Abstract:
An arthroscopic shaver with an inner cutting window having a plurality of teeth positioned along the lateral cutting edges, the teeth being configured for easy penetration into tissue to prevent ejection of tissue from the cutting window during closure. The inner cutting edges are formed in a milling operation using a milling cutter having an end radius equal to that of the surfaces forming the inner surfaces of the cutting edges. The teeth may be symmetrically or asymmetrically placed about the tube axis when viewed in a plan view.

Description:
[0001]     The present application claims the benefit of U.S. Provisional Application Ser. No. 60/651,646, filed on Feb. 11, 2005, the disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to arthroscopic surgery and, more particularly, to a shaver blade for arthroscopic surgery.  
       BACKGROUND OF THE INVENTION  
       [0003]     Resection of tissue by an arthroscopic shaver blade is accomplished by cooperative interaction between the edges of the inner and outer cutting windows. As the inner and outer windows come into alignment, tissue is drawn into the opening formed by these windows by a vacuum applied to the shaver inner lumen by an external vacuum source. Continued rotation of the inner member causes the inner cutting edges to approach the outer cutting edges. Tissue in the cutting window between the inner and outer edges is either trapped between the edges or ejected from the window. Tissue trapped between the edges is either cut by the edges as they approach each other, or torn by the cutting edges as they pass and rotate away from each other. The resected tissue is aspirated from the site through the inner lumen of the inner tube.  
         [0004]     Resection efficiency is improved by decreasing the relative portion of the material that is ejected from the window, and increasing the portion that is trapped between the edges and resected. Decreasing the relative portion ejected from the window is accomplished by increasing the sharpness of the cutting edges and adding teeth to either the inner cutting edges or outer cutting edges or both. Increasing the sharpness is accomplished by decreasing the included angle of a cutting edge by decreasing the edge radius, and/or by decreasing the roughness of the surfaces over which tissue must slide during resection. Shavers having inner cutting edges with teeth are well known in the art. U.S. Pat. No. 5,217,479 to Shuler and U.S. Pat. No. 5,269,798 to Winkler describe shavers having inner cutting edges with teeth which are formed by a “through-cutting” process such as wire Electrical Discharge Machining (wire EDM) or by grinding. The so-formed teeth are efficient at retaining tissue within the window so that it can be cut by the low included angle outer cutting edges as the inner and outer edges converge. The inner cutting edges do little cutting since the teeth form a very large included angle cutting edge. The Cuda™ by Linvatec Corporation (Largo, Fla.) and the Tomcat™ by Stryker Incorporated (Kalamazoo, Mich.) have teeth on both the inner and outer cutting edges, the edges being formed by a two-dimensional, through-cutting process such as grinding or wire EDM. The edges formed have large included angles, geometry inefficient for cutting tissue. Shavers having these two-dimensionally shaped teeth on the inner and outer cutting edges separate tissue principally by tearing, as the edges pass each other during closure of the cutting window. Such tearing is undesirable since the torn tissue may frequently become wrapped into the gap between the inner and outer tubes so as to prevent the tissue from aspirating from the site thereby clogging the instrument.  
         [0005]     Van Wyk et al., in U.S. Pat. No. 6,053,928, describe a shaver having a plurality of teeth on the laterally opposed cutting edges of an outer window, the cutting edges being symmetrical when viewed in a plane perpendicular to the axis of the tube. The cutting edges are formed so that, when viewed in any such plane, the edges have low included angles in the valleys between the teeth as well as the teeth. The Great White™ shaver by Linvatec, constructed in accordance with the principles of the Van Wyk patent, is very efficient at removing tissue and experiences reduced clogging due to the sharpness of the outer cutting edges which prevents tissue wrapping in the gap between the tubes.  
         [0006]     Improvements in the tissue removing efficiency of shaver blades have been accomplished primarily through improvements in the design and manufacturing of the outer cutting edges. It is much easier to produce advanced geometries on an outer cutting window than on an inner cutting window because edges on the outer window can be produced by grinding. Multi-axis Computer Numerically Controlled (CNC) grinding machines and grinding wheels with shaped peripheries are able to make a myriad of edge configurations. Conversely, it is only possible to grind inner cutting edges with large included-angles. These ground edges may have teeth of various sizes and shapes, but little other geometries are possible. And such teeth are quite efficient for preventing tissue from being ejected as the window closes.  
         [0007]     Advanced geometry outer cutting edges have little effect on the efficiency of a shaver when cutting bone. Bone is resected not by cooperative interaction of the inner and outer cutting edges, but by the inner cutting edges only. Accordingly, the geometry of the inner cutting edges has a much greater effect on the shaver performance when cutting bone.  
         [0008]     The process of cutting of bone by a cutting edge has two phases: initiation and propagation. Initiation of the cut refers to the penetration of the bone by the cutting edge. For resection of bone to occur, the cutting edge must penetrate the bone rather than bounce off. For this to occur, the compressive stress (force divided by area) generated at the cutting edge/bone interface must exceed the compressive strength of the bone. The force applied by the cutting edge is determined by the handpiece and operator. The applied stress at the cutting edge can be increased to the required level by decreasing the area over which the force is applied, that is, by “sharpening” the cutting edge. An axe head has two sides: a wedge-shaped side for cutting, and a blunt side for pounding. Both sides apply the same force. The compressive stress applied to a log is much higher on the wedge-shaped side since the area of the edge is much less. This higher pressure causes localized “failure” in the log thereby allowing the cutting edge to penetrate the log. In the same manner, it is necessary to minimize the area of the cutting edge on a shaver inner blade in order to initiate a cut in bone. This can be accomplished by making knife-like edges, two-dimensional teeth, or pyramid shaped teeth. All will cause localized failure in the bone which they encounter and will penetrate the bone.  
         [0009]     The second phase, propagation, is accomplished after initial penetration occurs. During propagation, the cutting edge advances further into the bone with spreading of the bone by the cutting edges causing a tensile failure in the material ahead of the edge. A “crack” is propagated ahead of the cutting edge. The direction of propagation is generally at a shallow angle with the surface of the bone. Accordingly, a “chip” of removed bone forms and slides along the surface of the cutting edge away from the edge. The propagation continues until the crack intersects a free surface so that a piece of material is removed, or localized fracturing of the chip occurs.  
         [0010]     While a variety of cutting edge configurations can cause initiation, propagation requires a wedge-shaped edge decreasing in width in the rotation direction and with an apex lying in a plane approximately parallel to the free surface of the bone. Decreasing the included angle of the edge increases both the ease or initiation and the ease or propagation.  
