Telemetric tibial tray

0001The present invention relates to orthopaedic components configured for implantation within a patient. In particular, the invention concerns systems for evaluating loads within a joint space, and more particularly in the knee.

0002Joint replacement surgery is quite common and enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints are normally composed of metallic and/or ceramic components that are fixed to existing bone.

0003Knee arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. Typical knee prostheses include a femoral component, a patella component, a tibial tray or plateau, and a tibial bearing member. The femoral component generally includes a pair of laterally spaced apart condylar portions, the inferior or distal surfaces of which articulate with complementary condylar elements formed in a tibial bearing component.

0004In a properly functioning artificial knee joint, the condylar portions of the femoral component must slide and roll freely over the articulation surface formed by the condylar elements of the tibial bearing member. Natural friction within a replaced, artificial joint can lead to the development of wear debris in which minute particles of debris (e.g., metal or plastic from the prosthesis) become dislodged and migrate within the joint. The phenomenon of wear debris within artificial joints is a serious problem that can inhibit the proper mechanical functioning of the joint. Moreover, wear debris can lead to osteolysis and bone deterioration. When wear debris develops within an artificial joint, surgical removal of the debris or subsequent replacement of the artificial joint is often necessary.

0005During normal usage of a properly implanted prosthetic knee joint, load and stress are placed on the tibial bearing member. The tibial bearing member is typically made of an ultrahigh molecular weight polyethylene (UHMWPE), and friction, continuous cycling and stress can cause some erosion and/or fracture of the tibial bearing member, thus leading to wear debris. The risk of wear debris can be even greater during misalignment of an artificial knee joint, which can result from normal usage or from imperfect and/or inaccurate implantation of the prosthesis within a patient. During misalignment the load upon the tibial bearing member is not evenly distributed. Instead, excess load is placed on certain areas of the tibial bearing member. This uneven distribution of load (or edge loading) can accelerate the development of wear debris. Contact stresses on the tibial bearing member increase substantially with misalignment of the joint, thus increasing the risk that wear debris will develop when a prosthetic knee joint is subjected to misalignment conditions.

0006Joint replacement surgery obviously requires a tremendous degree of precision to ensure that prosthetic components are properly sized, implanted, and aligned. Imperfect sizing, implantation and alignment can lead to inadequate performance of the knee joint as well as to the presence of high contact stresses in certain areas of the prosthesis, thus leading to the possible development of wear debris.

0007The anatomy of patients who undergo knee arthroplasty is widely variable and can lead to difficulty in matching the standard sized prosthetic components that form a prosthetic joint. Many prosthetic components are manufactured such that similarly sized components must be used together and implanted within a patient when replacing a natural joint. That is, the femoral component, tibial bearing member, and tibial plateau that form the artificial knee joint must normally be of a matched size. If the components are not size-matched, inappropriate edge loading may develop and accelerate wear.

0008<figref idref="f0001">FIG. 1</figref> illustrates three components found in a typical knee joint prosthesis 10. A femoral component 12 includes a superior surface 14 which is mountable within the distal end of a patient's femur and an inferior articulation surface 16. The articulation surface 16 includes adjacent condyles 18. The knee prosthesis 10 also includes a tibial tray or plateau 20 which includes a distally extending stem 22 that is mountable within the tibia of a patient. The proximal end of the tibial tray 20 includes a recessed region 24 within which a tibial bearing member 26 is mounted in a mechanical fit.

0009Tibial bearing member 26 includes a distal surface 30 mountable within the recessed region 24 of the proximal end of a tibial tray20plateau 24. The proximal face of tibial bearing member 26 forms articulation surfaces 28 that engage and articulate with the articulation surfaces 16 of femoral component 12. The articulation surfaces 28 of the tibial bearing member 26 are configured to correspond to the condyles 18 of the femoral component 12.

0010The articulation surface 16 of femoral component 12 and the articulation surfaces 28 of tibial bearing member 26 are configured such that the contact area is maximized. The greatest contact area is achieved in conditions of perfect alignment throughout the range of motion of the knee joint, and in certain conditions of misalignment, including varus-valgus lift and internal-external rotation. The ability to achieve a large contact area between the articulating surfaces is significant because contact stress on the prosthesis components is minimized, particularly the tibial bearing member. Most standard tibial bearing members are manufactured of polymeric materials, such as ultra-high molecular weight polyethylene (UHMWPE), ceramic or metal. Where loads are unevenly distributed or concentrated across the tibial bearing member during use of an artificial knee joint, edge loading can develop. Edge loading leads to the development of higher contact stresses in certain parts of the prosthesis which, in turn, can cause wear of the articulating surfaces. Debris resulting from this wear can develop within the joint, sometimes leading to osteolysis.

