Doctor's Answers (2)
With any refractive surgery, whether SMILE, LASIK, PRK or ICL surgery, there is a chance of over-correction or under-correction.
So if a person was myopic before surgery, an 'overcorrection' would cause hyperopia, and vice versa.
Usually, the degree of under or over-correction is minimal and is around 25 to 50 degrees or less (+/-0.25D to 0.50D). This means that vision is still very clear without glasses and is achieved in better than 95-97% of patients in general after a single treatment.
It is also important to realise that there is a 'margin of error' in measuring spectacle power, which is usually taken to be +/-0.25D or 25 degrees. Therefore, 25 degrees appearing on one examination may not be there on subsequent examinations.
Another important factor to consider is that spectacle power can fluctuate in the early period after any surgery. It is usually only stable after at least a month, and this period of time could potentially be longer for PRK.
A small amount of hyperopia eg +0.25D to +0.50D is not expected to cause any visual blurring in young people less than about 35 years old or so. In fact, there are also older people with this spectacle power who do not wear glasses for distance viewing.