A custom knee prosthetic includes a polycentric rolling contact joint, whose instantaneous center of rotation coincides with the instantaneous center of rotation of a patient's knee joint.

DETAILED DESCRIPTION

All documents mentioned herein are hereby incorporated in their entirety by reference. References to items in the singular should be understood to include items in the plural, and vice versa, unless explicitly stated otherwise or clear from the text. Grammatical conjunctions are intended to express any and all disjunctive and conjunctive combinations of conjoined clauses, sentences, words, and the like, unless otherwise stated or clear from the context. Thus the term “or” should generally be understood to mean “and/or” and so forth.

FIG. 1is a schematic view of a four bar linkage derived from a human knee. The linkage100is shown superimposed against a schematic lateral radiograph of a knee102(shown in dashed lines), including a tibia103aand a femur103bto illustrate the anatomical context. The linkage100is assumed to lie in a sagittal plane, and is comprised of projections of four anatomical points onto the plane. The four anatomical points are the origin104and the insertion106of the anterior cruciate ligament (ACL), and the origin108and insertion110of the posterior cruciate ligament (PCL). Any specific sagittal plane on which to project the anatomical points may be chosen; for example, any sagittal plane passing between the midpoints of any pair of anatomical points noted above, or any location derived from a weighted average of the locations of any two or more of the anatomical points. Unless otherwise specified in this document, reference to these anatomical points should be understood to refer to their corresponding projections in the selected sagittal plane.

The four bar linkage includes links between each of these anatomical points, except: ACL origin104to PCL origin108; and the ACL insertion106to the PCL insertion106. For convenience in what follows, it will be assumed that the link from the ACL origin104to the PCL insertion108is fixed, and to the extent necessary, a coordinate system can be defined implementing this assumption. In some implementations, the link from the ACL origin104to the PCL insertion108may be considered as the ground link in the four-bar mechanism. The links from the ACL origin104to the ACL insertion106and the PCL origin108to the PCL insertion110may be referred to in this document as grounded links. However, any coordinate system in which any link is fixed (or no link is fixed) may be adopted without departing from the scope of the invention.

The human knee's natural motion is close to, but distinct from, the motion of a pin joint (in which one of the joint's components may be considered fixed, and the other rotates about a fixed center of rotation linking the components). More specifically, during a period of flexion or extension, the tibia103aand femur103bundergo a “polycentric” rotational motion. That is, at any instant during flexion or extension, there is an “instantaneous center of rotation” about which the femur and/or tibia are rotating, but the location of this instantaneous center of rotation changes as the knee continues to flex or extend due to the variable radii of the human femoral condyles. In the four bar linkage100, this instantaneous center of rotation of the knee joint is approximated to a useful degree by the intersection point112of the links corresponding to the ACL and PCL (the grounded links).

It is advantageous for a knee prosthesis to mimic the natural kinematics of the knee, in particular accounting for a knee's polycentric rotation. Moreover, insofar as individuals' anthropometric variations are concerned, it is also advantageous for a particular prosthesis to mimic the specific kinematics of the patient for whom the prosthesis is intended. The techniques described herein are useful to produce such prosthetic devices or components thereof.

FIG. 2is a flowchart for creating a custom knee prosthesis for a given patient. Advantageously, the method200can be used to produce a prosthesis (or components thereof) that accounts for the individual's unique anatomy as described above.

In step202, an image of the patient's knee may be identified. In some implementations, the image may be a radiographic image such as an X-ray, magnetic resonance image (“MRI”), computed tomography (“CT”), or any other imaging modality sufficient to capture the requisite information about the ACL and PCL origins/insertions as described below. The image may be two dimensional or three dimensional. For two dimensional images, in some implementations, it is convenient to obtain the image in a sagittal plane so that positions of the anatomical points described above can be more easily located. For three dimensional images, the positions of the anatomical points described above can be projected onto a sagittal planar coordinate system.

In step204, the locations of the origins and insertions of both the ACL and the PCL may be identified within a selected sagittal plane. These locations can be used to define a model four bar linkage as described above. In step206, the trajectory of the instantaneous center of rotation (as the knee undergoes flexion or extension in a specified range of motion) are identified. In some implementations, the trajectory of the instantaneous center of rotation may be obtained by using the four bar linkage model described above. In some implementations, the instantaneous center of rotation may be obtained by other means.

The specified range of motion may include flexion from a tibio-femoral joint angle of 0 degrees (i.e., in which the knee is extended), to a joint angle of 175 degrees or more. In some implementations, it may be desirable to limit the range of motion (e.g., from 0 to 135 degrees) for the safety of the patient or for other reasons.

The instantaneous centers of rotation identified in step206collectively comprise a curve. However, the curve may appear differently when expressed in different coordinate systems or when different patients' anatomy is used to construct the four-bar linkage.

