Previous studies indicated that pathophysiological changes, such as disc degeneration and disc protrusion, were surprisingly common findings in young adults with neck pain [16, 17]. The course and outcome of CS are highly varied, especially in young patients . A minority of them have to accept surgical treatments due to severe pain and neurologic symptoms, while others do not. Several studies investigated that the risk factors about anatomical variables of the cervical vertebrae, such as anteroposterior vertebral body diameter, mid-sagittal vertebral canal diameter, and canal-body ratio [17, 18]. However, these anatomical variations just statically reveal their effects.
CS gradually occurs over time accompanied by dynamic activities that are complex anatomically and biomechanically. Cheng Huang reported that in a population of 10,930 dentists and 73,718 controls, younger dentists had a higher risk of developing cervical herniated intervertebral dis . Sustained contraction of the cervical muscles to keep forward-head postures that involve holding the neck and head in an unbalanced and unnatural forward position to gain better visibility is universally unavoidable in daily dental operations . The painless, insidious nature of repetitive minor trauma caused by such a prolonged static posture (PSP) may lead to the degeneration of spinal discs [17, 19, 20]. These suggested that there existed a significant role of the head’s movement in a pathophysiologic change of the cervical spine. This effect is produced through the craniovertebral junction (CVJ), including atlanto-occipital joint and its surrounding ligaments and muscles. DNFs are the only cervical muscles that lie closely in front of the cervical spine and has attachments confined to the vertebrae . DNFs, function as cervical segmental flexors that provide physical support to the cervical column and dynamically stabilize the neck, are considered necessary for the cervical spine [9–11, 20]. A compromised function is a feature of neck pain disorders, including whiplash-induced, idiopathic, work-related neck pain [10–13]. Retraining DCFs has been shown to decrease neck pain and improve the ability to maintain an upright posture of the cervical spine [12–14].
The force initiated from the head is transferred to the lower segments by atlanto-occipital joint. Therefore, stiff atlanto-occipital joint, represented by low FOC, makes increased mechanical stress downward. This may aggravate degeneration or herniation of the spinal discs [17, 21]. The presented case (Fig. 6), which should have been generally supposed to be remote from visible degenerative changes in cervical segments, has a limited FOC and this helps further support our present assumption. Previous studies reported that a restricted ROM at the C2-C7 angle in flexion after cervical laminoplasty induced the compensatory increase in the ROM in O-C2 flexion [21, 22]. Therefore, we believe that the middle-lower cervical segments and the upper one must interact with each other, as there exists a natural biomechanical balance.
Our results showed that the gap for the percentage of low FOC in 3 different age groups between cases and controls tended to be narrowed as the age increased (Fig. 4). Biological elements such as degenerated discs and ligaments gradually appear, reveal themselves, and ultimately developed into CS . However, these are unusually seen for the young. Hence, the larger gap in -30 years old group may help indicate the negative role of low FOC in the development of CS.
There existed a significant negative correlation between FOC and NDI (r = -0.451; p = 0.016) (Fig. 5). Mousavi reported that a 10-min static flexion could lead to changes in the neck proprioception and feed-forward control on account of mechanical and neuromuscular changes in the viscoelastic cervical spine structures and these changes in sensory-motor control could be a risk factor for neck pain and injury . Besides, PSPs are much more taxing than moving forces . Suffering from PSPs, the subsequent consequences of neck muscular tissues include muscle fatigue, protective muscle contraction, muscle imbalance, and finally spinal disc degeneration or herniation . In the light of the transferring role of CVJ, the stiff atlanto-occipital joint could make the middle-lower cervical segments experience more PSPs. In this way, therefore, FOC levels were negatively correlated with NDI.
This is the first study to demonstrate that the lower flexion function of atlanto-occipital joint could be a risk for accelerated degeneration of CS. The implication of this study lies in that individuals with low FOC can be screened by a cheap, easily accessible X-ray. These people should receive more education and care about musculoskeletal health related to the prevention of CS as early as possible. Currently, a need for flexion and even hyperflexion of the cervical segments have to increase, as many people use technological devices for recreation or/and work, especially smartphones for hours during the day. In 2014, the proportion of adolescents who used the smartphone was more than 60% in Shanghai, China, and this figure still keeps rising . Therefore, considering the adverse health effects of the increasing prevalence of smartphones on young adults, the present risk analysis of FOC about such an exposure, could be a promising new parameter in predicting CS, which is of great significance to spine surgeons.
The present study had a few limitations. First, the primary limitation is its retrospective case-control design, which was less eloquent to deduce causality from the results. Second, this is a retrospective study based on X-ray instead of CT, which reflect the real FOC in a more accurate way. Although X-ray is not the best choice to depict FOC, we identified the bone cortex as best as we could and meanwhile our results revealed that X-ray was also practical to detect individuals with low FOC. Last, there was inconsistency regarding the atlanto-occipital angles in flexion and neutral positions between our results and the Dohzono’s . Dohzono reported that the atlanto-occipital angle decreased when neck flexed from the mean of 12.4 degrees in the neutral position to 8.6 degrees in flexion. However, our result showed the opposite situation where the angle increased (Table 2). These limitations open the door for the future study.