Fig. 1.1
A diagram of a radial keratotomy with four incisions and the effect of the intraocular pressure. The cornea bends in the incised area and flattens in the not-incised center
The history of the RK is outlined as follows:
In the late nineteenth century, Snellen, Bates, and Lans made the first observations on the effect of RK.
In 1930, Sato experimented in the laboratory and clinically with up to 40 incisions.
In 1970, Fyodorov and Durnev designed a multifactorial formula with an optical area greater than 3 mm in diameter.
In 1978, Bores, Myers, and Cowden introduced the surgical technique in the USA.
In 1980, the NEI (National Eye Institute) of the USA conducted a prospective study in 9 clinical centers, yielding results in 1 and 3 years.
Since then several studies have been published based on different clinical nomograms (or diagrams of cuts), performed by several surgeons. Among them stand out those from Ruiz et al. [1]. The aim, later, was to use a few and shorter incisions, attempting to get minimally invasive procedures [2].
The calculation of the cuts in the RK and AK is mainly done by tables compiled by surgeons depending on cases [3, 4]. Some techniques incorporate closed mathematical formulas based on simple models [3]. Both of them exhibit a lack of prediction [4–6] because they do not consider the intrinsic behavior of the cornea [7] or the biomechanics of the cornea. Table 1.1 shows the results of three studies of RK, showing the percentage of eyes within the error of refraction [5].
Table 1.1
RK clinical studies
Estudio PERK (%) | Arrowsmith and Marks | Deitz and Sanders | |
|---|---|---|---|
±1.00 D | 69 | 48 % | 84 % |
±1.50 D | 86 | – | 94 % |
±2.00 D | 95 | 78 % | – |
The PERK study (Prospective Evaluation of Radial Keratotomy) [6] found that a surgeon could only be secured with a 90 % certainty that the refraction of a patient 1 year after having surgery is within 1.75 D (diopters, refractive power) of the predicted value. With regard to stability it was found that after a follow-up of 6 months to 4 years, the percentage of eyes with refractive change greater than one diopter was 28 %. The study enhanced the importance of preoperative variables on the outcome of surgery. It found that the main factors are the diameter of central clear area, age of the patient, and the depth of the incision. According to this statistical study, patient gender, initial keratometric power, corneal thickness, corneal diameter, intraocular pressure, and ocular rigidity did not have great influence.
Binder et al. [8] demonstrated, through a study with electron microscopy and histochemistry, that the incision was healed completely only after 5 years of surgery. The excess time of scarring caused by steroids was attributed for the slow healing. The study described the changes in the scar in the incision, possible causes of fluctuating vision and progressive hyperopia.
These techniques can leave the patient with significant overcorrections or undercorrections. This lack of accuracy is due to calculation methods that do not include the complexity of the cornea. With the incorporation of digital videokeratoscopy [9], the corneal topography has been adequately represented, especially after a surgical procedure. Furthermore, other studies have been performed about corneal anisotropy [10] and viscoelasticity [11]. Other studies need to confirm the influence on the outcome of the RS because of the sclera, ocular muscles, optic nerve, and eyelid.
2 Biomedical Engineering
Biomedical Engineering is an interdisciplinary science between medicine and engineering. It was originally conceived for the development of medical equipment. However, in these days, it covers a wide range of activities, from the theoretical analysis of physiological processes to the practical design of unconventional therapies. In this line, computer models have been useful in understanding the mechanics of physiological processes. These models are used to simulate changes in these processes, for example, when they are affected by medical therapies.
Biomechanics is the study of the mechanics of biological systems [12]. In the human body, it helps to understand their normal function, to predict changes due to alteration, and to propose methods of artificial intervention. Its history is ancient. Aristotle, Leonardo Da Vinci, Newton, Descartes, and Helmholtz have made contributions to it. Problems such as the analysis of human movement for rehabilitation, sports, and surgery are of great interest in this area.
Computational Mechanics formulates methods and algorithms to investigate complex mechanical processes. They can undertake various analyses (structural, fluid dynamics, thermal, electromagnetic) based on the discretization of the continuum and can be applied different computational approaches such as finite element, finite volume, and boundary element methods, among others.
In last decades, medical researchers and engineers have begun to introduce predictive computational tools in medical practice. The aim is to use advanced simulation techniques, together with the powerful computational resources currently investigating the biomechanical behavior and ultimately improving and anticipating the results of medical procedures. Computational Biomechanics, branch of Biomechanics and Computational Mechanics, has become important in medical specialties such as orthopedics and cardiology. In orthopedics, it has improved hip [13], knee, and foot prostheses through the simulation. In cardiology there has been performed simulations of blood flow through valves [14], stenosis, arterial bifurcations [15], bypasses, etc. In ophthalmology, there have been some computational studies for research, especially in the RS, for the estimation of elastic parameters (see Chap. 2) and simulation of surgical procedures on the cornea (see Chaps. 3–5).
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