Wilbrand Knee Revisited

Abstract

Background: Over a century ago, German ophthalmologist Hermann Wilbrand reported inferonasal crossing fibers within the chiasm curve anteriorly into the contralateral optic nerve. This anatomic bend, "Wilbrand knee," is classically cited as the explanation for the "junctional scotoma," a contralateral superotemporal visual field defect associated with lesions affecting the optic nerve at its junction with the chiasm. More recent reports have called into question the existence of Wilbrand knee or suggested that it may simply be an artifact.

Methods: Four human optic chiasms (obtained from cadaver donors with no reported premortem visual pathology) and 2 monkey chiasms were fixed and thin sectioned (40 µm), then examined using anisotropic scattering imaging , a novel technique that takes advantage of the fact that light reflects off well-defined linear structures (i.e., axonal tracts) in a predictable manner based on their orientation. Using this technique, tissue structures oriented in different directions can be distinguished at high resolution without the need for tissue staining.

Results: In all 4 human optic chiasms, thin fiber tracts consistent with, but less prominent than, those Wilbrand had described were observed. No such tracts were found in the monkey chiasms.

Conclusions: Wilbrand knee exists in humans but is modest in its anterior projection. Wilbrand knee does not seem to be present in monkeys, however, which may explain conflicting reports in the literature regarding its existence.

FIG. 1.

Anisotropic scattering imaging. Fiber bundles scatter light differently, based on different angles of illumination (A). These angle-dependent light reflections can be mapped, with different orientations associated with unique patterns (B). A thin unstained section of human brain imaged sequentially while being illuminated from 3 different angles (C).

FIG. 2.

Monkey optic chiasm, color-coded in spherical coordinates, displayed by azimuth and pitch.

FIG. 3.

Qualitative and quantitative anisotropic scattering imaging applied to a human optic chiasm sectioned in the axial plane. Wilbrand knee visualized qualitatively (A) and quantitatively (white arrows), color-coded by azimuth (B).

FIG. 4.

Qualitative and quantitative anisotropic scattering imaging applied to a monkey optic chiasm sectioned in the axial plane. No evidence of Wilbrand knee visualized qualitatively (left) or quantitatively (right).

FIG. 5.

Quantitative anisotropic scattering imaging applied to monkey and human optic chiasms sectioned in the coronal plane, color-coded by pitch. No evidence of Wilbrand knee in the monkey (A). Evidence of Wilbrand knee in the human (red), located ventrally, anterior to the chiasm (B).