- Email: [email protected]
Transmucosal implants of dense hydroxylapatite
Transmucosal implants of dense hydroxylapatite C. de Putter, D.D.S.,*
K. de Groot, Ph.D.,**
and P. A. E. Sillevis
Smitt, Ph.D., D.D.S.***
Free University, Schools of Dentistry and Medicine, Amsterdam, Holland
A
ccording to Hench et al.,’ medical implants are biocompatible when, among other requirements, the interface at the implant and surrounding tissue is characterized by histologic reactions that resemble nature as closely as possible. Bioactive materials for bone and tooth replacements developed thus far include calcium phosphate ceramics of apatite or whitlockite structure and bioglasses. Recent literature indicates that these materials have in common an apatite surface that is responsible for the observed tight bonding between the bone and the implant.2 Many authors have shown that this tight bond resembles ankylosis as found in natural teeth reimplanted without periodontal membrane.3-6These teeth usually become well fixed in the bone. However, the natural tooth roots resorb and after several years are replaced by bone, leaving the crown without support. In a previous study we showed that submerged tooth roots made of dense apatite ceramics do not resorb and thus tend to prevent resorption of the alveolar ridges.6 This article presents the results of a study aimed at answering the question of whether roots made of dense apatite ceramics implanted transmucosally rather than submerged could serve effectively as an abutment either for a single crown or for a fixed prosthesis. The following questions must be answered. 1. Does loading by chewing and other forces on transmucosal implants alter the tight bonding between the implant and the bone as observed with nonloaded calcium phosphate ceramics? 2. What is the nature of the transmucosal implant with respect to the attachment of epithelial and connective tissues to the implant surface? 3. Can the implant material withstand chewing forces? 4. What are the consequences of failure of the implant? *Research Associate, Department of Prosthodontics. **Professor, Department of Biomaterials. ***Professor, Department of Prosthodontics.
0022-3913/83/010087
+ 09$00.90/00
1983 The C. V. Mosby Co.
MATERIAL AND Implant material
METHODS
By compression and subsequent sintering of a commercial calcium phosphate salt, we obtained a 97% to 99% dense ceramic.6 The properties of this material strongly resemble those of dental enamel (compressive strength, 500” 100 MN/m2, Vickers hardness, 450 MN/m2; tensile strength 100” 50 MN/m2 and consisting of 90% apatite and 10% whitlockite). Animals Five mongrel dogs and seven beagle dogs were used in this study. For implantation procedures the dogs were premeditated intramuscularly with 1 ml Nesdonal (Specia Laboratories, Paris, France) and 0.5 ml atropine. They were intubated and anesthetized with a mixture of nitrous oxide, oxygen, and halothane. For evaluation procedures an extra intravenous dose of Nesdonal was used after premeditation as described. The animals received a normal diet of meat, standard dry dog food, and water. Methods
of implantation
Two methods of implantation were used: (1) placement of the implant immediately after extraction of the natural tooth, followed by placement of a crown on the implant, and (2) placement of the implant in a hole predrilled in an edentulous part of the mandible, followed by placement of a removable restoration with the implant used as an abutment. Method No. 1: Placement immediately after extraction. This experiment was performed in five mongrel dogs. With the aid of preliminary impressions, custom-made impression trays were prepared for the final impressions. On the casts obtained, the mandibular premolars (P2, P,, and PJ to be extracted later were reduced to a level of 1 mm above the gingival margin. Fixed restorations were made from the canine to the first molar. In the premolar region the fixed restorations were provided with removable parts to protect and fix the implants that would replace the premolars.
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Fig. 1. Three implants (i) 5 weeks after implantation
DE GROOT,
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(method No. 1).
Fig. 2. Three implants (i) provided with crowns, 11 weeks after implantation No. 1). With the aid of the casts and radiographs, 30 implants were modeled after the form of the teeth to be extracted. Because natural dog teeth have two roots diverging buccolingually and mesiodistally, they must be split before extraction. However, the implants must be placed intact so that only one root will accurately fit in the extraction site below the level of the furcation. Following extraction of the natural teeth and sterilization, the. implants were placed and adapted to the extraction wound. The height of the implant above the
AND
(method
gingiva was reduced to 1 to 1.5 mm Bands and removable restorations were placed. The implants were fixed and protected by an oral wound dressing (Coepak, no eugenol, Coe Laboratories, Inc., Chicago, Ill.). In this way chewing forces acted directly on the restorations and implants. Method No. 2: Placement in the edentulous mandible. The seven beagle dogs had been edentulous in the premolar region for at least 2 years. With slowly rotating instruments cooled with physiologic salt solu-
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time
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Fig. 3. Clinical results of implantation method No. 1. Horizontal axis indicates time in = Removal of wound dressing. weeks. Vertical axis indicates percent of implants. ---= = Fracture of implants Fixation achieved. ----- = Crowns placed. .-.-.loaded with all restorations. . . . . . . = Fracture of implants loaded with crowns only.
tion, four holes 10 to 13 mm in depth were drilled, two on each side of the mandible. Twenty-eight implants were fitted and placed in the prepared holes. The method of fixation and protection of the implants was the same as that used in method No. 1.
