Placing implants close to the alveolar nerve relying on 3D images and drill guides
Trauma of the alveolar nerve at implant insertion is a disaster not only for the patient and the clinician, but affects public opinion on implant dentistry and stomatology as a whole. Literature suggests that nerve trauma happens more often than we may think. Such damage can declare itself as "slight loss of sense" up to "complete loss of sense" and even occurrence of phantom pains.
A trauma can take place at any of the following stages:
- As a result of an inadvertent injection of anesthetic to the trunk of a nerve;
- As a result of tension or rupture of the collecting nerve at exfoliation and calcification of a rag;
- As a result of penetration of a drill to the mandibular canal during the implant bed formation;
- As a result of compression of a nerve by the implant.
A female patient, 43 years old, was directed after orthopedic treatment. The patient was unhappy about her prosthesis with ball attachments.
A CT study revealed a moderate bilateral distal mandibular atrophy, more expressed on the right (fig. 1A). The architectonic bone type according to Lekholm and Zarb's classification corresponded to class 3. The images show cross-sections in the distal mandible.(fig. 1B, 2) Due to the limited vertical bone height it was impossible to safely place implants in a "traditional" position.
Based on 3D images, it was decided to angle the implants to allow implant placement in the available bone, avoiding bone augmentation procedures. On the right, the implants were placed more to the vestibular, whereas on the left, the bone was at the lingual side. Picture 5 shows an overview.
Based on the SimPlant file, a stereo lithographic model of the mandible and matching bone-supported SurgiGuide drill guides were ordered. These models gave an exact realization of our plan prior to performing surgery (Picture. 3).
In the early and late postoperative period there were no clinical displays of a trauma or a compression of a nerve (Picture 4).
The mandibular canal can follow a variety of paths through the patient's anatomy. Locating and identifying the nerve in 3D is made possible and easy with the combination of CT data and interactive SimPlant software. With full knowledge of the nerve location, implant planning in the lateral sites of the distal mandible can be well prepared. SurgiGuide drill guides link the virtual treatment plan to the surgical arena further reducing the risk of complications. When the alveolar nerve is undamaged, the patient, the clinician, and the profession of implant dentistry are winners
Discussion: 3D images vs. 2D images
Many physicians don't realize the amount of distortion in panoramic x-ray imaging; however, they rely on 2D panoramic images to define the maximum implant length. Unfortunately, the panoramic picture does not give enough information and doesn't allow visualization of all zones suitable for implant placement. Crest shape and bone volume are especially hard to assess on a panoramic image.
It has been shown that the dimensions of bone structures shown in X-ray orthopantomography and kinear tomography are enlarged by 25-30 % in contrast to 0-4% in case of CT (computed axial tomography). CT provides reliable information not only with respect to the jaw dimensions, but also for visualizing anatomical structures such as the mandibular canal, mental foramen, mandibular foramen, incisive foramen and maxillary sinuses . This is important information even for the simplest of implant treatments and therefore surpasses conventional radiological methods.
In the course of our practice we are convinced of the advantages of spiral CT with 3D modelling of bone structures for the treatment planning of dental implants. CT allows for measurement of the thickness of an alveolar ridge and bone volume and we can immediately determine the diameter of dental implants to be placed. CT also allows for measurement from the upper edge of an alveolar ridge to the top wall of the mandibular canal. This enables us to choose the implant length more precisely, but also to avoid grafting procedures by tilting implants bucco-ligually. Using the SurgiGuides permits this difficult and demanding angulation to be transferred to surgery with confidence.
It is the opinion of the authors that nerve drawing tool is one of the most valuable assets of SimPlant when working in the mandible. The tool requires dropping points in the nerve canal and the software connects the points making a smooth highlighted line. When complete, the highlighted area is visible in each 2D view, as well as the 3D image. In all cases performed at this clinic, it was easily possible to define the path of the alveolar nerve, its position in relation to cortical plates, and its classification according to Solar.
The SurgiGuide drill guides allow "2D drill guidance". This means that the drill will enter the bone at the right position and the proper angulation. The surgeon controls the drilling depth using drill stops. 3i (Implant Innovation Inc. USA) manufactures plastic flanges which can be fixed on a drill. These will impede excessive osteotomy depths. Unfortunately, only few clinical physicians (even those who work with 3i implants) use these flanges in daily practice.