Abstract:
A method of assembling an implant in one embodiment includes aligning the connection features of a first implant component with the connection features of a second an implant component, placing an implant engagement surface of an impactor device and the first implant component in contact, striking an impaction surface of the impactor device, transferring force from the impaction surface to an impactor shaft, diverting the transferred force within the shaft, focusing a portion of the diverted force, and transferring the focused force from the shaft to the first implant component which brings the connection features on the first implant component into engagement with the connection features on the second implant component.

Description:
FIELD 
     This application relates to the field of impacting devices, such as those used to provide impact force to a prosthetic component in order to secure the prosthetic component to another device or to tissue. 
     BACKGROUND 
     Many orthopaedic procedures involve the implantation of prosthetic devices to replace badly damaged or diseased bone tissue. Common orthopaedic procedures that involve prosthetic devices include total or partial hip, knee and shoulder replacement. For example, a hip replacement often involves a prosthetic femoral implant. The femoral implant usually includes a rigid stem that is secured within the natural femur bone tissue. The femoral implant further includes a rounded head that is received by, and may pivot within, a natural or artificial hip socket. Shoulder replacement is somewhat similar, and typically includes a humeral implant that includes a rigid stem and a rounded head. The rigid stem is secured within the natural humerus bone tissue and the rounded head is pivotally received by a shoulder socket. 
     Increasingly, prosthetic devices are provided as subcomponents that are assembled during surgery. In particular, the different anatomies of different patients require that prosthetic devices such as femoral and humeral implants be available in different sizes and configurations. By way of simplified example, a humeral implant may be available in as many as six or more humeral head diameters. Stems may similarly vary in size and/or in shape. Because of differences in patients and individual conditions, it is advantageous that the surgeon have at her disposal many configurations and sizes of implants. Instead of providing a separate implant for each possible combination of features, implants are provided as modular kits of subcomponents that allow the surgeon to mix and match different subcomponents to achieve the most advantageous combination for the patient. Thus, the surgeon can pick from several sizes or configurations of each component and combine the components to form an implant having an optimal combination of features. 
     One example of a modular implant is the humeral implant  10  shown in  FIGS. 1 and 2 . The humeral implant  10  includes a humeral head  12  that may be assembled onto a humeral stem  14 . The humeral stem  14  is configured to be implanted in the intramedullary tissue of a natural humeral bone, while the humeral head  12  is configured to be received into the shoulder socket or glenoid cavity. 
     In the exemplary modular implant of  FIGS. 1 and 2 , an intermediate component  16  is provided between the humeral head  12  and the humeral stem  14 . The intermediate component  16  is a two part insert that includes a stem insert  17  and a head insert  19 . The stem insert  17  is provided within a cavity at the end of the stem  14 . The head insert  19  includes a truncated ball portion  21  and a frusto-conical portion  23 . The truncated ball portion  21  of the head insert is configured to fit within a receptacle in the stem insert  17 . The frustro-conical portion  23  serves as a tapered plug  16  that is designed to be received by a tapered receptable  28  in the humeral head  12 . It can be appreciated that the surgeon may secure alternative humeral head  12  designs on the same humeral stem  14 , thus providing the surgeon with a broad array of humeral head size options. 
     Once the components are selected, such as the humeral head  12 , the humeral stem  14 , and the intermediate component  16  of  FIGS. 1 and 2 , the components are assembled. One popular method of securing implant components together involves the use of a Morse taper. The components of  FIGS. 1 and 2  by way of example include a Morse taper arrangement. In particular, a Morse taper is a feature in which a tapered male component, e.g., the tapered plug  23  of the head insert  19 , is received into a tapered female component, e.g., the receptacle  28  of the humeral head  12 . The taper angle of the plug  23  is preferably, but need not be, slightly less than the taper angle of the receptacle  28 . In use, the plug  23  advances into the receptacle  28 , as indicated by arrow  29 , until it begins to engage the receptacle  28 . The further into the receptacle the plug  23  is forced, the more tightly it engages the humeral head  12 . 
