An implant

Abstract

An implant (1) has a proximal part (2) having a proximally facing saddle-shaped surface (7) for translational motion over the trapezium, a distal part (3) having spherical surface (5, and a neck (4) interconnecting the distal and proximal parts and having a longitudinal axis. The distal part has a diameter (W) in the range of 8 mm to 14 mm, and a dimension (L) between a distal-most surface of the implant and a proximal-most surface on the longitudinal axis is in the range of 6 mm to 12 mm. The implant is therefore configured for multi-axis motion in a distal bone and translational motion over a proximal bone, with the distal part bearing against hard cortical bone (105).

Description

[0001] “An Implant”

[0002] Introduction

[0003] The invention relates to an implant for a bone joint. In some examples it relates to implants in which there are multiple axes of rotation, such as those in which there is a dual axis hemiarthroplasty with two axes of rotation such as in the hand or elbow.

[0004] An example of an implant with multiple axes of rotation is one for a first carpometacarpal joint for spacing a trapezium bone from a first metacarpal bone (the CMC joint). In this case there is translational motion of a saddle-shaped surface of a proximal implant part over the trapezium and three-dimensional rotational movement of the distal part due to an articulated coupling such as a ball-and-socket j oint.

[0005] The thumb carpometacarpal (CMC) joint is often affected by osteoarthritis. In the CMC joint the locus for abduction and adduction is located inside the base of the first metacarpal and the locus for flexion and extension is located on or abouts the surface of the trapezium. These two loci are approximately 9mm apart. Historically, CMC implants have not addressed the fact that two loci exist, however the implants described in WO2017 / 137607 (NUIG) and W02020 / 193078 (Loci Orthopaedics Ltd.) do address this issue.

[0006] The invention is directed towards providing an implant which is simpler and yet effective for some applications, especially in some CMC joints.

[0007] Summary of the Invention

[0008] We describe an implant comprising a proximal part having a proximally-facing surface which is configured for translational motion over a proximal bone, a distal part having distally-facing surface which is at least partly spherical, and a neck interconnecting the distal and proximal parts and having a longitudinal axis, in which the distal part has a width dimension W in the range of 8 mm to 14 mm, and a dimension H between a distal-most surface of the implant and a proximal- most surface on the longitudinal axis is in the range of 6 mm to 12 mm, the implant being configured for multi-axis motion in a distal bone and translational motion over a proximal bone.

[0009] In some preferred examples, the distal part is substantially hemi-spherical. In some preferred examples, the proximal part proximally facing surface is at least partly saddle-shaped. Preferably, said saddle-shaped surface has a convex surface and a concave surface.

[0010] In some preferred examples, the implant is configured for a mammalian first carpometacarpal joint, in which the proximal part is saddle shaped for translational motion on the trapezium bone, and the distal part is configured so that at least a portion thereof is captured in a volume of first metacarpal bone in use.

[0011] In some preferred examples, the implant is of a material which has a modulus of elasticity substantially equal to that of cortical bone. The modulus of elasticity may preferably be in the range of 9 GPa to 16 GPa. In some preferred examples, the implant is of a polymer material. In some preferred examples, the implant is of material selected from pyrocarbon, PEEK, and CFR- PEEK.

[0012] In some preferred examples, the implant of if a metal material. In some preferred examples, the implant is of cobalt chrome and / or titanium material.

[0013] In some preferred examples, the implant is of unitary construction and being of the same material.

[0014] In some preferred examples, a dimension (L) which is the largest dimension of the proximal part across the longitudinal axis is in the range of 10 mm to 16 mm.

[0015] We also describe a method of inserting an implant of any example described herein in a hemiarthroplasty bone joint such as the CMC joint, the method comprising cutting across a proximal end of a distal bone and removing the proximal part, placing at least a portion of the distal part into the distal bone so that the distal part bears against cortical bone, and the proximal part is engaged with the proximal bone for translational movement.

[0016] In some preferred examples, the distal bone is the metacarpal and the proximal bone is the trapezium.

