Abstract
A partial hip prosthesis for reducing friction and wear in partial hip prosthesis by combining optimized geometry of the articulation and surface treatment of the prosthetic component. In the prosthesis, one of the articulating surfaces—either that of the reamed acetabulum, or that of the femoral head prosthesis is a-spherical so that a fluid-filled gap is formed at the area of major load transfer. The fluid-filled gap is sealed by an annular area of contact, over which the concave and the convex components are congruent. A prosthetic head is fixed to the femur by either a conventional stem, a perforated shell, or a femoral neck prosthesis screwed onto the femur so that it is partially covered by bone and partially exposed on the medial-inferior aspect, where it abuts the reamed cortex of the calcar region.
Claims
1. A partial hip prosthesis system, comprising (a) an articulation member, wherein said articulation member is spherical in shape and after implantation, allows for free rotation in a reamed acetabulum, and (b) an aspherical acetabulum reamer shaped to ream the bone of the acetabulum into an aspherical shape, so as to make, after implantation, an annular congruent band shaped contact to the articulation member which, after implantation, seals fluid in a gap between the articulation member and said reamed acetabulum, wherein the aspherical acetabulum reamer comprises a first radius, a second radius, a third radius, and a contour with a polar region and an equatorial region, wherein the first radius is located at the polar region, and the second radius is located at the equatorial region, and the third radius is located between the polar region and the equatorial region, wherein the third radius is equal to a radius of the articulation member, and a band of the aspherical acetabulum reamer having the third radius corresponds to the annular congruent band shaped contact, and wherein the first radius and the second radius are larger than the third radius.
2. A partial hip prosthesis system comprising (a) an articulation member, wherein said articulation member is spherical in shape and after implantation allows for free rotation in a reamed acetabulum, wherein a convex side of the articulation member is a prosthetic femoral head, and (b) an aspherical acetabulum reamer shaped to ream the bone of the acetabulum into an aspherical shape which contacts said prosthetic femoral head only at an annular congruent band, wherein a contour of the aspherical acetabulum reamer has a radius of curvature which is the same as that of a reamed acetabulum, and is centered about 30 to 55 degrees from the axis of revolution of the articulation member and its width is between 5 to 15 degrees, the radius of curvature corresponding to the annular congruent band shaped contact in the reamed acetabulum, wherein radii of curvature of the remaining contours of the aspherical acetabulum reamer are greater than the radius of the contour centered about 30 to 55 degrees from the axis of revolution of the articulation member.
3. A partial hip prosthesis system comprising (a) an articulation member, wherein said articulation member is spherical in shape and after implantation, allows for free rotation in a reamed acetabulum, wherein a convex side of the articulation member is a prosthetic femoral head, and (b) an aspherical acetabulum reamer shaped to ream the bone of the acetabulum into an aspherical shape so as to make, after implantation, an annular congruent band shaped contact to the articulation member, the annular congruent band shaped contact to the articulation member having a radius of curvature which is the same as that of a reamed acetabulum, and sealing, after implantation, fluid in a gap between the articulation member and said reamed acetabulum, wherein the aspherical acetabulum reamer comprises a first radius, a second radius, a third radius, and a contour with a polar region and an equatorial region, wherein the first radius is located at the polar region, the second radius is located at the equatorial region, and the third radius is located between the polar region and the equatorial region, wherein the third radius is equal to a radius of the prosthetic femoral head, wherein a band of the aspherical acetabulum reamer having the third radius corresponds to the annular congruent band shaped contact, and wherein the first radius and the second radius are larger than the third radius.
4. The partial hip prosthesis according to claim 1, wherein the prosthetic femoral head includes a diamond-like coating.
5. A partial hip prosthesis system comprising (a) an articulation member, wherein a convex side of the articulation member is a prosthetic femoral head which has a first shape; and (b) an acetabulum reamer shaped to ream the bone of the acetabulum into a shape which is different from said first shape, so as to make, after implantation, an annular congruent band shaped contact to the articulation member which, after implantation, seals fluid in a gap between the articulation member and a reamed acetabulum, wherein the annular congruent band shaped contact is centered about 30 to 55 degrees from the axis of revolution of the femoral head prosthesis, and its width is between 5 and 15 degrees of the femoral head prosthesis.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a schematic cross sectional view of the hip joint.
(2) FIG. 2 is a schematic cross sectional view of the proximal femur.
(3) FIG. 3 is a schematic cross sectional view of the proximal femur with the femoral head partially resected.
