Advanced: The Concept of Tarsal Tilt – Its Effects in Normal and Abnormal Clinical Conditions

Discrepancy between Observed Apparent Crease Height and True Anatomic Crease Height

We often see teaching staff demonstrating to house staff the nuances of measuring levator function (excursion) by placing a millimeter ruler along the face and eyelid, perhaps at the central portion of the upper lid margin. The measurement of the crease height is often taken in a similar position. To get the true anatomic crease height, we should have the patient looking downward, such that the upper lid pretarsal zone is almost vertical, or measure the eyelid crease height with the lids closed; we then obtain the true anatomic crease height, which is usually located at and corresponds to the height of the central tarsus.

In the figure we see here of a young adult ( Figure 20-2 ), the upper tarsus is measured to be at a tilt angle of 41° from the horizontal axis.

Asian Anatomy

Take for example a natural 7 mm crease for an Asian upper eyelid. Figure 20-3A shows the upper lid in a closed or down-turned position, while Figure 20-3B illustrates the lid in its normal, open position looking ahead. When the face is vertical and the eyes are looking ahead, the crease is optimally manifested and tucked in under its eyelid fold. The superior tarsal platform is angled supero-posteriorly in a tilted direction, close to a tilted incline angle (I) of 45°. The tarsus therefore manifests tarsal tilt.

Inclined crease height (Ich) or ‘tilted crease height’ (Tch) (blue in Figure 20-1 ) is the crease height as seen and measured vertically by an observer sitting across from the subject (eyes are open), and is always less than the true anatomic crease height.

An anatomic crease height of 7 mm (pretarsal skin or tarsus, red line in Figure 20-1 ) can be thought of as being aligned on the hypotenuse of a 45° isosceles triangle, while the two remaining sides of this hypothetical triangle are the vertical axis and the horizontal axis (each of the two sides will be approximately 7 mm × (1/√2), equaling 5 mm vertically and horizontally). Therefore a natural 7 mm crease will appear to the examiner as occupying 5 mm in vertical height from the most indented part of the crease to the eyelash margin (inclined or ‘tilted’ crease height, Tch), and about 3 mm only if there is 2 mm of eyelid fold overhanging it (the portion showing below the edge of the lid fold will be the clinically ‘apparent crease height’). Therefore it is quite normal for a single-eyelid patient to ask for a 3 mm crease for an end result; the practitioner should realize that it needs to come from a 7 mm anatomic crease placement.

or

implying that the surgical design of a crease height is inherently higher, up to a certain anatomic boundary, than what the patient observes or perceives.

The apparent height of the crease is less than the tilted crease height we see, by the millimeters of overhanging lid fold:

Tilted crease height is not worth measuring clinically, though it is usually approximately equal to 1/√2 (= 0.72 or five-sevenths) of the true anatomic crease height; the anatomic crease height should be measured with the eyelids closed or looking down.

Another crude method of measuring the tarsal tilt in an open eye is through MRI scan. In Figure 20-4 we see an image of a patient showing the measured angle of the open eyelid's tarsal segment to be 44.45°.

Methods of Investigation and Analysis: Mathematical Modeling

Mathematical modeling is often used by engineers and physicists to simulate real life scenarios when it is not feasible to measure complex events at the current stage of technology available, for example weather patterns, earthquake predictions, nuclear weapon testings, or aerodynamics of rocketry. Next to feasible actual measurement, it is considered the gold standard when it comes to accuracy; I will attempt to do the same here using basic geometry and trigonometry.

When the eyes are open, the tilt angle of the pretarsal segment of the upper lid (including skin, orbicularis, tarsal plate) as it lies on the eyeball may vary between 40 and 50° (assuming that the upper lid margin, hence tarsus, rests at the location of the upper corneal limbus and the tarsal plate extends superiorly beyond this point); we shall designate this tarsal resting angle as the incline angle, I.

Caucasian with 10 mm Tarsus ( Figure 20-5 )

Let us consider a Caucasian adult with the upper tarsus measuring 10 mm in vertical size (height), measured from the widest (central) portion of the tarsal plate. With the upper lid completely opened, assume the eyelid margin rests at the superior corneal limbus. Its crease will be 10 mm from the ciliary margin. The tarsus itself will subtend 46°of the circumference of the eye. The upper half of the cornea, which is 5 mm, will subtend 23°. The tarsus subtends 46°. From the knowledge that the two radii connecting from center of the globe to the lid margin and similarly to the crease indentation are equal, we can calculate the incline angle (I) relative to the horizontal axis for a Caucasian to be I = (180° − 46°)/2 − 23° = 44° (see Figure 20-5 ).

Figure 20-5 shows a model of a Caucasian eye with a 10 mm upper tarsus. The solid circle is the eyeball. The upper lid is not drawn here but its margin lies from the top of the superior corneal limbus on upward. The blue outline is an average clear cornea of 10 mm diameter. The 10 mm arc represents the upper lid tarsus when the eyelid is opened. (The horizontal line with the arrow is drawn parallel to the axial line that runs from the center of cornea to the back of the eyeball.) The dotted line is the slope of the tarsus rather than its true location in space (hence the tilt of the tarsus is I° when it indents to form the crease at the blue arrow where the upper border of the tarsus is located).

The magnitude of the tilt angle I (based on the lid position) can be used to relate what we apparently see (Tch) versus what we measure correctly when the lids are looking down (or closed): we may recall from trigonometry that {sine function of an angle = opposite/hypotenuse}.

Therefore, the sine function of value I (incline angle from tarsal tilt) is equal to the observed vertical component of the crease in space (Tch, which is not yet known), divided by the anatomic height (the hypotenuse) of the tarsus (whose superior tarsal border usually correspond to the location of the eyelid crease), which is known to be usually 10 mm in Caucasians (see also Figure 20-3 ):

$\begin{array}{c}\text{Sine}44°=\text{Tch}/10\text{mm}\\ \text{Sine}44°=0.69\\ \text{Therefore Tch}=10\text{mm}\times 0.69=6.9\text{mm for Caucasians}.\end{array}$

The Caucasian tarsus projects a 6.9 mm vertical component when examined with the eyelids opened, even though it is actually 10 mm. The crease height appears to be 6.9 mm when it should be 10 mm assuming that the crease folds in at an area along the superior tarsal border.