     The force applied to secure the plug  23  within the receptacle  28  is proportional to the retention force of the plug  23  within the receptacle  28 . Thus, if a sufficient amount of force is applied, then the humeral head  12  will be securely fastened to the humeral stem  14  via the insert  16 . Other prosthetic devices employ Morse tapers for substantially the same reasons. 
     To apply sufficient force to lock the Morse taper arrangement between the humeral head  12  and the plug  23 , it is known to impact the humeral head  12  such that the impact force directs the humeral head  12  toward the plug  23  and humeral stem  14 . The impact force drives the plug  23  into the receptacle  18  and forms the Morse taper lock. A hammer or mallet is typically struck directly on the head, or through an impactor device. 
     During assembly of the implant, the surgeon (or other person) may impact the prosthetic implant several times without knowing if sufficient force has been applied to lock the Morse taper sufficiently. In order to be sure that the Morse taper is locked, the surgeon or assistant may use excessive force. The use of excess force is undesirable because of the potential for damage to the bone tissue or the implant device. For example, the use of excess force may disengage the intermediate components between the head  12  and the stem  14 , such as the insert components  17  and  19 , from their locked position. 
     Thus, there is a need for assisting surgical personnel in applying the proper amount of force to a Morse taper to lock the Morse taper. In particular, it would be advantageous to provide an impactor device capable of dissipating the force that is transmitted through the impactor and to an implant when locking a Morse taper. Such an impactor would serve to limit the application of excessive force and any associated damage. The need for such a device is widespread as Morse tapers have commonly been used for connection of many types of implant devices. It would also be advantageous if such an impactor could be manufactured simply and at a low cost. 
     SUMMARY 
     A force dissipating impactor device is disclosed herein. The disclosed impactor device is configured to reduce the forces transmitted through the impactor device to an implant. The impactor device comprises a bar member comprising a hollow shaft, a first end, and a second end. The first end of the impactor device provides an impaction surface and the opposite second end of the impactor device provides an implant engagement surface. A plurality of holes are provided in the shaft and penetrate the surface of the bar member. 
     In one embodiment, the plurality of holes provided on the shaft surface of the bar member are arranged such that a line passing axially along the shaft surface intersects at least one of the plurality of holes. In this embodiment, the plurality of holes may be arranged in a staggered matrix that is provided around the shaft surface. In one embodiment, each row of the staggered matrix comprises four holes, and each hole in a row is situated ninety degrees relative to an adjacent hole in the row. 
     The implant engagement surface on the first end of the impactor device is contoured to mate with a surface of the implant member. Thus, in one embodiment, the implant engagement surface is rounded and concave and the surface of the implant member is rounded and convex. In one embodiment, the impactor device is between five and nine inches in length, and is preferably about seven inches in length. This length allows the impactor device to be easily handled by the surgeon. 
     The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary prior art humeral implant; 
         FIG. 2  shows a diagram of the humeral head, humeral stem, and insert for the humeral implant of  FIG. 1 ; 
         FIG. 3  shows a perspective view of a force dissipating impactor device; 
         FIG. 4  shows a side view of the impactor device of  FIG. 3 ; 
         FIG. 5  shows a perspective view of the impactor device of  FIG. 3  from an end portion of the impactor device; 
         FIG. 6  shows a side view of an alternative embodiment of the impactor device of  FIG. 3 ; 
         FIG. 7  shows a cross-sectional view of the impactor device along line VII-VII of  FIG. 6 ; 
         FIG. 8  shows a cross-sectional view of the impactor device along line VIII-VIII of  FIG. 6 ; 
         FIG. 9  shows a cross-sectional view of the impactor device along line IX-IX of  FIG. 6 ; and 
         FIG. 10  shows an expanded polar coordinate view of the shaft portion of the impactor device of  FIG. 3  with the holes arranged in a staggered matrix. 
