Inter-spinous process distance: a novel parameter predicting segmental lordosis during posterior cervical spine deformity surgery

Abstract

Purpose To investigate the normative value of distance between the spinous processes (inter-spinous process distance, ISPD), correlations between patients' demographics and ISPD, and determine cutoff ISPD values that result in segmental lordosis in posterior cervical-thoracic arthrodesis with instrumentation. Methods We reviewed patients with visible lower cervical and cervicothoracic spinous processes (C5-T2) on dynamic X-rays. Radiographic measurements included segmental Cobb angles (S-Cobb) and ISPD, defined as the distance between midpoints of the spinous processes. Correlations between patient demographics and ISPD were evaluated. In addition, the cutoff ISPD values that predict segmental lordosis (S-Cobb < 0°) were determined using the receiver operating characteristics (ROC) curve and linear regression model. Results We evaluated a total of 153 films on 51 patients. The mean ISPDs in neutral position were 16.0, 20.7, 23.2, and 26.0 mm at C5/6, C6/7, C7/T1, and T1/T2, respectively There were no or only weak correlations between ISPD and age, race, height, body weight, and BMI. We observed significant difference between male and female ISPDs at C7/T1 and T1/T2 (P < 0.05). The ISPDs had moderate/strong correlations with S-Cobb at each segment (− 0.450 ≤ r ≤ − 0.705). Based on the ROC curves, the following cutoff values of ISPD provided 100% positive predictive value for segmental lordosis: < 10 mm at C5-6; < 15 mm at each level from C6 to T2. Conclusion ISPD is a simple and useful parameter that correlates with the segmental Cobb angle. During posterior cervical deformity surgery, surgeons can intraoperatively predict adequate segmental lordosis by utilizing the vertebral level specific cutoff values of ISPD.