Abstract:
A joint prosthesis having an intramedullary insert. The intramedullary insert has an elongated body with an attached coil spring. The coil spring may be constructed to provide variable flexibility at different locations along the spring. A control rod is inserted through the coil spring and attached thereto further modifying the flexibility of the coil spring. The shape of the control rod may be varied throughout its length producing different flexibility. The selection of a particular coil spring and a particular control rod resulting in particular joint prosthesis.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to artificial joints and the structure by which they are secured in the natural bone of the host or patient. 
     BACKGROUND 
     The connection by which the artificial joint is held in the host bone is of critical importance to the long term success of any joint replacement surgery. The characteristics of the artificial joint must be matched with the characteristics of the naturally occurring host bone to prevent undue stress resulting in trauma to the natural bone and pain to the host. 
     Bone prosthesis are either secured in the host with bone cement or without the use of cement. While initially providing a useful connection between the prosthesis and the bone, the earlier cements tended to degrade over time with deleterious results. One approach to overcome this degradation problem was to not use cement at all. This invention relates to installing joint prosthesis without the use of bone cement. 
     When no cement is used, the connection or fit of the prosthesis within the intramedullary canal of the natural bone becomes more critical. Without the cushioning effect of cement surrounding the intramedullary insert of the prosthesis, there is a requirement for a more accurate matching of the prosthesis insert with the host bone. The prosthesis must, not only, conform to the size of the intramedullary canal of the host bone but to the inherent strength and flexibility or brittleness of the cancellous bone surrounding the intramedullary canal. 
     These different criteria can create mutually exclusive design requirements, for example, a large insert that must be very flexible because the characteristics of the host bone or a small insert that must be very stiff. 
     There are various patents and publications directed to the implantation of bone prosthesis without the use of cement each citing the attendant advantages of such procedures. Several patents are directed to the flexibility of joint prosthesis. 
     U. S. Pat. No. 5,549,702 to Ries et al discloses a one piece or two piece prosthesis in which one end or one portion of the prosthesis is inserted into the intramedullary canal of the host bone. The intramedullary insert of Ries et al has various modifications to achieve a specific flexibility. The insert may be a solid bar or it may be partially tubular. Among the modifications are spiral slots of varying depth. The slots may extend partially through the insert or may extend from a central bore through the entire thickness of the insert forming a coil structure. Other modifications include varying the thickness of the insert along the longitudinal axis. Once each of these variations is manufactured, the flexibility of the insert is permanently set. 
     U.S. Pat. No. 4,997,444 to Farling discloses a joint prosthesis in which the modulus of elasticity of the intramedullary insert may be varied along the length. The variation results from stacking disks having different characteristics upon a central strut or struts. Farling states that the disks and struts are connected together mechanically or metallurgically bonded together. This indicates that these devices are manufactured with various degrees of stiffness throughout their length and the resulting device cannot or should not be changed thereafter. 
     U.S. Pat. No. 5,062,851 to Branemark discloses a joint mechanism to be used in smaller bones. The tubular anchoring element has external screw threads for bone contact and a tapering wall thickness. The end portions of the tubular element have longitudinal slits and the wall thickness decreases toward the slit portion. The combination of the slits and thinner walls produce a more flexible end portion of the device. 
     These references are merely illustrative of the prior art and are not to be considered to be an exhaustive listing. 
     SUMMARY OF THE INVENTION 
     The instant invention is a joint prosthesis having an intramedullary insert. The intramedullary insert has an elongated body with an attached or integrated spring like helical construct having a continuous spiraling recess, that is, a coil spring like endpiece. For ease of explanation, the helical construct with recess will be referred to as a coil spring throughout this disclosure. The coil spring provides variable flexibility at different locations along the spring. A control rod is inserted through the coil spring and attached thereto further allows for the flexibility of the coil spring. The shape of the control rod may be varied throughout its length producing different flexibility. The instant invention allows the surgeon to match the stiffness of a particular prosthesis to the strength of the host bone by selecting certain combinations of interchangeable elements of the artificial joint. These elements are then assembled forming an artificial joint having the desired features for use in a particular host. The selection of a particular coil spring and a particular control rod allows joint prosthesis having varying flexibility. 
     Accordingly, it is an objective of the instant invention to provide a joint prosthesis which can be matched to the natural bone of a particular host. 
     It is a further objective of the instant invention to provide a joint prosthesis with an intramedullary insert which has a coil structure of particular flexibility. The flexibility results from the properties of the coil, such as the material from which the component is made, the helical slot width, helical slot pitch, cross sectional area of the coil, including outside diameter, inside diameter and width of the coil. 
