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Because the omnivorous Göttingen Minipig is small in size, has a low body weight, and resembles human anatomy in being diphyodont, heterodont, having brachydont incisives and bunodont molars, it is frequently used as large animal model in dental research, which often involves the in vivo testing of endosseus dental implants. Frequently, these interventions have failed entirely or have had less than satisfactory results. One reason was that the assumption of a similar anatomy as in humans often resulted in the penetration of the mandibular canal which caused injury to the neurovascular bundle (A., V. and N. alveolaris inferior), leading to bleeding, swelling, neurosensory alterations like hyperaesthesia and postoperative pain. In addition, it might negatively affect implant stability and can jeopardize the whole experiment. Because there is limited information about the size and shape of the mandibular canal in Göttingen Minipigs, the objective of this study was to provide detailed morphometric data using computed tomography and 3D visualization and compare it with human mandibles. By doing so, we contribute to the 3R principles. The results will help to avoid complications during surgery and to find animal-friendly surgical procedures and also facilitate the selection of suitable age groups.
MATERIALS AND METHODS
CT scans were collected from a group of 18 female minipigs consisting of six animals examined at the age of 12 months (357±39 d) (group 1) and 12 animals examined at 17 and 21 months (511±24 d and 620±37 d) (group 2 and group 3, respectively). Their body mass was measured using a decimal scale. The data was gathered on a 64-slice scanner (Lightspeed 64®; GE Medical Systems, Milwaukee, USA). Full-body CT scans with a slice thickness of 0.5 mm were taken in prone and supine positions. The image analysis was performed using Vitrea Advanced® from Vital Images Incorporated (Minnetonka, MN, USA). The following parameters of the mandibular canal and the alveolar ridge were defined and measured: Volume of the mandibular canal; Length of the mandibular canal; Maximal vertical depth; Maximal oblique depth; Maximal width of mandibular canal; Alveolar bone height; Inferior bone thickness and Alveolar ridge width.
The volume of the mandibular canal increases with age. Highly significant differences within animals of the same age groups could be observed. The biggest range was in group 3 (21 months), where the largest volume was 13.41 ml and the lowest one 4.71 ml, which is a percentage difference of 285%. In addition, the length and depth increase with age. The width, the inferior bone thickness and the alveolar ridge width do not significantly change over time. The alveolar bone height, a parameter that is very important when to correctly evaluate the available space for dental implant placement to avoid the penetration of the canal, diminishes over time, with the 12 months old minipigs having the highest distance. The body mass does not have an influence on any of the measured parameters.
Compared to humans, Göttingen Minipigs have a longer, deeper and wider mandibular canal. Humans have a circular shaped and very thin canal, whilst the canal in minipigs is oval-shaped and much larger. Older minipigs have a shorter alveolar bone height compared to humans. This should be taken into account when positioning a dental implant without risking the penetration of the mandibular canal. The increase in canal volume appears to be due to loss of deep spongy bone in the posterior premolar and molar regions. This reduces the available space for dental implantation, negatively affecting implant stability and potentially the integrity of the neurovascular
bundle. Dynamic anatomical changes occur until 21 months. This suggests, on ethical grounds, not using minipigs younger than 21 months in experimental implant dentistry. Paradoxically the measurements of the 12 months old pigs indicate a closer alignment of their mandibular anatomy to that of humans suggesting that they may be better models for implant studies. Given the variability in mandibular canal dimensions in similar age cohorts, the use of imaging techniques is essential for the selection of individual minipigs for dental prosthetic interventions as well as presumably for higher success rates and thus the use of fewer animals in the long run.