In recent years, increased attention has been given to the effects of chronic neck or lower back pain (LBP) which can not only bring about physical and psychological pain to the patients but also result in numerous socio-economic burdens. In natural populations, up to 70–80% of adults have experienced at least one incident of LBP in their lifetime [14]. For severe LBP with spinal degenerative diseases, fractures, tumors and deformities, surgical treatment, intervertebral fusion combined with screw fixation is often considered a reasonable indication and can obtain satisfactory clinical outcome. However, among elderly patients with osteoporosis, as the perioperative complication, failure or dissatisfaction of instrumentation is relatively often and clinically worrisome. Malposition and loosening of pedicle screw are main reasons of failure of internal fixation. The reported rate of pedicle screw malposition is 5–41% [15-17]. Numerous studies have proved the influencing factors of the pullout strength [18-19]. Nevertheless, few studies have looked at remedies for failed screw placement. Thus, this study has introduced the procedure of self-bone grafting in trajectory as a new remedial method for failed screw placement and compare pullout strength of adjusting pedicle screw with or without self-bone grafting in the previous trajectory using an osteoporotic human vertebral body to explore the feasible measures of adjusting screw in the osteoporotic vertebral body.
Osteoporosis is widely recognized in the scholarly literature as a premise and foundation of fragility fractures, which is most common in older people and needs to be corrected by surgery [20-21]. For the osteoporotic spine especially with vertebral instability, fracture, spinal deformity, tumor, multi-segment lumbar spinal stenosis and so on, fixation of pedicle screw is vital to maintain postoperative segment stability and ensure its rapid recovery [22-24].
Weinstein et al. [25] reported a biomechanical experiment of pedicle screws and found that 60% of the fixation strength required for pedicle screw was provided by pedicle, 15-20% by interaction of screw with cancellous bone, and another 20-25% by penetrating contralateral cortical bone. Many studies reported that osteoporosis could lead to the decrease of anchoring force of pedicle screw and the pedicle screw often loosen. Based on the results, we found that BMD had a high correlation with pedicle screw axial pullout force, which presented downward trends in osteoporotic vertebrae and was comparable with those by other authors [26, 27]. From CT scanning of lumbar vertebrae, exhibited signs of osteoporosis testifying to the bone tissue’s degradation were clearly observed in the osteoporosis group, which shown specificity as abnormal morphology and signal, a loss of well-distributed pattern, as well as a gradual reduction in the number of solidity bone tissue with some areas of disintegration. Moreover, the following specific changes were observed: the percentage of normal bone decreased significantly; the shapes of bone tissue were irregular and some of them were defected. We think it may be associated with the decrease of Minerals, like inorganic calcium and phosphorus and the disorder of bone trabecular structure. Varghese et al. found that BMD was an important factor affecting the fixation strength of pedicle screw, of which the greatest impacts on it involved insertion torque of screw (82%). The maximum pullout force (76%), stiffness (46%) and strain energy (85%), when synthetic osteoporotic cancellous bone block models with 3 different BMD of 80 kg/m-3, 160 kg/m-3, 240 kg/m-3 were utilized [28]. Ruffoni et al. through describing the three-dimensional lattice model of bone trabeculae found that the thinness degree of bone trabeculae mainly affected the pullout strength of screws, while the loss of bone trabeculae mainly affected the stiffness of screw [29]. As the similar results, we imagine that normal structure and function of bone trabeculae cannot be enough to maintained and lead to disintegration of internal structure, the decline of the ability to resist external forces and even the occurrence of micro fractures occur, once the loss of minerals to a certain extent.
In clinical practice, the elderly patients complicated with kyphosis, lateral bending and other spine malformations, increased the difficulty of screw placement and might involve secondary or even multiple insertions. Many studies reported that the biomechanical strength of a redirected pedicle screw is less than that of a correctly placed screw [30-32]. In the present study, we found that the pullout strength of reinserted pedicle screw seriously decreased by more than 40% in osteoporotic vertebral compared with inserted one time. This reduction in the pullout strength of the reinserted pedicle screw might be related to the compression of bone tissue in the pedicle and the loosening of the microstructure such as bone trabecula. Ferris et al. et al had preformed a study of evaluating the effect of various screw designs on the screw fixation strength and found that tapping a pilot hole does not increase pullout strength in bone with densities near 20 lb/ft3, which correlates with low-density cancellous or osteoporotic bone [33]. Purvi et al. found that pullout strength was obviously rising with the increased angle of the screw axis with the axis of pullout force for 10°, 20°, 30° and 40°, which might be related to the increased compressed resistance of the screw to the bone tissue above it [34]. The results, in our study, indicated that there was no significant difference in pedicle screw axial pullout strength between sagittal angle and axial angle reinserted, which could be related to the axial pullout of the screw. Axial pullout of the screw only needed to overcome the resistance of the screw thread trajectory without shear force. The results indicated that the use of the universal screw and crossbar might have a positive effect to avoid the axial pull-out of the screw, Inserted pedicle screws at the the first time formed a thread track matching the screw and the connection with the bone tissue was tight and firmly fixed. Inserted pedicle screws using the previous entry point easily caused compression and dissociation of previous trajectory due to too much spongy bone tissue decreased and the bone tissue around pedicle screw does not fit closely due to previous thread track broken down. At the same time, it speculated that the compression of the bone tissue surrounding screw resulted in uneven distribution of osteoporotic bone tissue and reduction of contact area of thread and bone, which needed to be confirmed in our future research.
Many studies have tried to solve the problem. Bone cement augmentation is considered an effective measure to improve the pullout strength in osteoporotic cancellous bone [35, 36]. However, when the screw position needs to be adjusted, it is very difficult to judge the integrity of the cortical bone as the poor bone quality. it is possible for the screw canal to communicate with the adjacent canal. There exists potential risks that redundant bone cement may be extruded and uncontrolled, resulting in bone cement leakage and nerve injury. Matsukawa et al. reported that in osteoporotic bone, increasing the diameter of pedicle screw larger than 5.5mm did not significantly increase the strength of screw fixation related to the thinning of bone in pedicle caused by osteoporosis, on the contrary, larger screw diameter might cause the risk of cortical fracture around pedicle [37]. Therefore, bone cement augmentation and increasing the diameter for reinserted pedicle screw may not be the best indication in osteoporotic vertebral body. We used the method of autologous spinous process replaced into the previous trajectory, which shown that the reduction of screw fixation strength caused by adjustment of trajectory could be effectively compensate by additional thread resistance from the self-bone grafting. In addition, the self-bone does not cause the rejection reaction and embolism. To self-bone grafting, the material is convenient to obtain and is relatively safe, and meanwhile, the operation time and the operation cost caused by the temporary change of treatment can be avoided.
There are still some limitations in this study. First, pullout force at certain angles to the screw axial direction was not involved, which may be more close to the biomechanical of the actual pedicle screw in vivo. Then, the adjustment of screw involved only two directions and optimal adjustment angle needed to be further explored. Finally, it is necessary to increase the sample size and expand the experimental conditions for further analysis.