         [0011]     Several currently available shavers have wedge-shaped inner cutting edges. Among these are the Full Radius Resector by Linvatec Corporation (Largo, Fla.), the Resector Full Radius by Stryker Corporation (Kalamazoo, Mich.), and the Full Radius Blade by Smith and Nephew (Andover, Mass.). These prior art shavers have low included-angle inner cutting edges formed by an axial channel cut into the distal end of the shaver, the channel width being approximately 70% of the inner tube diameter. The channel intersects the tube outer surface so as to form more or less wedge-shaped (knife-like) cutting edges from the proximal end of the window to the tangency of the distal radius. The edges formed are quite efficient for bone cutting. In the distal radius, however, the geometry is not well suited to bone cutting. The intersection of the channel with the spherical outer surface creates a transition from low included-angle cutting edges to high-included angle edges. This limits the relative angles between the surface of the bone to be resected and the axis of the shaver at which the shaver will cut effectively. When the angle between the axis of the shaver and the bone surface is low and cutting is done primarily with the more proximal knife-like portion of the edges, the shavers cut efficiently. However, when the angle between the axis and the bone surface is high, so that the distal radius of the shaver is brought into contact with bone, the high included angle portion of the edge does not penetrate the bone and causes the shaver to bounce away from the bone.  
         [0012]     Heisler et al., in U.S. Pat. No. 6,001,116, teaches a shaver inner cutting member with low included angle cutting edges which extend through the distal radius. The inner member, produced by wire EDM, has a “through cut” cutting window formed by two laterally opposed fingers which extend distally from the distal end of the inner tube to form two cutting windows. The resilient cutting edges so formed are able to deflect inward when subjected to high cutting forces, so as to increase the clearance between the cutting edges when cutting bone, for example. The geometry of the cutting edges, however, requires that they be produced by wire EDM. The edges produced are irregular and have rough surface finishes on the surfaces over which tissue must slide during the cutting process and this limits the efficiency of the shavers. The edge geometry is well suited for effective cutting to occur in the distal radius thereby making the shaver efficient at relatively large angles to the bone surface and an effective end cutter.  
         [0013]     Decreasing the included angle of a cutting edge to improve cut initiation in bone requires that the material of which the edge is made have a suitably high yield strength. The cutting edge is subjected to a compressive stress equal to that applied to the bone undergoing resection. Insufficient yield strength of the cutting edge material results in plastic deformation of the cutting edges. This “mushrooming,” in turn, results in dulling of the cutting edge and the generation of metallic debris as the deformed inner cutting edges interfere with the outer cutting edges and as the deformed metal causes galling of the inner surface of the outer tube in the region of the cutting window. Some shavers are made with the distal—most portion of the inner tube machined from a gall-resistant alloy such as Nitronic 60 or Gall-Tough to prevent galling of their distal bearing surfaces. These alloys generally have low yield strengths, not well suited to cutting edges for resection of bone. Other manufacturers produce their entire inner tube from an easily machined 300 series stainless steel, the distal portion being coated with a gall-resistant material. However, these 300 series stainless materials generally have low yield strengths, particularly in shavers in which the inner tube is formed from a single piece of tubing. This single-piece construction requires that the end of the tube be closed by plastic deformation. Closing of the tube in this manner requires a material which can undergo significant deformation without fracturing—a material with a low yield strength. Shavers made with 300 series inner cutting edges are not well suited to resection of bone since the edges undergo plastic deformation, unless the included angle of the cutting edges is increased to decrease the compressive stress at the edge.  
         [0014]     In view of the above, a shaver well suited to efficient resection of all tissue types including bone requires that the inner cutting edges have a low included angle and that the materials from the edges have a high yield strength. Producing such cutting edges, however, is problematic since, as noted previously, the geometry of inner cutting edges does not allow their manufacture by grinding. The inner cutting edges of currently available shavers with wedge-shaped inner cutting edges are produced by EDM, a spark-erosion process which uses a shaped electrode with a complementary form to produce contours on a partpiece. In EDM, pulses of high voltage are applied between the shaped electrode and the partpiece, each pulse producing an arc which vaporizes workpiece and electrode material, the vaporized material being flushed away by the dielectric fluid in which the process occurs. The surface finish produced on the partpiece by the process is a series of overlapping craters made by the arc, the roughness being determined by the parameters used in the process. The surface is also covered by resolidified metal (recast) which was not carried away by the flow of dielectric fluid during the forming process. This recast material is extremely brittle due to its rapid solidification rates, has cracks in it, and has portions which may be only loosely bonded to the surface of the partpiece. The EDM process is poorly suited to the manufacture of cutting edges, particularly those for use on tissue. The surfaces produced are rough, so as to cause tissue to embed in the surface rather than slide over it. The edges are irregular and have rounded portions due to localized melting and resolidification of the edge material.  
         [0015]     Milling is able to produce channels like those used in the prior art shavers currently available, however, the small size of the channels makes the milling of these channels problematic. A shaver with a 4 millimeter outer tube diameter will have an inner tube diameter of approximately 3.3 millimeters. The channel in a shaver of this size with parallel wedge-shaped cutting edges will be approximately 2 millimeters wide. Milling a channel of this width requires the use of an end-mill with a diameter of about 1.5 millimeters to allow roughing of the channel followed by finishing passes. An end-mill of this diameter is extremely fragile and requires high rotational speeds and low feed rates, both for preservation of the end-mill and to prevent the end-mill from flexing (wandering) and making products with a high degree of dimensional variability. Producing such a channel by milling with an end-mill is not economically feasible. This is especially true when the distal portion of the inner member is made from the high yield-strength required for a good bone cutter.  
         [0016]     Referring to  FIG. 1 , a prior art shaver  1  has an outer assembly  2  and an inner assembly  4  which is rotatably positioned therein. Inner assembly  4  has an elongated distal tubular portion  6 , and a proximal hub assembly  7  having an inner hub  8 , a spring  10 , and a spring retainer  12 . Hub assembly  8  is configured to transmit rotary motion from a powered handpiece to the inner assembly  4 . Inner tubular portion  6  has a distal end  14  which forms a cutting window. Outer assembly  2  has an elongated distal tubular portion  16  with a distal end  20  forming a cutting window, and a proximal hub assembly  18  suitable for removably mounting in a powered handpiece.  
         [0017]     As seen in  FIGS. 2 and 3  showing the distal end of prior art shaver  1 , outer cutting window  20  has a first lateral cutting edge  22  and a second lateral cutting edge  24  connected by a curvilinear proximal edge  26  and a curvilinear distal edge  28 . Inner cutting window  14  has a first lateral cutting edge  30 , a second lateral cutting edge  32 , a proximal curvilinear edge  34 , and a distal portion  36 . Shaver  1  is generally operated in an oscillate mode when cutting soft tissue. That is, inner assembly  4  is rotated within outer assembly  2  a predetermined number of revolutions in a first direction  38 , and then rotated a predetermined number of revolutions in a second, opposite direction  40 . The action is then repeated. When inner assembly  4  is rotated within outer assembly  2  in first direction  38 , suction in inner lumen  42  of inner tubular portion  6  draws tissue into the space between second inner lateral cutting edge  32  and first outer lateral cutting edge  22 , where it is trapped by the approaching edges and resected. When inner assembly  4  is rotated within outer assembly  2  in second direction  40 , suction in inner lumen  42  of inner tubular portion  6  draws tissue into the space between first inner lateral cutting edge  30  and second outer lateral cutting edge  24 , where it is trapped by the approaching edges and resected.  
