Previous reports have shown that pedicle screw loosening after surgery is a serious complication of spinal fixation surgery. Screw loosening causes nonunion back pain and sometimes neurological impairment, and it can be an indication for reoperation[13, 14]. Ohba et al. reported that pedicle screw pull-out was a risk factor for postoperative screw loosening. In the current study, we investigated the incidence of screw pull-out and attempted to identify relevant risk factors.
In our study, pull-out occurred for a total of 17 pedicle screws in 13 cases, giving an overall pedicle screw pull-out rate of 2.7%, which is relatively low compared with the rate of 16.2% reported previously. Although screw pull-out has been recognized as a distance of > 1 mm in previous studies, we defined it as ≥ 4 mm in our study. Considering that one pitch of the screw is 4 mm for the Voyager spinal system and 2.7 mm for the Reline spinal system, it seems reasonable to define pull-out as 1 or more pitches for the Voyager screw and as 2 or more pitches for the Reline screw. Thus, the inter-observer agreement was high (κ = 0.79) in this study. The difference in the cutoff value may explain why the incidence of screw pull-out was lower in this study than in previous investigations. Pedicle screw pull-out was detected in 19.7% of the patients in this study. Of note, all the postoperative CT images were obtained on postoperative day 2 before the patients started to mobilize, meaning that screw pull-out is most likely to occur during the operation. Other studies also mentioned that pedicle screw pull-out occurs during rod connection [10, 11]. Generally, screw pull-out can occur if there is a gap between the shape of the rod and the actual spinal alignment. In the PPS system especially, pull-out may easily occur at the time of inserting set screws because the gap between the rod and the screw head is not visible [10, 11]. Furthermore, we found that pedicle screw pull-out could occur at any of the levels including the cranial end, caudal end and inter-levels. The force in the direction in which the screw comes off is considered to vary according to the order in which the set screws are placed.
In this study, patients in the pull-out group were more likely to be elderly, to have a diagnosis of DISH or osteoporosis, and to have advanced elderly medical service system insurance cover. Other researchers have also identified that older age and osteoporosis are risk factors for pedicle screw loosening[13, 15]. Advanced elderly medical service system insurance is available for citizens over 75 years of age only, meaning that our patients with this type of insurance cover were indeed elderly. The number of fused segments was also associated with screw pull-out. In long fusion, force may be applied to the direction in which the rod does not fit the screw head and the force on the lever arm of the rod is increased, thereby increasing the risk of pedicle screw pull-out compared with short fusion. Additionally, in long spinal fusion, there are multiple screws to be connected to the rod, making it difficult to achieve appropriate rod-bending and to fit the rod completely to each screw head.
Logistic regression analysis identified advanced age and presence of DISH to be independent risk factors for pedicle screw pull-out. DISH appears on radiographs and CT images as ossification along the anterolateral aspect of the vertebral bodies[16, 17]. Therefore, movement of the spine becomes limited by spinal ossification. Given that DISH restricts the segmental motion of the spine, the screw-rod system applies force in the direction in which the screw comes off if the fused segments were over-corrected by de-kyphotic rod placement. Additionally, although there was no significant difference in DISH in this study, DISH is often performed PPS surgery, as it is considered that the screw comes off because the fitting between the rod and the screw head cannot be checked directly. Patients with DISH also tend to be elderly and have poor bone quality [18–20]. Therefore, posterior segmental fusion extending at least three levels above and below has been recommended in patients with DISH. To our knowledge, there have been no reports showing that the presence of DISH is associated with pedicle screw pull-out, and ours is the first to clearly demonstrate that DISH is a significant risk factor for intraoperative screw pull-out with an odds ratio of 6.10.
The most important factor is to ensure appropriate rod bending and gently connecting it to each screw head. As it is sometimes difficult to create a perfect curve by manual bending, a new automatic rod bending system may be useful to improve the fitting of the rod. Also, a new pedicle screw device using cement augmentation has recently become available as a strategy to prevent pull-out.
There are several limitations in this study. First, we did not evaluate the screw design, position, or trajectory as factors affecting pull-out. Second, radiation exposure is a risk for patients, although we used intraoperative CBCT, in which the radiation exposure is reduced. Third, the sample size is limited; Nevertheless, this study identified age and the presence of DISH as the important risk factors for screw pull-out during surgery.