     
    
    
     DESCRIPTION 
     With reference to  FIGS. 3-5 , an impactor device  50  is disclosed. The impactor device  50  is provided as an elongated bar member  52  that includes a shaft portion  54 , a grip  55 , a first end  56 , and a second end  58 . The first end  56  provides an impaction surface and the second end provides a force distributing surface in the form of an implant engagement surface. A plurality of holes  60  are formed in the shaft portion  54 . 
     In the embodiment of  FIGS. 3-5 , the shaft portion  54  of the impactor device  50  is generally cylindrical in shape. The shaft portion  54  is hollow with a channel  62  extending axially along the center of the shaft portion. The channel  62  is surrounded by an exterior wall  62 . Although the exterior wall  62  is cylindrical in the embodiment of  FIGS. 3-5 , one of skill in the art will recognize that the exterior wall may be any of numerous other shapes, such as box-shaped. 
     A plurality of holes  60  extend through the exterior wall  62  of the shaft portion  54  and into the axial channel  62 , resulting in a perforated shaft portion  54 . In the embodiment of  FIGS. 3-5 , the plurality of holes  60  are arranged on the shaft portion  54  such that any given line passing axially along the surface of the exterior wall will intersect at least one of the plurality of holes  60 . To obtain this result, the holes  60  on the shaft portion  54  may be arranged in a staggered matrix around the shaft. 
       FIG. 10  shows an expanded polar coordinate view of the shaft portion  54  further displaying the staggered matrix arrangement of the holes. This view shows the shaft portion  54  “unwrapped” along the central axis such that the leftmost position is a zero degree position and the right most position is a three hundred sixty degree position radially relative to the central axis of the shaft. As shown in  FIG. 10 , the holes  60  are arranged in a staggered matrix such that the holes  60  in one row are offset from the holes in an adjacent row. In the disclosed embodiment, seven rows of holes  60  are provided with four holes in each row. The holes  60  overlap in the axial direction such that a line extending axially along the shaft portion, such as line  90 , will intersect one or more of the holes  60 . With this arrangement, the holes in each row are situated at ninety degree increments around the shaft, as can be seen from  FIGS. 8 and 9 . In other words, the center of a hole in a row is ninety degrees removed from the center of an adjacent hole in the row. In the disclosed embodiment, the diameter of each hole is 0.379 inch. 
     With reference again to  FIGS. 3-5 , the grip  55  of the impactor device  50  is provided next to the shaft portion  54 , toward the first end  56  of the impactor device  50 . The grip  55  includes a plurality of fins  72  that extend axially along a length of the shaft surface. The fins  72  are separated by axial indentations  74 . The fins  72  and indentations  74  provide a knurled surface that provides an aid in gripping the impaction device  50 . 
     The first end  56  of the bar member  52  provides the impaction surface and is configured to receive a blow from a mallet or other striking device. In the embodiment of  FIGS. 3-5 , it can be seen that the impaction surface is generally flat. This flat surface helps prevent the surgeon or surgical assistant from hitting the impactor device off axis.  FIGS. 6-9  show a similar embodiment to that of  FIGS. 3-5 , and identical reference numerals are used to identify the same parts. However, in the embodiment of  FIGS. 6-9 , the impaction surface on the first end  56  of the bar member is convex. In this embodiment, the force of striking tool used by the surgeon is generally concentrated on a smaller area of the impaction surface. 
     The second end  58  of the bar member is positioned opposite the first end. The second end  58  of the bar member provides a force distributing surface. The force distributing surface is configured to engage an implant member, and thus serves as an implant engagement surface. If the implant member that will be contacted by the implant engagement surface is contoured, the implant engagement surface may be similarly contoured to mate with the surface of the implant member in a congruent fashion. The implant engagement surface shown in  FIGS. 3-5  is designed to engage a convex rounded surface, such as the spherical humeral head of a humeral implant. Thus, the implant engagement surface on the second end  58  of the bar member  52  provides a concave rounded surface. 