         [0018]     As seen in  FIGS. 4-7 , first lateral cutting edge  30  and second lateral cutting edge  32  have a length  50  and are separated by a distance  52 . Cutting edges  30  and  32  have linear portions  54  and  56  respectively, and distal curvilinear portions  58  and  60  respectively. Included angle  55  of linear portions  54  and  56  are the angle between tangencies of walls  62  and  64  respectively and cylindrical outer surface  65 . Curvilinear portions  58  and  60  are formed by the intersection of first lateral wall  62  and second lateral wall  64  with spherical outer surface  66  of inner tubular portion  6 . Distal portion  36  has an inlcued angle  39  formed by surface  37  and a tangent to spherical surface  66 . Angle  39  is greater than 90 degrees (obtuse).  
         [0019]     Inner window  14  in inner tubular portion  6  can be formed by milling using a conventional (that is “square end”) end mill, but is more generally made by EDM using an electrode having a width slightly less than width  52  of window  14 , and a length somewhat longer than length  50  of cutting edges  30  and  32 . Milling is not the preferred method of manufacture since the endmill diameter must be less than width  52  of window  14 . Endmills of these small diameters are prone to breakage and also flexing during use resulting in dimensional variation in the finished parts. The endmills dull quite rapidly during use, further exacerbating the breakage and flexure problems. EDM, however, produces rough surface finishes and irregular cutting edges. Also, the electrodes used to produce the EDMed windows erode during use and must be periodically refurbished. The EDM process is, however, with appropriate fixturing able to produce large numbers of parts in batch operations thereby improving the process economics.  
         [0020]     When cutting bone, a shaver is typically used with a constant forward or reverse rotation rather than in an oscillating mode. As described previously, resection of bone is accomplished by the inner cutting edges without cooperative interaction with the outer cutting edges. Linear portions  54  and  56  of edges  30  and  32  have low included angle geometry which will efficiently resect bone. While curvilinear portions  58  and  60  have good geometry in the portions adjacent to linear portions  54  and  56 , the portions of the edges adjacent to surface  36  have less cutting efficiency because of interaction between surface  36  and the bone. The distal portion of the cutting edge formed by surface  36  and spherical surface  66  has a large included angle which prevents it from cutting bone. When the portions of the curvilinear edge portions  58  and  60  adjacent to surface  36  penetrate bone, the penetration is limited by interference by surface  36 . Prior art shaver  1  is effective for resecting bone when the axis of the tube is substantially parallel to the surface of the bone, so that linear portions  54  and  56  of edges  30  and  32  do most of the resection. Shaver  1  is less effective when the axis of the tube is at a larger angle to the bone surface, so that curvilinear portions  58  and  60  of edges  30  and  32  and surface  36  interact with the bone.  
       SUMMARY OF THE INVENTION  
       [0021]     Accordingly, it is an object of the present invention to produce an arthroscopic shaver with improved efficiency when cutting bone.  
         [0022]     It is also an object of the present invention to produce an arthroscopic shaver which cuts bone efficiently regardless of the portion of the cutting edge in contact with the bone.  
         [0023]     It is also an object of the present invention to produce an arthroscopic shaver which efficiently cuts bone and soft tissue.  
         [0024]     It is further an object of the present invention to produce a low-cost arthroscopic shaver with improved efficiency when cutting bone.  
         [0025]     It is also an object of the present invention to produce a method for making the inner cutting window of an arthroscopic shaver with improved efficiency when cutting bone.  
         [0026]     These and other objects are achieved in the present invention by providing an arthroscopic shaver blade with an inner cutting window having two parallel lateral facing edges, a curvilinear distal edge joining the distal ends of the lateral edges, and a curvilinear proximal edge joining the lateral edges. In one embodiment, each lateral cutting edge is formed by the intersection of the cylindrical outer surface of the tubular member and a formed cylindrical surface having an axis approximately parallel to the tube axis. The curvilinear proximal edge is formed by the intersection of the outer cylindrical surface of the tube and a formed spherical surface having a center at the proximal end of the axis of the cylindrical surface which forms the lateral edges, the radius of the formed cylindrical surface and the spherical surface being equal. The curvilinear distal edge is formed by the intersection of the outer distal spherical surface of the tube and a formed spherical surface having a center at the distal end of the axis of the formed cylindrical radius which forms the lateral edges, the radius of the formed cylindrical surface and the formed spherical surface being equal. By varying the axial placement of the formed cylindrical surface forming the distal end of the window, different distal end cutting edge heights can be achieved. For instance, if the formed spherical radius is centered at the center of the tube distal radius, the cutting edge in the distal radius portion of the tube will have a uniform height equal to that of the longitudinal edges. By positioning the center of the spherical radius distal or proximal to the tube distal radius, the edge height in this region may be either respectively decreased or increased.  
         [0027]     The window may be formed in a milling operation using a milling cutter having a spherical end (commonly called a “ball-nose” endmill) having an end radius equal to that of the surfaces forming the inner surfaces of the cutting edges.  
         [0028]     In another embodiment, a cutting window formed in the manner previously described undergoes a second milling operations using a ball-nose endmill of a smaller diameter. The endmill follows the same path as that of the previous milling operation, however, the endmill is advanced axially (endmill axis) further into the tube to create additional clearance. The cylindrical and spherical surfaces created in this second milling operation intersect the surfaces created in the first milling operation. The resulting cutting edges have a high resistance to plastic deformation because of the larger included angle created by the surfaces of the first milling operation, and are aggressive when cutting soft tissue because of the clearance created by the second milling operation.  
         [0029]     In another embodiment, the proximal portion of the window is formed using a ball-nose endmill of a first size, and the distal portion of the window is formed by a ball-nose endmill of a second, smaller size, to create laterally opposed protrusions at the juncture between the proximal and distal portions. These protrusions are positioned approximately at the proximal end of the tube distal radius. The protrusions create areas of high compressive stress when they interact with the surface of bone during use thereby improving cut initiation and enhancing the efficiency of the resection process.  