LoE index as modified by van der Kuij’ was used to evaluate the clinical aspect of the gingival tissues around the implants and compare them with those around the adjacent natural teeth.
Clinical procedures
Six implants of one dog (method No. 1) were removed together with surrounding tissues 28 weeks after implantation (19 weeks after placement of crowns). These implants were studied histologically and compared with neighboring natural teeth. The tissue blocks were fixed in Susa Heidenhain fixing solution, decalcified in a mixture of picric acid and nitrous acid, and embedded in paraffin. Sections were stained with hematoxylin and eosin and investigated by light microscopy.
Once every week the implants were inspected and their supragingival surfaces carefully cleaned with 0.5% chlorine hexedine solution on surgical tabs. The wound dressing was renewed every week. As soon as the gingival tissues visually connected the implants, the wound dressing was omitted. When palpation with the thumb and forefinger indicated that the implants were fixed in the bone as tightly as the natural neighboring teeth, the restorations were removed. In method No. 1, after preparing the teeth for crowns with fast rotating instruments, metal crowns made of Ultratek (Whaledent International, Hamburg, W. Germany) were placed with zinc phosphate cement (Figs. 1 and 2). Because of the smaller sizes of the jaws of the beagle dogs, smaller implants were used in method No. 2. The forces of occlusion were applied to the implants indirectly with removable restorations due to the mechanical properties of the implantation material. For the same reason the implants of one dog implanted following method No. 1 were also loaded in this manner. A
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Histologic procedures
Treatment of failures In five instances where implants (method No. 2) were fractured, the parts that were still anchored in bone were removed by drilling and new implants inserted. One implant (method No. 2) that was not fixed to the surrounding bone after 12 weeks was removed. The cavity was prepared deeper and a new implant placed. Further clinical procedures were the same as previously described. When implants broke, the part that
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80 70 60 50 40 30 20 10
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Fig. 4. Clinical results of implantation method No. 2. Horizontal axis indicates time in weeks. Vertical axis indicates uercent of implants. = Removal of wound dressing. ---= Fixation achieved: --- = Fixed restorations placed. .-.-.= ImplaGt fracture.
was still fixed in the alveolar bone was submerged. In most implants the gingival tissues were already closed above the “root section.” Sometimes sharp edges were rounded with fast rotating instruments before suturing of the gingival tissues. In five implants (method No. 1) the upper part of the broken implant that was still connected to the gingival tissues was removed together with the surrounding tissues.
Clinical variables in time Variables that were measured in weeks were (1) the time of definitive removal of the wound dressing, (2) the time of fixation, (3) the time of placement under load (method No. 1, crowns; method No. 2, removable restorations), and (4) the time of fracture. The implants in method No. 1 were observed for at least 86 weeks. The implants in method No. 2 were observed for at least 62 weeks.
RESULTS Clinical aspects
Fig. 5. Decalcified natural incisor Cni) with
a welldeveloped junctional epithelium (je) and a limited field of inflammatory cells (id. (Magnification X30.)
90
The time of the removal of the wound dressing, the time of fixation, the time of placement of crowns, and the time of fracture for implantation with method No. 1 are presented in Fig. 3. It appears that for 80% of the implants the wound dressing could be omitted after 4 weeks, and for 100% after 9 weeks. About 80% of the
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Fig. 6. General view of mandibular bone with a decalcified two-rooted implant (i) 28 weeks after implantation (method No. 1). (Magnification ~12.)
implants were fixed in bone after 8 weeks and 100% after 13 weeks. Most of the crowns (87.5%) were placed within 11 weeks after implantation, and 100% within 17 weeks. The percent of fractured implants is symbolized by two lines. The first line (-.-.-.) includes the one dog that had had implants placed according to method No. 1 but was loaded indirectly with removable restorations. The second line (. . . . . .) excludes this dog. It appears that 30 weeks after implantation about 50% of the implants were broken, and after 1 year more than 70% were broken. Fig. 4 gives the results for the time of removal of the wound dressing, the time of fixation, and the time of loading and fracture for implantation method No. 2. It appears that at all implant sites the gingival tissues were healed after 1 week and the wound dressing could be omitted. Fixation of 85% of the implants was achieved 9 weeks after implantation and of 100% of the implants 30 weeks after implantation. All implants were loaded immediately after implantation. After 1 year 47% of them were broken. All six reimplanted implants showed fixation after 1 week. One implant broke again 5 weeks after implantation, and a new implant was placed for the third time. These implants survived for the following number of weeks: 29 weeks, 1 implant; 26 weeks, 1 implant; 16 weeks, 3 implants; and 15 weeks, 1 implant.
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The clinical aspect of the gingival tissues around the implants appeared the same as that around the neighboring teeth. The modified LoE index gave a score similar to that of the neighboring natural elements in all implants.