     In one embodiment, the impactor device  50  is designed to be somewhere between five and nine inches in length. This length generally facilitates ease of handling by the surgeon along with a sufficient size for many human implant devices. In one embodiment for use with a humeral implant, the impactor device  50  is about seven inches in length. Of course, one of skill in the art will recognize that the impactor device is not limited to a particular length and the impactor device may be designed to any number of different lengths. 
     The impactor device  50  may be comprised of any of several different materials. Preferably, the material will be moldable, offer high flexural fatigue strength, rigidity, low wear, toughness and resistance to repeated impact. In one embodiment, the impactor device  50  is comprised of an acetal copolymer such as Celcon®. The simplicity of the impactor device design and use of appropriate material will also allow the impactor device to be easily cleaned through autoclaving. 
     The impactor device  50  is used by a surgeon or other surgical personnel to assemble a prosthetic device to be implanted in a patient. To this end, the surgeon first chooses an appropriate design and size for the various components of the implant device based on the size and needs of the patient. The implant device comprises a first implant component and a second implant component to be connected by a Morse taper or similar arrangement where the implant components are configured for connection by forcing connection features on the first component into engagement with connection features on the second component. 
     After selecting appropriate implant components, the surgeon selects an impactor device as set forth above. The impactor device includes a shaft portion, a grip portion, a first end with an impact surface and a second end with an implant engagement surface. A plurality of holes are formed in the axial wall of the shaft portion. The implant engagement surface of the impactor device is configured to engage a surface of the first implant component in a congruent fashion. 
     The surgeon aligns the connection features of the first implant component with the connection features of the second an implant component. Next, the surgeon holds the impactor device by the grip portion  55  and brings the implant engagement surface  58  into contact with the first implant component (e.g., the head  12  of the humeral implant of  FIGS. 1 and 2 ). The axis of the impactor device is oriented on the first implant component such that a force transmitted through the impactor device will force the first implant component into full engagement with the second implant component. After properly aligning the impactor device, the surgeon strikes the impaction surface  56  on the impactor device, thus transmitting a force through the impaction device and to the first implant component (e.g., the plug  19  into engagement with the recess  28  in  FIG. 2 ). This force is intended to bring the connection features on the first implant component into engagement with the connection features on the second implant component. The surgeon may be required to strike the impaction surface  56  one or more times to bring the connection features on the first implant component into full engagement with the connection features on the second implant component. 
     When the surgeon strikes the impactor device, the impactor device dissipates the force transmitted through the bar member and to the implant. In particular, the holes  60  in the impactor device  50  provide voids in the shaft portion  54  so that the shaft portion  54  can compress and expand to dissipate energy. Furthermore, the orientation of the holes  60  not only limits the amount of force that is transmitted down the shaft portion, but also helps to maintain the integrity of the impactor device, such that the impactor device does not fracture, degrade or otherwise fail when struck with a mallet or other striking device. 
     The staggered matrix orientation and size of the holes on the shaft portion can effectively dissipate about forty percent of the impaction force imparted by a striking device. Thus, even if a five thousand pound force is delivered by a mallet strike, the impactor device  50  will reduce that force to around three thousand pounds, which would be more than enough force to cause the humeral head to engage the humeral insert for most implants. At the same time, the reduced force is much less likely to result in disengagement of or damage to the intermediate components in the implant device. 
     Although the present invention has been described with respect to certain preferred embodiments, it will be appreciated by those of skill in the art that other implementations and adaptations are possible. For example, the impactor may take the form of different shapes than those shown in the figures, may include different features, may be differently sized, or may be comprised of different materials than those disclosed herein. Moreover, there are advantages to individual advancements described herein that may be obtained without incorporating other aspects described above. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.