Minipig model of maxillary distraction osteogenesis: immunohistochemical and histomorphometric analysis of the sequence of osteogenesis

Cyclic loading failed to promote growth in a pig model of midfacial hypoplasia

Yucatan miniature pigs, often used as large animal models in clinical research, are distinguished by a breed-specific midfacial hypoplasia with anterior crossbite. Although this deformity can be corrected by distraction osteogenesis, a less invasive method is desirable. We chose a mechanical cyclic stimulation protocol that has been successful in enhancing sutural growth in small animals and in a pilot study on standard pigs. Yucatan minipigs (n = 14) were obtained in pairs, with one of each pair randomly assigned to sham or loaded groups. All animals had loading implants installed on the right nasal and frontal bones and received labels for cell proliferation and mineral apposition. After a week of healing and under anesthesia, experimental animals received cyclic tensile loads (2.5 Hz, 30 min) delivered to the right nasofrontal suture daily for 5 days. Sutural strains were recorded at the final session for experimental animals. Sham animals received the same treatment except without loading or strain gauge placement. In contrast to pilot results on standard pigs, the treatment did not produce the expected sutural widening and increased growth. Although sutures were not fused and strains were in the normal range, the targeted right nasofrontal suture was narrowed rather than widened, with no statistically significant changes in sutural cell proliferation, mineral apposition, or vascularity. In general, Yucatan minipig sutures were more vascular than those of standard pigs and also tended to have more proliferating cells. In conclusion, either because the sutures themselves are abnormal or because of growth restrictions elsewhere in the skull, this cyclic loading protocol was unable to produce the desired response of sutural widening and growth. This treatment, effective in normal animals, did not improve naturally occurring midfacial hypoplasia in Yucatan minipigs.

Skeletal and soft tissue response to automated, continuous, curvilinear distraction osteogenesis

PURPOSE

To document the bone formation and soft tissue changes in response to automated, continuous, curvilinear distraction osteogenesis (DO) at rates greater than 1 mm/day in a minipig model.

MATERIALS AND METHODS

Two groups of Yucatan minipigs underwent automated, continuous, curvilinear DO of the right mandible: group A, 1.5 mm/day (n = 5); and group B, 3.0 mm/day (n = 5). Each minipig underwent 12 mm of distraction followed by 24 days of fixation. The distracted and contralateral mandibles were harvested at the end of fixation. The percentage of surface area (PSA) of the regenerate occupied by bone, fibrous tissue, cartilage, and hematoma was determined using computerized histomorphometric analysis. The control groups consisted of DO wounds distracted discontinuously at 1 mm/day and the nonoperated contralateral mandible. The ipsilateral and contralateral digastric muscles were harvested and stained for proliferating cell nuclear antigen (PCNA), myogenic differentiation-1 (MyoD), and paired Box 7 protein (PAX7).

RESULTS

All 10 minipigs completed the distraction and fixation period. The PSA occupied by bone was similar for groups A (PSA 64.36% ± 5.87%) and B (PSA 63.83% ± 3.37%) and the control group (1 mm/day; PSA 64.89% ± 0.56%) but was less than that on the nonoperated side (PSA 84.67% ± 0.86%). The PSA occupied by cartilage and hematoma in all groups was minimal (<1.1%). The digastric muscles had no abnormal tissue or inflammation, and PAX7, MyoD, and PCNA expression had returned to the baseline levels.

CONCLUSIONS

The results of the present study indicate that bone formation in response to automated, continuous, and curvilinear DO at a rate of 1.5 and 3.0 mm/day is nearly identical to that with discontinuous DO at 1 mm/day. In addition, no deleterious effects were found on the digastric muscles.