Abstract:
A sigmoid notch resurfacing prosthesis for application to the sigmoid notch of the distal radius. The sigmoid notch prosthesis generally includes a saddle and a radius portion for attachment to the distal radius. The saddle may be formed from ultra high molecular weight polyethylene or another durable self-lubricating material. The saddle includes an at least partially concave contoured depression having rounded edges that is securable to the radius portion. The saddle may be secured by a sliding notch snap fit design. The prosthesis may be adapted for articulation with the natural head of the ulna or with an ulnar head prosthesis that has replaced the ulnar head.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains generally to orthopedic prostheses. More particularly, the present invention pertains to a joint prosthesis for the distal radioulnar joint.  
       BACKGROUND OF THE INVENTION  
       [0002]     The radius and ulna together form the bony structure of the forearm. The two bones articulate with one another at both their proximal and distal ends. The distal radioulnar joint is a “shallow socket” ball joint. The ulna, a relatively straight bone, translates dorsal-palmarly to accept the modestly bowed radius. The distal end of the radius articulates in pronation and supination on the distal head of the ulna at the sigmoid notch or fossa. The sigmoid notch socket in most wrists is relatively flat and a number of ligaments support the distal ulna where it meets the distal end of the radius. The supporting ligaments include the triangular fibrocartilage (TFC), the extensor carpi ulnaris (ECU) subsheath, and the ulnar collateral ligament complex. The stabilizing elements of the triangular fibrocartilage, the extensor carpi ulnaris subsheath, and the ulnar collateral complex work in cooperation with the distal ulna to transfer compressive loads between the ulnar carpus and the distal ulna across the distal radioulnar joint.  
         [0003]     Unfortunately, fractures of the distal radius and other injuries of the forearm commonly occur and may cause rotational instability. Following these injuries to the forearm, ligament disruption, ulnar styloid fractures, and fractures into the distal radioulnar joint commonly occur. Fracture or dislocation involving the distal radioulnar joint often results in a loss of forearm rotation related to either instability or incongruity between the sigmoid fossa of the distal radius and the head of the ulna. A variety of different fractures involving the distal radius can cause this condition including the Colles&#39; fracture and the Galeazzi fractures.  
         [0004]     When there is loss of stability of the distal radioulnar joint, subsequent weakness in grip and pinch as well as potential loss of forearm rotation occur. Instability can also be associated with an injury to the triangular fibrocartilage or to the ulnar styloid. When instability is present, a number of ligament reconstructive procedures have been devised to assist in treating the unstable distal ulna. Unfortunately, ligament reconstruction of the distal ulna often does not restore complete stability and joint replacement is often performed in an effort to stabilize the joint.  
         [0005]     Sometimes, when the distal ulna is damaged, the preferred treatment is hemiarthroplasty. That is to resect the head of the ulna and replace it with an ulnar head prosthesis which then is in contact with the natural bone of the radius. The prosthesis then articulates with the sigmoid notch to restore functionality to the distal radioulnar joint. Unfortunately, long term articulation between the man made implant and the natural bone may accelerate wear of the bone and lead to arthritic or degenerative change. This may cause the patient pain and restriction of motion.  
         [0006]     In other circumstances, both the head of the ulna and the distal radius may suffer injury, arthritic change or degenerative change simultaneously. Then, it would be desirable to replace the articular surfaces of both the head of the ulna and the sigmoid fossa. In addition, in some cases, replacement of the ulnar head with a prosthesis does not fully restore stability to the distal radioulnar joint. At present, patients that have had an ulnar head resection with implantation of an ulnar head prosthesis who still suffer from instability have few options.  
         [0007]     In light of the foregoing, the surgical arts would benefit from access to a prosthetic device that could be used to restore the function of a damaged sigmoid notch articular surface. The surgical arts would also benefit from the availability of a prosthetic combination to repair an injured or degenerated distal radioulnar joint.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention solves many of the above referenced problems. The invention includes a sigmoid notch resurfacing prosthesis for application to the sigmoid notch of the distal radius. The sigmoid notch prosthesis generally includes a saddle and a radius portion for attachment to the distal radius. The saddle is securable to the radius portion and includes an articular surface having an at least partially concave contoured depression having rounded edges. The saddle may be secured by a sliding notch snap fit design or by other techniques known to those skilled in the art. The saddle may be formed from ultra high molecular weight polyethylene or another durable self-lubricating material for articulation with the head of an ulnar head implant. At this time, it is thought that metallic materials such as stainless steel or titanium are preferable for articulation with natural bone if the sigmoid notch implant is used as a hemiarthroplasty.  
