Force and moment distributions among osseointegrated dental implants

Zygoma implant under function: biomechanical principles clarified

PURPOSE

The purpose of this document is to clarify the biomechanical principles involved when zygoma implants are placed under functional loads.

METHODS

Two independent reviewers conducted electronic search of the literature from January 2000 to February 2023 describing the biomechanical principles involved using the zygoma implant for maxillary reconstruction. Articles describing the stresses within the zygoma implant, the maxillary bone and the zygoma bone under functional loads were included.

RESULTS

The lack of maxillary boney support at the implant platform resulted in significant higher stress measured within the zygoma implant as well as the zygoma bone.

CONCLUSION

The maxilla is the primary support when zygoma implants are placed under functional loads. Quad-cortical stabilization of the zygoma implants and their cross-arch stabilization are recommended to reduce the degree of stress whenever possible.

The normal stiffness of the edentulous alveolar process

The normal stiffness of the jawbone is seldom considered, as opposed to the mechanical properties of its individual cortical and trabecular components. Our standpoint is essentially structural, rather than purely material-oriented, as the jawbone is considered as a natural load-bearing structure. Throughout the work, 3 representative sections in the mandible and the maxilla are modelled and compared. Specifically, we evaluate the sections' elastic structural stiffness numerically, according to the recent geometrical classification proposed by Shemtov Yona (2021). Each case is modelled using two extreme configurations for the cortical-trabecular interaction, namely bonded and unbonded. Those two configurations reflect extreme interfacial conditions, though the bonded one is more physical. For the unbonded cases, the structural stiffness is the sum of the individual stiffnesses of the components. By sharp contrast, the bonded case results in a much larger stiffness than that obtained by the simple sum of the individual stiffnesses, indicating a strong synergistic stiffening effect between the components through their interface. We also investigate the role of the elastic moduli, whose reported values vary widely in the literature, emphasizing the role of the trabecular Poisson's coefficient, whose stiffening effect is evidenced when it exceeds about 0.3. The bone's structural stiffness shown here complements the geometrical classification of the jawbone types with a fundamental mechanical/structural property delineating the coupling between the mechanical properties and the geometry. The adopted approach is not limited to the jawbone and applies in principle to other bone types. From a clinical standpoint, the results presented here complement not only the basic mechanical aspects of the geometrical characterization, but also provide a starting point for future studies on dental implant placement and stability, the latter being directly related to the structural stiffness.

Design and ex vivo characterization of narrow implants with custom piezo-activated osteotomy for patients with substantial bone loss

OBJECTIVE

Bone augmentation delays implant placement and increases risks due to additional surgeries. Implant systems compatible with reduced alveolar bone volume are required. To design, manufacture, and test a non-cylindrical dental implant system using piezotomes and custom-designed matching titanium mini-implants to address the needs of patients with missing teeth and narrow jawbone.

MATERIALS AND METHODS

Tapered mini-implants with a rectangular cross-section (4.6 mm × 2.1 mm) were machined with dimensions that could accommodate narrow alveolar ridges. The performance of the implants were tested in both static and fatigue cycle 30° compression tests. Tapered, rectangular cutting tools that matched the overall trapezoidal morphology of the implant were also designed. These novel tools were engineered to be compatible with commercially available piezoelectric osteotomes. Tools were optimized using finite element analysis and were manufactured accordingly and were used by a periodontal surgery team in a pork rib bone model to monitor utility of the device and ease of use.

RESULTS

The rectangular design of the implant allows for a full occlusal load due to the larger implant flexural rigidity compared to a similar diameter mini-implant with a standard cylindrical design. During 30° compression fatigue tests, the implant tested at 340 N did not fail after 5M cycles as shown in Kaplan-Meier survival curves. Finite element analysis allowed for functional optimization of the roughing and finishing tools. In the pork rib model, these tools successfully cut trapezoidal holes that matched the dimensions of the implant.

CONCLUSIONS

The implant system here demonstrates the feasibility of a mini-implant system that has superior flexural rigidity and potentially circumvents the need for patient bone augmentation.

