In this study, we determined the reference values for estimating the occipital-cervical distance in neutral, flexion, and extension positions by OC4D, a new radiographic parameter measurement method. The measurement method should provide a comprehensive and accurate estimation for vertical reduction of the occiput-cervical region while performing OCF. OC4D, a simple, convenient, and highly reliable measurement method for occiput-cervical distance, is not occluded by implants. More importantly, it is not affected by the change in head and neck position.
Conceptually, the occipitocervical neutral position is the functional and balanced position of the head on the cervical spine. We considered that patients should have a normal occipitocervical angle and occiput-cervical distance in this position. Sherekar et al .measured the occipito-C2 angle in 518 asymptomatic volunteers (261 male and 257 female subjects), and they obtained values of 14.66 ± 9.5° in males and 15.59 ± 8.26° in females. Many researchers reported that non-normal occipitocervical angles led to poor postoperative fusion, even severe dysphagia and/or dyspnea during OCF [3, 7–9]. However, it is still unknown whether dysphagia and/or dyspnea mostly due to mechanical airway obstruction caused by a non-normal occipitocervical angle. We believe that surgeons should pay attention to the lower cranial nerve stretch airway obstruction caused by over-distraction of the occiput-cervical vertical distance. Shigeto E et al. reported that the mechanism of dysphagia is not simply associated with the O-C2 angle, but it also involves the global craniocervical alignment in an individual patient, including the occiput-cervical distance .Wang Q et al. reported that performing OCF in the over-distraction position to treat vertical atlantoaxial dislocation may caudally displace the brainstem relative to the cranial base, resulting in traction injury to the 9th, 10th, and 11th lower cranial nerves .
In 1999, Phillips et al. first measured the occiput-cervical distance (OCD) by measuring the shortest distance from the most superior aspect of the C2 spinous process to the occipital protuberance in 30 asymptomatic subjects. The value of OCD in the neutral position was 21.5±1.22 mm, and it was significantly different from the values measured in flexion (28.0±1.32 mm) and extension (14.8±1.48 mm) . Seong et al. measured OCD in 200 normal, sagittal balanced patients (100 male and 100 female patients), and the mean neutral OCD was 22.98 ± 5.10 mm (range, 9.88–38.64 mm). Both these values were significantly different from those in flexion and extension positions .Unfortunately, the correlation between OCD and height, weight and BMI had not been reported in previous literature.Inour study, we measured the occiput-cervical distance by OC4D, and the mean neutral OC4D was 69.0±6.9 mm. However, this value was not significantly different from those measured in flexion (68.9±6.8 mm) and extension (68.1±6.9 mm). Seong et al. found that the posterior border of C4 as a landmark is the apex of cervical lordosis, and it is therefore the least affected by the cervical curve .We hypothesized that the C4 vertebral body, being the central point of the cervical sequence, is the least affected by motion of the cervical position. Hence, the shortest distance from the center of the C4 vertebral body to the McGregor’s line in different cervical positions can be regarded as the radius of a circle, and the circle is positioned at the center of the C4 vertebral body and tangent to the McGregor’s line (Fig. 2). Meanwhile, we found a positive correlation between OC4D and height as well as weight in this study, and we observed that it had a stronger correlation between OC4D and height compared to weight. But the correlation between OC4D and BMI was weak, and it was no statistically significant.
O-C4D, compared with OCD in previously reported literature [4, 5, 12], has its own unique advantages. First of all, the OC4D is a more accurate parameter. In OCD, significant inter-individual morphologic variation in the C2 spinous process and gender differences have been detected. Jiang wei T et al. found that variations in the C2 spinous process may affect the OCD value and there was a significant different between male and female subjects .The inter- and intra-observer reliabilities of OCD had ICC values of 0.651 and 0.754 in Seong's study .In this paper, we found that the posterior margin of the hard palate, occipital bone, and C4 vertebra, with less bone variation, were clear on lateral radiographs. The ICC values of inter- and intra-observer agreements for OC4D weremore than 0.93in neutral, flexion, and extension positions. Second, the OC4D, as an ingenious measurement method, is less affected by motion of the head and neck in neutral, flexion, or extension position. Obviously, the alignment of the subaxial spine can influence the occipitocervical alignment required to ensure a functional position of the occiput. This variable was not specifically measured in the current study. So far, only a few articles on the OCD measurements have been reported, and both of these articles showed that the mean neutral OCD value was significantly different from those in flexion and extension positions (Fig. 3) [4, 5, 9].However, there was no significant difference in the OC4D measurement method among neutral, flexion, and extension positions. It has clinical significance for guiding reduction during the operation but the occiput-cervical region is not in a neutral position. Third, the O-C4D is not occluded by implants and it may be a valuable intraoperative tool for designing of fusion implants and testing of the restoration condition in the operating room. Although it is already known, there are no reports showing that the C2 spinous process can be occluded by fixed implants during OCF and the implants could affect the measurement by the OCD method (Fig. 4). Previous literature has stated that it may be difficult to visualize the tip of the dens on radiographs, or the dens may be absent or fixed in an abnormal position in many conditions for which OCF is performed [13, 14]. Therefore, it may be difficult and inaccurate to evaluate vertical reduction of the occipitocervical region by the distance from the odontoid tip to the McGregor’s line during surgery. Wang Q et al. first described lower cranial nerve palsy following vertical over-distraction after OCF in 4 patients who had atlantoaxial dislocation with or without basilar invagination, and the symptoms of all of the patients were alleviated to different extents by releasing the screw cap and recovery to partial reduction of the occipitoatlantal anatomy . However, the OC4D method avoids the occlusion caused by implants and uncertainty of bony landmarks on radiographs, and it could be a useful tool to estimate and test the restoration condition of the occipitoatlantal anatomy by regulating the fixed implants.
The limitations of this study are as follows: the demographic data were not matched for age and the sample size was relatively small. In spite of these limitations, our study could present a new method for measurement of occipital-cervical distance, and it has practically valuable to guide and test the restoration condition of occipital-cervical region. Another limitation of this study is that only the cervical plain radiographs were analyzed, there was no data about the overall sagittal alignment of the spine in the study sample. Although previous literature has reported that cervical curvature could be affected by overall spinal sagittal imbalance [15–18], normal subjects with a normal cervical curvature were included in our study, and we found no difference between OC4D and the change of cervical curvature in neutral, flexion, and extension. However, we also recognized that cervical curvature changes can accelerate cervical degeneration, which maybe affect the results of OC4D measurements. Thus, prospective studies are needed to obtain more reliable measurements about cervical or overall spine sagittal alignment parameters, and further explore the effect of spinal sagittal parameters on OC4D.