Method and apparatus for determining acetabular component positioning

Abstract

An instrument for establishing orientation of a pelvic prosthesis comprises a tripod having an angularly adjustable guide rod on it. The tips of the legs define a plane, and the guide rod is set by the surgeon to a defined orientation with respect to this plane on the basis of preoperative studies. In use, two of the legs of the instrument are positioned by the surgeon at defined anatomical locations on the pelvis (e.g., a point in the region of the posterior/inferior acetabulum and a point on the anterior superior iliac spine). The third leg then lands on the pelvis at a point determined by the position of the first two points, as well as by the separations between the third leg and the other two legs. The position of the guide rod then defines with respect to the actual pelvis the direction for insertion of a prosthesis.

Claims

1. An instrument for establishing an ipsilateral hemipelvic reference plane for a pelvis, the pelvis including an acetabulum, an anterior superior iliac spine, an ischium, and an ilium, the instrument comprising: a hub having a vertical axis; first and second arms emanating from the hub, the first and second arms angularly separated from each other, having ends opposite the hub, and defining a nominal plane; first, second, and third legs, the first leg attached at or proximate to the end of the first arm, the second leg attached at or proximate to the end of the second arm, and the third leg attached at or proximate to the hub, the first, second, and third legs extending from the nominal plane defined by the first and second arms, and terminating in tips; and an alignment indicator attached to the hub and extending out from the hub, the alignment indicator rotatable in the nominal plane defined by the first and second arms to have an azimuth orientation, and the alignment indicator pivotable between the vertical axis of the hub and the nominal plane to have an elevation, wherein when the instrument is docked to the pelvis: the tip of the first leg contacts a region of the ischium where it joins a posterior wall of the acetabulum, the tip of the second leg contacts the anterior superior iliac spine, and the tip of the third leg contacts the ilium at a point free of identifiable landmarks, to establish the ipsilateral hemipelvic reference plane at the pelvis.

2. The instrument of claim 1 further comprising an angular indicator, the angular indicator including first and second angular scales for measuring the azimuth orientation and the elevation.

3. The instrument of claim 2 wherein the first and second angular scales are mounted on the instrument for movement relative to each other.

4. The instrument of claim 2 wherein the first angular scale is fixed to the instrument and the second angular scale is mounted for rotation with respect thereto.

5. The instrument of claim 4 wherein the alignment indicator is pivotally mounted on the second angular scale.

6. The instrument of claim 1 further comprising an angular indicator, the angular indicator including a first angular scale mounted on the instrument for measuring the azimuth orientation, and a second angular scale mounted for rotation in the nominal plane with respect to the first angular scale for measuring the elevation.

7. The instrument of claim 6 wherein the alignment indicator is pivotally mounted on the second angular scale.

8. The instrument of claim 1 wherein the first and second arms include arm segments that are extendable with respect to the first and second arms, the arm segments having scales thereon for indicating a degree of extension of the arm segments.

9. The instrument of claim 1 wherein a distance between at least two of the first, second, and third legs is adjustable.

10. The instrument of claim 1 wherein one or more of the first, second, and third legs is removably attached.

11. The instrument of claim 1 wherein a direction in space indicated by the alignment indicator is relative to the ipsilateral hemipelvic reference plane established by the instrument.

12. The instrument of claim 1 wherein the nominal plane defined by the first and second arms is parallel to the ipsilateral hemipelvic reference plane established by the instrument.

13. An instrument for establishing a hemipelvic reference plane for a pelvis having an ilium portion, the instrument comprising: a frame having a vertical axis and defining a nominal plane; first, second, and third legs having tips, the first, second, and third legs mounted to the frame, and having a spacing between the tips of the first, second, and third legs; and an alignment indicator attached to the frame, the alignment indicator extending out from the frame and pivotable between the vertical axis and the nominal plane to have an elevation, wherein the spacing between the tips of the first, second, and third legs is preconfigured prior to docking the instrument on the pelvis such that, when docked to the pelvis: the tip of one of the first, second, and third legs contacts the ilium portion of the pelvis at a point having no identifiable landmarks for identification by a surgeon, and the instrument establishes the hemipelvic reference plane for the pelvis.

14. The instrument of claim 13 wherein the tips of the first, second, and third legs are pointed tips for secure lodgment on a human body structure.

