In some situations, cervical fracture in AS patients might escape from recognition due to physicians’ lack of clinical experience or the limitations of the basic radiological examination. Therefore, it is imperative to have a high index of suspicion in AS patients who present with a history of trauma or without any neurological symptoms and to treat these patients as if they had a fracture until this had been excluded either by CT or MRI. X-ray is usually deemed as a primary option to discriminate the potential fracture in patients suffering from a trauma under emergency conditions but bidimensional images are easily influenced by abnormal ossification and position, which leads to the misdiagnosis of a minor fracture without an obvious fracture line and dislocation. However, based on our experience, certain X-ray imaging characteristics can offer more information beyond visible fractures; they can also warn for the possibility of a fracture. Hence, in the present study, we explored the predictive ability of radiological measurements used to establish the risk of cervical fracture in AS patients suffering LET.
An important reason why patients with AS are susceptible to LET is that the continuous ligament ossification and degenerative discs collectively reduce cervical elasticity, whose mechanical function behaves as a long force arm like extremities, acting as a rigid lever, incapable of appropriately dissipating the energy of a traumatic event . Besides, a thicker ossification is associated with higher stiffness and fragility; the fused spinal columns have lost their elasticity and movements resulting in altered biomechanics. In this study, to establish the severity of ligament ossification, measurements of the sagittal diameter of both the vertebral body and the canal were accomplished considering the ossification of the anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum. As can be seen in Fig. 2, thicker ossification of the anterior and posterior longitudinal ligament caused longer mVB (Fig. 2B), whereas thicker ossification of the posterior longitudinal ligament and ligamentum flavum caused shorter mVC (Fig. 2C). Additionally, thicker ossification of the three ligaments can cause both shorter mVC and longer mVB (Fig. 2D), and these three factors can decrease the Pavlov ratio (= mVC/mVB). According to the statistical results, the Pavlov ratios of C2 to C7 in Fracture group were all significantly lower than that in Non-fracture group (P < 0.05). Furthermore, the results of the binary logistic regression and ROC curve showed that the mean Pavlov ratio was significantly correlated to the occurrence of cervical fracture in AS patients who suffered from LET. The highest AUC of the mean Pavlov ratio manifested its best predictive ability among other parameters. The cut-off value of the mean Pavlov ratio was 0.72 (sensitivity = 0.829, specificity = 0.739), indicating that for patients with AS who encountered LET, those whose mean Pavlov ratio was less than 0.72 had a higher risk of cervical fracture. In this premise, if the X-ray examination appears normal, further CT and MRI are highly recommended.
The maintenance of cervical natural physiological lordosis contributes to buffering the action of a force when the skull and the neck suffer from trauma [16, 17]. The aggravating cervical rigidity caused by chronic inflammation leads to a reduction in the cervical buffer capacity. Besides, stress concentration becomes more pronounced under cervical rigidity, and the mobility trend in the vertical and horizontal directions becomes more obvious, which causes a state of instability of the cervical spine. In the present study, a total number of three parameters related to cervical curvature showed a significant difference between the two groups (P < 0.05), namely, Angle D (the intersection angle between the line parallel to the upper border of C1 body and the line passing through the anterior-inferior point and the posterior-inferior point of C7 body), Borden’s index, and Harrison’s value. In addition, straight cervical curvature for all three parameters (namely lesser cervical lordosis) was all exactly detected in the Fracture group. In the further analysis, Angle D and Borden’s index were incorporated into the binary logistic regression and ROC curve, and their AUC were 0.690 and 0.636, respectively. The cut-off value of Angle D and Borden’s index were 45.65° and 9.79 mm, indicating a higher possibility of cervical fracture in AS patients suffering from LET when Angle D and Borden’s index are lower than 45.65° and 9.79 mm.
Time interval between AS diagnosis and trauma is an objective reflection index for the severity of AS progression. Theoretically speaking, a longer time interval might be related to more seriousligament ossification and cervical rigidity. Besides, as previously reported, the risk of incurring a spine fracture after injury in AS grows gradually with time, and the risk of sustaining a vertebral fracture could reach an added 1.3% per year . A study conducted by Deminger et al.  explored the spinal radiographic progression in AS based on the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS). They found that the mean progression was 1.6 mSASSS units over five years (P < 0.001). Other studies revealed the progression of mean 4.2 per four years  and 1.3 mSASSS units per year . An investigation including 132 AS patients in the OASIS (Outcome in AS International Study) cohort further revealed that new syndesmophytes occurred in 33% and 48% of the patients after two and four years, respectively . Though the predictive ability of time interval between AS diagnosis and trauma was not high with its AUC of 0.636, the time interval of the Fracture group was significantly longer in the present study (21.5 years vs 16.6 years, P = 0.013). The cut-off value of the time interval between the AS diagnosis and the trauma was 15.50 years, indicating that if a patient who encountered LET more than 15.50 years after the diagnosis of AS should be considered with increased vigilance to have a potential cervical fracture to avoid missed diagnosis.
In the present study, the morbidity of the continuous bony bridge between C1 and C2 also had significant difference between the two groups (43.9% vs 13.6%, P < 0.001). The atlantoaxial joint plays an important role in the cervical natural motion, especially in the rotation function. A recent in vivo study showed that the flexion-extension of C1-2 was 13.7 ± 4.2°, accounting for 14.5% of the overall flexion-extension ROM; the lateral bending neck motion of C1-2 was 7.6 ± 2.7°, accounting for 13.2% of the overall lateral bending ROM; and the axial torsion neck motion of C1-2 was 72.9 ± 7.6°, accounting for 73.2% of the overall rotation ROM . Previous investigations also achieved similar results [23, 24, 25]. Once the motion of C1-2 joint is restricted by the anterior continuous bony bridge, the bearing force capacity from the skull and the neck is in turn reduced, which further increases the risk of cervical fracture.
However, there are some limitations in our study. On the one hand, the present study included a relatively small number of patients, a larger sample size and a multi-center study might make the results more convincing. On the other hand, this was a retrospective study, and a prospective study for predicting the possibility of CSF in AS patients suffering minor trauma might have the potential to provide more references to clinical practice.