         [0009]     The invention may also include an ulnar head prosthesis for replacing the distal head of the ulna. The ulnar head prosthesis includes a head and a stem to replace the distal ulnar head. The prosthesis head is formed with a curved surface for articulation with the sigmoid notch prosthesis when installed. The head presents a bore to allow for attachment of the head to the stem. The head may be formed with suture holes for anchoring the head to soft tissues that are exposed after resection of the distal ulna.  
         [0010]     The stem of the ulnar head prosthesis is elongated with an extended end for engaging within the intramedullary canal of the resected ulna. The stem of the ulnar head prosthesis also includes a distal end adapted for engagement with the bore in the head of the ulnar prosthesis. The stem further includes a collar between the proximal end of the stem and the distal end of the stem. The collar may be substantially flat. The proximal surface of the collar rests against the resected end of the distal ulna upon implantation to prevent the stem from penetrating excessively into the intramedullary canal of the ulna.  
         [0011]     The saddle plate of the sigmoid notch implant includes a stem extending outwardly from a reverse side thereof as well as a countersunk screw receptacle for receiving a low profile spherical head bone screw. Interconnecting the screw receptacle and the stem is a stiffening rib. The opposite surface of the radius portion includes a substantially flat platform and a saddle retainer that surrounds the saddle on three sides and is grooved to receive a portion of the saddle thereunder to hold the saddle in place. In addition, the saddle retainer includes a retaining ridge so that the saddle can be slid onto the retaining structure and retained by a snap fit. The saddle and the saddle plate can also be connectable in other way known to those skilled in the art.  
         [0012]     The sigmoid notch implant is implanted by first surgically accessing the distal radial ulnar joint. Once a surgeon decides the precise location for the sigmoid notch implant, the surgeon prepares the distal radius by drilling a hole to accept the stem of the saddle plate. A trial stem is placed in the hole in order to determine the proper location for pilot hole for a self-tapping bone screw. The trial stem has an undersized stem in order to preserve a press fit between the final implant and the bone of the distal radius.  
         [0013]     Once the location of the pilot hole for the self-tapping bone screw is completed the surgeon burrs down the sigmoid notch to provide a flat buttress for the back of the saddle plate. The surgeon also burrs a small countersink to accept a collar surrounding the head of the spherical screw and the stiffening rib between the screw hole and the stem. The radius portion is then placed into the drill hole and impacted to seat it at its final location. Once the radius implant is successfully located, the self-tapping bone screw is placed in the pilot hole and tightened. Once the radius plate is fully seated, the head of the ulnar head implant is returned to its proper location at the head of the ulna.  
