**3. Results**

#### *3.1. Demographic and Clinical Data*

The demographic and clinical characteristics of the patients are shown according to DISH status in Table 1. There was no significant difference in age, BMI, DM status, or cervical JOA score between the group with DISH (*n* = 107) and the group without DISH (*n* = 132). Table 1 shows the prevalence of pain and the JOA-CMEQ, JOA-BPEQ, and VAS scores for each domain. There was no significant between-group difference in these patient-reported outcomes except for lumbar spine function; however, there was a significant difference in the OP-index value.

**Table 1.** Demographic and clinical data for patients with OPLL according to presence or absence of DISH.


Data are expressed as the mean ± standard deviation or as the percentage. BPEQ, Back Pain Evaluation Questionnaire; CMEQ, Cervical Myelopathy Evaluation Questionnaire; DISH, diffuse idiopathic skeletal hyperostosis; JOA, Japanese Orthopaedic Association; OP-index, ossification of the posterior longitudinal ligament index; OPLL, ossification of the posterior longitudinal ligament; VAS, visual analog scale.; \* Significant at *p* < 0.05; \*\*\* significant at *p* < 0.001.

#### *3.2. Demographic and Clinical Characteristics by DISH Grade*

Patient demographics are shown according to DISH grade in Table 2 and Figure 2. There was a significant between-group difference in age (Figure 2a) but not in the sex distribution. No significant between-group difference was found in BMI (Figure 2b), DM status, or cervical JOA score among the three grades (Figure 2c). There was a significant correlation between the OP-index and DISH grade (Table 2).


**Table 2.** Demographics of patients with cervical OPLL according to DISH grade.

Data are expressed as the mean ± standard deviation or as the percentage, DISH, diffuse idiopathic skeletal hyperostosis; JOA, Japanese Orthopaedic Association; OP-index, ossification of the posterior longitudinal ligament index; OPLL, ossification of the posterior longitudinal ligament; \*\*\* significant at *p* < 0.001.

**Figure 2.** Relationship between basic demographic and clinical findings and DISH grade. (**a**) Patient age. (**b**) Body mass index. (**c**) JOA score. DISH, diffuse idiopathic skeletal hyperostosis; JOA, Japanese Orthopaedic Association.

#### *3.3. Severity of DISH Was Not Associated with Myelopathic Symptoms or Lumbar Spine Function in Patients with Cervical OPLL*

The score for each item in the JOA-CMEQ and JOA-BPEQ was evaluated to assess whether the severity of DISH in terms of cervical myelopathy and lumbar spine function affects the ability to perform activities of daily living. There were no significant correlations among the four groups for JOA-CMEQ scores (Figure 3a–e). Similarly, there were no significant differences among the three DISH grades in terms of lumbar spine function, social dysfunction, mentality, locomotive function, and body pain (Figure 4a–e).

**Figure 3.** Relationship between JOA-CMEQ scores and DISH grade. (**a**) Cervical function. (**b**) Upper extremity function. (**c**) Lower extremity function. (**d**) Bladder function. (**e**) Quality of life. DISH, diffuse idiopathic skeletal hyperostosis; JOA-CMEQ, Japanese Orthopaedic Association Cervical Myelopathy Evaluation Questionnaire.

**Figure 4.** Relationship between JOA-BPEQ and DISH grade. (**a**) Lumbar function. (**b**) Social dysfunction. (**c**) Mentality. (**d**) Locomotive function. (**e**) Body pain. DISH, diffuse idiopathic skeletal hyperostosis; JOA-BPEQ, Japanese Orthopaedic Association Back Pain Evaluation Questionnaire.

#### *3.4. Degree of DISH Correlated Negatively with Prevalence of Neck Pain but Not Back Pain or LBP in Patients with Cervical OPLL*

The prevalence of neck pain was significantly correlated with degree of DISH, but back pain and LBP were not (Table 3). Furthermore, although there was no statistically significant difference in LBP among the three grades of DISH, LBP tended to decrease with increasing grade.


**Table 3.** Prevalence of symptoms in patients with cervical OPLL according to DISH grade.

Data are expressed as the mean ± standard deviation or as the percentage, DISH, diffuse idiopathic skeletal hyperostosis; OPLL, ossification of the posterior longitudinal ligament; \* Significant at *p* < 0.05.

VAS scores from the JOA-CMEQ and JOA-BPEQ were investigated to clarify the relationship between degree of DISH and pain associated with cervical myelopathy. However, no significant difference was found in the VAS scores among the three DISH grades (Figures 5 and 6).

**Figure 5.** Relationship between the VAS scores included in the JOA-CMEQ and DISH grade. VAS scores for (**a**) neck pain, (**b**) upper extremity numbness, (**c**) chest constriction, and (**d**) numbness below the chest. DISH, diffuse idiopathic skeletal hyperostosis; JOA-CMEQ, Japanese Orthopaedic Association Cervical Myelopathy Evaluation Questionnaire; VAS, visual analog scale.

**Figure 6.** Relationship between the VAS scores included in the JOA-BPEQ and DISH grade. VAS scores for (**a**) low back pain, (**b**) lower extremity numbness, and (**c**) lower extremity pain. DISH, diffuse idiopathic skeletal hyperostosis; JOA-BPEQ, Japanese Orthopaedic Association Back Pain Evaluation Questionnaire; VAS, visual analog scale.
