Original study - ZZI 03/2012

Bionic design of small diameter dental implants

P. Streckbein1, J.-F. Wilbrand1, R. Streckbein2, H.-P. Howaldt1, M. Flach3

Introduction: Optimization strategies are an established part of product design processes in various industrial fields (automotive engineering, aerospace construction etc.). The bionic design method provides an efficient process to optimize the mechanical and biomechanical behaviour of dental implants following the example of nature. Optimization of dental implants using this method could result in a higher resistance to fracture and reduced strain in peri-implant bone.

Material and method: The bionic design method is applied to an axisymmetric finite element model to optimize the geometry of the implant. Stress distributions are calculated and analyzed for the optimized geometry of a reduced-diameter dental implant (BEGO Semados Mini-Implant, BEGO Implant Systems, Bremen, Germany) and the peri-implant bone. The optimized geometry is adopted in a CAD (computer-aided design) model and verified in a three-dimensional finite element analysis. The durability limits are determined by a final fatigue test according to the ISO 14801 standard “Dentistry – Fatigue test for endosseous dental implants”.

Results and discussion: By applying the bionic design method to the axisymmetric model, the stresses in the respective implant and the peri-implant bone can be reduced by at least 34 %. The verification of the CAD model shows an improvement to a more homogenous stress distribution. The ISO 14801 fatigue test shows high durability limits. The disadvantage of an expensive production process caused by complex bionic design geometries can be reduced using modern CAM (computer-aided manufacturing) production technologies.

Conclusion: The bionic design method is a very effective and easy to use procedure to design and optimize dental implants. The presented method provides a definite improvement for the construction process of dental implants.

Keywords: biomechanics; mini-implants; long-term success

Introduction

The treatment of patients with endosseous implants has become widespread in dentistry and is generally accepted. Over 150 implant systems are available today to the implantologist. The design of the implants varies greatly, ranging from a one-piece titanium implant to a two-piece zirconium implant. Implant systems are often developed on an empirical basis so the product characteristics can be evaluated only in experiments or clinical studies.

The design of an implant can be improved if the characteristics can be assessed in the construction phase. Virtual product development has long been established in many areas of technology (automotive engineering, aerospace construction etc.) and the development process can no longer be imagined without it [23].

Computer-aided optimization strategies are increasingly used in virtual product development to provide an optimal influence on product characteristics [18]. Initial attempts to design dental implant systems using optimization strategies are described in the literature [19]. However, reports on the influence of such results on the product development of implant systems are hitherto lacking.

Optimization strategies are particularly useful when the constraints for development are within narrow limits. This applies especially for reduced-diameter dental implants (< 3 mm) as these are particularly at risk of fracture. Reduced-diameter implants can be used for the minimally invasive management of narrow maxillas and mandibles without additional augmentation and therefore represent an additional treatment option with long-term stability [2, 4, 6, 13, 20, 25]. The particular challenge in developing implants with a small diameter consists in achieving high strength while at the same time keeping stress in the bone low. High marginal bone stresses lead to overloading of the bone and are the cause of marginal bone resorption from the biomechanical aspect [5, 7–9, 12, 21].

High forces and stresses occur at geometric notches. Notches always occur when a straight contour is abandoned. This also includes the outer thread of an endosseous implant.

The magnitude of the forces and stresses in the notch regions is very dependent upon their geometry. A distinction is made between deeply notched areas (higher forces and stresses) and shallowly notched areas (lower forces and stresses). Since the peri-implant remodeling processes follow the laws of physiological bone growth when osseointegrated implants are loaded, implant geometries and respective notch contours should be designed so as to minimize the forces and stresses.

The example of nature shows that notches can be shaped by natural growth processes so that no increases in forces and stresses occur. Figure 1 shows the notch regions of a femur and hip with the relevant notch contours. The notch contours do not follow a constant radius but adapt to the course of the force. Additional notch stresses can be avoided by this growth strategy. The principle of bone adaptation to mechanical stress has been known since Julius Wolff and takes place in the area of physiological stress [24]. Higher stresses lead to overstressing of the bone and there is a risk of cracking with subsequent fracture. In this area, known as “over-loading”, and below the physiological stress, that is, where there is “under-loading”, bone resorption occurs [5, 7–9, 12, 21]. The translation of the principles of nature into technical constructions comes into the domain of bionics.

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