Vision is one of our most vital senses. Precise ocular movements are critical in synchronized gaze direction.[1] One’s method of teaching the extraocular movements to the medical students has a great effect on their clinical practice, particularly among the young residents turned ophthalmologists. The direct-face-to-face teaching method is being used most commonly in medical education at all times.[2] This method of teaching provides students with a passive educational experience rather than hands-on training.[3]
Simulation with hands-on–based training has become an integral part of surgical education in recent years. Cataract surgery simulators have reduced the surgical complications and augmented the residents’ learning. For strabismus surgery, however, ophthalmology residency programs still conventionally depend on operating room knowledge taught by senior surgeons. This dependency is partly due to a lack of appropriate simulators. Adding to it, the literature search describing simulation in the field of strabismus is also negligible.[4]
One of the difficulties in training students on cranial nerve testing in relation to extraocular muscle movements is the unfamiliarity of Listing’s law. Listing’s law describes the three-dimensional orientation of an eyeball and its axes of rotation. The law states that the visual directions are related to the rotation of the eye, such that all the rotation axes lie in a plane.[3]
Regrettably, Listing’s law is conceptually difficult to deliver in classroom sessions without an actual demonstration, and currently, there is no eyeball model with extraocular muscles that can be used to demonstrate this law.
To overcome all the hitches and to make teaching a pleasurable experience, we have come up with an effective, low-cost, innovative, ready-to-make, Do It Yourself (DIY) model to describe the Listing’s law along with the extraocular muscle actions.
Innovation
A transparent ball with pupil markings, a printed fundus image to represent the retina, three 10-inch wooden skewer sticks, cardboard paper for the listing plane and cardboard sheet for the muscle plane, crepe bandage cut into small strips for extraocular muscles, and a plastic stand for mounting the ball for simple rotation are the materials used to construct our eyeball framework [Fig. 1].
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Figure 1: (a) The materials required for our eyeball model. (b) Assembled model of eyeball with muscles marked within it. IO = inferior oblique, IR = inferior rectus, LR = lateral rectus, MR = medial rectus, SO = superior oblique, SR = superior rectus
We have thoroughly covered each extraocular muscle’s muscular plane, axis of rotation, and activities in this document.
Axis of rotation
The axis of rotation acts perpendicular to the muscle plane. For horizontal muscles, it is around the Z-axis [Fig. 2]; for vertical muscles, it is 23° around the X-axis; and for oblique muscles, it is 36° around the Y-axis.
Figure 2: The muscle plane perpendicular to the horizontal muscle, which is parallel to the X-axis. Muscle action will be along the Z-axis
Muscle plane
Three muscle planes (i.e., tangential point to muscle insertion, along the axis of rotation), horizontal, vertical, and oblique plane, are being demonstrated with cardboard [Fig. 3a]. Listing’s plane is the plane which lies parallel to X- and Z-axes, but perpendicular to Y-axis [Fig. 3b]. Any movement into a tertiary position involves a movement about the Listing’s plane.
Figure 3: (a) The X-, Y-, and Z-axes, with wooden skewers placed along the horizontal, anteroposterior, and vertical axes, respectively. All three are made to meet at the center of rotation. (b) Listing’s plane with the axes. Listing’s plane is the plane that is parallel to the X- and Z-axes and perpendicular to the Y-axis
Movement to explain
Horizontal eye movements, such as adduction, a nasal movement, and abduction, a temporal movement, are represented as excursions around the vertical axis (Z-axis). Vertical eye movements, elevation, and depression are represented by rotations around the X-axis.
There are ocular excursions around the Y-axis as well, albeit these are more challenging to recognize clinically. Cycloduction is the term for the rotation of the eye around the Y-axis. Rotation of the 12 o’clock meridian nasally represents an incycloduction, while rotation of the 12 o’clock meridian temporally is referred to as an excycloduction.
Medial rectus and lateral rectus
It is easier to understand horizontal muscle motion. Primary positions of the medial rectus (MR) and lateral rectus (LR) muscles share a common horizontal muscle plane, which is parallel to the Y-axis and the X-axis [Fig. 4]. The axis of rotation at the primary position occurs only along the Z-axis. Because of this, the MR is a pure adductor, while the LR is a pure abductor.
Figure 4: Picture depicting horizontal muscle sharing a common muscle plane of X-axis and Y-axis, so that the axis of rotation is only along the Z-axis
Therefore, it only has a primary activity and no secondary or tertiary action (i.e., abduction and adduction for LR and MR muscles, respectively). By pulling the muscle medially from the initial position to test the MR muscle and laterally to test the LR muscle, the examiner can show its relative strength.
Vertical recti muscles
The primary position of the vertical muscle is not limited to one of the axes. So, the action is more complicated.
The superior rectus (SR) and inferior rectus (IR) muscle planes form an acute angle of 23° with the Y-axis and are inserted slightly anterior to the Z-axis. The axis of rotation does not coincide with the X-axis in the equatorial plane, but forms an acute angle of 23° with the X-axis.