15. The instrument of claim 13 wherein the frame includes first and second arms, and the first, second, and third legs are formed to extend fixed distances from a plane defined by the first and second arms, and the tips of the first, second, and third legs define the hemipelvic reference plane.

16. The instrument of claim 13 in which the first, second, and third legs are of equal length.

17. The instrument of claim 13 wherein at least one of the first, second, and third legs is shorter than the others.

18. The instrument of claim 13 wherein at least one of the first, second, and third legs is hollow to accommodate a rod affixable to a human body structure to facilitate placement of the instrument on the pelvis.

19. The instrument of claim 13 wherein the hemipelvic reference plane is established by the tips of the first, second, and third legs and the nominal plane defined by the frame is parallel to the hemipelvic reference plane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention description below refers to the accompanying drawings, of which:

(2) FIG. 1 is a schematized view of a left pelvis showing three points for defining an ipsilateral hemipelvic plane in accordance with the invention;

(3) FIG. 2 is a view in perspective of a prototype instrument used to test the concept of the invention;

(4) FIG. 3 is a view in perspective of one embodiment of an instrument suitable for use in establishing the reference planes and orientations in accordance with the invention;

(5) FIG. 3A is a partial view of the instrument of FIG. 3 form the direction 3A-#A in FIG. 3;

(6) FIG. 3B is an illustrative sketch showing the fitting of a leg to a conduit; and

(7) FIG. 4 is a view in perspective of yet another embodiment of an instrument suitable for use in establishing the reference planes and orientations in accordance with the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

(8) FIG. 1 is schematized view of a right pelvis showing the three points in question. Distance d.sub.1-2 is the distance between the first point, P.sub.1, (a point located on the ischium as it joins the posterior wall of the acetabulum) and the second point, P.sub.2, a point located on the lateral surface of the iliac wing, immediately adjacent to the anterior superior iliac spine. Distance d.sub.1-3 is the distance between the first point, P.sub.1, and the third point, P.sub.3 (on the ilium); and distance d.sub.2-3 is the distance between the second point, P.sub.2 and third point P.sub.3. The distance d.sub.1-2 is the baseline; the distances d.sub.1-3 and d.sub.2-3 are set at a distance of from 70% to 100% of the baseline distance, preferably about 80-90%. In FIG. 1 they are shown as being approximately 80-85% of the baseline distance. In FIG. 1, distances d.sub.1-3 and d.sub.1-3 are shown as approximately of equal length, although, as noted above, they need not necessarily be so. These three points, P.sub.1, P.sub.2, and P.sub.3 define an ipsilateral plane.

(9) The surgeon defines the desired orientation of the prosthetic acetabular components and the alignment indicator with respect to this ipsilateral pelvic plane. This may be done directly with respect to the ipsilateral pelvic plane so defined, or it may be done with reference to the anterior pelvic plane as established, for example, from a 3D computer model of the patient's pelvis based on CT, MR, or other imaging. While patient specific individual 3D models may be the most accurate method, other methods of determining the desired orientation of the alignment indicator and the ipsilateral pelvic plane based on statistical averages of pelvic structures and sizes of patients of similar size, sex, weight, age and/or other known characteristics may be used.

(10) FIG. 2 is a view in perspective of a prototype instrument used to test the concept of the present invention. The prototype was formed from readily-available components in order to quickly test the concept. The instrument has first and second arms in the form of metal rods 10, 12, respectively, emanating from a hub 14 comprising first and second adjustable clamps 14a and 14b to hold the arms 10, 12, respectively, in a desired positioned when tightened. Arm 10 was formed from rods 10a and 10b secured together by an adjustable clamp 10c to enable extension of rod 10b with respect to rod 10a to a desired position. Similarly, arm 12 was formed from rods 12a and 12b secured together by an adjustable clamp 12c to enable extension of rod 12b with respect to rod 12a to a desired position.

(11) At the outer extremity of arms 10b and 12b are legs in the form of rods 16 and 18, respectively, held in adjustable clamps 10d and 12d, respectively. Similarly, a leg in the form of a rod 20 extends from clamp 14a. In the prototype, the clamps were formed from readily available washers and nuts which can be loosened and tightened to hold a component in a desired position. The legs 16-20 were standard trochars having a pointed tip to enable extension through the skin of a patient and secure lodgment on the pelvic bone. The legs extended downwardly the same distance from the arms 10 and 12 and the hub 14 and thus the arms were essentially in a plane parallel to the plane defined by the tips of the legs.

