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Computed Tomographic Analysis of Outer Calvarial Thickness for Osseointegrated Bone-Anchored Hearing System Insertion

Introduction: Osseointegrated bone-anchored hearing systems (BAHSs) are a useful tool in auditory rehabilitation for single-sided deafness and conductive/mixed hearing loss. They rely on adequate osseointegration of titanium implants, which can be limited by calvarial thickness. This study examines adult computed tomographic (CT) temporal bone normative data for calvarial thickness in the areas commonly recommended for BAHS insertion and identifies hazards that may compromise the osseointegration process. Methods: Prospective analysis of 100 consecutive adult helical CT scans. Calvarial thickness was measured in axial and coronal planes 5.5 cm posterior to the superior external auditory canal at 6 vertical points (EAC, +1 cm, +2 cm, +3 cm, +4 cm, and +5 cm). Other parameters measured include temporal bone pneumatization and the presence of suture lines, bone marrow, and vascular structures. Results: A total of 195 temporal bones were examined in 100 patients. Mean patient age was 60.9 years, of whom 54.4% were men and 45.6% were women. Mean calvarial thickness was greatest at +1 cm above the EAC level (6.3 mm) and thinnest at +4 cm (5.1 mm). Of temporal bones, 55% had at least 1 site thinner than 4 mm and 21% had at least 1 site thinner than 3 mm. Air cells and suture lines were more likely to be encountered within 2 cm above the EAC level, and marrow space is more likely to be encountered 3 to 4 cm above the EAC level. Discussion: Selecting a position 3 to 4 cm above the level of the EAC is more likely to result in dural exposure for a 3-mm BAHS abutment, especially in men. Selecting a position near the level of the EAC provides thicker bone, but the surgeon is more likely to encounter suture line or enter mastoid air cells. Preoperative CT imaging may be a useful tool in evaluating calvarial thickness and hazards when planning BAHS insertion in the adult population.