     It is yet another objective of the instant invention to provide a control rod inserted through the coil which varies the flex through the shape of the rod. This also allows for differing amounts of flexibility at different points along the length of the intramedullary insert. 
     It is a still further objective of the invention to provide a plurality of prosthesis elements having differing structural characteristics in a kit. This allows the surgeon to custom build a joint prosthesis to fit the needs of the patient. 
     While the invention is directed to the major joints of the body, such as the hip, shoulder, elbow and knee, the same principles can be applied to artificial joints between the smaller bones. 
     Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a plan view of a joint prosthesis of this invention; 
     FIG. 2 shows a plan view of a modification of this invention partially in cross section; 
     FIG. 3 shows plan view of another modification of this invention partially in cross section; 
     FIG. 4 shows a plan view of another modification of this inanition partially in cross section; 
     FIG. 5 shows a plan view of another modification of this invention; 
     FIG. 6 shows a plan view of another modification of this invention; and 
     FIG. 7 shows a plan view of a control rod of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the invention will be described in terms of a specific embodiment, it will be readily apparent to those skilled in this art that various modifications, rearrangements and substitutions can be made without departing from the spirit of the invention. The scope of the invention is defined by the claims appended hereto. 
     The joint prosthesis  10  shown in FIG. 1 is directed to an artificial hip replacement. The joint replacement, in this case, a ball joint (not shown) is connected to the fitting  11  which forms one end of the elongated body  12 . The elongated body  12  is generally formed in an approximate shape of the particular bone portion that it is replacing, for example either end of the humerus, tibia, fibula, femur, radius, ulna, or smaller bones, to minimize the displacement of the surrounding soft tissues. The lower portion of the elongated body and the coil spring are inserted into the intamedullary canal of the host bone. The prosthesis  10  can be made of most any biocompatible material, including the conventional materials such as titanium, stainless steel, cobalt-chromium steel and composites. 
     It is most important that the intramedullary insert portion of the prosthesis accommodate, as closely as possible, the size of the intramedullary canal and the strength of the cancellous bone in the shaft of the host bone into which it is inserted. There are many structural variables in the naturally occurring bone, for example, thick or thin areas of the cancellous bone as well as conditions, such as pre-existing traumas or osteoporosis, that require the selection of a particular combination of prosthesis elements. 
     The other end of the elongated body  12  is formed in such as manner so as to resemble a coil spring. The flexibility and resiliency of the coil spring is partially determined by the materials from which it is made. The cross sectional area of the coils  13  can be adjusted to modify the flexibility and resilience of the coil spring. The cross sectional area of the coils is determined by the width of the coil  13 , the width of the helical slot  14  and the pitch of the helical slot  14 . The cross sectional area may also be changed by adjusting the outside diameter and inside diameter of the coil to change the thickness of the coil. The cross sectional area of the coils can be varied along the length of the coil spring to produce different flexibility in different areas, such as shown in FIG.  6 . The coil spring could be made in sections (not shown) with differing cross sectional areas and the sections could be assembled end to end to produce a coil spring with different flexibilities along its length. 
     Control rod  15  is inserted through the coil spring and connected to the other end of the elongated body  12 . The control rod  15  provides support and also varies the amount of flexibility along the length of the coil spring. The other end  16  of the control rod is shaped to conform with the other end of the coil spring and forms and end wall. When the control rod  15  is connected to the elongated body  12 , the end wall contacts the end of the coil spring and an adjustable amount of compression may be transferred longitudinally to the coil spring. This compression will directly affect the helical slot width. 
     As shown in FIG. 1, the control rod  15  and the coil spring are secured together to prevent longitudinal separation by a locking ring  24 ′. The locking ring  24 ′ is in the form of an annular ridge on the interior wall of the coil spring which resiliently fits into an annular depression  24  in the control rod  15 . As the control rod  15  is connected to the elongated body, the cooperating surfaces of the locking ring  24 ′ and  24  engage which prevents the control rod and elongated body from becoming disconnected after assembly. 
     The control rod  15 , as shown in FIG.  2  and FIG. 7, may be shaped with different diameters in different longitudinal areas to further modify the flexibility of the intramedullary insert. Such a configuration of the control rod could be in addition to or in lieu of the segmented coil spring construction mentioned above. The particular shape of the control rod used in a particular procedure will be determined by the conditions present along the shaft of the host bone. 
     In FIG. 2, the other end of the elongated body has a blind bore  19  extending along the axis of the bore of the coil spring. The blind bore  19  has internal screw threads  18  which cooperate with the screw threads  21  on the end of control rod  15 . In this modification, the control rod is shown with end portions having a larger diameter than the center portion. In this instance, the blind bore  19  is enlarged at  17  to accommodate the larger diameter portion  20  of the control rod. The blind bore is shown with an internal shoulder engaging the control rod however, the bore could be of uniform diameter and the end of the control rod could contact the end of the blind bore. 
