Abstract:
A prosthesis for the radius bone in the arm of a patient includes a head shaped for engagement with the capitellum of a humerus bone and the radial notch of the ulna. It also has a curved stem that extends from the head and is tapered with a distally decreasing cross section for insertion into the medullary canal of the radius. A system for implanting the prosthesis requires a resection guide for placing the forearm of the patient in a precise anatomical orientation for resection of the radius bone. The resection guide includes a notched alignment rod attached to an adjustable flange and a reference flange for aligning the forearm for a cut of the radius bone perpendicular with the axis of rotation of the radius bone about the ulna, and for measuring the size of the resection and prosthesis.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to prostheses. More specifically, the present invention pertains to prostheses which act as part of an elbow joint in a patient for motions which mimic anatomical movements. The present invention is particularly, but not exclusively, useful as a prosthesis for the radial head in the elbow joint of a patient. 
     BACKGROUND OF THE INVENTION 
     Like other joints and anatomical features of the human body, the elbow joint is exceedingly complex in its make-up and function. Also like the other joints and anatomical features of the human body, the elbow joint is unique unto itself and requires specific consideration for its reconstruction or replacement. The complexity and uniqueness of this joint are quite interesting and are, perhaps, best appreciated by considering the skeletal motions which are involved in its movement. 
     In the transition of the hand and forearm from pronation to supination the radius and ulna of the forearm transition from a crossed relationship to a side-by-side relationship. Thus, in this movement there is a relative rotation of the radius bone about the ulna. Also, but more subtly, during the transition between pronation and supination there is also some relative translational movement between the radius bone and the ulna. The consequence of all this is that from a reference point on the ulna, the radius bone appears to move with a general motion that includes both translation and rotation. The head of the radius interacts with the capitellum and the radial notch of the ulna during pronation and supination, providing elbow and forearm stability during rotation and translation. 
     In addition to its importance as a component of forearm function, the radial head is an equally important component of normal elbow function. Indeed, elbow function involves bending, lifting and twisting movements, all of which require joint stability. Because motions in the human body require the interaction of various anatomical components, it is crucial that replacement of a component be precise in form, size, and orientation. While the head of the radius bone directly engages the capitellum of the humerus and the radial notch of the ulna, it also relates indirectly to other anatomical components of the arm. Specifically, ligaments surrounding the radial head are essential to elbow stability. Further, misalignment of the radius bone will cause poor radial-capitellar joint contact, leading to subluxation, or poor alignment of the elbow. It follows that the wrist and shoulder joints are also affected by the alignment of the radius bone. 
     The importance of having a workable prosthesis for the head of the radius bone is underscored by the debilitating effects which commonly result when a joint becomes damaged due to fracture, arthritis, or osteochondrosis. It is well known that radial head resection, as seen in elbow injuries, results in persistent elbow instability. Additionally, forearm axial instability can result from radial head excision if the remaining stabilizers, the supporting ligaments, are compromised. Because this loss of stability affects the interdependent functions of the elbow and forearm, when the radial head is damaged, it is common to see further damage to other components of the radial ulnar joint system, including, but not limited to, the complex system of supporting ligaments that encase the elbow joint. It has been well demonstrated that damage of any one of the components of the radial ulnar joint system leads to pain, weakness, and loss of motion. It is, therefore, of great importance to the patient that damage to the radial head be remedied. 
     In light of the above, it is an object of the present invention to provide a prosthesis for engaging a radius bone in the forearm of a patient with the capitellum of a humerus bone and the radial notch of the ulna at the elbow of a patient. It is a further object of the present invention to provide a prosthesis that is shaped to establish a secure engagement with the radius bone and is capable of cooperating with the anatomic structure and function of the portion of the radius being replaced. Yet another object of the present invention is to provide a system for implanting the prosthesis which places the forearm in the precise anatomical orientation necessary for implantation of the prosthesis. It is also an object of the present invention to precisely measure the length of the radial bone to be resected, and to precisely measure the size of the prosthesis to be implanted in place of the resected bone. Another object of the present invention to provide a prosthesis that is relatively easy to manufacture, is simple to implant, and is comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, a prosthesis for replacing the head of a radius bone in the elbow of a patient includes a head and a curved stem which extends therefrom. Specifically, the head is shaped for simultaneous articulation with both the capitellum of the humerus bone and with the radial notch of the ulna. Also, the stem is curved, or arcuate shaped, in order to accommodate the anatomy of the proximal radius after resection of the radial head, and to securely anchor the prosthesis when the stem is inserted into the medullary canal of the radius bone. 
