Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the orthopaedic surgery art and more specifically to bone depth gauges. 
         [0003]    2. Description of the Related Art 
         [0004]    Installation of orthopaedic trauma reconstruction plates frequently require that holes be drilled through bones to allow the installation of screws that are used to restore the patient&#39;s natural anatomy. The depth of each hole must be measured so that the correct screw may be chosen to attach a support plate to the bone. Existing depth gauges used in re-sterilizable trays used for the operating room are usually far too expensive to discard after each use and are usually consigned to a hospital or surgical center. They are expected to be reused indefinitely. 
         [0005]    Devices of this type, because of the long expected use, are typically machined, stainless steel or anodized aluminum with a stainless steel tip hook. This style of depth gauge has a hook that extends from the distal end of a tip and which is intended to hook on the opposite edge of a hole drilled through a bone to determine the bone depth. The tip is typically manufactured from a rod of steel and adds significantly to the cost of the assembly. The tip has a substantially round cross-section except for the distal end where there is a hook feature. The maximum width of the tip must be less than the diameter of the drilled hole so that it may be introduced into the drilled hole and hook the far edge of the hole. An outer housing telescoped over the tip is pushed forward and a mark on the housing indicates, on a graduated scale, the depth of the hole. Frequently, the depth gauge is removed from the bone and then read which adds the necessity for some kind of friction to hold the sliding members in position as it is withdrawn. 
         [0006]    What is needed in the art, therefore, is an orthopaedic depth gauge in which the manufacturing costs of at least some of the components are reduced to the point that some of these may be made available as a single use item. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a cost effective bone depth gauge that may be available in a single use form. 
         [0008]    In one form, the invention is an orthopaedic bone depth gauge with a proximal handle and an elongated metal tip releasably connected to the handle with the tip having a hook on its distal end for engaging the opposite face of a bone through which a hole has been formed. The tip is formed by photo chemical machining A tubular element has a distal end telescoped over the tip and a proximal end telescoped over the handle with the tubular element being releasably retained by the tip. The tubular element and handle cooperate to provide indicia for measuring the bone depth when the hook of the tip engages the opposite face of the bone and the distal end of the tubular element engages the near face of the bone through which the hole has been formed. 
         [0009]    In another form, the invention is a tip for use with a bone depth gauge having a proximal handle. The tip includes a thin metal element having an elongated shaft and an integral hook at the distal end of the shaft, the element having a configuration integrally connected to the shaft at the proximal end thereof for releasable connection to the handle with the thin metal element being formed by photo chemical machining. 
         [0010]    In still another form, the invention is a method of manufacturing a tip for use with an orthpaedic depth gauge having a proximal handle and a tubular element telescoped over the handle and tip to measure bone depth. The method includes applying a photo resistant layer having a negative pattern of the tip configuration onto a sheet-like element and exposing the photo resist layer to light to harden the negative patterns. The non-hardened photo resistant portions are removed chemically and then the sheet-like element is chemically etched to remove all but the portion covered by the negative. Subsequently the photo resist material is chemically stripped from the sheet-like element to produce a final shape. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a longitudinal side view of an orthopaedic depth gauge embodying the present invention; 
           [0013]      FIG. 2  is a longitudinal section view taken on lines  2 - 2  of  FIG. 1 ; 
           [0014]      FIG. 3  is a side view of a tip used in the depth gauge of  FIGS. 1 and 2 ; 
           [0015]      FIG. 4  is a perspective view of the depth gauge tip and handle with a tubular element removed; and, 
           [0016]      FIGS. 5 and 6  show cross sections of the depth gauge tip of  FIG. 3 , taken on lines  5 - 5  and  6 - 6  of  FIG. 3 . 
       
