50 Chapter 3 density, measured using Hounsfield Units. They detected significantly higher angular deviations in the low bone density group when a semi-guided template was used. This may partly explain the slightly higher angular deviations found in our study, compared to others. However, we used an implant type that allows for a cutting action at the apex during placement. This enables the surgeon to make slight changes in the angular orientation, while maintaining the right position at the implant shoulder, so bone dehiscence can be prevented. Moreover, the clinical relevance of these angular deviation values is debatable. In edentulous cases that are rehabilitated with an implant overdenture, like in this study, the prosthodontist is able to overcome such minimal deviations. Technical complications One template could not be placed properly on the left side. An unplanned undercut of the clamp during the design process may have caused this. Additionally, model segmentation is crucial for proper template fit. During segmentation, the CBCT image is converted to a 3D model by manually choosing the grey values corresponding to the bone. Incidental underestimation of the representative bone is common as demonstrated by Vercruyssen et al.13 This could also cause an absence of small irregularities which may interfere with proper placement during surgery. In order to reduce the influence of segmentation error, this step and the 3D virtual planning were only performed by experienced users only and discussed before initiating production. A slight offset of the template was used to compensate for any irregularities. Limitations A known limitation in accuracy studies using CBCT is the technical steps that are needed to compare planned and placed implants. The post-operative CBCT scan needs to be segmented, which induces similar inaccuracies to those in the planning stage. Moreover, even though the post-operative scan is aligned with the pre-operative scan using a surface based algorithm, both scans differ slightly from each other. Lastly, the titanium implants cause a scattered image, which makes the alignment of the virtual analogues to the placed implants difficult. Since this last step is susceptible to inter-observer variation, it was validated in our study by repeating the alignment and measurement by a second observer (JK). While most studies did not validate this step, Vieira et al.5 tested intra-observer reproducibility through a Cohen’s kappa analysis. This resulted in a kappa-value of 0.72, which corresponds to a substantial agreement, comparable with our findings. Although other studies tried to define a mesio-distal and bucco-lingual plane of deviation, because they seem more clinically relevant than lateral deviation, we consciously did not apply these techniques. Defining the planes requires human interpretation, which again affects the validity of the data. The same applies for the defined depth deviation. However, since our study did not control the depth with guided placement, the depth-axis was extracted from the horizontal implant deviation to give a more valid outcome and interpretation.
RkJQdWJsaXNoZXIy MTk4NDMw