Abstract:
A system and method for trialing a modular hip replacement system permits evaluation and replication of the anatomic anteversion rotational angle of the femur. In one embodiment, a femoral hip implant kit includes at least one distal implant and a plurality of femoral heads, each of the plurality of femoral heads having a diameter different from the diameter of the other of the plurality of femoral heads. The kit includes a proximal trial housing with a bore within the housing, the bore configured to receive a portion of the distal implant, a collet located within the bore, the collet including an outer wall portion extending between a top surface portion and a bottom surface portion, a collapsing member for engaging the portion of the distal implant and for forcing the top surface portion of the collet toward the bottom surface portion of the collet along a first axis.

Description:
This application is a Continuation of application Ser. No. 11/605,099, filed on Nov. 28, 2006 (now U.S. Pat. No. 7,585,329 issued Sep. 8, 2009). 
    
    
     FIELD OF THE INVENTION 
     This invention relates to orthopedic appliances and, more particularly, to a system and method capable of gauging the degree of anteversion of the femur. 
     BACKGROUND 
     During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure, or joint arthroplasty, may involve the use of a prosthesis which is implanted into one of the patient&#39;s bones. In the case of a hip replacement procedure, a femoral prosthesis is implanted into the patient&#39;s thigh bone or femur. One type of early femoral prosthesis was typically constructed as a one-piece structure having an upper portion which includes a spherically-shaped head which bears against the patient&#39;s pelvis or acetabulum, along with an elongated intramedullary stem which is utilized to secure the femoral component to the patient&#39;s femur. In order to secure the prosthesis to the patient&#39;s femur, the medullary canal of the patient&#39;s femur is first surgically prepared (e.g. reamed and/or broached) such that the intramedullary stem of the femoral prosthesis may be subsequently implanted therein. The femoral prosthesis may be press fit into the medullary canal or, in the alternative, bone cement may be utilized to secure the femoral prosthesis within the medullary canal. 
     During performance of a joint replacement procedure, it is generally important to provide the orthopaedic surgeon with a certain degree of flexibility in the selection of a prosthetic device. In particular, the anatomy of the bone into which the prosthesis is to be implanted may vary somewhat from patient to patient. For example, in the case of a femoral prosthesis, the patient&#39;s femur may be relatively long or relatively short thereby requiring use of a femoral prosthesis which includes a stem that is relatively long or short, respectively. Moreover, in certain cases, such as when use of a relatively long stem length is required, the stem must also be bowed in order to conform to the anatomy of the patient&#39;s femur. 
     Such a need for prostheses of varying shapes and sizes can create a number of problems in regard to use of a one-piece prosthesis. For example, a hospital or surgery center must maintain a relatively large inventory of prostheses in order to have the requisite mix of prostheses needed for certain situations such as trauma situations and revision surgery. Moreover, since the bow of the stem must conform to the bow of the intramedullary canal of the patient&#39;s femur, rotational positioning of the upper portion (i.e. proximal end) of the prosthesis is limited thereby rendering precise locating of the upper portion and hence the head of the prosthesis very difficult. In addition, since corresponding bones of the left and right side of a patient&#39;s anatomy (e.g. left and right femur) may bow in opposite directions, it is necessary to produce “left” and “right” variations of the prosthesis in order to provide proper anteversion of the bowed stem thereby further increasing the inventory of prostheses which must be maintained. 
     As a result of these and other drawbacks, a number of modular prostheses have been designed. As its name implies, a modular prosthesis is constructed in modular form so that the individual elements or features of the prosthesis can be selected to fit the needs of a given patient&#39;s anatomy. For example, modular prostheses have been designed which include a proximal neck component which can be assembled to any one of numerous distal stem components in order to create an assembly which fits the needs of a given patient&#39;s anatomy. Such a design allows the distal stem component to be selected and thereafter implanted in the patient&#39;s bone in a position which conforms to the patient&#39;s anatomy while also allowing for a limited degree of independent positioning of the proximal neck component relative to the patient&#39;s pelvis. 