Histologic aspects The limited data from the six implants of one dog (method No. 1) allow only preliminary conclusions. The sulcus between the gingival epithelium and the implants seemed to have the same depth, and the inner epithelium bordering the implants seemed to have the same (average) length as that bordering the natural teeth. There was some proliferation of the epithelium bordering the implant as well as the natural tooth structures. The free gingival tissue around the implant surface had a field of inflammation as large as the free gingival tissue around the neighboring natural teeth. The connective tissues between the lower zone of epithelium and alveolar bone showed no inflammatory cells. The connective tissue fibers had a more or less perpendicular orientation without interruptions, giving the appearance of terminating in the superficial porosities (1% to 3%) or the rough surface of the implant. The bond between the implant and surrounding bone was tight and identical to that described for submerged tooth root implants.6 Apposition of new bone had taken place in the
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Fig. 7. Magnification of right part of Fig. 6. Gingiva (G) and alveolar bone CAB) show no histopathologic reactions. Along artificial root, apposition of new bone (NB) has taken nlace. I = Implant. (Magnification x30.)
subalveolar surface of the implant. There was no periodontal ligament between the alveolar bone and the opposite surface of the implant. Additional pertinent findings from the six implants that were studied histologically are found in Figs. 5 to 9.
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DISCUSSION
Bone-implant
interface
Horizontal forces tend to delay imf Blant fixation. This finding was illustrated by the fact that implants protected by adjacent first molars and canines were fixed earlier than nonprotected implants
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Fig. 8. Detail of Figs. 6 and 7 showing connective tissue (ct) fibers bordering implant (i) at cervical level below gingiva. Orientation of connective tissue fibers is more or less perpendicular to implant surface. Arrows indicate that fibers seem to terminate in superficial porosities or roughness of implant material. (Magnification x200.)
The dogs received normal food that was of hard consistency. Therefore, the chewing forces on implants were not much different from those on the natural teeth. The bonding of the bone to the implant obviously can withstand these forces, and hence the answer to the jrst question is that transmucosal implants of dense apatite clinically bond tightly to surrounding alveolar bone despite loading by chewing forces. The histologic results of this study illustrate these findings, showing the same interface of bone to implant found in the unloaded condition as published earlier.6
Soft tissue-implant
interface
The wound dressing was applied to induce a healing and protective effect, the latter because implants usually had a poor fit, especially in method No. 1. In method No. 2, where the fit was better, tissues were closed
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Fig. 9. Narrow encapsulation of implant (i) and connection with spongeous alveolar bone t’ab) by newly formed bone (nb). There is no connective tissue bordering implant. (Magnification x120.)
within 1 week. Thereafter the wound dressing could be omitted. Attachment of soft tissue to apatite ceramics seemsto be positively influenced by a certain surface roughness, while prevention of plaque is promoted by a smooth surface. These particular relationships will be studied further. Clinically, implants behaved like natural teeth. In both methods oral hygiene was of crucial importance for the health of gingival tissues. Factors that negatively influenced oral hygiene were the poor self-cleansing conditions near the crowns and restorations and the frequency of the oral hygiene provided (only once a week). Experimentally increasing the frecluency of the inspection and hygiene resulted in an improved clinical condition as well as in a better histologic aspect. The results of the histology of tissues around broken implants, studies with improved hygienic conditions, and further electron microscopic studies to evaluate the soft tissue-implant surface bond will be published separately. The fact that in five implants gingival tissues still
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GROOT,
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i 40
50 time
in
60
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Fig. 10. Fracture after fixation. Horizontal axis indicates time in weeks. Vertical axis indicates percent of implants. = Fracture after fixation, method No. 1. --= Fracture after fixation, method No. 2.
were connected with the upper part of the broken implants at the weekly inspection may illustrate the expected bond of the implants with the gingival tissues. Taking into account the preliminary nature of the histologic data, the answer to the second question stated in the introduction might be that attachment of gingival tissues to the implant surface under the conditions of this study occurred similarly to reattachment around the natural teeth. This is in agreement with studies of Ogiso et al., who reported that hemidesmosomes were present at the interfacial surface with a hydroxylapatite implant.
