Significance of the Posterior Fusion Mass Bone Density for the Onset of Mechanical Complications After a Thoracolumbar Three-Column Osteotomy in Adult Spinal Deformity Patients.

STUDY DESIGN: Retrospective comparative study.

OBJECTIVE: To assess the relationship of fusion mass bone density on CT and the development of rod fractures(RFs) and proximal junctional kyphosis(PJK).

SUMMARY OF BACKGROUND DATA: Few studies have evaluated the relationship of fusion mass bone density to mechanical complications.

METHODS: A retrospective review of adult spinal deformity patients who underwent thoracolumbar 3-column osteotomy from 2007 to 2017 was performed. All patients underwent routine 1-year CT imaging and had at least 24 months follow-up. Posterior fusion mass bone density was evaluated by measuring Hounsfield Unit(HU) on CT in 3 different regions (UIV, LIV, and osteotomy site), and were compared between patients with and without mechanical complications.

RESULTS: A total of 165 patients (63.2 years, 33.5% male) were included. Overall PJK rate was 18.8%, and 35.5% of these underwent PJK revision. There was significantly lower density of posterior fusion mass at the UIV in patients who experienced PJK compared to patients without PJK (431.5HU vs. 537.4HU, P=0.026). Overall RF rate was 34.5% and 61.4% of these underwent revision for RFs. Among 57 patients with RFs, 71.9% had pseudarthrosis. Fusion mass density did not differ between patients with or without RFs. However, in RF patients with pseudarthrosis, there was significantly higher bone mass density near the osteotomy compared to those without pseudarthrosis (515.7HU vs. 354.2HU, P=0.012). There were no differences in radiographic sagittal measures between the patients with and without RF or PJK.

CONCLUSIONS: Patients with PJK tend to have less dense posterior fusion mass at the UIV. Fusion mass density does not correlate with RF, but greater bone density near the osteotomy was correlated with accompanying pseudarthrosis in patients with RFs. Assessing density of posterior fusion mass on CT may be helpful in assessing risk for PJK and provide insight as to the causes of RFs.