HIP STEM

Abstract

A stem (100) for use in a joint prosthesis, such as a femoral stem for a hip joint prosthesis, the stem comprising: a solid central core (102); a proximal outer layer (127) disposed over a proximal portion (101a) of the central core, wherein the proximal outer layer comprises a set of longitudinal ribs (120), defining slots (130) there between; and a distal outer layer made of a deformable porous material disposed over a distal portion (101b) of the central core. The arrangement is such that the stem (100) can be made with a relatively large diameter yet without being excessively stiff, for cementless fixation in osteoporotic patients. The deformability of the distal outer layer also mitigates against the risk of intraoperative bone fractures.

Claims

1. A stem for use in a joint prosthesis, the stem comprising: a solid central core; a proximal outer layer disposed over a proximal portion of the central core, wherein the proximal outer layer comprises a set of longitudinal ribs, defining slots there between; and a distal outer layer made of a deformable porous material disposed over a distal portion of the central core.

2. The stem of claim 1, wherein the distal outer layer is disposed over the distal third to half of the central core, and the proximal outer layer is disposed over the corresponding proximal two thirds to half of the central core.

3. The stem of claim 1, wherein a distal end of the stem comprises a bullet-shaped or rounded tip.

4. The stem of claim 3, wherein the tip is also made of a deformable porous material.

5. The stem of claim 1, wherein the central core is tapered, narrowing towards the distal end.

6. The stem of claim 5, wherein the thickness of the distal outer layer increases towards the distal end of the central core.

7. The stem of claim 1, wherein the distal outer layer is substantially cylindrical or trapezoidal in cross-section.

8. The stem of claim 1, wherein the ribs are disposed radially about the core.

9. The stem of claim 1, wherein the ribs are disposed radially about the core except for at a proximal, medial portion of the stem.

10. The stem of claim 9, wherein the medial portion of the most proximal region of the proximal outer layer comprises a layer of porous material.

11. The stem of claim 1, wherein one or more of a bone stimulating material, a bone replacement material, and a bioactive bone substitute material is disposed within the slots.

12. The stem of claim 1, wherein one or more of the longitudinal ribs comprises a solid base portion and an outer face portion comprising a layer of porous material.

13. The stem of claim 12, wherein the layer of porous material is deformable.

14. The stem of claim 12, wherein the thickness, in a radial direction, of the solid base portion varies along the length of the stem such that the solid base portion is thicker towards the proximal end of the stem and thinner towards the distal end of the stem.

15. The stem of claim 14, wherein the thickness, in a radial direction, of the outer face portion varies along the length of the stem such that the outer face portion is thinner towards the proximal end of the stem and thicker towards the distal end of the stem.

16. The stem of claim 1, wherein the distal outer layer comprises a set of longitudinal ribs, defining slots there between.

17. The stem of claim 16, wherein the ribs and slots on the distal outer layer are arranged radially about the core.

18. The stem of claim 16, wherein one or more of a bone stimulating material, a bone replacement material, and a bioactive bone substitute material is disposed within the slots on the distal outer layer.

19. The stem of claim 1, wherein the pore size of the porous material varies along the length of the stem such that the pore size increases towards the distal end of the stem and decreases towards the proximal end of the stem.

20. The stem of claim 1, wherein the pore size of the porous material varies radially such that the pore size increases towards the centre of the stem and decreases towards the outer surface of the stem.

21-46. (canceled)

Description

BRIEF DESCRIPTION OF DRAWINGS

[0078] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0079] FIG. 1 shows a hip stem with longitudinal ribs extending along the majority of the length of the stem and terminating at a bullet-shaped tip, according to one aspect of the invention;

[0080] FIG. 2a shows a longitudinal cross-section through the hip stem of FIG. 1 according to one embodiment, in which the longitudinal ribs have a porous outer layer and a solid base layer;

[0081] FIG. 2b corresponds to FIG. 2a, but shows an alternative embodiment in which the longitudinal ribs are porous throughout their depth;

[0082] FIG. 3a is a transverse cross-section viewed on A-A of FIG. 2a;

[0083] FIG. 3b is a transverse cross-section viewed on A-A of FIG. 2b;

[0084] FIG. 4a is a transverse cross-section viewed on B-B of FIG. 2a;

[0085] FIG. 4b is a transverse cross-section viewed on B-B of FIG. 2b;

[0086] FIG. 5 shows an alternative hip stem with a longer, rounded-end tip (shown transparent) and longitudinal ribs extending through a thickened proximal medial portion;

[0087] FIG. 6 is a lateral elevation of the hip stem of FIG. 5;

[0088] FIG. 7a is a transverse cross-section viewed on C-C of FIG. 6 according to an embodiment in which the longitudinal ribs have a porous outer layer and a solid base layer; and

[0089] FIG. 7b is a transverse cross-section viewed on C-C of FIG. 6, according to an alternative embodiment in which the longitudinal ribs are porous throughout their depth.

