Effect of compromised cortical bone on implant load distribution.

PURPOSE: To investigate photoelastically the difference in load distribution of dental implants with different implant neck designs in intact and compromised bone. MATERIALS AND METHODS: Composite photoelastic models were fabricated using two different resins to simulate trabecular bone and a 1-mm thick layer of cortical bone. The following parallel-sided, threaded implants were centrally located in individual models representing intact and compromised cortical bone: Straumann (4.1-mm diameter x 12-mm length), AstraTech (4.0-mm diameter x 13-mm length), and 3i (3.75-mm diameter x 13-mm length). The compromised cortical bone condition was simulated by contaminating a 1-mm neck portion with Vaseline to impair the implant-resin interface. Vertical and oblique static loads were applied on the abutments, and the resulting stresses were monitored photoelastically and recorded photographically. RESULTS: For the fully intact condition, the highest stresses were observed around the crest and apical region for all implant designs under vertical and inclined loads. There were no appreciable differences in magnitude or distribution between implant types. With compromised cortical bone, for all designs and load directions, higher stresses in the supporting structures were observed. Increased stresses were noted especially at the cortical bone-trabecular bone interface. Somewhat lower stress levels were observed with the 3i implant. CONCLUSIONS: The condition of implant-cortical bone contact has considerable influence on stress distribution. A compromised cortical bone condition caused higher level stresses for all implant designs tested.

Effect of surgical techniques on primary implant stability and peri-implant bone.

PURPOSE: To evaluate the primary stability of dental implants placed with condensing-osteotome versus drilling-osteotome techniques and to explore peri-implant mircromorphologic consequences of lateral bone condensing. MATERIALS AND METHODS: The experimental model designed for the study comprised bilateral iliac crests from 3 fresh frozen human cadavers. Two AstraTech dental implants (AstraTech AB, Mölndal, Sweden) were consecutively placed with condensing- and drilling-osteotome techniques in bone with a 10-mm interimplant distance. Six experimental bone sites received a total of 12 implants. Installation torque values (ITVs) and implant stability quotients (ISQs) were measured to quantify primary implant stability. Bone specimens including implants were removed to quantify the peri-implant relative bone volume and bone microstructural parameters in the 1-mm circular vicinity of implants using desktop computed tomography (microCT). The Mann-Whitney U test was used to evaluate the differences in primary implant stability values and microCT data for the surgical placement techniques. RESULTS: ITVs and ISQs were similar for both surgical placement techniques without statistical significance (P > .05). Relative bone volumes around implants placed with the condensing-osteotome technique were significantly (P < .05) higher than those around implants placed with the drilling-osteotome technique. Microstructural parameters, such as trabecular thickness, separation, and number, differed significantly between the 2 osteotome techniques. CONCLUSIONS: Managing implant sites with the condensing-osteotome technique results in notable changes in peri-implant bone architecture but might not be as promising in improving primary implant stability compared with the drilling-osteotome technique.

Biomechanical aspects of initial intraosseous stability and implant design: a quantitative micro-morphometric analysis.

PURPOSE: The objective of this biomechanical study was to explore the effect of bone micro-morphology on initial intraosseous stability of implants with different designs. MATERIAL AND METHODS: Straumann and Astra Tech dental implants were placed into anterior and posterior regions of completely edentulous maxilla and mandible of a human cadaver. Experiments were undertaken to quantify initial implant stability and bone micro-morphology. Installation torque values (ITVs) and implant stability quotients (ISQs) were measured to determine initial intraosseous implant stability. For quantification of relative bone volume and micro-architecture, sectioned implant-bone and bone core specimens of each implant placement site were consecutively scanned and trabecular bone was analyzed in a micro-computed tomography (micro-CT) unit. Experimental outcomes were evaluated for correlations among implant designs, initial intraosseous implant stability and bone micro-structural parameters. RESULTS: ITVs correlated higher with bone volume fraction (BV/TV) than ISQs, at 88.1% and 68.9% levels, respectively. Correlations between ITVs and micro-morphometric parameters were significant at the 95% confidence level (P<0.05) while ISQs were not. Differences in ITVs, ISQs and BV/TV data in regards to implant designs used were not significant at the 95% confidence level (P>0.05). CONCLUSION: Bone micro-morphology has a prevailing effect over implant design on intraosseus initial implant stability, and ITV is more sensitive in terms of revealing biomechanical properties at the bone-implant interface in comparison with ISQ.

Influence of attachment systems on load transfer of an implant-assisted maxillary overdenture.

