SHOULDER ARTHROPLASTY DEVICE
Field of the Invention
The present invention relates to arthroplasty devices, and in particular to linked constrained shoulder arthroplasty devices.
Background
Current designs for constrained shoulder arthroplasty are not completely stable. Components are held together by Morse tapers and/or ci clips, which are not 100% effective and are at risk of becoming disconnected whilst in the arm which could lead to an otherwise avoidable revision surgery. Improved designs for a constrained shoulder arthroplasty device are needed.
Summary of the Invention
According to the present invention there is provided a shoulder arthroplasty device comprising: a base plate adapted to be attached to an area of bone, the base plate having a first screw thread provided thereon; a glenoid shell comprising a body component comprising: a second screw thread provided on a first end of the body component, in which the second screw thread is arranged to cooperatively engage the first screw thread on the base plate such that the glenoid shell may be fixed to the base plate by means of the first and second screw threads; a third screw thread provided on a second opposed end of the body component; and a recess provided on the second end of the body component in which the recess is arranged to receive a cup insert; a cup insert adapted to be received within the recess, in which the cup insert comprises a cup having a concave interior surface; a collar cap having a fourth screw thread arranged to cooperatively engage the third screw thread of the body component, and in which the collar cap further comprises 35 an aperture extending through the collar cap; a ball joint comprising a ball to interfit with the cup, in which the ball joint comprises a convex surface profile to match the profile of the cup, and a projection extending from the ball; and a stem fixably mountable onto the said projection.
The base plate may comprise a substantially planar component adapted to mount on an area of bone, such as the scapula of a patient, by having one or more apertures through which a screw may fit to fix the base plate to the bone.
In some embodiments the base plate comprises a recess in the planar component, the first screw thread is a female thread provided within the recess, and the second screw thread is a male thread provided on a projection from the first end of the body component of the glenoid shell.
In some embodiments the planar component may comprise a projection from the underside of the base plate, the recess being formed in the projection. The projection may be adapted to fit into a hole in the bone onto which the base plate is to be fixed.
In some embodiments the second end of the glenoid shell has a circular external cross-section, the third screw thread is a male thread provided on the outer circumference of the said second end, and the fourth screw thread is a female thread provided within a recess on the collar cap.
The cup may have a part-spherical interior surface and the ball may have a part-spherical convex surface. A part-spherical surface is a surface having a profile forming part of the surface of a sphere.
The stem may comprise an elongated component adapted to be fixed to a patient's humerus, for example inside the humerus. The stem may be adapted to be fixed to the projection and the device may comprise fixing means to fix the stem in place. In some embodiments the stem comprises a recess to receive the projection on the ball joint. The fixing means may comprise a threaded hole in the stem and a bolt configured to fit the threaded hole to bear on the said projection, or a clearance hole in the stem and a threaded hole in the projection to receive the bolt.
In some embodiments the stem comprises an angled stem head that fixes to the projection and an elongated stem body that mounts on the stem head such that the axis of the stem is at an angle to the axis of the projection when fixed onto the ball joint. The stem head may be provided with an angle selected in a range, such as for example 5 to 20 degrees.
In use, the base plate may be secured to the scapula, for example by means of screws through apertures provided in the base plate. The glenoid shell then screws on to the thread on the base plate, and the cup insert is placed into the recess of the glenoid shell. The ball joint projection is passed through the aperture in the collar cap, and the collar cap is screwed onto the thread on the glenoid shell to fix the ball joint in place in the cup. The stem, which may be selected to suit the humerus of the patient, for example to suit the size of the humerus and its natural orientation relative to the shoulder joint of the patient, is then fixed to the projection of the ball joint using the fixing means, such as a bolt.
In this way the invention provides an improved shoulder arthroplasty device and method for shoulder joint replacement, in which the device has the following advantages over prior art devices: (a) the device comprises screw fittings between the glenoid shell component and the base plate, which may be tightened using a tool, for example to a known torque and then locked into place by means of grub screw, and which does not risk becoming disengaged by a movement of the arm, in contrast with the push-fit arrangement typical of the prior art; (b) the device comprises a screw-fit arrangement between a collar to retain the ball joint in the insert cup, which again allows the collar to be tightened in a controlled way, and provides a fixing that is resistant to disengagement in movement of the arm, and which is easy to use in practice, in contrast with push-fit and clip-fitting means to retain the ball in the cup typical of the prior art; (c) the device allows selection of the stem to fit an individual patient, independently of the other components of the device, so providing ease of use to the operating surgeon.
According to a further aspect, the invention provides a method of providing a 35 shoulder arthroplasty using a device as described herein.
Brief Description of Figures
Figure 1 shows a side view of an embodiment of the invention.
Figure 2 shows a top side isometric view of the embodiment shown in figure 1.