     In detail, the head of the prosthesis of the present invention has a proximal surface that is formed as a substantially concave recess, and it has a distal surface that is formed with a hole. Preferably, the proximal surface of the head is made of highly polished cobalt chrome or a ceramic material to facilitate articulation of the head with the capitellum and with the radial notch of the ulna bone. More specifically, as envisioned for the present invention this articulation involves a relative sliding motion between the head of the prosthesis and both the capitellum and the radial notch of the ulna. 
     The stem of the prosthesis has a proximal end and a distal end. Also, the stem is preferably made of a material such as cobalt chrome or titanium. Further, it has an extension at its proximal end that is slightly tapered with a proximally decreasing cross section to facilitate insertion of the extension into the hole in the distal surface of the head. Preferably, the extension is configured as a morse taper. In any event, as intended for the present invention, the proximally decreasing cross section of the extension establishes an interference fit between the extension of the stem and the head that helps to hold the head on the stem. 
     As indicated above, the stem of the prosthesis is curved in shape and is generally arcuate. Importantly, the configuration of the stem is intended to structurally mimic the shape of the medullary canal of the radius bone. In general, to accomplish this the stem is formed with a first portion and a second portion. Specifically, the first portion is attached to the head and extends distally therefrom substantially along a first axis. The second portion then extends distally from the first portion substantially along a second axis. In their relation to each other, the first axis and the second axis define an angle α therebetween. Preferably, the angle α will be in a range of about 5 degrees to approximately 25 degrees. The curve of the stem that is established by the angle α can be achieved using a radius of curvature that is in a range of about 0.5 inch to approximately 3 inches. Further, as it extends distally along the first and second axes, the stem is tapered with a distally decreasing cross section. This facilitates insertion of the stem into the medullary canal of the radius bone and helps to anchor the prosthesis on the radius bone. The stem also has a roughened or textured surface, which interacts with the medullary canal of the radius bone to help hold the stem on the radius. For purposes of the present invention, the surface of the stem can be coated with a material such as titanium, cobalt-chrome beads or hydroxyapatite. In addition to these structural features, the stem also has a collar that is formed on the first portion of the stem. As intended for the present invention, this collar is positioned to limit insertion of the stem into the medullary canal. 
     In order to implant the prosthesis of the present invention into the radius bone of a patient, it is first necessary to do a resection of the radius by removing the radial head and exposing the medullary canal. When doing this, it is important that the prosthesis have the same functional dimensions as the amount of bone that is to be removed. For the present invention, the required accuracy for this exchange is achieved by using a resection guide. 
     In accordance with the present invention, the resection guide includes an alignment rod which has a series of notches on its surface near the proximal end. The resection guide also includes an adjustable flange and a reference flange. Specifically, the reference flange is pre-positioned at a fixed location between the distal and proximal ends of the alignment rod. When the adjustable flange is engaged with the alignment rod, it can be selectively fixed at one of the notches on the alignment rod. 
     As intended for the present invention, the notches on the alignment rod are located relative to the reference flange so that, when the adjustable flange is affixed to the alignment rod, the distance between the adjustable flange and the reference flange will correspond with a particular head size of the prosthesis. Thus, upon the engagement of the adjustable flange with a notch, the size of the head of the prosthesis can be determined. 
     For the operation of the resection guide, after an incision has been made to establish access into the elbow of the patient, the adjustable flange is engaged with the proximal end of the alignment rod. The adjustable flange is then positioned against the articular surface of the capitellum. Also, the distal end of the alignment rod is positioned over the ulna styloid process. When so positioned, the alignment rod establishes the axis of rotation of the radius about the ulna. Stated differently, the adjustable flange is oriented to align the alignment rod with the center of rotation of the capitellum and with the styloid of the ulna. Next, once the alignment rod is properly positioned between the capitellum and the styloid of the ulna, the alignment rod and the reference flange that is attached to the alignment rod are guided proximally or distally relative to the adjustable flange until the adjustable flange is located at a notch on the alignment rod and the reference flange is positioned at a desired location on the radius. At this point, the reference flange is positioned to establish a distance between the adjustable flange and the reference flange that is equal to a desired length for radial head resection. 
