Patent Application
20130123931
The present invention relates to a revision knee prosthesis that provides decreasing coronal and transverse rotational constraint as the knee rotates from extension to flexion, and then increasing constraint as the knee rotates from flexion to full extension.
Knee revision surgery, also known as revision total knee arthroplasty, is a procedure in which the surgeon removes a previously implanted artificial knee joint, or “primary” prosthesis, and replaces it with a new “revision” prosthesis. Replacement of the “primary” prosthesis is required when it becomes defective or its connection to the femur and/or tibia becomes compromised (loosened), both of which may occur for several reasons. For example, the primary prosthesis may simply wear out or be damaged from patient trauma. However, the most common condition requiring primary prosthesis replacement is weakening or degradation of the anatomy to which the primary prosthesis is attached, which is commonly caused by osteolysis. Osteolysis is an inflammatory response that occurs when tiny fragments of polymer liner are absorbed by tissue cells around the knee joint. The inflammatory response dissolves the bone around the prosthesis and eventually loosens its connection thereto. In such case, it is usually not possible to surgically re-attach the primary prosthesis since the bone has typically degraded to a condition that is incompatible for re-connection to a primary device. Furthermore, the patient's bone is further degraded when the primary prosthesis, which is typically cemented to the bone, is removed by chipping away at the bone.
While the articulating surfaces of a revision prosthesis are similar to a primary prosthesis, the design of a revision prosthesis accommodates the fact that there is typically less bone to which the prosthesis is attached. For example, a revision prosthesis typically includes integrated stems that are inserted into the intermedullary canal of the femur and tibia for better support and stability. A revision prosthesis may also include attachable augments of varying thicknesses connected to the tibial base plate or the femoral component to accommodate irregularities in the bone profile. These components enable the bone to better support the device.
In addition to providing greater support on the bones, the design of a revision prosthesis typically provides greater constraint than a primary prosthesis. As the knee is flexed, the most obvious rotation of the tibia occurs in the sagittal plane about a coronal axis relative to the femur. However, the femur also translates posteriorly on the tibia and the tibia also rotates internally about its longitudinal axis. The complex articulation path of the human knee is dictated by the geometry of the distal femur and proximal tibia, as well as the arrangement of ligaments surrounding and connecting the distal femur and proximal tibia. The collateral ligaments provide stability to the knee in varus and valgus stresses. The cruciate ligaments provide stability in the anterior and posterior direction, and also allow the tibia to rotate axially, i.e., about its longitudinal axis. Thus, as the knee is flexed, the tibia undergoes internal rotation about its longitudinal axis.
During primary knee replacement surgery, the anterior cruciate ligament (ACL), and sometimes posterior cruciate ligament (PCL), is removed but the collateral ligaments remain intact. During revision knee surgery, the PCL (if remaining) and the collateral ligaments are almost always removed. Therefore, the stability/constraint that was previously provided by these ligaments must be provided by the design of the revision prosthesis itself
To provide stability, prior art revision prostheses include a femoral component with a guide box and a tibial liner with a central a central post, which articulates within the guide box. The post and guide box of prior art revision prostheses are designed and dimensioned to allow free rotation of the tibia in the sagittal plane but little rotation in the transverse or coronal plane. Any rotation in the transverse or coronal plane is due to the clearance or laxity between the engaging surfaces of the post and guide box, which enables some movement in both the coronal and transverse planes.
In the prior art, the amount of clearance intentionally designed between the post and the guide box is dictated by two competing considerations based on the natural articulation of the knee. To mimic the complex articulation path of the human knee, a revision prosthesis should provide high restraint during full extension but provide some laxity during flexion. Some prior art revision prostheses have very little clearance between the post and the guide box, which provides the desirable high level of constraint in full extension but undesirable high level of constraint during flexion. On the other hand, other prior art revision prostheses have greater clearance between the post and the guide box, which provides undesirable laxity in full extension but desirable laxity during flexion. It is believed, however, that none of the prior art revision prostheses provide high restraint during full extension and reduced restraint during flexion. Therefore, it would be desirable to provide a revision prosthesis, which more accurately facilitates the motion of the natural knee by providing a high degree of restraint during extension and less restraint during flexion.