     In another type of modular implant, three components (in addition to the head) are utilized: a distal stem component that is engaged within the femur, a proximal metaphyseal filling component, and an intermediate neck component that supports the head component on the distal stem component. The provision of three components has greatly increased the degree of flexibility in producing a total hip implant that most closely approximates the patient&#39;s skeletal anatomy and normal joint movement. One such system is the S-ROM® total hip system marketed by DePuy Orthopaedics, Inc. The S-ROM® total hip system offers neck and head components having different lengths, different lateral offsets of the neck relative to the stem, as well as different stem configurations. 
     In order to properly size the final implant, many systems utilize trial implants, commonly referred to as simply trials. Thus, in modular systems such as the S-ROM® instrument system, neck trials, proximal body trials, distal stem trials, head trials and sleeve trials can be provided. Each trial is provided in a number of sizes and geometries to give the surgeon a wide range of combination from which to choose. The trials afford the orthopaedic surgeon the opportunity to assess the fit and position of a final implant without having to complete the fixation. Like the implant systems itself, the trials are modular to reduce the inventory of components and the complexity of the trialing process. 
     Success of the hip replacement procedure depends in large part on the technical precision with which the final implant is inserted and the modular components oriented relative to each other. Current trialing systems have performed well in assessing implant size and gross orientation, relying primarily on laser marking, for example. There remains, however, an unfulfilled need for a trialing system (as well as a modular implant system) that is able to more accurately reproduce the anteversion angle of the femur. The anteversion angle is an angle of rotation between the ball end of the femur and the plane of the intramedullary canal of the bone. In the context of the modular implant, the anteversion angle is the relative angular rotation of the proximal neck component relative to the distal stem component. Proper rotational position, or anteversion angle, allows for accurate and stable reproduction of the mechanical orientation and function of the reconstructed hip joint. 
     For implants having a straight distal stem, the proper anteversion angle can be obtained by simply spinning the distal stem within the prepared bore in the femur. In a typical case, the surgeon can visually evaluate the orientation of the proximal body relative to the surrounding anatomy. If a trial is used, the trial is removed and the final prosthesis is implanted as close to the trial position as possible. Sometimes, x-rays are used to verify the rotational alignment, while some systems rely upon external references to verify alignment. 
     The S-ROM® total hip system described above utilizes laser markings on the proximal end of the distal stem and on the proximal sleeve. These markings enable the surgeon to measure relative anteversion rotational alignment between the components. Since the sleeve has infinite anteversion it is not necessarily oriented relative to a bony landmark that can be used to define the anteversion angle. In fact, for simplicity, most current sleeves are oriented with the laser marking pointing directly laterally into the remaining available bone of the femur. 
     The problem of ensuring proper anteversion alignment is exacerbated where the modular system includes a curved distal stem. As explained above, where a long distal stem is utilized, it must be curved to follow the natural shape of the femur. Obviously, the rotational alignment of a curved stem cannot be modified once the curved stem is implanted. 
     In accordance with some prior art modular systems, the anteversion is determined by using a variety of trial femoral heads which allow a surgeon to vary the position of the head center along the axis of the neck thereby altering the head offset in both the horizontal and vertical directions simultaneously. Other systems provide a combination of trial femoral heads with two or three trial neck components that allow pure horizontal variability. Some systems even offer independent horizontal and vertical control in addition to the effects of the trial heads. 
     More recently, an additional degree of freedom has been provided by allowing modifications to the anteversion angle about the axis of the stem. A trialing system of this type, with four independent degrees of freedom (i.e., translation along the neck axis, horizontal offset, vertical offset, and anteversion angle), gives the surgeon tremendous flexibility in positioning the head center, thus providing an improved opportunity for optimization of the joint biomechanics. 
     While the foregoing systems are useful, they suffer from various limitations. For example, the anteversion angle in known systems can be changed in either discrete, indexable jumps (e.g., 10 degree increments) or with infinitely fine, continuous movement. The precision of systems using discrete jumps is limited by the size of the jump. This presents a limitation in that even rotational changes on the order of about 5 degrees can appreciably affect impingement, dislocation rate, and the range of motion of a joint. 
