Preoperative Low Psoas Muscle Index and Moderate to High Screw Angulation at the Upper Instrumented Vertebra Predicts Proximal Junctional Kyphosis and Failure After Adult Spinal Deformity Surgery

Introduction: Proximal junctional kyphosis and failure (PJK; PJF) can be catastrophic complications associated with adult spine deformity (ASD) surgery. Sarcopenia can influence these complications, leading to mechanical failure. We aim to assess sarcopenia with the recently introduced Magnetic Resonance Imaging (MRI)- based Psoas Muscle Index (PMI) in patients undergoing spinal deformity surgery.

Methods: Retrospective analysis of patients aged 50 years and older who underwent ASD surgery involving >5 thoracolumbar levels, with a minimum follow-up of two years. Demographic information, spinopelvic parameters, and procedure-related variables were collected for each patient. The PMI for each patient was calculated using preoperative T1-weighted MRI scans, which involved determining the average of the maximum right and left anteroposterior and transverse diameters of the psoas muscle at the L3 vertebra and dividing it by the patient's body mass index. Univariate analysis was performed to identify potential risk factors associated with PJK/PJF.

Results: 116 patients were included in the analysis, with a mean age of 63.0±7 years. Among them, 38 patients (32%) developed PJK/PJF. Significant differences between the groups included the mean psoas muscle index, 0.41±0.16 vs 0.3±0.16 (p < 0.001), for patients without PJK/PJF versus those with PJK/PJF, respectively. Additionally, patients with PJK/PJF exhibited a higher UIV screw/superior endplate angle of 9.6°±7.4° compared to 4.8°±5.6° in the non-PJK/PJF group (p < 0.003). Furthermore, patients with PJK had a lower UIV screw-rod angulation, with 83.3°±11.5° versus 90.3°±7.0° in the non-PJK/PJF group.

Conclusion : Patients who developed PJK/PJF exhibited a significantly lower PMI, indicative of sarcopenia. Additionally, patients with PJK/PJF showed higher UIV screw/superior endplate angle and lower screw-rod angulations. These findings suggest sarcopenia may predict PJK/PJF. Moreover, maintaining an appropriate contour of the proximal rod, coupled with minimal angulation of the UIV screws and harmonious alignment between the rod and the screws, are essential factors in preventing PJK/PJF.