Medical knowledge | Dentistry » Williams-Murphy - A finite element stress analysis of the transmandibular implant system

A Finite Element Stress Analysis of the Transmandibular Implant System Keith R. Williams, BSc, PhDa William M. Murphy, BDSb Purpose: The aim of this work was a simulation of the onset of osseointegration of the transmandibular implant system in the mandible. This was achieved by imposing joint elements at the implant-bone interface. Materials and Methods: The geometric model was derived from the anatomic measurement of several mandibles by tomographic scanning. The spring constants of the joint elements interposed at the implant-bone interface were varied between 1 N/mm and 109 N/mm to represent the conditions leading to osseointegration. Results: Increasing the value of the spring constant at the joint elements gradually increases the effective stress in an increasing volume of crestal cortical bone. Additionally, a larger volume of crestal cortical bone assumes a higher stress value as the simulation proceeds. Conclusion: This work indicates that considerable changes in stress

magnitude and distribution occur in the crestal cortical bone margins as osseointegration is simulated, which may be the necessary stimulus for bone remodeling. Int J Prosthodont 2001;14:115–119. T alveolar bone resorption.1–8 Finite element analysis has been used to help our understanding of the biomechanical issues related to the design, shape, positioning, and function of various implant systems.9–15 The main clinical indication for use of the transmandibular implant (TMI) system16,17 has been stated as a situation with insufficient bone height for conventional implants. It has been claimed that the Bosker TMI can function in extremely atrophic mandibles (as small as 2 mm) without mandibular augmentation.18 Following surgery, osseous wound healing adjacent to the posts and screws provides support for the implant prostheses. Additionally, it has been shown that bone regeneration of the atrophic mandible progresses with time and function.19,20 The increase in bone height is

commonly observed in the premolar region and occasionally mesial to the most distal abutment. The finite element method is now widely used and applied to the design and function of implant systems in bone tissue.9–15,21 In this work, rather than assume complete fixity of the metal posts and screws in the he progressive reduction of the residual alveolar ridge following tooth loss has been well documented. Clinicians are well aware of the problems posed by these changes, and a great number of hours are spent in treating the condition. Retention, stability, comfort, and appearance can all be compromised in attempting to provide satisfactory complete dentures in these situations. Implants have certainly helped to improve the oral rehabilitation of patients with severe a Reader, Department of Basic Dental Science, Dental School, University of Wales College of Medicine, Heath Park, Cardiff, Wales. b Senior Lecturer, Department of Prosthetic Dentistry, Dental School, University of Wales

College of Medicine, Heath Park, Cardiff, Wales. Reprint requests: Dr K. R Williams, Department of Basic Dental Science, Dental School, University of Wales College of Medicine, Heath Park, Cardiff, Wales CF4 4XY, United Kingdom. Fax: + 02920 744509. e-mail: wbdkrw@thorcfacuk This work was presented at the British Society for Dental Research meeting, 10–13 April 2000, Lancaster University, United Kingdom. Volume 14, Number 2, 2001 115 The International Journal of Prosthodontics Finite Element Analysis of Transmandibular Implants Fig 1 Williams/Murphy of 0.5 mm to a position local to the distal implant From this point to the condyle, the cortical bone thickness gradually diminishes and was therefore not included. The current model included posts of one length only, at 22 mm, and screws of 16-mm length. The elements employed were mainly eight-noded brick and six-noded pentahedral parabolic elements for the bone and metal components, amounting to a total of 829 elements

incorporating 3,555 nodes. Osseointegration was modeled by merging the nodes at the interface region of the implant and surrounding bone. Partial osseointegration leading to complete integration during the healing period was simulated by the use of three-dimensional joint elements interposed between the implant and surrounding bone. Values of spring constants used in the joint elements were between 1 N/mm (essentially equivalent to the elastic modulus of collagen) to represent fibrous encapsulation and 109 N/mm (equivalent to the modulus of cortical bone) to represent complete fixity or osseointegration. This numeric procedure was thought reasonable in the present model, effectively tracking the gradual onset of mineralization local to the implant. Support of the model was provided by using either spring or completely fixed conditions at the nodes representing the condyle and at the nodes of the symmetry position. These two support conditions represented the extremes thought possible