[0017] Additional Statements

[0018] We describe an implant comprising a proximal part having a proximally-facing surface which is configured for translational motion over a proximal bone, a distal part having distally-facing surface which is at least partly spherical, and a neck interconnecting the distal and proximal parts and having a longitudinal axis, in which the distal part has a width dimension (W) in the range of 8 mm to 14 mm, and a dimension (H) between a distal-most surface of the implant and a proximal- most surface on the longitudinal axis is in the range of 6 mm to 12 mm, the implant being configured for multi-axis motion in a distal bone and translational motion over a proximal bone.

[0019] Preferably, the distal part is substantially hemi- spherical.

[0020] Preferably, the proximal part proximally facing surface is at least partly saddle-shaped.

[0021] Preferably, said saddle-shaped surface has a convex surface and a concave surface.

[0022] Preferably, the implant is configured for a mammalian first carpometacarpal joint, in which the proximal part is configured for translational motion on the trapezium bone, and the distal partis configured to be captured in a volume of first metacarpal bone.

[0023] Preferably, the implant is of a material which has a modulus of elasticity substantially equal to that of cortical bone.

[0024] Preferably, the modulus of elasticity is in the range of 9 GPa to 16 GPa. Preferably, the implant is of a polymer material. Preferably, the implant is of material selected from pyrocarbon, PEEK, and CFR-PEEK. Preferably, the implant is of unitary construction being of the same material.

[0025] We also describe a method of inserting an implant of any preceding claim in a hemi-arthroplasty bone joint such as the CMC joint, the method comprising cutting across a proximal end of a distal bone and removing the proximal part, placing the distal part of the implant into the distal bone so that the distal part bears against cortical bone, and the proximal part is engaged with the proximal bone for translational movement.

[0026] Detailed Description of the Invention

[0027] The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:

[0028] Fig. l is a side view of an implant of the invention, and Fig. 2 is a diagrammatic sectional side view of the from an angle at 90° to the view of Fig. 1, Fig. 3 is a side view similar to that of Fig. 1, in this case showing dimension parameters,

[0029] Fig. 4 is a cross sectional view of cutting across the proximal end of the metacarpal in a first step for use of the implant of Figs. 1 to 3, and

[0030] Figs. 5 and 6 are diagrammatic views showing the implant in in use in the metacarpal.

[0031] Referring to Figs. 1 to 3 an implant 1 is for a CMC joint in this example. In this example it is of unitary integral construction. It has a proximal part 2 for translational motion over the trapezium, a distal part 3 for engagement within a volume of the proximal end of the metacarpal, and an interconnecting neck 4. The proximal part 2 has a proximally facing saddle-shaped surface 7 which is configured for translational motion over a proximal bone, in this case the trapezium. This part also has a generally convex distally facing surface 6. The distal part 3 has a distally facing surface 5 which is spherical. In this case the surface 5 forms a hemisphere, however in other examples it may form more than a hemisphere, however this is not essential because it does not need to fit into a socket as is the case for prior implants with ball and socket joints.

[0032] The implant is unitary and does not need a stem for the distal bone such as the metacarpal, and indeed it does not need any other part to be inserted into the distal bone. The spherical surface 5 engages with a proximal surface of the distal bone and is retained in place with compression in directions around the axial direction. The axial, or longitudinal, direction is defined as the vertical axis through the implant as shown in any of Figs. 1 to 3.

[0033] Referring to Fig. 3 important parameters of the implant 1 are the distraction distance or height dimension H between the on-axis proximal surface 7 and the distal surface 5, and the lateral or width maximum dimension of the distal part 3. The dimension W is the diameter of the distal part 3. The dimension L is the longest length of the proximal part 2, across the longitudinal axis.

[0034] These dimensions are such that the proximal part 2 can move translationally over the trapezium as is the case for the implants described in WO2017 / 137607 (NUIG) and W02020 / 193078 (Loci Orthopaedics Ltd.). However, the distal part 3 is wider. The distraction height dimension H is preferably in the range of 6 mm to 12 mm, and the dimension W is preferably in the range of 8 mm to 14 mm. The dimension L is preferably 12, and more generally may be in the range of 10 mm to 16 mm. The length of the neck 4 is set by the dimensions H and W, which are in turn chosen to suit the anatomy of the patient so that the surface 5 can rotate within the metacarpal with bearing primarily on the (harder) cortical bone around the periphery. Not all combinations of W and H would work, for example W=14 would not work with H=6, so the combination of H and W are subject to the constraint that the neck has a value of at least zero. For example, H=6 and W=8 gives rise to a situation that the combined axial dimension of the part 2 thickness and the neck 4 length is 2 mm, in which case the neck has negligible length.

