Alignment guide for slotted prosthetic stem

Abstract: An alignable orthopedic prosthesis system includes a prosthesis and an alignment guide (910). The prosthesis includes a stem (230) having at least one slot (234) extending along its length. The alignment guide has an alignment body with a guide surface extending from an end thereof, namely a key element formed as a blade (215) for engaging and guiding the slot to orient the prosthesis stem. The alignment body also fits against a machined bone surface so that the blade is positioned in a plane that is oriented to guide the stem smoothly into contact with a recess formed in the bone.

Description: FIELD OF THE INVENTION The present invention relates to an alignment guide for ensuring proper alignment or orientation of a prosthetic stem in bone. BACKGROUND OF THE INVENTION Stems are used in prosthetic joint implants to anchor the implant in a bone cavity. The bone for receiving the stem is typically prepared by drilling a hole in the bone and creating an opening sized and contoured to receive the stem of the implant. The stem is inserted into a prepared cavity of a bone and a joint bearing surface attached or coupled to the stem, extends out to the cavity. Many stems are fluted, i.e., they have distally extending longitudinal grooves in the stems that provide greater stability and anchoring of the stem once it is inserted into the prepared cavity. Typically, once a fluted stem is inserted into a prepared cavity, it is extremely difficult to rotate the stem to properly reorient it. In order to do so the stem must be removed and reinserted which can damage bone and increase operative time. Typically, the stem is aligned by marking the bone and the stem and using the marks to align the stem with respect to the bone. The drawback to this method is the potential imprecision in the alignment. Because the landmark on the stem and bone are not in close proximity to each other, parallax and other problems associated with alignment by eye may result. Also, the stem may move from its aligned position as it is inserted. In EP-A-0850610, there is shown a means for aligning a fluted stem of an orthopedic implant prior to and as it is being inserted into a prepared bone cavity. Accordingly, it is an object of the present invention to provide an improved means for aligning a fluted stem of an orthopedic implant prior to and as it is being inserted into a prepared bone cavity. SUMMARY OF THE INVENTION According to the invention, there is provided an alignable orthopedic prosthesis system as set forth in claim 1. According to the invention, there is further provided a prosthetic stem system as set forth in claim 11. Further aspects of the invention are provided as set forth in claims 2 to 10 and 12 to 21. In one embodiment, the alignment guide keys to the stem by a blade which fits within a longitudinal slot of the stem to fix its rotational orientation. The slot is parallel along the central plane of the spout, and the blade, which rigidly extends from the guide, is centrally aligned by means of an opposed guide surface that fits the prepared bone, so as to define an insertion orientation to guide the proximal geometry of the stem into the prepared contour. This embodiment is preferably implemented as an elongated handle, having a proximal grip portion, a blade extending from the distal end, and a middle shoulder which fits the prepared bone. Alternatively, it may be implemented as a smaller body having an outer surface portion which fits the prepared bone surface, and an inner surface portion rigidly extending in a precise plane across the prepared bone canal to engage the slotted stem. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which: FIG. 1 illustrates an exploded perspective view of a bone with a partial break away of a prepared cavity, a prior art alignment guide and an implant inserted through the alignment guide; FIG. 2 illustrates a perspective view of the bone, alignment guide and implant of Figure 1 with the alignment guide in place and the implant inserted partially into the bone through the alignment guide; FIG. 3 illustrates a perspective view of the alignment guide of figures 1 and 2. FIG. 4 is a perspective view of another prior art alignment guide; FIG. 5 is a perspective view of the alignment guide of FIG. 4 in contact with a fluted stem of a hip prosthesis femoral component; FIG. 6 is a side view of the alignment guide and fluted stem of FIG. 5; FIG. 7 is a cross sectional view of the alignment guide and fluted stem of FIG. 6 taken along line 7-7; FIG. 8 is a side view of the alignment guide of figures 5 to 7 and prosthetic stem component being inserted into a bone; FIG. 9 is a perspective view of an alignment guide of the invention: FIG 10 is a view of the guide of FIG. 9 engaging a slotted prosthetic stem; FIGS. 11A and 11B illustrate stem insertion with the guide of FIG. 9 in a prepared bone; and FIG. 12 illustrates another alignment guide of the invention. DETAILED DESCRIPTION OF THE INVENTION The following is a description of a preferred embodiment of the present invention in.use with a hip stem implant. Referring now to Figures 1 through 3, a prior art alignment guide 10 is illustrated. The alignment guide 10 comprises a body a guide portion 12, and an opening 14. The body and guide portion 12 form an outer shape that