J Korean Assoc Oral Maxillofac Surg.  2019 Apr;45(2):97-107. 10.5125/jkaoms.2019.45.2.97.

Are critical size bone notch defects possible in the rabbit mandible?

• Carlisle P
• Guda T
• Silliman D
• Hale RG
• Brown Baer PR

• KMID: 2467153
• DOI: http://doi.org/10.5125/jkaoms.2019.45.2.97

Abstract

OBJECTIVES
Small animal maxillofacial models, such as non-segmental critical size defects (CSDs) in the rabbit mandible, need to be standardized for use as preclinical models of bone regeneration to mimic clinical conditions such as maxillofacial trauma. The objective of this study is the establishment of a mechanically competent CSD model in the rabbit mandible to allow standardized evaluation of bone regeneration therapies.
MATERIALS AND METHODS
Three sizes of bony defect were generated in the mandibular body of rabbit hemi-mandibles: 12 mm×5 mm, 12 mm×8 mm, and 15 mm×10 mm. The hemi-mandibles were tested to failure in 3-point flexure. The 12 mm×5 mm defect was then chosen for the defect size created in the mandibles of 26 rabbits with or without cautery of the defect margins and bone regeneration was assessed after 6 and 12 weeks. Regenerated bone density and volume were evaluated using radiography, micro-computed tomography, and histology.
RESULTS
Flexural strength of the 12 mm×5 mm defect was similar to its contralateral; whereas the 12 mm×8 mm and 15 mm×10 mm groups carried significantly less load than their respective contralaterals (P<0.05). This demonstrated that the 12 mm×5 mm defect did not significantly compromise mandibular mechanical integrity. Significantly less (P<0.05) bone was regenerated at 6 weeks in cauterized defect margins compared to controls without cautery. After 12 weeks, the bone volume of the group with cautery increased to that of the control without cautery after 6 weeks.
CONCLUSION
An empty defect size of 12 mm×5 mm in the rabbit mandibular model maintains sufficient mechanical stability to not require additional stabilization. However, this defect size allows for bone regeneration across the defect. Cautery of the defect only delays regeneration by 6 weeks suggesting that the performance of bone graft materials in mandibular defects of this size should be considered with caution.

Keyword

Bone regeneration; Rabbit; Mandible; Micro-computed tomography; Cautery

MeSH Terms

Animals
Bone Density
Bone Regeneration
Cautery
Mandible*
Rabbits
Radiography
Regeneration
Transplants

Figure

• 

  Fig. 1 Bone defect creation in the mandibular body. A, B. The 12 mm×5 mm defect was created in the region just anterior to the antegonial notch and gutta-percha markers were included 5 mm above the initial defect corners as radiographic markers. C, D. The bone defect is created by making two vertical cuts 12 mm apart and 5 mm deep using an oscillating saw to remove the inferior cortex and marrow of the mandibular body.

• 

  Fig. 2 Radiographic and micro-computed tomography (micro-CT) assessment of hemi-mandibles. Radiographs of the contralateral hemimandibles showing intact architecture and representative images from the three experimental groups showing regenerated bone after 6 or 12 weeks post defect creation. Micro-CT longitudinal sectional images of representative animals from each of the experimental groups showing bone regeneration within defect sites (boxed) compared to non-surgical control sites shown in the contralateral hemi-mandible (boxed).

• 

  Fig. 3 Bone volume at defect site in micro-computed tomography (micro-CT) analysis. The bone volumes in the defect groups without cautery at 6 weeks and with cautery at 12 weeks were not significantly different from their contralateral controls, but the bone volume in the defect group with cautery was significantly lower compared to the contralateral hemi-mandible after 6 weeks (*P<0.05).

• 

  Fig. 4 Bone regeneration patterns within the defect in micro-computed tomography (micro-CT) analysis. The bone volume regenerated within each group was evaluated at every 17 µm along the defect from the posterior to the anterior margin. This evaluation showed that while there was no difference in regeneration patterns between the defect with no cautery at 6 weeks (in blue) and the defects with cautery after 12 weeks (in green), significantly less bone was regenerated at every location in the defects with cautery after 6 weeks (in red) compared to the other two groups.

• 

  Fig. 5 Histological evaluation showing bone regeneration, intercanal ossification, and cellular debris. Histological sections were prepared along the longitudinal axis of the hemi-mandibles and stained with Sanderson's Rapid Bone stain and counterstained with van Geison's stain to label mineralized tissue pink and soft tissue blue (×2). While a ridge of bone regenerated the cortex in the defects without cautery at 6 weeks and in the defects with cautery at 12 weeks, no such regeneration was seen in the defects with cautery after 6 weeks (orange arrows). At the defect margins, cellular debris was observed in the groups with cautery of the margins (yellow arrows). While the hemimandibles had clear tooth canals, the tooth canals in the defects without cautery had ossification after 6 weeks (black arrows), possibly due to a robust regenerative response from the periodontal tissues (white arrows).

• 

  Fig. 6 Histomorphometric analysis of bone area regenerated at the defect site. While no significant differences were observed between the different groups in terms of the area of bone regeneration, a strong trend (P=0.16) of lower mineralized area was observed in the groups with cautery compared to the group without cautery or the contralateral hemi-mandible.

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