(4) FIG. 4 is a schematic cross sectional view of the proximal femur with the femoral head partially resected and the femoral neck reamed over a guide pin.
(5) FIGS. 5a-5b are perspective views of the femoral head prosthesis and the femoral neck prosthesis according to the invention.
(6) FIG. 6 is a schematic cross sectional view of the proximal femur with the femoral head prosthesis and the femoral neck prosthesis affixed to the femur.
(7) FIG. 7 is a schematic perspective view of the proximal femur with the femoral head prosthesis and the femoral neck prosthesis affixed to the femur.
(8) FIG. 8 is a schematic perspective view of the proximal femur with the femoral head prosthesis and the femoral neck prosthesis affixed to the femur, wherein the flattened head feature of the prosthetic head is offset from the neck axis to line up along the major joint force vector.
(9) FIG. 9 is schematic partial cross sectional view of the acetabulum reamed-out to bone to create a spherical cavity for the head.
(10) FIG. 10 is a schematic partial cross sectional view of the hip prosthesis according to the invention articulated within a spherically reamed acetabulum.
(11) FIG. 11 is a schematic cross sectional view of the femoral head spherically reamed to remove the surface layer of damaged cartilage and bone.
(12) FIG. 12 is schematic cross sectional view of the femoral head resurfaced by the femoral head prosthesis with the perforated inner shell.
(13) FIG. 13 is a schematic cross sectional view of the acetabulum a-spherically reamed to remove the surface layers of damaged cartilage and bone and to create a cavity for receiving the spherical head prosthesis.
(14) FIG. 14 is a schematic cross sectional view of the spherical femoral head prosthesis, affixed to a conventional femoral stem, articulating within the a-spherically reamed acetabulum.
DETAILED DISCLOSURE
(15) For a simple and clear presentation, a human total hip joint articulation has been chosen for this disclosure, but the same technical arguments and design approaches can be used for a hip prosthesis for animals, specifically for dogs and cats.
(16) The present invention is an extension of a prior invention by the inventor as set forth in PCT Patent Application No. WO2008/058756, published on May 22, 2008, which is incorporated herein, in its entirety, by reference (“the Tepic Application”). The Tepic Application discloses a joint prosthesis, such as a hip prosthesis, in which the convex and concave components have differences in shape to provide a broad contact surface. As set forth in the Tepic Application, the differences in shape between the components further provide improved lubrication of the components and particularly the contact surface. While that structure results in significantly reduced wear, it may still be of a concern, particularly when the concave component is formed of UHMWPE. The wear can be further reduced by the so present invention, in which the head is treated by ADLC and, most importantly, reduced to wear of only the biological materials present in bone, which are readily re-absorbed by natural mechanisms.
(17) Major reduction in production and clinical application costs are expected in partial hip replacement according to the invention. Surgical time can be reduced by about 15 minutes. Post surgical complications will also be less likely, particularly dislocation of the hip and incidence of lung embolism. The procedure is well suited to so-called minimally invasive approaches to the hip joint. Of the traditional approaches, both the anterior-lateral and the posterior approach can be used, i.e. no new surgical skills are necessary. Should a revision be indicated, it would be a simple matter to convert a partial prosthesis according to this invention to a conventional total hip replacement.
(18) FIG. 1 shows a schematic cross sectional view of the human hip joint. The femur 1 on its proximal medial aspect comprises the femoral head 2 and the femoral neck 3. The greater trochanter 4 anchors most of the tendons of the muscles pulling towards the pelvis, e.g. piriformis, gluteus minimus, gluteus medius, quadratus femoris, obturator externus and internus, but also and in continuity, of the tendons of the muscles acting in the distal direction towards the knee joint, particularly of vastus lateralis. The lesser trochanter 5 anchors the tendons of psoas major and iliacus. The femoral head is covered by cartilage 6. The medullary cavity 10 of the shaft of the femur in the cross sectional view is defined by the medial cortex 7 and the lateral cortex 8. It is filled by fatty marrow, which poses a high risk of lung embolism if extruded into circulation via distal venous drainage by pressurization of the cavity during preparation for and/or insertion of a classical stemmed femoral component. Proximal aspect 9 of the femur is filled by cancellous bone. The pelvis 11 receives the head in its concave acetabulum, defined by the subchondral bone 12, covered by cartilage 13.
(19) FIG. 2 shows a schematic cross sectional view of the proximal femur alone. The head 2 is covered by cartilage 6 over little more than a hemisphere facing superiorly, medially and slightly anteriorly. Fovea 14 of the femoral head receives the ligament of the head of the femur.
