Donaldson's Line

The landmark is constructed by projecting a course from the lateral semicircular canal that runs parallel to the posterior semicircular canal. Where this projected line meets the posterior labyrinth, it provides a directional estimate toward the vestibular aqueduct and endolymphatic sac. This geometric relationship lets the surgeon use recognizable labyrinthine structures to orient the search rather than approach the sac without a fixed reference.

These structures serve as neighboring orientation points during temporal-bone work. Keeping the sigmoid sinus, dura, and semicircular canals spatially accounted for helps the surgeon interpret the projected landmark within the surrounding anatomy. Donaldson’s line therefore does not function alone; its value comes from relating the estimated endolymphatic pathway to major structures that must remain identifiable during dissection.

Unlike a directly exposed anatomic structure, Donaldson’s line is an imaginary projection used to estimate location. Its usefulness lies in reproducibility: the same relationship to the lateral and posterior semicircular canals can be applied during temporal-bone dissection and surgical training. It should therefore be understood as an orientation aid, not as the duct or sac itself.

A practical sequence begins by identifying the lateral semicircular canal, then projecting a line parallel to the posterior semicircular canal. The point where that projection intersects the posterior labyrinth provides guidance toward the vestibular aqueduct and endolymphatic sac. Throughout the dissection, the sigmoid sinus, dura, and semicircular canals help preserve spatial orientation around the intended area.

In a transmastoid approach, the landmark helps identify the expected region of the endolymphatic duct and sac before decompression. Its relationship to the semicircular canals and posterior labyrinth supplies a reproducible route for orienting the dissection, while nearby structures provide additional spatial context. This application is particularly relevant to procedures performed for Ménière disease.

Donaldson’s line gives trainees a consistent reference for practicing temporal-bone dissection and interpreting the spatial relationships of the labyrinth. Rather than relying only on an isolated target, learners can connect the projected line with the lateral and posterior semicircular canals, posterior labyrinth, sigmoid sinus, and dura. That framework supports more systematic identification of the endolymphatic region.