Micromotion of Dental Implants: Basic Mechanical Considerations

Abstract

Micromotion of dental implants may interfere with the process of osseointegration. Using three different types of virtual biomechanical models, varying contact types between implant and bone were simulated, and implant deformation, bone deformation, and stress at the implant-bone interface were recorded under an axial load of 200 N, which reflects a common biting force. Without friction between implant and bone, a symmetric loading situation of the bone with maximum loading and displacement at the apex of the implant was recorded. The addition of threads led to a decrease in loading and displacement at the apical part, but loading and displacement were also observed at the vertical walls of the implants. Introducing friction between implant and bone decreased global displacement. In a force fit situation, load transfer predominantly occurred in the cervical area of the implant. For freshly inserted implants, micromotion was constant along the vertical walls of the implant, whereas, for osseointegrated implants, the distribution of micromotion depended on the location. In the cervical aspect some minor micromotion in the range of 0.75 μm could be found, while at the most apical part almost no relative displacement between implant and bone occurred.

Figure 1

Single tooth implant used for replacing the first molar in the lower left mandible. An axial force acting on the occlusal surface of the restorations may displace the implant relative to the surrounding bone.

Figure 2

Description of scenario 1 with a dental implant resting on a fixed apical surface, with no contact existing between the vertical implant walls and the walls of the bony socket (left). When the implant is loaded vertically, deformation of the implant occurs mainly in the coronal part and decreases towards the apex. Similarly, relative displacement between implant and bone diminishes towards the apex (right).

Figure 3

Description of scenario 2, where the implant rests on a layer of elastic trabecular bone with no contact existing between the vertical implant walls and the walls of the bony socket (left). The apically located layer of bone may be substituted by a spring which is compressed when an axial load is applied on the implant (center). Due to the great difference in elastic modulus between implant and trabecular bone, relative implant displacement is independent from the region of the implant considered (right).

Figure 4

Scenario 3 showing an implant elastically supported by cortical and trabecular bone (a). The elastic support in the different regions can be replaced by a system of springs (b).

Figure 5

Without contact between implant and bone, an axial force acting on the implant causes implant dislocation as a result of elastic deformation of bone predominantly in the periapical region of the implant. Left: unloaded implant; right: loaded implant with implant displacement Δu and displacement of cortical bone Δu _b (displacement of a reference mark on bone).

Figure 6

Considering an osseointegrated implant with contact between the implant surfaces and bone, axial implant loading causes elastic deformation of bone in all areas but no relative displacement between implant and bony socket, that is, no micromotion, occurs. Left: unloaded implant; right: loaded implant with implant displacement Δu and displacement of cortical bone Δu _b (displacement of a reference mark on bone).

Figure 7

(a) Three-dimensional finite element models of dental implants with and without threads [12]. (b) Three-dimensional finite element model of a bony implant socket with cortical and trabecular bone. Areas (1) and (2) surrounding the implant are designed as an intermediate layer allowing the elastic modulus to be set independently from areas (3) and (4) representing native bone which is not affected by healing processes occurring during osseointegration [12]. (c) Three-dimensional finite element model of a single implant embedded in a bone segment consisting of cortical and trabecular bone (calculations were done on a complete model; for illustration purposes the model is cut in half) [12].

Figure 8

Definition of micromotion at the implant bone interface. Six corresponding nodes on the implant and on the bone were used as reference marks. For determining the relative displacement of two corresponding nodes on bone and implant, the displacement of a specific reference mark on the bone was subtracted from the displacement of the corresponding reference mark on the implant.

Figure 9

Distribution of von Mises equivalent stress around implants loaded with 200 N axial vertical force [12]: cylindrical implant without friction between implant and bone (a), threaded implant without friction between implant and bone (b), threaded implant with friction between implant and bone (coefficient of friction: 0.3) (c), and threaded implant with force fit between implant and bone (d).

Figure 10

Distribution of global displacement around implants loaded with 200 N axial vertical force [12]: cylindrical implant without friction between implant and bone (a), threaded implant without friction between implant and bone (b), threaded implant with friction between implant and bone (coefficient of friction: 0.3) (c), and threaded implant with force fit between implant and bone (d).

Figure 11

Displacement of corresponding reference marks on bone and implant for freshly inserted implants and resulting micromotion: data recorded from model without friction between bone and implant (a), data recorded from model with friction between bone and implant (b) (note the different scales).

Figure 12

Displacement of corresponding reference marks on bone and implant for osseointegrated implants and resulting micromotion: data recorded from model without friction between bone and implant (a), data recorded from model with friction between bone and implant (b), note the different scales!