PURPOSE: This photoelastic study compared the load transfer characteristics of 2 retention mechanisms in an implant-assisted overdenture prosthesis. MATERIALS AND METHODS: Four implants were incorporated into a photoelastic model of a moderately resorbed edentulous human maxilla. Two retention mechanisms were studied by changing components on the same model and the palateless overdenture. The retention mechanisms studied were bar splint with anterior clip and distal resilient attachments, and solitary ball/O-ring attachments. Loads, ranging from 1.4 to 14.4 kg, were applied to the palatal incline of central incisors and buccal incline of premolars with and without balancing contacts. Stresses developed around all the implants under each loading condition were photographed in the field of a circular polariscope. RESULTS: With both retention mechanisms, protrusive and laterotrusive loads without balancing contacts caused instability of the overdenture, producing minimal stress around the implants in the supporting structure. High intensity stresses indicating intrusion of the posterior implants were noted when the bar/distal resilient attachment overdenture had balancing contacts for protrusive and laterotrusive loads. The posterior implants of ball/O-ring attachment overdenture exhibited high intensity stresses indicating not only intrusion, but also bending, when the occlusion was balanced. CONCLUSIONS: Balanced occlusion was required in both retention mechanisms for stability of the implant-assisted overdenture when clinically acceptable loads were applied. The protrusive and laterotrusive loads were not distributed equitably in either mechanism, since highest stresses occurred at the posterior implants.

Implant biomechanics in grafted sinus: a finite element analysis.

This in vitro study investigated the stress distribution in the bone surrounding an implant that is placed in a posterior edentulous maxilla with a sinus graft. The standard threaded implant and anatomy of the crestal cortical bone, cancellous bone, sinus floor cortical bone, and grafted bone were represented in the 3-dimensional finite element models. The thickness of the crestal cortical bone and stiffness of the graft were varied in the models to simulate different clinical scenarios, representing variation in the anatomy and graft quality. Axial and lateral loads were considered and the stresses developed in the supporting structures were analyzed. The finite element models showed different stress patterns associated with helical threads. The von Mises stress distribution indicated that stress was maximal around the top of the implant with varying intensities in both loading cases. The stress was highest in the cortical bone, lower in the grafted bone, and lowest in the cancellous bone. When the stiffness of the grafted bone approximated the cortical bone, axial loading resulted in stress reduction in all the native bone layers; however, lateral loading produced stress reduction in only the cancellous bone. When the stiffness of the graft was less than that of the cancellous bone, the graft assumed a lesser proportion of axial loads. Thus, it caused a concomitant stress increase in all the native bones, whereas this phenomenon was observed in only the cancellous bone with lateral loading. The crestal cortical bone, though receiving the highest intensity stresses, affected the overall stress distribution less than the grafted bone. The stress from the lateral load was up to 11 times higher than that of the axial load around the implant. These findings suggest that the type of loading affects the load distribution more than the variations in bone, and native bone is the primary supporting structure.

Three-dimensional morphometric analysis of human cadaver bone: microstructural data from maxilla and mandible.

The mechanical behavior of bone is a critical factor in the attainment and maintenance of osseointegration. Current procedures and classifications in assessing bone quality have limitations. Micro-computed tomography (microCT) is a new method to image and quantify bone with very high resolution. This study was aimed to analyze cadaveric maxillary and mandibular trabecular bone with 3-D morphometric data acquired through microCT and correlate with conventional bone assessment methods. Moderately resorbed edentulous maxilla and mandible from a human cadaver were scanned in the conventional CT unit with opaque markers indicating specific anterior and posterior sites. These sites were then sectioned and standard periapical radiographs were obtained. Bone cores were harvested from the sectioned sites and scanned in the microCT unit. Bone density values based on the Hounsfield scale ranged from 51 to 529 in the mandible and 186 to 389 in the maxilla, anterior sites being higher in both. Periapical images did not yield distinct differences. 3-D morphometric analysis in microCT produced a range of values with anterior specimens being favorable: bone volume density (0.12-0.291), trabecular thickness (0.12-0.16 mm), trabecular separation (0.46-0.82 mm), trabecular number (1.08-2.071/mm) and structural model index (0.29-1.27). General agreement between bone density and microCT indices was noted; however, subtle differences have to be studied with larger samples. This preliminary study suggests that the understanding of mechanical competence of trabecular bone might reveal further information about the prognosis of implant therapy, advancements in implant design, surgical techniques and grafting.

Load transfer by an implant in a sinus-grafted maxillary model.

PURPOSE: This in vitro study determined the stress distribution around an implant placed in a posterior edentulous maxillary model with simulated sinus grafts that had different degrees of stiffness. MATERIALS AND METHODS: The composite photoelastic model with a standard threaded implant consisted of simulated crestal cortical, cancellous, sinus cortical, and grafted bone. The graft maturation process and inherent graft quality were represented in the model by varying the stiffness of the graft. Prior to placement of the simulated graft, axial and inclined loads were applied to the implant The stresses that developed in the supporting structures were analyzed photoelastically. The graft was then placed and the testing procedure was repeated over 4 consecutive days, during which time the simulated graft stiffened. RESULTS: The stress analysis indicated that before placement of the simulated graft, loading on the implant transferred the highest stresses to cortical bone. The presence of the simulated graft transferred stress from the native bone simulants to the simulated grafted bone. DISCUSSION: As the stiffness of the graft increased, a more equitable stress distribution was observed in the multilayer bone surrounding the implant. CONCLUSION: Loading of an implant in a less stiff grafted sinus could lead to overloading of the native bone as well as the maturing grafted bone.