<ul>
 <li>To predict postoperative refraction for an IOL implant</li>
</ul>
<Note>
 The formulae only predict the post-operative spherical equivalent
</Note>
<ul>
 <li>Partial coherence laser interferometry is superseding ultrasound biometry</li>
 <ul>
 <li>Greater accuracy</li>
 <li>Greater reproducibility</li>
 <li>Measures true foveal axial length</li>
 </ul>
 <li>Principles of Zeiss IOLMaster (partial coherence, non-contact biometry)</li>
 <ul>
 <li>Two coaxial laser beams that are partially coherent</li>
 <li>An interference pattern is produced</li>
 <li>This pattern is used to interpret measurements</li>
 </ul>
</ul>

## Parameters in biometry
<ul>
 <li>Axial length</li>
 <ul>
 <li>
 Significant disparity between the two eyes should be noted (ie. greater than 0.2mm difference)
 </li>
 <li>Aphakia, pseudophakia and silicone oil can all affect the readings generated</li>
 <li>Hoffer Q (or Haigis) tends to be used for shorter eyes</li>
 </ul>
 <li>Average keratometry: the mean of the powers of the highest and lowest curvatures</li>
 <ul>
 <li>If astigmatism is highly irregular, corneal topography must be studied preoperatively</li>
 <li>
 Patients with previous keratorefractive surgery pose a challenge and biometry is much less
 accurate. Haigis-L formula is used for such patients
 </li>
 </ul>
 <li>A constant</li>
 <ul>
 <li>Correction factor applied to each lens type to achieve accuracy</li>
 <li>
 Reflects position within the eye ie. AC lenses have a different (lower) A constant compared to
 PC lenses
 </li>
 </ul>
</ul>
<HotTopic>
 
 The more posterior a lens is placed, the stronger it needs to be. Hence if after posterior
 capsule rupture an AC lens is to be placed, it needs to be weaker than the planned intracapsular
 lens. A planned intracapsular lens in the sulus can produce myopic shift since it will be
 slightly too strong for this more anterior position
 
</HotTopic>
<ul>
 <li>
 For the original SRK and the newer SRK-T formulae there is a 1:1 relationship between the
 A-constant and the IOL power. Therefore if two lenses have the same predicted outcome, but
 different IOL powers, this difference will be the same between the A-constants.
 </li>
</ul>
<ul>
 <li>
 SRK-I formula: IOL power = A constant - 2.5 (axial length in mm) - 0.9 (keratometry reading)
 </li>
 <li>
 The SRK-II formula introduced a modification scale for the A constant depending on the axial
 length.
 </li>
</ul>
<HotTopic>
 When to repeat biometry (RCOphth guidelines)
 <ul>
 <li>AL &lt;21.2 or &gt; 26.6</li>
 <li>Mean K of &lt;41 or &gt;47</li>
 <li>Difference in inter-ocular mean K of &gt;0.9</li>
 <li>Delta K or &gt;2.5 (astigmatism)</li>
 <li>Difference in AL of &gt;0.7</li>
 </ul>
</HotTopic>

## Inaccurate AL
<ul>
 <li>Too short</li>
 <ul>
 <li>Corneal compression in contact scans (eg. A scan)</li>
 <li>Beam misalignment</li>
 <li>Asteroid hyalosis</li>
 <li>Phakic settings in a pseudophakic eye</li>
 </ul>
 <li>Too long</li>
 <ul>
 <li>Silicone oil filled eye</li>
 </ul>
</ul>

Clinical Techniques

Biometry

Intro

FRCOphth Part 1 Study Notes

Orbit and ocular adnexae

Orbit

Extraocular Muscles

Orbital Blood Vessels

Paranasal Sinuses

Lacrimal Glands

Tear Film

Eyelids

Blinking

Anterior segment - ocular surface

Conjunctiva

Conjunctivitis

Sclera

Cornea

Corneal Structure And Development

Corneal Physiology

AC to Lens

Angle

Ciliary Body

Aqueous Humour Dynamics

Iris

Pupil

Lens

Posterior segment

Choroid

Vitreous

Retinal Macrostructure

Neurosensory Retina

Retinal Pigment Epithelium

Neuroanatomy

Venous Sinuses

Cerebral Ventricles

Cerebral Blood Supply

Supranuclear Gaze Pathways

Head And Neck Autonomic Nervous Supply

Head And Neck Glands

Osteology

Nerves

Muscles And Pharyngeal Arches

Extraocular Nerves

Sensory Nerves

Physiology

Physics Of Blood Flow

Neuronal Control Of Blood Vessels

Oxygen-Haemoglobin Dissociation Curve

Cytochrome p450

Pituitary Gland

Thyroid Hormone Production

Insulin

Glucagon

Parathyroid Hormone

Corticosteroids

Catecholamines

Cardiac Cycle

Renal Physiology

Fluid Balance

Acid-Base Balance

Muscle Physiology

Sensory Fibres

Cutaneous Receptors

Endocytosis

Glycosaminoglycans

Condensed Ocular Biochemistry

Eye Movements

Accomodation

Vision

Visual Cycle And Phototransuction

Visual Pathway

Visual Acuity

Colour Vision

Binocular Vision

Pathology

Collagen

Tissue Damage

Inflammation

Disease Entities

Wound Healing

Thrombosis / Atherosclerosis

Cell Growth

Neoplasia

Histopathology

Nystagmus

Free Radicals And Antioxidants

Immunology

Ocular Immunology

Microbiology

Viral Infections

Bacterial Infections

Fungal Infections

Protozoa

Nematodes

Antimicrobials

Genetics

Chromosomes

Mitosis

Meiosis

Genes

Polypeptide Synthesis

Histones

MicroRNA

Hardy-Weinberg Equilibrium

Mutations

Chromosome Defects

Autosomal Inheritance

X-Linked Inheritance

Mitochondrial Inheritance

Multifactorial Inheritance