         [0030]     In another embodiment, the lateral edges are formed by the intersection of a series of overlapping spherical surfaces formed in the tube with the outer surface of the tube, the centers of the spherical surfaces being displaced from each other axially (tube axis) so as to form lateral cutting edges having a plurality of teeth. The tooth spacing and included angles are determined by the radius of the spherical pockets, the depth of the pockets in the tube, and the axial spacing between pockets. During use, the teeth create regions of localized high compressive stress when they interact with bone. This aids the initiation portion of the cutting process and enhances the bone cutting efficiency of the shaver. The teeth also aid in preventing soft tissue from being ejected from the window as the cutting edges approach, thereby improving the effectiveness of the resection process. The spherical surfaces are formed using a ball-nose endmill or similar cutter. The cutter is positioned at a first predetermined location and advanced distally in a drilling motion a predetermined distance to form a first spherical surface. The cutter is then withdrawn and repositioned to a second predetermined location and advanced distally to form a second spherical surface. This sequence of metal removal and positioning operations is repeated until all the spherical surfaces are formed.  
         [0031]     In another embodiment, the lateral edges are formed by the intersection of a series of overlapping spherical surfaces formed in the tube and the outer surface of the tube, the centers of the spherical surfaces being displaced from each other axially and laterally so as to form lateral cutting edges having a plurality of teeth, the placement of the teeth being asymmetrical about the tube axis when viewed in a plan view. The tooth spacing and included angles of the cutting edges are determined by the radius of the spherical surfaces, the depth of the surfaces in the tube, and the axial and lateral spacing between the spherical surfaces. The function of the teeth is the same as that of the teeth of the previous embodiment when cutting bone and soft tissue. The asymmetric placement of the teeth, however, provides additional benefit when cutting soft tissue since, when used in oscillate mode, the teeth of one direction of rotation interact with tissue portions between those affected by the teeth of the previous, opposite rotation. This is more fully described in co-pending application 10/937,210, incorporated by reference.  
         [0032]     In yet another embodiment, the cutting edges are formed by the intersection of a surface formed by a spherical radius as it traverses a planar non-linear path, and the outer surface of the tube. The nonlinear path may create, for example, a plurality of teeth, or other shapes to form cutting edges optimized for certain applications.  
         [0033]     Other embodiments are made using milling cutters having distal end geometries other than spherical. For instance, distal ends formed to the frustrum of a cone, formed to adjacent conical portions, and formed to geometries having linear and curvilinear portions when viewed in profile are used.  
         [0034]     Arthroscopic shavers with inner cutting edges formed in accordance with the principles of this invention are efficient bone cutters because of their low included-angle cutting edges. Because the cutting edges have low included angles in all portions of the cutting window, the shavers are effective resectors of tissue and bone in orientations in which prior art shavers were ineffective. This is particularly true when resecting bone in applications in which resection is accomplished using primarily the distal portion of the cutting edges. Because the cutting edges have low included-angle cutting edges throughtout, they are also efficient tissue cutters. This efficiency is further increased in embodiments having teeth or other protrusions on the cutting edges. The method of forming the window, milling using a cutter with an end formed to a spherical surface, is able to produce cutting edges with high repeatability at low cost. Because the windows are formed by milling, the machined surfaces of the cutting edges have smoother surface finishes and the edges have smaller edge radii than prior art devices which are formed by EDM. This further enhances the efficiency of the shaver when cutting bone or soft tissue.  
         [0035]     These and other features and advantages of the invention will be more apparent from the following detailed description that is provided in connection with the accompanying drawings and illustrated exemplary embodiments of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]      FIG. 1  is a disassembled view of a prior art arthroscopic shaver.  
         [0037]      FIG. 2  is an expanded perspective view of the distal end of the shaver of  FIG. 1 .  
         [0038]      FIG. 3  is a distal end view of the distal end of the shaver of  FIG. 2 .  
         [0039]      FIG. 4  is a plan view of the distal end of the inner member of the shaver of  FIG. 1 .  
         [0040]      FIG. 5  is a side, elevational view of the inner member of  FIG. 4 .  
         [0041]      FIGS. 6A and 6B  are an axial distal end views of the inner member of  FIG. 4 .  
         [0042]      FIG. 7  is a perspective view of the inner member of  FIG. 4 .  
         [0043]      FIG. 8  is an expanded, perspective view of the distal portion of a shaver constructed in accordance with the principles of the present invention.  
         [0044]      FIG. 9  is an axial distal end view of the shaver of  FIG. 8 .  
         [0045]      FIG. 10  is a plan view of the distal portion of the inner member of the shaver of  FIG. 8 .  
         [0046]      FIG. 11  is a side elevational sectional view of the inner member of  FIG. 10  at location B-B of  FIG. 10 .  
         [0047]      FIG. 12  is an axial distal sectional view of the inner member of  FIG. 10  at location A-A of  FIG. 10 .  
         [0048]      FIG. 13  is a perspective view of the inner member of  FIG. 10 .  
         [0049]      FIG. 14  is a plan view of an alternate embodiment of the distal portion of an inner member of a shaver according to the present invention.  
         [0050]      FIG. 15  is a perspective view of the inner member of  FIG. 14 .  
         [0051]      FIG. 16 a  side elevational sectional view of the inner member of  FIG. 14  at location A-A  FIG. 14 .  
         [0052]      FIGS. 17A and 17B  are distal axial sectional views of the inner member of  FIG. 14  at location B-B of  FIG. 14 .  
         [0053]      FIGS. 18A and 18B  are distal axial sectional views of the inner member of  FIG. 14  at location C-C of  FIG. 14 .  
         [0054]      FIG. 19  is a plan view of an alternate embodiment of the distal portion of an inner member of a shaver according to the present invention.  
         [0055]      FIG. 20  is a side elevational sectional view of the inner member of  FIG. 19  at location A-A of  FIG. 19 .  
         [0056]      FIG. 21  is an expanded distal axial sectional view of the inner member of  FIG. 20  at Location B-B of  FIG. 19 .  
         [0057]      FIG. 22  is a perspective view of the inner member of  FIG. 19 .  
         [0058]      FIG. 23  is a plan view of an alternate embodiment of the distal portion of an inner member of a shaver according to the present invention.  
         [0059]      FIG. 24  is a perspective view of the inner member of  FIG. 23 .  
         [0060]      FIG. 25  is a side elevational sectional view of the inner member of  FIG. 23  at location A-A of  FIG. 23 .  
         [0061]      FIG. 26  is a distal axial sectional view of the inner member of  FIG. 23  at location B-B of  FIG. 23 .  
         [0062]      FIG. 27  is a distal axial sectional view of the inner member of  FIG. 23  at location C-C of  FIG. 23 .  
         [0063]      FIG. 28  is a plan view of an alternate embodiment of the distal portion of an inner member of a shaver according to the present invention.  
         [0064]      FIG. 29  is a perspective view of the inner member of  FIG. 28 .  
         [0065]      FIG. 30A and 30B  are expanded distal axial sectional views of the inner member of  FIG. 28  at location B-B of  FIG. 28 .  
