Cemented calcar replacement varying height trial

Abstract: A calcar trial includes a stem having a proximal end and a distal end, and a body. The body is engaged with the stem near the proximal end of the stem and is slidable with respect to the stem. A locking mechanism inhibits movement of the body with respect to the stem when desired. A collar extends radially outward from the stem to surround a portion of the body. The body and stem are configured so that the diameter of the calcar trial at a point between the collar and the distal end of the stem increases as the body is moved toward the distal end of the stem and decreases as the body is moved away from the distal end of the stem. Additionally, the proximal end of the stem can include opposed notches that are egagable by a forked tool.

Description: FIELD OF THE INVENTION The present invention relates to a device used in hip arthroplasty, such as a trial for determining the required dimensions of a prosthetic femoral component, and more particularly to a trial for a calcar stem. BACKGROUND OF THE INVENTION A successful hip replacement or arthroplasty procedure results, in part, from selection of prosthetic joint components that are dimensioned and positioned to closely approximate or replicate the geometry and functional characteristics of a natural, healthy hip joint. Typically, the component selection process includes a pre-operative analysis of joint images. However, it has been discovered that a valuable adjunct to image analysis is the temporary fixation of one or more provisional components to a bone or bones of interest at a stage of the arthroplasty procedure prior to permanent fixation of the prosthetic joint. The provisional components are intended to mimic certain aspects of the permanent prosthetic joint in order for a surgeon to validate measurements and to test or "try-out" several different possible component sizes and configurations. Hence, provisional components are aptly known as "trials." In a known procedure, a trial for a femoral component is used in the following manner. The proximal end of a femur is resectioned and the medullary canal of the femur is reamed. A broach is inserted into the resected proximal end of the femur to provide a cavity within the bone dimensioned and contoured to receive a femoral stem. However, prior to removing the broach, a trial neck or trunnion and trial head can be secured to the broach to simulate a complete femoral stem. Normally, several neck and head trials of varying lengths and geometries are successively joined to the broach in an attempt to determine an appropriate neck length and overall femoral stem length. Once these lengths have been determined, the trial neck and head are removed from the broach and the broach is removed from the femur. Subsequently, a femoral stem of the appropriate length is selected for insertion into the cavity defined by the broach using techniques known to those skilled in the art. Other techniques require that the broach be removed from the medullary canal to allow a trial having a stem portion to be used, in addition to a trial head and neck. For example, United States Patent No. 5,100,407 discloses a system including a group of variously sized trial neck/body portions and a group of differing length trial stem portions which are mixed and matched to create a suitable trial. However, repetitive removal and insertion of successions of trial stems accompanied by successive assembly and disassembly with respect to the body can consume a lengthy and costly period of time. Another known trial includes a stem to which a collar is secured at successive points along the length of the trial until an appropriate neck length and stem length have been ascertained. Undesireably, this type of trial induces measurement inaccuracies resulting from stem movement as the collar is repeatedly engaged with and disengaged from the stem. Additionally, as the collar is moved toward the distal end of the stem, less and less of the stem is disposed within the medullary canal, causing the trial to become increasingly unstable and rendering accurate measurements very difficult to achieve. In addition to other deficiencies of known trials, it is believed that a trial system consisting of numerous parts that must be selected and mated in various combinations, possibly many times, is cumbersome, unnecessarily complex and a waste of valuable time. In US-A- 5569263 (HEIN) there is disclosed a calcar trial according to the pre-characterizing part of claim 1 appended hereto. SUMMARY OF THE INVENTION The present invention overcomes the disadvantages of known trials by providing a unified assembly that facilitates very accurate measurements in a convenient, easy to use manner. The trial does not require repeated assembly and disassembly, and it is uniquely able to provide a geometry that closely approximates a broached cavity, regardless of the height of the trial. According to the present invention there is provided a calcar trial as set forth in claim 1. A collar can extend radially outward from the stem to surround a portion of the body. The body and stem can be configured so that the diameter of the calcar trial at a point between the collar and the distal end of the stem increases as the body is moved toward the distal end of the stem and decreases as the body is moved away from the distal end of the stem. Additionally, the proximal end of the stem can include engagement structures such as opposed notches that are engagable by a forked tool. Further preferred features of the invention are set forth in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present invention and the attendant advantages and features thereof will be more readily understood by reference to the following detailed description when it is considered in conjunction with the accompanying drawings, wherein: FIG. 1 is an exploded view of the trial in accordance with the invention that illustrates a body portion and a stem portion; FIG. 2 is a perspective view of the stem portion of the trial shown in FIG. 1; FIG. 3 is a top view of the body portion illustrated in FIG. 1; FIG. 4 is