Patent Publication Number: US-2005131433-A1

Title: Microkeratome cutting-blade assembly using staking and adhesive

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to cutting-blade assemblies and specifically, cutting-blade assemblies for use in a microkeratome for use in ophthalmic surgery.  
      2. Description of the Related Art  
      Laser-Assisted In-situ Keratomileusis or LASIK surgery has become a widespread and effective eye correction surgical procedure in the last several years. Before a laser ablates a portion of a patient&#39;s corneal tissue to correct that patient&#39;s vision, a flap of the patient&#39;s cornea must be formed.  
      A typical cornea, on average, is about 520 microns thick. A typical flap thickness for the corneal flap, that is formed prior to laser ablation and LASIK surgery, is desired to be on the order of 160 to 200 microns. As is well known, these corneal flaps are made using microkeratomes that travel in a linear, arcuate, or even in a horizontally hinged path. A microkeratome typically cuts the corneal flap using a cutting-blade assembly made with standard razor blade stock available from any of numerous razor blade manufacturers, though other materials such as ceramics or plastics may be used. It is also typical that the cutting-blade is oscillated to aid in the cutting, while the cutting-blade is translated across the cornea to form a corneal flap.  
      A rather accurate measurement of the corneal thickness prior to LASIK surgery is obtainable through any number of known measurement methods, such as the use of an ORBSCAN™ Topography System available from Bausch &amp; Lomb Incorporated. After the corneal thickness measurement has been obtained, depending on the surgeon&#39;s preference and the amount of correction needed, a flap thickness determination is then chosen by the surgeon.  
      Typically, in the prior art, each microkeratome comes with a variety of cutting heads, which are precisely manufactured to obtain different flap thicknesses, such as cuts of 160 microns, 180 microns, and 200 microns. Again, in the prior art, a single cutting-blade assembly has been used with these different precision cutting heads to obtain the different flap thicknesses.  
      One variation to this is from Med-Logics, Inc. Med-Logics currently manufactures LASIK blades, which consist of a piano or nominal length blade and a plus and a minus blade, wherein the blade extensions vary from the piano extension either plus or minus 20 microns. According to Med-Logics, this then allows the doctor to produce a flap of thinner or thicker thickness from the piano blade using a given cutting head.  
      A problem with all prior art microkeratome cutting-blade assemblies has been the consistency of the blade extension of the cutting head of the cutting-blade assembly. The blade extension is defined as the distance from the cutting tip of the blade to the nearest point of the blade holder. A microkeratome cutting head is precisely machined to applanate the cornea a given amount and to hold the blade holder within fairly tight tolerances. However to this point, the blade extension has not been held to a tight enough tolerance to give a consistent flap thickness cut. The criticality of the blade extension consistency has only recently become understood. The importance of blade extension consistency and a method of achieving such consistency are described in detail in co-pending U.S. patent application Ser. No. 10/334,358, filed Dec. 30, 2002, and entitled Microkeratome Cutting Blade Assembly and is hereby incorporated in its entirety by reference. It has always been a goal to provide a consistent and predictable flap thickness with a given cutting-blade in a given microkeratome cutting head.  
      The consistency of the flap thickness cut is crucial for several reasons. The reasons include that the laser ablation algorithm is based on the patient&#39;s need for correction and the amount of stromal bed left to be ablated after the flap has been created. This is critical to achieving an acceptable outcome for the patient. If too much corneal bed is ablated and not enough corneal bed thickness is left, the patient&#39;s intraocular pressure could cause serious change to the cornea. Conversely, if the corneal flap is too thin the flap could easily tear or it could be difficult to adequately correct the patient&#39;s vision without complications such as halos.  
      While it is easy to obtain a corneal thickness measurement before LASIK surgery, it has proven extremely difficult to measure corneal thickness of an eye with a corneal flap laid back over, and it is equally difficult to obtain a reliable corneal flap thickness measurement due to changes in hydration of the corneal flap and the cornea which occur quite rapidly under the surgical lights of an operating room.  
      If the corneal flap is thinner or thicker than desired by the surgeon and a patient&#39;s cornea is on the thin side to begin with, then serious complications could result from a flap that is thicker than desired. Therefore, it is desirable to provide a microkeratome cutting-blade assembly having a tightly controlled blade extension and to provide an easily accomplished method of producing such a tight blade extension.  
