Patent Application
20070129797
The field of the invention relates generally to corneal implants, and more particularly, to intracorneal inlays.
As is well known, abnormalities in the human eye can lead to vision impairment. Some typical abnormalities include variations in the shape of the eye, which can lead to myopia (near-sightedness), hyperopia (far-sightedness) and astigmatism as well as variations in the tissue present throughout the eye, such as a reduction in the elasticity of the lens, which can lead to presbyopia. Corneal implants have been used successfully to treat these and other types of vision impairment.
Corneal implants typically correct vision impairment by altering the shape of the cornea. Corneal implants can be classified as an onlay and an inlay. An onlay is an implant that is placed over the cornea such that the outer layer of the cornea, e.g., the epithelium, can grow over and encompass the implant. An inlay is an implant that is surgically implanted into the cornea beneath a portion of the corneal tissue by, for example, cutting a flap in the cornea and inserting the inlay beneath the flap. The inlay in the cornea alters the shape of the cornea's anterior surface, and therefore the refractive power of the cornea. Since the cornea is the strongest refracting optical element in the human ocular system, altering the cornea's anterior surface is a particularly useful method for correcting vision impairments caused by refractive errors. Inlays are also useful for correcting other visual impairments including presbyopia.
An essential step in using an inlay to correct vision impairment is designing the inlay to produce a desired shape of the anterior corneal surface, which requires accurately modeling the effect of the inlay's geometry on the shape of the anterior corneal surface. One model was proposed in Warsky, M., el al, “Predicting Refractive Alterations with Hydrogel Keratophakia”, Investigative Ophthalmology & Visual Science, Vol 26, No 2, 1985, pp 240-243. In this model (“Warsky model”), the predicted radius of curvature of the anterior corneal surface is equal to the sum of the radius of curvature of the inlay and the thickness of the overlaying flap. However, in clinical tests, the Warsky model did not accurately predict the actual post-operative refractive outcome of inlay implantation in the correction of hyperopia. This suggests that the Warsky model does not provide a good basis for the design of intracorneal inlays.
Accordingly, there is a need for improved designs of intracorneal inlays that more accurately produce desired shapes of the anterior corneal surface for correcting vision impairments.
The present invention provides improved designs of intracorneal inlays to produce desired shapes of the anterior corneal surface for correcting vision impairments.
In an exemplarily embodiment, an inlay is designed for a patient experiencing vision impairment. First, a desired post-operative anterior corneal surface that corrects the patient's vision impairment is determined. A thickness profile is then defined by a difference between the desired post-operative anterior corneal surface and the patient's pre-operative anterior corneal surface. The inlay is then dimensioned to substantially have the same thickness profile. When implanted in the patient's cornea, the thickness profile of the inlay is substantially transferred to the anterior corneal surface through the intervening corneal tissue, thereby producing the desired post-operative anterior corneal surface.
The inlay designs according to the invention are useful for correcting a wide range of vision impairments by varying the shape of the desired anterior corneal surface used to define the thickness profile of the inlay. For example, the desired anterior corneal surface can be spherical to correct refractive errors due to, for example, hyperopia or myopia. For another example, the desired anterior corneal surface can be aspheric to correct refractive errors and higher-order aberrations of the patient's ocular system. For yet another example, the desired corneal surface can form a two-zone, bifocal simultaneous vision, surface to correct both prebyopes and hyperopia. The desired anterior corneal surface can be of any other shape for the correction of other vision impairments.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. It is also intended that the invention not be limited to the details of the example embodiments.
a is a cross-sectional view of a cornea with a flap cut into the cornea using an intralase keratome.
b is a cross-sectional view of a cornea with a flap cut into the cornea using a mechanical keratome.
a is a cross-sectional view of the cornea of
b is a cross-sectional view of the cornea of
Described herein are designs of intracorneal inlays to produce desired shapes of the anterior corneal surface for correcting vision impairments.
