Resident Mayo Clinic Mayo Clinic Rochester, MN, US
Introduction: Transforaminal lumbar interbody fusion (TLIF) offers indirect decompression and segmental correction through restoration of disc height. Stresses exerted on the vertebral endplates by the interbody device can result in implant subsidence and loss of correction. This time-to-event analysis aims to identify predictors of interbody subsidence.
Methods: Patients who underwent 1- or 2-level TLIF were identified. Data on demographics, surgical details, preoperative bone quality using CT-based Hounsfield units (HU), and pre- and postoperative lumbopelvic parameters were collected. Univariable analyses were conducted to identify predictors of subsidence (≥2mm intrusion of interbody into cranial or caudal vertebrae) and significant subsidence (≥4mm). Multivariable Cox regression was performed to identify independent predictors of subsidence, expressed as hazard ratios with 95% confidence intervals.
Results: 198 patients (age 65.5±9.6yr; 56.6% female) treated at 241 levels were included. Ninety-two levels (38.2%) experienced subsidence and 25 (10.4%) experienced significant subsidence. Subsided levels (≥2mm) were associated with lower HU in cranial and caudal vertebrae, had interbodies positioned farther from the anterior apophyseal ring, were implanted with taller, more lordotic interbodies, and underwent greater disc height restoration. Levels showing significant subsidence were similarly associated with lower HU in the cranial and caudal vertebrae, had interbodies positioned farther from the anterior apophyseal ring of the caudal vertebrae, and underwent more greater disc height restoration. Multivariable Cox regression showed time-to-subsidence was predicted by greater implant height (HR 1.20/mm; [1.05, 1.38]; p=0.009), greater postoperative disc height (HR 1.21/mm; [1.09, 1.34]; p< 0.001), and greater disc height restoration (HR 1.11/mm; [1.04, 1.19]; p=0.002). The time-to-subsidence analysis for significant (≥4mm) subsidence showed it was predicted by lower HU in the cranial vertebrae (HR 0.98/unit; [0.97, 0.99]; p=0.001), increasing number of levels instrumented (HR 1.26; [1.04, 1.52]; p=0.016), and greater disc height restoration (HR 1.33/mm; [1.18, 1.51]; p< 0.001).
Conclusion : This time-to-event analysis suggests interbody subsidence following TLIF is best predicted by implantation of a taller interbody and aggressive disc height restoration. Significant subsidence is similarly predicted by aggressive disc height restoration along with poor baseline bone quality. The results suggest the need to balance aggressive correction at the time of surgery against the increased risk of subsequent interbody subsidence.