Figure 3 shows a bottom side isometric view of the embodiment shown in figure 1.
Figure 4 shows a cross-section through the embodiment shown in figure 1.
Figure 5 shows a top side isometric exploded view of the embodiment shown in figure 1.
Figure 6 shows a bottom side isometric exploded view of the embodiment shown in figure 1.
Detailed Description of Figures
Referring to figures 1 to 6, an embodiment of a device according to the invention 15 comprises: a base plate 10 adapted to be attached to an area of bone, the base plate having a having a first screw thread 12 provided thereon, a glenoid shell 14 comprising a body component 16 having: a first end 18 having a second screw thread 20 provided thereon to match the first screw thread on the base plate, such that the glenoid shell may be fixed to the base plate by means of the first and second screw threads, a second end 22 having a third screw thread 24 provided thereon, and comprising a recess 26 to receive a cup insert 28, the second end having a circular external cross-section 30, a cup insert 28 adapted to fit the said recess 26, comprising a cup 32 having a concave interior surface 34, a collar cap 36 having a fourth screw thread 38 to interfit with the third screw thread and an aperture 40 through the collar cap, a ball joint 42 comprising a ball 44 having a convex surface 46 having a profile to match the profile of the cup such that the ball may interfit into the cup and a projection 48 extending from the ball, a stem 50 comprising an elongated component configured to mount on the projection, and a fixing means 52 in the form of a socket screw to fix the stem onto the projection.
The base plate comprises a substantially planar component adapted to mount on an area of bone, such as the scapula of a patient, by having one or more apertures 66 through which a screw may fit to fix the base plate to the bone. In this embodiment the base plate comprises a recess 68 in the planar component, the first screw thread 5 12 is a female thread provided within the recess, and the second screw thread 20 is a male thread provided on a projection 54 from the first end of the body component of the glenoid shell. The base plate comprises a projection 56 from its underside, the recess being formed in the projection. The projection 56 is adapted to fit into a hole formed in the bone onto which the base plate is to be fixed. The base plate may be 10 fixed to the scapula by a set of screws, for example in this embodiment a central 6.5mm screw 58 in the recess 68, and four 4.75mm screws 60 through apertures 66.
The glenoid shell may be fixed to the base plate by screwing the male second thread 20 into the first female thread 12, tightened using a spanner on the hex flats 70 (for example 23mm across flats) on the body component and 72 (for example 28mm across flats) on the base plate, and then locked in place with a grub screw 62, such as a 4mm grub screw, provided in a threaded hole 64 in the base plate. This provides a secure fixing, that is less prone to error in fixing and is prevented from becoming loose by the dual fixing means comprising screw fitting means 70, 72 and grub screw 62.
The cup insert 28 may then be placed in the recess 26 of the glenoid shell to provide a bearing surface for the ball joint. The projection 48 of the ball joint is passed through the aperture 40, the ball is placed in the cup, and fourth female thread 38 provided on the inside of the outer rim of the collar cap 36 is then screwed onto the external male thread 24 provided on the outer circumference of the glenoid shell. Holes 74 are provided for a pin spanner, such as a 35mm diameter pin spanner, to tighten the collar cap. This provides an improvement over prior art fixing designs using a push in ball joint and a spring clip, which is difficult to fix securely during surgery and risks coming undone if suddenly stressed in use.
The stem 50 is then mounted on the projection 48 and fixed in place using the fixing bolt 52 through hole 76 and threaded hole 78 on the projection. The stem may be selected to suit the patient, and comprises a stem head 80 that fixes to the projection 48 and a stem body 82 that mounts on the stem head, for example by means of a tapered end on the stem body and a matching tapered recess on the stem head as shown in figure 4. In this way the device provides for interchangeable stem heads and stem bodies to be selected, to allow for the optimal angle of the stem head and length and diameter of the stem body to be chosen.
The device may be dimensioned appropriately for use in the shoulder of a patient. The fixings and dimensions across flats on the components may be as described above to allow a standard tool set to be used.
The components of the device may be formed from materials as used in prior art 10 prostheses. The base plate may have a coating of hydroxyapatite over porous plasma spray to promote adhesion to the bone. The cup insert may be polyethylene. The metal components may comprise titanium.
The invention has been described by way of examples only and it will be appreciated that variation may be made to the above-mentioned embodiments without departing from the scope of invention. Firstly it will be understood that any features described in relation to any particular embodiment may be featured in combinations with other embodiments.
With respect to the specification therefore, it is to be realised that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use (including using it on a different joint like the hip for example), are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention.
Therefore, the foregoing is considered as illustrative only of the principles of the invention, with variation and implementation obvious and clear on the basis of either common general knowledge or of expert knowledge in the field concerned. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention as set out in the accompanying claims.