Key Engineering Materials Vols. 396-398 (2009) pp 331-335 online at http://www.scientific.net © (2009) Trans Tech Publications, Switzerland
Hydroxyapatite coating improves bone integration and interface strength of polymer implants in bone J. Brandt1, a, M.Pfennig1 , C.Bieroegel2, c, W.Grellmann2, d, A. Bernstein1, e 1
University of Halle, Department of Orthopaedics, Magdeburger Straße 22, 06097 Halle/Saale Germany 2 University of Halle, Department of Materials Science, Geusaer Straße 88, 06217 Merseburg Germany a
[email protected],
[email protected], c
[email protected],
[email protected],
Keywords: PEEK, HA coating, polymer implants, histomorphometry, interface
Introduction Many attempts had been made to improve the durability of artificial joint replacement and other orthopaedic implants by approaching the mechanical properties of bone and artificial material. The most joint prostheses used today are manufactured of metal alloys based on cobalt, chromium or titanium. The mechanical stiffness of these materials is much higher than that of natural bone resulting in adverse effects such as local overloading on one hand or stress shielding phenomena with the lack of adequate mechanical load on the other. Both mechanisms contribute to earl loosening and failure of implants. Polymer materials may deliver mechanical properties very similar to bone and their mechanical behaviour may be modified in a wide range during the process of manufacturing. First attempts to lower the stiffness of the implant material and to gain the stiffness range of natural bone were made in the seventies by R. Matthys with his concept of “isoelastic hip prosthesis”. In this prosthesis the femoral stem was manufactured of polyacetal, a thermoplastic polymer with very good biocompatibility and elastic properties which are much nearer to bone than common metal alloys. While the prosthesis showed good results during the mechanical testing the clinical use in vivo became a disaster. Shortly after implantation polyacetal was degraded in the body and broke down under the immense loading of the human hip joint. Later attempts to use polymer materials alone for load bearing implants also failed in clinical practice over a long time because the mechanical interlocking between bone and implant was not sufficient for the biological demand. To make the outstanding properties of polymer materials useable for load bearing implants they are backed with metal alloys (as polyethylene for hip joint cups) until the presence. Only recent developments of polymer science succeeded in the use of polymers for loaded implants. One of the most interesting materials seems to be the polyetheretherketone (PEEK) which is successfully used for spinal fusion cages [2] and computerdesigned individual implants for defect reconstruction in the skull [4] meanwhile. A pre-clinical study of a new anatomically shaped flexible acetabular cup reported satisfactory results recently [3]. Materials and Methods Medical grade (PEEK) was reinforced with short length carbon fibers and extruded in a common procedure to small cylindric specimens of 5 mm diameter and a length of 20 mm. The cylinders were either coated with medical grade titanium and hydroxyapatite or left native without any coating. The metal and ceramic coating could be perfomed in the plasma spraying procedure because PEEK is a polymer with relatively high resistance against short heating. Medical grade titanium rods with the same dimensions served as a control. All rods were gas sterilized using ethylene oxide and kept in sterile package until the operation procedure. All specimens were implanted into the distal femora of New Zealand white rabbits with the same surgical procedure. Under general anaesthesia and aseptic surgical conditions the lateral aspect of the distal femur was All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 141.48.250.91-10/09/08,15:09:58)
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dissected and a 5,2 mm diameter hole was drilled into the distal femoral condyles using drills with stepwise increasing diameter to avoid fractures or cracks in the very brittle cortical bone of the rabbits. After intense rinsing the drill holes with Ringer’s solution to remove the shavings as good as possible. The specimens were than anchored in the defect by press fitting and the wound was closed according to the anatomical layers and covered with a sterile wound sealing spray. The animals got an intramuscular analgetic twice a day for a period of three days to avoid pain reactions. The rabbits had been sacrificed after 2, 4, 6, 8 and 12 weeks and the formation of bone was evaluated in the defect zone around the implant. The bilateral surgical procedure was performed in a total of 8 animals per polymer material and implantation period, every animal received a control implant of titanium. After sacrificing the animals femoral bone was dissected from the surrounding soft tissue and processed for histological staining by fixation with increasing concentrations of ethanol (40%-100%) prior to defatting in Rotihistol® and embedding in polymethylmethacrylate (PMMA) resin. After polymerisation the samples were sliced perpendicular to the long axis using a diamond cutting device (Exakt, Norderstedt, Germany). Subsequently the specimens had been grinded stepwise and finally polished with a diamond paste upto a surface roughness of about 0.5µm. The slices had been mountd on slides using a polymer glue. After sure fixation the slides were stained with Masson´s trichrome staining for examination in conventional light microscopy. Using a Zeiss Axioplan 2 microscope the fate of implants as well the reaction of the host bone to the implanted material had been assessed by conventional qualitative evaluation at first. We characterized the process of bone formation around the implants starting with the generation of osteoid and its stepwise calcification. In the surrounding of the material we looked for the formation of bonding tissue, the presence of inflammatory reactions or multinucleated giant cells a. In a second step we performed an quantification of new bone formation around the implant