(12) The legs are shown positioned on a model of the pelvic anatomy for performance of a surgical procedure on the right acetabulum. The first leg, 16, rests on a point, P1, the anchor point, located on the ischium as it joins the posterior wall of the acetabulum. The second leg, 18, rests on a point, P2, located on the lateral surface of the iliac wing, immediately adjacent to the anterior superior iliac spine; the third leg, 20, rests on a point, P3, on the ilium; the location of this point is determined by the length of the arms 10 and 12 as set by the surgeon, as well as the relative angular orientation of these arms and the surface of the bone.

(13) A direction guide 22 is adjustably secured to the frame. This guide will define the orientation for insertion of a prosthesis or the performance of other surgical procedures. The orientation of the guide with respect to the reference points P.sub.1, P.sub.2, and P.sub.3 is established by the surgeon in pre-operative or intra-operative planning. When placed on a patient in the manner described above, the instrument defines an ipsilateral pelvic plane with respect to which the orientation of the guide 22 may readily be established by imaging, by direct measurement, or by other known means. From CT or other imaging studies, the orientation of this plane with respect to the commonly used anterior pelvic plane may be determined, and thus the orientation of the guide with respect to the anterior pelvic plane may be established if desired. During subsequent surgery, when the instrument is positioned on the patient in accordance with the reference points, the guide provides the surgeon with a reference axis for insertion of a prosthesis such as an acetabulum at a desired orientation. In the prototype, the guide was secured to one of the arms (e.g., arm 12) by means of an adjustable clamp 24. Although the guide is shown in FIG. 2 as a simple solid rod, it may advantageously be a hollow tube through which a pointed rod may be inserted into the patient. The rod is screwed into the bone and the instrument then removed, leaving the rod as the guide for acetabular component insertion or other procedure. Alternatively, a hollow tube may be mounted parallel to the guide and a pin inserted through this tube into the patient's bone to set the desired orientation for insertion of a component or for other surgical procedure.

(14) In use, the surgeon places the patient in an appropriate position for the procedure to be performed. For example, in replacing the acetabulum of the right pelvis of a patient, the patient may be placed on his or her left side and a surgical field of sufficient size is created in order to expose the right acetabulum. The surgeon selects the point P1, preferably in the region of the ischium as it joins the posterior wall of the acetabulum. This is the Anchor Point, and is generally exposed in the surgical field during the operation, so that it is readily located. Leg 16 is positioned on this point. The surgeon next selects a point, P2, on the lateral surface of the iliac wing, preferably immediately adjacent to the anterior superior iliac spine. This region is usually readily located by palpation, and need not be within the exposed incision, but rather simply within the sterile surgical field. The surgeon then adjusts the length of arm 10 (and, if necessary, arm 12) to position leg 18 on P2, while ensuring that leg 20 lands on the hemipelvis, and preferably on a relatively flat area so that the leg will not slip. This defines point P.sub.3.

(15) Placing the first and second legs on the patient as discussed above, and fixing the distances d.sub.1-3 and d.sub.2-3 between the legs in the manner discussed above, fixes the point at which the third leg of the instrument will land on the patient. For acetabular component placement, this point will preferably be on the ilium, generally above the sciatic notch. The area in which it lands is a relatively strong portion of the anatomy, and thus is amenable to receiving the third leg to provide a stable reference plane on the patient. However, it is devoid of significant landmarks, and thus without the aid of the present instrument would not serve as an area which would help to establish a reference for component placement.

(16) As noted previously, a simple but effective rule for adjusting the lengths of the arms 10, 12 is to set the distance d.sub.1-3 between P1 and P3, as well as the distance d.sub.2-3 between P2 and P3, at some ratio of the distance d.sub.1-2 (the baseline distance) between P1 and P2. A ratio of from about 70% to 100% appears to work satisfactorily, but I have found that a ratio of about 80-90% works well in most cases.