(20) FIG. 3 shows the first step of the surgical procedure required to attach the prosthesis to the femur—the head 2 is resected from the neck 3 by a saw cut along the plane 20, approximately along its “equator” and at approximately 90 degrees to the axis 21 of the neck 3 of the femur.
(21) FIG. 4 shows the next step of surgical femur preparation. A guide pin 22 is inserted along the neck axis 21 and a cannulated cylindrical reamer 23 is used to cut a cylindrical groove 27 centered onto the pin 22. With the proper placement of the guide pin 22 and the proper size of the reamer 23, the neck of the femur will end up having a proximal, fully reamed portion 24, a partially intact medial-inferior cortex 25 and a distal abutment 26 in the calcar region of the femur, just above the lesser trochanter 5.
(22) FIG. 5a shows a perspective view of the prosthesis head 40. The section of the head along the band 51 has a radius of curvature 41, equal to the radius of the spherically reamed acetabulum. The polar region of the head denoted by 52, has a radius of curvature 42, which is larger than 41. The lower section of the head has the radius of curvature 43, which is smaller than 41. For production convenience the three sections (with radii 41, 42 and 43) can all be spherical in shape and their centers can lie on the main axis of the head. The transition band 54 between the sections 51 and 52 has a small radius of curvature 44. The transition band 55 between the section 51 and the lower section of the head, has a radius 45. With these transitions the finished head has a smooth, axisymmetric outer contour. Inside, the head is provided with a conical recess 46. Several lengths of the head can be provided, as usual in the hip systems, by machining the conical recess deeper or shallower into the head.
(23) A complete system for treating patients with the partial hip prosthesis according to the invention would have base diameter of the head (2 times the radius 41) in the range from about 40 to about 70 mm, in increments of either 1 or 2 mm. The angle defining the position of the middle of the band 51, measured from the axis of the head should be in the range from about 30 to about 55 degrees, the theoretical best being at 45 degrees. The width of the band 51 should be from about 5 to about 15 degrees. The radius 42 at the polar zone can be several millimeters larger than the radius 41, e.g. 2 to 4 mm, while the radius 43 should be smaller by about 0.2 to 1 mm.
(24) FIG. 5b shows a perspective view of the prosthesis neck 30. The upper part 34 of the neck prosthesis is conical in shape, 31, adapted to fit into the conical recess 46 of the head 40. A central hole 32 allows for insertion into/onto the reamed neck of the femur over the guide pin 22. Recesses 33 provide means for engaging the implant with an instrument to screw it into/onto the neck of the femur. The lower part 35 of the neck prosthesis engages the bone by both outer threads 36 and inner threads 37 of the same pitch. A number of optional holes 38 preferably placed between the threads are intended for better vascular supply of the new bone which will form around the implant.
(25) FIG. 6 shows a cross sectional view of the proximal femur 1 implanted with a prosthesis according to the invention. The prosthesis comprises the head 40 and the neck 30. On the medial-inferior aspect 60, the prosthetic neck skims over the outside of the natural femoral neck, engaging it with its inner threads, and abuts the cortex at the reamed abutment 26. On the lateral-superior aspect 61, the prosthetic neck engages the bone of the resected and reamed natural neck by both inside and outside threads. The cross section along the plane A-A shows how the prosthetic neck transsects the natural neck at its anterior and posterior cortices so that at the aspect 60 of the prosthetic neck is outside the natural neck.
(26) This is a unique, novel and fundamentally important feature of the prosthesis neck according to the invention. It allows for load transfer to the strong bone of the medial cortex at the abutment 26, while providing stability against tilting medially over the abutment by anchorage to the lateral aspect of the natural neck and by transecting the cortices of the neck. The cancellous bone of the neck remains relatively intact, disturbed by only the guide pin.
(27) FIG. 7 shows a perspective view of the anterior aspect of the proximal femur 1 implanted with a prosthetic neck 30 and a prosthetic head 40. The neck 30 is visible on the medial-inferior aspect 60 and hidden by bone on the lateral-superior aspect 61.
