This study found that individuals with chronic idiopathic neck pain had greater MFI in their deep extensor muscle (multifidus) as compared to age and sex-matched controls. This difference was apparent, even when accounting for differences in age and BMI, factors believed to affect MFI.[20, 29, 30] MFI values differed depending on the spinal level measured, with more caudad spinal levels generally displaying greater MFI than more cephalad levels between C3 and TI. MFI differed between muscles, with the multifidus having the highest MFI. The multifidus had a larger muscle volume with greater MFI in individuals with chronic idiopathic neck pain compared to controls, with no between-group difference in relative volume. Thus, MFI may be one factor that may identify individuals with chronic idiopathic neck pain. The development of clinical tools to identify MFI in individuals with chronic pain may lead to personalised interventions and the ability to direct treatment resources effectively.
Evidence for the clinical correlates of MFI has largely been derived from studies of individuals with low back pain[29–32] or types of neck pain other than idiopathic neck pain (e.g., whiplash associated disorder, cervical myelopathy).[14, 33–37] These studies have shown that MFI appears to be associated with higher levels of disability in patients with WAD[36, 38] or cervical myelopathy. Greater MFI is associated with postural instability and poor balance in patients with radicular spondylopathy. Functional recovery after surgical decompression is worse with higher MFI. Relationships between clinical findings and MFI are reported more frequently for the multifidus compared to other muscles in both the cervical and lumbar spines. These findings suggest that MFI in the multifidus may be radiologic sign, potentially identifying patients with a less favourable prognosis.
The one identified previous study of MFI that included individuals with idiopathic neck pain found that MFI at C2/3 and C5/6 was similar to healthy controls and less than that observed in those with whiplash-associated disorder. That study was limited to females and had a smaller sample of participants with idiopathic neck pain than the current study. It also had a smaller number of participants with idiopathic neck pain than their comparison groups, possibly affecting ability to detect a significant difference. The participants in the current study had a longer duration of neck pain, on average, than the previous study (68 vs 34 months), possibly accounting for differences in findings. The lack of studies investigating muscle composition in individuals with idiopathic neck pain suggests more research is warranted.
The current study found that MFI in the multifidus of individuals with idiopathic neck pain was greater than in asymptomatic matched controls, accounting for both age and BMI. Older age is associated with increased MFI in the lumbar spine,[29, 30] and this was consistent in the current study. Higher BMI has also been associated with greater MFI in the lumbar and cervical spines previously, and in the current study. After adjusting for age and BMI, the greater MFI in the multifidus remained in those with chronic neck pain. While not significant, the unadjusted values for the semispinalis, splenius capitis and longus colli also showed greater MFI in individuals with neck pain compared to controls (Supplementary Table 3 in Additional File 1). This may suggest that age and BMI could account for the between-group differences in MFI in those muscles. Alternatively, it may mean the differences between people with and without neck pain were not large enough, or the lack of homogeneity proved a challenge to detect significance. Nonetheless, the greater MFI in the multifidus regardless of age and BMI highlights the complex uniqueness of the multifidus muscle. Indeed, there is evidence that the deep cervical muscles function differently to the superficial muscles during a motor skill task.
As the current study was cross-sectional, it cannot determine if MFI is a cause or an effect of pain. There is some evidence that MFI increases in healthy individuals after 4 weeks of immobilisation, and 12 weeks of strength training can decrease MFI in the thigh muscles of older individuals. These findings suggest that future research should investigate interventions that might have the potential to reduce MFI in the cervical multifidus to determine any effect on neck pain. Importantly, muscle volume and MFI differed between muscles, and for each muscle values varied depending on spinal level measured. Thus, studies of muscle size and composition should include as many muscles and spinal levels as is feasible, and studies of single muscles or spinal levels should not be generalized to the health of the entire cervical spine.
The strengths of this study include the measurement of muscle volumes and MFI from multiple cervical muscles across multiple spinal levels, allowing quantification of the majority of existing muscle covering the cervical spine. This allowed comparisons across muscles and spinal levels. It is, to our knowledge, only the second study to examine these variables in individuals with idiopathic neck pain, and the first to include all spinal levels from C3-T1. Results are limited to this sample of individuals with chronic idiopathic neck pain. Participants in this study reported an average duration of neck pain of 63 months, with half of the sample reporting they had experienced neck pain for greater than five years. It is unknown if changes in MFI might be recognised earlier in individuals who go on to develop persistent neck pain symptoms, potentially enabling targeted interventions.
Future research should develop methods to enable muscle volume and MFI to be quantified in the clinical setting, potentially through automated methods that eliminate the time needed to manually contour muscle boundaries. As MFI varies based on age and BMI, a large normative database is needed to effectively identify deviations from normal. Finally, investigations of interventions that may reduce MFI, such as resistance training or specific muscle retraining, need to be conducted to determine whether MFI and neck pain can both be reduced with intervention.