Neck Disability Index, short form-36 physical component summary, and pain scales for neck and arm pain: the minimum clinically important difference and substantial clinical benefit after cervical spine fusion

Background context: The Neck Disability Index (NDI), the short form-36 (SF-36) physical component summary (PCS), and pain scales for arm and neck pain are increasingly used to evaluate treatment effectiveness after cervical spine surgery. The minimum clinically important difference (MCID) is a threshold of improvement that is clinically relevant to the patient. However, the true goal is to provide the patient with a substantial clinical benefit (SCB).

Purpose: This study determines the MCID and SCB using common anchor-based methods for NDI, PCS, and pain scales for arm and neck pain in patients undergoing cervical spine fusion for degenerative disorders.

Study design/setting: The study setting is a longitudinal cohort in a multisurgeon spine specialty clinic.

Patient sample: The sample comprises 505 patients who underwent a cervical fusion for degenerative spine conditions and who have prospectively collected outcome scores with a minimum 1-year follow-up.

Outcome measures: The outcome measures of the study were NDI, SF-36, and numeric rating scales for arm and neck pain.

Methods: The MCID and SCB values for NDI, PCS, and pain scales for arm and neck pain were determined using receiver operating characteristic (ROC) curve analysis with the Health Transition Item of the SF-36 as an anchor. The Health Transition Item asks a patient "Compared to one year ago, how would you rate your health in general now?" with answers ranging from "Much Better," "Somewhat Better," "About the Same," "Somewhat Worse," to "Much Worse." An ROC curve was constructed for each measure. The ROC curve-derived MCID was the change score with equal sensitivity and specificity to distinguish the "Somewhat Better" from the "About the Same" patients. The ROC curve-derived SCB was the change score with equal sensitivity and specificity to distinguish the "Much Better" from the "Somewhat Better" patients. Distribution-based methods including the standard error of the mean and the minimum detectable change were also used to calculate MCID.

Results: The calculated MCID is 7.5 for the NDI, 4.1 for SF-36 PCS, and 2.5 for arm and neck pain. The calculated SCB is 9.5 for the NDI, 6.5 for SF-36 PCS, and 3.5 for arm and neck pain.

Conclusions: Patients with an eight-point decrease in NDI, a 4.1-point increase in PCS, and a three-point decrease in arm or neck pain can detect a minimally clinically important change. Patients with a 10-point decrease in NDI, a 6.5-point increase in PCS, and a four-point decrease in arm or neck pain can detect an SCB after cervical spine fusion.