in the context of the current simulation. For both simulations, a simple 10-N load was applied along the axis of the post adjacent to the symmetry position. Because in this investigation a linear elastic analysis was employed, doubling the applied load will simply double the calculated interface stresses. The aim was to simulate the effects of a gradually changing mechanical property value of tissue local to the implant surface and the corresponding changes in local stress patterns. These changing stress patterns will depend only on the local tissue properties, with only the magnitude of the stress depending on applied load. The elastic modulus of the type IV gold alloy required for the input data file to the finite element program is well documented at a value of 91 GPa, while those for the cortical and cancelleous bone were chosen at 16.25 GPa and 210 GPa, respectively.22 Because the analysis is a linear elastic calculation, the bone was necessarily considered homogeneous and

isotropic in this work, which is probably a reasonable assumption. The stress contour patterns generated when there is complete fixity at the condyle and symmetry position will not be discussed. Under these severe boundary conditions, the stress levels are small and relatively uniform throughout the model discretization. Such boundary conditions are expected to be unrealistic because Transmandibular implant system. mandible following surgery, a gradual increase in the level of osseointegration was assumed with time. This was achieved by introducing spring elements at the metal-bone interface. By varying the values of these spring constants, it was possible to progressively increase the level of implant osseointegration. In this way, the changing stress magnitude and patterns can be observed in the cortical bone from levels of poor integration through to complete implant fixity. Bone cell behavior is influenced by complex local and systemic factors. In relation to endosseous dental

implants, stress transmitted via the prosthesis is clearly one of these factors, but little is known about the levels of stress that influence bone remodeling. The claim that the TMI system increases bone regeneration19,20 has been of particular interest and was the stimulation for the present work. The stress levels generated in cortical bone following TMI surgery have been calculated and analyzed using the noninvasive finite element method. Materials and Methods The TMI system is shown in Fig 1 and comprises a base plate, four posts of various lengths depending on the height of the remaining alveolar bone, and medial and lateral cortical screws of sizes specified by the mandibular anatomy. A bar connects the posts through locking nuts, screws, and sleeves, which in turn support the prosthesis. The metal superstructure is a gold-copper alloy similar to type IV gold inlay and crown material. The finite element geometric model used in the present work derives from the anatomic

measurement of several mandibles by tomographic scanning. The mandible and metal superstructure are assumed to be symmetric; accordingly, only one half is discretized. The thickness of the cortical layer, although known to be variable, was modeled with a mean dimension The International Journal of Prosthodontics 116 Volume 14, Number 2, 2001 Williams/Murphy Finite Element Analysis of Transmandibular Implants Fig 2 Effective stress generated in the cortical bone with the spring constant set at 1 N/mm, simulating fibrous encapsulation. Fig 3 Effective stress generated in the cortical bone with a spring constant of 105 N/mm, simulating partial mineralization of adjacent tissue. Maximum effective shear stress (MPa) 18 16 Maximum effective basal stress (MPa) 14 Maximum effective crestal stress (MPa) 12 10 8 6 4 2 0 Log spring constant Fig 4 Effective stress generated in the cortical bone with a spring constant set at 109 N/mm, simulating complete integration. Fig 5 Maximum