     The systems that provide infinitely fine, continuous movement overcome the limitation of using discrete angles. The continuous movement systems, however, are also limited. Specifically, once the configuration of the trial has been established, it is necessary to perform a trial reduction of the joint to verify that the optimum configuration has been achieved. Prior art continuous movement systems, however, do not provide a sufficiently strong coupling between the trial and the distal body. Accordingly, the continuous movement systems frequently fail to hold rotational stability during the trial reduction. This requires the anteversion to be re-established and another trial reduction performed, prolonging the surgical procedure. 
     What is needed is a system which allows a proximal trial to be attached to a distal body with a full 360 degree freedom of rotation and which provides for sufficient stability to resist trial failure. A further need exists for a system that may be used with a distal body that is one or more of a broach, a reamer, a trial and an implant. 
     SUMMARY 
     A system and method for trialing a modular hip replacement system permits evaluation and replication of the anatomic anteversion rotational angle of the femur. In one embodiment, a femoral hip implant kit includes at least one distal implant and a plurality of femoral heads, each of the plurality of femoral heads having a diameter different from the diameter of the other of the plurality of femoral heads. The kit includes a proximal trial housing with a bore within the housing, the bore configured to receive a portion of the distal implant, a collet located within the bore, the collet including an outer wall portion extending between a top surface portion and a bottom surface portion, a collapsing member for engaging the portion of the distal implant and for forcing the top surface portion of the collet toward the bottom surface portion of the collet along a first axis, thereby forcing the outer wall portion in a direction generally perpendicular to the first axis such that the outer wall portion is located outwardly of the top surface portion and the bottom surface portion with respect to the first axis. 
     In another embodiment, a femoral hip implant proximal trial includes an housing and a bore within the housing that is configured to receive a portion of a distal implant. A resiliently collapsible collet is located within the bore and a collapsing member engages the portion of the distal implant and is operable to resiliently collapse the collet within the bore such that the collet is resiliently pressed against the wall of the bore, so as to angularly lock the housing with the distal implant. 
     One method in accordance with the invention includes providing a bore within a proximal trial housing, receiving a portion of a distal implant within the bore, engaging the portion of the distal implant with a collapsing member, and moving an upper portion of the collet closer to a bottom portion of the collet along a first axis. The method includes forcing an outer wall portion of the collet in a direction generally perpendicular to the first axis in response to the moving of the upper portion of the collet; and contacting the wall of the bore with the outer wall portion of the collet such that the distal implant is angularly locked to the proximal trial housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a side plan view of a modular trial assembly in accordance with principles of the present invention; 
         FIG. 2  depicts a side plan view of the components of an exemplary modular trial kit in accordance with principles of the present invention; 
         FIG. 3  depicts a side cross-section view of the proximal trial body of  FIG. 1  with the distal trial stem and collet removed; 
         FIG. 4  depicts a side plan view of a collet that may be used with the proximal trial body of  FIG. 1  in accordance with principles of the present invention; 
         FIG. 5  depicts a side cross-section view of the proximal trial body of  FIG. 1  with the collet of  FIG. 4  and a spring washer within a main bore portion and a spring within a cavity which extends outwardly from the bore is which is separated from the main bore portion by a lip; 
         FIGS. 6-8  depict a distal trial implant being inserted and angularly locked within the proximal trial body of  FIG. 1 ; 
         FIG. 9  depicts an alternative embodiment of a proximal trial body with a separately formed nut and collet used to angularly lock the proximal trial body with a distal implant in accordance with principles of the present invention; 
         FIG. 10  depicts a perspective view of the collet of  FIG. 9  showing the walls and ribs of the collet; 
         FIG. 11  depicts a side cross-section view of the collet of  FIG. 9 ; 
         FIG. 12  depicts an alternative embodiment of a proximal trial body with a separately formed nut and collet used to angularly lock the proximal trial body with a broach that includes an expandable flange in accordance with principles of the present invention; and 
         FIG. 13  depicts an alternative embodiment of a proximal trial body with a separately formed nut and collet used to angularly lock the proximal trial body with a reamer that includes an expandable flange in accordance with principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     Referring now to  FIG. 1 , there is shown a modular trial assembly  100  according to one embodiment of the invention, including a distal trial stem  102 , a proximal trial body  104  and a locking nut  106 . The proximal trial body  104  includes a neck segment  108 . The distal trial stem  102  is separate from the proximal trial body  104 . In an alternative embodiment, the neck segment  108  is also separate from the proximal trial body  104 . The modular trial assembly  100  can be made of stainless steel or another suitable material such as titanium, cobalt, etc. 
