Surgical implications of anatomical landmarks on the lateral surface of the mastoid bone

The aim of this study was to examine the relationships of the surgical landmarks on the lateral surface of the mastoid bone with the landmarks in a deeper location. Simple mastoidectomy was carried out without drilling over the linea temporalis inferior (LTI) on 20 adult temporal bones. The suprameatal spine, i.e., Henle spine (HS), variants were noted. Morphometric measurements were performed between these surgical landmarks, and their variations with pneumatization or HS types were evaluated. Three types of HS were identified: triangular, crest, absent. The HS-lateral semicircular canal distance was 15 mm on average and longer in bones with a triangular HS than a crest type HS (16.4 vs. 14.3 mm). The LTI was found to be located on average 4.7 mm inferior to the middle fossa dural plate (MFD). The LTI-MFD distance had a tendency to be longer in bones without an HS than with a crest type of HS (5.9 vs. 3.9 mm). Chorda tympani emerged from the facial nerve at the stylomastoid foramen in five specimens (25%). This anatomical organization was not correlated with the type of HS. Korner's septum (KS) was identified in nine bones (45%). It was present in eight of 16 (50%) bones with good pneumatization. No tendency for the existence of KS was found for any specific type of HS. This study confirms that the mastoid antrum is located 15 mm deep to the lateral surface of the mastoid bone. It should be expected to be longer in bones with a triangular HS. In addition, the MFD is located on average 5 mm above the LTI, which could be useful information for beginners or inexperienced surgeons. The ear surgeon should anticipate that the MFD might be longer in bones without an HS.