A first step in the design of an inlay in accordance with the invention is determining a profile thickness that the inlay must induce on the anterior corneal surface to produce a desired anterior corneal surface. Although not so limited, the thickness profile may be illustrated by way of an example. In this example, a thickness profile is specified for the correction of hyperopia (far-sightedness). Hyperopia occurs when light entering the eye focuses behind the retina. Hyperopia can be corrected by increasing the curvature of the anterior corneal surface and thereby the refractive power of the cornea, which causes light to bend more in the cornea and focus at the retina. This type of correction is known as hyperopic correction.
The objective in this example is to increase the curvature of the cornea's anterior surface to achieve the desired hyperoptic correction.
A first step is to determine an anterior radius of curvature, r′a, that provides the desired refractive change, ΔRx=Rxpre−Target, where Rexpre is a pre-operative spherical equivalent of the cornea's refractive power and Target is a targeted post-operative equivalent of the cornea's refractive power. The equivalent change in the cornea's refractive power, ΔKequiv, at the anterior surface is given by:
where V is a spectacle vertex distance, e.g., 0.012 meters, from a spectacle to the cornea's anterior surface. The spectacle vertex distance, V, takes into account that measurements of the cornea's refractive power are typically taken with a spectacle located a distance from the cornea's anterior surface, and translates these power measurements to the equivalent power at the cornea's anterior surface.
The pre-operative refractive power at the anterior corneal surface may be approximated by Kavg−Kpost, where Kavg is the average corneal refractive power and Kpost is a posterior corneal refractive power. The desired radius of curvature, r′a, of the anterior surface may be given by:
For purposes of design and analysis, Kpost may be approximated as −6 diopters. The pre-operative radius of curvature, rpreop, may be approximated by:
rpreop=(1.376−1)/(Kavg−Kpost) Equation 3
In
L(r)=Lc+Zpreop(r)−Zanew(r) and Lc=Zanew(dz/2)−Zpreop(dz/2) Equation 4
where Lc is the center thickness of the thickness profile 18, Zpreop(r) is the pre-operative anterior corneal surface 15 as a function of r, Zanew (r) is the desired anterior corneal surface 17 as a function of r, and dz is the diameter of the optical zone. The optical zone diameter dz is the diameter of the post-operative surface 17 which produces acceptable image quality, e.g., 5.0 mm. The optical zone for intracorneal inlays correcting hyperopia may range from about 4 mm to 6 mm in diameter. For the case of hyperoptic correction, Zanew has a radius of curvature equal to r′a, and Zpreop has a radius of curvature equal to rpreop.
In the example above, the anterior surfaces Zanew and Zpreop were assumed to be spherical. This need not be the case. The anterior surfaces may also be aspheric. More generally, the desired anterior surface Zanew may be a function of hyperoptic correction and also more complex design parameters, e.g., an aspheric surface for higher-order aberration correction, or a more complex zonal surface for presbyoptic correction. Also, the pre-operative anterior surface Zpreop is generally aspheric. For designs requiring aspheric surfaces, the surface function Z(r) may be given by the general aspheric form:
where: rc is the radius of curvature
The above expressions for the thickness profile are intended to be exemplary only. Other mathematical expressions or parameters may be used to describe similar or other thickness profiles. Therefore, the invention is not limited to particular mathematical expressions or parameters for describing the thickness profile. In addition, the thickness profile may be specified by surfaces of any other shape for the correction of other vision impairments. For example, the thickness profile may be specified by multi-focal surfaces, simultaneous vision surfaces, multi-zonal surfaces, and the like.
For example,
For another example,
After the required thickness profile is determined, the inlay is dimensioned to have substantially the same thickness profile, e.g., to within about a micron. The inlay may have a diameter of, e.g., 5 mm, and a center thickness, e.g., in the range of 5 μm to 100 μm. When implanted in the cornea, the thickness profile of the inlay is substantially transferred to the anterior corneal surface through the intervening corneal tissue, thereby producing the desired post-operative anterior corneal surface. The transfer of the inlay's thickness profile to the anterior corneal surface may be illustrated by way of examples.
a and 5b show cross-sectional views of lamellar flaps 526a and 526b cut into the cornea. The flaps 526a and 526b are used to implant the inlay in the interior, e.g., stroma, of the cornea and may have a depth of, e.g., 100 μm. The flaps typically remain attached to the cornea by a hinge (not shown).