effective stress generated in the basal and crestal cortical bone versus the log of the spring constant values of the joint elements used to simulate osseointegration. from 1 N/mm to 109 N/mm, then a changing stress pattern and magnitude is observed in the basal and cortical bone. On increasing the spring constants to 105 N/mm, the stress magnitude increases in the crestal cortical bone as shown in Fig 3 until, on complete integration at 109 N/mm, the stress pattern and magnitude shown in Fig 4 is observed. The maximum effective stress in the basal and crestal cortical bone is plotted against the spring constants chosen in the joint elements as shown in Fig 5. In each of these simulations, it is assumed that all tissue in contact with the implant takes on a value of the prescribed spring constant. Decreasing the spring support conditions at the condyle and symmetry positions also diminishes marginally the magnitude of stress in the cortical bone. However, for the purposes of this

work, only a single value of spring support, 100 N/mm, was used. it has been shown that translational and rotational displacements occur along the mandible during function.16,17 Accordingly, only the spring-supported conditions at the condyle and symmetry plane maintained at a constant value of 100 N/mm were employed. Results An example of the stress patterns and magnitudes generated when implants are assumed to be nonintegrated, ie, surrounded by a fibrous capsule, is shown in Fig 2. The maximum effective stress is small and generally uniform over the volume of the crestal cortical bone. If a situation leading to final osseointegration is assumed to occur by gradually increasing the properties of the tissue local to the implant surface through a series of joint elements with spring constants ranging Volume 14, Number 2, 2001 117 The International Journal of Prosthodontics Finite Element Analysis of Transmandibular Implants Williams/Murphy Discussion markedly changing stress

contour pattern across the crestal cortical margins, may well be the stimulus responsible for the bone deposition observed clinically.18–20 The significantly changed stresses observed when the geometric model was supported by a spring constant of 100 N/mm at the condyle and symmetry positions rather than rigidly fixed may be a consequence of flexural bending of the mandible under the more relaxed support conditions provided. This is entirely feasible because bending of the mandible has been observed during function and has been modeled using finite element methodology.16,17 Because this program was based on a linear elastic analysis with no imposed fracture criteria, there was no cut-off mechanism within the program to prevent stresses from increasing beyond a supposed fracture value. It is therefore significant that the basal cortical bone is likely to fracture if loads are applied prior to the osseointegration of the posts and screws. Further work on the stress levels responsible

for bone remodeling and also on the local bone fracture stress magnitude is essential so that the finite element methodology can be fully exploited in the design and clinical positioning of dental and medical prostheses for optimum reliability. This study does not presume to fix the level of bone stress leading to bone deposition or resorption, but rather simulates stress patterns that are likely to be responsible for bone volume changes. The model relies, in common with other studies, on homogeneous, isotropic, and elastic behavior of the constituents that are considered reasonably accurate.23 Furthermore, in most finite element analysis studies, complete contact with bone is presumed over the total implant surface, a situation rarely achieved clinically. In this analysis, a gradual fixation of the implant to surrounding bone is modeled by interposing joint elements at the implant-bone interface. The joint element allows spring support in three dimensions, and by varying the value of

the spring constants, the implant can be assumed to be fibrously encapsulated, partially integrated, and up to completely osseointegrated. In this way, it is possible to simulate the gradual osseointegration of the posts and screws in the mandible following surgery. Figure 2 shows the effective stress level and pattern developed when the posts are poorly integrated. In this condition, a remarkably high stress develops in the basal cortical bone in the area around the loaded post. It is thought that this condition arises because of the fact that all of the applied load must be supported by the base plate attached to the loaded post, which then bends elastically, also bending the basal cortical bone to which it is attached by the screws. Under these boundary conditions, the stress in the crestal cortical bone is small and uniform, as indicated in Fig 2. On increasing the spring connectivity of the joint elements between post and bone to 105 N/mm, a significant change in stress pattern

and magnitude emerges. The maximum basal cortical bone stress falls, while the crestal bone stress increases. The stress on the crestal cortical bone is also distributed around the most distal implant, as shown in Fig 3. A further increase in spring connectivity of the joint elements to 109 N/mm indicates that the majority of loading is now taken up by the crestal cortical bone, as shown in Fig 4. Additionally, the relatively constant stress in the crestal bone found at low values of connectivity has now given way to a markedly varying pattern of stress. Despite the amount of published skeletal research, the magnitude and rate of mechanical loading that generates remodeling remain unknown, as does the mechanism whereby loads are transduced to cellular activity.24 However, what is clear from the present analysis is that the stress in the crestal cortical bone gradually increases as osseointegration of the posts is simulated. This increase in stress, together with the The International