(17) FIG. 3 is a view in perspective of another embodiment of an instrument in accordance with the invention that is more suited for replication and widespread use. A turret 50 is pivotally mounted on a base 52 for rotation about a vertical axis 54. (In this connection, it will be helpful to view also FIG. 3A which is a partial view of the instrument from the direction 3A-3A in FIG. 3). Wings 56, 58 on the turret mount a guide 60 for rotation in a vertical plane about a horizontal axis 55 that is transverse to axis 54. First and second extensible arms 62, 64 are mounted in base 52 for rotation with respect to the base and thus to each other. This enables the surgeon to adjust the angle a between the arms as desired. Of course, this may also be accomplished with one fixed arm and one rotatable arm. (It will also be understood that both arms may be fixed at a set angle .)

(18) Extensions 62a, 64a, extend from arms 62, 64, respectively. The extensions 62a, 64a may telescope from their respective arms as shown in FIG. 3, for example, in a manner similar to that of a camera tripod. Alternatively, they may be mounted for sliding overlap with these arms or may use other known forms of extension. Mounted at the ends of extensions 62a, 64a are hollow guides 66, 68, respectively. Legs 70, 72, have sharpened tips 70a, 72a, respectively, and heads 70b, 72b, respectively. The legs preferably also have a short boss 70c (see FIG. 3B) for mating with a corresponding groove on the interior surface of conduits 66, 68. The legs extend downwardly from the arms and the hub to the same extent, and thus the arms are essentially in a plane parallel to the ipsilateral plane formed by the tips of the legs.

(19) In use, the surgeon grasps the heads 70b, 72b of the legs 70, 72 and inserts them into the conduits 66, 68 so that the bosses 70c, 72c align with the corresponding grooves in the respective conduit. He or she then rotates the heads to thereby removably lock the legs into the guides. In similar fashion, a leg 74 having a sharpened tip 74a and a head 74b is removably insertable into, and lockable within, the base 52. This construction facilitates cleaning of the instrument by enabling the rapid disassembly and reassembly of the legs. Of course, the legs could also be permanently fixed to the respective arms.

(20) The tips 70a, 72a, and 74a of the legs 70, 72, 74, respectively, define a plane, the ipsilateral plane. Axis 54 is perpendicular to this plane, while axis 55 is parallel to it.

(21) Thus, the orientation of guide 60 relative to the ipsilateral plane is defined by the orientation of the guide relative to axis 55 (which defines the azimuthal orientation or angle of the pointer) and axis 54 (which defines the elevational orientation or angle of the pointer). To facilitate setting or determining these angles, scales (not shown) may be attached to, or marked on, the turret 50 and the base 52.

(22) The instrument of FIG. 3 is used in the same manner as that of FIG. 2, i.e., for operation on the right hip, the tip of leg 70 is located on the selected point P1, the tip of leg 72 is located on selected point P2, and the lengths of these legs (as well as the angle if appropriate) is adjusted to ensure that the tip of leg 74 lands on the hemipelvis, preferably in a relatively flat region, to thereby fix point P.sub.3. This orientation of the instrument thereby recreates the ipsilateral plane defined in the preoperative studies. When the guide 60 is then set to the angles determined as desirable in the preoperative studies, it is oriented with respect to the patient in the desired manner. It may then be used to guide the component insertion tools along the appropriate direction. To do this, the surgeon may simply visually align the tool with the guide during the insertion. Preferably, however, the guide 60 is hollow and an elongated pin is extended down the guide while it is in the desired position on the patient, and the pin is then anchored into the patient. The instrument may then be removed, the pin preserving the desired orientation for placement of the prosthesis.

(23) The important parameters of the instrument can be easily adjusted so that the instrument can effectively be customized to each patient in a very short period of time, taking just a minute or two, and can be done while the instrument is sterile on the operating table. Important adjustments are the lengths of arms 62 and 64, the angle between these arms, and the orientation of the alignment indicator. While the angle (alpha) between arm 1 and arm 2 can be adjustable, the preferred angle can be fixed at approximately 67.5 degrees, which is the angle defined if the distances between P1 and P3 and between P2 and P3 are both 0.9 times the distance between P1 and P2. Similarly, while the length of arms 60 and 62 need not be the same, the preferred embodiment of the instrument is such that the arms are of approximately the same length relative to each other, although the length of arm 1 and arm 2 varies from one individual to the next.

