Nanosized Hydroxyapatite Coating on PEEK Implants Enhances Early Bone Formation: A Histological and Three-Dimensional Investigation in Rabbit Bone

Abstract

Polyether ether ketone (PEEK) has been frequently used in spinal surgery with good clinical results. The material has a low elastic modulus and is radiolucent. However, in oral implantology PEEK has displayed inferior ability to osseointegrate compared to titanium materials. One idea to reinforce PEEK would be to coat it with hydroxyapatite (HA), a ceramic material of good biocompatibility. In the present study we analyzed HA-coated PEEK tibial implants via histology and radiography when following up at 3 and 12 weeks. Of the 48 implants, 24 were HA-coated PEEK screws (test) and another 24 implants served as uncoated PEEK controls. HA-coated PEEK implants were always osseointegrated. The total bone area (BA) was higher for test compared to control implants at 3 (p < 0.05) and 12 weeks (p < 0.05). Mean bone implant contact (BIC) percentage was significantly higher (p = 0.024) for the test compared to control implants at 3 weeks and higher without statistical significance at 12 weeks. The effect of HA-coating was concluded to be significant with respect to early bone formation, and HA-coated PEEK implants may represent a good material to serve as bone anchored clinical devices.

Keywords: HA; biomaterial; histology; polyether ether ketone.

Figure 1

Insertion torque vs. distance for the coated and uncoated implants. The implant was inserted with its top at the level of the surrounding bone surface at 3.5 turns.

Figure 2

SEM images of a PEEK implant at the thread valleys (A) before insertion and (B) after insertion and removal. Scale bar = 200 nm.

Figure 3

SEM images of thread edge of implant after insertion and removal. (A) scale bar = 10 µm and (B) scale bar = 1 µm.

Figure 4

Schematic plan of the subdivision of micro-CT volume of interest (VOI). D1 is the closest to the threads and D3 is the one furthest away.

Figure 5

Histologic section of all threads presenting manifest bone ingrowth in all threads, indicative of osteoconductive features of the PEEK material (arrow). This actual section was from an HA-coated implant retrieved 12 weeks after surgery.

Figure 6

Histologic results. Masson Goldner-Trichrome stained sections of the implant after 3 and 12 weeks of healing. No soft tissue, or only sparse amounts (green), have migrated down between the implant and bone. Fibrous tissue (orange, arrow) can be viewed at the gap between the bone and implant, more commonly at the 3 week samples. (A) 3 weeks test (HA); (B) 3 weeks control; (C) 12 weeks test (HA); (D) 12 weeks control. Scale bar = 50 µm.

Figure 7

Bar graphs displaying the percentage of (A) bone area (BA) and (B) bone-to-implant contact (BIC) for test and control after 3 and 12 weeks of healing. Data are presented as mean with standard deviation. Mann Whitney U test was used for testing level of significance. (*, p ≤ 0.05).

Figure 8

3D rendered image of the PEEK implant.

Figure 9

Illustration of implant location (A) and exposed surgical site with implant in position (B).