a sectional view of a trial showing a locking mechanism in an engaged position; FIG. 5 is a sectional view of a trial showing the locking mechanism in a disengaged position; FIG. 6 is a side view of the trial of FIG. 1 in an assembled configuration; FIG. 7 is a side view of the trial in accordance with the invention inserted into a femur at a first body height; FIG. 8 illustrates a trial in accordance with the invention inserted into a femur at a second body height; FIG. 9 is a perspective view of an inserter/extractor tool; FIG. 10 illustrates the tool of FIG. 9 engaged with a trial for insertion of the trial into a femur; and FIG. 11 illustrates the insertion/extraction tool in an extraction position. DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, a trial in accordance with the invention is shown in an exploded view to show a stem 10, a body 12, a pin 14, a spring 16, and a pushbutton 18. The stem 10 has a proximal end 20 and a distal end 22, and the body 12 has a proximal end defining a trunnion 24 and a distal end 26. The body 12 is engagable with the stem 10 near the proximal end of the stem so as to be slidable with respect to the stem a predetermined distance between the proximal end and the distal end of the stem. A guide or track can be associated with either or both of the body 12 and the stem 10 to guide movement of body with respect to the stem. For example, as shown in FIG. 2, the stem 10 includes a track 28 protruding from a first face 30 of the stem and extending a predetermined distance between the proximal end 20 and the distal end 22 of the stem. As shown in FIG. 1, the body 10 includes a notch 32 for receiving the track 28. However, in other embodiments, the body 12 includes a raised portion that is engagable with a track that is recessed within the stem. Regardless of its configuration, the complimentary guide/track/notch of the body and stem serve to limit movement of the body 12 along a predetermined path, such as longitudinal movement, as well as to inhibit undesired movements such as twisting or lateral displacement. Additionally, a locking mechanism can be provided for inhibiting movement of the body 12 with respect to the stem 10. As shown in FIGS. 1 and 3, the body 12 can include a channel 34 that is transverse to the notch 32 and which is adapted to receive an elongate portion of the button 18 therein. The button 18 is movable within the channel from a first position, wherein a portion of the button contacts and engages a portion of the track 28 of the stem, to a second position wherein the button is disengaged from the track. As shown in FIG. 4, the button 18 is biased to the first position by the spring 16. FIG. 5 illustrates the button 18 in the second position. The button 18 includes an expanded head portion 36 that engages the body 12 to limit insertion depth of the button 18 into the body. The button 18 also includes a cut-out portion 38 into which the pin 14 and a portion of the track 28 are received. It will be noted most clearly in FIG. 2 that the track includes first and second transverse grooves 40 and 42. When the body 12 and the stem 10 are caused to slide with respect to each other, the cut-out portion 38 rides over/along the track 28 until a groove 40, 42 is reached, whereupon an end portion 44 of the button is biased into the groove 40, 42. The end portion 44 is released from the groove 40, 42 by pushing the button 18 into the body 12 with enough force to overcome the bias force of the spring 16. Referring again to FIGS. 1 and 2, a collar 46 extends radially outward from the stem. The collar 16 surrounds a portion of the body 12, as shown in FIG. 6, and it is dimensioned to be disposed on a resectioned bone surface as shown in FIGS. 7 and 8. The collar 46 and the distal end of the stem 22 are a fixed distance apart. The collar defines an aperture 48 having curves to compliment the shape of the body 12 and into which the body is received. Turning now to FIGS. 7 and 8, use of the calcar trial is illustrated with respect to resectioned femurs 50 and 52 respectively. Once the femur has been prepared to receive the trial, the distal end 22 of the trial is inserted into the medullary canal 54, 56. It should be noted that in both FIGS. 7 and 8 the full length of the stem 10 from the collar 46 to the distal end of the stem 22 is inserted into the medullary canal. This enables a surgeon to verify that the medullary canal has been reamed to a sufficient depth and width to accommodate a replacement hip stem. The surgeon then slides the body 12 with respect to the stem as required to adjust the head height or distance between a reference point 58 on a head 60 affixed to the body 12 and the collar 46 to determine a prosthetic hip stem length. In FIG. 7, the body 12 is positioned with respect to the stem 10 at a head height of 45mm, whereas in FIG. 8 the head height is 55mm. The trial is then removed from the medullary canal and a prosthetic hip stem having the determined length is selected from a group of hip stems. The selected hip stem is cemented into the medullary canal. It should be noted in these illustrations that the proximal portion of the trial underneath the collar is wider than an intermediate portion of the stem or its distal end 22 to ensure a tight fit of the trial within the femur. The trial is slightly larger than an actual replacement stem in the proximal section below the collar to allow the trial to fill the medullary canal which has been reamed to be slightly larger than an actual replacement stem (to leave room for bone cement to surround the replacement stem). Also, the stem 10 and the body 12 are configured so that a diameter of the trial at a point between the collar and the distal end of the stem increases as the body is moved toward the distal end of the stem and decreases as the body is moved away from the distal end of the stem. Thus, regardless of where a resection cut is made, the dimensions and shape of the trial correspond to the dimensions and shape of the broached medullary canal. As the trial is usually seated within the medullary canal very snugly, the trial in accordance with the invention