Abstract:
The present invention is directed to a modular shoulder prosthesis measuring device having an adjustable radial offset provided by relative rotation of an adapter interdisposed between the stem and the head. Specifically, the interface configuration between the stem and the adapter, as well as between the adapter and the head are designed such that relative positioning of these components provides a continuous adjustment in the radial offset. Indicia are provided to precisely determine the magnitude and direction of the adjustment being made.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a prosthesis for replacing and reconstructing a portion of the humerus and more specifically to a trialing system for a modular humeral prosthesis which allows for shoulder joint replacement. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a prosthesis for replacing and reconstructing a portion of the humerus and more specifically to a modular humeral prosthesis which allows for shoulder joint replacement. 
     The shoulder joint is considered to be one of the most complex joints in the body. The scapula, the clavicle and the humerus all meet at the shoulder joint. The head of the humerus fits into a shallow socket of the scapula called the glenoid fossa to form a mobile joint. When the joint is articulated, the humeral head moves in the glenoid fossa to provide a wide range of motion. The shoulder joint may suffer from various maladies including rheumatoid arthritis, osteoarthritis, rotator cuff arthropathy, avascular necrosis, bone fracture or failure of previous joint implants. If severe joint damage occurs and no other means of treatment is found to be effective, then shoulder reconstruction may be necessary. 
     A shoulder joint prosthesis generally includes the replacement of the ball of the humerus and, optionally, the socket of the shoulder blade with specially designed artificial components. The bio-kinematics, and thus the range of motion in the shoulder vary greatly among prospective patients for reconstruction shoulder surgery. The humeral component typically has a metal shaft or stem with a body portion that is embedded in the resected humerus and a generally hemispherical head portion supported on the stem. The head slidingly engages a glenoid implant on the glenoid fossa. During reconstructive surgery, the components of the prosthesis are matched with the bio-kinematics of the patient in an effort to maintain the natural range of motion of a healthy shoulder joint. Thus, a shoulder prosthesis design must be readily adaptable to a wide range of bio-kinematics for prospective patients. 
     In this regard, shoulder prostheses are generally available as either unitary structures or modular components. With unitary shoulder prosthesis, a large inventory of differently sized prostheses must be maintained to accommodate the different bone sizes and joint configurations of the prospective patients. With such unitary shoulder prosthesis, the patient is typically evaluated by x-ray to determine the approximate prostheses size needed for reconstruction. A number of differently sized prostheses are selected as possible candidates based upon this preliminary evaluation. Final selection of the appropriately sized prosthesis is made during the surgery. With unitary shoulder prosthesis, each design may represent a compromise that is unable to achieve all of the natural range of motion of a healthy shoulder joint because of the fixed geometric configuration in their design. 
     Modular prostheses systems which reduce the need to maintain large inventories of various sized components are well known in the art. Conventionally, the humeral prosthesis includes two components—a humeral stem component and a spherical head releasably coupled to the stem. Alternately, a three component design is known in which the stem and shoulder are interconnected with an adapter. In either of the two-piece or three-piece designs, a radial offset or angulator inclination of the head relative to the stem is provided in individual components. Different radial offsets or angular inclinations are achieved through the use of different adapters or heads. In this regard, conventional modular shoulder prosthesis kits include multiple redundant components such as adapters and heads to achieve a range of prosthetic options. 
     While providing an advantage over the unitary design in reducing the number of components needed, a rather large inventory of head components and/or adapter components must be maintained to provide the desired range of geometric configurations with the conventional modular shoulder prostheses. These components are readily adaptable to provide a range of geometric configurations, i.e. radial offsets of angular inclination while minimizing the number of components required. There is, therefore, a need for a trialing system and method for determining which of these components are needed and their specific orientation. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention a modular joint prosthesis system is provided. Specifically, a humeral component for a shoulder prosthesis includes an adapter and a head component which cooperate to provide a range of radial offsets and/or angular inclinations and which are adapted to be used in conjunction with a stem. 
     In a first embodiment, a measuring instrument for humeral component for a shoulder prosthesis is provided for determining the needed adjustable radial offset of the head with respect to the stem. The present invention includes an adapter interposed between a stem and a head. The adapter is slidably coupled to the head such that relative linear positioning of the adapter on the head will effect a first adjustment in the radial offset. Likewise, the adapter component is rotationally coupled to the stem as such that relative angular position of the adapter will effect a rotational offset adjustment. By selectively positioning the adapter with respect to the head, an infinite adjustment of the radial offset within a given range may be achieved. Indicia are provided at the interface between the adapter and the head to indicate the offset vector (i.e., offset amount and direction). 
     In a second embodiment, a measuring instrument for a humeral component for a shoulder prosthesis is provided for determining the adjustable radial offset of the head with respect to the stem. The present invention includes an adapter interposed between a stem and a head. The adapter is slidably coupled to a cavity formed in the head such that relative linear positioning of the adapter on the head will effect a first adjustment in the radial offset of the head. Likewise, the adapter component is rotationally coupled to the stem as such that relative angular position of the stem on the adapter will effect a rotational offset adjustment. A fastener is provided to fix the location of the head to the adapter. As presently preferred, indicia are provided on the adapter and the head to indicate the offset vector. 