Optimal placement of the two anterior implants for the mandibular All-on-4 concept

The novelty of the All-on-4 concept for a mandibular implant-supported fixed dental prosthesis is the inclination of the posterior implants. Typically, the anterior implants are placed lingually relative to the canine/incisor teeth and perpendicular relative to the occlusal plane. According to the laws of elementary biomechanics, the long axis of the implant unit should be aligned to the axis of the occlusal loading forces during clenching in the maximal intercuspal position. When several implants are connected by a prosthesis, the mean axis of the overall occlusal loading must be taken into account. The objective of this report was to propose a different position for anterior implants by tilting them labially to counterbalance the distal inclination of the posterior implants.

Finite Element Analyses of Two Antirotational Designs of Implant Fixtures

The purpose of this study was to compare the effect of cyclic compressive forces on loosening of the abutment retaining screw of dental implant fixtures with two different antirotational designs using the finite element analysis. A three-dimensional model of externally hexed and trichannel dental implant fixtures with their corresponding abutments and retaining screws was developed. Comparison between the two designs was carried out using finite element analysis. The results revealed that the externally hexed design has significantly higher overall stress, contact stress, and deflection compared with the trichannel design. The trichannel antirotational design has the least potential for fracture of the implant/abutment assembly in addition to its capability for preventing rotation of the prosthesis and loosening of the screw.

A novel computational method for real-time preoperative assessment of primary dental implant stability

A novel methodology which allows for fast and fully automatic structural analysis during preoperative planning for dental implant surgery is presented. This method integrates a fully automatic fast finite element solver within the framework of new concepts in computer-assisted preoperative planning for implant surgery. The planning system including optimized structural planning was validated by experimental results. Nine implants were placed in pig mandibles and mechanically loaded using a testing rig. The resulting displacements were measured and compared with those predicted by numerical analysis during planning. The results show that there were no statistically significant differences (P = 0.65) between the results of the models and the experiments. The results show that fast structural analysis can be integrated with surgical planning software allowing the initial axial implant stability to be predicted in real time during planning. It is believed that such a system could be used to select patients for immediate implant loading and, when further developed, be useful in other areas of preoperative surgical planning.

Biomechanical stress in bone surrounding an implant under simulated chewing

The concept of reducing nonaxial loading of dental implants has been widely regarded as the standard procedure. The aim of this study was to reveal the biomechanical stress distribution in supporting bone around an implant and a natural tooth under chewing function. Three-dimensional finite element models of the mandibular first molar and the titanium implant both with the mandible in the molar region were constructed. The directions of displacement constraints were determined according to the angles of the closing pathways of chopping type and grinding type chewing patterns. The tooth model showed smooth stress distribution in the supporting bone with low stress concentration around the neck of the tooth. The implant model showed stress concentration in the supporting bone around the neck of the implant, especially in the buccal area. The grinding type model of the implant showed higher tensile stress concentration than the chopping type model at the lingual neck of the implant. The results of this study suggested the importance of considering occlusion under chewing function for understanding the biomechanics of oral implants.

Anisotropic elasticity of cortical and cancellous bone in the posterior mandible increases peri-implant stress and strain under oblique loading

The aim of this study was to compare implant-bone interface stresses and peri-implant principal strains in anisotropic versus isotropic three-dimensional finite element models of an osseointegrated implant in the posterior mandible. We obtained anisotropic (transversely isotropic) elastic constants for mandibular bone and derived equivalent isotropic constants by averaging over all possible spatial orientations. A finite element model was constructed using ten-node tetrahedral p-elements, providing curved edges where necessary and increasing the accuracy of the results in regions of high stress gradients. Perfect bonding was assumed at the implant-bone interface. An oblique load was applied at the coronal aspect of the crown with 100 N vertical and 20 N bucco-to-lingual components. Implant-bone interface stresses exceeded reported bond strengths and principal strains reached yield strain levels in the cortical crest. Anisotropy increased what were already high levels of stress and strain in the isotropic case by 20 to 30% in the cortical crest. In cancellous bone, anisotropy increased what were relatively low levels of interface stress in the isotropic case by three- to four-fold to exceed bond strength levels. Anisotropy has subtle, yet significant effects on interface stresses and peri-implant strains and careful consideration should be given to its use in finite element studies of dental implants.