DETAILED DESCRIPTION

[0090] FIGS. 1 and 2a, 3a and 4b illustrate a first exemplary hip stem 100. The hip stem 100 comprises a central core 102 extending longitudinally within the stem from a proximal end 104 to a distal end 106. A tip 108 is located at the distal end 106 and a collar 110 is located at the proximal end 104. An attachment portion 112 extends from the collar 110 at the proximal end 104 of the stem 100 at an angle of approximately 45 degrees from a longitudinal axis of the stem in the medial direction, for the attachment thereto of a femoral head component (not shown), as known in the art. A thickened proximal medial portion 116 is located towards the proximal end 104, forming a buttress connecting the collar 110 with the main part of the stem for supporting load transfer to the proximal medial wall of the femur when the stem is located in situ within a patient's femoral canal. A lip 117 may be formed between a peripheral edge of the proximal end of the thickened proximal medial portion 116 and the underside of the collar 110, to prevent initial stem migration, as is known in the art.

[0091] The core 102 is generally cylindrical; with a circular transverse cross-section, from the distal end 106 through to near the proximal end 104. Where the core extends through the thickened proximal medial portion 116, the core is itself thickened in a medial direction, forming an ovalised portion 103, as best seen in FIG. 4a, that increases in cross-sectional area towards the proximal end 104, as best seen in FIG. 2a.

[0092] The core 102 may be made of Ti or beta Ti alloy. Alternatively, the core 102 may be made of a porous material which may be deformable. Alternatively, the core 102 may be made of more of trabecular or porous titanium, titanium alloy or tantalum.

[0093] The collar 110 may be formed integrally with the core 102 or may be secured thereto in a separate step. In the latter case, the respective components may be formed of different materials, which may be selected to have particularly useful characteristics for their intended purpose.

[0094] A number of ribs 120 are disposed on the stem 100, extending radially from the core 102. On the lateral, anterior and posterior sides, the ribs 120a extend all the way from an underside of the collar 110 at the proximal end 104 to the tip 108 at the distal end 106. On the medial side, the ribs 120b in this embodiment run from the bottom end of the thickened proximal medial portion 116 to the tip 108.

[0095] As illustrated, there are eight ribs 120 in total, comprising 5 full-length ribs 120a disposed around the lateral, anterior and posterior sides, and 3 shorter ribs 120b on the medial side. The ribs 120 are disposed at equal angular intervals around the core 102. However, it will be appreciated that greater or fewer ribs 120 may be provided and that they need not be arranged with such rotational symmetry, nor project radially. Indeed, according to certain embodiments, it can be envisaged that asymmetrical arrangements of the ribs could be advantageous in order to better match the stiffness or bending strength of the device to the surrounding bone or the loads imposed.

[0096] As illustrated, the ribs 120 extend contiguously over both a proximal portion 101a and a distal portion 101b of the stem 100. However, in certain embodiments the ribs 120 would only extend along the proximal portion 101a, which may, for example, comprise the proximal-most half to two-thirds of the stem from beneath the collar 110 to the distal tip 108.

[0097] An inner, base portion 124 of each rib is, according to this embodiment, formed of a solid material, forming an intermediate layer 125 over the core 102. The intermediate layer 125 may be comprised of the same material as the core 102, which may be made of a light weight metal. The material of the core 102 and/or the intermediate layer 125 may be Ti or beta Ti alloy. The intermediate layer 125 may be fixed to the core 102 or it may be integrated with at least the proximal portion 101a of the core 102 such that the core 102 and the intermediate layer 125 form a single component.

[0098] An outer, face portion 126 of each rib comprises a porous or other low-stiffness material, such as a polymer: PEEK or polyethylene, by way of example; or a polymer composite: PEEK with granules of hydroxyl apatite embedded therein or polyethylene with granules of hydroxyl apatite embedded therein, by way of example. Likewise, the thickened proximal medial portion 116 comprises a solid inner portion 113 and a porous outer portion 115. The porous outer portions 126 and 115 together form a porous outer coating layer 127.

