Relative Contribution of Trabecular and Cortical Bone to Primary Implant Stability: An In Vitro Model Study
The specific aim of this study was to examine the relative contributions to the implant insertion torque value (ITV) by cortical and trabecular components of an in vitro bone model. Simulated bone blocks of polyurethane were used with 2 densities of foam (0.08 g/cm3 to mimic trabecular bone and 0.64 g/cm3 to mimic cortical bone). We have developed a new platform technology to collect data that enables quantitative evaluation of ITV at different implant locations. Seven groups were used to model varying thicknesses of cortical bone over a lower-quality trabecular bone that have clinical significance: a solid 0.08 g/cm3 block; 1 mm, 2 mm, and 3 mm thick 0.64 g/cm3 sheets with no underlayer; and 1 mm, 2 mm, and 3 mm thick 0.64 g/cm3 sheets laminated on top of a 4 cm thick 0.08 g/cm3 block. The ITVs were recorded as a function of insertion displacement distance. Relative contributions of ITV ranged from 3% to 18% from trabecular bone, and 62% to 74% from cortical bone depending on the thickness of the cortical layer. Inserting an implant into 2-mm and 3-mm cortical layers laminated atop trabecular blocks had a synergistic effect on ITVs. Finally, an implant with a reverse bevel design near the abutment showed final average torque values that were 14% to 34% less than their maximum torque values. This work provides basic quantitative information for clinicians to understand the influence of composite layers of bone in relation to mechanical torque resistances during implant insertion in order to obtain desired primary implant stability.
Setup used to measure torque vs displacement curves when drilling and placing implants into simulated bone blocks. (a) Custom-made platform that rotates the sample (b) at a pre-set rate. The implant driver (c) is attached to the lower chuck and is shown driving an implant into a sample. A modified compact disc (d) is used as a reflecting surface that moves in lock step with the implant driver. The axial compensator (e) allows the implant driver to move freely up and down while the sample is rotating. The lower bar of the compensator moves while the upper bar remains stationary and transmits the torque felt by the implant to the torque sensor (g) via the upper chuck located between the sensor and the compensator. Displacements are measured using a laser sensor (f) that is fixed in the lab frame by a ring stand and probes the position of the reflecting surface. The implant is initially brought into contact with the sample and the lower bar of the compensator raised to nearly touch the upper bar using the axial actuator handle (h).
Multiple torque vs displacement curves were taken for every combination of implant type (regular platform and narrow platform) and every sample condition (simulated trabecular bone, simulated cortical bone of varying thicknesses, and simulated thin cortical bone overlayers affixed atop simulated trabecular bone). (a) Example showing 18 torque vs displacement curves using a regular platform (RP) implant being inserted into a 1-mm-thick dense cortical overlayer (#40 Sawbones) fixed atop a thick simulated trabecular underlayer (#5 Sawbones). To visualize multiple different conditions on the same plot, we simplified the presentation of these data by plotting the mean torque value at each displacement to obtain 1 curve from the multiple runs. (b) Standard deviation bars indicate the distribution of the data about this mean.
The regular platform (RP) implants showed a distinct drop in insertion torque in the last few millimeters of travel when being inserted into a dense cortical overlayer affixed atop a low-density trabecular underlayer. The narrow platform implants showed no such drop. This figure shows data for a 1-mm-thick #40 (dense/cortical) Sawbones overlayer affixed atop a #5 (low-density/trabecular) Sawbones underlayer. The inset shows the RP implant being placed into a pure low-density trabecular layer showing no drop in torque. The effect is thus due to the thin dense cortical overlayer. NP indicates narrow platform.
Images of the 2 implant types used in this study: Nobel Biocare regular platform (RP) and narrow platform (NP). The RP implant has a retrograde slope near the top of the implant while the NP implant does not. In addition, the thread patterns are clearly complex, which adds structure to the torque vs displacement data not typical of a more simply threaded screw.
Increasingly thick dense cortical overlayers atop low-density trabecular underlayers resulted in increasing values of insertion torque. The overall magnitude of the drop in insertion torque consistently occurred near the same value of insertion displacement, consistent with this drop being the result of the geometric design of the implant. In addition, the magnitude of the drop in insertion torque remained constant as the overlayer thickness increased. RP indicates regular platform.
A synergistic effect occurs when inserting implants into thin dense overlayers atop lower-density underlayers. For example, looking at the final insertion torques, the underlayer alone (regular platform [RP] #5, 3.8 N-cm) plus the overlayer alone (RP #0 2 mm, 35 N-cm) is only 71% of the final torque measured when inserting the implant into the combined layers (RP #5 2 mm, 49 N-cm). The small arrow to the right of the (RP #0 2 mm) curve shows the additional torque from the underlayer alone. The distance along the torque axis from the tip of the arrow to the (RP #5 2 mm) curve represents the synergistic effect.
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