Impaired bone mineral density as a predictor of graft subsidence following minimally invasive transpsoas lateral lumbar interbody fusion

Purpose The LLIF procedure is a useful stand-alone and adjunct surgical approach for many spinal conditions. One complication of LLIF is subsidence of the interbody graft into the vertebral bodies, resulting in severe pain, impaired arthrodesis and potentially fracture of the body. Low bone density, as measured by T score on DEXA scanning, has also been postulated to increase the risk of subsidence. Methods A retrospective review of prospectively collected data was performed on all patients who underwent LLIF at this institution consisting of 712 levels in 335 patients. Patients with subsidence following LLIF were recorded. We utilized the T score obtained from the femoral neck DEXA scans, which is used to determine overall fracture risk. The T score of patients with subsidence was compared to those without subsidence. Results 20 of 57 (35 %) patients without subsidence had a DEXA T score between -1.0 and -2.4 consistent osteopenia, one patient (1.8 %) exhibited a T score less than -2.5, consistent with osteoporosis. 13 patients of 23 (57 %) with subsidence exhibited a T score between -1.0 and -2.4, consistent with osteopenia, five (22 %) exhibited a T score of -2.5 or less, consistent with osteoporosis. The mean DEXA T score in patients with subsidence was -1.65 (SD 1.04) compared to -0.45 (SD 0.97) in patients without subsidence (p<0.01). The area under the receiver operating characteristic curve for patients with a T score of -1.0 or less was 80.1 %. Conclusions Patients with DEXA T scores less than -1.0 who undergo stand-alone LLIF are at a much higher risk of developing graft subsidence. Further, they are at an increased risk of requiring additional surgery. In patients with poor bone quality, consideration could be made to supplement the LLIF cage with posterior instrumentation.