[0099] Such an intermediate layer 125 at the base of one or more of the ribs 120 would provide additional proximal stiffness and strength to that provided by the core 102. The intermediate layer 125 may be gradually tapered in a longitudinal direction such that the diameter thereof decreases towards the distal end 108, thus giving a gradual reduction of bending stiffness along the stem.

[0100] The core 102 and the inner portions 124 of the ribs 120, as well as the inner portion 113 of the thickened proximal medial portion 116, may be formed integrally in a single manufacturing step.

[0101] According to an alternative embodiment, as shown in FIGS. 2b, 3b and 4b, the stem 100′ is the same as the stem 100 described above with reference to FIGS. 2a, 3a and 4a (and like parts are referenced by common reference signs), except for the fact that the intermediate layer 125′ is not solid like the core 102, but is instead made of a porous material. Each rib 120′ thus comprises an inner, base portion 124′ that is formed of porous material and an outer, face portion 126′ that is also formed of porous material. In other words, each rib 120′ is formed of porous material throughout its depth, extending fully to the core 102. Likewise, the thickened proximal medial portion 116 comprises a porous inner portion 113′ and a porous outer portion 115′.

[0102] The inner and outer porous portions 124′, 113′ and 126′, 115′ (and therefore the intermediate and outer porous layers 125′, 127′) may be formed of the same porous material and be of the same density or may be formed of different porous materials and/or have different densities, preferentially with the least-stiff material at the outer surface. Where the inner and outer porous portions are formed of the same material, there is essentially no intermediate layer present, only the solid core 102 and a porous outer layer 127′.

[0103] A further alternative embodiment of a stem 200 is shown in FIGS. 5 to 7b. The hip stem 200 comprises many of the same features as the stems 100, 100′ shown in FIGS. 1 to 4b (and like parts are referenced by common reference signs, albeit with the preceding ‘1’ replaced by a ‘2’), although the ribs 220 and associated slots 230 only extend along a proximal portion 201a of the stem, from the collar 210 to approximately half to two-thirds of the way to the distal end 206, with the distal portion 201b forming the remaining third to half of the length. Also, all of the ribs 220, including those on the medial side, extend right from the underside of the collar 210. The medial ribs 220b and the anterior and posterior ribs 220c each curve outwards in the medial direction at their proximal ends to form buttresses defining the thickened proximal medial portion 217.

[0104] Each of the anterior and posterior ribs includes a fin portion 221 at its proximal end beneath the collar 210. As best seen in FIGS. 5 and 6, the fins 221 project outwardly beyond the profile of the collar 210 and the rest of the generally cylindrical profile of the stem to provide rotational stability of the stem when located in situ within the medullary canal. This is particularly advantageous for stems having a generally cylindrical cross section. A further fin (not shown) may project from the lateral side too.

[0105] Alternatively, rotational stability may be provided by having a non-cylindrical cross sectional profile at the proximal end 104, 204 of the stem. For example, a stem with a trapezoidal cross section would resist rotation in situ within a patient's medullary canal.

[0106] In a first arrangement, as shown in FIG. 7a, each of the ribs 220a, b, c comprises a solid, base portion 224 and an outer, face portion 226 of a porous material. In an alternative arrangement, as shown in FIG. 7b, each of the ribs 220a′, b′, c′ comprises a porous base portion 224′ as well as a porous outer, face portion 226′. The description above in connection with the constitution of the materials in the base and outer portions of the alternative embodiments set out in FIGS. 3a and 3b applies equally here.

[0107] The distal portion 201b of the stem 200 comprises a cylindrical sheath of porous material disposed around the distal portion of the core 202.

[0108] Hence, in the first arrangement of FIG. 7a, the stem 200 comprises a solid core 202, an intermediate layer 225 comprising base portions 224 of the ribs 220 over a proximal portion 201a of the stem, and an outer layer 227 comprising the porous outer, face portions 226 of the ribs 220 in the proximal portion 201a in conjunction with the cylindrical sheath of porous material disposed over the distal portion 201b. Likewise, in the arrangement of FIG. 7b, the stem 200 comprises a solid core 202, and an outer layer 227′ comprising the porous outer, face portions 226′ of the ribs 220′ in the proximal portion 201a in conjunction with the cylindrical sheath of porous material disposed around the distal portion 201b.

[0109] For all embodiments, the porous outer layer 127, 127′, 227 may comprise a coating layer which may, for example, be made of Trabecular Titanium (TT), porous titanium alloy, or Porous Tantalum (PT). Such a porous outer layer 127, 127′, 227 provides a low-modulus anchorage area to encourage bone in-growth from the surrounding bone cortex when the stem is inserted in a patient, and thus to secure the stem in situ.