[0035] Fig. 4 shows initial cutting across the proximal end of the metacarpal 100 to remove the proximal and 102. This leaves open access to the soft cancellous bone 106 surrounded by the cortical bone 105. As shown in Figs. 5 and 6 the distal part 3 can bear against the cortical bone 105, which provides sufficient strength for the spherical surface 5 to bear against.

[0036] The implant 1 preferably is of a material which has an elastic modulus which is close to that of the cortical bone, thereby not damaging the bone and causing it to re-model. Examples are Pyrocarbon, PEEK, and CFR-PEEK. Parameters of the material are preferably, though not required to be, of a modulus of elasticity of bone. This may vary between a material of 9 GPa to a material of 16 GPa, or a material which has properties that can provide a similar range of elastic properties.

[0037] By fabricating the implant 1 from a single material which only articulates on bone, the risk of debris formation is significantly reduced, and any debris formed by bony particles is intrinsically inert.

[0038] Advantageously, the distal part 3 being hemispherical permits free rotation of the metacarpal, and this hemisphere is integral with the proximal part of the implant. The required functions of both axes of movement on an independent basis, necessary to fulfil the native requirements of the CMC joint are thus fulfilled by one single component. This provides quality improvements in terms of simplicity and the maximum reduction of component part.

[0039] The distal part 3 has a proximally facing surface 8 extending at an angle in excess of 30° to the longitudinal axis towards the neck. Therefore, it does not present a proximally facing surface for articulation as would be the case in a ball-and-socket joint articulation coupling. The articulation is achieved merely by ability of the distal part 3 to pivot within the distal bone’s recess, the joint being under compression. The base of the metacarpal within the CMC joint consists chiefly of hard cortical bone, especially around the periphery, and to take advantage of this hard-wearing surface, the diameter (maximum width) of the distal part 3 is in the range of 8 mm to 14 mm, to be selected in use depending on bone size. In this example, the saddle-shaped surface 7 of the proximal part 2 has a convex surface and a concave surface. The implant 1 is configured for multi-axis motion in a distal bone (for example the metacarpal) and translational motion over a proximal bone (for example the trapezium). The implant is configured such that a desired cone of motion is provided by pivotal motion of the implant about a locus of the distal part 3, the locus being a distal-most point on the convex surface 5. As the bone forms the “socket” which articulates on the hemisphere, theoretically it can have a 180° cone of motion.

[0040] In this example, the implant 1 is configured for a mammalian first carpometacarpal (“CMC”) joint. However, in other examples the proximal part 2 may be configured for translational motion on any surface to which it is not affixed, and the distal part may be configured to be captured in a volume of any adjacent bone.

[0041] The implant 1 is preferably of pyrocarbon, ceramic or any engineering plastic such as PEEK to match the bone elasticity properties. The material may be a material fabricated by functional gradient manufacturing or similar process to provide variable elastic properties.

[0042] The implant may alternatively be of a biocompatible metal such as titanium or cobalt chrome.

[0043] The implant may be referred to as a “dual-axis mono-block” (DAM), for use as a semi-constrained internal distraction implant for the treatment of moderate to severe osteo-arthritis. The distal part is preferably hemispherical in shape to reside in the proximal end of the distal bone but is not fixed to it. The proximal part is dimensioned such that it articulates over the distal end of the proximal bone. In a healthy joint, both sides of the joint are covered with cartilage which provides a smooth surface over which each of the bones articulate. Degenerative arthritis causes the wearing away of this cartilage, or the wearing away of this cartilage causes degenerative arthritis. In any case, the pain associated with osteoarthritis is thought to be due to the rubbing of bony surfaces over each other without the smooth and protective element of the cartilage. By removing at least one of these rough bony surfaces and replacing it with a prosthetic smooth or polished surface, the pain and degeneration pathway is significantly improved.

[0044] The implant has dimensions: distal part 3 diameter (W), 8.00 mm - 14 mm, proximal part 2 longest dimension (shown in Figs. 1 and 3), 12.00 mm, proximal part 2 shorter width dimension (shown in Fig. 2), 7 mm neck 4 narrowest width, 2.00 mm.