0011More significantly, undue bearing wear can result in conditions requiring that the joint endoprosthesis be removed and replaced in a revision procedure. Accordingly, early determination of unacceptable wear conditions is critical. Misalignment of the joint prosthesis components can be detected during the implantation procedure and during rehabilitation of the new joint. Various measurements and templates can evaluate proper positioning and spacing of the components.

0012Another important indicator of proper or improper alignment is the distribution and transfer of loads across the prosthesis. In particular, loads experienced by the tibial tray 20 can provide the earliest indication of bad joint "fit". In order to evaluate these loads, telemetric implant components have been developed, such as the dual tray telemetric implant described in <patcit id="pcit0001" dnum="US5360016A"><text>US-5360016</text></patcit>. A force transducer is incorporated into the proximal tibial component of the implant. The force transducer uses strain gauges to generate output signals indicative of force measurement data that can be used to assess pressure differences across the surface of the tibial tray which may be indicative of an improperly aligned implant.

0013The introduction of telemetric implants has provided a means for evaluating the loads actually experienced by an endoprosthesis. This evaluation can occur in real-time as the joint is exercised and loaded. However, since the primary function of the implant is to serve as a prosthetic joint, and not simply as a data transmission device, the implant must be able to withstand joint loads without failure. Load is transmitted from the femur to the tibia through the large articulating surface areas of the condylar surfaces and the bearing surfaces. However, once the load reaches the tibial tray, such as the tibial tray, the force is transmitted through four support posts into the tibia. Therefore, it can be appreciated that the strength of these posts is critical to the strength of the implant.

0014In conflict with need for structural strength is the need to generate sufficient strain in the diaphragms such that a measurable strain differential may be detected between the strain gauges. The ability to measure accurately the forces transmitted across the joint space is enhanced as the magnitude of the strain differential increases. The trade-off for a stronger implant has been a reduction in diaphragm strain and a sacrifice in accuracy of the load measurement. The introduction of the no-load posts is an effort to recapture some accuracy in the load measurement capabilities of the strain gauge arrays. There remains room for improvement in both the strength of the telemetric implant component as well as the ability of the transducer component to provide a true measure of the loads transmitted across the joint.

0015The telemetric tibial tray of the present invention provides an optimum balance of implant strength and accuracy in load measurement. In accordance with one embodiment of the invention, the cross-sectional area of the support posts is increased over the prior art devices. Moreover, in lieu of the square cross-section of the prior art support posts, the support posts in the present invention are circular, which maximizes the load-bearing area of the posts without sacrificing flex responsiveness of the load diaphragm to which the posts are attached.

0016In another feature of the invention, the no-load post is eliminated so that the cavity-side face of the load diaphragm is featureless. Removing the no-load posts eliminates stress risers at the junction between the no-load posts and the diaphragms and significantly reduces the risk of fracture at the base of the support posts.

0017In addition, removing the no-load posts allows repositioning of the strain gauge array from the pattern employed in the prior art. It has been found that the radial strain pattern across the load diaphragm exhibits significant micro-strain behaviour at the centre of the diaphragm. Removing the no-load posts allows placement of the radially inboard strain gauges as close to the centre of the diaphragm as possible. At each circumferential position, there is more room for the radially outermost strain gauge so that the two strain gauges at each circumferential position yield a more accurate differential strain reading, which translates into a more accurate measure of the diaphragm strain.

0018The radial position of the inner and outer strain gauges is also calibrated according to the micro-strain response of the transducer cavity diaphragm to load. In one aspect of the invention, the inner strain gauge is positioned to span the radial location at which the maximum positive micro-strain occurs. The outer strain gauge is positioned at a radial location between the zero crossing point and the outer wall of the cylindrical transducer cavity. In a more specific aspect, the outer strain gauge is positioned to span the radial location of the maximum negative micro-strain response of the diaphragm. These optimized locations produce the greatest differential strain value, which leads to greater strain sensitivity of the force measurement features of the invention.

0019Another aspect of the strain gauge pattern diverges from the compass point arrangement of the prior art telemetric implants. Rather than align the radial strain gauges in planes parallel to the sagittal and lateral planes through the joint, the strain gauges of the present invention are rotated at 45°. It has been found that this orientation of the radial strain gauges increases the strain sensitivity of the transducer component, especially when the joint is flexed or extended.

