On hydroxyapatite modified peek implants for bone applications

Abstract: People are living longer today than ever before, and the number of older adults will increase exponentially over the next coming years. However, that does not necessarily mean that they are living healthier. An aging population presents many opportunities but also challenges in the public health care with a sharp increase in incidence of many diseases associated to the bone, teeth and joint systems. Orthopedic and dental implants commonly used mostly consist of metals, composites, polymers and ceramics. Titanium has been widely used in the orthopedic and dental field to replace or support mineralized structures. Titanium possesses good mechanical, physical and biologic properties. However, titanium has in the clinic presented some critical drawbacks. The modulus of elasticity is higher than that of bone which may cause stress shielding and bone resorption ultimately jeopardizing the implant retention. In addition, with rare cases of titanium allergy and patient desires for non-metallic treatments, alternatives to titanium are being studied. Furthermore, the radiopacity of titanium causes interference with the radiographic evaluation and the non-esthetic color may discolor the gingiva of thin biotypes. Recently, advanced polymeric implantable materials have been introduced into the medical field. Polyetherether ketone (PEEK) is widely used in various orthopedic applications, such as spine implants, joint replacements and fracture fixations. In dentistry, PEEK can be manufactured as prosthetic components, fixed and removable bridges and dentures. PEEK is a thermoplastic polymer with mechanical properties that are close to those of human bone, for example the elastic modulus. This will minimize the stress shielding upon loading and stress distribution. Besides, PEEK holds excellent biocompatibility with no toxic or mutagenic effects in vitro and in vivo. However, unmodified PEEK is biologically inert with limited pro-tein and cell adhesion to the surface. Therefore, improving the bio-activity of PEEK is a significant challenge and a prerequisite to possess all of its potentials as a biomaterial. There are currently 2 methods to improve the bioactivity of PEEK; surface modification and composite preparation. Hydroxyapatite (HA) is the most widely used material due to its biocompatibility and osteoconductive potential in association with biomaterials. There are many developed and evaluated methods to deposit HA on metallic implants, but the only commercially used one is plasma spraying. However, the technique has some severe disadvantages such as irregular coating thickness with deficient adherence to the implant surface. Clinical complications have been confirmed with inflammation caused by fragments of delaminated coating. To challenge these disadvantages, several alternative coating techniques have been introduced. The aim of this thesis was to evaluate a novel surface modification of bioactive hydroxyapatite deposited by a spincoating technique. The technique generates a crystalline, nanosized coating homogenously distributed over the implant surface. In particular, the thesis encompasses a comprehensive topographic analysis to evaluate the coating properties. In addition, the HAPEEK implant will be compared in vivo with an unmodified control with respect to its oste-oconductive and bioactive properties. In study I, the surface topography was characterized using interferometry (IFM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) of the same types of implants that were evaluated in the experimental animal models. The chemical characterization before and after the coating procedure was evaluated with X-ray photon spectroscopy (XPS). Mechanical testing of implants were completed in accordance with the tensile testing standard (ISO 527-2). Furthermore, the hydrophilic appearance after coating was estimated by measuring the contact angle. The results from the ma-terial characterization reveal a hydrophilic coating with a minimally rough surface which preserved its mechanical properties after the coating procedure. The chemical analysis revealed that only Ca, P, C and O were present at the surface of HA-PEEK with a proportion as that of human bone. In study I and IV, PEEK implants were implanted in the rabbit tibia and femur and retrieved after 3, 12 and 20 weeks of submerged heal-ing and subjected to biomechanical analysis using a removal torque device. The removal torque required to unscrew the implants was significantly higher for HA-PEEK after the 3 and 12 week retrieval time points. However, the absolute torque values were lower at 12 weeks compared to earlier time points, presumably caused by absence of primary stability and dissolution of the HA-coating. After 20 weeks of healing the absolute values were even lower than for the implants recovered at 3 and 12 weeks. However, the retention was still significantly increased for the HA-PEEK implants. In study II, III and IV, the osseointegration was histologically evaluated with respect to boneimplant contact (BIC) and bone area (BA) after the same healing time points as aforementioned. The performance of the HA-coating was most significant 3 weeks after implantation. However, after 12 and 20 weeks, the BIC turned out to be comparable between the groups. Furthermore, the implants implanted in femur were designed with an apical perforation to evalu-ate the osteoconductivity in terms of area of bone. After 3 and 12 weeks the results revealed a significant improvement on the HA-PEEK. However, after an extended healing period of 20 weeks, the unmodified implants were presenting comparable results as the HA-PEEK group. A more comprehensive description of the osseointegration can be achieved using computed microtomography. Three different diameters around the implant and inside the apical perforation were evaluated with respect to bone density and trabecular properties. However, there were no detectable differences between the PEEK and HA-PEEK. The outcome was limited to the resolution of X-rays. There was a minor contrast between resin and PEEK. The implant surface, particularly the thread edges are subjected to mechanical forces during implant placement and removal. The stability of the coating was evaluated in study II. The coating was wellpreserved in deformities of the surface, but on the flat edges of the threads there were signs of deformation and absence of HA crystals. This thesis demonstrated that a nanosized HA-coating can be pro-cessed using a spin-coating technique with nanostructures and suffi-cient adhesion to the substrate. Furthermore, the performance of the HA-coating was found to improve the surface osteoconductivity by increasing the level and speed of osseointegration. However, the effect was most prominent in the early stages of healing whereas the implants from the extended time points showed osteoconductive properties comparable to unmodified PEEK. In addition, the ability of HA-coating to improve bone fusion inside a perforation was the most clinical relevant outcome. Unmodified PEEK implants are today widely used in spine surgery and the present surface modifications may improve the clinical outcomes where a rapid bone formation is essential.