Superior rectus
SR has three distinct actions. The primary action is elevation along the X-axis, the secondary action is intortion along the Y-axis, and the third action is adduction along the Z-axis. At the primary position, the main pulling power of the SR is medial to the center of rotation, which accounts for the adduction action of the muscle.
When the transparent ball is rotated at 23° abduction, the visual axis will be in line with the muscle plane, with the X-axis and the axis of rotation coinciding with each other, making SR a pure elevator, with its elevation action being maximum at this point.
With 67° adduction, the SR muscle plane coincides with the X-axis, but is perpendicular to the Y-axis (visual axis). At this position, SR becomes a pure incycloductor, but with minimal elevation because it cannot be adducted beyond that [Fig. 5].
Figure 5: EOM of SR muscle: (a) In the primary position, (b) on abduction 23°, (c) acts as a pure elevator and (d) on adduction 67°, (e) acts as a pure intorter with minimal elevation. EOM = extraocular muscle, SR = superior rectus
Inferior rectus
Similar to SR, IR has three muscle actions. Primary action is depression along the X-axis, secondary action is extortion along the Y-axis, and tertiary action is adduction along the Z-axis.
When the eye is rotated to 23° abduction, the muscle plane and the visual axis (i.e., the X-axis and the axis of rotation) coincide. At this position, IR becomes a pure depressor and its depressor action is maximum.
With 67° adduction, the visual axis is perpendicular to the IR muscle plane, making IR act as a pure extortor.
Oblique muscle
The superior oblique (SO) and inferior oblique (IO) muscle planes go in a direction from the anteromedial aspect of the globe to the posterolateral aspect. So, the muscle plane coincides neither with the median plane of the globe nor with the axis of rotation.
The muscle planes of SO and IO muscles form an angle of 54° and 51°, respectively, with the visual axis. Because of this large angle, the oblique muscle mainly exhibits cyclorotation.
Superior oblique
In the primary position, SO causes incycloduction, depression, and abduction.
So, with 54° of adduction, SO would be a pure depressor, with the muscle plane in line with the visual axis. Also, with 36° of abduction, its action would be pure incycloduction.
Inferior oblique
The primary position of IO causes excycloduction, elevation, and abduction.
Similar to SO, with 51° adduction, IO would be a pure elevator and with 39° of abduction, its action would be pure excycloduction.
Angle demonstration
The different angles involved in strabismus are explained using our model. The angle gamma (g) is created when the black wooden stick is brought to meet the three other sticks (X, Y, and Z axes) at the center of rotation. The angle alpha (α) is formed when the black stick is brought anteriorly to meet at the second black point, designated as the nodal point in the pupillary axis, and the angle kappa (κ) is formed when the black stick is brought even more anteriorly to meet at the first black point, which is the pupil’s center [Fig. 6].
Figure 6: Demonstration of angles: (a) angle gamma, (b) angle alpha, and (c) angle kappa. Arrow points represent the points of intersection
Discussion
Simulation in medical education has been established as a safe and efficient adjunct to learning. Simulation may improve students’ clinical skills through practicing and mastering specific knowledge of extraocular muscles and their innervation. Even if a trainee or a resident does not want to be a strabismic surgeon, basic knowledge about strabismus is essential to one’s routine practice in ophthalmology.
Cutting-edge technologies are filling the classroom and have been quickly adopted by instructors and students. However, the higher the technology, the higher the cost will be. Low-cost models help institutions reduce the costs of education, such as the materials used in this study. This model allows students to detach and reattach the muscle insertions, which may further strengthen their understanding of Listing’s law. Use of such low-cost simulations and easy-to-use models could also make the process of teaching and learning more varied, interesting, and effective.[3]
Our main aim was to design and validate a high-fidelity model useful to a range of learners, from junior residents to experienced surgeons, aiming to improve knowledge about strabismus.[4] Our eyeball model is of low cost, easy to construct, requires minimal assembly, is easy to practice, and has a wider range of applications. All the materials needed to make this model are readily available and inexpensive. It also allows for easy transportation outside the wet lab and can be used in any setting to teach the residents.
Conclusion
Simulation is becoming a standard in medical education. It is important that in ophthalmology residency programs, they prioritize simulation in all its subspecialties, with an emphasis on strabismus, for a smooth residency learning experience.
In conclusion, this model is a cost-effective, readily available, multipurpose eye model that serves as a good alternative to the commercially available ones in teaching not only the residents, but can also be used in teaching undergraduates and optometrists.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References
1. Wei Q, Sueda S, Pai DK. Physically-based modeling and simulation of extraocular muscles Prog Biophys Mol Biol 2010;103:273–83.
2. Lam CK, Sundaraj K, Sulaiman MN. Virtual simulation of eyeball and extraocular muscle reaction during cataract surgery Procedia Eng 2012;41:150–5.
3. Zhang N, He X. Understanding the extraocular muscles and oculomotor, trochlear, and abducens nerves through a simulation in physical examination training:an innovative approach J Chiropr Educ 2010;24:153–8.
4. Jagan L, Turk W, Petropolis C, Egan R, Cofie N, Wright KW, et al. Validation of a novel
strabismus surgery 3D-printed silicone eye model for simulation training J AAPOS 2020;24:3–e1.