corresponds to at least a portion of a machined cavity 21 in the bone 20. The guide portion 12 has an inner circumference 15 having grooves or notches 16 for receiving flutes 31 of the stem 30 an implant 50. Protrusions 13 in the guide portion 12 may contact the stem 30 of the implant 50. In this embodiment, the grooves 16 and the protrusions 13 form a key for guiding insertion of the stem 30 into the bone cavity. In variations of this embodiment grooves 16, protrusions 13 or the like may be used alone or together to key insertion of the keyed stem 30. Figure 1 illustrates a prepared bone 20 having a machined cavity 21 and a longitudinal cavity 22 formed in the canal of the bone. At least a portion of the machined cavity 21 has a shape corresponding to the shape of the alignment guide 10. Figure 1 further illustrates an implant 50 comprising a stem 30 having flutes 31 distally extending along the length of the stem 30; a protrusion or proximal geometry 35 shaped to fit within the machined cavity 21 of the bone; and a neck 33 extending from the proximal end of the implant for receiving a ball portion of a ball and socket joint. The notches 16 in the inner circumference 15 of the guide are placed so that when the stem 30 is inserted into the alignment guide 10, the flutes 31 of the stem 30 fit within the notches 16 in the alignment guide 10 to hold or guide the stem 30 in an appropriate or aligned position within the bone cavity. In this embodiment the shape of the machined bone cavity 21 is generally tapered to form an elliptical end portion 23. The alignment guide 10 is similarly shaped with the body 11 forming an elliptical end portion 18 on the proximal end of the body. The implant 50 as well includes a similarly shaped proximal section with an elliptical end portion 34. The bone cavity 21, further comprises a cut 24 corresponding to tapers 19, 35 on the alignment guide 10 and the implant 50 respectively. The cut 24 of the bone cavity 21 and the taper 19 of the alignment guide 10 prevent the alignment guide 10 from being inserted too far into the bone cavities 21, 22. The cut 24 of the bone cavity 21 and the taper 35 of the implant 50 similarly prevent the implant 50 from being inserted too far into the bone cavities, 21, 22 and thus prevent the neck 33 from being in an improper position. In use, the alignment guide 10 is inserted into the machined bone cavity 21 with the elliptical end portion 18 of the alignment guide 10 in alignment with the elliptical end portion 23 of the machined cavity 21. The stem 30 of the implant 50 is placed in the inner circumference 15 of the guide portion 12 of the alignment guide 10 with the elliptical end portion 34 of the implant 50 generally in line with a groove 9 across the elliptical end portion 18 of the alignment guide 10. In order to insert the implant 50 properly aligned, the proximal geometry 32 is generally aligned so that it will fit within the machined bone cavity 21. The flutes 31 are placed within the notches 16 of the guide portion 12 and the implant 50 is driven into the bone cavity 22 guided by the alignment guide 10. When the implant 50 is generally in the bone cavities 21, 22 but prior to complete insertion, as illustrated in Figure 2, the alignment guide 10 is removed by inserting a device into the opening 14 in the guide 10 to remove it from the bone cavity 21 and from the stem 30 of the implant 50. The implant 50 is then further driven into place with the elliptical end portion 34 fitting within the elliptical end portion 23 of the bone and the taper 35 abutting adjacent a cut 24 of the bone cavity 21. An additional prior art alignment guide 110, illustrated in FIG. 4, has a handle 112 and an alignment body 114. The handle 112 is generally shaped so as to be easily grasped by a surgeon and has proximal and distal ends 116, 118. The alignment body 114 is located at the distal end 118 of the handle 112 and has a guide surface 120 disposed on an inner portion 122 thereof. The guide surface 120 is releasably engageable with an outer surface 124 of a prosthetic stem 126 as shown in FIG. 5. The illustrated stem 126 is adapted for use as the femoral component of a hip prosthesis, however, the system of the invention is useful with a variety of prosthetic implants, especially those suited for implantation in a long bone. The prosthetic stem 126 may have proximal and distal portions 128, 130 and have at least one flute 132 formed on its outer surface 124. The distal portion 128 of the stem 126 is substantially cylindrical and may also include a slot 134, in particular, a coronal slot. The proximal portion 128 of the stem may have an angled region 136 or proximal spout having an elliptical cross-section leading to a neck 138. The preparation of a bone cavity (such as bone cavity 21 of FIG. 1) adapted to such a geometry is illustrated in U.S. Patent No. 4,790,852 to Noiles. A person of ordinary skill in the art will recognize that other reaming or broaching techniques may also be used to create a bone geometry corresponding to the illustrated stem 126. For use, stem 126 has at least one surface element or keyed component, such as flute 132, disposed on its outer surface 124 that is angled with respect to the outer surface 124 and extends in the direction of a longitudinal axis 140 (PIG. 6) of the stem 126. As illustrated in FIGS. 1-2 