The invention comprises a revision knee prosthesis that provides variable restraint restraint as the knee rotates through its normal range of motion. In a first embodiment, the revision knee replacement prosthesis provides decreased restraint (increased laxity) as the knee rotates from full extension to flexion, and then provides increased restraint (decreased laxity) as the knee rotates from flexion to full extension. In a preferred embodiment, variation in restraint relates to valgus-varus laxity (rotation or tilt in the coronal plane) and/or medial-lateral laxity (rotation in the transverse plane).
The novel prosthesis comprises a femoral component that articulates with a tibial component and the natural or prosthetic patella. In a first embodiment, the revision knee prosthesis comprises a femoral component that connects to the distal end of a resected femur and a tibial component that connects to the proximal end of a resected tibia. The femoral component includes medial and lateral condyles having distal, articulating condylar surfaces, and a patellar flange having an articulating patellar surface. The tibial component includes a proximal bearing surface with medial and lateral concavities that articulate with the medial and lateral condyles. The condylar surfaces and concavities enable anterior-posterior translation of the femur relative to the tibia, and enable the tibia to rotate about its longitudinal axis during flexion of the knee.
The femoral component includes a guide box fixed to the femoral mounting surfaces intermediate the condyles. The tibial component includes a central post fixed intermediate the tibial concavities, which articulates within the guide box and constrains rotational movement of the femoral and tibial components relative to one another in the coronal plane and transverse planes. The post and guide box are constructed and arranged to provide progressively decreasing and then progressively increasing rotational restraint of the femoral component in the coronal and transverse planes as the femoral component rotates in the sagittal plane between full extension to a position in flexion, and then back to full extension, respectively. In a preferred embodiment, the femoral component is restrained from rotating or tilting more than about 2 degrees in the coronal plane when the prosthesis is positioned in full extension and from rotating or tilting more than about 7 degrees in the coronal plane when the prosthesis is positioned in full flexion.
In a preferred embodiment, the femoral component is preferably restrained from rotating more than about 1 degree in the transverse plane when the prosthesis is positioned in full extension and from rotating more than about 4 degrees in the transverse plane when the prosthesis is positioned in full flexion.
In one embodiment, the change in restraint δR on the femoral component is constant over the full range of flexion. Alternatively, the change in restraint δR on the femoral component is variable over the full range of flexion. In another embodiment, the change in restraint δR on the femoral component has a first profile over a first range of flexion, and has a second profile over a second range of flexion.
The guide box has opposed side walls with interior surfaces, and the post has opposed exterior surfaces that engage the interior surfaces of the guide box. The guide box interior surfaces have a minimum interior width between them at an anterior end (AW) and a maximum interior width at a posterior end (PW), and the post has exterior surfaces with a constant width (CW) that is less than AW. In one embodiment the guide box side walls have exterior surfaces that are parallel to one another and interior side walls that are skew. The interior surfaces may have a planar surface or an irregular surface profile. In another embodiment, a cam is fixed to each of the interior surfaces of the side walls. The cams have interior surfaces that engage the exterior surfaces of the post. The interior cam surfaces have a minimum interior width between them at an anterior end (AW) and a maximum interior width at a posterior end (PW). The post has exterior surfaces have a constant width (CW) between them that is less than AW. The interior surfaces of the cams may also have a planar or an irregular surface profile.
The femoral component includes a cam connecting the posterior ends of the condyles. Anterior and posterior translation of the femoral component relative to the tibial component is controlled by the cam and central post. Rotation of the tibia about its longitudinal axis is also controlled, at least in part, by the cam and central post. In one embodiment contact between the cam and post occurs at knee flexion greater than about 30 degrees. Posterior translation caused by the cam and post is about 1-2 millimeters after the prosthesis is fully flexed.
