Screw loosening is common as reported in PLIF with an incidence of 1%-60%7 10 13. Risk factors as explored are connected with osteoporosis, incorrect failing loading scenario, insufficient fusion or screw stress distribution7, however, most of researches have not reached consensus. CBT screw has comparable pullout resistance and stability as TPS since 2009 by Santoni et al1. It can provide enhanced screw purchase and preferable interface strength attribute to characteristics of engaging higher density cortical bone even in osteoporosis patients21–23. Perez-Orribo et al explored the biomechanics of TPS and CBT and concluded that equivalent stability was found between TPS and CBT fixation3. Matsukawa et al found the screw insertion torque of CBT was 1.71 times higher compared to TPS24. Thus, theoretically, CBT screw has been proposed to promote pull-out strength and enhance the construct stability. In the present study, we found a 26.6% incidence of screw loosening in a total of 79 sample (11.4% in 316 screws). To investigate the risk factors of screw loosening, we documented and analyzed the mentioned parameters of each screw which would be more beneficial for surgery and the results of risk factor analysis showed three main factors (FS1, CA and CBCL) mainly constituted the predict scoring nomogram.
The odds ratio of FS1 was highest compared other parameters (OR = 3.82). In our study, there were 22 screws fixed in S1 vertebra, and 9 of them (40.9%) were found obvious lucent zone in CT scan. Grigoryan et al25conducted a cadaveric biomechanical study and considered lumbosacral fixation with CBT screw was stable against loosening, which is contrary compared the results of our study. The reasons of FS1 was concluded as a risk factor of screw loosening were assessed: First, lumbosacral fixation is inherently thought to have higher risk of screw loosening due to alignment restoration and holding strength26–28. Second, the learning curve of lumbosacral fixation with CBT is relative higher. Matsukawa et al29elucidated the penetrating S1 endplate CBT technique with a mean cephalad angle of 30.7° could provide favorable stability for lumbosacral fixation, while during our work, especially for early cases, it was hard to identify a content position for instrument in S1, and repeating screw track adjustment might result in instability, and this also occurred in other segment for early cases .Therefore we considered fixation to S1 need experienced surgeons to perform, and despite the result was not good for FS1, we believe CBT screw for S1 is an alternative method for fixation due to the reduction for paraspinal dissection and facility for retraction in sacrum.
With regard to CA and CBCL. CBCL possessed an important role in screw loosening of CBT screw according to the nomogram. Typical trajectory of CBT contains four parts as the cortical bone to increase the stability of fixation, and among these, the lateral par as the start point is essential. The lateral par is an identifiable structure as entry point and is less influenced by degenerative change to provide good bony reference in the surgery30 31. And the start point could also have influence on CA. Literatures recommended an approximate 10° -14°angle to medial32 33, and in our study, the mean CA was 10.36° in Non-SL group and 13.94° in SL group, which concluded similar to the previous studies. Matsukawa4 stated that CA was more variable than SA, and CA might have been derived from differences in the location of the starting point. We believe biochemical studies would be performed to clarify the mechanism in the future.
In the present study, we have documented and provided a reference for the measurement of SA as angle between screw line and vertical line because we think this might reduce the error for measurement of wedge-shaped vertebra in some cases, while some authors recommended a measurement method of angle between screw and vertebral endplate33 34. However, the results showed no statistically significant difference between the two groups but there was no denial that SA was an important parameter.
Lower BMD evaluated by dual x-ray absorptiometry (DXA) was significantly associated with screw loosening by influencing the pullout strength1 35 36, nevertheless, it was an average value to assess BMD by DXA. In addition of DXA, the use of HU based on CT scan has been applied and clarified to be a reliable method for BMD evaluation37–39, which can be used to assess the region involved by each screw. However, literatures revealed that there was no consensus for a value of HU to evaluate a low BMD as a risk for osteoporosis. In the current study, BMD around the screw was assessed by HU value to explore if BMD would be a risk factor for screw loosening and the result was negative. This demonstrated that the BMD of the region where screw threaded could not make much difference. Lee et al33 have reported a HU measurement of cortical bone, however, we have attempted to make the repetition, and the results showed a poor Inter-rater reproducibility due to the thin wall of cortical bone and we didn’t adopt the method to replace CBCL.
The study had some limitations, mainly due to retrospective analysis with a small sample size. The surgery with CBT technique was performed during learning curve of the early period, and this might contribute to the loosening of S1 screw. Further studies with experienced surgical technique will be performed to validate the present study.