Readers' round table

To THE EDITOR: I recently read with interest the article “Eleven-year study of hydroxyapatite implants” by Drs. Denissen, Kalk, Veldhuis, and van den Hooff (J PROSTHE’I‘ DENT 1989:61: 706-12). The report of the use of bulk hydroxyapatite implants and hydroxyapatite-coated titanium implants was interesting and worthwhile. However, I do have one significant concern that I believe deserves comment. The authors present their data over the “long-term” use of the hydroxyapatite-coated titanium implants. They stated that 12 of the titanium cores required removal because of “loosening of the cement layer between the t itanium and hydroxyapatite coating.” From the data Jnresented in the table, it appears that 17 of the hydroxyapatite-coated titanium implants were lost over the approximately S-year time period. The authors also make the comment that of the 15 implants placed for single tcboth replacements, only three were left free-standing. The other 12 were apparently splinted to the adjacent tooth bec;,use “it was safer.” However, in the authors’ conclusion, they stated that the design and technique of the hydroxyapatite-coated t itanium implant is considered satisfactory. They felt !hat “loaded and unloaded hydroxyapatite implants have introduced to dentistry reliable natural tooth root sulstitutes.” When I read this report my reaction was that the hydroxyapatite-coated implants are not a satisfac? ory substitute for tooth replacement when compared with other systems that have been followed for comparable and longer periods of time, such as the Branemark system. Ihe authors make their conclusion while at the same time they stated that “since cement fractures occasionally occur. we now prefer plasma-sprayed coatings of hydroxyapatitf’ on the titanium cores, thus avoiding the cement layer.” The authors may be finding that the plasma-sprayeri titanium implants have success rates comparable to other reported systems and studies, but their report does not (lea1 with that issue. It deals only with the hydroxyapa,itecoated implants, which do not have a satisfactory 8 Tear success rate compared with other systems that are avail;< ble. The only logical conclusion that can come from thi- report that evaluates the HA-coated titanium implant is ‘hat these implants did not provide as predictable an outc ime as other systems have shown. Further long-term stutlies, perhaps with plasma-sprayed coatings of hydroxyapatite, are indicated. and 1 would urge the authors to cant nue their work and to report those findings. However, I dc not believe that their present conclusions are supported b\ the data presented. RUSSELLA. WILLIAMS, D.M.D.,lM.S. 997 (‘l.o~IcTo\r EH DR.. Sl,F. 1’ SI’I~INGI;II4.I~,II. tit!70-l

he has shown in our research on hydroxyapatite (HA) implants. We agree with Dr. Williams t.hat osseointegrated types of implants give predictable results. However. these implants depend on mechanical means of retention contrary to HA implants, which are chemically bonded to the bone. We also agree that the cement failure that occurred negatively affected the results and urged us 5 years ago to find an alternative way to bond HA to titanium. However, the cement problem only concerned a shortcoming in the bonding of HA to titanium. It did not contradict our implant concept, which was to achieve an attachment to the bone without mechanical retention. The concept was to use a nonvital HA ceramic in living bone as a natural tooth root substitute to allow function. Therefore, we support. after 11 years of research, our conclusion that HA implants have introduced to dentistry reliable natural tooth rorjt substitutes hy the following data: 1. Seventy-five percent of the bulk HA implants were retained under lower complete dentures, while maintaining the volume of denture- bearing region of the mandible. Loss after many years was a logical outcome of the pressure of the removable denture. 2. One-hundred percent of the bulk HA implants were retained under fixed partial dent.ures. 3. The cylindrical design and the insertion technique of the HA-coated implants were satisfactory because implantation was easy with little trauma to the bone and few instruments compared with osseointegrated types of implants. The diameter of the HA implant was only :3mm. All coated implants were strongly bonded to living bone at the time of post connection. In the event cement failure occurred the HA coating was retained and ankylosed to the bone comparable to the bulk HA implants. Unfortunately recementation of the titanium core was too difficult a procedure. We do not want to conceal that the cement fracture was a great disappointment f’or us. In my lectures I always compare this problem with the initial fracture problems of porcelain-fused-to-metal restorations. Porcelain was the best material and also the concept of bonding porcelain to a metal substrate was correct. Only the bonding itself has to be improved, not the system. In the case of HA bonding to titanium, the plasma spray procedure also seems to be such an improvement. However. a plasma spray HA coating is only a technical variation, it does not change our implant philosophy, which is based on HA ceramic interfacing with alveolar bone. HARRY W. DENISSEN, D.D.S., PH.D.

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REPLY

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To THE EDITOR: I appreciate the opportunity to respond to the letter of Dr. Williams and I would like to thank him for the intc,rest THE

JOURNAL

OF PROSTHETIC

DENTISTRY

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