Figure 1. Topography acquisition time with the MS39 is 1.0 seconds making the MS39 user and patient friendly.
Refractive surgery demands the highest level of diagnostic accuracy, repeatability, and efficiency. Failure to adequately diagnose an underlying corneal ectasia, for example, can have terrible consequences for the patient, the surgeon, and the refractive business.
Corneal topography, coupled with corneal and whole-eye wavefront analyses, are the cornerstone of refractive diagnostics.
Improved screening tools and algorithms have reduced the chance of error and improved our understanding of patients’ symptoms and outcomes from refractive surgery.
CSO, a manufacturer of diagnostic ophthalmic instruments, has developed two devices that, when used in combination, create the ultimate refractive surgery workplace. We have had the MS39 in our practice for four years and recently added the Osiris aberrometer to complete the refractive surgery toolbox.
The MS39 is a combination placido disc topographer plus anterior segment fourier domain optical coherence tomographer. This multifunctional device has remarkable capabilities and boasts a resolution of 3.6 μm.
The Osiris measures whole eye aberrations with up to 45,000-point resolution. It measures the total wavefront at 33 images per second.
REFRACTIVE SURGERY SCREENING
Assessing whether a person is suitable to have refractive surgery is a time-consuming, labour-intensive exercise. Getting it right is vital in ensuring safety standards are maintained and results are accurate.
All patients attending our clinic for a laser vision correction assessment have the full suite of tests performed on the MS39. This device is user-friendly, and acquisition is extremely quick: a topography scan takes one second only and the entire suite of tests can be over in less than two minutes.
Topography
Topography is performed with a combination placido/disc plus optical coherence tomography (OCT) analysis. The placido disc provides anterior corneal curvature, while the OCT measures actual corneal thickness, posterior elevation, and delineates the components of the cornea to allow for actual visualisation of the epithelium and stromal surfaces individually.
The Phoenix software allows the user to isolate the different maps and highlight any areas of concern.
Epithelium Mapping
Epithelium mapping is an essential tool for any refractive surgeon. The role of the epithelium is to create a smooth anterior surface, so the epithelium will be thicker over areas of stromal depression and thinner over areas of stromal elevation. As a result, epithelium mapping is essential for diagnosing which corneal curvature maps are normal (i.e; explained by epithelium thickening) and which are of concern.
Epithelium mapping is also essential for performing any therapeutic refractive procedures, such as trans epithelial phototherapeutic keratectomy.
Keratoconus Screening
The Phoenix software has a proprietary keratoconus screening map. This includes anterior and posterior corneal curvature, epithelium maps, stromal thickness maps, and corneal thickness maps. These have Gaussian and best fit sphere maps, which highlight any variations from normal.
Artificial intelligence tools produce a diagnostic report of each map, flagging any variables that may indicate underlying keratoconus.
Figure 2. This keratoconus screening map shows a ‘crab-claw’ anterior curvature map, that could indicate pellucid marginal degeneration. There is no posterior elevation or stromal thinning, and the epithelium map is thickened over the suspicious zone. This is, therefore, not an ecstatic cornea.
Tear Film Analysis
Dry eye management is vital in any form of refractive surgery. The tear film break-up time is easily measured and indicates which patients require tear film management before and after surgery. The display is easy to understand and is a very useful educational tool for patients suffering with dry eye.
Pupillography
Dynamic pupil measurements in scotopic, mesopic and photopic conditions are easily measured and assessed. This is useful when determining which procedure is most suitable for each patient and can help with intraocular lens selection in lens-based surgeries.
Corneal Aberrometry Analysis
Aberrometry is used in both treatment planning and post-operative analysis. Presbyond surgery (ZEISS Medical Technology), also known as laser blended vision or presbyopia surgery, requires measurement of the corneal spherical aberration used in treatment planning.
Post-operative aberrometry helps analyse and diagnose undesirable visual symptoms.
Whole eye aberrometry is measured with the Osiris and corneal aberrometry with the MS39. The combination allows the user to determine where the aberrations originate from.
Anterior Segment OCT
Anterior segment OCT has multiple vital uses for refractive surgeons, none more so than in the planning and post-op diagnosis of patients undergoing implantable collamer lens surgery. Formulae based on OCT analysis help with implantable collamer lens sizing, which allows for much safer surgery.
Other uses include glaucoma diagnostics and locating old LASIK flap depths.
OSIRIS
The addition of the Osiris aberrometer has completed the refractive surgery toolbox. The Osiris aberrometer measures whole eye aberrometry and when used in combination with the MS39 can delineate where the aberrations are coming from.
The Osiris has multiple functions (point spread function, modulation transfer function, dynamic accommodation) that help the clinician understand patient symptoms, individual eye physiology, and surgical outcomes.
Figure 3. Whole eye aberrometry (Osiris) shows a bifocal IOL explaining the patient’s dysphotopsia at night.
CONCLUSION
Ensuring a patient is suitable to have a refractive procedure and determining the appropriate procedure is not always clearcut. Having the best available tools to aid in accurate diagnosis and therapeutics is essential in any refractive surgery practice.
The CSO combination of MS39 plus Osiris provides the complete diagnostic toolbox for refractive surgery.
Dr Anton van Heerden FRANZCO is a cataract and refractive surgeon. He was Head of Unit, Surgical Ophthalmology Services at the Royal Victorian Eye and Ear Hospital 2014–2020. He is a Director of Armadale Eye Clinic, Mornington Peninsula Eye Clinic, and Eye Laser Specialists.