(28) FIG. 8 shows a perspective view of the anterior aspect of the proximal femur implanted with a prosthetic neck 30 and a prosthetic head 40. The aspherical feature of the head is now centered on axis 73, offset from the axis of the neck 21 by an angle 74. The angle 70 between the femur shaft axis 71 and the femur neck axis 21 is typically about 135 degrees, but it shows significant patient-to-patient variation. The purpose of the offset angle 74 is to bring the flattened section of the head surrounded by the annular section 75, which closely fits the spherical cavity of the acetabulum and thus seals the pool of fluid trapped between the head and the acetabulum, in closer alignment to the joint load 72. Direction of the load 72 does change with activities of the patient, but with an offset of about 15 to 35 degrees, preferably about 25 degrees, the chances are good that the pool will remain sealed for the majority of the angles and forces across the hip joint. The angle 74 should not be in the frontal plane, but rather within a pine with about 15 degrees of antiversion. For the surgeon to place the head into an optimal position the head needs a marking, best on the bottom of it facing the bone once attached to the neck, for the intended placement with the flattened feature facing anteriorly, medially and superiorly.
(29) FIG. 9 shows the surgical preparation of the acetabulum in the pelvic bone 11 with a spherical reamer 101. The layer of cartilage 13, usually severely damaged by arthosis of the joint, and some of the subchondral bone 12 are reamed out to a spherical shape 100 with a radius 102. As mentioned earlier, the radius 102 should be equal to the radius 41 of the head, FIG. 5.
(30) FIG. 10 shows a perspective view of the anterior aspect of the proximal femur 1 implanted with a prosthetic head 40, placed within the reamed acetabulum, which is shown in a schematic cross sectional view. Between the head 40 and the reamed bone, there is a gap 110, which holds lubricating synovial fluid, sealed therein by the band of contact, which the matching section 51 of the head 40 makes with the reamed acetabulum. The major joint load 72 is offset from the neck axis 21 by the angle 74.
(31) FIG. 11 shows an alternative surgical preparation of the head of the femur 2. The layer of cartilage 6 is reamed together with some supporting cancellous bone to the spherical-cylindrical shape 120, by the reamer 121. The reaming is performed over a guide pin 122; the serrated sleeve 123 can also be used to guide the reamer 121 centered on the head 2.
(32) FIG. 12 shows a schematic cross sectional view of another embodiment of the invention, wherein the prosthetic head 140 is affixed to the femur via an inner, perforated shell 150, press-fitted onto the reamed head of the femur. For this fixation to work well, the bone of the femoral head needs to be in good condition, with good vascular supply, limiting the application to only a small subset of the patients. The prosthetic head is aspherical, the radius 141 over the band 151, closely matches the radius of the reamed acatabulum. The radius 142 over the polar region 152 is larger than the radius 141. The transition zone 154 has a small radius of curvature 144; the transition zone 155 the radius 145. The lower section of the prosthetic head 140 is also spherical with the radius 143 smaller than 141, to prevent jamming of the head in the acetabulum reamed out to the radius 102, see FIG. 9, which is the same as 141.
(33) Double shell fixation of this type is the subject of the pending application WO/2005/094731, “Double shell implant for cementless anchorage of joint prostheses”, by Slobodan Tepic and Henrik Malchau.
(34) FIG. 13 shows yet another embodiment of the invention, whereby the acetabulum is reamed to an aspherical shape, while the prosthetic head is spherical with the radius 205. The reamer 200 reams the acetabulum 201 to an exact contour 204 defined by the shape of the reamer. The radius 202 over a band 203 is equal to the radius 205 of the prosthetic head. At the pole, the reamer removes more bone than a spherical reamer would do. The same is true at the equator of the reamer. The contact to the prosthetic head is thus limited to the band 203.
(35) FIG. 14 shows a perspective view of the proximal femur 1 with a spherical .sub.prosthetic head 300 affixed to the bone via a conventional stern 310. The acetabulum is shown in a cross sectional view of the pelvis 11, with a gap 301 formed between the articulating surfaces.
(36) In all cases described, two essential conditions are met in order for the joint articulation according to the invention to function satisfactorily: (1) a gap in the area of otherwise maximal contact stresses within the articulation is provided for the purpose of supplying lubrication to the area of contact surrounding the gap, which is achieved by targeted congruency over a band of contact; (2) the surface of the prosthetic head which articulates against the reamed bone exhibits very low coefficient of friction, the preferred surface of the implant being amorphous diamond-like coating on a metallic substrate. Pyrolytic carbon is a costlier alternative.
(37) The prosthetic femoral neck described is a preferred, novel solution for affixing the prosthetic head to the remaining bone of the femur. The transcortical concept allows approach to the dense cortical bone in the calcar region with a geometrically simple shape of the implant. The true and proven concept of threaded implants is extended by providing both external and internal threads to engage the bone for maximum stability without undue damage to the vascular supply of the involved bone.