     As the alignment rod is guided to position the reference flange at a desired location on the radius for resection, the distal end of the alignment rod is simultaneously positioned at the styloid process of the ulna. Thus, the forearm of the patient is positioned for an osteotomy in a plane that is substantially perpendicular to the axis of rotation of the radius bone about the ulna. In this position, the reference flange can be used as a guide to establish a cutting line for the resection. Specifically, the reference flange will guide the cutting of the radius in a plane that will be perpendicular to the axis of rotation of the radius bone about the ulna. The cut thus made to the radius bone will facilitate insertion of the curved stem of the prosthesis into the medullary canal of the radius bone. 
     Once the radial head has been removed, it can be replaced by the prosthesis. Specifically, this is done by inserting the stem of the prosthesis into the medullary canal and positioning the head of the prosthesis on the stem for articulation with the capitellum and the ulnar notch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
     FIG. 1 is a perspective representation of the prosthesis; 
     FIG. 2 is a perspective view of the head of the prosthesis; 
     FIG. 3A is an exploded elevation view of the head of the prosthesis and the stem of the prosthesis positioned for insertion into the medullary canal of a radius bone; 
     FIG. 3B is an exploded side elevation view showing the stem of the prosthesis inserted into the medullary canal of a radius bone; 
     FIG. 3C is a side elevation view of the prosthesis after implantation into the radius bone; 
     FIG. 4 is an exploded perspective view of a portion of the resection guide of the present invention; and 
     FIG. 5 is a perspective view of the resection guide of the present invention in position on the forearm of a patient for performing a resection of the radial head. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A prosthesis in accordance with the present invention is shown in FIG.  1  and is generally designated  10 . As shown, the prosthesis  10  essentially includes a head  12  and a curved stem  14 . For the purposes of the present invention, the curved stem  14  is made of a material such as cobalt chrome or titanium. Further, the prosthesis  10  includes a collar  16  that is integral with the stem  14 . Preferably, both the surface  18  of the stem  14  and the surface  20  of collar  16  are roughened, or textured. For the purpose of this invention the surface  20  of the stem  14  and collar  16  can be coated with a material such as titanium, cobalt-chrome beads, or hydroxyapatite. For purposes of this disclosure, and specifically for relative directional references between components of the present invention, the head  12  is considered to be proximal to the stem  14 . Accordingly, the stem  14  is considered to be distal to the head  12 . 
     In FIG. 2 it can be seen that the head  12  is formed with a recess  22 , and that the recess  22  is generally concave in shape. For the purposes of the present invention the head  12 , and particularly the proximal surface  24  of recess  22 , is made of a highly polished cobalt chrome or a ceramic material. In FIG. 3A, it can be seen that the head  12  is also formed with a distal hole (i.e. bore)  26  that is on the opposite side of the head  12  from the recess  22 . 
     Still referring to FIG. 3A, the structure of stem  14  for the present invention is seen to generally include two distinct portions. A first (proximal) portion  28  is substantially aligned along an axis  30 . Also there is a second (distal) portion  32  that is substantially aligned along an axis  34 . An angle, α, is formed between the axis  30  and the axis  34  which gives the stem  14  a curved or arcuate appearance. For an alternate description of the stem  14 , it can be noted that the second portion  32  of the stem  14  is bent relative to the first portion  28 . Specifically, as shown in FIG. 3A, at the transition between the first portion  28  and the second portion  32 , there is an effective radius of curvature, R, which will establish the particular configuration for the stem  14 . 
     In addition to the curved or arcuate configuration for the stem  14 , it is to be appreciated that the stem  14  is tapered with a distally decreasing cross section. Further, as shown in FIG. 3A, the stem  14  includes an extension  40  that is actually the part of first portion  28  that extends proximally from the collar  16 . As intended for the present invention, the extension  40  can be slightly tapered with a proximally decreasing cross section. The purpose of the tapered extension  40  is to facilitate the engagement of the stem  14  with the head  12 . 
     Regardless whether the curved or arcuate configuration for the stem  14  is described in terms of the angle “α,” or the radius of curvature “R,” and regardless of the amount of taper provided for the stem  14  or the extension  40 , it is important that the resulting configuration for the stem  14  effectively conform to the medullary canal  36  of the radius bone  38 . For the present invention, this means that the angle α will be generally in a range from about five degrees to approximately twenty-five degrees (α=between 5° and 25°). The curve of the stem  14  that is established by the angle α can be achieved using a radius of curvature that is in a range of about 0.5 inch to approximately 3 inches. 