and 6-8, the stem surface elements may include a plurality of flutes. Generally, these angled surface features 132 may include any combination of indentations or protrusions from the outer surface 124 suitable to hold the stem 126 against rotational misalignment when engaged with the alignment guide 110. The guide surface 120 of the alignment guide 110 engages the outer surface 124 of the stem 126, as shown in FIGS. 6-7, around the cylindrical stem 126 generally up to about 180°. The guide surface 120 also includes one or more angled guide surface features 142 which form a key to engage the stem surface features 132 to hold the stem 126 against rotation. The angled guide surface features 142 are also formed so that the guide 110 may be removed from its engagement with the stem 126 in a plane transverse to the longitudinal axis 140 as illustrated by the lateral axis 144 and referred to herein as a lateral direction, which, in the illustrated embodiment, is directed from lateral to medial for a hip prosthesis embodiment. In the illustrated embodiment, the outermost guide surface features 142 (FIG. 7) each have an outer angled surface 146 formed to allow lateral disengagement. That is, rather than contacting a stem surface feature 132, the outer angled surface 146 is generally in the direction of the lateral axis 144 so as not to engage a lateral facing surface 148 of the stem surface element 132, thus permitting lateral removal of the alignment guide 110 from the stem 126. In the embodiment illustrated in FIGS. 4 to 8, the alignment guide 110 may have a tapered outer shape 150 to correspond to the shape of the bone cavity into which the stem 126 is being implanted. The alignment guide 110 may also have an alignment marker 152 located on a proximal surface 154 of the guide body 114 to provide a visual aid for engaging the stem 126 with the alignment guide 110. A method for inserting a prosthetic stem 126 into a long bone begins with the preparation of a suitable cavity in the long bone. Where, as illustrated in FIG. 8, the stem 126 includes features such as a tapered proximal spout 136, the cavity should be formed so as to correspond to such features. The prosthetic stem 126 is placed at the opening of the cavity in the long bone in a predetermined rotational orientation and is engaged by the alignment guide 110. The prosthetic stem 126 is inserted into the cavity while maintaining contact with the alignment guide 110. A surgeon may apply lateral pressure using the handle 112 in order to ensure a sufficient engagement between the prosthetic stem 126 and the alignment guide 110 to prevent undesired rotation of the stem 126. The alignment guide 110 may be removed from the stem 126 prior to final seating of the stem 126 within the cavity. In addition, surface features such as flutes 132 may be longitudinally extended in a proximal direction in the area where the guide 110 meets the stem 126 in order to provide a greater duration of contact between the stem 126 and guide 110. Although the alignment guide of the present invention is described with respect to a particular embodiment, i.e., in use with a fluted hip stem using the flutes of the stem as a key or reference, it will be apparent to those skilled in the art that various modifications may be made without departing from the character and scope of the invention as defined by the appended claims. For example, the insertion guide may be used to assist the insertion of any stem like component having a keyed feature on the outer surface of the stem to key the insertion of the stem in a properly selected aligned position. The alignment guide may have any number of indentations or protrusions to key in the proper insertion of the stem of the implant. In particular, the invention employs a blade or other protrusion fitted to a slot in the stem to orient and guide the stem into a fitted insertion position. Embodiment of the invention allow the stem flutes to be optimized for broaching or cutting a fitted passage and securing the stem to the bone without requiring the flutes to also sustain shear forces imposed during insertion by a guiding surface. As seen in FIG. 5, a suitable slot 134 may be already provided on some stems, and this slot is generally oriented to reduce stem stiffness along one direction, while allowing some flexibility of fit of the distal stem as it enters the prepared bone bore. FIG 9 illustrates an alignment guide 210 of the invention configured for slot engagement. In this embodiment, the guide has a blade 215 which protrudes from one end of a handle 217, which as illustrated is provided with gripping features such as knurling, finger grooves or the like. An intermediate portion of the guide has a surface 219 shaped to nest or fit within the prepared curved and inclined spout bore of the resected bone end, e.g., against surface 24 (FIG. 1). Surface 219 thus orients blade 215 along a central parallel plane. Similarly the prosthesis slot precisely fits the blade, and is oriented along a central plane of symmetry of the prosthesis post and shoulder, so that the prosthesis is thus oriented to seat correctly when fully inserted. As further illustrated in FIG. 9, the main body of the handle 217 has a distal end face 217a which crosses the handle longitudinal axis substantially parallel to the bone insertion axis, i.e., at an