     The joint prosthesis measurement system of the present invention provides great flexibility in the adjustment of important bio-kinematic parameters for the prosthesis systems while allowing for the minimizing the number of components required for the modular system. These and other features of the present invention will become apparent from the description and especially taken in conjunction with the accompanying drawings illustrating the invention and the preferred embodiment. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is an exploded front view of a modular shoulder prosthesis measurement system in accordance with the present invention; 
         FIG. 2  is a perspective view of the adapter and head components of the device illustrated in  FIG. 1  shown in an assembled state; 
         FIG. 3  is a bottom view of the embodiment of the present invention illustrated in  FIG. 1 ; 
         FIGS. 4A-4C  are views of the adapter shown in  FIG. 1 ; 
         FIGS. 5A-5D  are views of the head shown in  FIG. 1 ; 
         FIG. 6  represents the implantation of the measurement head into a stem component; 
         FIG. 7  is cross-sectional view of the trial head coupled to an implanted stem; 
         FIG. 8  is cross-sectional view of the trial head coupled to an implanted stem and being positioned into a glenoid; 
         FIGS. 9 and 10  represent the adjustment of the head with respect to the adapter; and 
         FIG. 11  represents a kit of components. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
       FIG. 1  is an exploded front view of a modular shoulder prosthesis measurement system according to the teachings of the present invention. The measuring device  10  is formed of an adapter  12 , a head  14 , and a coupling member  16 . The adapter  12  is preferably formed of a polymer material, which allow its relative rotation with respect to a fixation member or stem  18 . The measuring device  10  is configured to determine both the needed radial offset of an implant head with respect to an implanted fixation member, and also the rotational offset of the head with respect to the fixation member. As further described below, the adapter  12  is slidably coupled to the head  14  such that relative linear positioning of the adapter  12  with the head  14  will affect a first adjustment in the radial offset. Selected positioning of the adapter  12  with respect to the head  14  gives an infinite adjustment of the radial offset within a given range. 
     Referring generally to  FIG. 1 ,  FIGS. 4A-4C  and  FIG. 6 , the adapter  12  has a body portion  24 , having a first pair of bearing surfaces  26  and  28 . The first pair of bearing surfaces  26  and  28  are slidably coupled to a second pair of bearing surfaces  30  and  32  defined on the head  14 . The body portion  24  further has a flat stop surface  34  and a circular stop surface  36  which function to limit the movement of the adapter  12  with respect to the head  14 . The adapter  12  further defines a coupling member accepting bore  38  which is optionally threaded. A tapered coupling portion  40  is configured to interface with a Morse taper coupling feature on the stem  18 . This tapered coupling portion  40 , while shown as a male taper, may optionally be a female taper configured to interface with a male Morse taper formed on the stem  18  or any other connection member. 
     As shown in  FIGS. 2 and 3 , the bottom surface  34  of the adapter  12  and a bottom surface  22  of the head  14  each have indicia  46  and  48  which indicate the relative positioning of the head  14  with respect to the adapter  12 . Additionally, the outer spherical surface  20  has the rotational indicia  43  which is used to determine the relative rotation of the head  14  with respect to the stem  18 . 
       FIGS. 5A-5D  represent the head  14  shown in  FIG. 1 . Defined on the bottom surface  22  is an adapter accepting cavity  50 . The cavity  50  has the second pair of bearing surfaces  30  and  32 . Additionally, the cavity  50  has flat and curved bearing surfaces  52  and  55  which are configured to interface with the flat and circular bearing surfaces  34  and  36  of the adapter. 
     The head  14  further defines a through bore  54 . The through bore  54  passes through the outer spherical surface  20  and the adapter accepting cavity  50 . The through bore  54  has a defined shelf  56  which is configured to support a head portion  57  of the coupling member  16 . The through bore  54  further has a slot portion  58  and a circular portion  60  which facilitate transverse movement of the coupling member  16  within the through bore  54 . As the cavity  50  has a length L 1  which is longer than the length L 2  of the adapter  12 , the adapter  12  is configured to move transversely within the head  14 . The difference in L 1  and L 2  is the distance of the linear offset of the system. The first pair of bearing surfaces  26  and  28  and second pair of bearing surfaces  30  and  32  are configured so as to prevent relative rotational movement between the adapter  12  and the head  14 . 
       FIGS. 6-8  show views of the relationship of the measuring device  10  in its environmental surroundings. The taper portion  40  of the adapter  12  is positioned within the taper  42  of the stem  18 . Coupling member  16  passes through the bore  54  of the head  14  to loosely couple the head  14  to the adapter  12 . After, the head  14  is then positioned against a glenoid  62  which can be natural or an implant, and the kinematic action of the head is then tested. 
     As seen in  FIGS. 8 and 9 , should a physician determine that adjustment is necessary, the radial offset  49  of the head  14  can be accomplished by moving it in a first degree of freedom relative to the adapter  12 . After this adjustment is made, the physician will then tighten the coupling member  16  to fix the radial position of the head  14  with respect to the adapter  12 . The physician can then use the indicia  46  and  48  on the lower stem engaging surface  22  to determine the appropriate implant to use. 
     As seen in  FIG. 9 , the adapter  12  and head  14  can be rotated  51  in a second degree of freedom with respect to the stem  18 . The rotational indicia  43  on the outer spherical surface  20  can be used to mark the relative location of the implant measuring device  10  with respect to the stem  18 . This marking can optionally be made on the biologic tissue surrounding the stem  18 . This relative rotation marking is then used by the physician to determine the rotational alignment of the offset implant prior to implantation. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.