Adhesive improvement of the mechanical properties of a dense HA-cemented Ti dental implant

PHASE STABILITY AND MICROSTRUCTURAL DEVELOPMENT OF Y-TZP–DOPED HYDROXYAPATITE

Hydroxyapatite containing yttria-tetragonal zirconia polycrystals (Y-TZP) with different concentrations were prepared by using the conventional ball-milling technique. Samples prepared from these powders were compacted and sintered at three different temperatures (1250°C, 1400°C and 1600°C). The sintering behaviour of the composites were studied in terms of phase stability, densification and microstructural evolution. By X-ray diffraction, tetragonal zirconia as well as HA was identified as the major phases in the composites. In addition, small diffraction peaks corresponding to α-TCP was evident indicating sign of HA decomposition. Reaction between the zirconia and HA to form CaZrO2 was also detected in the composite system containing up to 20 wt% Y-TZP. SEM examination of the sintered composites at room temperature suggested the involvement of a reactive liquid phase during sintering. The presence of zirconia phase in hydroxyapatite matrix played an important role in the stability of the HA phase.

Effects of installation depth on survival of an hydroxyapatite-coated Bicon implant for single-tooth restoration

PURPOSE

The present retrospective study evaluated various implant surface factors associated with Bicon implant survival for single-tooth restoration in the healthy individual.

PATIENTS AND METHODS

A retrospective cohort study design was used. A total of 613 Bicon (Bicon System, Boston, MA) implants (272 patients) were included. Because the use of hydroxyapatite (HA) coating has been controversial, the surface type was chosen according to the patient's preference. A total of 308 HA-coated implants and 305 titanium plasma-sprayed (TPS) implants were used. The macroanatomy of both implant types was identical. Patients who had systemic disease, poor quality bone, or removable prosthetics were excluded from the present study. A chart review was conducted to record age, gender, implant diameter, implant length, installation depth, installation location, and the use of bone grafting. Implant failure was recorded, and the data were analyzed using the chi(2) test and logistic regression analysis.

RESULTS

The installation depth was an important prognostic factor in the HA-coated implants. The failure rate for the HA-coated implants installed at margin level and 2 mm below level was 10.29% and 3.01%, respectively (chi(2) = 6.035, P = .014). The implant length was an important prognostic factor for the TPS-treated implants. The failure rate recorded for the TPS-treated implants installed with a length of less than 10 mm and 10 mm or longer was 15.46% and 2.40%, respectively (chi(2) = 18.414, P < .001).

CONCLUSION

Many factors can influence the failure rate of TPS and HA-coated Bicon implants. Among these, installation depth played an especially significant role in the success of HA-coated Bicon implants in the present study.

Integrin mediated attachment of periodontal ligament to titanium surfaces

OBJECTIVE

Reducing the force between the implant and the bone by recapitulating a similar matrix has the potential to reduce implant failure. To begin to pursue the goal of creating a periodontal ligament interface between a dental implant and bone, the mechanism of cellular attachment to dental implant surfaces must be characterized.

METHODS

In this study we examined the role of integrin receptors in the attachment of periodontal ligament fibroblasts to titanium surfaces utilized on dental implants; those surfaces included smooth polished titanium, acid pickled titanium, ground titanium, sandblasted and acid etched titanium, non-oxidized titanium that has been sandblasted and acid etched, hydroxyapatite coated titanium, titanium plasma sprayed or uncoated titanium. For these studies integrin mediated fibroblast attachment was blocked by the integrin blocking peptide GRGDSP or anti-integrin beta1 antibody or a combination of the two. Quantitation of periodontal ligament fibroblast attachment was completed by counting cells on the various implant surfaces after culturing in vitro for 24h with and without the integrin receptor blockers.

RESULTS

Antibody and peptide treatment significantly reduced the number of fibroblasts cells attached to the various implant surfaces but this effect varied significantly depending on the surface. Moreover, increased levels of peptide further decreased fibroblasts attachment in a dose dependent manner.

SIGNIFICANCE

Blocking studies suggest first, that integrin receptors function in periodontal ligament attachment to titanium surfaces and second, that different integrin subunits are important in attachment to a particular surface.

Reassessment of long-term use of dense HA as dental implant: case report

Dense hydroxyapatite (HA) is widely believed to be unsuitable for clinical use as dental implants due to its poor mechanical properties, although it has excellent biocompatibility and is chemically stable and nonresorbable in vivo. However, the case in this article is one in which the patient's dense HA implants are still stable and in good functional condition 16.5 years after he received four pieces of a one-piece dense HA implant in both sides of his lower molar regions. Furthermore, almost no radiolucency is evident along the root portions of the implant sites in the bone. These findings imply that dense HA can be clinically useful and should be reevaluated as a dental implant material.

