LASIK後のフリーキャップ

Key Points of This Condition

• Free cap is a rare intraoperative complication of LASIK, in which the corneal flap hinge separates and the flap becomes completely free.
• It occurs more often with microkeratome use, and the incidence has dramatically decreased with the introduction of femtosecond lasers.
• Flat cornea (less than 40 D) and insufficient suction are the main risk factors.
• If the stromal bed is smooth, laser ablation can continue, and the free cap is repositioned and fixed with a bandage contact lens.
• With proper management, severe permanent vision loss is rare.

1. What is a Free Cap?

A free cap is one of the intraoperative complications of LASIK (laser in situ keratomileusis). In normal LASIK, a corneal flap is created with a hinge left intact, and an excimer laser is applied to the exposed stromal bed. When the hinge separates and the flap becomes completely free, this condition is called a free cap.

The incidence of free cap with microkeratome LASIK is reported to be 0.004% to 1.31%. Since the introduction of femtosecond lasers, flap-related intraoperative complications have dramatically decreased.

Q Does a free cap mean the surgery failed?

A

A free cap is an unexpected complication, but with appropriate management, good visual outcomes can be achieved. If the stromal bed is in good condition, surgery can continue, and the free cap is repositioned and secured. Severe permanent vision loss is rare.

2. Main Symptoms and Clinical Findings

Free cap is an intraoperative complication, confirmed as a flap that has completely lost its hinge after microkeratome pass ¹⁾. Its incidence in microkeratome LASIK is rare, ranging from 0.004% to 1.31%. In a large analysis of 55,700 eyes, the incidence with SBK microkeratome was 0.276%, and risk factors identified include thin central corneal thickness, extreme ring height values, and high stop values ²⁾.

Postoperative findings may include irregular astigmatism, flap striae, and epithelial ingrowth if the free cap is repositioned improperly.

3. Causes and Risk Factors

Risk factors for free cap are classified into anatomical and mechanical factors.

Anatomical Factors

Flat cornea: Corneas less than 40 D tend to have smaller flap diameters.

Deep orbit: Makes proper placement of the microkeratome difficult.

Mechanical Factors

Use of microkeratome: Higher risk compared to femtosecond laser.

Insufficient suction: Inadequate suction fixation can cause flap failure.

Eccentric suction ring: Eccentric ring placement prevents proper hinge formation.

Blade defect: Defective microkeratome blade causes abnormal incision.

Prevention Tips

Prevention of free cap is most important. Appropriate selection of suction ring and stop size based on keratometry, ensuring adequate suction pressure, use of large-diameter suction ring for flat corneas less than 42 D, and avoidance of flap creation for corneas less than 38 D are recommended. Applying asymmetric markings before flap lifting helps correct repositioning in case of free cap.

4. Diagnosis and Examination Methods

Diagnosis of free cap is made by direct intraoperative observation. The diagnosis is confirmed when the flap is found to be completely free after passing the microkeratome.

If irregular astigmatism or flap wrinkles are suspected postoperatively, evaluation is performed using slit-lamp microscopy, corneal topography, and anterior segment OCT.

5. Standard Treatment

Intraoperative Decision

When a free cap occurs, the surgeon decides whether to continue or abort laser ablation.

If the stromal bed is smooth, ablation can be continued. The free cap is handled by placing it on a drop of BSS (balanced salt solution) to prevent drying. After ablation is complete, the free cap is repositioned with the epithelial side up and aligned correctly. Preoperative markings help confirm alignment ¹⁾.

If the stromal bed is irregular, the flap is repositioned without laser ablation. If no ablation is performed, refractive error changes or significant vision loss usually do not occur.

Post-repositioning Management

After repositioning, a bandage contact lens (BCL) is often used. The corneal endothelial pump function firmly reattaches the cap. Keeping the eye closed with two crossed adhesive tapes for about 30 minutes postoperatively is effective in preventing disc dislocation.

Management of Complications

Complication	Management
Disc dislocation	Early refixation
Irregular astigmatism	Refixation or repeat LASIK
Epithelial ingrowth	Flap lift and irrigation

There are reports of good visual acuity achieved by re-fixation within 4 days of disc detachment. For irregular astigmatism, methods such as reoperation with a deep flap after 6 months or more, or use of a homologous corneal cap have been reported.

Caution

If the free cap is placed with the epithelial side and stromal side reversed, epithelium may become engrafted between the layers, potentially leading to disc loss. Preoperative asymmetric marking is essential to confirm correct orientation.

Q Is there a way to prevent free cap?

A

Yes, prevention is quite possible. The basics are selecting an appropriate suction ring according to corneal curvature and ensuring sufficient suction pressure. Especially for flat corneas less than 40 D, a large suction ring is recommended; for less than 38 D, it is advisable to avoid flap creation altogether. In recent years, femtosecond laser flap creation has become mainstream, significantly reducing the occurrence of this complication.

Q Can vision recover even if a free cap occurs?

A

With appropriate management, good visual acuity can often be achieved. If the stromal bed is intact, laser irradiation can continue, and the free cap can be repositioned and fixed correctly. Severe permanent vision loss is very rare. However, irregular astigmatism or flap wrinkles may persist, so management by an experienced surgeon is important.

For patients: Please read

This article is intended for healthcare professionals and is not a substitute for diagnosis or treatment. For complications and risks of LASIK surgery, please consult directly with your refractive surgery specialist.

---

References

1. Paryani M, Israni N, Kochar S, Aggarwal K. Microkeratome chronicles: management of a free flap. Indian J Ophthalmol. 2023;71(7):2926-2927.
2. Katz T, Druckiv V, Siebelmann S, Frings A, Skevas C. Prediction model of free flaps in microkeratome-assisted LASIK. PLoS One. 2021;16(9):e0255525.