Fracture of implants After a few months of functional use the implants began to fracture. Fig. 10 gives the time of fracture after the time of fixation in the alveolar bone, illustrates the difference between the times of fracture, and presents the percent of fractured implants between the implants loaded with crowns (method No. 1) and the implants loaded with removable restorations (method No. 2). It appears that 30 weeks after fixation about 80% of the implants loaded with crowns were broken, as opposed to less than 40% of the implants loaded under the removable restorations. The differences between the two methods are due to the different stress intensities on the implants, higher on the crowned implants (method No. 1) than on the implants loaded with the removable restorations (method No. 2) where
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horizontal and vertical forces are also transmitted to the strong natural teeth (canines and first molars). All fractures occurred at the site of greatest stress intensity. In the transmucosal implants that site is the place where the implant leaves the alveolar bone. It is known that ceramic materials are weak in tension and that continuous tensile loading under the proportional limit may lead to a fatigue failure.’ This happened with the implants in this study. Factors influencing fatigue failure also include the absence of a buffering periodontal membrane, the ratio of volume to surface of the implant, the shape of the implant, and the nature of the surrounding elements (blood, saliva, etc.). Soft food, for example, would have delayed the average time of fracture. There are no reasons to believe that stress intensities are lower in humans than in dogs. Chewing forces are larger in dogs, but the different occlusion and articulation patterns preclude such conclusions with respect to stress intensities and subsequent fatigue failure of apatite ceramic implants. Until a method is developed with better resistance to force intensities leading to less or no fatigue failure, calcium phosphate ceramics with mechanical properties as described in this article must not be considered for use as loaded abutments in prosthetic dentistry. This conclusion answers the third question negatively and is in agreement with earlier suggestions that apatite ceramics have at the moment no clinical use at
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sites where other forces than compression play a role. It is therefore important to improve the resistance of calcium phosphate ceramics to fatigue failure. Currently we are studying prestressed implants by comparing them with prestressing materials and techniques in the building industry (prestressed concrete). Results on such improved material will be published separately.
Consequences of failure After fracture, implants can be easily shortened so that they may survive as submerged root implants or be removed by means of fast rotating instruments cooled with physiologic salt solution, Then a new implant can be inserted. The tight bonding with bone prohibits removal by extraction but also renders mechanical retention unnecessary. Failure of apatite implants thus results in minimal bone loss because of the minimal amount of alveolar bone needed for retention of the implants.
CONCLUSIONS 1. Loaded transmucosal apatite ceramic implants act as ankylotic elements and adhere similarly to bone in an unloaded situation. 2. The clinical aspect of gingival tissues around implants is the same as around natural teeth. The limited histologic data indicating a good attachment of gingival tissues to the implant warrant further histologic study. 3. Implants cannot withstand chewing forces because of fatigue failure. 4. Implants can easily be submerged, removed, and/or substituted with minimal loss of bone or other severe damage.
SUMMARY Transmucosal application of implants of dense hydroxylapatite has been studied in a long-term animal experiment. Implants were loaded with crowns or removable restorations. It appeared that transmucosal implants adhered to bone similarly as in the submucosal application. The clinical aspect of gingival tissues
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around the implants was the same as that around the adjacent natural teeth. Histologically, there were indications for a good attachment of gingival tissues to the implants. Implants fractured due to fatigue failure of the ceramic material. Prestressing of the implants may be the solution to this problem. Failing implants can clinically be treated by submerging, removal, or substitution. REFERENCES 1
Hench, L. L., Greenke, T. K., Paschal], H. A., Allen, W. C., and Piotrowski, G.: Reports 1-6, 1970-1975, U.S. Army Medical Research and Development Command, Contract DADA-17-70-C-0001. 2 de Groot, K.: Bioceramics consisting of calcium phosphate salts. Biomaterials 1:47, 1980. 3 Aoki, H., Kato, K., and Tabata, T.: Osteocompatibility of apatite ceramics in mandibles. Report of the Institute for Medical Dental Engineering, Tokyo, 11:33, 1977. 4 Jarcho, M., Kay, J. F., Gumaer, K. I., Dot-emus, R. H., and Drobeck, H. P.: Tissue, cellular and subcellular events at a bone-ceramic hydroxylapatire interface. J Bioeng 1:79, 1977. 5 Denissen, H. W.; and de Groot, K.: Immediate dental root implants from synthetic dense calcium hydroxylapatite. J PROSTHETDENT 42551, 1979. 6 Denissen, H. W., Veldhuis, A. A. H., Makkes, P. Ch., van den Hooff, A., and de Groot, K.: Dense apatite implants in preventive prosthetic dentistry. J Clin Prev Dent 2:23, 1980. 7. van der Kuij, P.: Reducing Residual Ridge Reduction. Thesis, Free University, Amsterdam, Netherlands, 1080. 8. Ogiso, M., Kaneda, H., Arasaki, J., Ishida, K., Shiota, M., Mituwa, T., Asano, K., Tabata, T., Yarnazaki, Y., and Hidaka, T.: Affinity of epithelial tissue with hydroxylapatite ceramics for the purpose of dental implantation. J Dent Res GO(Special issue A):419, 1981. 9. Bornhauser, A., and Pabst, R. F.: Evaluation of subcritical crack extension parameters with ceramic biomaterials. A critical examination. Abstract P. 1.3. Book of Abstracts First World Biomaterials Congress, Baden, near Vienna, April 1980.