     In the embodiment of  FIG. 1 , the proximal trial body  104  is shown in use with the distal trial stem  102 . The proximal trial body  104  may also be used with other trial components. As shown in  FIG. 2 , a trial system kit  110  can include multiple sizes of components. In this embodiment, the trial system kit  110  includes the proximal trial body  104  and distal trial stems  102 ,  114  and  116  which have different sizes and/or shapes. Each of the distal trial stems  102 ,  114  and  116  include an identically threaded upper portion  118 ,  120  and  122 , respectively and identical flanges  124 ,  126  and  128 . A neck such as the neck  112  extends between each of the threaded upper portions  118 ,  120  and  122  and flanges  124 ,  126  and  128 . Additionally, the trial system kit  110  includes femoral heads  130  and  132  which are of different sizes. The femoral heads  130  and  132  include internal bores  134  and  136 , respectively, which are sized to receive the neck segment  108 . 
     In kits wherein a separate neck segment is provided, necks of various lengths may also be included. Additionally, a variety of proximal trial bodies may be provided. The proximal trial bodies may have different outer diametric and/or geometric sizes, various vertical heights and various lateral offsets. Further embodiments include proximal trial bodies with geometric features such as calcar bodies, metaphyseal filling and conical shapes. Moreover, the kit may include reamers, broaches and implants, each of which include threaded upper portions and each of which may or may not include a flange. Thus, different sizes and shapes of components can be mixed and matched with one another to produce a modular trial assembly that matches the size and shape of a patient&#39;s joint anatomy. 
     Referring now to  FIG. 3 , the proximal trial body  104  is shown in cross-section with various components removed for clarity. The proximal trial body  104  includes an internal bore  138 . A cavity  140  extends outwardly from the internal bore  138 . The internal bore  138  further includes a main bore portion  142  which is separated from the cavity  140  by a lip  144  which protrudes into the internal bore  138 . 
     The nut  106 , shown in  FIG. 4 , includes a main body  146  which is sized to be received within the main bore portion  142 . The nut  106  includes one end portion  148  which is configured to be rotated by a tool such as a wrench. A bore  150  (see  FIG. 5 ) extends from the end portion  148  to another end portion  152  where the bore  150  expands to form a collet  154 . The collet  154  includes an upper portion  156  and a bottom portion  158  separated by a wall  160 . A plurality of cutouts  162  are located about the wall  160 . 
     A threaded portion  164  is located within the bore  148 . The threaded portion  154  is configured to threadedly engage the threaded portions  118 ,  120  and  122  of the distal trial stems  102 ,  114  and  116 . The threaded portion  164  may be further configured to threadingly engage one or more of an implant, a broach and a reamer. In this embodiment, the upper portion  156  and the bottom portion  158  of the collet  154  are curvilinear and the entire collet  154  is formed from series 300 stainless steel, preferably with a wall thickness of between about 15/1000 of an inch and 60/1000 of an inch. 
     When the trial proximal body  104  is assembled as shown in  FIG. 5 , the main body  146  of the nut  106  is positioned within the main bore portion  142 . The bottom portion  158  of the collet  154  is located within the main bore portion  142  above the lip  144 . A spring washer  166  is located between the bottom portion  158  of the collet  154  and the lip  144 . The bottom portion  158  of the collet  154  is sized such that the bottom portion  158  of the collet  154  cannot pass beyond the lip  144 . The nut  106  is maintained within the bore  138  by a weld  168 . Additionally, a spring  170  is located within the cavity  140 . 
     Referring now to  FIGS. 6-8 , an exemplary operation of the proximal trial mount  104  is explained. Initially, a patient is prepared for surgery and an incision is made to access the bone which is to receive a distal implant. Once the medullary canal of the bone is accessed, the bone is reamed in preparation of receiving the distal implant. A distal member is then positioned within the medullary canal. In various embodiments, a reamer, a broach or an implant is configured to be used as a distal trial stem. 