further includes features that are of use when inserting the trial into or extracting the trial from the medullary canal. For example, referring to FIG. 2, the proximal end of the stem includes a tool engagement structure, such as a pair of opposed notches 62 and 64 on the stem. A tool 66, shown in FIG. 10, includes a first fork portion or furcation 68 and a second furcation 70 for engaging the opposed notches 62 and 63. The space between the furcations corresponds to the shape of the body 12 to allow the tool to snugly interfit with the body. This ensures that the tool remains axially aligned with the body 12 and the stem 10. Each furcation 68 and 70 can include angled end portions or tines 72 and 74, respectively. A handle 76 provides an easily graspable structure for pulling or pushing tool 66 as well as a suitable surface for mallet striking. Because the notches 62, 64 are similar to the notches of a replacement stem, used to orient cerclage cables, the same tool 66 can be used to insert and extract both the trial and the replacement stem. FIG. 10 shows the tool 66 positioned with respect to the trial for insertion of the trial into a medullary canal, wherein the tines 72, 74 are not engaged with the notches 62, 64, but rest directly upon the collar 46 on opposite sides of the body 12. FIG. 11 shows the tool 66 positioned with respect to the trial for extraction of the trial from a medullary canal, wherein the tines 72, 74 are engaged with the notches 62, 64. The forked tool thus allows an even and distributed force to be applied to the stem 10 during both insertion and extraction. Although the invention has been shown and described with respect to exemplary embodiments thereof, various other changes, omissions and additions in form and detail thereof may be made without departing from the scope of the claims. It will be understood that the reference numerals in the claims are purely for illustration, and are not to be taken as having a limiting effect on the claims.

Claims: A calcar trial comprising: a stem (10) having a proximal end (20) and a distal end (22); and a body (12) defining a trunnion (24), the body engaged with the stem near the proximal end of the stem and slidable with respect to the stem a predetermined distance between the proximal end and the distal end of the stem characterized by the body carrying a biased locking mechanism (14,16,18) for inhibiting movement of the body with respect to the stem ,the locking mechanism being biased from an unlocked state which permits sliding movement of the body with respect to the stem to a locked state which stops sliding movement of the body with respect to the stem when the body is positioned relative to the stem at one of two predetermined locations. The calcar trial of claim 1, wherein the stem includes a track (28) protruding from a first face (30) of the stem and extending a predetermined distance between the proximal end and the distal end of the stem, and wherein the body includes a notch (32) for receiving the track. The calcar trial of claim 2, wherein the body includes a channel (34) that is transverse to the notch and an elongate button (18) disposed in the channel, the elongate button having a first position wherein a portion of the elongate button contacts and engages a portion of the track of the stem, and a second position wherein the elongate button is disengaged from the track. The calcar trial of claim 3, wherein the elongate button is biased to the first position. The calcar trial of claim 4, wherein a portion of the track of the stem extends into notch of the body, the portion of the track includes a plurality of transverse grooves (40,42), and wherein a portion of the elongate button is receivable within a selected one of the transverse grooves when the elongate button is biased to the first position. The calcar trial of claim 1, wherein the stem includes a collar (46) extending radially outward from the stem and surrounding a portion of the body, the collar and the distal end of the stem defining a fixed distance. The calcar trial of claim 6, wherein a diameter of the calcar trial at a point between the collar and the distal end of the stem increases as the body is moved toward the distal end of the stem and decreases as the body is moved away from the distal end of the stem. The calcar trial of claim 6, wherein the stem includes an intermediate region between the collar and a selected point, and a distal region between the selected point and the distal end, and wherein the diameter of the intermediate region is greater than the diameter of the distal region. The calcar trial of claim 1, wherein the body includes an arcuate face between a point proximate the trunnion and the distal end of the body. The calcar trial of claim 1, wherein the proximal end of the stem includes a tool engagement structure (62,64). The calcar trial of claim 10, wherein the proximal end of the stem is wider than the body and wherein the tool engagement structure includes a pair of opposed notches (62,64) in the stem on opposite sides of the body. A calcar trial according to any one of claims 1 to 10, wherein the body has a proximal end which defines the trunnion and a distal end. The calcar trial of claim 12, further comprising a vertical flange defined by the stem, the vertical flange being wider than the body and including a pair of opposed notches (62,64) on opposites side of the body. A hip replacement system comprising: a calcar trial for a prosthetic femoral implant according to any one of claims 1 to 9 in which the sliding of the body with respect to the stem increases and decreases the length of the trial; a vertical flange that is wider than the body and which includes a pair of opposed notches on opposites side of the body; a forked tool having a pair of furcations for engaging the opposed notches of the body; and a plurality of different length hip stems.

Inventors: HAMMOND DAN, MCDOWELL CHRIS, RO GLORIA, WILSON STEPHEN, HAMMOND, DAN, MCDOWELL, CHRIS, RO, GLORIA, WILSON, STEPHEN
Applicants: DEPUY PRODUCTS INC, DEPUY PRODUCTS, INC.
Application year: 1998
Main classification: A61F2
IPC codes: A61F2