Reprmt requeststo: DR. C. DE PUTTER FREE UNIVERSITY SCHOOLOF DENTISTRY 1007 MC AMSTERDAM HOLLAND
95
K. de Groot, Ph.D.,**
and P. A. E. Sillevis
Smitt, Ph.D., D.D.S.***
Free University, Schools of Dentistry and Medicine, Amsterdam, Holland
A
ccording to Hench et al.,’ medical implants are biocompatible when, among other requirements, the interface at the implant and surrounding tissue is characterized by histologic reactions that resemble nature as closely as possible. Bioactive materials for bone and tooth replacements developed thus far include calcium phosphate ceramics of apatite or whitlockite structure and bioglasses. Recent literature indicates that these materials have in common an apatite surface that is responsible for the observed tight bonding between the bone and the implant.2 Many authors have shown that this tight bond resembles ankylosis as found in natural teeth reimplanted without periodontal membrane.3-6These teeth usually become well fixed in the bone. However, the natural tooth roots resorb and after several years are replaced by bone, leaving the crown without support. In a previous study we showed that submerged tooth roots made of dense apatite ceramics do not resorb and thus tend to prevent resorption of the alveolar ridges.6 This article presents the results of a study aimed at answering the question of whether roots made of dense apatite ceramics implanted transmucosally rather than submerged could serve effectively as an abutment either for a single crown or for a fixed prosthesis. The following questions must be answered. 1. Does loading by chewing and other forces on transmucosal implants alter the tight bonding between the implant and the bone as observed with nonloaded calcium phosphate ceramics? 2. What is the nature of the transmucosal implant with respect to the attachment of epithelial and connective tissues to the implant surface? 3. Can the implant material withstand chewing forces? 4. What are the consequences of failure of the implant? *Research Associate, Department of Prosthodontics. **Professor, Department of Biomaterials. ***Professor, Department of Prosthodontics.
0022-3913/83/010087
+ 09$00.90/00
1983 The C. V. Mosby Co.
MATERIAL AND Implant material
METHODS
By compression and subsequent sintering of a commercial calcium phosphate salt, we obtained a 97% to 99% dense ceramic.6 The properties of this material strongly resemble those of dental enamel (compressive strength, 500” 100 MN/m2, Vickers hardness, 450 MN/m2; tensile strength 100” 50 MN/m2 and consisting of 90% apatite and 10% whitlockite). Animals Five mongrel dogs and seven beagle dogs were used in this study. For implantation procedures the dogs were premeditated intramuscularly with 1 ml Nesdonal (Specia Laboratories, Paris, France) and 0.5 ml atropine. They were intubated and anesthetized with a mixture of nitrous oxide, oxygen, and halothane. For evaluation procedures an extra intravenous dose of Nesdonal was used after premeditation as described. The animals received a normal diet of meat, standard dry dog food, and water. Methods
of implantation
Two methods of implantation were used: (1) placement of the implant immediately after extraction of the natural tooth, followed by placement of a crown on the implant, and (2) placement of the implant in a hole predrilled in an edentulous part of the mandible, followed by placement of a removable restoration with the implant used as an abutment. Method No. 1: Placement immediately after extraction. This experiment was performed in five mongrel dogs. With the aid of preliminary impressions, custom-made impression trays were prepared for the final impressions. On the casts obtained, the mandibular premolars (P2, P,, and PJ to be extracted later were reduced to a level of 1 mm above the gingival margin. Fixed restorations were made from the canine to the first molar. In the premolar region the fixed restorations were provided with removable parts to protect and fix the implants that would replace the premolars.
THE JOURNAL
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DE PUTTER,
Fig. 1. Three implants (i) 5 weeks after implantation
DE GROOT,
88
SILLEVIS
SMITT
(method No. 1).
Fig. 2. Three implants (i) provided with crowns, 11 weeks after implantation No. 1). With the aid of the casts and radiographs, 30 implants were modeled after the form of the teeth to be extracted. Because natural dog teeth have two roots diverging buccolingually and mesiodistally, they must be split before extraction. However, the implants must be placed intact so that only one root will accurately fit in the extraction site below the level of the furcation. Following extraction of the natural teeth and sterilization, the. implants were placed and adapted to the extraction wound. The height of the implant above the
AND
(method
gingiva was reduced to 1 to 1.5 mm Bands and removable restorations were placed. The implants were fixed and protected by an oral wound dressing (Coepak, no eugenol, Coe Laboratories, Inc., Chicago, Ill.). In this way chewing forces acted directly on the restorations and implants. Method No. 2: Placement in the edentulous mandible. The seven beagle dogs had been edentulous in the premolar region for at least 2 years. With slowly rotating instruments cooled with physiologic salt solu-
JANUARY
1983
VOLUME
49
NUMBER
1
DENSE HYDROXYLAPATITE
TRANSMUCOSAL
IMPLANTS
time
in
weeks
Fig. 3. Clinical results of implantation method No. 1. Horizontal axis indicates time in = Removal of wound dressing. weeks. Vertical axis indicates percent of implants. ---= = Fracture of implants Fixation achieved. ----- = Crowns placed. .-.-.loaded with all restorations. . . . . . . = Fracture of implants loaded with crowns only.
tion, four holes 10 to 13 mm in depth were drilled, two on each side of the mandible. Twenty-eight implants were fitted and placed in the prepared holes. The method of fixation and protection of the implants was the same as that used in method No. 1.
