BI, accompanied by IAAD, invariably results in evolutionary compression of the medulla, leading to severe neurological damage and even death. Surgical treatment is often required for most patients to restore the dislocation, reconstruct stability, relieve clinical symptoms . At present, OF operation is the most popular posterior surgical procedure for treating BI with IAAD using plate-screw-rod instruments [9,10,11,12,13]. The TARP system was designed by Yin et al. in 2004, and it is an effective surgical approach for treating irreducible atlantoaxial dislocation caused by congenital developmental anomaly, tumour, trauma, and so on, completing release, reduction, decompression, fixation, and fusion in one step with an anterior-only approach [15,16,17]. For treatment of BI with IAAD, the TARP system possesses distinctive advantages, because it can move down the odontoid process from the foramen magnum and achieve reduction to directly relieve the compression anterior to the spinal cord [5, 6].
Several clinical studies have confirmed the effectiveness of TARP and OF operation for the treatment of BI with IAAD, but all involved a single surgical procedure [5,6,7,8,9,10,11,12,13]. We retrospectively analyzed the clinical profiles of 56 patients diagnosed with BI with IAAD who underwent a TARP or OF operation, with the aim of comparing the two treatments for BI with IAAD.
In this study, patients undergoing TARP or OF operation obtained significant radiological improvement in postoperative ADI, CCA, CL and CMA demonstrating the effect of two surgical approaches for reduction and decompression. Moreover, postoperative JOA scores in all patients obviously improved. The aforementioned result explains that the use of a TARP or OF operation is an effective surgical approach for the treatment of BI with IAAD, consistent with previous studies.
Found in comparative analysis of surgical characteristics, the operative time and blood loss in the TARP group were less than those in the OF group, which might because the TARP operation can be performed with an transoral anterior-only approach [5,6,7,8, 15,16,17], while OF operation often requires concomitant transoral anterior release before the posterior approach is made in order to achieve satisfactory reduction and decompression [9,10,11,12,13]. Otherwise, operative time and blood loss would be increased, leading to a wider range of surgical trauma and a higher risk of infection. Furthermore, the spinal cord might have been injured while in the transforming operative position after transoral anterior release when the atlantoaxial joint was extremely unstable .
Although no significant differences were found in the preoperative radiographic measurements of ADI, CCA CL, and CMA between the TARP and OF groups, compared with OF, the improvements of postoperative ADI, CMA, CCA, and CL in the TARP group were superior. This indicates that TARP operation was more effective than OF operation in reducing BI with IAAD, bound up with a more direct method of reduction by TARP than by OF. By using reduction instruments directly in TARP procedures, we were able to separate the atlantoaxial junction and pull the axis with the odontoid process inferiorly and subsequently push C1 backward relative to C2. We thus achieved atlantoaxial reduction, while OF procedures reduced the atlantoaxial junction to allow the axis, with the odontoid process, to descend indirectly via the strength generated by the posterior plate-screw-rod instruments.
Satisfactory neurological improvements were achieved in both the TARP and OF groups after operations. Despite no difference in preoperative JOA scores between two groups at discharge, the JOA score of the TARP group was significantly higher than that in the OF group at the 3-, 6- and 12-month follow-ups. Patients with BI and IAAD suffered from ventral spinal cord compression induced by ascent of the odontoid process. Jiang et al.  reported that the craniocervical volume change rate is a positive predictor for evaluating the improvement of postoperative neurological function in patients with BI. The study by Wei et al.  confirmed the negative correlation between the CL and the craniocervical volume improvement rate. The postoperative CL in TARP group was significantly less than that in OF group, so that, the function of TARP operation for improving craniocervical volume could be more obvious than that of OF, which could be more beneficial for recovering of impaired spinal nerves.
We also found that the bone fusion rate was much higher at the early stage postoperatively in the TARP group than that in the OF group. It might be related to methods of bone grafting in two procedures. Although, both TARP and OF fixation had a excellent biomechanical stability proved by previous researches [19,20,21], more pressure was put on bone grafts in TARP procedure than in OF operation, because most bone was grafted into the lateral mass joint space in TARP operation , while all bone was grafted on the surfaces of C1 posterior arch, C2 lamina, and lower part of the occipital in OF operation .
Wound infection is another concern. Most surgeons select OF fixation mainly because of this complication. TARP fixation might be more possible to cause the occurrence and spread of infection [24,25,26]. But, with proper preoperative preparation and postoperative care, the complication rate can be reduced. In this study, no patient had wound infection in TARP group.
There are several limitations in this study. Selection bias exists in this study due to the single-center analysis, that hints the requirement of a multicenter, large sample study in the future. Additionally, the present study is retrospective in nature; future prospective studies may better control for follow-up timing intervals and may have the potential to include more standardized outcome measures.