A modified measurement method for functional spinal unit ROM of cervical spine

The interspinous process motion (ISM) method can provide a more accurate assessment of postoperative subaxial cervical fusion than Cobb angle method which is used more commonly in clinical practice. However, the ISM method presents the measurement results in millimeters which cannot be directly compared with the Cobb angle measurement data. We proposed a modified measurement method for cervical functional spinal unit range of motion  (FSU ROM) and evaluate its repeatability and reliability in measuring the ROM of the surgical segment after cervical artificial disc replacement surgery. A total of 81 patients who underwent cervical artificial disc replacement surgery in our department were retrospectively reviewed. Postoperative flexion-extension dynamic cervical radiographs were used for the measurement of FSU ROM of the surgical segment. The modified measurement method (M1) and the traditional Cobb angle measurement method (M2) were used. In the comparative analysis, there was no statistically significant difference between the two measurement methods (P > 0.05). Linear correlation study showed significant correlation between the two methods (P < 0.01), with a correlation coefficient of 0.947. Bland-Altman analysis showed the average difference between the two methods was 0.13°, the most of the measured data points fell within the 95% confidence interval. The M1 method demonstrated high intra-observer reliability (ICC = 0.957) and inter-observer reliability (ICC = 0.947). The M2 method showed good intra-observer reliability (ICC = 0.889) and good inter-observer reliability (ICC = 0.874). The M1 method yielded an SEM of 0.64° and an MDC of 1.49° for FSU ROM measurement, while the M2 method resulted in an SEM of 1.14° with an MDC of 2.66°.The modified measurement method for assessing cervical FSU ROM demonstrates satisfactory repeatability and reliability, provide higher measurement precision.