LoE index as modified by van der Kuij’ was used to evaluate the clinical aspect of the gingival tissues around the implants and compare them with those around the adjacent natural teeth.
Clinical procedures
Six implants of one dog (method No. 1) were removed together with surrounding tissues 28 weeks after implantation (19 weeks after placement of crowns). These implants were studied histologically and compared with neighboring natural teeth. The tissue blocks were fixed in Susa Heidenhain fixing solution, decalcified in a mixture of picric acid and nitrous acid, and embedded in paraffin. Sections were stained with hematoxylin and eosin and investigated by light microscopy.
Once every week the implants were inspected and their supragingival surfaces carefully cleaned with 0.5% chlorine hexedine solution on surgical tabs. The wound dressing was renewed every week. As soon as the gingival tissues visually connected the implants, the wound dressing was omitted. When palpation with the thumb and forefinger indicated that the implants were fixed in the bone as tightly as the natural neighboring teeth, the restorations were removed. In method No. 1, after preparing the teeth for crowns with fast rotating instruments, metal crowns made of Ultratek (Whaledent International, Hamburg, W. Germany) were placed with zinc phosphate cement (Figs. 1 and 2). Because of the smaller sizes of the jaws of the beagle dogs, smaller implants were used in method No. 2. The forces of occlusion were applied to the implants indirectly with removable restorations due to the mechanical properties of the implantation material. For the same reason the implants of one dog implanted following method No. 1 were also loaded in this manner. A
THE JOURNAL
OF PROSTHETIC
DENTISTRY
Histologic procedures
Treatment of failures In five instances where implants (method No. 2) were fractured, the parts that were still anchored in bone were removed by drilling and new implants inserted. One implant (method No. 2) that was not fixed to the surrounding bone after 12 weeks was removed. The cavity was prepared deeper and a new implant placed. Further clinical procedures were the same as previously described. When implants broke, the part that
89
DE PUTTER,
DE GROOT,
AND
SILLEVIS
SMITT
80 70 60 50 40 30 20 10
-i 30
40
50 time
in
60
70
80
90
104
weeks
Fig. 4. Clinical results of implantation method No. 2. Horizontal axis indicates time in weeks. Vertical axis indicates uercent of implants. = Removal of wound dressing. ---= Fixation achieved: --- = Fixed restorations placed. .-.-.= ImplaGt fracture.
was still fixed in the alveolar bone was submerged. In most implants the gingival tissues were already closed above the “root section.” Sometimes sharp edges were rounded with fast rotating instruments before suturing of the gingival tissues. In five implants (method No. 1) the upper part of the broken implant that was still connected to the gingival tissues was removed together with the surrounding tissues.
Clinical variables in time Variables that were measured in weeks were (1) the time of definitive removal of the wound dressing, (2) the time of fixation, (3) the time of placement under load (method No. 1, crowns; method No. 2, removable restorations), and (4) the time of fracture. The implants in method No. 1 were observed for at least 86 weeks. The implants in method No. 2 were observed for at least 62 weeks.
RESULTS Clinical aspects
Fig. 5. Decalcified natural incisor Cni) with
a welldeveloped junctional epithelium (je) and a limited field of inflammatory cells (id. (Magnification X30.)
90
The time of the removal of the wound dressing, the time of fixation, the time of placement of crowns, and the time of fracture for implantation with method No. 1 are presented in Fig. 3. It appears that for 80% of the implants the wound dressing could be omitted after 4 weeks, and for 100% after 9 weeks. About 80% of the
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Fig. 6. General view of mandibular bone with a decalcified two-rooted implant (i) 28 weeks after implantation (method No. 1). (Magnification ~12.)
implants were fixed in bone after 8 weeks and 100% after 13 weeks. Most of the crowns (87.5%) were placed within 11 weeks after implantation, and 100% within 17 weeks. The percent of fractured implants is symbolized by two lines. The first line (-.-.-.) includes the one dog that had had implants placed according to method No. 1 but was loaded indirectly with removable restorations. The second line (. . . . . .) excludes this dog. It appears that 30 weeks after implantation about 50% of the implants were broken, and after 1 year more than 70% were broken. Fig. 4 gives the results for the time of removal of the wound dressing, the time of fixation, and the time of loading and fracture for implantation method No. 2. It appears that at all implant sites the gingival tissues were healed after 1 week and the wound dressing could be omitted. Fixation of 85% of the implants was achieved 9 weeks after implantation and of 100% of the implants 30 weeks after implantation. All implants were loaded immediately after implantation. After 1 year 47% of them were broken. All six reimplanted implants showed fixation after 1 week. One implant broke again 5 weeks after implantation, and a new implant was placed for the third time. These implants survived for the following number of weeks: 29 weeks, 1 implant; 26 weeks, 1 implant; 16 weeks, 3 implants; and 15 weeks, 1 implant.