0020A further improvement provided by the present invention is in the location of the wiring channels in the tibial tray. The wiring channels, although necessary for connection of the strain gauge wiring to the central circuit board, disrupt the transducer cavities and produce non-uniform strain patterns across the load diaphragm. In accordance with one feature of the invention, the wiring channels intersecting each transducer cavity is at 45° relative to the sagittal and lateral planes. It has been found that this positioning of the wiring channels also increases the strain sensitivity of the transducer.

0021It is one object of the present invention to provide a telemetric tibial tray that has increased strength characteristics over prior telemetric components. Another object is to increase the strain sensitivity of the telemetric component over prior devices.

0022Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is an exploded perspective view of the components of a knee prosthesis.</li><li><figref idref="f0002">FIG. 2</figref> is a side view of a tibial component of a knee prosthesis.</li><li><figref idref="f0002">FIG. 3</figref> is a bottom view of a lower transducer plate forming part of the tibial component shown in <figref idref="f0002">FIG. 2</figref>.</li><li><figref idref="f0002">FIG. 4</figref> is a bottom view of a lower transducer plate for a telemetric tibial tray in accordance with one embodiment of the present invention.</li><li><figref idref="f0003">FIG. 5</figref> is a partial cross-sectional view of the lower transducer plate shown in <figref idref="f0002">FIG. 4</figref>.</li><li><figref idref="f0003">FIG. 6</figref> is a graph of micro-strain as a function of radial distance in a transducer cavity of the plate shown in <figref idref="f0002">FIG. 4</figref>.</li></ul>

0023Referring to the drawings, <figref idref="f0002">FIGS. 2 and 3</figref> show a tibial component 40 which includes a stem 42 configured to be engaged within the tibia. A tibial tray 44 is mounted on the stem, and includes a cover plate 46 that is directly attached to the stem. A lower plate 48 is mounted on the cover plate, while an upper plate 50 is supported on the lower plate by a plurality of support posts 52. As best seen in <figref idref="f0002">FIG. 3</figref>, the lower plate 48 includes a perimeter wall 54 configured to engage the cover plate 46. Fasteners (not shown) are used to fasten the two plates together.

0024The lower plate defines a plurality of transducer cavities 56, each corresponding to a support post. The base of each cavity defines a diaphragm 63 to which a corresponding support post 52 is attached or integrally formed. The support posts are preferably integral with the lower plate 48 and the upper plate 50 but are configured to separate the two plates by a gap 53. Load applied to the upper plate 50 is transmitted through the support posts 52 to the integral diaphragms 63 which flex in relation to the transmitted load.

0025In order to measure the deflection of these diaphragms, a force sensing element is disposed within each transducer cavity. More specifically, the force sensing elements include an array of strain gauges that are affixed to the diaphragm at the base of each transducer cavity 63. As shown in <figref idref="f0002">FIG. 3</figref>, each strain gauge array includes four radially inner strain gauges 67 and four radially outer gauges 69 disposed at the four compass points around the cavity. More specifically, the strain gauges are arranged on axes that are at 90° or 180° to the sagittal and/or lateral planes of the knee joint prosthesis.

0026The strain gauges include wiring 71 that passes through wiring channels 60 and 61 to a centrally located circuitry cavity 58. A processing circuit board 73 is disposed within this cavity and includes electrical components and/or integrated circuits adapted to process the output of the strain gauges and facilitate translation of that output into load information. In some implants, such as the force transducer disclosed in <patcit id="pcit0002" dnum="US5360016A"><text>US-5360016</text></patcit>, the circuit board 73 serves to condition the strain gauge signals and to provide a wiring harness for connection to an external processor or computer. In other implants, the circuit board 73 prepares the strain gauge signals for transmission by a transmission device. In some implants, the circuit board includes a telemetry device and a power supply. In other implants, the stem 42 (<figref idref="f0002">FIG. 2</figref>) carries a telemetry device 75 and associated power source 77 adapted to transmit the strain gauge output signals to an external processor where the signals are evaluated.

0027In the telemetric tibial component 40 shown in <figref idref="f0002">FIG. 3</figref>, a no-load post 65 projects from the diaphragm 63 between the inner strain gauges 67. It can be appreciated that the no load posts 65 are essentially co-linear with the support posts 52, although the two sets of posts reside on opposite sides of the diaphragm 63 of each transducer cavity 56. The no-load posts are believed to promote a circumferentially symmetric strain pattern within each cavity.

0028<figref idref="f0002">FIG. 3</figref> shows a lower transducer plate 48 of a telemetric tibial tray which utilizes support posts 52 that are square in cross-section. These support posts typically have a dimension of about 2.5 mm on each side. Each support post is integral with the load diaphragm 63 and is aligned with the no-load post 65 projecting into the transducer cavity. The no-load posts produce stress risers at the junction with the load diaphragm. The strain pattern across the diaphragm is disrupted by the presence of the no-load posts.