     Once the desired distal member is positioned within the medullary canal, which in this example is the distal trial stem  102 , the proximal trial body  104  is placed on the distal member. Specifically, the distal trial stem  102  is received into the internal bore  138 . As the threaded upper portion  118  passes into the main bore portion  142 , the flange  124  contacts the spring  170 , forcing the spring  170  outwardly from the longitudinal axis of the bore  138  into the cavity  140 . When the spring  170  is forced into the cavity  140 , the flange  124  is allowed to pass beyond the spring  170 . As the flange  124  passes the spring  170 , the spring  170  resiliently returns to its original shape, thus trapping the flange  124  within the bore  138  until such time as sufficient force is applied to the distal trial stem  102  to force the flange  124  past the spring  170 . 
     In this condition, the distal trial stem  102  is loosely engaged with the proximal trial body  104 . The proximal trial body  104  may then be rotated with respect to the distal trial stem  102  to the desired configuration. Alternatively, the locking nut  106  may be rotated in the clockwise direction. The rotation of the locking nut  106  in the clockwise direction causes the threaded portion  164  of the locking nut  106  to engage the threaded upper portion  118  of the distal trial stem  102 . 
     Continued rotation of the nut  106  in the clockwise direction pulls the locking nut  106  downwardly toward the distal trial stem  102 . As the nut  106  travels downwardly, the bottom portion  158  of the collet  154  compresses the spring washer  166  against the lip  144 . Once the spring washer  166  is sufficiently compressed, movement of the bottom portion  158  of the collet  154  is restricted. The material, wall thickness and the cutouts  162  of the collet  154  are selected such that further rotation of the locking nut  106  causes elastic deformation of the collet  154 . Specifically, the upper portion  156  of the collet  154  is forced toward the bottom portion  158  causing the wall  160  of the collet  154  to move outwardly from the longitudinal axis of the distal trial stem  102 . 
     The outwardly movement of the wall  160  forces the wall  160  against the wall of the internal bore  138  as shown in  FIG. 5 . The height of the wall  160  provides a large contact area between the collet  154  and the internal bore  138 . Accordingly, a strong frictional lock is provided between the proximal trial body  104  and the distal trial stem  102  through the collet  154  and the nut  106 . 
     Once the distal trial stem  102  and the proximal trial body  104  are angularly locked, and a femoral head, such as femoral head  130 , is placed onto the neck segment  108 , the surgeon may perform a trial reduction to assess the desirability of the configuration of the modular trial assembly  100 . Once the assessment is completed, the proximal trial body  104  is initially loosened by rotating the nut  106  in the counter-clockwise direction. The counter-clockwise rotation causes the nut  106  to move upwardly, away from the lip  144 . This allows the collet  154  to elastically decompress. The decompression of the collet  154  continues until the nut  106  is no longer compressing the spring washer  166 , thereby removing the angular lock between the distal trial stem  102  and the proximal trial body  104 . 
     If desired, the angle between the distal trial stem  102  and the proximal trial body  104  may be modified. For example, one or more trial components may be replaced, or the angle between the distal trial stem  102  and the proximal trial body  104  may be altered and a subsequent trial reduction performed. 
     When the configuration provided by the modular trial assembly  100  is determined to be optimal, the modular trial assembly  100  may be removed and replaced with permanent implants mimicking the configuration of the modular trial assembly  100 . This is accomplished by continued rotation of the nut  106  in the counterclockwise direction until the upper threaded portion  118  of the distal trial stem  102  is no longer engaged with the threaded portion  164 . The proximal trial body  104  is then removed from the distal trial stem  102  by moving the proximal trial body  104  upwardly until the flange  124  contacts the spring  170  forcing the spring  170  into the cavity  140 . The proximal trial body  104  is then separated from the distal trial stem  102 . 
