Evaluation of soft tissue coverage over porous polymethylmethacrylate space maintainers within nonhealing alveolar bone defects

Abstract

Current treatment of traumatic craniofacial injuries often involves early free tissue transfer, even if the recipient site is contaminated or lacks soft tissue coverage. There are no current tissue engineering strategies to definitively regenerate tissues in such an environment at an early time point. For a tissue engineering approach to be employed in the treatment of such injuries, a two-stage approach could potentially be used. The present study describes methods for fabrication, characterization, and processing of porous polymethylmethacrylate (PMMA) space maintainers for temporary retention of space in bony craniofacial defects. Carboxymethylcellulose hydrogels were used as a porogen. Implants with controlled porosity and pore interconnectivity were fabricated by varying the ratio of hydrogel:polymer and the amount of carboxymethylcellulose within the hydrogel. The in vivo tissue response to the implants was observed by implanting solid, low-porosity, and high-porosity implants (n = 6) within a nonhealing rabbit mandibular defect that included an oral mucosal defect to allow open communication between the oral cavity and the mandibular defect. Oral mucosal wound healing was observed after 12 weeks and was complete in 3/6 defects filled with solid PMMA implants and 5/6 defects filled with either a low- or high-porosity PMMA implant. The tissue response around and within the pores of the two formulations of porous implants tested in vivo was characterized, with the low-porosity implants surrounded by a minimal but well-formed fibrous capsule in contrast to the high-porosity implants, which were surrounded and invaded by almost exclusively inflammatory tissue. On the basis of these results, PMMA implants with limited porosity hold promise for temporary implantation and space maintenance within clean/contaminated bone defects.

FIG. 1.

Porosity values as calculated by microcomputed tomography (μCT). Samples were scanned and the resultant scans were reconstructed, reoriented, and binarized. Implant porosity was determined using a cylindrical (9 mm diameter ×5 mm height) volume of interest slightly smaller than the implant dimensions to eliminate edge effects. Data are reported as means ± standard deviation (n = 3). The asterisk over a bar denotes a statistically significant difference (p < 0.05) as detected using analysis of variance and Tukey's post hoc tests between the group marked and the group with the same % carboxymethylcellulose (CMC) but lower aqueous phase in relation to polymer phase. No statistically significant differences in porosity were found as a result of changing the % carboxymethylcellulose from 7% to 9%.

FIG. 2.

Implant interconnectivity percentages as a function of the minimum interconnection size. Samples were scanned and processed as reported in the Materials and Methods section, and a built-in software package was used to determine the percentage of the implant porosity that was accessible from outside the volume of interest. Data are reported as means ± standard deviation (n = 3).

FIG. 3.

Representative images of implant cross sections and surfaces. Cylindrical implants (10 mm diameter × 6 mm height) from each experimental group were scanned by μCT or scanning electron microscopy (SEM). Virtual μCT cross sections of the implants were made by slicing through the center of the axially oriented implant. For electron micrographs, the scale bars represent 500 μm.

FIG. 4.

Representative gross views of harvested tissue covering the alveolus and implant. (A) Failure of wound healing over a solid polymethylmethacrylate (PMMA) implant is shown. The exposed implant is denoted by white arrows. (B, C) Well-healed soft tissue covering the intraoral exposure is seen where dentition is missing over low-porosity (B) and high-porosity (C) implants.

FIG. 5.

Representative light micrographs (25 × magnification) of coronally sectioned tissue samples through the center of the (A) solid PMMA, (B) low-porosity, (C) high-porosity space maintainers. The intraoral exposure of the solid implant (A) is shown with black arrows. Blue arrows denote the titanium plate used to stabilize the mandible. Tissue ingrowth is seen within both porous implants; for all implants, the original defect space appears well maintained with minimal tissue collapse or contracture. In (B) and (C), soft tissue discontinuities at the left (buccal) side of the implant capsule are due to embedding and processing artifacts. Scale bars represent 1 mm.

FIG. 6.

Representative light micrographs (200 × magnification) of the lingual surface of coronally sectioned tissue samples through the center of the implanted (A) solid PMMA, (B) low-porosity, and (C) high-porosity space maintainers. Regenerated bone is seen near the surface of all implants. A well-formed capsule is seen in (A), while only a thin layer of loosely organized fibrous is seen at the surface of the low-porosity space maintainer (B). An abundance of plasma cells is seen at the surface and penetrating the surface porosity of the highly porous space maintainer (C). Scale bars represent 100 μm for the larger images; the inset scale bar represents 25 μm.

FIG. 7.

Score distributions for the graded (A) implant interface for all formulations tested in vivo and (B) tissue response within pores for the two porous implant formulations. Scoring criteria are defined in Table 1. Statistically significant differences (p < 0.05) between groups, denoted by asterisk, were determined using pairwise Dwass–Steel–Critchlow–Fligner tests for the implant interface scoring (A) and a Mann–Whitney U-test for the tissue response within pores (B).