The Minimum Clinically Important Difference of the modified Japanese Orthopaedic Association Scale in Patients with Degenerative Cervical Myelopathy

Abstract

Introduction The modified Japanese Orthopaedic Association (mJOA) score is the most frequently used clinician-administered tool to assess functional status in patients with degenerative cervical myelopathy (DCM). By defining the minimum clinically important difference (MCID) for this scale, clinicians can evaluate treatment outcomes for this condition and better interpret evidence from clinical studies. This study aims to establish the MCID of the mJOA in patients with CSM. Material and Methods Three different methods were used to determine the MCID of the mJOA: 1) distribution-based, 2) anchor-based and receiver operating characteristic (ROC) analysis and 3) professional opinion. The first two methods were accomplished using data from 517 patients enrolled in the AOSpine CSM-North America or CSM-International studies. Distribution-based methods were used to estimate the MCID by computing the half standard deviation and standard error of measurement. Using anchor-based methods, mJOA at 12-months after surgery was compared between patients who “slightly improved” on the Neck Disability Index (NDI) and those who were “unchanged.” ROC analysis was then performed to compute a discrete integer value for the MCID that yielded the smallest difference between sensitivity and specificity. Finally, MCID estimates were obtained by surveying members of AOSpine International. We repeated the anchor-based methods for patients with mild (mJOA: 15–17), moderate (mJOA: 12–14) and severe disease (mJOA < 12). Results Our cohort consisted of 315 men and 202 women, with ages ranging from 21 to 86 years (mean age: 56.37 ± 11.60). The mean baseline mJOA score was 12.48 ± 2.71. One hundred and twenty-nine patients were classified as mild (mJOA = 15–17) preoperatively, 208 as moderate (mJOA = 12–14) and 180 as severe (mJOA < 12). Based on the NDI at 12-months following surgery, 76 (14.70%) patients worsened (NDI < −7.5), 130 (25.15%) were unchanged (−7.5≤NDI < 7.5), 87 (16.83%) slightly improved (7.5≤NDI < 15) and 224 (43.33) showed marked improvements (15≤NDI). The half standard deviation of the baseline mJOA was 1.36 and the standard error of measurement was 1.21. The difference in mJOA between patients who “slightly improved” on the NDI and those who were “unchanged” was 1.11. ROC analysis yielded a value of 2 for the MCID (Fig. 1). The survey of 416 spine professionals confirmed these estimates: The mean response was 1.65 ± 0.66, although the most commonly selected answer was 2 (39.42%). The MCID significantly varied depending on myelopathy severity: ROC analysis yielded a threshold of 1 for mild patients, 2 for moderate patients and 3 for severe patient. Conclusion The MCID of the mJOA is estimated to be between 1 and 2 points and varies significantly with myelopathy severity. This knowledge will enable clinicians to identify meaningful functional improvements in surgically treated CSM patients.