HYDROXYLAPATITE ––– MAGIC MATERIAL OR MIGHTY MARKETING?
Letter to the Editor, Journal of the American Dental Association
In the interview article on dental implants in the December issue, the JADA questioned five dentists and a biomaterials expert on Implantology. One question posed by the JADA was: "What types of implants and materials are currently used?"
To this, Dr. Thomas Golec, an Oral Surgeon from San Diego, responded: "Today, most if not all subperiosteal implants are made from surgical Vitallium. This material should be coated with hydroxylapatite. Most endosseous implants, whether blade- or cylinder-type implants, are made from commercially available pure titanium. Again, these should be coated with hydroxylapatite to increase speed and predictability of integration."
A close review of the research does not confirm Dr. Golec’s claims of the advantages of coating endosseous implants with hydroxylapatite (HA), and no clinical or animal studies can be found in the literature related to its efficacy on subperiosteal implants.
Dr. Dan Laskin, Editor of the Journal of Oral and Maxillofacial Surgery, stated in an editorial entitled, "Implantology ‘86" (November issue[Vol. 44, No. 11]): "It is understandable that manufacturers praise their own products, but we would hope that we can at least turn to our professional colleagues for a less biased appraisal...Unfortunately, the impact of commercialism, and the political and economic conflicts within our own profession related to the use of implants, often seems to delay rather than to hasten the needed progress...The time has come for us to take a harder look at what is going on in the field."
ANIMAL RESEARCH ON HYDROXYLAPATITE-COATED IMPLANTS:
The results of dog studies by Drs. John Kent, Chairman of Oral Surgery, LSU, Michael Jarcho, President of Cacitek, and others were published in the Lsu Implant Symposium Abstract Book (March 1986) and in transcripts of other meetings (Trans. 12th Ann. Mtg. Soc. Biomaterials, 14, 1986). They reported on push-out tests of HA-coated and uncoated titanium cylindrical implants with three different surface characteristics –– smooth, fluted and porous. All were tested after various unloaded healing periods in dogs. One finding was: "On the uncoated dense (smooth) titanium surfaces, a thin fibrous interface was present around all specimens. The HA coated dense surfaces, however, demonstrated no fibrous tissue layers at any time period and demonstrated mineralization directly onto the surface of the implant."
The HA-coated titanium cylinders demonstrated osseointegration (absence of fibrous tissue at the interface of bone and implant) while the uncoated smooth titanium implants did not. The claim is made by the researchers that the difference in surface responses was due to the difference in materials. A more likely explanation is that the smooth configuration of the titanium cylinder did not allow for the immediate fixation essential for osseointegration. Adding HA coating to titanium cylinders would increase surface texture, ensuring the needed initial fixation. Previous research by Dr. Steve cook, one of the co-researchers with Kent and Jarcho, compared smooth and grit-blasted titanium implants in dogs (Journal of Biomedical Materials Research, Vol. 19, 1985) and proved that osseointegration was highly dependent on surface irregularities: "It was found that rough surfaced implants exhibited apparent direct bone apposition, whereas smooth surfaced implants exhibited varying degrees of fibrous encasement."
CLAIM OF STRONGER BONDING WITH HA COATING ON TITANIUM:
The claim of stronger bonding of bone to HA-coated endosseous implants is based on push-out strength tests. Kent et al. compared non-osseointegrated, smooth titanium cylinders to osseointegrated HA-coated cylinders and, as would be expected, the smooth design of the uncoated, non-osseointegrated implant was less resistent to displacement than the osseointegrated HA-coated implant: "The dense (smooth) titanium systems with HA coatings showed an approximately 2-3 times increase (8 MPa [1200 psi] vs 2-3 MPa [300-500 psi]) in interface attachment strength when compared to the non-HA coated systems."
The claim that the surface coating of HA, in itself, provides greater attachment, is not substantiated by the other findings of their study. With the porous surfaces, both coated and uncoated implants osseointegrated and the push-out strengths were equal. The magnitudes far exceeded that of the smooth cylinders, which more closely conform to the commercially available design of HA-coated implants: "The mechanical testing for the porous system with and without the HA coating showed that the long-term maximum interface strengths were equivalent at about 20 MPa (2,900 psi)."
Cook’s presentation at the LSU Implant Symposium in March, 1986, showed histological evidence that, under push-out tests with the fluted titanium implants, failure occurred at the HA metal interface with the coated implants. The fluted titanium implants without HA coating maintained the titanium-bone integrity with fracture consistently occurring in the bone itself. To date, this observation, made in front of a large audience, has not been published. My attempts to obtain copies of the histological slides from one of the co-researchers has met with refusal on the basis that Calcitek funded the study and I would have to get their permission. Clinical experience with loaded HA-coated implants is very limited but, already, early reports by Dr. Dan Patrick of Long Beach, California, and Dr. Charley Dodge of Walnut Creek, California (personal communication) indicate that separation of the HA from the metal has been observed clinically.
