INTRODUCTION

The deep suboccipital muscles including the rectus capitis posterior major (RCPma), rectus capitis posterior minor (RCPmi), obliquus capitis superior (OCS),and obliqus capitis inferior (OCI), located between the inferior nuchal line of the occipital bone and the dorsal aspect of the axis, are indispensable in maintaining head posture and in proprioception. ^{Hack et al. (1995}) first proposed the concept of the myodural bridge (MDB) as a dense fibrous connection between the RCPmi,posterior atlanto-occipital membrane (PAOM) and dura mater (DM) Hack et al., 1995. Subsequent researches have shown that the RCPma (^{Scali et al., 2013}), OCI (^{Pontell et al., 2013}) and ligamentum nuchae (LN) participate in forming the MDB. Researchers believe that these deep suboccipital muscles via the MDB perform important physiological roles such as preventing DM in folding, keeping the cerebellomedullary cistern patent and maintaining the normal flow of cerebrospinal fluid (CSF),transmitting DM tension and adjusting head posture (Hack et al., 1995). Recently, ^{Sui et al., proposed that the MDB may be a dynamic source of CSF circulation (Sui et al., 2013}; ^{Zheng et al., 2014}). Some clinical studies suggested that there is a relationship between the deep suboccipital muscles and headache (^{McPartland et al., 1997}; ^{Fernández-de-Las-Peñas et al., 2007}, 2008). To understand the morphology and human distribution characteristics of RCPmiRCPma and OCI, imaging anatomical data is significantly meaningful in functional studies of the MDB,especially in identifying pathologic changes of the deep suboccipital muscles.

Based on the MRI imaging on healthy adults, Xu et al. (2013) provided an imaging reference values of the length and sectional area of the RCPmi in male and female subjects. However, morphological data on the RCPma and OCI in healthy adults have not been reported to date. In this study, T2-weighted MRI images was used to measure the RCPma and OCI in healthy adults. This study provides morphometric data on RCPma and OCI in healthy adults, and lays a foundation for functional and clinical research on the suboccipital muscles.

MATERIAL AND METHOD

Approval for this study was obtained from the Ethics Committee for Research of Basic Medical College of Dalian Medical University. One hundred and nine (109) healthy adults including 47 males (age range from 19 to 70 with an average of 40.0±14.1 years) and 62 females (age range from 20 to 67 years with an average of 38.1±13.9 years) participated in this study.

Scanning equipment and parameters. A 0.35T MRI scanner (BTI-035, Shengzhen Basda Medical Apparatus Co., Ltd.) was used with a neck coil phased array. The scanning sequence T2WI/FSE (TR/TE 3,200 ms/ 115 ms) was employed with a thickness of 5 mm, interval of 1 mm, 256 X 279 matrix, 272 X 272 mm view field, and two activations.

Image acquisition. Bilateral 0-degree (sagittal) and 30degree oblique sagittal scanning sectional areas of RCPma and OCIwas imaged by T2-weighted scanning as stipulated by Xu et al. The bilateral 0-degree (sagittal) scanning sectional area of RCPma and OCI has five millimeter bias from the median sagittal plane,and the 30-degree oblique sagittal scanning sectional area of RCPma and OCI is in the plane with the maximum area of RCPma.

Measurement of RCPma and OCI sectional area. The boundary of selected sagittal and oblique sagittal scanning images of RCPma and OCI was identified via MR Workstation PACS system, and the sectional area of the muscles was measured automatically.

Analysis of data. The range, 95 % confidence interval, mean,and standard deviation of the morphological parameters of the RCPma and OCI were calculated. Comparisons between the sexes were tested using OneWay ANOVA analysis and the bilateral symmetry of muscles were tested using a paired sample T-test. A p value of less than 0.05 indicated a statistically significant difference.

Fig 1 The MR scanning reference lines and bilateralT2WI image with 5 mm bias from the median sagittal plane. A. The MR scanning reference lines B. Left T2WI image with 5 mm bias from the median sagittal plane C. Right T2WI image with 5 mm bias from the median sagittal plane. ∆ Cross section of RCPma. Cross section of OCI. 

Fig 2 The MR scanning reference lines and T2WI image including suboccipital muscles with 30-degree bias from the oblique sagittal plane A. The MR scanning reference lines B. T2WI image with 30-degree bias from the oblique sagittal plane including suboccipital muscle. The sectional area of RCPma is maximum. ∆ Cross section of RCPma. Cross section of OCI. 

Fig 3 The T2WI image of RCPma and OCI with 30-degree bias from the oblique sagittal plane (The sctional area of RCPma is maximum). 1. RCPmi, 2.RCPma, 3OCI; The range of area measurement representing the muscle cross-section was outlined by the red line. 