         [0066]      FIG. 31  is a side elevational sectional view of the inner member of  FIG. 28  at location A-A of  FIG. 28 .  
         [0067]      FIG. 32  is a plan view of an alternate embodiment of a shaver inner member constructed in accordance with the principles of the present invention.  
         [0068]      FIG. 33  is a perspective view of the inner member of  FIG. 32 .  
         [0069]      FIG. 34  is a side elevational sectional view of the inner member of  FIG. 32  at location A-A of  FIG. 32 .  
         [0070]      FIG. 35  is an expanded distal axial sectional view of the inner member of  FIG. 32  at location B-B of  FIG. 32 .  
         [0071]      FIG. 36  is a plan view of an alternate embodiment of a shaver inner member constructed in accordance with the principles of the present invention.  
         [0072]      FIG. 37  is a perspective view of the objects of  FIG. 36 .  
         [0073]      FIG. 38  is a side elevational sectional view of the objects of  FIG. 36  at location A-A of  FIG. 36 .  
         [0074]      FIG. 39  is an axial sectional view of the objects of  FIG. 36  showing the profile of the distal end of the cutter used to produce the cutting window.  
         [0075]      FIG. 40  is a perspective view of an alternate embodiment of a shaver inner member constructed in accordance with the principles of the present invention.  
         [0076]      FIG. 41  is a plan view of the objects of  FIG. 40 .  
         [0077]      FIG. 42  is a side elevational view of the objects of  FIG. 40 .  
         [0078]      FIG. 43  is an axial sectional view of the objects of  FIG. 40  showing the profile of the distal end of the cutter used to produce the cutting window. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0079]      FIGS. 8 and 9  depict the distal portion of a shaver  100  formed in accordance with the principles of the present invention and having an inner assembly  102  with an inner cutting window, and outer assembly  104  with an outer cutting window. Shaver  100  is operated in a manner identical to that of prior art shaver  1 , that is, in oscillate mode, tissue is trapped between opposing inner and outer lateral cutting edges as they approach, first during rotation of the inner assembly in a first direction, and again when the rotation of the inner is reversed. When cutting bone, shaver  100  is used with a constant forward or reverse rotation.  
         [0080]     Referring to  FIGS. 10-13 , inner cutting window  110  has a first lateral cutting edge  112  and a second lateral cutting edge  114  joined by a curvilinear proximal edge  116  and a curvilinear distal edge  118 . As seen in  FIG. 11  (showing a side elevational sectional view in direction B-B ( FIG. 10 )) and in  FIG. 12  (showing an axial sectional view in direction A-A ( FIG. 10 )), lateral cutting edges  112  and  114  have low included angles throughout, the edges being formed by the intersection of the cylindrical outer surface  120  of the tubular member  122  and cylindrical surfaces  124  of first lateral edge  112  and  126  of second lateral edge  114 , cylindrical surfaces  124  and  126  of length  125  being approximately coaxial, and of radius  127 . The included angle  113  of first lateral cutting edge  112  is defined as the angle between the tangencies of outer surface  120  and surface  124  at the cutting edge. The included angle of second lateral edge  114  is defined as the angle between the tangents of outer surface  120  and cylindrical surface  126  and is equal to angle  113 . Angle  113  is preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0081]     Curvilinear distal edge  118  has a low included angle throughout, edge  118  being formed by the intersection of spherical outer surface  128  of tubular member  122 , and spherical surface  130  of radius  132  equal to radius  127 . The included angle  119  of distal edge  118  is defined as the angle between the tangents of spherical outer surface  128  and spherical surface  130  at any point on edge  118 . Angle  119  is preferably between 20 and 75 degrees, and more preferably between 30 and 70 degrees.  
         [0082]     Center  134  of spherical surface  130  is at distal end  136  of axis  138  of cylindrical surfaces  124  and  126 . Axis  138  is parallel to axis  139  of tubular member  122  and displaced therefrom a distance  141 . Curvilinear proximal edge  116  is formed by the intersection of cylindrical outer surface  120  of tubular member  122 , and spherical surface  140  of radius  142  equal to radii  127  and  132 . Center  144  of spherical surface  140  is at proximal end  146  of axis  138  of cylindrical surfaces  124  and  126 . When viewed in a plan view as in  FIG. 10 , distal spherical surface  130  of edge  118  is centered within distal spherical surface  128  of outer tube  122  so that distal edge  118  is of constant height  119 , as seen in  FIGS. 11 and 12 . In other embodiments, when viewed in plan view, the center of inner spherical surface  130  is distal to the center of outer spherical surface  128  so that the distal portion of cutting edge  118  is not of constant height, but rather has a lower height at its distal end. In other embodiments, inner surface  130  is proximal to the center of surface  128  to make the distal portion of edge  118  higher than the more proximal portions of the cutting window.  
         [0083]     A preferred method for forming inner cutting window  110  is by milling with an endmill having a spherical, distal end cutting surface ( commonly called a “ball nose” end mill), the radius of the spherical tip being equal to radii  127 ,  132 , and  142 . The endmill is oriented with its axis perpendicular to the tube axis, centered with the tube axis and the endmill is advanced axially to machine into tube  122  until the center of the spherical radius is distance  141  from axis  139  of tube  122 . The endmill is then fed linearly parallel to axis  139  of tube  122  distance  125 , the distance between the centers of spherical distal surface  130  and spherical proximal surface  140 .  
         [0084]     Because the window is formed by a portion of the distal radius of an endmill with a spherical end, the endmill diameter can be larger than that used to form the cutting window of the prior art device. This results in less breakage of the endmill and decreased flexing of the endmill during machining of the window. This, in turn, allows the use of more aggressive feed rates, decreases the cycle times and makes forming of the window more economical.  
         [0085]     Because distal edge  118  has a low included angle throughout, inner window  110  is able to resect bone more effectively than prior art shavers when the angle between the axis  139  of inner tube  122  and the bone surface is such that contact between the bone and cutting window  110  occurs primarily in the distal portion of the cutting edge.  
         [0086]     Cutting edges  112 ,  114  and  118  of window  110  have a uniform height throughout. Creating protrusions or irregularities on the cutting edges creates areas of localized higher compressive stress when resecting bone thereby improving the cut initiation and resection efficiency. Accordingly, other embodiments have protrusions or teeth which aid in resecting bone.  
