This study showed that low bone quality, as assessed by clinical and image methods, is related with low primary stability of 6-mm short implants placed at the posterior region of the maxilla and mandible. Higher insertion torque values were associated with better bone types and higher mean gray values in CT images. Insertion torque had a negative moderate association with bone type categorization by intra-surgical tactile evaluation and visual assessment of CT images. In addition, a positive moderate association was found between insertion torque and average mean gray values, as well as for segmental mean gray values in the axial, coronal, and sagittal ROIs.

The present study assessed the variation in normalized mean gray values to evaluate the bone quality in preoperative CT images using the axial, coronal, and sagittal sections, and the average of the three ROIs. Several studies have investigated the potential clinical application of CBCT mean gray values, especially for bone density evaluation in comparison with various clinical bone parameters [19,20,21,22,23,24,25,26,27]. However, before using CBCT gray values for bone density estimations, a histogram calibration is needed.

One single cone beam scanner (i-CAT, Imaging Sciences Intl, Hatfield, PA, USA) and one single multislice scanner (Elscint CT Twin II, Elscint Ltd., Haifa, Israel) were used in this study. Even though quantitative differences in absolute numbers might be expected using distinct imaging modalities, a clinical approach was established to clinically comparable values. Using rescaled gray values through a pseudo-Hounsfield scale, the small differences between them were minimized.

In addition to the known CBCT exposure factors that contribute to the deviation of gray values, e.g., noise, beam hardening, limited FOV, local tomography effect, and the position inside the FOV, the machines appear to have incorporated a “histogram shift” in their reconstruction algorithm. This implies that the gray values are distributed based on the contents of the scan. The contrast of each individual scan is optimized, but gray values differ between scans containing low- or high-density materials. The presence of high-density objects in the scan shifts the histogram, leading to lower gray values throughout the image [28]. Previous studies tried to correct the inconsistency and calibrate gray values along a Hounsfield unit or a density scale [13, 28,29,30].