     The other end of the coil has a fitting for accepting a locking device to hold the coil and the control rod together. A through bore with internal screw threads  22  is shown. When assembled, the inner end of the through bore  22  is located adjacent the annular grove  24  in the control rod  15 . A locking screw  23  is threaded through the bore into the annular grove  24  fixing the coil and rod together. Obviously, other conventional locking devices, such as, pins and adhesives could be used. 
     The other end of the control rod  15  is shown with a fitting  25  to accommodate a tool for assembly of the elements. The particular fitting is not critical but merely has to cooperate with the chosen tools. 
     The length of the elongated body may be adjusted to more closely comply with the host bone. The condition of the host bone may dictate a longer or shorter portion of the host bone be removed and in some instances it may be desired to insert more or less of the elongated body into the intramedullary canal. In FIG. 3 a modified elongated body is illustrated in which the overall length of the elongated may be changed by the use of an insert  26 . of course to change the overall length, the elongated body may be used without the insert, as shown in FIG. 2, or it may be used with inserts of different lengths. In FIG. 3, the lower portion of the elongated body has an insert  26  fitted between it and the coil spring. The insert  26  has a through bore to accommodate the passage of the control rod  15 . One end of the through bore has internal screw threads  29  which cooperate with screw threads  21  on control rod  15 . The end of control rod  15  extends through the insert  26  and engages the internal screw threads  30  in the blind bore  19 . 
     The control rod  15  is shown with a larger diameter portion  20 , in FIG. 3, and a reduced diameter end carrying screw threads  21 . The transition between these diameters forms a external shoulder on the control rod. The insert  26  is formed with a similar internal shoulder between the large diameter smooth through bore and the smaller diameter screw threaded bore. This modification allows the rotational orientation of the insert to be fixed during assembly. However, the through bore may be of a larger diameter throughout its length. 
     The other end of the insert  26  has a friction fitting in the form of a tapered annular ring  28 . The tapered annular ring  28  fits into a tapered annular cup  27  on the end of the coil spring. The cup  27  and the ring  28  form a friction fit between the insert and the coil spring. Obviously, the friction elements may be reversed. 
     FIG. 4 shows another modification of the prostheses  31 . One end of the intramedullary insert carries a friction fitting in the form of a tapered ring  32  for engaging a cooperating fitting on the other end of the elongated body  12  or an insert  26 . The control rod  34  has one end  36  which terminates in and conforms to the blind bore  35  formed in the end of the coil spring  33 . The other end of the control rod forms the end wall  37  for the coil spring  33 . 
     FIG. 5 shows another modification of the prosthesis in which the flexibility of the coil spring is varied about its circumference. The intramedullary insert  41  of FIG. 5 has longitudinal grooves formed in the coil spring. These grooves result from different thickness of the coils in certain areas about their circumference. The individual coils of the coil spring have thick portions  43  and thin portions  44 . The end of the coil spring is formed with a friction fitting  42  for attachment to the elongated body or to an adapter. The other end of the control rod forms the end wall  45  for the intramedullary insert. 
     Another modification for changing the flexibility of the intramedullary insert throughout its length is shown in FIG.  6 . The coil spring  51  is longitudinally tapered. The coils  54  have decreasing diameter from one end to the other end. The other end has a locking element  52  and a smooth end wall for engaging the control rod  15 . 
     A specific modification of the control rod is shown in FIG.  7 . One end of the control rod has screw threads  64  for engaging an elongated body or an insert or both. The other end of the control rod  61  has an end wall  62 . The end wall is formed with a fitting  63  therein for use in assembly of the prosthesis. Along the length of the control rod are portions having differing diameters. One end has a larger diameter  65 , the other end has a larger diameter which may be the same or different than the diameter  65 . The central portion has a smaller diameter  66 . The control rod shown in FIG. 7 would clearly be less flexible at the end portions than in the middle. The control rod  61  can also be modified to have the larger diameter in the middle and smaller diameters in the end portions. The use of a tapered control rod is also contemplated wherein the larger diameter may be at either end portion and the smaller diameter at the opposite end. 
     While the different control rod shapes have been discussed in relation to varying the flexibility of the intramedullary insert, it is clear that structural requirements of the intramedullary canal may dictate a particular control rod. 
     It is to be understood that while a certain form of the invention has been illustrated, it is not to be limited to the specific form or arrangement of parts herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is described in the specification and drawings.