Introduction

Several studies have identified potential risk factors for proximal junctional kyphosis (PJK) and proximal junctional failure (PJF) following thoracolumbar posterior instrumented fusion including low bone mineral density (BMD), persistent sagittal plane deformity, and severe frailty. However, no study has investigated the interplay between these variables or the impact of paraspinal sarcopenia on the development of PJK and PJF. The purpose of the present study was to determine demographic and radiographic variables that predict an increased risk of PJK or PJF.

Methods

We retrospectively reviewed a cohort of patients greater than 50 years of age who underwent posterior instrumented fusion with pelvic fixation and a construct that terminated proximally between T10 to L2 between the years 2013-2020. Patient demographic information was collected and the Modified Frailty Index (mFI) and Charlson Comorbidity Index (CCI) scores were calculated for each patient. Patients were subdivided into three groups: (1) no PJK or PJF, (2) PJK without PJF, and (3) PJF. Preoperative and 1-year postoperative sagittal alignment parameters were compared between subgroups. Bone mineral density (BMD), based upon both DEXA scan measurements and Hounsfield Units (HU) at the upper instrumented vertebra (UIV) and UIV+1, was compared between subgroups. Paraspinal sarcopenia was assessed using both qualitative (cross-sectional area) and quantitative (Goutalier grade) methodologies and compared between subgroups. We utilized student’s T-test and ANOVA to compare means within and between groups, respectively. Multivariable analyses were performed to determine risk factors for PJK and PJF. P values <0.05 were considered significant.

Results

We identified 150 patients for inclusion in this study with a mean age of 67.0 years and an average follow-up of 32 months. There was no difference in baseline demographic variables or radiographic variables between subgroups, except that patients with PJF had a higher preoperative and postoperative pelvic tilt. The subgroup of patients with no PJK/PJF demonstrated a significantly higher HU at the UIV (148.3±34.5) than patients who developed PJK (117.8 ±41.9) or PJF (118.8 ±41.8; P<0.001). There was a much higher rate of severe multifidus fatty infiltration observed in patients who developed PJF (78.9%) or PJK (76.0%) than in patients who did not develop PJK/PJF (34.0%; P<0.001). Furthermore, no patient that developed PJK or PJF had normal multifidus quality. Multivariate analysis comparing patients without PJK/PJF to those who developed PJK demonstrated that both mean UIV HU (0.80, 95% CI 0.69-0.93; P<0.001) and moderate-severe multifidus sarcopenia (5.40, 95% CI 1.8-16.1; P<0.001) were independent predictors of increased risk of PJK. Similarly, mean UIV HU (0.79, 95% CI 0.66-0.94; P=0.01) and moderate-severe multifidus sarcopenia (7.69, 95% CI 1.96-30.18; P<0.001) were identified as independent risk factors predicting an increased risk of developing PJF.

Discussion

Patients with lower mean HU at the UIV and a higher degree of paraspinal multifidus fatty infiltration are at increased risk of PJK and PJF following thoracolumbar fusions that terminate proximally between T10 and L2.