Force Required to Luxate a Newly Placed Dental Implant in Bone: An In vitro Pilot Study

Edentulous dental patients are generally disappointed when they learn of the long healing period required for osseointegration of a dental implant. They have been known to apply verbal pressure to the treating dentist to speed treatment time so that their new implants may be able to function sooner. There have been studies that examine the success of immediately functionally loaded implants.1 Implant immobility is necessary for proper osseointegration to occur. It may be the case that if a healing implant undergoes a micromovement (displacement) larger than 50 to 150 μm, a microhemorrhage may occur, which induces subsequent fibrous growth that may cause failure of the osseointegration of the implant.2 

The occlusal forces generated by the jaws are more powerful in the posterior than the anterior area. Forces in the molar area are in the range of 244 to 1243 Newtons (N), while the anterior forces are generally about a third of that.3,^–5 During function, the jaws impart a load to prostheses and the supporting implants. A healing implant that is not yet osseointegrated may be susceptible to luxation during chewing or parafunction.6,^–8 Depending on a number of factors, including the jaw force generated by a patient, a newly installed implant may move in the bone and fail to osseointegrate. Thus, depending on how much occlusal force a patient can exert, this force may luxate a newly placed single implant and displace it by 50 to 150 μm, which, in turn, will induce a failure to osseointegrate.1^,2 Clearly, splinting newly placed implants may increase the critical force needed to luxate the implants.9^,10 

The goal of this in vitro pilot project was to determine the amount of lateral force required to displace a dental implant 50 μm, measured at the surface of the bone. It is suspected that dental implants are more likely to fail to osseointegrate if they are displaced more than 50 μm during the healing process. The movement may disturb the relatively slow progress of osseous healing, causing fibrous growth instead of bone growth. Proper osseointegration does not occur in the presence of fibrous tissue, and the implant therefore fails.

The range of biting forces that prevent osseointegration of newly placed implants is currently unknown,11 as is the force required to displace a newly placed implant by 50 μm. The purpose of this pilot study was to determine the amount of horizontal force necessary to move a nonosseointegrated implant 50 μm and to form a basis for further studies. The final aim is to have the resulting data extrapolated and correlated to an individual patient's measured biting force that would induce a 50-μm luxation. Thus, a patient may be measured to determine his or her candidacy for immediate functional implant loading.

A fresh bovine mandible was secured from the University of Connecticut Pathobiology Department (Figure 1). A section of ramus was cut with a bone saw and stored at 3°C in humid conditions. At the time of testing, sections of the ramus were cut into strips for osteotomies and implant placement (Practice Implant Screwline, CamLog, Henry Schein, Melville, NY) (Figure 2). Because bone type density could not be accurately determined, it was not factored into the testing.

A device was built to apply a lateral force to one side of the implant and to measure the displacement of the implant on the opposite side, relative to the bone into which the implant was screwed (Figure 3). The primary challenge was maintaining the accuracy of the implant measurement by holding the displacement of the device and of the bone to negligible values, so that their influence on the measured values would be minimal. In other words, the device had to be capable of isolating the displacement of the implant. The device had to accommodate sequential implant testing for implants placed at various heights relative to a datum reference. Different measurement devices were considered and evaluated according to their accuracy and cost. The final design of our device met our accuracy criteria and was capable of applying horizontal loads from 0 to 2000 N, while measuring displacements of up to 200 μm for implants mounted in various bone thicknesses.

In such a scenario, the energy generated by the force applied to the implant mounted in the bone will be primarily absorbed by the bone, since the implant and the device itself are practically rigid compared to the rigidity of the bone. Consequently, this has several effects on the experiment. As the force is applied, and as the implant starts to dislocate, the bone behind the implant is compressed and starts to deform both locally (local deformation near the implant-bone interface) and globally (bending of the bone). Thus, the final device had to mount the bone itself in such a way that the global bone deformation would remain negligible. In order to isolate the measurement of the implant dislocation (displacement), the measurement sensor and the force applicator were mounted on opposite sides of the implant (Figure 3). Furthermore, the measurement sensors were placed as close to the implant as physically possible to eliminate any other deformations of the device from the measured data.