Differences in microstructural characteristics of dense HA and HA coating

Two implant types of hydroxyapatite (HA) currently are available for dental implants: dense HA-cemented titanium (Ti) and HA-coated. It has been shown in previous reports that there are differences in the chemical and mechanical stabilities between the dense HA and HA coated. The differences are thought to be due to structural differences between the two ceramic types. The aim of this study was to investigate the differences in microstructural characteristics of currently available dense HA and HA coated implants before implantation and at periods of 3 weeks and 10 months after implantation in canine bone. X-ray diffractometry, infrared analysis, transmission electron microscopy, and energy dispersive X-ray analysis were used. The dense HA is composed of crystal grains, with a well crystallized structure of HA, closely bound to each other and approximately 0.4-0.6 micron in size. Implantation did not change the original sintered structure of the dense HA. The HA coating was composed of an amorphous phase with a Ca/P ratio of 1.46 and a crystal phase consisting of oxyhydroxyapatite, tricalcium phosphate, tetracalcium phosphate, and CaO, with a Ca/P ratio of 1.57. In the amorphous phase, compared to other portions in the amorphous phase, there were some layers with lower atomic density and with no significant difference in Ca/P ratio. After implantation, the crystallization of super fine crystals of approximately 4-5 nm in thickness occurred in the amorphous phase, and with time it progressed and spread from the surface to the deeper portion of the HA coating. A Ca/P ratio of 1.58 in the crystallized portion was close to the ratio (1.60) in the dense HA, suggesting that the super fine crystals were HA. This crystallization cannot significantly decrease the solubility of the amorphous phase portion and poses risks of stress accumulation within the coating and a decrease of binding strength between the HA coating and the substrate.

The process of physical weakening and dissolution of the HA-coated implant in bone and soft tissue

Hydroxyapatite (HA)-coated implants were developed to promote osseointegration of titanium implants and to overcome the mechanical drawbacks of solid HA implants. Although many clinical reports on the prognosis of HA-coated implants have reported high success rates, the risks of dissolution and weakening of the coating have been noted. We hypothesized that the chemical and mechanical stability of HA coating are affected by its microstructural characteristics. The present study investigates differences in the microstructures of available HA-coated implants, before and after implantation into the coxal bones of dogs for periods ranging from 3 weeks to 10 months and under the coxal periosteum of dogs for 10 months. The results of transmission electron microscopy and energy-dispersive x-ray analysis revealed that crystallization of super-fine HA crystals occurred in the amorphous phase of the HA coating and progressed over time. This crystallization weakens HA-coated implants by making the amorphous phase brittle, causing stress accumulation within the coating, and causing a decrease in the binding strength between the coating and the substrate. Furthermore, the HA coating dissolved in soft tissue. Dissolution started with the super-fine HA crystals in the crystallized portion that was originally part of the amorphous phase.

Comparative push-out test of dense HA implants and HA-coated implants: findings in a canine study

Two types of hydroxyapatite (HA) implants have been developed: an HA-coated implant and a dense HA implant. For a longer in situ life span, the HA implant must remain chemically stable and possess high resistance to occlusal force. To determine which type of HA implant shows better durability, this comparative dog study was done to evaluate push-out test results of HA-coated implants and dense HA implants of approximately the same size after implantation in the mandibular and coxal bones for periods ranging from 3 weeks to 10 months. The findings revealed that for the mandibular implants, the push-out values of HA-coated implants were significantly higher than those of dense HA implants at 2 and 4 months after implantation, with significance levels of p < .001 and p < 0.05, respectively. However, there was no significant difference between the two implant types at 10 months. As for the coxal implants, no significant differences were noted for any period. Furthermore, the ratio of push-out values of the dense HA implants to those of the HA-coated implants situated in the same position bilaterally in each bone of the body for each implantation period rose with the passage of time, especially in the mandible. In the mandibular implants, the correlation coefficient of the relationship between the ratio and duration of implantation was highly significant (p < 0.001). Push-out testing caused detachment of the surface portion of the HA coating that was bound to the dense bone from the HA-coated implant at 2, 4, and 10 months after implantation. Furthermore, at 10 months the HA-coated layer in the wide areas of the implants had completely detached from the metal substrate, in contrast to the dense HA implants, which remained durable throughout the test period.

Microstructural changes in bone of HA-coated implants

In our previous comparative push-out test of HA-coated implants and dense HA implants in dog bone, the ratio of the push-out value of the HA-coated implant to that of the dense HA implant decreased with time due to weakening of the HA coating as compared to the dense, more durable HA. The aim of this study was to investigate by histological examination of HA-coated implants in dog bone, using TEM, how this weakening of the HA coating occurs. The HA coating before implantation is composed of an amorphous glassy phase and a crystal phase scattered within the glassy phase. After implantation, the crystal phase remained almost unchanged. However, in the glassy phase, crystallization occurred and progressed with time. By 3 weeks after implantation, this crystallization already had started in the surface portion of the HA coating where it was covered by bone and also where it still touched the soft tissue. By 10 months, the crystallization had progressed to the deeper portion of the HA coating and had expanded to most of the glassy phase except for the narrow portions along the substrate-coating interface. These findings suggest that a progression of crystallization in the glassy phase causes stress accumulation within the HA coating, especially in the interface between the HA coating and the substrate, and that this stress accumulation results in a weakening of the HA-coated implant.