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 in the repair these injuries is to resolve the restriction of prolapsed orbital soft tissue without inducing iatrogenic impingement at the time of implant placement. Forced duction testing, consisting of grasping the conjunctiva with forceps and manually manipulating the globe, is the current gold standard for assessing mechanical restriction of eye movement. However, forced ductions result in binary designations of “positive” or “negative,” as determined by an individual surgeon's subjective perception, 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 a vertical translational motor stage via a load cell that senses vertical resistance ([Fig. 1]). The horizontal translational stage is coupled to a load cell that senses horizontal resistance, with a custom 3D-printed rail and shuttle interface system that creates 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 quick intraoperative positioning, neutralizing recoil force from motor motion, and eliminating user movement interference. In addition, 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 along each orbit clock hour, of force applied normal to the ocular surface, and of globe torque; this enables rapid mapping of soft tissue resistance and range of motion with respect to translation, cyclotorsion, and retropulsion. [Fig. 4] depicts a proposed color-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 stacked translational and rotational motor stages. We demonstrated the feasibility of manipulating the globe through the suction mechanism and the ability of the load cell to measure resistance to translational force, distinguishing the entrapped state in a cadaveric fracture model. The need to gauge soft-tissue resistance across greater ranges of motion, and to remove user error stemming from the handheld operation of the previous iteration, led to the creation of the present version. Though we have constructed a fully functioning prototype, cadaveric and animal testing have not been performed due to COVID-19 restrictions.

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 other pathologies involving mechanical restriction of ocular motion including thyroid eye disease, orbital tumors, and iatrogenic injury. We envision this device to be used for both preoperative and intraoperative assessment of these conditions and their surgical correction.

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Publication History

Article published online:
12 February 2021