Force and Movement of Non-Osseointegrated Implants: An In Vitro Study

Dental implants have enabled a dramatic increase in the quality of life for many partially edentulous and edentulous patents. Immediate loading of newly placed dental implants is a recent advancement that attempts to meet patient demand. However, immediate loading of a just placed implant may induce implant failure to osseointegrate. Jaw forces may induce movement of the implant, thus arresting osseointegration. Some patients can generate a biting force in the posterior jaws that can reach approximately 1300 Newtons (N).1–3 The forces of the jaws are greater in the posterior than in the anterior region. Forces in the molar area are in the range of 244–1243 N. In the anterior, forces generally are about a third of that.3–5 Bone ultimately resists the forces of occlusion.

The magnitude of bite force that would cause failure of osseointegration of newly placed implants is currently unknown. It has been proposed that osseointegration would fail if an implant is luxated in bone more than 50–150 microns.1,2 A microhemorrhage may occur, and fibrous tissue, not bone, would form. Implant immobility is a requirement for appropriate osseous integration.

This study investigated the amount of off-axial force required to move a nonosseointegrated 4.3 × 13 mm implant 50 microns. The previously published pilot study for this present study found that the amount of horizontal force required to displace an implant 50 microns was approximately 150 N.6 

A newly placed implant may luxate during chewing or parafunction.7–9 Depending on a number of parameters, the implant may move within the bone and fail to osseointegrate. Thus, depending on how much occlusal force a patient can exert, the implant can be displaced by 50–150 microns, which will induce a failure to ossoeintegrate.1,2 Dental implants are unlikely to osseointegrate if they are displaced more than 50 microns during the healing process. Splinting implants may increase the force needed to move the implants within bone.10,11 

The range of biting forces that prevent osseointegration of newly placed implants is currently not known,12 nor is the magnitude of force required to displace a newly placed implant 50 microns.

The objective of this study was to determine the amount of lateral force from various directions that would be needed to displace a newly placed dental implant 50 microns measured at the surface of the bone. Eventually, data generated by this and future studies may correlate with an individual's measured biting force for immediate, true functional implant loading.

A fresh bovine mandible was secured from a local beef slaughtering plant. A section of ramus was cut with a bone saw and stored at approximately 3°C in humid conditions. At the time of testing, sections of ramus were cut into strips for osteotomies, and implants were placed in the conventional manner (Figure 1). Thirty-four titanium alloy 4.3 × 13 mm implants were purchased and installed into 9 separate sections of the bovine rami (Practice Implant Screwline, Camlog Biotechnologies, Henry Schein, Melville, NY).

Because accurate bone density could not be determined, it was not included in the testing. It was assumed to be type 1 (Misch), based on perceived qualitative density during the osteotomies.

A new measurement device was used that differed from the device in the pilot study (Figures 2 through 9). This new device comprises a series of clamps, a stationary vise, a force transducer, a displacement transducer, and a sliding vise table that is capable of changing the angle of approach at 0, 22, 45, and 60 degrees. The sliding vise pushed the secured implant into a fixed push rod that transferred the force to the force transducer. Several different angles were used to apply forces to the implants. A complete description of this device is available from the author.

A LabVIEW 8.2.1 program was developed to perform calibration of the force and displacement transducers, as well as to record the results. The 2 transducers were connected to LabVIEW (MathWorks, San Jose, Calif), which recorded the experimental data.

All but two tests were successful, for a total number of 32 successful tests (Table). Trend lines were developed from the data (Figure 10). Two tests were unsuccessful because of seating difficulties in the apparatus. The test results reported are representative of most of the tests, although some showed a more linear trend between force and displacement. Because 1000 samples were taken each second, every thousandth point was used to create these graphs. To represent the data with a trend line, a second order polynomial curve was used. This was done even for tests that showed a more linear trend, because it was assumed that this trend was seen through the 100 micron range and would have leveled off, as did most. With use of these trend lines, the force required to move each implant 100 µm within the bone was calculated.

Tested angles and thicknesses of bone implant sites were noted.

The tests yielded a force vs displacement trend (Figure 10). Force related to the angle of the applied force was developed (Figure 11). A three-dimensional representation of force vs angle vs displacement is presented in Figure 12a and b. A graph was made to show displacement compared with bone thickness (Figure 13). Displacement correlations at 100 microns were graphed (Figure 14).

Although most tests yielded a force vs displacement trend as shown in Figure 10, some displayed slightly different behavior. Tests 3, 6, 11, 14, and 24 were represented by linear trend lines, rather than by polynomial lines, because their data happened to fall in such a way that polynomial trend lines would curve up, rather than level off. The data for these tests seem to follow a linear trend; therefore it was assumed that they had not a reached leveling off point before the test was ended.