0029Moreover, the posts 65 limit the radial space available in the cavity 56 for placing the radial strain gauges 67, 69. In prior devices, two strain gauges are placed diametrically opposite each other, as reflected in <patcit id="pcit0003" dnum="US5360016A"><text>US-5360016</text></patcit>. In later devices, two strain gauges have been placed along each radial extent to measure the differential strain at different circumferential positions around the diaphragm. The presence of the no-load posts 65 in these prior devices limits the space available for the radial strain gauge pairs.

0030In the prior devices, such as the tibial component 40 illustrated in <figref idref="f0002">FIG. 3</figref>, the strain gauge pairs are aligned along the compass points. More succinctly, the strain gauges are positioned in planes that are parallel with the sagittal and lateral planes of the joint. Similarly, the wiring channels 60, 61 of the prior device 40 are also aligned with the sagittal and lateral planes. The wiring channels interrupt the transducer cavity and disrupt the strain sensitivity at that intersection.

0031Referring to <figref idref="f0002">FIGS. 4</figref> and <figref idref="f0003">5</figref>, a lower plate 81 is provided as part of a telemetric tibial tray. The lower plate 81 and an integral upper plate 79 (<figref idref="f0003">FIG. 5</figref>) can be substituted for the like components of the tibial tray 44 shown in <figref idref="f0002">FIG. 2</figref>. Thus, the lower plate 81 defines a perimeter wall 82 that is configured to engage the cover plate 46 so that the cover plate can shield the electrical components carried by the lower plate. The upper plate 79 includes a recess 80 configured to receive the tibial bearing member 26 depicted in <figref idref="f0001">FIG. 1</figref>.

0032The lower plate 81 includes a plurality of cylindrical transducer cavities 83 and a centrally located circuitry cavity 84. The upper and lower plates are integrally attached by four support posts 86 projecting from a circular load diaphragm 88 in each transducer cavity. As understood, the diaphragms 88 flex when subjected to forces transmitted through the support posts 86. However, unlike the prior art, the support posts have a circular cross-section, as best seen in <figref idref="f0002">FIG. 4</figref>. Moreover, the cross-sectional area of these support posts 86 is significantly increased over the support posts of the prior art, such as the square posts shown in <figref idref="f0002">FIG. 3</figref>. In the preferred embodiment of the invention, the support posts 86 have a diameter of about 5.0 mm. The cross-sectional area of these posts is about 20 mm², which is over three times larger than the cross-sectional area (6.25 mm²) of the prior devices. This significantly greater cross-sectional area means that the support posts 86 have greater load bearing capacity with a longer fatigue life than the prior devices. In a preferred embodiment of the invention, the diameter of the support posts is about one third of the diameter of the transducer cavity.

0033In a further feature of the invention, the load diaphragm 88 does not include a no-load post at the centre of the diaphragm. Eliminating the no-load posts found in the prior devices (see <figref idref="f0002">FIG. 3</figref>) eliminates the stress risers and the potential locus for fatigue that accompanies those load posts. Moreover, removing the no-load posts frees the centre of the diaphragm for an optimum placement of radial strain gauges around the transducer cavities 83.

0034Turning to the graph of <figref idref="f0003">FIG. 6</figref>, it can be seen that the micro-strain as a function of radial distance exhibits a high response at the centre of the load diaphragm 88. The micro-strain remains at this high level for about half the radial distance to the perimeter of the transducer cavity and exhibits a positive maximum value at a radial location M⁺ offset from the centre of the diaphragm. The zero-crossing represents the point at which the micro-strain reverses sign from a positive magnitude to a negative magnitude. The zero crossing point Z for the micro-strain is nearer the radial edge of the cavity. The micro-strain response of the diaphragm also exhibits a negative maximum value point - between the zero crossing point and the outer wall 89 of the cylindrical cavity. This strain graph provides a guideline for optimum placement of the force sensing elements within the cavities, or more particularly the radial strain gauges, namely the inner gauges 90 and radially outer gauges 91.

0035As is known in the art, the strain gauge array measures differential strain across the diaphragm, which can then be translated directly into a measure of the forces imposed on the diaphragm as the knee prosthesis is loaded. Depending upon the arrangement of the array, the measured strains can be used to calculate the load imposed on the tibial tray, including its magnitude, direction and location. These calculations can be made in an external processor, such as a computer, upon receiving the data transmitted form the telemetric implant in a known manner. As is known in the art, providing a cylindrical transducer cavity and circular load diaphragm allows placement of the strain gauges in a circumferential pattern about the centre of the diaphragm to evaluate the radial differential strain across the diaphragm.

