Impact of annual bone loss and different bone quality on dental implant success – A finite element study
Introduction
The failure of osseointegrated implants is often caused by peri-implant bone loss [1], [2], [3]. The annual amount of bone loss less than 0.2 mm following the first year of implant service is recommended as one of the criteria for implant success by Albrektsson et al. [4]. Some researchers call for a revision of this specific criterion [5], however it is still used in commonly accepted studies on dental implant success [6], [7], [8], [9], [10]. The major causes of bone loss are peri-implantitis and mechanical overload [10]. The latter, in turn, occurs due to excessive bone stresses and strains caused by poor bone quality and inadequate implant dimensions.
For long-term implant success, a good quality (density) of bone surrounding an implant and a sound interface between them should be achieved. Bone quality describes its internal structure and reflects its elasticity and mechanical strength [11], [12], [13], which have a wide range of variability [14], [15], [16], [17]. Overall, bone quality is a crucial factor in selecting an implant and surgical procedure, determining the healing period and implant loading [18], [19], [20], [21].
Bone quality itself depends on the appropriate level of bone modeling/remodeling necessary to maintain bone density and manage bone microdamage. When bone tissue is incapable of coping with the repair of microdamage, the latter might accumulate and cause bone failure. In the “Mechanostat” proposal, Frost suggests that unrepaired bone microdamage starts to accumulate when strains exceed 3000 μstrain [22], [23]. According to an alternative viewpoint [24], stresses in bone should not exceed its ultimate strength (100–190 MPa for cortical bone and 1–5 MPa for cancellous bone) [24], [25], [26], [27], which also depends on bone quality. Furthermore, bone quality influences stress and strain concentrations around endosseous implants [13], [17], [24], [28], [29], [30], [31], [32], [33], [34]. Increased bone strains and decreased implant stability are associated with low bone density, while high bone density improves stress transmission and distribution from an implant to adjacent bone. A common way to reduce bone stresses and strains is to increase implant dimensions.
Since inadequate implant dimensions are the main cause of peri-implant bone overload leading to bone loss and subsequent implant failure [27], [32], [33], [34], [35], effective strategies for selecting implants are crucial [17], [24], [28]. To evaluate the load-carrying capacity of different-sized implants under different bone quality conditions, a methodology based on correlating implant dimensions with stress, strain, and ultimate functional load values was proposed [28], [36], [37]. This is supported by the linear elasticity hypothesis, making it possible to calculate the ultimate value of functional load, which in turn determines the load-carrying capacity of an implant. Since the reduction, due to bone loss, of implant ultimate functional load to values below experimentally obtained functional load values would jeopardize implant longevity, assessment of reduced ultimate functional loads for different-sized implants can help in estimating the implant lifetime prognosis.
The finite element (FE) method is a unique instrument for the biomechanical analysis of dental implant longevity in view of bone loss. It was proposed to simulate the mechanical behavior of dental systems under various loading conditions [38], [39]. Numerical simulation was successfully applied to obtain unique results in bone-implant interface studies [34], [40], [41], [42], [43], [44].
Since partially osseointegrated implants (implants with partial loss of adjacent bone) cannot withstand the full functional loading due to a reduced strength of the bone-implant interface and excessive strains, bone loss study is a relevant subject of research in oral implantology. The aim of this study is to compare the load-carrying capacities of different-sized fully and partially osseointegrated implants in order to predict their long-term success and to help dentists in implant selection.
Section snippets
FE modeling principles
Nine 3D geometrical models of a bone segment with an inserted implant were generated using computed tomography (CT) images to determine the cortical bone thickness of four types of mandibular bone according to Lekholm and Zarb classification [45]. The diameter and the height of each model were set to 22 mm, chosen so to localize stress fields around implants (Fig. 1). Gingival soft tissues were not modeled. M1 bone model with type I bone contained compact bone only (Fig. 1 A, B), while M2 with
Results
High stresses and strains were localized in cortical and cancellous bone in the plane of critical bone-implant interface, where mobile points of critical stresses and strains were found. They are illustrated in Fig. 2 for a bone-implant assembly with 4.1 × 12.0 mm implant, two stages of bone loss (0.8 mm and 1.6 mm), and I–IV bone quality types. Von Mises stress and first principal strain values were calculated for each critical point of bone-implant interface.
For types I and II bone, the
Discussion
Occlusal overload is one of the most controversial issues in oral implantology. As follows from this study results, the overload is generated by excessive stresses in adjacent bone, which are, in fact, the flip side of excessive strains (Fig. 3, Fig. 4). Both stresses and strains reflect the mechanical reactions of bone tissues to loading, which are described in biomechanics in terms of constitutive equations. They are the simplest if linear relationships between stresses and strains, as well
Conclusions
With bone loss, both bone quality and implant dimensions have a major impact on implant lifetime. The long-term success of implant treatment is ensured by keeping stresses and strains within safe limits, choosing adequate implants, and dealing only with types I–III bone. Furthermore, the longevity of implants in types II and III bone depends on the peri-implant cortical bone loss period. Due to bone loss, type IV bone is unsuitable for implant placement. To achieve a high success rate in oral
Conflict of interest statement
None Declared.
Acknowledgements
All authors of this paper disclose no conflict of interest with any institutions.
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