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The clinical aspect of the gingival tissues around the implants appeared the same as that around the neighboring teeth. The modified LoE index gave a score similar to that of the neighboring natural elements in all implants.
Histologic aspects The limited data from the six implants of one dog (method No. 1) allow only preliminary conclusions. The sulcus between the gingival epithelium and the implants seemed to have the same depth, and the inner epithelium bordering the implants seemed to have the same (average) length as that bordering the natural teeth. There was some proliferation of the epithelium bordering the implant as well as the natural tooth structures. The free gingival tissue around the implant surface had a field of inflammation as large as the free gingival tissue around the neighboring natural teeth. The connective tissues between the lower zone of epithelium and alveolar bone showed no inflammatory cells. The connective tissue fibers had a more or less perpendicular orientation without interruptions, giving the appearance of terminating in the superficial porosities (1% to 3%) or the rough surface of the implant. The bond between the implant and surrounding bone was tight and identical to that described for submerged tooth root implants.6 Apposition of new bone had taken place in the
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Fig. 7. Magnification of right part of Fig. 6. Gingiva (G) and alveolar bone CAB) show no histopathologic reactions. Along artificial root, apposition of new bone (NB) has taken nlace. I = Implant. (Magnification x30.)
subalveolar surface of the implant. There was no periodontal ligament between the alveolar bone and the opposite surface of the implant. Additional pertinent findings from the six implants that were studied histologically are found in Figs. 5 to 9.
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DISCUSSION
Bone-implant
interface
Horizontal forces tend to delay imf Blant fixation. This finding was illustrated by the fact that implants protected by adjacent first molars and canines were fixed earlier than nonprotected implants
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Fig. 8. Detail of Figs. 6 and 7 showing connective tissue (ct) fibers bordering implant (i) at cervical level below gingiva. Orientation of connective tissue fibers is more or less perpendicular to implant surface. Arrows indicate that fibers seem to terminate in superficial porosities or roughness of implant material. (Magnification x200.)
The dogs received normal food that was of hard consistency. Therefore, the chewing forces on implants were not much different from those on the natural teeth. The bonding of the bone to the implant obviously can withstand these forces, and hence the answer to the jrst question is that transmucosal implants of dense apatite clinically bond tightly to surrounding alveolar bone despite loading by chewing forces. The histologic results of this study illustrate these findings, showing the same interface of bone to implant found in the unloaded condition as published earlier.6
Soft tissue-implant
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The wound dressing was applied to induce a healing and protective effect, the latter because implants usually had a poor fit, especially in method No. 1. In method No. 2, where the fit was better, tissues were closed
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Fig. 9. Narrow encapsulation of implant (i) and connection with spongeous alveolar bone t’ab) by newly formed bone (nb). There is no connective tissue bordering implant. (Magnification x120.)
within 1 week. Thereafter the wound dressing could be omitted. Attachment of soft tissue to apatite ceramics seemsto be positively influenced by a certain surface roughness, while prevention of plaque is promoted by a smooth surface. These particular relationships will be studied further. Clinically, implants behaved like natural teeth. In both methods oral hygiene was of crucial importance for the health of gingival tissues. Factors that negatively influenced oral hygiene were the poor self-cleansing conditions near the crowns and restorations and the frequency of the oral hygiene provided (only once a week). Experimentally increasing the frecluency of the inspection and hygiene resulted in an improved clinical condition as well as in a better histologic aspect. The results of the histology of tissues around broken implants, studies with improved hygienic conditions, and further electron microscopic studies to evaluate the soft tissue-implant surface bond will be published separately. The fact that in five implants gingival tissues still
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i 40
50 time
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Fig. 10. Fracture after fixation. Horizontal axis indicates time in weeks. Vertical axis indicates percent of implants. = Fracture after fixation, method No. 1. --= Fracture after fixation, method No. 2.
were connected with the upper part of the broken implants at the weekly inspection may illustrate the expected bond of the implants with the gingival tissues. Taking into account the preliminary nature of the histologic data, the answer to the second question stated in the introduction might be that attachment of gingival tissues to the implant surface under the conditions of this study occurred similarly to reattachment around the natural teeth. This is in agreement with studies of Ogiso et al., who reported that hemidesmosomes were present at the interfacial surface with a hydroxylapatite implant.
