A single straight expandable cage via a hybrid posterior-transforaminal approach with rhBMP-2 or allograft provides high fusion rates with low risk of subsidence.

Background: Due to the ongoing debate surrounding the clinical impact of surgical technique; cage type (expandable vs. static), cage shape (straight vs. banana), or technique [posterior lumbar interbody fusion (PLIF) vs. transforaminal lumbar interbody fusion (TLIF)], the aim of this study was to evaluate the mid-term clinical and radiographic outcomes of patients who underwent a hybrid posterior-TLIF (P-TLIF) with a single straight expandable titanium cage using recombinant human bone morphogenetic protein-2 (rhBMP-2) or demineralised bone allograft (DBA) bone substitute.

Methods: A retrospective analysis of data from consecutive patients who underwent a hybrid P-TLIF by a senior spine surgeon between August 2017 and May 2022. A single straight expandable interbody cage was inserted obliquely after laminectomy and bilateral facetectomies. Cages were packed with either rhBMP-2 or DBA. Consecutive patients received rhBMP-2 prior to withdrawal (Australia, March 2020), and then DBA was used. Patient-reported outcome measures (PROMs) included visual analogue scale (VAS) back and leg pain, Oswestry disability index (ODI) and 12-Item Short Form Survey (SF-12) measured at preoperative, postoperative 6-week, 6-month, 12-month, and 24-month. Computed tomography (CT) imaging, assessed by an independent radiologist, was conducted postoperative day-2 for instrumentation positioning then at either 6-, 12-, or 24-month to assess subsidence and interbody/posterolateral fusion (Bridwell classification). If fusion was achieved no further CTs were undertaken.

Results: This cohort consisted of 81 (54.3% female) patients with a mean age of 57.3±12.5 years. rhBMP-2 was used in 60 (74.1%) and DBA in 21 (25.9%) patients. Total clinical complication rate was 27.2% including five patients requiring reoperation. Asymptomatic radiologic subsidence rate was 7.4% and clinical subsidence rate was 1.2%. Total (interbody and posterolateral) fusion was achieved at 6-month in 34.4% and 55.7%, 12-month in 76.8% and 88.4%, and 24-month in 86.3% and 93.2% of patients. There was a non-significant difference in fusion rates at each timepoint between rhBMP-2 and DBA. Preoperative pain, disability, and function all significantly improved postoperatively. Mean VAS back/leg (7.8±0.8, 7.7±0.9), and ODI (35.8±6.6) significantly (P<0.001) decreased (2.7±1.8, 1.9±2.3, 13.6±5.8); SF-12 physical/mental (27.4±3.8)/(38.1±8.3) showed significant improvements (P<0.001) at 12-month follow-up (47.1±8.8, 52.1±8.7). The mean follow-up time was 20.3±6.1 [12-24] months.

Conclusions: A hybrid P-TLIF with a single straight titanium expandable cage permitted safe cage insertion, guided repositioning, and controlled expansion. Patients demonstrated significant improvements in pain, disability and function with low subsidence and high CT fusion rates over 24-month follow-up. The use of DBA in this cohort showed no significant difference in fusion rates across 24-month when compared to rhBMP-2.