Overdenture attachment selection and the loading of implant and denture-bearing area. Part 1: In vivo verification of stereolithographic model

Preliminary to a study investigating the force transfer from osseointegrated dental implants to the surrounding bone via various types of overdenture attachment, a stereolithographic model (SL-model) was constructed and compared to an in vivo situation in order to confirm the validity of the modeling technique for the planned measurements of implant strain and denture-bearing area loading. The SL-model was generated using the patient's computer tomographic data and duplicated in a material of known elastic properties. The model was fitted with sensors to measure strains in the peri-implant bone and loading forces within the posterior mandibular bone, i.e. the denture-bearing area of the mandible. Special telescopic copings were constructed to measure implant strain in this model as well as in vivo. Using these copings under identical overdenture loading conditions, the strains measured at the implants in vivo and in vitro were the same and never exceeded a tolerance of two standard deviations or a mean difference of -8.5% of the in vitro value. This indicates that the model was reliable for the measurement of implant strain. Denture-bearing area loading within the alveolar ridge cannot be measured in vivo. Instead, a method of extrapolating in vivo denture-bearing area loading figures from implant strain readings was developed and tested (better than 90% accuracy). These in vivo extrapolated figures were then compared to in vitro readings under otherwise identical loading conditions. The result indicated that the SL-model is reliable for measurements of denture-bearing area loading with an error of 10 to 20%.

Theoretical cantilever lengths versus clinical variables in fifty-five clinical cases

STATEMENT OF PROBLEM

Cantilever loading increases loads distributed to implants, potentially causing biomechanical complications. The implemented length is often less than what is considered to be optimal.

PURPOSE

This study investigated the effects of clinical variables on predicted cantilever lengths. Theoretically, calculated maximum cantilever was defined as the length that would not cause gold screw loosening or fatigue failure. The variables investigated included number and distribution of implants, arches placed, and the clinician's "optimal" cantilevers.

MATERIAL AND METHODS

Implant and prosthesis location coordinates of 55 clinical cases were determined from casts. The distribution of an applied 143 N vertical load to implants was calculated through the Skalak model for more than 500 loading sites. Gold screw joint overload was assumed to occur at 200 and 250 N in compression and tension. Calculated lengths were compared with clinical variables.

RESULTS

For a set number of implants, the relationship between calculated cantilever length and anterior-posterior spread was linear. The sum of length on both sides versus prosthesis length between the most distal implants was linear, regardless of the number of implants. Predicted satisfaction was defined as calculated length greater than the clinicians' optimal length. Satisfaction rates were 100%, 56%, 33%, 8%, and 0% for cases supported by 8 and 7, 6, 5, 4, and 3 implants (44% overall), respectively. Ninety-eight percent of cases with anterior-posterior spreads greater than 11.1 mm were satisfied.

CONCLUSION

Within the limitations of the model, predicted complications of the gold screw joint may be reduced if: (1) cantilever length is less than calculated from linear equations, and (2) anterior-posterior spread is greater than 11.1 mm.

One clinical visit for a multiple implant restoration master cast fabrication

The making of a one-piece, long-span, implant-supported prosthesis with conventional procedures frequently has difficulties associated with the accuracy of fit. This article presents a clinical and laboratory procedure for making an accurate implant working cast that facilitates fabrication of the casting on the master cast. The procedure demonstrates the process of sectioning and rejoining of the resin between the transfer copings and then pouring the impression by first joining the analogs alone with impression plaster, sectioning it, and rejoining it again to stabilize the analogs, and finally, using dental stone to pour the impression. Clinical, radiographic, and laboratory (optical microscope) measurements for one clinical implant restoration confirm the accuracy of fit of this one prosthesis made with this procedure. Its advantage is that it can allow fabrication of the final casting on the cast, thereby eliminating the clinical time necessary to obtain repetitive solder indexes, and thus minimizing inconvenience to the patient.

Dental materials: 1995 literature review

This critical review of the published literature on dental materials for the year 1995 has been compiled by the Dental Materials Panel of the United Kingdom. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, resin composites, dentine bonding, dental amalgam, endodontic materials, casting alloys, investment materials, resin-bonded bridges and ceramo-metallic restorations, all ceramic restorations, denture base and soft lining materials, impression materials, dental implants, orthodontic materials and biomechanics). Three hundred and thirty articles published in 68 titles have been reviewed.