angle of about 30° corresponding to the angle between the bone canal axis and the axis of the prepared bone spout bore. This allows the prosthesis stem to slide along the blade 215 close to and substantially parallel to the face 217a, providing maximum strength, rigidity and precision of the guide mechanism. By way of example, the thickness of blade 215, and the corresponding stem slot width, may be on the order of 2-5 millimeters. The handle may be 10-15 centimeters long, with the blade extending an additional 1-4 centimeters, preferably resulting in a relatively short, maneuverable guide with a total length under about 14 centimeters. The guide is advantageously machined or fabricated of a solid metal, such as titanium or a stainless steel or surgical alloy. FIG 10 shows the guide of FIG. 9 in engagement with a prosthesis stem 230 similar to the stem of FIG. 5. As shown, the blade 215 fits precisely in slot 234, thus aligning and centering the prosthesis surface 239 with guide surface 219. As the stem is inserted in bone along an insertion axis A, the blade 215 rides up the slot 234. Once the stem is securely tracking as it is inserted in the bone the blade 215 may be withdrawn along the direction of the handle axis H i . For ease of withdrawal, the handle has grip-enhancing features at its proximal end, and is also provided with a cross-hole 211 (FIG. 9) which may receive a T-bar or be engaged by an engagement hook of an impact puller, to exert a withdrawal force along axis H i . As noted above, in existing slotted stem prostheses, the slot serves a compliance function allowing a reduction of bending stiffness; it may also be compressed as the stem tip is driven in and subjected to radial compressive force. To prevent binding of the guide in the stem as the slot is closed, it is therefore desirable to fabricate the blade or slot with this added consideration in mind. For example, width dimensions may be set to initially achieve a slight clearance therebetween, such that as the distal tip commences to cut its way along the machined bone canal, the two components acquire a precise fit. Alternatively, the slot may taper wider toward the stem proximal end to facilitate withdrawal once the stem has been sufficiently inserted. Similarly, the blade to stem contact area may be reduced, for example by hollow-grinding the central face of the blade, so that despite high compressive forces the blade may slide along the slot and be easily withdrawn. Furthermore, in conventional prostheses, such as titanium prostheses, the slot extends only partially up the distal stem, and this slot length may be inadequate for achieving a sufficiently stable insert direction before the blade has to be withdrawn. In the prototype alignment guide shown in FIG. 9, the distance during which the stem remains precisely guided during insertion is made long or maximized by positioning the blade vertically offset along the stem insertion axis - i.e, by having it project from the lower portion of the handle cross section. This limitation may also be addressed by providing a slot 234 which extends further up the prosthesis stem, a procedure which may be readily implemented for stiffer, stronger prosthesis alloys such as cobalt chrome. In other embodiments, the distal stem flutes are made slightly larger in diameter to assure sufficient bite in the bone canal early in insertion to guarantee straight insertion after being guided only a short distance by the alignment tool. Thus, blade offset and stem and slot width and length may each be adjusted to assure that the blade functions as an effective guide. FIGS 11A and 11B illustrate the use of the alignment guide 210 of FIG. 9. As shown in FIG. 11A, the guide is nested against the cylindrical prepared surface of the angularly oriented spout channel machined in the proximal femur 20, and the prosthesis stem 230 is lowered over the blade 215 while care is taken to visually align the stem along the axis of the bone. Thereafter, the stem is driven into the bone, while the blade 215 provides a firm and precise guide surface, constraining the stem of the prosthesis in the desired guide plane while allowing straight movement deeper into the bone canal. In addition to the embodiment of FIG. 9, the present invention contemplates an embodiment of the alignment guide wherein a structure corresponding to the hand-grip portion of the handle is optional, and , if provided, is a separate element or tool of the system. In this embodiment, an essential portion of the guide includes only a body that seats in a defined orientation against the prepared bone surface, and that possesses a blade protruding therefrom and positioned across the central plane in which the stem alignment slot is to be positioned. Such an alignment guide embodiment 310 is shown in FIG. 12. As seen therein, the guide possesses a slot-engaging blade 315 that extends radially across the bone canal from the edge of an arm 316, which in turn extends downwardly from a positioning body 318. As in the embodiment of FIG. 9, the positioning body 318 possesses a surface 319 which aligns against a prepared bone surface. It also has a gripping hole 323 for attaching a handle or extractor to allow removal of the body 318 along an extraction direction parallel to the spout bore, after the stem has been inserted to a substantial depth.