         [0087]     Referring now to  FIGS. 14-18  showing the distal portion  200  of the inner tubular member  202  of an alternate embodiment, cutting window  204  has a first or proximal portion  206  having a first cutting edge  208  and a second cutting edge  210  joined by proximal edge  212 . Linear portion  209  of edge  208  is formed by the intersection of cylindrical surface  214  and outer cylindrical surface  216  of tubular member  202 . Linear portion  211  of edge  210  is formed by the intersection of cylindrical surface  218  with outer cylindrical surface  216  of tubular member  202 . Cylindrical surfaces  214  and  218  have a common axis  222 . Included angle  221  of linear portion  211  of edge  210  is defined as the angle between the tangencies of cylindrical surfaces  218  and  216 . The included angle of linear portion  209  of edge  208  is defined as the angle between the tangencies of cylindrical surfaces  214  and  216 , and is equal to angle  221 . Angle  221  is preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0088]     Curvilinear portion  207  of edge  208  is formed by the intersection of spherical surface  205  and outer cylindrical surface  216  of tubular member  202 . Curvilinear portion  213  of edge  210  is formed by the intersection of spherical surface  215  and outer cylindrical surface  216  of tubular member  202 . Proximal edge  212  is formed by the intersection of spherical surface  220  with outer surface  216  of tubular member  202 . Center  224  of spherical surface  220  is at the proximal end of axis  222  of cylindrical surfaces  214  and  218 . Center  225  of spherical surfaces  205  and  215  is at the distal end of axis  222 . Axis  222  is parallel to axis  226  of tubular member  202  and displaced from axis  226  a distance  228 . Radius  232  of cylindrical surfaces  214  and  218  is equal to radius  230  of spherical surfaces  205 ,  215  and  220 .  
         [0089]     Second or distal portion  240  of window  204  has a curvilinear cutting edge  233  formed by the intersection of spherical surface  242  of radius  244  and laterally opposed parallel planar surfaces  246  and  248  with outer spherical surface  250  of tubular member  202 . Distance  252  between surfaces  246  and  248  is equal to twice radius  244 . Center  254  of spherical surface  242  is displaced a distance  256  from axis  226  of tubular member  202 , and is displaced axially a distance  258  distal to the center of spherical outer surface  250  of tube  202 .  
         [0090]     Window  204  is formed using a two-step milling process. Proximal portion  206  is formed using a ball-nose end mill having a spherical radius equal to radius  232  of cylindrical surfaces  214  and  218  and spherical radius  230  of spherical surfaces  205 ,  215  and  220  in the same manner as inner cutting window  110  ( FIGS. 10-13 ). Distal portion  240  of window  204  is formed using a ball nose endmill having a spherical radius equal to radius  244  of spherical surface  242 . Proximal portion  206  is machined first using the larger diameter endmill to minimize the amount of material which is removed using the smaller diameter endmill to form distal portion  240  of window  204 . In this way, the metal removal rate during machining of the window is maximized and the machining cost minimized.  
         [0091]     Window  204  has a first protrusion (or tooth)  260  formed by curvilinear portion  207  of first edge  208  of proximal window portion  206 , and first proximal edge portion  262  of cutting edge  233  of distal portion  240  of window  206 . Window  204  also has a second protrusion (or tooth)  264  formed by curvilinear portion  213  of second edge  210  of proximal window portion  206 , and second proximal edge portion  266  of cutting edge  233  of distal portion  240  of window  206 . Protrusions  260  and  264  have low included angles and act as teeth which readily penetrate soft tissue so as to aid in preventing soft tissue from being ejected as inner and outer cutting edges approach each other during use. Protrusions  260  and  264  also create areas of localized high compressive stress in bone when resecting bone so as to aid in the initiation of resection. The distal placement of the protrusions allows the shaver to resect bone more effectively than prior art shavers when the angle between the axis  226  of inner tube  202  and the bone surface is such that contact between the bone surface and cutting window  204  occurs primarily in the distal portion of the cutting edge. All cutting edges of window  204  have low included angles throughout.  
         [0092]     An embodiment having multiple protrusions for enhanced efficiency when cutting tissue or bone is depicted in  FIGS. 19-22 . Distal end  302  of inner tube  300  has a cutting window  304  formed by the intersection of spherical surfaces  306 ,  308 ,  310  and  312  with cylindrical outer surface  314  and spherical distal surface  316  of tube  300 . The centers of spherical surfaces  306  through  312  are separated axially by distance  318 . Center  320  of surface  312  is displaced distally from the center of spherical distal surface  316  of tube  300  by distance  322 . The centers of spherical surfaces  306  through  312  lie on a line  313  which is parallel to axis  324  of tube  300  and displaced a distance  326  therefrom. Spherical surfaces  306  through  312  have a common radius  328 . The intersection of spherical surfaces  306  and  308  form teeth  330 , the intersection of radii  308  and  310  form teeth  332 , and the intersection of radii  310  and  312  form teeth  334 . Cutting window  304  has low included angles  311  throughout, the angles being defined as the angle between the tangencies of cylindrical surfaces  306 ,  308 ,  310 ,  312  and cylindrical outer surface  314  or spherical distal surface  316 . Angle  311  is preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0093]     Window  304  is formed by a four-step operation using a ball-nose endmill having a spherical radius equal to radius  328  of spherical surfaces  306  through  312 . The endmill is positioned with its axis perpendicular to axis  324  of tube  300  and positioned a distance  322  distal from the center of spherical surface  316  of tube  300 . While rotating, the endmill is advanced distally along its axis until the center of the end mill spherical radius is a distance  326  from axis  324  of tube  300  to form spherical surface  312 . The endmill is retracted and repositioned by moving proximally distance  318  along tube axis  324 . The endmill is advanced distally along its axis until the center of the endmill spherical radius is distance  326  from axis  324  of tube  300  to form spherical surface  310 . This sequence of repositioning and machining operations is repeated so as to form spherical radii  308  and  306 . Because the spherical surfaces are formed by the spherical end of a ball-nose endmill, the diameter of the endmill is greater than that of endmills necessary to produce prior art shaver inners. Additionally, because the endmill forms the spherical surfaces through a drilling (axially advancing) motion, wear of the end mill cutting edges occurs across the entire portion of the endmill spherical radius which forms the pocket, rather than being concentrated in the portion which intersects the tube wall when an endmill is moved laterally along a predetermined path. This minimizes localized wear which affects product surface quality and consistency.  
         [0094]     As with the previous embodiment illustrated in  FIGS. 14-18  above, the protrusions (teeth) on the cutting edges minimize the ejection of soft tissue from the cutting windows as the cutting edges approach each other. Also, when cutting bone, the teeth produce locally concentrated areas of high compressive stress when the teeth contact a bone surface. This aids the initiation of resection and enhances the efficiency of the removal process. All cutting edges of this embodiment have low included angles throughout, preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0095]     In yet another embodiment shown in  FIGS. 23-27 , distal end  402  of inner tube  400  has a cutting window  404  formed by the intersection of spherical surfaces  406  through  418  with cylindrical outer surface  420  and spherical surface  422  of tube  400 . When viewed in a plan view as in  FIG. 23 , centers  426  through  438  of spherical surfaces  406  through  418 , respectively, are spaced axially a distance  440 . Center  432  of spherical surface  412  is displaced axially distance  441  distal to center  443  of spherical surface  422  of tube  400 . Centers  426 ,  428 ,  430  and  432  of spherical surfaces  406 ,  408 ,  410  and  412  are laterally offset in a first direction a distance  442  from axis  443  of tube  400 . Centers  434 ,  436  and  438  of spherical surfaces  414 ,  416  and  418  are laterally offset in a second direction a distance  444 . Spherical surfaces  406  through  418  have a common radius  446 .  