Tests 7, 18, 29, and 30 displayed similar behavior as well. In this case, negligible amounts of force were able to displace the implant for significant amounts of the 100 micron range. These implants became mobile within the bone. To account for this, trend lines were adjusted to include only the data generated after the force began to increase.

Another atypical test result occurred at implant site No. 9. In this test, the slope of the trend took a sharp turn midway through the test. The data suggest that the bone yielded to allow the implant to move the remainder of the 100 micron range without an increase in force.

Implant site No. 16 showed some yielding as well. This case was particularly interesting because after the yielding, there was a drop in force, and the data continue the polynomial trend.

To explore trends between the force required to move the implant 100 µm and the angle of applied force, a scatter plot was made that included all tests done (Figure 11).

The red lines represent upper and lower bounds (respective red lines), which define 3 different regions. To form the bounds, the highest and lowest forces needed to move the implant the critical amount at each angle were used. The 2 blue x's under the lower bound at angle 22 were unsuccessful tests and thus were not used to define the lower bound. The upper bound in this plot ranges from 69.75 N to 135.18 N, and the lower bound ranges from 23.25 N to 50.31 N. More specifically, as the angle was increased, the horizontal (parallel to the flat plane of the ramus bone) component was reduced, and therefore a higher force was required to move the implant the critical amount. This trend is shown in the plot, with the exception of the lower bound at the 60 degree angle. It can be seen that most of the points at this angle withstood forces greater than 100 N. Only the lowest 3 points at this angle do not contribute to the trend that was expected. After investigation, however, all 3 of these points were determined to be from the same section of bone, and the lowest 2 points from adjacent implant sites. The natural bone properties of this section may have differed from those of the other sections, as did the resulting values. A strong correlation was noted between the angle of applied force and the force needed to move the implant the critical amount.

A three-dimensional surface plot was generated using Matlab R2007b to show the forces and displacements throughout the entire range of 100 µm for each of the angles. This plot shows only upper and lower bounds (Figure 12).

It is important to note that the upper and lower bounds do not represent failure zones for an osseointegrated implant. These bounds represent the maximum and minimum force required to displace a nonosseointegrated implant by the amount shown on the plot. To determine the values used to generate the surface plots, points were taken from the upper and lower bounds (Figure 11). Equations from the trend lines correspond to these points and were used to determine the values at increments of 10 µm through a range of 10–100 µm. In some cases, equations generated by other tests would yield forces lower than the lower bound for displacements less than 100 µm, only to fall within the bound again at 100 µm. In these cases, the lowest points were used at each displacement. As a result, some of the lines in the surface plot for specific angles were generated using data from two separate tests. This was also done in one case for the upper bound. It is important to note that the colors indicated on the plots are not representative of what the actual forces are on that portion of the surface. To generate this plot, Matlab takes an average value for each rectangular section and assigns the color of the rectangle to reflect that average value.

Through this experimental testing, it was determined that the average force required to displace an implant the critical amount at an angle of 0 degrees was 50 N (range 23–79 N). When the angle was increased to 22 degrees, the average force increased to 52 N (range 27–70 N). At the 45 degree testing angle, the average force was calculated to be 62 N (range 48–86 N). Finally, when the angle was increased to 60 degrees, the average force was determined to be 87 N (range 33–135 N).

If these forces were to be applied at their respective angles, the osseointegration process is likely to fail. The results are shown graphically in Figure 10. It can be seen that as the angle is increased, the force required to displace an implant 100 µm increases exponentially.

On Figures 11 and 12, only a slight trend can be observed between the thickness of the bone and the force required to displace an implant the critical amount. A more significant trend may have been identified if it was not hidden by the stronger correlation between force and angle.

Figures 11 and 12, shown below, are the scatter plots that were introduced in the “Results” section. The upper and lower bounds on each plot define 3 distinct regions. Above the upper line guarantees movement, below the lines does not produce movement, and between lines, movement may or may not occur. The first region is referred to as the safe zone. This region is the area below the lower bound on the scatter plot and is the region where data collected suggest that the implant will not exceed a displacement of 100 µm in the bone. The second region is termed the risk zone and is the area between the upper and lower bounds. In this region, the osseointegration has a chance of failure. The closer the loading is to the upper bound, the greater is the chance that the implant will displace the critical amount. This region is formed by other variables that play a role, such as bone thickness, bone type, and seating of the implant. The final region, called the prohibitive region, is defined as the area above the upper bound. This is the area in which the data collected suggest that the implant will move the critical 100 µm.

This in vitro study points to the need to control occlusal force during implant healing. Most patients can generate enough force to move a newly placed implant. Thus, loading of just placed implants should be completely avoided by patients who can generate this magnitude of occlusal force while waiting for osseointegration to occur. However, many patients may not have this much oral strength and may be able to have immediately loaded implants. Additional studies are needed to pursue this concept before clinical parameters can be established.