From all Lasik flap complication, LASIK flap buttonholes are the ones that lead most often to loss of best-corrected visual acuity(BCVA).
Why do they happen? Thin, irregular and perforated flaps seem to result from a common etiology; an inadequate coupling of the blade to the cornea.
Steep corneas have been compared to tennis balls that would buckle centrally upon applanating pressure.
This results in a central dimple missed by the blade leading to a buttonhole.
Another theory is that higher keratometric values offer increased resistance to cutting when applanated, leading to upwards movement of the blade.
The latter is probably more applicable to keratomes with lower oscillation rates.
Similarly, flat corneas may result in a thin and/or small flap as they could be below the adequate cutting level in certain locations.
Inadequate blade to cornea coupling is often due to poor suction (sunken globe/small diameter corneas with inadequate suction ring placement, conjunctival incarceration in the suction port...
).
Non-angled blades have equal chances of moving upwards towards the surface or downwards towards the stromal side if faced with resistance.
On the other hand, inferiorly angled blades are more likely to be driven towards the stroma.
If a buttonhole is encountered (especially centrally), most surgeons prefer to abort the procedure, replace the flap and re cut a deeper flap (20-60 ?m deeper) approximately 10-12 weeks later.
While some advocate proceeding with scraping the epithelium and performing a PRK laser ablation, we believe this approach may not be feasible in higher myopes due to the appearance of unexpected haze.
A higher index for epithelial ingrowth should maintained around the margins of the buttonhole.
The incidence of perforated flaps (as well as thin and irregular ones) may be reduced if the surgeon ensures adequate suction, inspects the blades and adjusts the keratome plate thickness according to corneal curvature.
Other helpful measures include ensuring adequate intraocular pressure before cutting the flap.
Measurement may be most valuable with a pneumotonometer as other means were reported to provide imprecise readings at times.
Care should be taken to avoid conjunctival clogging in the suction port, which could lead to discrepancy between the intraocular pressure and the suction pressure recorded on the microkeratome vacuum console.
Newer microkeratomes have a safety mechanism to automatically abort the procedure or to activate additional suction but are also prone to similar problems if IOP measurements are not obtained to ensure adequate suction.
Some surgeons inspect the microkeratome blade under the operating microscope before engaging it in the suction ring in order to rule out manufacturing or other preoperative damage to the blade.
It is best to keep the microkeratome away from hard surfaces after assembly to avoid subsequent blade damage.
Why do they happen? Thin, irregular and perforated flaps seem to result from a common etiology; an inadequate coupling of the blade to the cornea.
Steep corneas have been compared to tennis balls that would buckle centrally upon applanating pressure.
This results in a central dimple missed by the blade leading to a buttonhole.
Another theory is that higher keratometric values offer increased resistance to cutting when applanated, leading to upwards movement of the blade.
The latter is probably more applicable to keratomes with lower oscillation rates.
Similarly, flat corneas may result in a thin and/or small flap as they could be below the adequate cutting level in certain locations.
Inadequate blade to cornea coupling is often due to poor suction (sunken globe/small diameter corneas with inadequate suction ring placement, conjunctival incarceration in the suction port...
).
Non-angled blades have equal chances of moving upwards towards the surface or downwards towards the stromal side if faced with resistance.
On the other hand, inferiorly angled blades are more likely to be driven towards the stroma.
If a buttonhole is encountered (especially centrally), most surgeons prefer to abort the procedure, replace the flap and re cut a deeper flap (20-60 ?m deeper) approximately 10-12 weeks later.
While some advocate proceeding with scraping the epithelium and performing a PRK laser ablation, we believe this approach may not be feasible in higher myopes due to the appearance of unexpected haze.
A higher index for epithelial ingrowth should maintained around the margins of the buttonhole.
The incidence of perforated flaps (as well as thin and irregular ones) may be reduced if the surgeon ensures adequate suction, inspects the blades and adjusts the keratome plate thickness according to corneal curvature.
Other helpful measures include ensuring adequate intraocular pressure before cutting the flap.
Measurement may be most valuable with a pneumotonometer as other means were reported to provide imprecise readings at times.
Care should be taken to avoid conjunctival clogging in the suction port, which could lead to discrepancy between the intraocular pressure and the suction pressure recorded on the microkeratome vacuum console.
Newer microkeratomes have a safety mechanism to automatically abort the procedure or to activate additional suction but are also prone to similar problems if IOP measurements are not obtained to ensure adequate suction.
Some surgeons inspect the microkeratome blade under the operating microscope before engaging it in the suction ring in order to rule out manufacturing or other preoperative damage to the blade.
It is best to keep the microkeratome away from hard surfaces after assembly to avoid subsequent blade damage.