This study found that the insertion time were significantly difference between v-thread cylinder-core pedicle screw (22,94 s) with commercially available pedicle screw (15,86 s) (p < 0,05). The pull-out strength was significantly difference between commercially available pedicle screw (408,60 N) with square-thread conical pedicle screw (836,60 N) (p < 0,05). The square-thread conical-core group have the highest interface area (1486,21 mm2).
The insertion time of pedicle screw is important because it can be a factor that influence how fast a surgeon can finish the operation. The longer the time taken to insert a pedicle screw means the longer the duration of surgery and the higher the risk of complications. Cheng Hang had reported that every additional of 15 minutes for surgery times can increase the infection rate by 13%.8
Study by Higashino K showed that pedicle screw removal was found to be 435.6 Newton in osteoporotic vertebra,9 which is comparable to this study for measurement of commercially available pedicle screw group (408.6 Newton). This study propose that best combination of pedicle screw design is square-thread conical-core design regarding the statistical analysis due to its comparable insertion time and higher pull-out strength compare to commercially available pedicle screw.
Screw loosening was found between 0.6–11% and might be higher in osteoporotic bone. Re operation rate because pedicle screw loosening was found between 14–27%.10 Fixation failure because of screw loosening might change the spinal alignment and fixation stability that might harm the patient.
The geometry of the pedicle screw core can be conical, cylindrical or combination of both. The three types have different mechanical strengths. Abshire et al. Compared the conical and cylindrical core designs and concluded that the conical screw has a better pull-out strength than the core cylinder.3 However, Kwok et al carried out a similar study and found that there was no significant difference in the pull-out strength between screw with conical design and cylinder core.11 Meanwhile, Yaman et al examined the pull-out strength of a pedicle screw with dual core and found that the design had a higher pull-out strength value compared to the screw with conical and cylinder cores.1 The limitation in this study is that we cannot use a pedicle screw with dual cores due to the limited production equipment.
Kim et al examined the shape of the thread on a screw and its effect on pull-out strength and found that threads with a v-thread design had a higher pull-out strength compared to square threads. According to their study, this is influenced by the flank overlap area (FOA) and thread pitch where the screw which has a larger flank overlap area and a smaller thread pitch has a higher pull-out strength value.4 This is different from the results we obtained in this study where the pull-out strength value of a pedicle screw with a square thread has a higher pull-out strength value than a pedicle screw with a v-thread. This may be influenced by the contact surface area between the screw and the larger media on the pedicle screw with the square thread which in this study was successfully measured using the solid works software program. This difference certainly requires further study and further research with a larger sample size may be required.