         [0096]     As illustrated in  FIGS. 25-27 , the plane containing centers  426  through  438  is displaced a distance  448  from axis  443  of tube  400 . Tooth  456  is formed by the intersection of spherical surfaces  406  and  408 ; tooth  458  is formed by the intersection of spherical surfaces  408  and  410 ; tooth  460  is formed by the intersection of spherical surfaces  410  and  412 ; tooth  462  is formed by the intersection of spherical surfaces  412  and  414 ; tooth  464  is formed by the intersection of spherical surfaces  414  and  416 ; tooth  466  is formed by the intersection of spherical surfaces  416  and  418 . All cutting edges of window  404  have low included angles throughout, the angles being defined as the angle between the tangencies of spherical surfaces  406  through  418  and cylindrical outer surface  420  or spherical outer surface  422  at any point on the cutting edges. The included angles are preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0097]     Window  404  is formed by a multi-step operation using a ball-nose endmill in the same manner as the previous embodiment shown in  FIGS. 19-22 , using a ball-nose endmill with a spherical radius equal to radius  446  of spherical surfaces  406  through  418 . The endmill is positioned with its axis perpendicular to axis  443  of tube  400  and positioned a distance  441  distal from the center of spherical surface  422  of tube  400  and a distance  442  laterally in a first direction from axis  443 . While rotating, the endmill is advanced distally until the center of the endmill spherical radius is a distance  448  from the plane normal to the cutter axis containing axis  443  of tube  400  so as to form spherical surface  412 . The endmill is retracted and repositioned to center  434  of spherical surface  414 . The endmill is advanced distally to form spherical surface  414 . This sequence of repositioning and machining operations is repeated to form spherical surfaces  410 ,  416 ,  408 ,  418 , and  406 . Because the spherical surfaces are formed by the spherical end of a ball-nose end mill, the diameter of the endmill can be greater than that of end mills necessary to produce prior art shaver inners. Additionally, because the endmill forms the spherical surfaces through a drilling (axially advancing) motion, wear of the end mill cutting edges occurs across the entire portion of the endmill spherical radius which forms the pocket, rather than being concentrated in the portion which intersects the tube wall when an endmill is moved laterally along a predetermined path. This minimizes localized wear which affects product surface quality and consistency.  
         [0098]     As with the previous embodiment shown in  FIGS. 19-22  above, the protrusions (teeth) on the cutting edges minimize the ejection of soft tissue from the cutting windows as the cutting edges approach each other. The asymmetric positioning of the teeth when viewed in a plan view also enhances the efficiency of the shaver when used on tissue in an oscillate mode. When the inner member is rotated in a first direction, the teeth on a first side of the cutting window engage tissue, the teeth preferentially removing tissue in tissue regions which the teeth engage. When the inner rotation is reversed, the teeth of the opposite cutting edge engage regions which were not affected by the teeth of the previous rotation direction. This results in reduced ejection of tissue from the cutting window as the cutting edges approach each other. Also, when cutting bone, the teeth produce locally concentrated areas of high compressive stress in bone when the teeth contact a bone surface. This helps the initiation of resection and enhances the efficiency of the removal process.  
         [0099]     In certain circumstances, it is desirable to have cutting edges with high strength for cutting bone, and also low included angles for aggressive resection of soft tissue. This is accomplished in the embodiment shown in  FIGS. 28-31 . The inner member is a modification of the shaver inner member  102  shown in  FIGS. 10-13 . Specifically, a portion of the cylindrical surfaces (which in combination with tube outer surface form the lateral linear cutting edges) and a portion of the spherical surface (which in combination with the outer spherical surface of the inner tube form the distal curvilinear cutting edge) are removed to facilitate the flow of tissue into the inner lumen for resection and aspiration from the site.  
         [0100]     Referring now to  FIGS. 28-31 , distal portion  502  of inner tube  500  has a cutting window  504 . First lateral cutting edge  506  and second lateral cutting edge  508  of length  509  are formed by the intersections of first cylindrical surfaces  512  and  514  with cylindrical surface  516  of inner tube  500 . First cylindrical surfaces  512  and  514  extend from cutting edges  506  and  508  to second cylindrical surfaces  522  and  524  respectively. First cylindrical surfaces  512  and  514  are coaxial, have a radius  526  and have an axis  527  displaced distance  525  from axis  529  of tube  500 . Second cylindrical surfaces  522  and  524  are coaxial, have a radius  528 , and an axis  531  displaced distances  533  from axis  529  of tube  500 . Distal cutting edge  510  is formed by the intersection of first spherical surface  530  with spherical surface  532  of tube  500 . First spherical surface  530  extends from cutting edge  510  to second spherical surface  534 . Center  535  of first spherical surface  530  is at the distal end of axis  527  of first cylindrical surfaces  512  and  514 . Center  536  of second spherical surface  534  is at the distal end of axis  531  of second cylindrical surfaces  522  and  524 . Proximal edge  540  of window  504  is formed by the intersection of spherical surface  542  with outer cylindrical surface  516 . Spherical surface  542  of radius  544  has a center  546  at the proximal end of axis  527  of first and second cylindrical surfaces  512  and  514 . As seen in  FIG. 30B , included angle  551  is defined as the angle between the tangencies of cylindrical surfaces  512  and  514  and outer cylindrical surface  516 , and is equal to the included angle of distal cutting edge  510  which is defined as the angle between the tangencies of cylindrical surface  530  and spherical surface  532  at any point on edge  510 . Angle  551  is preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0101]     Window  504  is formed in a two-step milling operation. A first operation forms first cylindrical surfaces  512  and  514 , and spherical surface  530  in a milling operation identical to that used to form window  110  of the previous embodiment shown in  FIGS. 10-13 . Second cylindrical surfaces  522  and  524 , and spherical surface  534  are formed in a second milling operation using a ball-nose endmill with a spherical radius equal to radius  528  of spherical surface  534 . The endmill is positioned with its axis perpendicular to axis  529  of tube  500 , and centered with spherical surface  534 . While rotating, the endmill is advanced until the center of the endmill radius is a distance  533  from tube radius  529 . The endmill is then fed proximally parallel to the tube axis  529  distance  509 , and retracted at the completion of the motion.  
