Evaluation of tissue ingrowth and reaction of a porous polyethylene block as an onlay bone graft in rabbit posterior mandible

Abstract

Purpose: A new form of porous polyethylene, characterized by higher porosity and pore interconnectivity, was developed for use as a tissue-integrated implant. This study evaluated the effectiveness of porous polyethylene blocks used as an onlay bone graft in rabbit mandible in terms of tissue reaction, bone ingrowth, fibrovascularization, and graft-bone interfacial integrity.

Methods: Twelve New Zealand white rabbits were randomized into 3 treatment groups according to the study period (4, 12, or 24 weeks). Cylindrical specimens measuring 5 mm in diameter and 4.5 mm in thickness were placed directly on the body of the mandible without bone bed decortication, fixed in place with a titanium screw, and covered with a collagen membrane. Histologic and histomorphometric analyses were done using hematoxylin and eosin-stained bone slices. Interfacial shear strength was tested to quantify graft-bone interfacial integrity.

Results: The porous polyethylene graft was observed to integrate with the mandibular bone and exhibited tissue-bridge connections. At all postoperative time points, it was noted that the host tissues had grown deep into the pores of the porous polyethylene in the direction from the interface to the center of the graft. Both fibrovascular tissue and bone were found within the pores, but most bone ingrowth was observed at the graft-mandibular bone interface. Bone ingrowth depth and interfacial shear strength were in the range of 2.76-3.89 mm and 1.11-1.43 MPa, respectively. No significant differences among post-implantation time points were found for tissue ingrowth percentage and interfacial shear strength (P>0.05).

Conclusions: Within the limits of the study, the present study revealed that the new porous polyethylene did not provoke any adverse systemic reactions. The material promoted fibrovascularization and displayed osteoconductive and osteogenic properties within and outside the contact interface. Stable interfacial integration between the graft and bone also took place.

Keywords: Bone regeneration; Bone-implant Interface; Histology; Interfacial shear strength; Polyethylenes; Rabbit.

Figure 1. The onlay bone graft implantation procedure in which the cylindrical specimens were directly fixed on the mandible body of the rabbit by a titanium screw.

Figure 2. Scanning electron micrographs depicting the microstructure of porous polyethylene. (A) Low magnification. (B) High magnification.

Figure 3. Histological analysis of the ingrowth of tissues in porous polyethylene after it was onlay-grafted on rabbit mandible. Hematoxylin and eosin staining demonstrates the ingrowth of NB (arrows or NB) and F in the pores of PE. (A) Gross appearance. (B) Four weeks after implantation. (C) Twelve weeks after implantation. (D) Twenty-four weeks after implantation. (E) Enlarged view of fibrous tissue with abundant blood vessels growing into the pores of porous polyethylene. (F) Enlarged view of the ingrowth of NB displaying an oriented lamellar structure and a lining of osteoblasts (arrow heads) at the edges. The dotted oval line indicates blood vessels.

NB: new bone, F: fibrous tissue, PE: polyethylene.

Figure 4. Ingrowth depth of fibrous tissue and bone from the implant-mandibular bone interface into porous polyethylene at each time point after implantation (error bars= standard deviation, n=3). No significant differences were found between 4 weeks and 24 weeks for fibrous tissue, between 4 weeks and 12 weeks for bone, or between 4 weeks and 24 weeks for bone (P=0.414, P=0.437, and P=0.222 respectively).

^a)Significant difference, P<0.05.

Figure 5. Shear strength of the implant-mandibular bone interface at each time point after implantation (error bars=standard deviation, n=2). No significant differences among time points were found (P=0.270).

Figure 6. Representative SEM and their corresponding EDS mapping images of elemental calcium on the fracture surfaces of the porous polyethylene side after interfacial shear testing. Evidence is shown of ingrowth of tissue into the pores of polyethylene (*) and elongation of the tissue due to the breakage. Some yielding and deformation of the polyethylene structure due to the breakage is also observed (arrows). (A) SEM, 4 weeks after implantation. (B) EDS mapping image, 4 weeks after implantation. (C) SEM, 12 weeks after implantation. (D) EDS mapping image, 12 weeks after implantation. (E) SEM, 24 weeks after implantation. (F) EDS mapping image, 24 weeks after implantation.

SEM: scanning electron microscope, EDS: energy-dispersive X-ray spectroscopy.

Figure 7. Representative scanning electron micrographs of the fracture surfaces of the mandibular bone side after interfacial shear testing. The location of the fixation of the cylindrical implant on the M is clearly evidenced. NB formation encompassing the outer surface of the porous polyethylene implant is also identified. (A) Four weeks after implantation. (B) Twelve weeks after implantation. (C) Twenty-four weeks after implantation.

M: mandible, NB: new bone.