Primary Stability of Dental Implants in Human Jawbones: Experiments & FE Analyses

Abstract

Objectives: Primary stability (PS) is a key factor for promoting osseointegration and long-term success of dental implants particularly for immediate loading protocols. Beyond the current assessments of PS, an accurate pre-operative evaluation of PS would contribute to the improvement of surgical planning and treatment outcome. This study used biomechanical testing and homogenized finite element (hFE) analysis to objectively measure PS in the laboratory, and digitally estimate PS from prior μCT reconstructions.

Material and methods: Thirty-five bone samples extracted from the jaws of two donors were examined. Twenty-two were finally evaluated for PS. After scanning of the samples with μCT, implants were inserted by two experienced surgeons, and various metrics such as μCT-based bone volume fraction (BV/TV), insertion torque (IT), and resonance frequency analysis (RFA) were assessed to determine PS. Mechanical tests were conducted to measure ultimate force (UFexp) as an objective indicator of PS while the hFE simulations were performed to estimate this same ultimate force (UFsim).

Results: Higher correlation was found between UFsim and UFexp (R² = 0.85) than between BV/TV and UFexp (R² = 0.61), IT and UFexp (R² = 0.50), and RFA and UFexp (R² = 0.38). All variables demonstrated a statistically significant linear correlation with UFexp (p < 0.01).

Conclusion: UFsim turns out to be a more reliable and objective indicator of PS than IT and RFA. The hFE analysis requires prior μCT reconstructions and is currently limited by numerical convergence problems. Despite these limitations, pre-operative hFE analysis emerges as a promising tool with a higher accuracy for estimation of PS than state of care techniques.

Keywords: FE‐analysis; dental implants; human jaw; insertion torque; primary stability prediction; ultimate force.

FIGURE 1

Overview of the sequence steps performed from the extracted samples of an 82‐year‐old human jaw (n = 19) and an 86‐year‐old human jaw (n = 16): from extractions and μCT scan, to experimental test and simulation prediction of primary stability. Only n = 22 samples were valid for prediction as 3 samples had to be excluded by the experiment and 10 by simulation.

FIGURE 2

Left: Tooth extraction and flattening by milling of the anatomical samples, when a tooth has been extracted. Middle: In PMMA embedded bone sample that did not require milling. After pilot drilling, the anatomical samples were cut in segments for each implant site and embedded in PMMA, for final drilling, implant placement, and mechanical testing. Right: Setup of the mechanical tests.

FIGURE 3

Top left: Registered μCT images to localize implant position and narrowing down of the ROI. The cropped image was segmented and used for BV/TV mapping. Top right: Structure of the simulation with the boundary conditions. The displacement for the mechanical testing was along the z‐axis with a 30° inclination of the implant. Bottom: Force displacement curve of the experiment (left) and simulation (right). The red point denotes the ultimate force.

FIGURE 4

Left: Evaluated samples according to their anatomical location. The color scheme indicates average BV/TV of each sample ranging from 15% (yellow) to 75% (red). Right: Box plot of the BV/TV distribution along the osteotomy of the pilot drill. Samples marked with * or × correspond to the excluded samples by the experiment or the simulation, respectively.

FIGURE 5

Linear regression plot of each model with the 95% confidence interval (dashed line). The x‐ and y‐axis are transformed to the log scale for the power models.