Claims: An alignable orthopedic prosthesis system for implantation into a prepared bone cavity, said system comprising: (a) a prosthesis including a stem (230) having a proximal portion, a distal portion, and an axis (A i ), with a slot (234) extending along the axis of the stem, and (b) an alignment guide including an alignment handle (217) having an outer shaped surface portion (219 ; 319) corresponding to at least a portion of the prepared bone cavity, characterised in that said alignment guide further includes a guide blade (215; 315) disposed on an inner portion of the alignment handle and keyed for engaging the slot (234) of the prosthesis, said outer shaped surface portion (219; 319) being positioned such that when said shaped portion seats against the prepared bone cavity, the blade (215; 315) is located in a plane effective to fix rotational alignment of the stem (230) while allowing sliding insertion. The prosthesis system of claim 1, wherein the prosthesis comprises a stem (230) of a prosthetic joint assembly. The prosthesis system of claim 1 or 2, wherein the blade (215; 315) comprises at least one relief portion for reducing binding in the slot (234). The prosthesis system of claim 1, 2 or 3 wherein the blade (215; 315) effectively fills the cross-section of the slot (234). The prosthesis system of any preceding claim, wherein the blade (215; 315) extends transverse to the stem (230). The prosthesis system of any preceding claim, wherein the alignment guide (210; 310) further comprises means (217; 211; 323) for effecting removal of the guide from the stem (230) along an axis (H i ) parallel to or a direction away from the prepared bone surface. The prosthesis system of any preceding claim, wherein the proximal portion of the stem (230) comprises a proximal geometry shaped to fit said prepared bone cavity. The prosthesis system of any preceding claim, wherein the outer shaped surface portion (219; 319) fits against a cylindrical prepared bone surfaces (21) to align the blade (215; 315) along an axis (H i ). The prosthesis system of claim 8, wherein the handle (217) and blade (215; 315) of the alignment guide both extend alon said axis (H i ). The prothesis system of any preceding claim, wherein an outer portion of the handle is removably attached to the remainder of the guide (318). A prosthetic stem system comprising: (a) a prosthetic stem (230) including a longitudinal axis (A i ) a slot (234) extending along the axis, and an outer surface (239) having at least one stem surface feature extending along the outer surface in a substantially longitudinal direction for penetrating and gripping bone (20), and (b) an alignement guide (210; 310) including a handle (217) having proximal and distal ends, characterised in that said alignment guide further includes an alignment blade (215 ; 315) disposed on the distal end of the handle to releasably engage the slot (234) as a guide surface that extends parallel to the handle across the slot axis to engage inside slot surfaces and provide rotational stability to the prosthetic stem during insertion of the stem into a bone (20), said alignment guide (210; 310) being oriented by contact with a prepared bone surface to guide the slot during prosthesis insertion to fit a final insertion position. The stem system of claim 11, wherein the slem (230) is fluted. The stem system of claim 11 or 12, wherein the blade (215 ; 315) is oriented to be in plane aligned with the spout of a femoral prosthesis. The stem system of claim 11, 12 or 13, wherein the blade (215; 315) is laterally disengageable from the stem by movement along the axis (H i ) of the handle (217). The stem system of any one of claims 11 to 14, wherein the stem (230) is elongated, substantially cylindrical and implantable within a prepared bone cavity of a long bone (20). The stem system of claim 15, wherein the stem (230) forms at least part of a femoral component for a hip prosthesis. The stem system of any one of claims 11 to 16, wherein the stem (230) includes a proximal spout. The stem system of any one of claims 11 to 17, wherein the stem (230) includes plural distal slots. The stem system of any one of claims 11 to 18 wherein the alignment guide handle (217) comprises an outer surface shape (219) corresponding to at least a portion of the prepared bone cavity for orienting the blade (215; 315). The stem system of any one of claims 11 to 19, wherein the slot (234) provides bending fit of the stem (230) and the slot tapers wider toward its proximal end to reduce binding on the blades (215; 315). The stem system of any one of claims 11 to 20, wherein the proximal end of the handle is removably attached to the remainder of the guide (318).

Inventors: JAGGERS CHARLES W, MANASAS MARK ALLAN, OSTIGUY PIERRE S JR, SALVAS PAUL, VARIEUR MICHAEL S, JAGGERS, CHARLES W., MANASAS, MARK ALLAN, OSTIGUY, PIERRE S., JR., SALVAS, PAUL, VARIEUR, MICHAEL S.
Applicants: DEPUY PRODUCTS INC, DEPUY PRODUCTS, INC.
Application year: 1999
Main classification: A61F2
IPC codes: A61F2