In some implementations, the solution of the forward kinematics problem in different coordinate systems may be used to design different components of a knee prosthesis. For example, a coordinate system in which the tibia is fixed (as described above) can be used to describe a surface of a joint component that can be advantageously coupled to the tibia. Similarly, a coordinate system in which the femur is fixed can be used to described a surface of a compatible joint component that can advantageously be coupled to the femur. For joints having mating surfaces constructed this way, the point of rolling contact between the joint components is, to a useful degree, coincident with the instantaneous center of rotation of the physical knee joint during flexure and extension.

However, the technique described in the previous paragraph is not the only way to arrive at joint component geometry. In particular, the joint components that result from the previous paragraph are necessarily of complementary convexity (i.e., one joint surface is convex, and it mates with a concave joint surface). By contrast, joints having two mating convex surfaces are possible, which still possess the property that the point of rolling contact between the joint surfaces is coincident to a useful degree with the instantaneous center of rotation of the physical knee joint.

Referring toFIG. 3, one way to construct such “convex/convex” joints is illustrated.FIG. 3is a plot of a typical curve302of the instantaneous centers of rotation (“COR”). The coordinate system ofFIG. 3is such that origin is coincident with the insertion of the ACL on the anterior tibia, the Y-axis is orthogonal to the X-axis, which is parallel to the horizontal, and the coordinate system lies in the relevant sagittal plane. An arbitrary reference curve304(e.g., a convex curve) may be defined to describe one of the joint's surface's geometry. The reference curve304may be coincident with at least one point of the COR curve302. Then, a difference between the COR curve302and the reference curve304can be computed, yielding a difference curve (not shown). By adding a constant term, the difference curve can be shifted so that it is coincident with both the COR curve302and the reference curve304. The shifted curve306describes the shape of a surface that mates with the surface described by the reference curve304. For a typical COR curve302, a convex reference curve304may yield a convex curve306. In some implementations, the reference curve304may be a circular arc.

Techniques for designing joint surface geometries are described further in Rolling Contact Orthopaedic Joint Design by Alexander Henry Slocum Jr. (PhD Thesis, Mechanical Engineering Department, Massachusetts Institute of Technology, 2013), the entirety of which is incorporated by references herein.

Referring back toFIG. 2, in step208, joints having components with appropriate mating surfaces are constructed, such that the point of contact as the interface of mated components coincides with the instantaneous center of rotation of the knee joint. Appropriate materials for joint construction are describe in more detail below.

Joints fabricated according to method200also possess the property that the surfaces roll (as opposed to slip) relative to each other during articulation. In practice, small design or manufacturing deviations from the above description may introduce a small degree of slippage during joint articulation. In this document, a degree of slippage during articulation that does not result in additional discomfort or adverse medical consequences (vs. no slippage) to a patient during the anticipated lifetime of the fabricated joint is regarded as insubstantial.

FIG. 4is a side view of an exemplary rolling contact joint. The joint400may include a first component402having a first surface404and a second component having a second surface406. In this exemplary joint400, the surfaces404and406have been constructed using the technique of expressing the curve of instantaneous centers of rotation in coordinates in which the femur (for surface404) and the tibia (for surface406) remain fixed. As described above, during articulation of the joint400, the first component402rolls across the second component406at a point of contact between the surfaces404and408.

Each component402,406may include a protrusion410operable to limit the joint's range of motion in one or both directions. In some implementations, the protrusions410are positioned to permit a range of motion corresponding to a tibio-femoral angle range of a typical healthy knee (i.e., between 0 degrees and 175 degrees). In some implementations, the protrusions410may be positioned to permit a range of motion corresponding to some other range of tibio-femoral angle; e.g., 0 to 135 degrees. A lesser angle range can be advantageous to mitigate the risk of certain injuries with certain patients.

Although the protrusion410is shown as structure that physically interferes with the motion of the joint400, other implementations are possible. For example, corresponding structures may be employed to mechanically resist (or entirely limit) motion beyond a pre-defined range. More particularly, magnetic or electromagnetic structures, hydraulics, actuators, springs, or the like may be used to provide a resistive or limiting reactionary force to motion beyond a pre-defined threshold.

The joint400may include a pre-load spring412. The spring412may be operable to bias the component second component406towards the first component402, thereby mitigating the risk of undesirable separation during use. In order to effectively provide the pre-load, one end of the spring412may be mechanically coupled to the second component406, while another end of the spring412may be coupled to a different component. (SeeFIGS. 6A and 6Bfor exemplary embodiments.)

In some implementations, the pre-load force provided by the spring412may be great enough to maintain engagement of the components402,406under worst case conditions with respect to the risk of the components being separated (e.g., motions akin to deep knee squats). In some implementations, the biasing force may be between 40N and 60N. In some implementations, the biasing force may be determined for the particular patient based on the patient's individual requirements.