with halfautomized histomorphometry. Therefore the specimens were scanned using a digital video camera and an electrically driven scanning table. A macro based software was developed to define a region of interest independent of the investigator which comprehends the near sourrounding of the implant and covering about the space of the former drill hole. By image processing in this region of interest we segmented in different steps unmineralized (osteoid), mineralized bone and the implant itself by colour detection using the HLS colour space. The process of segmentation runs automatically but with the possibility of interactive intervention to verify the segmented tissue regions thus minimizing influences of staining variations. Once gaining a digital image of the segmented region the software automatically runs up to the definite measurement value which is stored in a database. To evaluate the quantity of bone healing we computed common histomorphometric values as bone volume/total voulme coefficient and the percentage of implant surface covered by bone. To evalute the qualitiy of mechanical interlocking of bone and implant the shear strength of the interface had been quantified by push-out test procedure in specimens left native. For push-out testing we used a Zwick Z 020 testing machine (Fa. Zwick, Ulm) with a special specimen holder developed for small animal investigation and described previously [1]. Results and Discussion The process of bone healing showed no principal differences between titanium and polymer implants. A physiological progress could be observed concerning the formation of new bone in the defect zone around the implant and the development and maturation of organic and inorganic bone matrix. All materials were fully biocompatible, extended inflammatory reactions or other adverse effects did not happen. Two weeks after implantation of we found in the titanium implants thin, single standing trabeculae of young woven bone which are overall covered by osteoid. The trabeculae are lined in a ring-shaped, concentric pattern around the implant. The newly formed bone shows slight contact points to the implant surface on one hand and to the surrounding host bone on the other. In the uncoated PEEK implants we could observe a similar ring of small trabeculae with osteoid at the surface but in contrast to the titanium they had mostly no direct contact to the implant
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surface. In HA-coated PEEK implants we found broader trabeculae in intense contact with the implant surface forming pseudopodia-like pedicles which will be characteristic for all other times of investigation as well (fig. 1).
Fig.1: Left: PEEK implant, 2 weeks after implantation, Mason-Goldner, 10x Right: intense contact of bone trabeculae to the PEEK/HA surface (arrows) two weeks after implantation, Mason-Goldner, 40x. bone: dark grey, osteoid: light grey, implant: black
Four weeks after implantation in titanium implants the trabeculae enlarged in width and number and showed broader connections to the host bone. At the contact areas to the implant often remains a tiny gap. Also around the PEEK implants the trabeculae are widened and better connected and a small gap between bone and implant could often be observed. In the HA coated implants the trabeculae show pseudopodia-like enlargement at the surface and seem to creep over it. After eight weeks implantation the trabeculae in the drill hole around the implant reached nearly the same size like the original bone stock and anchored with broad pedicles immediately at the surface of titanium implants. Lamellar bone can be found in the interior of many trabeculae. In PEEK implants the trabeculae found direct contact to the implant surface only in part. Often remained a small gap between bone and implant. In case of HA coating the contact between bone and implant was more intensively, we observed mainly direct contact and the trabeculae enclosed the implant surface with broad pseudopodia-like pedicles. At the survival of twelve weeks the picture was similar to the findings after eight weeks, but we observed trabeculae mainly composed of matured lamellar bone. Histomorphometry revealed a high amount of the newly formed bone two weeks after implantation followed by a slight decrease up to the fourth week and again a slight increase until the end of the trial. This pattern could be observed uniformly in all types of implants and showed no significant differences of the parameter bone volume/total volume in the statistical analysis which characterizes the amount of bone modelling.
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Fig. 2 Histomorphometry of the bone-implant interface: development of the bone volume / total volume (left) and the implant surface covered by bone (contact - right diagram). Straight line: titanium, dotted line: PEEK, dots and little lines: PEEK/HA coating
Concerning the percentage of implant surface covered by bone we found only slight differences between the three materials during the time of implantation. While titanium and coated PEEK specimens showed a slight but constant increase of bone covering until the end of the study uncoated PEEK implants are characterized by a degree from the fourth week on. It seems that with the replacement of primary woven bone by matured lamellar bone witch represents a higher level of organization a process of self optimization adopts the newly formed bone to the biomechanical requirements. The process of bone remodelling appear to be the “fine tuning” of the interaction between bone and the implant material. All measurements showed large interindividual variances in the histomorphometric values. Although the mentioned trend is clearly visible in the diagram a statistical significant difference in the percentage of implant surfaces covered by bone could be proofed only at week twelve. At this time the mean bone coverage of the titanium surface was 55.2% (± 2.5%), the PEEK/HA was covered at 45.4% (± 5,6%) of the surface and uncoated PEEK showed a coverage of only 29.8% (±2.4%). The difference was statistically significant between titanium and PEEK (p