     Those of ordinary skill in the art will appreciate that a number of variations of the invention are possible. By way of example,  FIG. 9  shows an alternative proximal trial body  180 . The proximal trial body  180  is shown with a nut  182  that includes a lower portion  184  that is configured to engage a collet  186  in order to angularly lock the proximal trial body  180  with a distal implant  188 . 
     As shown in  FIG. 10 , the collet  186  includes an upper portion  190  and a bottom portion  192  separated by a wall  194 . A plurality of ribs  196  are located about the wall  194 . The upper portion  190  and the bottom portion  192  define openings  198  and  200  (see  FIG. 11 ), respectively. The upper portion  190  is configured to be generally complementary with the lower portion  184  of the nut  182 . Additionally, the openings  198  and  200  are configured to be just slightly larger in diameter than the portion of the distal implant  188  that extends through the collet  186  as shown in  FIG. 9 . 
     The embodiment shown in  FIGS. 9-11  is operated in much the same manner as the modular trial assembly  100  of  FIG. 1 . One difference is that as the collet  186  is collapsed, the ribs  196  provide the binding contact with the proximal trial body  180 . Additionally, the resilient deformation of the collet  186  causes the openings  198  and  200  to bind with the distal implant  188  providing a frictional lock in addition to the frictional lock between the nut  182  and the distal implant  188 . Accordingly, a strong angular lock between the distal implant  188  and the proximal trial body  180  is achieved. 
     A further alternative embodiment is shown in  FIG. 12 . The proximal trial body  210  includes a main bore portion  212  and a lower bore portion  214 . A collet  216  is located within the main bore portion  212 . The collet  216  includes a bottom portion  218 , a top portion  220  and an outer wall  222 . A nut  224  is also located within the main bore portion  212 . The broach  226  which is used with the proximal trial body  210  includes an expandable flange  228  and a threaded portion  230 . 
     The, the flange  228  is configured to expand within the main bore portion  212  of the proximal trial body  210 . Thus, when angularly locking the proximal trial body  210  and the broach  226 , the collet  216  is compressed between the nut  224  and the expandable flange  228 . Another variation in the embodiment of  FIG. 12  is that the bottom portion  218  of the collet  216  is not symmetrical with the top portion  220 . This is because the bottom portion  218  is configured to extend only partially over the lower bore portion  214  while the outer wall  222  of the collet  216  extends outwardly of the lower bore portion  214  when the collet  210  is not compressed. Thus, the outer wall  222  prevents the collet  216  from falling through the lower bore portion  214  when the broach  226  is not inserted within the proximal trial body  210 . Moreover, the partial extension of the bottom portion  218  over the lower bore portion  214  allows the threaded portion  230  of the broach  226  to pass through the collet  216  while ensuring that the flange  228  entraps the collet  216  between the flange  228  and the nut  224  when the flange  228  is inserted into the main bore portion  212 . 
     Additionally, in this embodiment, the collet  216  does not have ribs. The outer wall  222  of the collet  216 , however, is slightly curved in an outwardly direction when the collet  216  is in an uncompressed state. Thus, as the collet  216  is elastically deformed, the surface area of the wall  222  that contacts the wall of the main bore portion  212  increases. 
     Yet another alternative embodiment is shown in  FIG. 13 . The proximal trial body  234  includes a main bore portion  236  and a lower bore portion  238  separated by a lip  240 . A collet  242  is located within the lower bore portion  238 . The collet  242  is similar to the collet  216 . A nut  244  is also located within the main bore portion  236 . The reamer  246  which is used with the proximal trial body  234  includes an expandable flange  248  and a threaded portion  250 . 
     The embodiment of  FIG. 13  works similarly to the embodiment of  FIG. 12 . The main difference is that the collet  242  is compressed between the flange  248  and the lip  240 . Thus, the nut  244  compresses the collet  242  indirectly. 
     While the present invention has been illustrated by the description of exemplary processes and system components, and while the various processes and components have been described in considerable detail, applicant does not intend to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will also readily appear to those ordinarily skilled in the art. The invention in its broadest aspects is therefore not limited to the specific details, implementations, or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept. By way of example, but not of limitation, the system described herein may be applied to other bones and joints besides the hip. Such bones may include tibial and humerus bones.