0036Increasing the strain sensitivity of the strain gauge array will produce a more accurate measure of the differential strain at various points around the load diaphragm. It has been found in accordance with the present invention that placing the radially inner strain gauges 90 close to the centre of the diaphragm increases the strain sensitivity of the gauge array. In a preferred embodiment, the inner gauges are positioned to span the maximum micro-strain point M⁺, which in a specific embodiment is within 2.5 mm of centre. Removal of the no-load post allows this more radially inboard position for the inner strain gauge.

0037In addition, the present invention contemplates positioning the outer strain gauges as close as possible to or immediately adjacent the outer wall 89 (<figref idref="f0003">FIG. 5</figref>) of the cylindrical cavity 83. With this position, the outer gauges 91 will be positioned beyond the zero crossing point Z for the micro-strain across the diaphragm. Preferably, the outer gauge is positioned to span the negative maximum micro-strain point. This placement of the inner and outer strain gauges 90, 91 produces the largest differential strain, and consequently the greatest strain sensitivity. As an additional improvement, the diameter of the transducer cavities is increased from the prior art devices. Specifically, the diameter is increased from 13.4 mm to about 15.0 mm. This larger diameter provides more radial space for placement of the outer strain gauge 91, which assures that the outer gauge will be well beyond the zero-crossing for the micro-strain, as reflected in the graph of <figref idref="f0003">FIG. 6</figref>. The larger diameter transducer cavity consequently yields greater differential strain values, which improves the accuracy of the load measurements.

0038The preferred embodiment of the invention yields even greater improvements in load measurement accuracy by optimizing the orientation of the strain gauge arrays. It has been found that rotating the diametrically opposed inner/outer gauge pairs by 45° further increases measurement sensitivity. Thus, as shown in <figref idref="f0002">FIG. 4</figref>, the inner and outer gauges 90, 91, respectively, are arranged at 45° relative to the sagittal and lateral planes. Rotating the position of the radial strain gauge pairs increases the differential strain measured between the inner and outer gauges, relative to the conventional placed gauges of the prior art. Again, increases in differential strain translate directly into more accurate load measurements in each load diaphragm.

0039Additional improvement is realized by orienting the strain gauges at 45° relative to the wiring channels. In one embodiment of the invention, the strain gauges 90, 91 are oriented as shown in <figref idref="f0002">FIG. 4</figref>, while the wiring channels 60 are oriented as shown in <figref idref="f0002">FIG. 3</figref>. In other words, the wiring channels are arranged at 90° or 180° relative to the sagittal plane for the joint or prosthesis, while the strain gauges 90, 91 are oriented at 45° or 135° to the same plane. Thus, with this specific embodiment, no strain gauge is aligned with the interface between a transducer cavity and a wiring channel.

0040In another aspect of the invention, arrangement of the wiring channels was also found to contribute to the strain sensitivity of the telemetric tibial tray. The wiring channels 85 provide a path for the strain gauge wiring to connect to the circuit board 93 disposed within the central cavity 84. (Note that the wires are not depicted within the channels 85 in <figref idref="f0002">FIG. 4</figref> for clarity). It can be appreciated that the intersection of a wiring channel with the transducer cavity creates a localized disruption in the strain pattern across the diaphragm 88; however, the wiring channels are necessary (absent the cost-prohibitive approach of burying the wires within the body of the lower plate 81). The present invention contemplates optimum positioning of the wiring channels at 45° to the sagittal and lateral planes, as illustrated in <figref idref="f0002">FIG. 4</figref>. This orientation of the wiring channel produces greater differential strains than the prior conventional channel placement.

0041With the wiring channel arrangement shown in <figref idref="f0002">FIG. 4</figref>, it is preferable that the strain gauges be oriented at an offset angle relative to the channels. In other words, it is preferable that no strain gauge be aligned with the intersection between a wiring channel and a corresponding transducer cavity. Thus, in a specific embodiment, the wiring channels 85 are angularly oriented as shown in <figref idref="f0002">FIG. 4</figref> at 45° or 135° relative to the sagittal plane, while the strain gauges can be arranged like the gauges 67, 69 shown in <figref idref="f0002">FIG. 3</figref> at 0 or 180° relative to the same plane. In certain alternatives, improvements in differential strain sensitivity may still be accomplished if the strain gauges and wiring channels are both oriented at 45° and 135°.