a and 6b show inlays 531a and 531b according to the invention implanted in the cornea of
The transfer of the inlay's thickness profile to the anterior corneal surface through the flap material may be based on the following assumptions:
a. The inlay thickness profile remains approximately constant as the inlay conforms to the bed shape of the flap when inserted below the flap. This appears reasonable to first order because the inlay is typically more than 70% water (e.g., incompressible) and the forces exerted on the inlay are not great enough to change the inlay hydration. Note that the flap is first order a “free” piece of tissue. The lamella cut during the flap creation do not re-attach in a way to recover the tension along the sclera-to-sclera arc.
b. The inlay conforms to the bed geometry of the flap.
c. The deformation of the anterior corneal surface by the inlay and flap geometries is approximately along the optical axis instead of radially. To radially push the overlying flap volume, the geometry must allow the periphery of the flap to expand radially. The uncut cornea, peripheral to the keratome cut forming the flap, presents a barrier to that increased diameter. Additionally, the corneal stroma (flap material) is composed of layers of lamellar fibers, roughly perpendicular to the optical axis. As such, elastic deformation along the lamellae (e.g., radial expansion) is more difficult than deformation perpendicular to the fibers, suggesting a preferential tendency for axial displacement due to the inlay.
d. The bed shape is proportional to the anterior corneal curvature. Lower power corneas have flatter bed shapes and stronger power corneas have steeper bed shapes, regardless of the flap cut geometry.
The inlay designs of the invention differ from the Warsky model in several respects. First, the inlay designs of the invention assumes that the inlay substantially displaces the overlying flap axially, whereas, the Warksy model implies that the inlay displaces the overlying flap radially. Consequently, the inlay designs of the invention predict that the thickness profile of the inlay is substantially transferred to the anterior cornea surface. In contrast, the Warksy model predicts that the thickness profile induced at the anterior cornea surface expands radially from the thickness profile of the inlay. Also, in the Warsky model, the radius of curvature of the anterior corneal surface due to an inlay depends on flap thickness. The inlay designs of the invention are approximately independent of flap thickness.
In a clinical test, an exemplary inlay design of the invention was compared to an inlay based on the Warsky model. In the test, the inlay design and the Warksy model were used to predict the expected post-operative refractive outcome (Rxpost) for subjects with implants of known geometry and known flap geometry. The standard deviation of the difference between the actual post-operative refractive outcome (Rxactual) and Rxpost was used as a measure of the accuracy.
Table 1 below summarizes the standard deviations of the actual post-operative refractive outcome Rxactual, and the predicted post-operative refractive outcome Rxpost of the inlay design of the invention and the Warksy model. The data is given for mechanical and Intralase keratomes separately, since each creates a different flap geometry. The data below includes optimization of two general constants, attempting to make the Warsky model as accurate as possible. The constants include a coefficient to the inlay's designed anterior radius of curvature to account for potential changes in the inlay's anterior radius of curvature when the inlay conforms to the bed geometry. Another additive constant was added to the clinically measured flap thickness to account for methodological errors in the measurement or consistent offsets to the flap thickness due to biomechanical properties.
Clearly, the inlay design of the invention, based on axial displacement of flap material, achieves the same predictability as the actual results. In contrast, the Warsky model has significantly higher standard deviations, suggesting that the model is less accurate. Consistent with these results, the same standard deviation of clinical data provided in the Warsky paper demonstrate a value of 1.52 diopters, similar the results in Table 1.
In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, the invention does not require that the inlay's thickness profile be exactly the same as the desired thickness profile. The thickness profiles may be substantially the same while differing slightly depending on the precision with which the inlays can be fabricated. In addition, minor adjustments may be made to the inlay's thickness profile to compensate for, e.g., edge thickness of the inlay, without departing from the scope of the invention. As another example, each feature of one embodiment can be mixed and matched with other features shown in other embodiments. As yet another example, the order of steps of method embodiments may be changed. Features and processes known to those of ordinary skill may similarly be incorporated as desired. Additionally and obviously, features may be added or subtracted as desired. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.