Although a spring412is shown in the exemplaryFIG. 4, in general any structure operable to bias one component402,406into the other may be employed. Such structures include, but are not limited to: magnets, tension bands or straps, linkages, or any such manner of constraining two cam surfaces relative to one another.

FIG. 5is a side view of an exemplary geared joint. The geared joint500includes a first component502having a first geared surface and a second component504having a second geared surface508. The surfaces504,508can be defined as in the previous rolling contact joint example, except for the presence of mating trapezoidal gear teeth. Similarly to the previous example, the components502,506may each include a protrusion510to limit flexure within a desired range, and the geared joint500may include a pre-load spring512.

The geared joint500may also include additional couplings514extending away from the gears. These couplings may be useful for attaching the geared joint500to other structures. For example, in embodiments in which a geared joint500is included in a knee brace, the additional couplings514are useful to attach to cuffs or similar structures that are operable to removably couple the knee brace to a patient's tibia or femur. In embodiments in each a geared joint500is included in an in vivo application such as a knee replacement, the additional couplings514can be used to irremovably couple the components502,506to a patient's femur or tibia.

AlthoughFIG. 4andFIG. 5illustrated a rolling contact joint and a geared joint respectively, it may be understood that hybrids of these exemplary structures may be used. For example, a hybrid joint may have components with “partially geared” surfaces that articulate compatibly with the natural kinematics of a patient's knee joint.

FIG. 6Ais an exploded view of an exemplary rolling contact joint assembly.FIG. 6Bis a non-exploded view of the same assembly, except for a face plate that has been suppressed in order to reveal the remaining components. The face plate may be employed in order to mitigate parasitic torques on the joint's articulating surfaces.

The assembly600may include two face plates602(only one of which is shown inFIG. 6B), a pre-load spring case604, a pre-load spring606, two rolling contact joints608, and a geared joint610. The various joints608-610may include custom surfaces (or geared surfaces) that each, as described above, articulate in a way that matches the kinematics of a patient's knee. Although two rolling contact joints608and a single geared joint610are shown inFIGS. 6A and 6B, in principle any number of either component (or hybrids thereof) can be used.

Using a combination of a rolling contact joint608and a geared joint610has certain advantages over using only one type of either joint. For example, a geared joint610has the advantage that it is relatively resistant (compared to a rolling contact joint only) to parasitic torques. However, a rolling contact joint is relatively resistant to increased loads, e.g. from the weight of the patient. Thus, in combination, one or more rolling contact joints can advantageously help bear a patient's load (thereby prolonging the expected life of the geared joints), while one or more geared joints can advantageously mitigate undesirable torque, thereby prolonging the expected life of the rolling contact joints and the constraint mechanisms.

The face plates602may provide various advantages. For example, the face plates may shield the moving surfaces of the various joints608,610in the mechanism600, thereby mitigating the risk of damage to the mechanism or injury to the patient or third party. Moreover, one or both face plates602may advantageously serve as an anchor for certain components of the mechanism and help to mitigate parasitic torques. For example, the bolt612couples one component of each joint608,610to the face plates602, while the other component of each joint608,610can “float” relative to the face plates602. At the same time, the pre-load spring contacts the floating components of each of the joints608,610, thereby helping to ensure adequate engagement of the relevant components of each joint608,610.

In various embodiments, the structures described inFIGS. 3-6A,6B can be constructed from any suitable material or combination of materials. In some implementations, the materials can include metal (e.g., stainless steel, aluminum, titanium, etc.). The materials can also include non-metals such as ceramics, plastics, or still other materials. The particular choice of material from which to fabricate a component is influenced by conventional design considerations, such as the intended component lifetime, cost sensitivity, operating environment (e.g., propensity to corrode certain metals in external applications; risk of causing an adverse reaction in a patient for in vivo applications, etc.), load capacity and other required mechanical tolerances, and the like.

The meanings of method steps of the invention(s) described herein are intended to include any suitable method of causing one or more other parties or entities to perform the steps, consistent with the patentability of the following claims, unless a different meaning is expressly provided or otherwise clear from the context. Such parties or entities need not be under the direction or control of any other party or entity, and need not be located within a particular jurisdiction.

Thus for example, a description or recitation of “adding a first number to a second number” includes causing one or more parties or entities to add the two numbers together. For example, if person X engages in an arm's length transaction with person Y to add the two numbers, and person Y indeed adds the two numbers, then both persons X and Y perform the step as recited: person Y by virtue of the fact that he actually added the numbers, and person X by virtue of the fact that he caused person Y to add the numbers. Furthermore, if person X is located within the United States and person Y is located outside the United States, then the method is performed in the United States by virtue of person X′s participation in causing the step to be performed.

While particular embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications in form and details may be made therein without departing from the spirit and scope of this disclosure and are intended to form a part of the invention as defined by the following claims, which are to be interpreted in the broadest sense allowable by law.