# Torque Requirements and the Influence of Pilot Holes on Orthodontic Miniscrew Microdamage
## 4 Discussion  
Four of the five damage characteristics showed no reduction in microdamage when a pilot hole was drilled. There was a significant decrease in maximum crack length, but the difference was small and may not hold clinical significance. These findings differ from an in vitro canine study that found less microdamage for 2.2 mm cortical bone when a pilot hole was used [9]. Similarly, a further canine study showed reduced microdamage when a pilot hole was pre-drilled [23]; however, bone specimens were prepared for analysis after insertion of OMIs and for which preparation damage was not controlled.  
Reductions in all damage characteristics were observed as cortical bone thickness decreased. The reduced microdamage with decreasing cortical bone thickness may be explained by fewer threads of the OMI engaging with the cortical bone [20]. The OMIs used in this study had a thread pitch (distance between threads) of 0.685 mm, and fewer threads were embedded in the bone as the thickness decreased by 0.5 mm increments [24]. Another study found no correlation between cortical bone thickness and microdamage when an Aarhus 1.5 mm diameter OMI was inserted into porcine bone [13]; however,  
Figure 4: Box plot of maximum insertion torque (Ncm) comparing no pilot hole (NoPH) to pilot hole (PH) for each bone thickness.  
the reported study evaluated microdamage using scanning electron microscopy, which potentially introduced cracks from sample dehydration.  
There was no difference in maximum insertion torque according to pilot hole status, which confirms the findings of previous research [25]. Insertion torque increased with the depth of insertion, with the maximum insertion torque occurring at the end of OMI insertion as the neck of the screw comes into contact with the bone [11]. In the present study, the OMIs were inserted to a maximum depth, at which point the neck of the OMI made contact with bone, to ensure complete insertion, regardless of pilot hole status. This contradicts the findings of [PERSON] and [PERSON] [5] and [PERSON] et al. [17], who reported a decrease of 20-40% in maximum insertion torque when a pilot hole protocol was used. Clinically, it is difficult to determine the depth of insertion due to the variation in overlying soft tissue thickness. These other studies did not account for the confounding factor of OMI insertion depth, which would be difficult to control in vivo due to the soft tissues.  
The present study clearly found an increase in maximum insertion torque as cortical bone thickness increased. This confirms the findings of a study that measured maximum insertion torque during OMI insertion into 1.5 and 2.0 mm artificial cortical bone [26]. These findings are relevant for clinicians inserting OMIs into thicker cortical bone, found in the posterior mandible, where the maximum insertion torque will increase. In addition, higher maximum insertion torque was related to increased microdamage characteristics, specifically total damage area and diffuse damage area. As previous studies found that maximum insertion torque is related to lower OMI success rates [3, 4], this may be explained by an increase in microdamage created during OMI insertion. A survey of orthodontic practitioners who measured insertion torque during placement of OMIs had greater overall success rates compared to those who did not measure torque [27]  
Porcine bone has been used in previous studies for the detection of microdamage and OMI torque measurements [6, 20, 28], but it is likely that the cortical bone of the maxilla and mandible behaves differently to that of porcine tibia bone [29, 30, 31]. However, the availability and ease of sample preparation with fresh porcine bone make it an ideal experimentation model. Future research to validate the use of porcine bone as a model for human cortical bone would be beneficial to support the clinical applicability of the present results.