[0110] The pore size of the porous material may range between 300 to 1000 microns, preferably between 300 to 600 microns, or more preferably between 300 to 500 microns. The density and/or pore size of the porous material may vary along the length of the stem, being more dense and/or porous (i.e. having less pores and/or having larger pore sizes) distally. It may also vary radially, being denser and/or less porous near to the central core and least dense and/or more porous at the outer surface, for example.

[0111] At least the porous outer layer of the distal portion 101b, 201b is deformable such that it reduces stress-concentration, distal load transfer and risk of intraoperative and postoperative femoral fracture.

[0112] The central core 202 may taper along the length of the prosthesis, such that it has a smaller diameter at the distal tip. It may also not extend the full length of the stem, leaving only a porous structure at the distal tip in zone 201b.

[0113] The ribs 120, 220 may range from 2 to 5 mm in circumferential width and may range from 3 mm to 8 mm in radial height. Accordingly, the outer layer 127, 127′, 227 may have a thickness in the range of 3 to 8 mm or, preferably, 4 to 7 mm. The outer diameter of the porous outer layer 127, 127′, 227 (and thus of the stem 100, 100′, 200) may be in the range of 16 to 28 mm, which may be the same size as or a bit smaller than that of the diameter of a femoral canal into which it is to be inserted. The stems 100,100′, 200 may be designed such that they are undersized, i.e. so that there is no contact between the stem and the surrounding bone, thus avoiding initial distal load transfer from the stem to the bone.

[0114] The diameter of the core 102, 202 may be in the range of 8 to 14 mm, but this is dependent upon the overall size of the stem 100, 100′ and 200. The core 102, 202 may be tapered such that it is thicker at the proximal end 104, 204 of the stem 100 and 200 and thinner at the distal end.

[0115] Slots 130, 230 are formed in the spaces between the ribs 120, 220. The slots may be 4 to 7 mm deep (from the height of the ribs 120, 220 to the outer edge of the core 102, 202). Depending on the arrangement of the ribs 120, 220, the slots may have an angular spacing of, for example, 30 to 45 degrees.

[0116] The slots 130, 230 reduce the stiffness of the proximal portion 101a, 201a of the stem 100, 100′, 200 (in comparison to a stem having the same diameter but being solid, rather than a having a rib/slot arrangement). Where the ribs/slots extend over the distal portion 101b too, the stiffness of that distal portion 101b of the stem 100, 100′ is also reduced. Moreover, the slots 130, 230 create spaces in which a special coating or filling can be placed; for example, a bone stimulating/replacement material which provides a high rate of osseointegration, even for osteoporotic or poor quality bone. One or more of any suitable known bioactive bone substitute materials may also be placed in the longitudinal slots 130, 230. Such a filling would encourage bone formation around the stem 100, 100′, 200. In certain embodiments, some or all of the slots 130, 230 may be left empty.

[0117] In one embodiment (not shown), the distal portion 201b is as described above, but rather than the ribs 220 over the proximal portion 201a having a porous outer face 226, they are solid, typically being formed of the same material as the central core 202.

[0118] Common to all embodiments is the combination of a solid core and at least a distal-most portion that is formed of a deformable porous material.

[0119] The distal tip 108 of the embodiments of FIGS. 1 to 4b is bullet shaped, whereas the distal tip 208 of the embodiments of FIGS. 5 to 7b is rounded. It will be understood, however, that any of the stems herein described may comprise either a bullet-shaped or a rounded tip.

[0120] The distal tip 108, 208 is preferably made of a porous material; most preferably the same as that of the porous outer layer 127, 127′, 227. Likewise, the underside of the collar 110, 210 including the lip 117, 217 if present, may be formed of a similar porous material to encourage bone in-growth for anchoring the stem in situ, mitigating against the usual scenario where bone-collar contact is lost due to bone resorption post-surgery.

[0121] The stems 100, 100′, 200 are typically symmetrical in the anterior/posterior plane, allowing for use on either the left or right side of the body, thereby avoiding the need for surgeons to have access to separate inventories for left and right side operations. However, it can be envisaged that stems could be designed asymmetrically for specific left or right side use. The proximal portion 101a, 201a of the stem may be curved to match medullary canal geometry, as known in the art.

[0122] It will be understood that any number of ribs 120, 220 may be provided around any of the central cores 102, 202 described herein, although, preferably, there are 8 to 12 ribs.

