Predictors of Bubble Formation and Type Obtained with Pneumatic Dissection During Deep Anterior Lamellar Keratoplasty in Keratoconus

PURPOSE: To identify predictors of bubble formation and type during big-bubble deep anterior lamellar keratoplasty (BB-DALK) performed in keratoconus at different stages of severity. DESIGN: Retrospective Cohort Study. METHODS: Setting: University Magna Græcia (Catanzaro, Italy); Study Population: Consecutive keratoconus patients undergoing BB-DALK from September 2014 to Feb- ruary 2019; Observation Procedure: Keratometric astigmatism, mean keratometry value (K-mean), highest keratometry value (K-max), thinnest point, anterior segment Optical Coherence Tomography (AS-OCT)-based stage of ectasia. Main Outcome Measures: Rate of bubble formation and type; number and fate of micro/macro-perforation; conver- sion to mushroom keratoplasty (MK); comparison of parameters in patients with bubble formation vs failure and in type 1 vs type 2 bubble; areas under the curves (AUC) of preoperative parameters for distinguishing between bubble types. RESULTS: Pneumatic dissection succeeded in 113/155 eyes (72.9%), with 100 type 1 bubbles (88.4%), 11 type 2 (9.8%) and 2 mixed-type (1.8%). Micro-perforations were managed conservatively in type-1 bubbles; macro-perforations occurring in both types of bubbles required conversion to MK. Preoperative K-mean and K-max values were significantly higher in eyes in which bubble formation succeeded (respectively, P=0.006 and P<0.013). Type 1 bubbles formed in eyes with significantly lower age, K-mean, AS- OCT stages and higher pachymetric values (always P<0.029). Age had the highest di- agnostic power for discrimination between bubble types, followed by AS-OCT stage, pachymetry, K-mean and astigmatism (respectively, AUC=0.861; 0.779; 0.748; 0.700; 0.670). CONCLUSIONS: Older age and advanced stages of keratoconus were predictors of type 2 bubble formation during BB-DALK that was associated with an increased risk of complications.