Influence of coronal-morphology of endplate and intervertebral space to cage subsidence and fusion following oblique lumbar interbody fusion

Background: The morphology of endplate was considered to be one of the influencing factors of Cage subsidence after lumbar interbody fusion (LIF). Previous radiographic evaluations on endplate mostly used sagittal X-ray or MRI. Whereas, there are few studies on the CT evaluation of endplate and intervertebral space (IVS), especially the evaluation of coronal morphology and its influence on subsidence and fusion after LIF. We aimed to measure and classify the shapes of the endplate and IVS using coronal CT imaging and evaluate the radiographic and clinical outcomes of different shapes of the endplate/IVS following oblique lateral lumbar interbody fusion (OLIF). Methods: ： A total of 137 patients who underwent L4-5 OLIF combined with anterolateral fixation from June 2018 to June 2020 were included. The endplate concavity depth (ECD) was measured on the preoperative coronal CT image. According to ECD, the endplate was classified as flat ( 4 mm). The L4-5 IVS was further classified according to endplate type. The disc height (DH), DH changes，subsidence rate, fusion rate, and Oswestry Disability Index (ODI) in different endplate/IVS shapes were evaluated during 1-year follow up. Results: : The ECD of L4 inferior endplate (IEP) was significantly deeper than that of L5 superior endplate (SEP) (4.20±1.07 vs 1.63±0.83, P < 0.001). Four types of L4-5 IVS were identified: shallow-shallow (16, 11.68%), shallow-flat (45, 32.85%), deep-shallow (32, 23.36%), and deep-flat (44, 32.12%). A total of 45 (32.85%) cases of cage subsidence were observed. Only one (6.25%) subsidence occurred in the shallow-shallow group，which was significantly lower than in the other three groups (19 shallow-flat, 6 deep-shallow, and 19 deep-flat ) (P < 0.05). Meanwhile，the shallow-shallow group had the highest fusion rate (15, 93.75%) and the highest rate of reach minimal clinically important difference (MCID) ODI among the four types. The shape of L4 IEP mainly affects the final fusion rate, which is higher in the shallow shape than deep ( OR=2.845, p=0.034). Conversely, L5 SEP mainly affects the rate of cage subsidence, which was flat shape higher than shallow (OR=4.364, p=0.001). Conclusion: The L4-5 IVS is an irregular shape that tends to be fornix-above and flat-down on coronal CT view. The shallow-shallow IVS has the lowest subsidence rate and best fusion result, which is possibly because it has a relatively good degree in matching either the upper or lower interface of the cage and endplates. These findings provide a basis for the further improvements in the design of the OLIF cage.