CLAIM OF FASTER OSSEOINTEGRATION WITH HA COATING ON TITANIUM:
The research report of S.D. Cook, J. Kay and M. Jarcho et al. (Kay and Jarcho are employees of Calcitek) (Trans. 12th Ann. Mtg. Soc. Biomaterials, 14, 1986) stated: "In the case of porous implants, the long-term (32 weeks) maximum interface strengths were equivalent for both coated and non-coated implants. However, the HA-coated porous implants reached this maximum strength approximately twice as fast as the uncoated porous implants."
This statement implies that it took 32 weeks to reach "maximum interface strengths" for the non coated porous titanium implants, but half this time for HA-coated porous implants. This is contradictory by Cook’s other report (abstract published in LSU Implant Symposium Book, March 22, 1986): "Results of the mechanical testing for the porous system with and without the HA coating showed that the long-term maximum interface strengths were equivalent at about 20 MPa; however, the rate of development of the maximum interface strength was approximately halved. This time period was approximately 3-6 weeks for the HA-coated porous titanium systems as compared to the 6-12 weeks for the non-HA-coated system."
This statement confirms that the maximum interface strength of both types of implants was reached within 6-12 weeks, not the 32 weeks implied in the previous report. Since the time periods overlap, it is doubtful that a statistically significant difference existed between test results. In any case, there is no proven clinical advantage, since the same 3-4 month submerged healing period is recommended for the HA-coated implant from Calcitek as for the Core-Vent and Biotes implants. Histological reports from Ken et al. (LSU Symposium Abstract) confirm the need to wait this time period. "The early (1 month) specimens (HA-coated) were not completely encased by bone. Bony trabeculae extended to the implant, with these trabecular attachments to the implant separated from each other by cancellous marrow elements. By four months, the bone implant interface was more consolidated with the formation of a layer of lamellar bone."
Calcitek’s instruction manual confirms that the HA coating does not shorten the healing period: "As a general guideline, a healing period of approximately three to four months is allowed for optimum osseointegration and bone bonding to the Calcitite-coated Integral implant."
Note that "bone bonding" and "osseointegration" are considered as two separate interfacial entities by Calcitek.
OSSEOINTEGRATION –– THE LEGACY:
In a recent article by Albrektsson, Zarb et al. in the Journal of Oral and Maxillofacial Implants, Vol. 1, 1986, the authors stated: "The Brånemark results...clearly underscore the basic concept of osseointegration as being the major, if not the exclusive, reason for a successful long-term dental implant attachment."
Brånemark and his co-researchers have proven, with 20 years of clinical testing, the long-term predictability of a titanium oxide-bone interface. This same titanium oxide surface, as now proven with Auger analysis, also exists on the surface of medical grade titanium alloy. The Core-Vent Implant (titanium alloy) has demonstrated achievement and maintenance of osseointegration in 5 years of wide clinical use (now in 3,000 dental offices). The interfacial phenomenon referred to as osseointegration demonstrates under SEM 20-40 Ångstroms of glycoprotein between the bone and the titanium oxide surface. If "bone bonding" exists and is different than osseointegration (manufacturer claims it’s better), how then can the long-term predictability of osseointegration be extrapolated to the HA surface?
RATIONALE FOR HA COATING OF ENDOSSEOUS IMPLANTS:
The question that must ultimately be answered relative to the efficacy of HA coating of titanium or titanium alloy is: Does HA coating solve a problem that doesn’t exist while creating potential problems of its own? It appears that the rationale for coating implants with HA is based on the "chicken soup" theory ––– i.e. it may not help but it can’t hurt. At this point, we don’t know what the long-term ramifications of "bone bonding" will be or even the long-term maintenance of the HA-metal interface.
One of the five dentists questioned in the JADA interview was Dr. Roland Meffert, Chairman of the Graduate Periodontic Program at LSU. This school has been involved with HA coating on Calcitek’s implants since the implant’s creation. He stated at the American Association of Periodontists National Meeting in Cleveland, September, 1986: "We’ve just been working with the HA (Calcitek Implant)...I think the longest case we have restored now is a year, a year plus...we don’t know two years from now, three years from now, four years from now what is going to happen."
The other argument for its use is that it may provide an extra margin for error if one of the principles proven essential for osseointegration is violated. The compromise paid in implant design in order to provide that flat surface needed for coating is the elimination of threads. Threads on an implant ensure immediate fixation, increased surface area for load distribution and facilitate insertion and immediate removal for repositioning if this is necessary. These real clinical factors far outweigh any theoretical gains based on a few dog studies of unloaded implants. In fact, the studies showed that the interfacial strength increased 250% by using a porous design over the smooth cylinder.
The recent professional and public acceptance of implantology as a viable alternative to conventional dental procedures offers dentists a new challenge. Dr. Dan Laskin put it succinctly in his editorial in the November 1986 issue of the Journal of Oral and Maxillofacial Surgery:
Although we are making great strides in our understanding of implant design and the proper use of biomaterials, we still have much to learn in both areas. Unfortunately, the impact of commercialism, and the political and economic conflicts within our own profession related to the use of implants, often seems to delay rather than to hasten the needed progress.
Gerald A. Niznick, DMD, MSD