Skeletal unit construction of rat mandible based on the masticatory muscle anatomy and double microcomputed tomography

Transition of endochondral bone formation at the normal and botulinum-treated mandibular condyle of growing juvenile rat

OBJECTIVE

The aim of this study was to understand the temporal and spatial distribution of canonical endochondral ossification (CEO) and non-canonical endochondral ossification (NCEO) of the normal growing rat condyle, and to evaluate their histomorphological changes following the simultaneous hypotrophy of the unilateral masticatory closing muscles with botulinum toxin (BTX).

DESIGN

46 rats at postnatal 4 weeks were used for the experiment and euthanized at postnatal 4, 8, and 16 weeks. The right masticatory muscles of rats in experimental group were injected with BTX, the left being injected with saline as a control. The samples were evaluated using 3D morphometric, histological, and immunohistochemical analysis with three-dimensional regional mapping of endochondral ossifications.

RESULTS

The results showed that condylar endochondral ossification changed from CEO to NCEO at the main articulating surface during the experimental period and that the BTX-treated condyle presented a retroclined smaller condyle with an anteriorly-shifted narrower articulating surface. This articulating region showed a thinner layer of the endochondral cells, and a compact distribution of flattened cells. These were related to the load concentration, decreased cellular proliferation with thin cellular layers, reduced extracellular matrix, increased cellular differentiation toward the osteoblastic bone formation, and accelerated transition of the ossification types from CEO to NCEO.

CONCLUSION

The results suggest that endochondral ossification under loading tended to show more NCEO, and that masticatory muscular hypofunction by BTX had deleterious effects on endochondral bone formation and changed the condylar growth vector, resulting in a retroclined, smaller, asymmetrical, and deformed condyle with thin cartilage.

Three-dimensional radiographic and histological tracking of rat mandibular defect repair after inferior alveolar nerve axotomy

OBJECTIVE

To sequentially track mandibular defect repair by using radiographic and histological techniques, so as to compare repair patterns of sensory denervated versus innervated mandibles.

DESIGN

Forty Sprague-Dawley rats were subjected to unilateral inferior alveolar nerve (IAN) axotomy and bilateral 3 mm full-thickness circular osteotomy of their mandibles. Micro-CT and histological staining were applied to track the repair process of the mandibular defects at 1, 2, 4, and 8 weeks after surgery.

RESULTS

The bone volume of both sides increased by 2 weeks post-operation, and then gradually decreased. The new bone volumes of the axotomy side were significantly less than that of the sham side at 1, 2, and 4 weeks post-surgery, whereas no significant differences were detected at 8 weeks post-surgery. Meanwhile, there were no significant differences in bone mineral density between the two sides during repair. Noteworthy, the repaired bone remained more vertically than horizontally aligned throughout the repair process.

CONCLUSION

IAN axotomy decreases the quantity of bone calluses during the early stage of mandibular defect repair, but with no effect on the degree of mineralization. The shape of the defect area appeared to be aligned with the direction of local mechanical force produced by masticatory muscles.