Fracture of implants After a few months of functional use the implants began to fracture. Fig. 10 gives the time of fracture after the time of fixation in the alveolar bone, illustrates the difference between the times of fracture, and presents the percent of fractured implants between the implants loaded with crowns (method No. 1) and the implants loaded with removable restorations (method No. 2). It appears that 30 weeks after fixation about 80% of the implants loaded with crowns were broken, as opposed to less than 40% of the implants loaded under the removable restorations. The differences between the two methods are due to the different stress intensities on the implants, higher on the crowned implants (method No. 1) than on the implants loaded with the removable restorations (method No. 2) where
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horizontal and vertical forces are also transmitted to the strong natural teeth (canines and first molars). All fractures occurred at the site of greatest stress intensity. In the transmucosal implants that site is the place where the implant leaves the alveolar bone. It is known that ceramic materials are weak in tension and that continuous tensile loading under the proportional limit may lead to a fatigue failure.’ This happened with the implants in this study. Factors influencing fatigue failure also include the absence of a buffering periodontal membrane, the ratio of volume to surface of the implant, the shape of the implant, and the nature of the surrounding elements (blood, saliva, etc.). Soft food, for example, would have delayed the average time of fracture. There are no reasons to believe that stress intensities are lower in humans than in dogs. Chewing forces are larger in dogs, but the different occlusion and articulation patterns preclude such conclusions with respect to stress intensities and subsequent fatigue failure of apatite ceramic implants. Until a method is developed with better resistance to force intensities leading to less or no fatigue failure, calcium phosphate ceramics with mechanical properties as described in this article must not be considered for use as loaded abutments in prosthetic dentistry. This conclusion answers the third question negatively and is in agreement with earlier suggestions that apatite ceramics have at the moment no clinical use at
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sites where other forces than compression play a role. It is therefore important to improve the resistance of calcium phosphate ceramics to fatigue failure. Currently we are studying prestressed implants by comparing them with prestressing materials and techniques in the building industry (prestressed concrete). Results on such improved material will be published separately.
Consequences of failure After fracture, implants can be easily shortened so that they may survive as submerged root implants or be removed by means of fast rotating instruments cooled with physiologic salt solution, Then a new implant can be inserted. The tight bonding with bone prohibits removal by extraction but also renders mechanical retention unnecessary. Failure of apatite implants thus results in minimal bone loss because of the minimal amount of alveolar bone needed for retention of the implants.
CONCLUSIONS 1. Loaded transmucosal apatite ceramic implants act as ankylotic elements and adhere similarly to bone in an unloaded situation. 2. The clinical aspect of gingival tissues around implants is the same as around natural teeth. The limited histologic data indicating a good attachment of gingival tissues to the implant warrant further histologic study. 3. Implants cannot withstand chewing forces because of fatigue failure. 4. Implants can easily be submerged, removed, and/or substituted with minimal loss of bone or other severe damage.
SUMMARY Transmucosal application of implants of dense hydroxylapatite has been studied in a long-term animal experiment. Implants were loaded with crowns or removable restorations. It appeared that transmucosal implants adhered to bone similarly as in the submucosal application. The clinical aspect of gingival tissues
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around the implants was the same as that around the adjacent natural teeth. Histologically, there were indications for a good attachment of gingival tissues to the implants. Implants fractured due to fatigue failure of the ceramic material. Prestressing of the implants may be the solution to this problem. Failing implants can clinically be treated by submerging, removal, or substitution. REFERENCES 1
Hench, L. L., Greenke, T. K., Paschal], H. A., Allen, W. C., and Piotrowski, G.: Reports 1-6, 1970-1975, U.S. Army Medical Research and Development Command, Contract DADA-17-70-C-0001. 2 de Groot, K.: Bioceramics consisting of calcium phosphate salts. Biomaterials 1:47, 1980. 3 Aoki, H., Kato, K., and Tabata, T.: Osteocompatibility of apatite ceramics in mandibles. Report of the Institute for Medical Dental Engineering, Tokyo, 11:33, 1977. 4 Jarcho, M., Kay, J. F., Gumaer, K. I., Dot-emus, R. H., and Drobeck, H. P.: Tissue, cellular and subcellular events at a bone-ceramic hydroxylapatire interface. J Bioeng 1:79, 1977. 5 Denissen, H. W.; and de Groot, K.: Immediate dental root implants from synthetic dense calcium hydroxylapatite. J PROSTHETDENT 42551, 1979. 6 Denissen, H. W., Veldhuis, A. A. H., Makkes, P. Ch., van den Hooff, A., and de Groot, K.: Dense apatite implants in preventive prosthetic dentistry. J Clin Prev Dent 2:23, 1980. 7. van der Kuij, P.: Reducing Residual Ridge Reduction. Thesis, Free University, Amsterdam, Netherlands, 1080. 8. Ogiso, M., Kaneda, H., Arasaki, J., Ishida, K., Shiota, M., Mituwa, T., Asano, K., Tabata, T., Yarnazaki, Y., and Hidaka, T.: Affinity of epithelial tissue with hydroxylapatite ceramics for the purpose of dental implantation. J Dent Res GO(Special issue A):419, 1981. 9. Bornhauser, A., and Pabst, R. F.: Evaluation of subcritical crack extension parameters with ceramic biomaterials. A critical examination. Abstract P. 1.3. Book of Abstracts First World Biomaterials Congress, Baden, near Vienna, April 1980.
Reprmt requeststo: DR. C. DE PUTTER FREE UNIVERSITY SCHOOLOF DENTISTRY 1007 MC AMSTERDAM HOLLAND
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