Biomechanical study of rotational micromovement of the pedicle screw
© The Author(s) 2016
Received: 21 December 2015
Accepted: 28 June 2016
Published: 8 July 2016
In regard to the fixation using a pedicle screw (PS) and rod system, the mechanism from the onset of the clear zone up to the development of loosening of the pedicle screw is not completely clarified. The purpose of this study is to determine the cause of the pedicle screw loosening by performing a biomechanical study with three-dimensional movie analysis.
Ten PS fixation model of the lumbar spines (L3–4) of boar cadavers were used. The rotational angles of the L3 and L4 vertebral body and the screw at the time of applying a ±5 Nm load in the left anterior and right posterior flexion directions respectively were calculated based on those at the time of applying no load. The absolute value of the difference in the rotational angles between each vertebral body with left anterior flexion and right posterior flexion and the inserted screws was defined as rotational micromovement.
In both the left anterior and right posterior flexion directions, there were significant differences (p < 0.05) in the rotational angles between the screw and the vertebral body for both the L3 and L4 vertebral bodies.
Our biomechanical results showed that rotational micromovement occurred between the PS and the vertebral body, and repeated rotational micromovement might cause loosening of the screw or pullout of PS fixation.
Loosening of the screw or pullout of PS fixation occurs in some patients postoperatively (Aghayev et al. 2014; Sandén et al. 2004; Schatzker et al. 1975). The biomechanical mechanism of development of loosening of the pedicle screw is little known (Galbusera et al. 2015; Tokuhashi et al. 2008). We have investigated the biomechanical study and produced the results of weakness of PS fixation under rotational stress, and it may be the cause of loosening of the screw. Thus, in order to study the biomechanical cause of the pedicle screw loosening, a study with three-dimensional movie analysis using a functional spinal unit (hereinafter, FSU) of the lumbar spine of cadaver boars was performed.
The animal experiments in this paper comply with the Principles of Laboratory Animal Care (NIH publication No. 85–23, revised 1985), the OPRR Public Health Service Policy on the Humane Care and Use of Laboratory Animals (revised 1986) and the U.S. Animal Welfare Act, as amended, were followed, as well as specific national laws. And, this study was performed with the approval (No. 1449) of the ethics committee of our university.
In the left anterior flexion direction, the mean rotational angle of the L3 vertebral body was 2.1° ± 0.5° (mean ± SD), the mean rotational angle of the screw inserted into L3 was 0.9° ± 0.3°, the mean rotational angle of the L4 vertebral body was 0° because it was defined as the fixed end, and the mean rotational angle of the screw inserted into L4 was 0.5° ± 0.3°. These results showed that there was a significant difference (p < 0.05) in the rotational angle between the screw and the vertebral body for both the L3 and L4 vertebral bodies, and mean rotational micromovement was 1.2° ± 0.5° at L3 and 0.5° ± 0.3° at L4.
In the right posterior flexion direction, the mean rotational angle of the L3 vertebral body was 1.8° ± 0.6°, the mean rotational angle of the screw inserted into L3 was 0.9° ± 0.2°, the rotational angle of the L4 vertebral body was 0° because it was defined as the fixed end, and the mean rotational angle of the screw inserted into L4 was 0.7° ± 0.2°. These results showed that there was a significant difference (p < 0.05) in the rotational angle between the screw and the vertebral body in both the L3 and L4 vertebral bodies, and mean rotational micromovement was 0.9° ± 0.4° at L3 and 0.7° ± 0.2° at L4.
Pedicle screw loosening is one of the most frequently reported complication of spinal fixation. Ohtori et al. (2013) reported 15 (14.7 %) loosened pedicle screws in a total of 102 patients with osteoporosis. The screw loosening is usually a consequence of pseudoarthrosis and may be occasionally associated with screw breakage and progressive kyphosis (Berjano et al. 2013; McLain et al. 1993). It is also known that this screw loosening often occurs in patients with multilevel fusion (Schatzker et al. 1975) besides osteoporotic patients.
However, the cause to loosening pedicle screws has not been sufficiently clarified (Mehmanparast et al. 2014). As for the loosening of the screw, Inceoğlu et al. (2008) and Costa et al. (2013) reported that various factors, such as the diameter and material of the screw, or the material and angle of the bone, are involved. Law et al. (1993) and Okuyama et al. (2000) reported that cyclic caudocephalad toggling caused by the craniocaudal screw may be the cause of loosening. We have investigated the biomechanical study and produced the results of weakness of PS fixation under rotational stress, and it may be the cause of loosening of the screw. In our results, there were significant differences (p < 0.05) in the rotational angles between the screw and the vertebral body for both the L3 and L4 vertebral bodies. As far as we have been able to determine, no biomechanical study has been conducted from the viewpoint of rotational micromovement between the screw and the vertebral body, and this paper may be the first study to confirm that there is rotational micromovement between the vertebral body and the screw. This showed the possibility that occurrence of this rotational micromovement caused repeated friction between the bones and the screw within the living body, which led to loosening of the screw or pullout of PS fixation. For this problem, we suggest some ideas. First, expandable screw are useful, because the screws expand in vertebral body and have large contact area to bone. It is under experiment in our laboratory. Additional fixation like sublaminar wirings and trial of raising bone density are also useful.
In this experiment, since it was difficult to obtain the spines of the human cadavers, the spines of boar cadavers were used instead. The advantages on the use of cadaveric boar spines are that the resource of destructive animals can be utilized effectively and that the spines can be obtained easily at very low price. The disadvantage is that the anatomy of the vertebral body of boar spines is significantly different to those of humans. Thus, the results should be interpreted as a proportion or trend rather than quantitatively as angles or ROM (Wasinpongwanich et al. 2014). We would like to perform the same experiment using the spines of human cadavers and conduct a detailed study on rotational micromovement between the pedicle screw and vertebral body by attempting to create a model using the finite element method in the future. The rotational micromovement introduced in this study should be considered in the development and study of new spine instrumentation in the future.
The behavior of the PS and the vertebral body was biomechanically observed using a PS fixation model of the lumbar spines of boar cadavers. The results showed that rotational micromovement occurred between the PS and the vertebral body, and repeated rotational micromovement might cause loosening of the screw or pullout of PS fixation.
TM drafted the manuscript, did first selection of articles, and assessed the quality of the papers. TY, TI and TS gave important inputs for the methodic part of this paper, assessed the quality of the papers, performed the statistical analysis, and revised the manuscript critically for its content. YK helped to draft and to correct the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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