The bone was mounted in a rectangular Die-Keen hard dental stone (Die Keen, Heraeus Kulzer, Hanau, Germany) (Figure 4), which was secured in a table vise to hold the bone securely for testing. The hard dental stone provides the most secure yet flexible method of attachment for the bone and, at the same time, increases the rigidity of the mounted bone. Our table vise was controlled by a lead screw, allowing transversal movement of the bone relative to the point of force application. Consequently, this design allowed several mounted implants to be tested sequentially without removing the bone from the vise.

The force was applied mechanically using controlled levers and weights via a lead screw. A force sensor (Omega LC302–500, with an accuracy of ±10 N at 2200 N, Omega Engineering, Inc, Stamford, Conn) was used to measure the force applied to the implant. A linear displacement sensor (Omega GP911–1-S, with an accuracy of ±2 μm, Omega Engineering) was used to measure the displacement of the implant. Our design allowed the control of the vertical position (relative to the implant) of both the force application and the displacement measurement in order to support testing of various bone thicknesses.

Our 2 sensors were connected via data acquisition cards to a commercial software (Labview, National Instruments, Austin, Tex) to acquire in real time and analyze and present the measured forces and displacements. Our sensors were calibrated, within the Labview environment, against certified calibration weights and thickness gauges before testing was initiated.

As mentioned above, the bone strips were set in dental stone. After the stone had set for 45 minutes, a grinding wheel was used to shape the stone to fit into the table vise. Osteotomies were drilled to accept the dental implants, and the implants were placed at 30 N cm of torque (CamLog, Henry Schein). Each osteotomy/implant was given a letter designation. The bone thickness for each testing site was measured with a Boley gauge. The implants were then installed into the bone osteotomies. The 13-mm–length implants that we used had a diameter of 4.3 mm.

After the implants were installed, the bone/stone was clamped to the cross-slide table. The implants were placed, leaving the machined collar exposed above the bone surface. The lateral force was applied to the implants, monotonically increasing until the deflection readings reached 150 μm. The data acquisition program saved the data, and the sensors were reinitialized. This procedure was repeated for the remaining implants. Figure 5 shows the final setup of our testing device. The force was applied to the implant collar. Abutments were not placed on the implants.

Force-displacement curves were collected for each implant mounted in the bone (see, for example, Figure 6, corresponding to a thickness of the bone of 10.0 mm). By postprocessing the collected data, we identified the forces required to displace an implant in 10-μm increments up to 50 μm. The filtered data are presented in Table 1. We found that a range of force resulted: 50 to 150 N. The quality of the encasing bone was indeterminate, but based on a visual assessment, a qualitative judgment by one author (D.F.) indicated that the bone was Misch type I.

The data shown in Table 1 were used to generate the combined 3-dimensional plot shown in Figure 7, which shows how the measured horizontal force varied with the measured displacement and thickness of the bone. Observe that the force increases monotonically with the displacement but varies significantly for different thicknesses of the bone, as measured at the mounting location of each implant. Our results indicate that these variations are due to the inhomogeneity of the bone properties across the bone thickness, which practically changes the (local) load-carrying capacity of the bone.

One way to decode this variation of the load-carrying capacity of the bone with the bone thickness is to construct a separate plot that displays upper and lower bounding surfaces for the force, as shown in Figure 8. These surfaces show the range of horizontal forces that affect the osseointegration of the implant. Specifically, the interpretation of these bounding surfaces is as follows: for a given displacement of d μm and a bone thickness of t mm, the lower bounding surface shows the value of the horizontal force up to which the displacement of the implant remains smaller than the prescribed value of d μm, so that the osseointegration is not compromised. The upper bounding surface shows the value of the horizontal force above which the displacement of the implant exceeds the prescribed value of d μm, which implies that for horizontal forces that are above this upper bound, the osseointegration will always be compromised. Finally, for horizontal forces that are below the upper bounding surface, but larger than the lower bounding surface, the osseointegration may or may not be compromised, depending on the local load-carrying capacity of the bone. The numerical values used to create the upper and lower bounding surfaces have been extracted from Table 1 and are shown in Table 2.

Finally, a scatterplot containing the measured horizontal forces that would produce 50-μm and 150-μm displacements for various bone thicknesses is shown in Figure 9. The 2 upper and lower bounds for the 50-μm displacement were superimposed in this figure.