         [0102]     The cutting edges of window  504  have a higher resistance to plastic deformation than the edges of window  110  depicted inFigures  10  through  13  because of the larger included-angle, and increased aggressiveness when cutting soft tissue due to the low included angle portions of the cutting edges formed by second cylindrical surfaces  522  and  524 , and by second spherical surface  534 . The cutting edges of this embodiment, while more resistant to edge deformation when cutting bone, are able to penetrate tissue efficiently because of the decreased included angle of portions of the cutting edge formed by cylindrical surfaces  522  and  524  and spherical surface  534 . All cutting edges have low included angles throughout, preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0103]     In yet another embodiment shown in  FIGS. 32-35  depicting the distal end  602  of inner tubular member  600  with a cutting window  604  formed therein, window  604  is formed by the intersection of outer cylindrical surface  606  and spherical surface  608  of tube  600  with cylindrical surfaces  610  and spherical surfaces  612  formed in tube  600 . The included angles of the cutting edges is low throughout, including in the far distal portion of the window. The included angles may be varied by changing the size of radius  614 , height  618 , and path  616 . The included angles are preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0104]     Cylindrical surfaces  610  and spherical surfaces  612  are formed by a milling cutter having a spherical cutting surface formed at its distal end, the radius of the distal end being equal to radius  614  of surfaces  610  and  612 . The milling cutter is advanced distally into tube  600  at first location  616  until the center of the cutter distal radius is a distance  618  above the plane normal to the cutter axis containing axis  619  of tube  600 . The cutter is then moved along cutter path  620  to location  622  and retracted. This embodiment is used in the same manner as the previous embodiments, the teeth having the same function as the teeth in the previous embodiments, that is, to prevent the ejection of tissue from the window when cutting soft tissue, and to create areas of high compressive stress at the bone surface when cutting bone so as to increase cut initiation efficiency. The shape of window  604  is determined by cutter path  620 , the configuration of the milling cutter, and distance  618 . By varying these parameters, the number of teeth may be increased or decreased, the cutting edges may have larger or smaller included angles, or edge configurations with irregular contours may be created as specific applications require.  
         [0105]     The embodiments described above have been formed using milling cutters having their distal ends formed to a radius. Other embodiments are formed using milling cutters having distal ends formed to other geometries. For instance,  FIGS. 36 through 39  depict the distal end  702  of a shaver inner tubular member having a cutting window  704  formed by milling using a cutter having a distal end formed to the frustrum of a cone. Cutting window  704  has a first lateral cutting edge  706  and a second lateral cutting edge  708 , a proximal edge  710  connecting edges  706  and  708 , and a distal edge  712  connecting the distal ends of edges  706  and  708 . Lateral edges  706  and  708  are formed by the intersection of planar surfaces  714  and  716  respectively with tube cylindrical outer surface  718 . Distal cutting edge  712  is formed by the intersection of conical surface  720  and tube spherical surface  722 . Lateral edges  706  and  708 , have an included angle  724  defined as the angle between the tangencies to cylindrical surface  718  and surfaces  714  and  716  respectively. Distal edge  712  has an included angle  726  defined as the angle between a tangency to spherical surface  722  and conical surface  720  at any point on edge  712 . Angles  724  and  726  are preferably between 20 and 85 degrees, and more preferably between 30 and 70 degrees.  
         [0106]     The milling cutter used to produce window  704 , the profile of the distal-most portion of which is depicted in  FIG. 39 , has a distal end  750  forming the frustrum of a cone having an included angle  730  to form surfaces  714  and  716  to the same included angle. Angle  732  of conical surface  720  is equal to about half of included angle  730  of the conical portion of cutter distal end  750 . Window  704  is formed in the same manner as the previous embodiments, that is, cutter  750  is positioned at a first location centered above distal surface  722 . While rotating, cutter  730  is advanced into tubular member  500  to a predetermined depth. When the depth is reached, the cutter is fed axially (tube axis) until lateral edges  706  and  708  are the desired length. The cutter is then withdrawn.  
         [0107]     In yet another embodiment depicted in  FIGS. 41-43 , the window is formed by a milling cutter having a distal portion formed of adjacent conical portions, the distal conical portion having a larger included angle than the proximal conical portion. The window formed has a decreased included angle at the cutting edges so as to provide enhanced performance when cutting tissue, and improved efficiency when resecting bone since the decreased included angle increases the compressive stress at the bone surface for improved cut initiation. Distal portion  802  of shaver tubular inner member  800  has a window  803  having a first lateral edge  804 , a second lateral edge  806 , a curvilinear proximal edge  808  and a curvilinear distal edge  810 . First lateral edge  804  and second lateral edge  806  are formed by the intersection of first planar surface  812  and second planar surface  814  respectively with outer cylindrical surface  816  of tubular member  800 . Curvilinear proximal edge  808  is formed by the intersection of conical surface  818  with outer surface  816 . Curvilinear distal edge  810  is formed by the intersection of conical surface  820  with spherical outer surface  822 . Included angle  824  of first lateral edge  804  is defined as the angle between first planar surface  812  and a tangent to cylindrical surface  816 . The included angle of second lateral edge  806  is similarly defined and is equal to angle  824 . The included angle of curvilinear distal edge  810  is defined as the angle between conical surface  820  and a tangent to spherical outer surface  822  at any point on edge  810  and is equal to angle  824 . The milling cutter used to produce window  804 , the profile  828  of the distal portion of which is depicted in  FIG. 43 , has a proximal conical portion  830  with an included angle  832  and a distal conical portion  834  with an included angle  836 . Included angle  832  of proximal portion  830  determines the angle of inclination  840  of planar surfaces  812  and  814  and conical surface  820  which is half of angle  832 . Included angle  836  of distal conical portion  834  determines the angle of inclination  842  of second planar surfaces  844  and  846  and second conical surface  848 . Milling of window  803  is accomplished in the same manner as previous embodiment  702  shown in  FIGS. 36 through 39 .  
         [0108]     The included angles of cutting edges produced in accordance with the principles of this invention are determined by the configuration of the distal end of the milling cutter used to produce the window, and the characteristics of the millng process. The distal end of the milling cutter used to produce all embodiments herein described, both with and without teeth, may be spherical, elliptical, conical, or have a complex geometry wherein the profile when viewed in section is comprised of linear and curvilinear segments. The milling processes as described herein may be accomplished in a single operation, or may comprise roughing and finishing passes to produce the finished geometry.  
         [0109]     The cylindrical machined surfaces formed in the embodiments described above have been described as being coaxial and having axes parallel to the tube axis. In certain applications, however, it may be advantageous for the cylindrical surfaces to have axes that are not parallel to the tube axis, or to have machined cylindrical surfaces which are not coaxial. Such configurations are anticipated and within the scope of this invention.  
         [0110]     The above description and drawings illustrate preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.