The Impact of Biodegradable Barrier Membrane in Tissue Engineering-Based Reconstruction of Critical-Size Mandibular Defects

Recent research has shown some promises of using tissue engineering techniques for regenerating of large craniofacial bone defects. A typical tissue engineering procedure involves delivery of stem cells and/or growth factors (GFs) with scaffold materials. As a common scaffold material, β-tricalcium phosphate (βTCP) granules are osteoconductive, biodegradable and affinitive to cells, but long-term retention of the βTCP granules within the defect area can be uncertain especially with active jaw movements during chewing. A potential solution to this problem is to use a biodegradable barrier membrane to seal the defect after impacting it with βTCP granules. The aim of this study was to assess bone regeneration and βTCP degradation at the defect sites with or without overlying biodegradable barrier membrane.

Six three-month-old domestic pigs underwent bilateral mandibular osteotomy for creation of critical-size (5cm³) defects, which were supraperiosteal buccally but subperiosteal lingually. All defects were subsequently grafted with one of three construct combinations: βTCP granules alone (control), βTCP+autologous mesenchymal stem cells (MSCs) (T1) and βTCP+MSCs+controlled-release GFs (T2). Autologous MSCs were isolated and expanded from bone marrow of the tibia. An average of 3.5x10⁷MSCs were integrated with 5cc CELLstart-coated-βTCP and cultured with serum-free media 24h before transplantation. GFs bone morphogenetic protein-2 (BMP-2) and vascular endothelial growth factor (VEGF) were encapsulated in biodegradable poly(lactic-co-glycolic acid) polymeric microspheres. GFs were mixed with βTCP+MSCs immediately before transplanting into surgical site. After impacting the graft materials, the buccal side of all βTCP (control) and half of T1/T2 surgical sites was sealed by Tissell and closed by suturing overlying soft tissues; while the other half of T1/T2 sites were sealed with an Inion GTR membrane before soft-tissue closure. Twelve weeks after surgery, specimens containing the critical-size defect areas underwent computed tomography (CT) scan for geometric analysis, micro-CT scan for bone density analysis, and histology for analysis of βTCP degradation. Abundance of βTCP regions were graded by two raters semiquantitatively.

Kruskal-Wallis tests were used to compare the differences of all parameters among varied construct groups (control, T1, T2) and between different defect closure methods (with/without membrane). Significance level was 0.05.

Volumetric CT images showed that the number of surgical sites with through-and-through defect reduced with MSC treatments, while adding GFs further augment its efficiency. Experimental sites closed with membranes showed smaller gap width/volume than those without membranes. Bone mineral density reflected by the average gray level values showed statistical differences among different groups in the middle portion of the defects, where the measurements were the highest in T2, followed by T1, then the control group. Experimental sites closed with and without the use of membranes showed similar average gray values, which were both greater than that of the control group. The relative amount of residual βTCP inside the defect regions were higher in T1 and T2 than in the control group, but after separating the experimental sites into subgroups according to membrane use, a significantly higher residual βTCP was seen in sites closed with membranes than those without. 

In conclusion, it is feasible to use βTCP granules integrated with autologous MSCs and slow-release growth factors to reconstruct critical-size jaw defects. Using biodegradable membranes to seal the grafts helps retention of βTCP granules but also prolongs the time of their degradation. 

References:

1. Seebach C, Henrich D, Kahling C, et al. (2010) Endothelial progenitor cells and mesenchymal stem cells seeded onto beta-TCP granules enhance early vascularization and bone healing in a critical-sized bone defect in rats. Tissue Eng Part A 16: 1961–1970

2. Zhang W, Zhu C, Wu Y, et al. (2014) VEGF and BMP-2 promote bone regeneration by facilitating bone marrow stem cell homing and differentiation. Eur Cell Mater 27: 1–12