         [0014]     In this embodiment, the saddle is inserted into the retaining structure of the radius plate and advance distally. Assuming the saddle is properly aligned with the radius plate, the saddle will move distally until it is about seventy five percent engaged, at which point it will rise up on a ramp in the bottom of the saddle as the ramp passes over the interference ridge on the surface of the radius plate. Once the saddle is completely seated in the saddle plate the surgeon reduces the joint and assesses range of motion. Assuming that relative motion of the ulnar head implant and the sigmoid notch implant is satisfactory, the surgeon repairs the joint capsule and closes the incision and the procedure is complete. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a sigmoid notch implant in accordance with the present invention;  
         [0016]      FIG. 2  is another perspective view of the sigmoid notch implant;  
         [0017]      FIG. 3  is a front elevational view of the sigmoid notch implant;  
         [0018]      FIG. 4  is a plan view of the sigmoid notch implant;  
         [0019]      FIG. 5  is a rear elevational view of the sigmoid notch implant;  
         [0020]      FIG. 6  is a side elevational view of the sigmoid notch implant;  
         [0021]      FIG. 7  is a perspective view of a saddle in accordance with the present invention;  
         [0022]      FIG. 8  is a front elevational view of the saddle in accordance with the present invention;  
         [0023]      FIG. 9  is a perspective view of the saddle in accordance with the present invention;  
         [0024]      FIG. 10  is a plan view of the saddle with phantom lines showing internal structures;  
         [0025]      FIG. 11  is a rear elevational view of the saddle;  
         [0026]      FIG. 12  is a side elevational view of the saddle with phantom lines depicting internal structures;  
         [0027]      FIG. 13  is a plan view of a radius component of the sigmoid notch implant;  
         [0028]      FIG. 14  is a front elevational view of the radius component;  
         [0029]      FIG. 15  is a side elevational view of the radius component;  
         [0030]      FIG. 16  is a rear elevational view of the radius component;  
         [0031]      FIG. 17  is a perspective view of the radius component;  
         [0032]      FIG. 18  is another perspective view of the radius component;  
         [0033]      FIG. 19  is an elevational view of the sigmoid notch implant and an ulnar head implant with the saddle of the sigmoid notch implant partially installed; and  
         [0034]      FIG. 20  is an elevational view of the sigmoid notch implant and an ulnar head implant with the saddle completely installed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]     An embodiment of the sigmoid notch implant  30  generally includes radius portion  32 , saddle  34  and bone screw  36 . Referring to  FIGS. 18 and 19  the sigmoid notch implant  30  is generally utilized along with an ulnar head implant  38 . A typical ulnar head implant  38  includes a head portion  40  and a stem portion  42 . One exemplary ulnar head prosthesis is disclosed in U.S. Pat. No. 6,302,915. The contents of that U.S. patent are incorporated herein by reference.  
         [0036]     Referring to  FIGS. 1-6  and  13 - 18 , radius portion  32  generally includes stem  44  and saddle plate  46 . Stem  44  may extend outwardly from saddle plate  46  at a substantially right angle.  
         [0037]     Stem  44  includes cylindrical portion  48  and rounded end  50 . Stem  44  joins saddle plate  46  at fillet  52 . Stem  44  may also have a tapered shape or include ridges or surface texturing thereon.  
         [0038]     Referring to  FIGS. 13-18  saddle plate  46  is roughly trapezoidally shaped and may be integrally formed with stem  44 . Saddle plate  46  presents saddle retainer  54  and retainer ridge  56  on a side opposite from stem  44 . Saddle plate  46  also defines screw hole  58  there through.  
         [0039]     In one embodiment, saddle retainer  54  is structured to substantially surround saddle  34  on three sides. Saddle retainer  54  includes ridge  60  on three sides thereof, which together define a three-sided groove  62 . Surface  64  of saddle plate  46  is substantially planar. Retaining ridge  56  extends upwardly from surface  64  at an end of saddle plate  46  substantially opposite from saddle retainer  54 . Retaining ridge  56  may have a bevel  66  on the top thereof.  
         [0040]     Screw hole  58  passes through saddle plate  46 . Screw hole  58  desirably includes spherical countersink  68 . As seen in  FIGS. 14-16  and  FIG. 18 , screw hole  58  also passes through collar  70  which may be integrally formed with saddle plate  46  and which joins saddle plate  46  at circular fillet  74 . Collar  70  is connected to stem  44  by stiffening rib  76  which interconnects circular fillet  74  with fillet  52 . Stiffening rib  76  may also be integrally formed with saddle plate  46 . Screw hole  58  may be located beneath saddle  34  when saddle  34  is assembled to saddle plate  46  or screw hole  58  may be in a location left exposed when saddle  34  is assembled to saddle plate  58 .  
         [0041]     Screw hole  58  defines cylindrical portion  78  on its inner aspect. Spherical countersink  68  defines circular bevel  80  where it meets surface  64 .  
         [0042]     Radius portion  32  is desirably machined, cast, molded or otherwise formed from a single piece of material. Radius portion  32  may be manufactured from implant grade 316L stainless steel or other biocompatible materials such as titanium. Biocompatible polymer or composite materials may be used as well.  
