A device to quantify orbital compliance and soft-tissue restriction

Introduction: Facial trauma can often lead to functionally significant fractures of the orbital bones. A significant challenge inthe repair these injuries is to resolve the restriction of prolapsed orbital soft tissue without inducing iatrogenic impingementat the time of implant placement. Forced duction testing, consisting of grasping the conjunctiva with forceps and manuallymanipulating the globe, is the current gold standard for assessing mechanical restriction of eye movement. However, forcedductions result in binary designations of positive or negative, as determined by an individual surgeon's subjectiveperception, which is highly variable and experience-dependent. Here, we present a device to determine quantitative and reproducible measurements of orbital compliance and orbital soft tissue restriction. Method: The device consists of stacked rotational and horizontal translational piezoelectric motor stages, attached to avertical translational motor stage via a load cell that senses vertical resistance ([ Fig. 1 ]). The horizontal translational stageis coupled to a load cell that senses horizontal resistance, with a custom 3D-printed rail and shuttle interface system thatcreates a curvilinear motion for appropriate globe manipulation. The ocular surface is engaged via vacuum-assisted suction([ Fig. 2 ]). The apparatus is mounted on a locking gooseneck arm that can be fastened to the operating table, allowing quickintraoperative positioning, neutralizing recoil force from motor motion, and eliminating user movement interference. Inaddition, we have devised a component for sensing resistance to cyclotorsion ([ Fig. 3 ]), to be affixed between the rotational and horizontal translational motor stages. We designed the device to automate scanning of resistance to translation alongeach orbit clock hour, of force applied normal to the ocular surface, and of globe torque;this enables rapid mapping of softtissue resistance and range of motion with respect to translation, cyclotorsion, and retropulsion. [ Fig. 4 ] depicts a proposedcolor-coded system for visualizing data on translational range of motion and resistance. Results: We had performed cadaveric testing of a previous handheld iteration of the device, consisting of stackedtranslational and rotational motor stages. We demonstrated the feasibility of manipulating the globe through the suctionmechanism and the ability of the load cell to measure resistance to translational force, distinguishing the entrapped state ina cadaveric fracture model. The need to gauge soft-tissue resistance across greater ranges of motion, and to remove usererror stemming from the handheld operation of the previous iteration, led to the creation of the present version. Though wehave constructed a fully functioning prototype, cadaveric and animal testing have not been performed due to COVID-19restrictions. Conclusion: Our device allows for the quantitative characterization of all extraocular muscles and resistance to retropulsion,which can assist in the evaluation not only of soft tissue entrapment secondary to orbital fractures, but also otherpathologies involving mechanical restriction of ocular motion including thyroid eye disease, orbital tumors, and iatrogenicinjury. We envision this device to be used for both preoperative and intraoperative assessment of these conditions and the irsurgical correction.