Purpose
This study aimed to evaluate the predictive performance of preoperative computed tomography–based Hounsfield unit (HU) and magnetic resonance imaging-based vertebral bone quality score (VBQS) for cage subsidence after 1- to 2-level oblique lumbar interbody fusion (OLIF), and whether it differs by intraoperative cage placement position.
Methods
Ninety-one OLIF levels in 54 patients (2015–2022) were retrospectively reviewed in this single-center cohort. Subsidence was defined as ≥2 mm middle disc-height reduction or ≥2 mm cage protrusion at 1 year. Multivariable logistic regression with cluster-robust standard errors estimated adjusted odds ratios (OR); the pre-specified primary test was the lower-instrumented-vertebra HU×cage-position interaction.
Results
Subsidence occurred in 24 of 91 levels (26.4%). Each one-standard-deviation decrease in lower-instrumented-vertebra HU was independently associated with subsidence (adjusted OR, 0.40; 95% confidence interval, 0.20 to 0.81; p=0.011); VBQS was not. The subsidence group included more osteoporosis-range levels (<110 HU; 62.5% vs. 25.4%, p=0.003). The interaction was not significant (p=0.553), but the HU effect concentrated in middle-placed cages (adjusted OR, 0.345; p=0.012) and attenuated in anterior-placed cages (OR, 0.50; p=0.185).
Conclusion
Lower-instrumented-vertebra HU is an independent predictor of cage subsidence after 1- to 2-level OLIF, most evident in middle-placed cages.

Background
Minimally invasive transpsoas or antepsoas lateral lumbar interbody fusion (MI-LLIF) has been reported as an effective surgical option for various lumbar diseases. Many researchers reported high fusion rate and clinical excellence of LLIF with the use of bone morphogenic protein (BMP). However, there have been paucity of studies regarding LLIF without the use of BMP. Therefore, this study aimed to analyze radiologic and clinical results of patients who underwent minimally invasive lateral lumbar interbody fusion without the use of BMP. Furthermore, a further analysis was conducted regarding the frequency of cage subsidence and its impact on the radiologic and clinical outcome.
Materials and Methods
Fifty patients and 109 levels treated by MI-LLIF with postoperative follow-up of at least 2 years were included. Radiologic evaluation included intervertebral disc height, segmental lordosis, lumbar lordosis, fusion rate, cage subsidence grade, and the bone mineral density. Radiologic fusion was determined by modified Bridwell’s grade, and cage subsidence by Marchi’s grade. Clinical outcome was evaluated by VAS of low back pain (LBP) and leg pain, and ODI score. The above clinical and radiologic variables were analyzed statistically for comparison of cage subsidence and nonsubsidence groups.
Results
There were 20 male and 30 female patients with the average age of 69 years. Average follow-up period was 29.6 months(24-42 months). Graft material used for PEEK cage was autogenous bone only in 9 levels, autogenous bone and DBM in 19 levels, and DBM only in 81 levels. Twelve patients received surgery on 1 level, 20 patients on 2 levels, 17 patients on 3 levels, and 1 patient on 4 levels, respectively. Operated levels were L1-2 in 7 cases, L2-3 in 27, L3-4 in 41, and L4-5 in 34, respectively. Mean low back pain (LBP) VAS decreased from preoperative 5.5 to 2.2 at the final follow-up, leg pain from 6.1 to 1.7, and ODI score from 25.6 to 13.7, with statistical significance (p<0.001). Mean disc height increased from preoperative 5.9 mm to postoperative 11.5 mm, and subsequently decreased to 9.6mm at the final follow-up. Average lumbar lordosis increased from preoperative 18.6 degrees to postoperative 37.0 degrees, and 35.9 degree at final follow-up. Radiologic union rate was 90.8%. Cage subsidence was observed in 6 levels (5.5%) on immediate postoperative radiographs; and in 20 levels (18.3%) at final follow-up. Comparison between cage subsidence and non-subsidence groups revealed no significant difference in age and BMD. Immediate postoperative segmental lordosis was considered as a risk factor of cage subsidence (p=0.005, odds ratio 0.813, CI 0.703~0.940). Furthermore, the preoperative and the final follow-up measurement of disc height, VAS score of LBP and leg pain, and ODI score were not different between the two groups. However, pseudoarthrosis rate was higher in subsidence group.
Conclusions
Minimally invasive LLIF was an effective surgical option with high fusion rate even without the use of BMP. Although cage subsidence also increases the frequency of pseudarthrosis, it does not significantly deteriorate the lumbar lordosis correction and clinical outcome.

A 77-year-old female suffering from severe degenerative scoliosis, spinal stenosis and lumbar disc herniation underwent Direct lateral lumbar interbody fusion (DLIF) at L2-4. On the 3rd postoperative day, she complained of severe back pain without any trauma history. Simple radiograph revealed L3 vertebral fracture and cage subsidence. Pain was subsided after conservative treatment including TLSO and medication. Radiographic union was achieved at fractured vertebra after 3 months. Solid fusion was observed at operated level after 6 months. Patient has visited our clinic without any pain. DLIF is one of novel minimally invasive spine procedures available today. It is designed to maximize benefits and minimize risks of other traditional techniques such as anterior approach and posterior approach. However, there can be some risk of cage subsidence and vertebral fracture after DLIF. Therefore, care should be taken to avoid cage subsidence during the operation.