(24) The patient-specific adjustable variables for each operation then are the lengths of the arms and the orientation of the alignment indicator or guide relative to the plane of the instrument body. The lengths of the arms can be determined before surgery or even during surgery using a number of methods, each with a differing degree of precision. The most precise method of determining the desired lengths of the arms is by three dimensional imaging of the individual patient, typically by CT or MR imaging. In this way, points P1 and P2 can be determined by the surgeon on the computer model and the location of P3 can be automatically calculated as the unique point lying on the bone surface at a specified distance from P1 and P2. Less precise, but potentially suitable methods of determining the lengths of the arms include: 1. An overall average of all patients; 2. An overall average of all patients of the same sex, height, weight, diagnosis; 3. As statistically predicted from measurements from magnification corrected plain radiographs of the patient to be treated and matching those radiographs statistically with radiographs and three-dimensional reconstructions of similar patients who have had CT, MR, or other methods of deriving three-dimensional models. This method can be referred to as 2D to 3D statistical modeling; or 4. Directly measuring the distance between P1 and P2 at surgery, which can even be performed using the instrument itself.

(25) Next, the orientation of the alignment indicator relative to the plane of the instrument body must be decided. This determination is based on two or more factors. The first is the relative orientation of the hemipelvic plane (P1, P2, P3) relative to the overall pelvis and/or the anterior pelvic plane. The second is the surgeon's desired cup position relative to the overall pelvis and/or the anterior pelvic plane in the case of acetabular cup replacement surgery. Finally, an additional variable might include an adjustment for anticipated orientation of the overall pelvis for various activities and positions after surgery. This third variable is mentioned because there are variations in the way different patients' pelvises are orientated, some with more pelvic tilt and some with less pelvic tilt. (Klingenstein G, Eckman K, Jaramaz B, Murphy S. Pelvic Tilt Before and After Total Hip Arthroplasty, International Society for Computer Assisted Orthopedic Surgery, 2008.) This is particularly true of patients with a fused lumbosacral spine. If the individual postoperative patient pelvic orientation can be predicted preoperatively, then this factor can be incorporated into the planning of the desired orientation of the alignment indicator.

(26) As with determination of the lengths of the arms, orientation of the alignment indicator can be determined by the above combined with knowledge of the anatomy of the individual patient's pelvis, aided by determination of one or more of the factors discussed above. Using these methods of adjusting the instrument body, adjusting the alignment indicator, and applying the instrument in surgery, the desired orientation of an acetabular component can be rapidly and reliable determined during surgery. Further, the method avoids the unreliable influences of local anatomy that is frequently deformed and further distorted by the arthritic process.

(27) FIG. 4 shows yet another embodiment of the invention in which the extensible arms are formed by slidable beams and in which the conduits holdings the legs are themselves removable for ease in cleaning. In particular, the instrument has first and second arms 102 and 104 formed from first and second arm segments 102a and 104a, respectively, fixed to and extending from a hub in the shape of a cylindrical conduit 105 having a central bore extending therethrough. Markers 102c, 104c on arm segments 102b, 104b, respectively, indicate the amount of extension of the arm segments, and thus the length of the arms. In the configuration shown, the arm segments 102a, 104a are positioned at a fixed angle to each other, advantageously on the order of 67.5 degrees.

(28) A mating rod segment 110 having a leg 116 extending therefrom is snugly but removably press-fitted into the conduit 105 to form one leg of the tripod 100. Similarly, rod segments 106 and 108, having legs 112, 114, respectively, extending therefrom are snugly but removably press-fit into hollow-bored conduits 107, 109, respectively, formed on the ends of arms 102b, 104b, respectively. Removal of the legs facilitates sterilization of the instrument before each use, and also makes the instrument more compact for storage.