    
    
       [0017]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one embodiment of the invention and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1  shows an orthopaedic bone depth gauge  10  which includes a proximal handle  12  providing a mounting for a tip  14  on the distal end  16  of handle  12 . A tubular element  18  is telescoped over the assembly and has a proximal end  20  over handle  12  and a distal end  22  over tip  14 . 
         [0019]    Referring now to  FIG. 3 , tip  14  is made of thin metal sheet up to around 1 mm in thickness, preferably up to 1.07 mm thickness). The configuration illustrated in  FIG. 3  is a final configuration before assembly with the handle  12  and tubular element  18 . Tip  14  has an elongated shaft  24  terminating in a hook  26  at its distal end. At the proximal end  28  there is a window  30  sized and configured to be received over a post  32  integral with handle  12 . A tab  34  is coplanar with the tip  14  and has a flexible section configured to be received in a notch  36  on handle  12 . A tab  38 , integral with tab  34 , permits an operator to depress tab  34  sufficiently to clear notch  36  and remove tip  14  laterally from post  32 . A friction arm  40  is also integral with the proximal end  28  and extends longitudinally in a slightly radially outward direction and has a tip  42  for engaging the inner surface of tubular element  18  thus providing friction against movement and in addition to provide a limit to the displacement of tubular element  18  away from the proximal end of handle  12  by abutting a window feature  44  on the proximal end of tubular element  18  that tip  42  flexes into under spring force. Tip  42  has a ramp feature  46  enabling the tubular element  18  to be slipped over arm  40  during assembly. A tab  48  extends sufficiently beyond the distal end of tubular element  18  when window  44  abuts tip  42  to enable the arm  40  to be displaced radially inward and permit disassembly of the unit. 
         [0020]      FIG. 4  shows the interconnection between the tip  14  and handle  12  and illustrates the post  32  over which the window  30  is moved and the notch in the handle  12  for receiving tab  34 . 
         [0021]    Referring now to  FIG. 2  the shaft  24  of tip  14  is introduced into a hole  50  that has been drilled through a bone  52 , both shown by dashed lines. The hook  26  is engaged with the opposite face of the bone  52  and the tubular element  18  is displaced to the left until its distal end  22  abuts the near face of the bone  52 . A graduated scale  54  on handle  12  lines up with the proximal end  20  of tubular element  18  to indicate the depth of the hole  50 , typically in millimeters. Tabs  56  extending from tubular element  18  may have marks on them for easy determination of the bone size. This enables the proper screw selection, taking into account the thickness of any support device for the particular fixture. Tubular element  18  and handle  54  may be formed from widely available surgical plastic material for example (Radel) plastic. This plastic is re-sterilizable and reusable. 
         [0022]    In accordance with another aspect of the present invention, the tip  14  is formed by photo chemical machining This process enables significant reductions in manufacturing costs and reduces the unit cost of tip  14  to the point where it is economically feasible to provide the tip  14  in a one use package. The process begins with providing a sheet of metallic material, typically stainless steel used in surgery. A photo sensitive sheet with an adhesive back is rolled preferably on both sides of the sheet and then a machine loads a photo negative of the material of the ultimate configuration in a glass press and this is done on both sides. Subsequently, both sides are exposed to a light source such as ultraviolet to harden the exposed portion of the tip configuration, namely what is contained outside of the negative. The material that has not been hardened is then stripped by a bath which immerses the sides in a chemical that strips the exposed areas away, leaving the configuration of the part as shown in  FIG. 3 . The sheet material is then etched in a system which uses high pressure acid traversing the sheet to eat away at the portions of the material not covered by the photo resist. Typically, on each side, the etching extends to about 60% of the thickness of the material. When it is done to opposite sides, it cuts through the material. One of the features of the photo chemical process is that a portion of the chemical etching slips underneath the photo resist so that the end configuration is a semi-hexagonal shape as shown in  FIGS. 5 and 6 . This is particularly advantageous when the tip is used for entry through the hole  50  since it more closely approximates a round configuration. Subsequent to the photo etching process, the photo resist is stripped from the sheet and then the entire finished part is subjected to a brief acid etching, referred to as a post etch, to remove sharp edges that may be a cutting hazard. 
         [0023]    The net result of the process used in the formation of tip  14  enables significant reductions in the cost of manufacturing the tip  14 . In fact, the manufacturing cost is reduced sufficiently so that the tip  14  may be provided in a one use system in which the tip is prepackaged and then subjected to gamma rays for complete sterilization. Thus, the user has the option of either re-sterilizing the tip or using the tip  14  in a one use system. In either application, the tip  14  has significant benefits in terms of ease of disassembly and a simplified structure that provides retention and friction characteristics for the device. 
         [0024]    While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

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