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NL, Ismail YH, Zaki HS, Pipko D Three-dimensional finite element stress analysis in and around the Screw-Vent implant. Int J Oral Maxillofac Implants 1991;6:391–398 13. Rieger MR, Adams WK, Kinzel GL A finite element survey of eleven endosseous implants. J Prosthet Dent 1990;63:457–465 14. Meijer HJA, Kuiper JH, Starmans FJM, Bosman F Stress distribution around dental implants: Influence of superstructure, length of implants, and height of mandible. J Prosthet Dent 1992;68:96–102 15. Meijer HJA, Kuiper JH, Starmans FJM, Bosman F A comparison of three finite element models of an edentulous mandible provided with implants. J Oral Rehabil 1992;20:147–157 16. Bosker H The Transmandibular Implant [thesis] Univ of Utrecht, 1986. 17. Bosker H, Powers MP The TMI reconstruction system In: Fonseca RJ, Davis WH (eds). Reconstructive Preprosthetic Oral and Maxillofacial Surgery, ed 2. Philadelphia: Saunders, 1995: 565–668. 18. Bosker H, Jordan RD, Sindet-Pedersen S, Koole R The

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Literature Abstract Biological and technical complications and failures with fixed partial dentures (FPD) on implants and teeth after four to five years of function. The aims of this study were to compare the prevalence of biologic and technical complications and failures with fixed partial dentures (FPD) on implants and teeth and as mixed tooth-implant–supported reconstructions after 4 to 5 years of function, and to associate biologic and technical complications with risk factors. A total of 85 partially edentulous patients (53 women and 32 men) were recruited for the study. The mean age of the patients was 55 years All implants belonged to the ITI dental implant system (Straumann) The patients were divided into three groups. Group I-I consisted of 33 patients with 40 FPDs on implants, group T-T consisted of 40 patients with 58 FPDs on teeth, and group I-T included 15 patients with 18 mixed tooth-implant–supported FPDs. Complete failure resulted in the loss of one FPD in each

group Two implants were lost because of fracture secondary to development of a bone defect One tooth had a vertical fracture, and one tooth was lost because of periodontitis. Biologic complications (periimplantitis) occurred at 5% to 10% of the implants Biologic complications occurred at 12% of the abutment teeth; 3% had secondary caries, 5% had endodontic problems, and 4% had periodontitis. Ten of 32 patients with a general health problem indicated a biologic complication More technical complications were found in FPDs on implants and were associated with bruxism. Six of ten bruxers had a technical complication, whereas 13 of 75 nonbruxers had such a complication. Extensions were associated with more technical complications It was concluded that favorable clinical conditions were found at tooth and implant abutments after 4 to 5 years of function. Loss of FPDs over 4 to 5 years occurred at a similar rate with mixed-, implant-, or tooth-supported reconstructions. Significantly more

porcelain fractures were found in FPDs on implants. Impaired general health status was not significantly associated with more biologic failures, but bruxism as well as extensions were associated with more technical failures Brägger U, Aeschlimann S, Bürgin W, Hämmerle CHF, Lang N. Clin Oral Implants Res 2001;12:26–34 References: 44. Reprints: Prof Dr Urs Brägger, Department of Periodontology and Fixed Prosthodontics, University of Berne, Freiburgstrasse 7, CH-3010 Berne, Switzerland. e-mail: ursbraegger@zmkunibechAW Volume 14, Number 2, 2001 119 The International Journal of Prosthodontics