Ophthalmic Biometry

KEYWORDS

Biometry IOL calculation Refractive surgery

Cataract surgery and intraocular lens (IOL) implantation are currently evolving into a refractive procedure. The precision of biometry is crucial for meeting expectations of patients undergoing cataract surgery. Moreover, the optimal results for new IOLs being developed, such as toric, multifocal, accommodative, and aspheric, all depend on the accuracy of biometry measurements. For all of these reasons, biometry is an important and currently relevant topic to be discussed.

The fundamental points for accurate biometry include the axial length measurements, corneal power calculation, and IOL position (effective lens position [ELP]), the selection of the most appropriate formula, and its clinical application.

AXIAL EYE LENGTH MEASUREMENTS

Instruments and Methods

The measurement of axial eye length is one of the most important steps for IOL lens power calculation. An error in axial length measurement of 1 mm can cause an error in IOL power of 2.5 D (approximately). The continual refinements of ultrasound use in ophthalmology are important in minimizing this error.

Although laser interferometry (IOL Master, Zeiss, Germany) was developed to increase the accuracy of biometry measurements, in some eyes that are unable to fixate, it is not possible to perform accurate examinations with this method.1 Optical coherence biometry is a noncontact and operator-independent method that emits an infrared beam which is reflected back from the retinal pigment epithelium. The reflected light beam is captured by the instrument, and the axial length is calculated by the interferometer. The patient needs to fixate on the device’s internal light to allow for axiality with the fovea. Patients with dense nuclear or subcapsular cataracts, patients

with retinal detachment, and patients for whom cooperation is poor need to be evaluated by the ultrasonic methods.

A-scan Biometry

An A-scan is currently used for biometric calculations, but in some cases the precision of the measurements can be optimized by a B-scan. Clinical decisions can be made during dynamic examinations. In eyes with staphyloma or nanophthalmos, the A-scan can be guided by the B-scan. A-scan biometry includes two main techniques: the contact method and the immersion technique.

Contact method

In the contact (applanation) method, the ultrasound probe directly touches the cornea. In the echogram for the axial eye length measurement, the first spike represents the probe tip placed on the cornea, followed by the anterior lens capsule, posterior lens capsule, vitreous cavity, retina, sclera, and orbital tissue echoes (Fig. 1). The contact technique is completely examiner dependent because it requires direct contact and anterior compression of the cornea. Previous studies have demonstrated a mean shortening of axial length by 0.1 to 0.33 mm using the contact technique compared with immersion technique.2–5

Immersion technique

Because the immersion method eliminates compression of the globe, this technique has been shown to be more precise than contact biometry (Fig. 2). In the immersion technique, a scleral shell filled with fluid is placed over the cornea while the patient lies supine. The probe is immersed in the fluid overlying the cornea. Clinically, this method is important in eyes with a small axial length (high hyperopia, microphthalmos, nanophthalmos).