[0123] Where this document refers to porous material, it is primarily used to encourage bone ingrowth and also to provide reduced material and structural stiffness, but the parts described above as being porous may also be made of other low-stiffness material or composite such as a polymer, that may not be porous, yet still have the desired structural effect of reduced stiffness when compared against a solid cylindrical component. Examples of such materials include the polymers PEEK and polyethylene, as well as the polymer composites PEEK with granules of hydroxyl apatite in it and polyethylene with granules of hydroxyl apatite in it.

[0124] The stems 100, 100′, 200 shown in the Figures have a substantially cylindrical geometry below their thickened proximal medial portions 116 and 216. The diameter of the stems 100, 100′, 200 may range from 14 to 28 mm. Such a cylindrical geometry facilitates easy insertion of the stem into a prepared femoral canal, thus reducing the risk of intra-operative femoral fracture. Where the core 102, 202 is tapered, in order for the stem to have a cylindrical outer profile, the intermediate and outer layers will be correspondingly tapered in the opposite sense: i.e. thicker at the distal end 106, 206. This may be achieved by having the base 124, 224 with a constant depth and the outer face 126, 226 getting thicker (in depth) towards the distal end, or by having the outer face 126, 226 of a constant depth and the base 124, 224 getting thicker (in depth) towards the distal end, or a combination of the two.

[0125] Alternatively, the stems could also have a tapered geometry below their thickened proximal medial portions 116, 216, such that the diameters of the stems decrease towards the distal end 106, 206. Such a profile would also facilitate easy insertion of the stem into a prepared femoral canal.

[0126] The overall shape of the proximal part of the stems 100 and 200 may be configured to fully fill the femoral medullary canal. This may be achieved by providing a stem 100, 100′, 200 having a geometry which matches, for example, either a ‘normal’ or an osteoporotic femoral canal geometry. By way of example, the geometry may be adapted to a smaller intramedullary canal diameter and to a different pattern of taper along its length.

[0127] Multiple sizes of stems 100, 100′, 200 may be provided as standard to fit the variable sizes of different patients. For example, the stems may be available in a range of ‘small’, ‘medium’ and ‘large’ sizes.

[0128] The stems 100, 100′, 200 may be shortened so that they fill only a small metaphyseal zone of the proximal end of the femur, or even fit only into the neck of the femur.

[0129] The stem 100,100′, 200 may be used as a conventional stem, in which case it would be in the range of 120 mm to 170 mm long. The stem 100,100′, 200 may instead be used as short-stem prostheses, in which case it may be in the range of 70 mm to 100 mm long. According to a further embodiment, the stem 100, 100′, 200 may be used as a much more localised hip femoral component, which places a short stem coaxial with the neck of the femur, in which case it would be in the range of 50 mm to 70 mm long.

[0130] The porous material of the stems may have variable structure, such that it is stronger/stiffer in some zones, and softer in others, depending on the load transfer requirements. For example, the distal tip may be relatively soft/deformable, to reduce load/stress concentration here. This type of micro-structural adaptation may be built using ‘rapid prototyping’ methods, in material such as titanium alloy.

[0131] The bone cavity into which the stem 100, 100′, 200 is to be inserted may be prepared such that it is circular in cross-section, by a drilling type operation.

[0132] Alternatively, a more complex bone cavity could be made by a broaching tool, such that the cavity fits to any ribs and/or slots present in the stem 100, 100′, 200, or to match the trapezoidal geometry where included.

[0133] According to certain embodiments, the stem could be designed and manufactured for patient-specific application. As known in the art, a scan of the patient can be made (e.g. a CT scan) to collate scan data that can then be used in a planning/design phase to design a stem to suit a particular patient-specific need, optionally taking into account a surgeon's expert input. For example, the scan data can reveal the local joint topography and the surgeon can identify which parts of the bone should be resected during a joint replacement or repair operation. Optionally, the data can be used to match stem stiffness as closely as possible to the surrounding joint physiognomy. Using those inputs, a patient-specific stem can be designed to match the existing joint topography and to replace those portions of bone that are to be resected. Once designed, the stem can be manufactured using known manufacturing techniques, such as rapid prototyping or additive manufacturing, which are particularly suited to providing stems having variable structure throughout, such as having varying porosity to create areas with a range of stiffness, in order to optimise load transfer and reduce stress-shielding and stress concentrations. Software may be provided to assist in the design and operational planning phases.

[0134] Although the invention has been described in the context of a femoral stem for a hip joint prosthesis, the skilled person will appreciate that the teaching herein may instead be applied mutatis mutandis to other joint prostheses, such as either or both of a tibial or femoral stem for a knee joint prosthesis, or for the proximal humerus in shoulder replacement.