RESULTS

The MR images of 109 healthy adults were collected via 0-degree (sagittal) and 30-degree oblique sagittal MRI scanning angle of the nape. In the section with 5 mm bias from the median sagittal plane, the RCPma is a narrow triangle or prolate spindle shaped,the parameters of sectional area are 93.14-353.10 mm2 on the left and 92.01391.73 mm2 on the right with a significant difference (P<0.05). (Table I). The OCI is found to be spindle shaped, and the sectional area parameters are 110.63-403.67 mm2 on the left and 111.77-390.85 mm2 on the right with no significant difference (P>0.05) (Table II). In the section with 30-degree bias from the oblique sagittal plane, the RCPma has the shape of an elongated triangle,the sectional area parameters are 183.30± 42.24 mm2 in males and 133.05± 26.44 mm2 in females with a significant sex difference (P<0.0) (Table III). The OCI is oval shaped,the sectional area parameters are 254.81± 46.20 mm2 in males and 167.42± 27.85 mm2 in females with a significant sex difference (P<0.05) (Table IV).The sectional areas of RCPma and OCI present no significant difference between the group greater than 45 years old and less than or equal to 45 years old in males or females. (Tables V and VI).

Table I The sectional area of the RCPma with 0-degree bias from oblique sagittal plane (mm²). 

Table II The sectional area of the OCI with 0-degree bias from oblique sagittal plane. (mm²). 

Table III The sectional area of the RCPma with 30-degree bias from oblique sagittal plane. (mm²). 

Table IV The sectional area of the OCI with 30-degree bias from oblique sagittal plane. (mm²). 

Table V The comparsion of sectional area of the RCPma with 30-degree bias from oblique sagittal plane in different age groups. mm². 

Table VI The comparsion of sectional area of the OCI with 30-degree bias from oblique sagittal plane in different age groups. mm². 

DISCUSSION

This study collected the T2-weighted MR images of the RCPma and OCI of 109 healthy adults via 0-degree (sagittal) and 30-degree oblique sagittal continuous MRI scanning. The MR image features and imaging anatomical data of RCPma and OCI was acquired. Significant sex difference was found in the sectional areas of RCPma and OCI respectively, those of males were significantly higher than females. Xu et al. also found significant sex differences in the MRI parameters of RCPmi. In this study, we also found that the RCPma was bilateral asymmetric, the sectional area on the right is larger than that of the left, but the OCI is symmetric. The discrepancy in the left and right RCPma may be influenced by head posture and handedness. The sectional areas of RCPma and OCI present no significant difference between the group greater than 45 years old and less than or equal to 45 years old. Studies by Xu et al. found that the parameters of the RCPmi was not age related which implies that the deep suboccipital muscles do not significantly atrophy due to normal aging.

Since the concept of the MDB was conceived ^{Hack et al., (1995}), the functions of the deep suboccipital muscles have been extended. The MDB is proposed to prevent DM folding, maintaing patency of the subarachnoid space and the normal flow of cerebrospinal fluid(CSF), transmitting tension of DM and adjusting the head position (Hack et al., 1995; ^{Scali et al., 2015}). Recently, Sui et al. proposed that the MDB may be a dynamic source of CSF circulation in the spinal subarachnoid space (Sui et al.; Zheng et al.). The RCPmi is connected to the cervical spinal dura mater via the MDB through the PAOM (Hack et al., 1995; ^{Rutten et al., 1997}; ^{Alix et al., 1999}; ^{Nash et al., 2005}; ^{Zumpano et al., 2006}; Scali et al., 2015; ^{Yuan et al., 2016}) and the RCPma and OCI is connected to the cervical spinal dura mater via the MDB through the posterior atlanto-axial membrane (PAAM) (Hack et al., 1995; Rutten et al.; Alix et al.; Nash et al.; Zumpano et al.; Scali et al., 2005; Yuan et al.). The connections between these structures are speculative anatomic bases for the above functions and may be of clinical significance. Literatures put forward that chronic headache is associated with pathological changes of the MDB and RCPmi (McPartland et al.; Alix et al.; Fernández-de-LasPeñas et al., 2007, 2008), when lesion occurs in the RCPmi, and the DM which is pain sensitive is pulled, headache may occur (Hack et al., 2004) This study presented the MRI image features, imaging anatomical data and sexual dimorphism of RCPma and OCI. Moreover, this imaging anatomical data will be useful for functional and clinical studies on RCPma, OCI, patients with muscular atrophy and neck pain,and the MDB.

CONCLUSION

The study presented the MR image features and imaging anatomical data of the RCPma and OCI in healthy adults.Significant sex difference exist in the RCPma and OCI muscles respectively.However, there was no age-related significant difference. This study also showed that the RCPma is bilaterally asymmetric.