The fact that the bone is essentially a heterogeneous material implies an uneven distribution of all its material properties, including density, hardness, and yield strength. Since no evaluation of material properties was performed, our results incorporate the effects of material property variations on the measured displacements. In human bone there are bone qualities found that would offer less support for a newly placed implant. Lesser bone density may provide less resistance to horizontal as well as forced-form other off-axial directions. The lesser support by bone may allow more implant movement with less force.

This pilot study was aimed at determining the amount of horizontal off-axial force required to move a nonosseointegrated implant 50 μm, as measured at the surface of the bone. Experimental mechanical design was also developed for future studies. The initial data show that the amount of horizontal force required to displace a 4.3 × 13–mm implant by 50 μm is on the order of 150 N. Assuming that the angle between the direction of the biting force and the vertical is between 0° and 20° (see Figure 10), our data show that a 4.3 × 13–mm implant may fail to osseointegrate for biting forces that are as low as 440 N.

Figure 11 shows the variation of the magnitude of the biting force required to produce a horizontal force of 150 N as a function of the angle between the biting force and the vertical. The relationship between an applied force and its projection onto the horizontal plane can be written as

where F_h is the projection of a force F, the direction of which makes an angle α with the horizontal plane (see Figure 10). One can easily see that this value of the biting force greatly depends on the angle between the direction of the biting force and the vertical direction (which is hard to predict in general, since it is a function of the tooth morphology, the individual's anatomy, and, last but not least, of what the patient is biting on). This figure shows that a 1300-N biting force produces a horizontal force of 150 N (corresponding to our measurements for 4.3 × 13–mm implants) for an angle α of approximately 6.6° with the vertical, which is fairly close to a vertical biting force.

Note that an implant that has a diameter smaller than 4.3 mm (used in this study) has a smaller contact area with the bone than does the 4.3-mm–diameter implant and that the ratio between the 2 contact areas is proportional to the ratio of the 2 diameters.11 Therefore, such an implant will require an even smaller horizontal force than the 4.3-mm–diameter implant in order to undergo a displacement of 50 μm. Note that the smaller the horizontal force, the smaller the maximum angle α would be for the same value of the biting force.

Consequently, our data indicate that a patient developing 1300 N of jaw force should probably not be considered for immediate function loading of newly placed dental implants, even in the anterior. Furthermore, our results indicate the existence of 3 distinct load zones for these horizontal forces (Figure 8):

• a “safe” zone that contains the values of the horizontal forces producing displacements smaller than the prescribed displacements of concern;

• a “prohibitive” zone containing force values that are high enough to provide large displacements that would compromise the osseointegration; and

• finally, a “risk” zone that contains all the forces that could produce displacements compromising the osseointegration.

Future studies should involve performing tests on more implants and bone samples, which will provide more data points and, hence, a more complete characterization of these forces. Additionally, force placed at the polished collar may not be indicative of clinical forces. However, future studies may require abutment placement to more closely replicate the clinical configuration.

For the thicker sections with soft cancellous bone in the center, it may be helpful to note the thickness of the cortical sections, along with the thickness of the cancellous section in the center. However, a comprehensive quantification of these 3 load zones would require a consideration of additional factors. Importantly, the resulting data could be extrapolated to correlate an individual patient's measured biting force to the ability to induce a 50-μm luxation and a subsequent failure to osseointegrate. Even though a commercial device for measuring a patient's biting force does not seem to exist at the present time, the technology required to build such a device is available, and such a device can be manufactured if required by potential end-users. Such a device would allow a patient's biting force to be measured in order to determine his or her candidacy for immediate functional implant loading.

Immediate loading of newly placed dental implants is a consideration in meeting patients' demands.6,^–10 However, immediate loading may induce implant failure to osseointegrate. Patient bite force may reach 1300 N in the posterior jaws. It is suspected that if an implant is luxated in bone more than 50 μm, the osseointegration may fail, and any forming bone will be replaced with fibrous tissue. This pilot study was done to examine the amount of force required to move a newly placed dental implant 50 μm in the encasing bone and to facilitate further study.

We conclude that it seems that a range horizontal force of 50 to 150 N can move a newly placed implant in bovine bone 50 μm. However, different qualities of bone density may influence the range of magnitude of force. Future study may use the mechanical design for this project, and bone quality is an important issue for accurately relating the force magnitude to human oral conditions.