BLOG: Why are flap tears shaped like a horseshoe?

ByDoug Rett, OD, FAAO

Most of us have seen a flap tear in the peripheral retina. If it’s in the superior retina, it’s invariably “U” or “V” shaped. If it’s in the inferior retina, it’s shaped like an upside “U” or “V.”

But can it be an upside down “U” in the superior retina? Or a regular “U” in the inferior retina? In a word: no.

The reason tears get their shape has to do with why they form in the first place. Retinal tears occur from vitreous traction, and 74% of tears occur within 2 weeks of the patient’s posterior vitreous detachment (PVD). When the retina tears, it does so because as the vitreous is detaching away from the internal limiting membrane (ILM), it essentially hits a snag. This snag could be a vitreal retinal tuft (VRT), the strong vitreal-retinal (V-R) adhesion of a lattice lesion or simply an area of retina previously devoid of any overt traction. In fact, 70% of retinal tears occur at locations where no lesion (VRT, lattice, etc) was previously noted.

Once the vitreous hits this discontinuity in the V-R interface, one of two things can happen: The vitreous can pop cleanly off, leaving the retina unaffected or the vitreous can tear the retina as it detaches from the ILM.

Flap tear

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If the second option happens, then one of two things will follow: The retina can tear 360 degrees around the discontinuity (creating a hole) or the edges of the retina tear will not meet in a circle, and the tear will continue to be adherent to the vitreous as it continues to detach. This is why flap tears are horseshoe shaped. The vitreous will always detach in the same direction: from the posterior towards the anterior. From the macula towards the vitreous base. So, the direction of the vector of force will always point toward the ora.

So, when the vitreous detachment hits that snag (or the point/base of the “V” or “U”) if it tears, it will always tear continuing toward the ora. Thus, the horseshoe will always “point” toward the posterior pole, and it can’t be the opposite because the vitreous will always detach in the same direction.

Why is it always peripheral retina?

Why don’t we get tears in our posterior pole, like a big horseshoe tear next to the macula? It’s almost always in the periphery or midperiphery.

The first reason is that the peripheral retina is thinner than that of the posterior pole. You’ve probably noticed this while looking at posterior pole optical coherence tomography scans. The farther your scan gets from the macula, then thinner the retinal tissue is. And if you attempt a more peripheral scan, you’ll see that the far retina is quite thin, indeed. So, thinner retina is much easier to break.

However, there is another, more complex, reason why the peripheral retina is the first to break, and that has to do with the insertion of the vitreous to the retina. The vitreal fibers run anterior to posterior within the vitreous body. That means when the vitreous cortex (a.k.a. hyaloid face) is still attached to the retina, the fibers at the fovea are coming in perpendicular to the face of the retina, and the fibers at the equator have an oblique insertion. Now, remember what we said about the direction of the vector of force that the PVD has: it always detaches from posterior to anterior, toward the vitreous base. That means that the strongest force of V-R adhesion is right at the fovea. And, sure enough, vitreomacular traction causes macular holes. But excluding macular holes, the PVD almost never causes a break in the posterior pole.

This is because the angle is wrong for tearing. If you were trying to tear the retina, you wouldn’t come at it from such a straight-on angle. You would grab ahold of that V-R tuft or that discontinuity of the retina and pull on it with an angle more acute/oblique to the surface of the retina. So, the angle that the posterior pole retina makes with the direction of the vector of force of the PVD is closer to 90 degrees, not ideal for tearing. However, the angle that the equatorial retina makes with the direction of the vector of force of the PVD is much more acute and the ideal angle to tear something. So, the angle of the vitreous insertion, in combination with the thinner retinal tissue, is why the peripheral or midperipheral retina is most always the spot for retinal breaks.

Clinical practice guidelines

Finally, I’d like to call your attention to the Clinical Practice Guidelines, put out by the American Optometric Association and the Preferred Practice Patterns, put out by the American Academy of Ophthalmology. These documents are essentially the standard of care in our field and are the standard to which we are held as practicing eye doctors. I urge you all to read them, as they are very comprehensive, very well-written and one of the best places to keep up-to-date on how you should manage your patients.

Specifically for the peripheral retina, both sites have entries that include a table documenting how one should follow/refer certain peripheral retinal conditions. There is a wealth of information on peripheral retinal lesions, how likely each is to cause retinal detachment, how soon each should be treated and the proper follow-up to give each lesion. It’s important for all of us to review this information, because if (as the statistics say) one in 10 patients will have a retinal break sometime in their life, then I dare say that one of you reading this will have a patient with a retinal tear this very day. And the more you know, the better your patient will be managed.

Also, if you’re interested, Google Norman E. Byer’s work on the peripheral retina. Dr. Byer is a giant in the field of peripheral retina, and while most of his work was done in the 1980s and 1990s, it still reads as very relevant today. He writes with a good voice and has done some amazing work. It’s humbling for me to think about all the work that our colleagues in past decades have provided for us. We only need to carry on the tradition.

References:

American Academy of Ophthalmology. Preferred Practice Patterns. http://www.aao.org/guidelines-browse?filter=preferredpracticepatternsguideline. Accessed September 9, 2015.

American Optometric Association. Clinical Practice Guidelines. http://www.aoa.org/optometrists/clinical-resources/clinical-care-publications/clinical-practice-guidelines?sso=y. Accessed September 9, 2015.