lumbar spinal fusion was conducted using the PubMed, Google Scholar, and EBSCO databases adhering to PRISMA guidelines. Data were collected and analyzed from more than 20 publications that contained pertinent information on the efficacy of different imaging modalities, classification systems, clinical presentations, and the normal course of healing in relation to spinal fusion. The mean methodological index for nonrandomized studies score was 18 ± 2.5. Furthermore, industry experts and board-certified spinal surgeons were consulted in the development of a proposed definition of successful spinal fusion. RESULTS: A total of 20 studies evaluating 1324 spinal fusion procedures were included in the final analysis. Based on the available literature, a clinical algorithm that physicians can implement in their practice to determine whether a spinal fusion procedure may be deemed successful was created. The algorithm begins broadly by stratifying patients as either symptomatic or asymptomatic. Asymptomatic patients can be considered as having successful fusions after 12 months. If patients are symptomatic, the imaging modality and healing characteristics are based on the quality of pain experienced. For radicular pain, fusion evaluation includes a flexion/extension (F/E) radiograph to assess for foraminal compression, trabecular bridging, minimal angular rotation, minimal translational movement, and minimal halo sign. For axial pain, a helical CT scan is recommended, with characteristics of success that include trabecular bridging, lack of radiolucent shadowing, lack of visible bone or hardware fracture, lack of cystic or sclerotic changes, low subsidence level near the graft, and minimal screw-rod construct migration. Spinal fusion is considered "unsuccessful" if symptoms persist beyond a year postoperatively, regardless of radiographic findings. CONCLUSIONS: The authors have constructed a systematic, standardized method for evaluating spinal fusion success that incorporates clinical symptoms, various imaging modalities, and the natural course of bone healing. A potential limitation of this algorithm is its reliance on radiographic imaging and heterogeneous data. However, the authors believe that implementation of this algorithm and a widespread unified fusion definition will lead to better postoperative evaluation, better surgical outcomes, and a standardized metric to assess developments in spinal fusion procedures and technology.