         [0043]     Radius portion  32  may be mirror polished over surface  64 , saddle retainer  54  and retaining ridge  56  and any other surfaces that do not make direct contact with the bone of the radius. The surfaces of stem  44 , collar  70 , stiffening rib  76 , fillet  52  and circular fillet  74  and any other surface that makes contact with the bone of the radius may be roughened to encourage osseointegration such as by the application of commercially pure titanium plasma coating.  
         [0044]     Saddle  34  presents articular portion  82  and securing portion  84 . Referring to  FIGS. 8-12 , articular portion  82  presents articular face  85  which includes concave portion  86  and convex portion  88 . Concave portion  86  and convex portion  88  are contoured so that articular face  85  substantially conforms to the shape of an ulnar head implant  38 . Articular portion  82 , desirably, has a radiused edge  90 . Perimeter  92  of saddle  34  substantially conforms to the shape of the perimeter of saddle plate  46 . The outline of saddle  36  is generally trapezoidal and includes rounded corners  94 .  
         [0045]     In an exemplary embodiment, concave portion  86  may be substantially spherical and have a radius of curvature of about 0.709 inches. Convex portion  88  may have a radius of curvature of about 0.5 inches. These values are exemplary and should not be considered limiting. Adjacent to convex portion  88  is sloped portion  96  which maybe sloped at about sevently degrees relative to the perimeter  92  of saddle  34 . Saddle  36  can be adjusted in size, thickness and shape to conform to the natural head of the radius or to various ulnar head implants  38 .  
         [0046]     Securing portion  84  of saddle  34  presents bottom face  98  surrounded by tongue  100 . Tongue along with perimeter  92  define groove  102 . Groove  102  extends substantially around perimeter  92  and is sized and structured to receive ridge  60  while tongue  102  fits into groove  62 . Tongue  100  partially surrounds the edge of bottom face  98  on three sides. Tongue  100  extends outwardly from bottom face  98  and defines adjacent groove  102  which also extends around three sides of bottom face  98 . Tongue  100  and groove  102  are dimensioned to mate with ridge  60  and groove  62  of saddle retainer  54 .  
         [0047]     Bottom face  98  is substantially planar and further presents recess  104 . Recess  104  may be substantially rectangular in shape and includes flat portion  106  and sloped portion  108 . Adjacent to recess  104  and beyond the edge of sloped portion  108  is retaining slot  110 . Recess  104  is dimensioned so that flat portion  106  can receive retaining ridge  56  therein when tongue  100  and groove  102  are aligned with ridge  60  and groove  62  and saddle  34  is slidably engaged with saddle retainer  54 . Retaining slot  110  is dimensioned to receive retaining ridge  56  therein when saddle  34  is slidably secured to radius portion  32 . Note that saddle plate  46  and/or saddle  34  resiliently flex to make the engagement between retaining slot  110  and retaining ridge  56   
         [0048]     Saddle  34  may be formed from ultra high molecular weight polyethylene or another self-lubricating material. Saddle  34  may also be from other polymers, composite or metallic material. It is generally believed that biocompatible metallic materials are preferred for articulation with the natural head of the ulna if the sigmoid notch implant  30  is used for hemiarthroplasty. It is specifically contemplated that saddle  34  may be joined to radius portion  32  by many other techniques as understood by those of ordinary skill in the art such as the use of screws, clamps or interference fit techniques.  
         [0049]     Bone screw  36  is a spherical head bone screw. Bone screw  36  includes spherical head  112  and shaft  114 . Shaft includes threaded portion  116  and unthreaded portion  118 . Bone screw  36  desirably includes flutes  20  to facilitate a self-threading design. Bone screw  36  may be manufactured to the standards of ISO 5835. Referring to  FIG. 5 , bone screw  36  may angulate in a conical fashion at an angle alpha within spherical countersink  68 . For example, bone screw  36  may articulate conically at a solid angle alpha of about thirty degrees.  
