Evaluation methods in diagnosing AAD
Clinically, AAD is generally divided into reducible, irreducible and fixed atlantoaxial dislocation according to the degree of reduction difficulty. The reduction of reducible atlantoaxial dislocation is relatively easy to be treated with the application of conventional posterior reduction and screw rod fixation. For irreducible atlantoaxial dislocation, transoral or posterior atlantoaxial release is required before effective reduction and fixation [6–8]. Because of the complexity of irreducible atlantoaxial dislocation, its diagnosis and treatment are still challenges in the clinical setting. The previous methods used to evaluate the irreducible AAD mainly include routine cervical traction before operation, bidirectional cervical traction before operation and rapid cervical traction under general anesthesia during operation. The first method [9] is to perform routine skull traction on the patient before operation. The traction weight is generally about 1/10 ~ 1/8 of the patient's weight, and then take a bedside X-ray to evaluate the effect of traction reduction. This method is simple and feasible, but for more complex dislocations, unidirectional cervical traction is often difficult to obtain an ideal reduction and the resolution of bedside X-ray is too poor to make an accurate evaluation. To overcome that, Liu [10, 11] proposed to use bidirectional cervical traction technology to implement atlantoaxial reduction, and then evaluate the reduction effect and degree of reduction difficulty. This technique suggests that the effect of reduction should be observed by taking a bedside X-ray 2 weeks after bidirectional traction. Compared with the conventional unidirectional cervical traction, it has significantly improved the reduction effect of atlantoaxial dislocation, reduced the misdiagnosis of some pseudo irreducible atlantoaxial dislocation, and avoided many unnecessary releasing operations. But the traction time in bed is too long to tolerate for the elderly patients. Other scholars [12] proposed to observe the reduction effect by rapid cervical traction radiograph under general anesthesia before surgery as a pre-operative method, which could provide from the pain of traction for a long time before surgery. Additionally, the muscles are relatively loose under anesthesia, which is easy to reduction. But it is limited by image overlap of intraoperative radiograph and the unsatisfactory quantitative accuracy. In addition, because the traction time is too short, the contracture tissue and scar have not been fully opened that causes the improper diagnose. There is still a deviation in diagnosing the irreducible AAD, due to the different evaluation criteria of traction weight, time, traction method and reduction degree.
Feasibility And Clinical Significance Of Quantitative Evaluation Method Based On Ct Image
The pathological mechanism of irreducible atlantoaxial dislocation is complex. It is generally believed that soft tissue factors and bone factors are the two reasons for the difficulty of reduction. Soft tissue factors include: 1) Contracture tension band, including contracture musculus longus capitis, anterior longitudinal ligament, lateral mass joint capsule and apical odontoid ligament; 2) Hypertrophic scar, including hypertrophic scar tissue around the joint of atlantoaxial lateral mass; 3) Obstructive scar, mainly the hypertrophic scar tissue behind the atlanto-odontoid joint. The principle of tradition atlantoaxial release surgery is to cut off the contracture tissue that hinders the reduction with an electric knife, insert the reamer into the lateral mass joints to distract and release, release the contracture tension band in front of the atlantoaxial, and create conditions for reduction. In recent years, many scholars have realized the more important factors that impede AAD reduction are caused by bone other than soft tissue [13–17], including the C2 upper facet joint sloping, hyperplastic osteophytes and callus between lateral mass joints, obstruction caused by lateral mass joint deformity, hyperplasia, deformity and fusion between the atlanto-odontoid joint. It is necessary to grind, cut, disperse and reshape with the help of speed burr and ultrasonic osteotomy to eliminate the structural obstacles, transform the irreducible AAD into reducible type, and create conditions for further surgery. So it needs a proper method to aid in determining the procedure.
To solve the diagnosis dilemma, we firstly proposed a quantitative evaluation method for the irreducible AAD reduction based on pre-operative CT image characteristics, including: the C2 upper facet joint sloping, osteophyte between the lateral mass joints, atlantoaxial lateral mass joint deformity, vertical interlocking of the lateral mass joints, hypertrophic callus and blocking or fusion between atlantoaxial lateral mass joint. Each characteristic is assigned with different weighted scores, so that AAD patients can accumulate total scores according to the imaging characteristics of the lateral mass joints on both sides and atlanto-odontoid joint.
In addition, we used bidirectional cervical traction technology, which is more powerful and scientific than unidirectional traction technology and thin-slice CT scan, which could obtain higher-resolution image data than X-ray. AAD contains two components: vertical dislocation and anteroposterior dislocation. It is not perfect to evaluate the reduction difficulty of one component only, so we comprehensively evaluate the two components and propose the overall reduction rate as the evaluation index. We observed the CT image characteristics and analyzed the reduction rate of AAD in different grades. The results showed that AAD patients of grade I and II had a higher reduction rate, and the reduction rate of AAD patients of grade III and IV was significantly lower than that of AAD patients of grade I and II. The reduction rate of grade III and IV bidirectional traction have not exceeded 40%. It is suggested that the higher the preoperative score, the lower the reduction rate of traction.
In this cohort of 82 patients, 29 patients with grade I and II presented with congenital loose odontoid process combined with AAD and traumatic AAD. There were fewer atlantoaxial joint deformities and hypertrophic callus behind the atlanto-odontoid joint. Therefore, the overall score was low, and the traction reduction rate was high, which could be determined as reducible atlantoaxial dislocation. Most of the patients in grade III and IV were diagnosed with congenital craniocervical malformation combined with BI, and their atlantoaxial lateral mass joints and atlanto-odontoid joints had varying degrees of deformities, callus hyperplasia and other factors hindering reduction. Although using bidirectional cervical traction technology, the reduction rate is significantly lower than that of grade I and II patients. They could be determined as irreducible atlantoaxial dislocation. It was recommended that atlantoaxial transoral or posterior release should be performed previously, and then reduction and internal fixation.
This study provides a systematic and non-invasive quantitative evaluation method for spine surgeons to evaluate the difficulty of AAD reduction, which can be used as a supplement to existing traction evaluation techniques. However, there are still some defects in the study: if vertical traction cannot be achieved during CT scanning, we can only use cervical hyperextension position to maintain the traction effect in the early stage. The degree of anteroposterior reduction will be lost to some extent. In addition, the samples still need to be expanded and verified in clinical application in the future.