Author Response: Motion-Encoded MRIs Provide Evidence against Orbital Pulleys

The authors thank you for the opportunity to bring motion-encoded magnetic resonance imaging (MRI) of the orbit to wider scientific attention. 

As you correctly state, the published data are in agreement with Sherrington's Law of Reciprocal Innervation. However, computed tomography and magnetic resonance imaging (MRI) cross-sectional studies of extraocular muscles before and after ocular movement led scientific understanding to the same conclusion years ago. Motion-encoded MRI is the latest technique of image analysis. Its main strength is depiction of the change in muscle unit length within the muscle while the eye is moving. 

We agree that the word choice “normal deformation” has potential for improvement. However, the word stem “conform” does not seem to precisely express our findings. In our normal population, we illustrated physiological muscle contraction and relaxation of the horizontal rectus muscles over time. We were also able to depict an abnormal innervation pattern in patients with Duane's syndrome type I, which affected specific extraocular muscle segments only. 

Motion-encoded MRI measures distances and not tensions or forces, a fact that we strictly adhered to while interpreting the image data. To mix these entities, as was the case with ultrasound imaging, would have resulted in substantial misconceptions. 

The MRI method has known technical limitations in demonstrating the pulley structures directly. Its signal is optimized in muscle tissue. Therefore, the precise visibility of tendons and connective tissue bands is strongly reduced. Imaging advantages in characterizing the pulleys have been attributed to secondary and tertiary gaze positions. Hence, ocular movement amplitudes larger than those investigated in our current publication are necessary to provide better insight. 

At present, spatial resolution of the published images generally is not high enough to separate horizontal extraocular muscles into functional subunits, such as the global and the orbital layers. Clear spaces between tissue borders may also be an expression of the water–fat shift of the MR signal. 

The strong interest in the technique motivates us to further invest in it. Especially in the MR technique there is potential to improve the image quality and therefore even smaller changes may be visible in the future. 

To summarize, motion-encoded MRI in its current technical state neither supports nor contradicts the active pulley hypothesis.