(29) A first plate 120 is fixed to the hub 105; the plate has a scale 120a thereon. A second plate 122 is pivotally mounted on the hub 105 for rotation in the horizontal plane about a vertical axis 124 with respect to the first plate 120. A releasable lock 123 fixes is the angular orientation of the plate at the orientation set by the surgeon. A guide 128 is pivotally mounted on the second plate for rotation in the vertical plane about a horizontal axis 126. Plate 120 has a scale 120a for indicating the angular orientation of the plate 122 with respect to it (the azimuthal angle). Similarly, plate 122 has a scale 122a for indicating the angular orientation of the guide 128 with respect to the plate (the elevation angle). As was previously the case, the tips 112a, 114a, and 116a of legs 112, 114, 116, respectively, define a plane (the ipsilateral hemipelvic plane) and the arms 102 and 104 are parallel to this plane. Thus, the orientation of the guide can be referred to this plane and thus also to the anterior pelvic plane if desired.

(30) In this embodiment, the conduits 112, 114, 116 are detachable from the arms 102 and 104 and the hub 105, respectively. Advantageously, they may simply form a force-fit, although other means of connation may be used. This facilitates sterilizing the instrument for repeated use, while enabling its ready reassembly in the operating room. It also enables the instrument to be stored in a more compact package.

(31) In still a further variation of the instrument of FIG. 4, one of the legs, e.g., the leg 112, rather than being removable, may in fact comprise a hollow conduit of the same length as the legs 114 and 116. With this configuration, the surgeon may first select the anchor point P.sub.1, insert a pointed rod (trochar) into the anchor point, slide the hub 106 onto the rod, and then position the legs accordingly as described above, while being sure that the instrument frame will not slide from the hip during positioning.

(32) A further variation in the use of the instrument involves its combination with computer-assisted surgical navigation using optical, electromagnetic, or other means of tracking. In any of these processes, a coordinate system for the pelvis must be defined, is where this coordinate system be the anterior pelvic plane or another plane. Since the mathematical relationship between the ipsilateral hemipelvic plane, as defined by the mecahnical instrument, and any other plane, such as the anterior pelvic plane, can be determined preoperatively, placement of the instrument onto the ipsilateral hemipelvis and then measurement of its location using a navigation system can be used to rapidly determine the orientation of the pelvis. The mechanical instrument can then be removed and surgical navigation can proceed as usual thereafter.

(33) Similarly, surgical navigation can be facilitated by rapidly determining the overall orientation of the pelvis using the same method as above, but with a virtual mechanical instrument. In this way, P1 is determined by the surgeon using a digitizer (optical, electromagnetic, or other). Next, the surgeon defines P2 using the digitizer. The virtual instrument defines the distance P1-P2 and so P2 must lie on a sphere whose radius is P1-P2 and must also lie on the bone surface. As such, two of three degrees of freedom for determining the point P2 are predetermined and only one of the three degrees of freedom for determining the point P2 is subject to surgeon choice. This greatly improves the accuracy of determining this point. Finally, the surgeon defines P3 using a digitizer. Using the virtual instrument, the location of point P3 must lie on the bone surface, must be a specified distance from point P1, and must be a specified distance from point P2. Thus, the virtual mechanical instrument defines all the three degrees of freedom that determine the location of the point P3 and its location is not subject to surgeon choice. Further, this very specific point can be determined on a flat, indistinct surface where there are no palpable landmarks.

(34) The instrument described herein is well suited to a variety of surgical approaches is to hip arthroplasty. Commonly, the legs will be of equal length and perpendicular to the arms. However, in some cases, such as when using an anterolateral exposure, it may be desirable to tilt the plane of the arms so as to provide further clearance for optimum positioning of the surgical instruments. This is readily accomplished with the instrument described herein by shortening one or more of the legs.

(35) Specifically, I have found that shortening the leg that is to be positioned adjacent the ASIS point, thereby tilting the plane defined by the arms of the instrument with respect to the plane defined by the tips of the legs, can enhance use of the instrument when performing surgery using an anterolateral exposure. It does this by providing additional clearance between the instrument and the surgical instruments in the positions that they are commonly used for this type of procedure.

(36) With this structure, the plane of the tips of the legs will no longer be parallel to the plane of the arms and the direction indicator (e.g., the indicator formed by plates 120 and 122 of FIG. 4) will not provide the same readings as when the legs are of equal length. However, knowing the lengths of the legs, the orientation of the direction indicator with respect to the plane of the tips of the legs (and thus to the anterior pelvic plane) may readily be determined and the azimuthal and elevational angles corresponding to a desired orientation with respect to the anterior pelvic plane set correspondingly.