Quantitative Analysis of Temporal Bone Density and Thickness for Robotic Ear Surgery

Abstract

Background and Objective: Quantitative assessment of bone density and thickness in computed-tomography images offers great potential for preoperative planning procedures in robotic ear surgery. Methods: We retrospectively analyzed computed-tomography scans of subjects undergoing cochlear implantation (N = 39). In addition, scans of Thiel-fixated ex-vivo specimens were analyzed (N = 15). To estimate bone mineral density, quantitative computed-tomography data were obtained using a calibration phantom. The temporal bone thickness and cortical bone density were systematically assessed at retroauricular positions using an automated algorithm referenced by an anatomy-based coordinate system. Two indices are proposed to include information of bone density and thickness for the preoperative assessment of safe screw positions (Screw Implantation Safety Index, SISI) and mass distribution (Column Density Index, CODI). Linear mixed-effects models were used to assess the effects of age, gender, ear side and position on bone thickness, cortical bone density and the distribution of the indices. Results: Age, gender, and ear side only had negligible effects on temporal bone thickness and cortical bone density. The average radiodensity of cortical bone was 1,511 Hounsfield units, corresponding to a bone mineral density of 1,145 mg HA/cm³. Temporal bone thickness and cortical bone density depend on the distance from Henle's spine in posterior direction. Moreover, safe screw placement locations can be identified by computation of the SISI distribution. A local maximum in mass distribution was observed posteriorly to the supramastoid crest. Conclusions: We provide quantitative information about temporal bone density and thickness for applications in robotic and computer-assisted ear surgery. The proposed preoperative indices (SISI and CODI) can be applied to patient-specific cases to identify optimal regions with respect to bone density and thickness for safe screw placement and effective implant positioning.

Keywords: BAHA; bone conduction implants; bone mineral density; bone thickness; calibrated Hounsfield units; quantitative computed-tomography; screw safety.

Figure 1

Definition of the retroauricular coordinate system and grid specification for the analyzed region of interest (ROI). The origin of the coordinate system lies at Henle's spine. The x-axis is oriented along the zygomatic process, as specified by two landmarks. The x/y-plane is perpendicular to the transversal image plane as defined by the clinical protocol. The red square indicates the ROI containing the 8 × 8 probe grid. The probe positions are equally spaced by a distance of 5 mm, resulting in a covered area of 35 × 35 mm². The lower anterior corner of the ROI is positioned at x = 4 mm and y = −10 mm.

Figure 2

Calibration scale between the radiodensity in the applied high-resolution CT imaging protocol (in HU) and the actual bone mineral density. A linear relation can be observed with a scaling factor of 1.32 between HU and mg HA/cm³.

Figure 3

Left: Three-dimensional visualization of the projection of grid points onto the outer surface of the temporal bone mesh. Right: Probe evaluation in a section of the temporal bone. Blue lines represent the probes that from the mask intercept the outer bone surface. Red lines represent the normal direction to the external surface, along which thickness and densities are computed. In green are highlighted the points where density is measured for every probe, spaced between each other by 0.15 mm.

Figure 4

Exemplary course of radiodensity (in HU) and bone mineral density (in mg HA/cm³) along a probe trajectory. The temporal bone thickness (d_TB) along the trajectory is indicated by a dashed line. The green shaded area indicates the region considered for external cortical bone density computation.

Figure 5

Heat map visualization of temporal bone thickness in the retroauricular region of interest averaged across all subjects.

Figure 6

Heat map visualization of cortical bone density (in HU) in the retroauricular region of interest averaged across all subjects.

Figure 7

Relation between age and cortical bone density for male (circles) and female subjects (triangles), as well as Thiel fixed ex-vivo specimens (crosses). The solid black line indicates the model described by (7). The line is dashed for the prediction of the model.

Figure 8

Visualization of the screw implantation safety index (SISI) for a screw length of 4 mm in the region of interest, averaged across all subjects.

Figure 9

Visualization of the screw implantation safety index (SISI) for a screw length of 5 mm in the region of interest, averaged across all subjects.

Figure 10

Visualization of the column density index (CODI) in the region of interest, averaged across all subjects.