         [0050]     In operation, sigmoid notch implant  30  articulates with ulnar head implant  38  to restore stability and pain free natural motion to the distal radial ulnar joint. To implant sigmoid notch implant  30  the surgeon first exposes the distal radial ulnar joint. In this discussion of the implantation of sigmoid notch implant  30  it will be assumed that an ulnar head implant  38  having a metallic articular surface has already been implanted to replace the head of the ulna. It is specifically contemplated that sigmoid notch implant  30  may also be implanted to articulate with the natural head of the ulna as a hemiarthroplasty. If this aspect of the invention is practiced it is to be understood that saddle  34  may be formed of a metallic material or another biocompatible material appropriate to articulate with living bone. At this time, it is generally thought that for two surface arthroplasty a metal to polymer interface is preferred and for hemiarthroplasty a metallic to bone interface is preferred but these beliefs should not be considered to be limiting.  
         [0051]     Once the joint is exposed the surgeon removes the head portion  40  of the ulnar head implant  38  and sets it aside. First however, the surgeon measures from the distal face of the existing ulnar head implant  38  to determine the location for drilling a hole to accept the stem  44  of radius portion  32 . The surgeon should estimate the drilled depth required to accept stem  44 . The surgeon then drills a hole in the distal radius utilizing, for example, a 3.5-millimeter drill.  
         [0052]     Once the hole to receive stem  44  is drilled the surgeon will use a trial radius portion (not shown) which has an undersized trial stem to preserve a press fit for the sigmoid notch implant  30 . Once the trial implant is satisfactorily placed in the drilled hole the surgeon drills a pilot hole for the self-tapping bone screw  36  using an appropriately sized drill. The pilot hole is located so that the pilot hole is substantially centered in screw hole  58 . The pilot hole may be angled for optimal placement of bone screw  35 . The pilot hole may be angled as needed to avoid pre-existing implant hardware, or to assist in fracture fixation or to avoid fractured portions of the bone. If the pilot hole is drilled non-parallel to the hole to receive stem  44  axial pullout strength is increased.  
         [0053]     Once the pilot hole for bone screw  36  is made, the surgeon removed the trial radius portion and burrs down the sigmoid notch to provide a flat buttress for saddle plate  46 . The surgeon also burrs a small countersink to receive collar  70  and a space to receive stiffening rib  76 .  
         [0054]     The surgeon places radius portion  32  of sigmoid notch implant  30  so that stem  44  is in the drilled hole. The surgeon then impacts radius portion  32  until it is secured by press fit in the predrilled hole by stem  44  and flush against the flat buttress surface of the radius. If the hole is too small to receive stem  44 , the surgeon should consider removing radius portion  32  and redrilling to remove debris rather than applying excessive force to radius portion  32  in an effort to insert it.  
         [0055]     Once radius portion  32  is in place, bone screw  36  is inserted and tightened. It is important that bone screw  36  be tightened evenly and that saddle plate  46  be evenly supported against the radius to avoid bending saddle plate  46 . In addition, care should be taken to protect the polished surfaces of the ulnar head implant  38  and the radius portion  32  (for example by handling head portion  40  carefully). Any scratches on the polished surfaces of the ulnar head implant  38  may decrease the wear life of saddle  34 . Scratches on portions of the components that articulate with surrounding tissues may encourage inflammation.  
         [0056]     Referring to  FIGS. 18 and 19 , the surgeon slides saddle  34  (of UHMWPE or other polymer material) into radius portion  32  so that tongue  100  and groove  102  mate with groove  62  and ridge  60 . When the saddle  34  is about  75  percent engaged sloped portion  96  of saddle  34  will engage bevel  66  of retaining ridge  56 . If need be, the surgeon can stake an osteotome into the radius and pry against saddle  34  to overcome the resistance of retaining ridge  56  against sloped portion  108 . Once saddle  34  snaps into place in saddle retainer  54  implantation of the sigmoid notch implant  30  is complete.  
         [0057]     The surgeon then replaces the head portion  40  of ulnar head implant  38  and reduces the joint to assess range of motion. Assuming that range of motion and alignment is acceptable, the surgeon repairs the joint capsule and closes the skin.  
         [0058]     If the invention is practiced as a hemiarthroplasty, saddle may be formed of metallic material and be secured to radius portion  32  by another technique as discussed above. In a hemiarthroplasty, the head of the ulna will, of course remain intact.  
         [0059]     The present invention may be embodied in other specific forms without departing from the central attributes thereof, therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than the foregoing description to indicate the scope of the invention.