[0045] Length L, 6 mm to 12 mm.

[0046] One example of its application is the CMC joint where the distal part resides in a generally hemispherical cavity fashioned with appropriate orthopaedic tooling in the proximal end of the first metacarpal. The cavity so formed is a close fit for the distal part of the DAM 1 such that it provides free rotation and a consequent cone of motion for the implant with the centre of the distal area as the locus, as shown in Figs. 5 and 6. The proximal part 2 is then in a position to space the trapezium bone from the first metacarpal bone and translate freely over the distal end of the trapezium. Thus, the implant permits independent or concurrent motion and two loci and permits either bone to move independently or concurrently.

[0047] The implant 1 is not subject to this risk of subsidence with respect to the proximal bone since the proximal area of the implant is free to translate over the distal end of the proximal bone, and this also reduces any such subsidence risk with respect to the first bone.

[0048] The implant may advantageously be used instead of known implants because it is of single component construction and thus offers inherent simplicity and reduced risk, it provides ease of implantation as it does not require precision location in the long axis of the metacarpal as required by other known stem-like implants, and it simultaneously provides rotational and translational motion capability from two separate loci, thus facilitating complex joint motion.

[0049] This implant fulfils the requirement for two loci of motion which can act independently and concurrently.

[0050] The hemispherical element provides a means for the base of the metacarpal, when suitably configured by a burr, to rotate and thus facilitate the adduction and abduction movements of the metacarpal. Similarly, the base of the proximal part provides a means of translation over the trapezium and thus flexion and extension of the thumb.

[0051] The invention is not limited to the embodiments described but may be varied in construction and detail within the scope of the claims.

Claims

Claims1. An implant (1) comprising a proximal part (2) having a proximally-facing surface (7) which is configured for translational motion over a proximal bone, a distal part (3) having distally-facing surface (5) which is at least partly spherical, and a neck (4) interconnecting the distal and proximal parts and having a longitudinal axis, in which the distal part has a width dimension (W) in the range of 8 mm to 14 mm, and a dimension (H) between a distal- most surface of the implant and a proximal-most surface on the longitudinal axis is in the range of 6 mm to 12 mm, the implant being configured for multi-axis motion in a distal bone and translational motion over a proximal bone.

2. An implant as claimed in claim 1, wherein the distal part (3) is substantially hemi- spherical.

3. An implant as claimed in claims 1 or 2, wherein the proximal part proximally facing surface (7) is at least partly saddle-shaped.

4. An implant as claimed in claim 3, wherein said saddle-shaped surface (7) has a convex surface and a concave surface.

5. An implant as claimed in any preceding claim, wherein the implant is configured for a mammalian first carpometacarpal joint, in which the proximal part (2) is saddle shaped for translational motion on the trapezium bone, and the distal part (3) is configured so that at least a portion thereof is captured in a volume of first metacarpal bone in use.

6. An implant as claimed in any preceding claim, wherein the implant (1) is of a material which has a modulus of elasticity substantially equal to that of cortical bone.

7. An implant as claimed in claim 6, wherein the modulus of elasticity is in the range of 9 GPa to 16 GPa.

8. An implant as claimed in claim 6 or claim 7, where in the implant is of a polymer material.

9. An implant as claimed in claim 8, wherein the implant is of material selected from pyrocarbon, PEEK, and CFR-PEEK.

10. An implant as claimed in any of claims 1 to 5, wherein the implant of if a metal material.

11. An implant as claimed in claim 10, wherein the implant is of cobalt chrome and / or titanium material.

12. An implant as claimed in any preceding claim, wherein the implant is of unitary construction and being of the same material.

13. An implant as claimed in any preceding claim, wherein a dimension (L) which is the largest dimension of the proximal part (2) across the longitudinal axis is in the range of 10 mm to 16 mm.

14. A method of inserting an implant of any preceding claim in a hemi-arthroplasty bone joint such as the CMC joint, the method comprising cutting across a proximal end of a distal bone and removing the proximal part, placing at least a portion of the distal part into the distal bone so that the distal part bears against cortical bone, and the proximal part is engaged with the proximal bone for translational movement.

15. A method as claimed in claim 14, wherein the distal bone is the metacarpal and the proximal bone is the trapezium.

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