Which NDI domains best predict change in physical function in patients undergoing cervical spine surgery?

Physical function is a critical aspect of patient outcomes. NDI is a widely validated outcome measure in cervical spine disease, yet to what extent its individual domains predict changes in physical function remains unknown.

To examine the impact of individual NDI domains on changes in physical function in patients undergoing cervical spine surgery.

Prospective Cohort Study.

Adult patients undergoing cervical spine surgery, excluding those undergoing surgery for instability due to trauma.

Absolute change in outcome measures (Patient Reported Outcomes Measurement Information System [PROMIS] Physical Function [PF], Short Form 36 [SF-36] Physical Component Score [PCS], and Neck Disability Index [NDI]) from pre- to postoperatively, correlation of NDI individual domains with PROMIS PF and SF-36 PCS (preoperatively, postoperatively, and change from pre- to postoperatively).

Patients undergoing cervical spine surgery between 2016 and 2018 were prospectively enrolled. Patients completed questionnaires (NDI, SF-36 PCS, and PROMIS PF) preoperatively and at 6 months postoperatively. Patient demographics, including age, body mass index, Charlson Comorbidity Index, and underlying diagnoses were collected. Comparisons between NDI scores preoperatively versus postoperatively were conducted using Wilcoxon signed rank sum test. Correlations of NDI individual items and PROMIS/SF-36 were assessed using Pearson correlation. A stepwise linear regression analysis was performed to identify NDI items that are independently predictive of PROMIS PF and SF-36 PCS.

A total of 137 patients were included in the study, with mean age 56.9 years (range 24.4-84.9). Each of the NDI domains as well as PROMIS PF and SF-36 PCS demonstrated significant improvement following cervical spine surgery (p<.001). Changes in all NDI domains demonstrated significant negative correlation with changes in PROMIS PF, with recreation (R=-0.537, p<.001), work (R=-0.514, p<.001), and pain intensity (R=-0.488, p<.001) having the greatest negative correlation. Changes in all NDI domains demonstrated significant negative correlation with changes in SF-36 PCS, with recreation (R=-0.451, p<.001), work (R=-0.443, p<.001), lifting (R=-0.373, p<.001), and driving (R=-0.373, p<.001) having the greatest negative correlation. For PROMIS PF, the NDI domains that were independently associated with changes in PF were work (R=-0.092, p=.001), pain intensity (R=-0.089, p=.003), and recreation (R=-0.067, p=.004). For SF-36 PCS, the NDI items that were independently associated with changes in PCS were work (R=-0.269, p=.003) and recreation (R=-0.215, p=.002).

All NDI domains improve significantly after cervical spine surgery and demonstrate significant correlation with changes in PROMIS PF and SF-36 PCS. The work, recreation, and pain intensity domains were the only independent predictors of physical function changes postoperatively. Considering physical function, our findings highlight the importance of presenting changes in individual NDI domains in addition to the total score.