     The actual engagement of the prosthesis  10  with a radius bone  38  will, perhaps, be best appreciated by collectively considering FIGS. 3A,  3 B and  3 C. When doing this it is to be appreciated that the radius bone  38  has previously been resectioned. Further, it will be noted that the configuration for stem  14  is chosen to establish an interference, or friction fit with the radius bone  38 . Preferably, this can be done without using cement. If necessary, however, the present invention envisions that a cement of a type well known in the pertinent art can be used to anchor the stem  14  in the medulilary canal  36  of the radius bone  38 . Once the stem  14  has been engaged with the radius bone  38  (FIG.  3 B), the head  12  can then be engaged with the stem  14  (FIG.  3 C). Specifically, engagement of the head  12  onto the stem  14  is done by simply inserting the extension  40  of the stem  14  into the hole  26  of the head  12 . The result is that the head  12  will extend from the radius bone  38  through a distance  42 . 
     It is an important aspect of the present invention that the distance  42  through which the head  12  extends from the radius bone  38  be anatomically correct. To ensure this, the present invention envisions the use of a resection guide such as the one shown in FIG.  4  and generally designated  44 . As shown, the resection guide  44  includes an alignment rod  46  which is formed with a plurality of notches  48  near its proximal end  60  (the specific notches  48  a-c are only exemplary). The resection guide  44  also includes an adjustable flange  50  that is formed with a slot  52  and it includes a reference flange  54  that is pre-positioned at a fixed location between the notches  48  and the distal end  68  of the alignment rod  46 . Further, when a notch  48  on the alignment rod  46  is inserted through the slot  52  of the adjustable flange  50 , the nut  56  can be engaged with the thread  58  at the proximal end  60  of the alignment rod  46  to fixedly hold the adjustable flange  50  to the alignment rod  46 . As intended for the present invention, the notches  48  are located relative to the reference flange  54  so that, when the adjustable flange  50  is affixed to the alignment rod  46 , the distance between the adjustable flange  50  and the reference flange  54  will correspond with a particular head  12  size of the prosthesis  10 . Thus, upon the engagement of the adjustable flange  50  with a notch  48 , the size of the head  12  of the prosthesis  10  can be determined. 
     For the operation of the resection guide  44 , the resection guide  44  is initially assembled as described above. After an incision  62  has been made to establish access into the elbow  64  of a patient, the adjustable flange  50 , is positioned against the capitellum  66 . The alignment rod  46  with the reference flange  54  attached, is then engaged with the adjustable flange  50 . Next, the alignment rod  46  and attached reference flange  54  are guided proximally or distally relative to the adjustable flange  50  which is located at a notch  48  on the alignment rod  46  until the reference flange  54  is positioned at a desired location on the radius  38 . At this point, the reference flange  54  is positioned to establish a distance between the adjustable flange  50  and the reference flange  54  that is equal to a desired length for a radial head resection. 
     While the alignment rod  46  and attached reference flange  54  is guided to position the reference flange  54  at a desired location on the radius  38  for resection, the distal end  68  of the alignment rod  46  is positioned over the ulna styloid process  70 . Thus, the forearm  69  of the patient is positioned for an osteotomy in a plane that is perpendicular to the axis of rotation of the radius bone  38  about the ulna  72 . In this position, the reference flange  54  can be used as a guide to establish a cutting line for the resection. Specifically, close to the position of the resection guide  44 , the reference flange  54  will guide the cutting of the radius  38  in a plane that will be perpendicular to the axis of rotation of the radius bone  38  about the ulna  72 . 
     As intended for the present invention, the spacing between the adjustable flange  50  and the reference flange  54  will determine the distance  44  for the prosthesis  10 . Stated differently, when resection of the radius bone  38  is accomplished at the reference flange  54 , the amount of radius bone  38  that is removed will be precisely known. Importantly, this removed bone must be compatible with the prosthesis  10  that is to be implanted. Furthermore, the cut on the radius bone  38  has been made so that the curved stem  14  of the prosthesis  10  can then be engaged with radius bone  38  as disclosed above with reference to FIGS. 3A,  3 B and  3 C. 
     While the particular Radial Implant System as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.