      It has been found that attaching a blade holder to a cutting-blade by known methods such as cold staking, heat staking, or adhesive bonding do not provide a robust enough bond to maintain the precise blade extensions desired under certain circumstances. Therefore, it would be desirable to provide an attachment between the blade holder and cutting-blade that is robust but yet economical to manufacture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side view of a prior art cutting-blade assembly;  
       FIG. 2  is a bottom view of  FIG. 1 ;  
       FIG. 3  is a bottom view of a cutting-blade assembly in accordance with the present invention; and  
       FIG. 4  is a perspective view of an alternate embodiment of a cutting-blade assembly in accordance with the present invention; 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a microkeratome cutting-blade assembly  10  in accordance with the present invention. Assembly  10  includes a cutting-blade  12  and a blade holder  14  attached to the cutting-blade  12 . Preferably, blade holder  14  is attached to cutting-blade  12  through an aperture or through-hole in cutting-blade  12  (not shown) via post member  16  through a commonly known procedure such as heat staking. However, other means of attachment, such as cold staking, or other means are also possible. In addition, the aperture does not need to be a through-hole but rather could be mating indentations and raised portions in the blade holder and blade, as is known. Preferably, a blade extension represented by number  18  is controlled to within at least six (6) ten-thousandths of an inch of a target extension length for assisting and providing a consistent, predictable corneal-flap thickness. Blade extension  18  may also be measured from a front surface of holder  14  to a line parallel to the front surface and passing through the cutting tip of blade  12 . Such tight tolerances and blade extensions may be very important as explained in detail in the above cited co-pending patent application. After staking, such as by the preferred heat staking, voids or gaps may form between the blade holder  14  and cutting-blade  12  as shown at  19 . These gaps  19  are shown for illustrative purposes only. The gaps  19  in practice may not be seen from a visual inspection. These gaps reduce the strength of the possible bond between the holder  14  and blade  12 . In fact, under certain conditions the bond may not be strong enough with just heat staking to hold the tight tolerances desired. These gaps are a by-product of achieving the desired tight blade extension tolerances. This is because the post member  16  must be moveable within the through-hole so that a precise blade extension can be achieved; there is simply not enough material in post  116  to fill all the gaps.  
       FIG. 2  is a bottom view of the assembly  10  of  FIG. 1 . The blade  12  is placed over post  16  of holder  14  as shown. The view of  FIG. 2  is after the heat staking, and in this way notches  20  are partially seen. The purpose of notches  20  is to allow the material of post  16  upon heat staking to flow into the notches  20  and ensure attachment of the blade  12  to the blade holder  14  and the blade holder of the present invention. However, it may be preferable not to form notches  20  in blade  12 . Preferably, blade holder  14  is made of Lubiloy™ and is molded or machined. Lubiloy™ is a polycarbonate material, which is preferred for blade holder  14 , though any known suitable material is acceptable for blade holder  14 , such as Delrin™. As previously discussed, cutting-blade  12  is preferably formed from razor blade stock widely available from a number of manufacturers, though a number of other materials are also possible.  
       FIG. 3  shows a cutting-blade assembly  31 ,in accordance with the present invention. A post member  32  of blade holder  33  is preferably heat or cold staked to cutting-blade  34  in a manner described in the above cited co-pending application to form cutting-blade assembly  31 . Preferably, post member  32  is heat staked to cutting-blade  34  at between 350-425° F. at 10 psi and most preferably at 425° F. Adhesive  36  is then applied to blade assembly  31  to fill gaps between the blade holder  33  and cutting-blade  34  for forming a stronger bond than can be achieved with staking alone. Adhesive  36  is applied by any known method from a source  38  and is preferably #4304 available from Loctite but may be other adhesives suitable for surgical applications. Capillary action is believed to draw adhesive under the deformed post  32  and aids in adding lateral and axial strength to the assembly.  
      A gap must exist between the post  32  and through-holes (not shown) in cutting-blade  34  to allow the desired tight tolerance on blade extension to be achieved in a manufacturing environment. During assembly, as described in the cited co-pending application, the holder  33  moves relative to the blade  34  so that the desired blade extension can be achieved be staking the holder  33  to the blade  34 . It has been found that because of the necessary gap between the post  32  and cutting-blade  34  heat staking will not sufficiently fill up the gap to create a strong enough bond. It has been found that the addition of adhesive can create a bond several times stronger than the bond achieved with staking or adhesive alone. In this way the tight blade extension tolerances desired may be maintained throughout operation and use of the cutting-blade assemblies.  
       FIG. 4  shows an alternative embodiment of a microkeratome cutting-blade assembly in accordance with the present invention. A blade  42  is connected to a blade holder  44  via post  46 , preferably by heat staking as described above, in addition to the use of adhesive  49 . Adhesive  49  is preferably the same as adhesive  36 .  FIG. 4  also shows an insertion tool hole  48 , such as known in the prior art and described in U.S. Pat. No. 6,051,009 to Hellenkamp, et al. Blade  42  has a back datum surface  50  and blade  42  is keyed by radius  52  being offset along back surface  50 .