a-20c are illustrations of the progressively decreasing medial-lateral (transverse plane) rotational restraint of the femoral component as the femoral component rotates from full extension to an intermediate position, and then rotates to a full flexion;
a-23c are illustrations of the progressively decreasing valgus-varus (coronal plane) rotational laxity of the femoral component as the femoral component rotates from full extension to an intermediate position, and then rotates to a full flexion;
For the purpose of illustrating the invention, several embodiments of the invention are shown in the accompanying drawings. However, it should be understood by those of ordinary skill in the art that the invention is not limited to the precise arrangements and instrumentalities shown therein and described below. Throughout the specification, like reference numerals are used to designate like elements. Numerous changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Unless otherwise defined, all technical and scientific terms used herein in their various grammatical forms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terms varus, valgus, anterior, posterior, proximal, distal, medial, lateral, sagittal, coronal, and transverse are used herein with their conventional medical/anatomical meaning as defined, for example, in Dorland's Illustrated Medical Dictionary. The term laxity shall mean slackness or displacement in the rotational motion of the knee joint. The term restraint shall mean restriction or confinement in the rotational motion of the knee joint. Valgus-varus restraint shall mean rotational or tilting restriction or confinement in the coronal plane. Medial-lateral restraint shall mean rotational restriction or confinement in the transverse plane. The term “change in restraint ER” shall mean the increase or decrease in valgus-varus restraint or medial-lateral restraint or degree of rotation of the prosthesis in the sagittal plane. Because the components rotate, the orientation of various features is described with reference to the configuration in full flexion as show in
A revision knee prosthesis in accordance with a first preferred embodiment of the invention is illustrated in
The femoral component 20 has a medial condylar portion or condyle 22, a lateral condylar portion or condyle 24, a patellar flange portion or flange 26, which bridges the anterior ends 28, 30 of the medial 22 and lateral 24 condyles, respectively, and a cam 47 that bridges and connects the posterior ends 29, 31 of the medial 22 and lateral 24 condyles. The medial 22 and lateral 24 condyles are arranged in substantially parallel relationship to each other and define an intercondylar notch 32 there between. As the prosthesis flexes, different sections of the curved condylar portions engage and articulate with the tibial component 52.
The patellar flange 26 includes a patellar groove 42, which is flanked by a medial trochlear surface 44 and a lateral 46 trochlear surface. The patellar flange is designed to articulate with either the natural patella or a patellar component. The patellar flange transitions smoothly with the condyles 22, 24. The patellar flange is constructed by approximating the geometry of the distal anterior surface of a natural femur. As a result, the patellar flange has natural tracking of the prosthetic or natural patella.
Each condyle 22, 24 generally comprises an anterior 34, 36 and posterior 38, 40 surface, which blend smoothly with each other without any abrupt transition. In a preferred embodiment shown in
The condyles 22, 24 have a radius about the minor axis (minor axis) in the coronal plane. In a preferred embodiment, the condyles have a constant radius of curvature in the coronal plane. However, the condylar surfaces could have a more complex geometry wherein the radius of curvature in the coronal plane can vary. In particular, the posterior condylar surfaces can be designed with varying radii in all three planes.
The interior boundaries of the medial 22 and lateral 24 condyles define an intercondylar notch 32 there between. A guide box 100 is centrally fixed to the proximal mounting surface 21 of the femoral component 20 and straddles the intercondylar notch 32. The guide box 100 is formed from a medial 102 and lateral 104 web that extend proximally in generally-parallel sagittal planes along the interior edge of each condylar portion 22, 24. Referring to
Referring to
As best seen in
A boss 112 is fixed to the proximal surface of the bridge plate 106. The boss 112 has a generally cylindrical shape and is dimensioned to receive an intermedullary mounting stem 9 shown in
Referring to
Referring to
The liner 60 has a proximal bearing surface 62, which articulates with the femoral component 20, and a distal surface 64, which abuts and is fixed to the tibial platform 52. The tibial component 50 also has a medial side 66, a lateral side 68, an anterior side 70, and a posterior side 71. The tibial component is generally symmetrical about a central sagittal axis running anterior to posterior.
A medial concavity 72 and a lateral concavity 74 are formed in the medial and lateral sides of the proximal surface 62, respectively. The medial 72 and lateral 74 concavities engage the medial 22 and lateral 24 condyles of the femoral component 20 as the components articulate relative to one another. In general, the concavities 72, 74 are shallower than the depth of the femoral condyles 22, 24.
Each concavity 72, 74 generally comprises an anterior 76, 78 and posterior 80, 82 surface, respectively, that resemble toroidal sections, which blend together at an intermediate boundary. In a preferred embodiment, the anterior surfaces 76, 78 have a major radius of curvature oriented substantially in the sagittal plane. Preferably, the posterior surfaces 80, 82 have a major radius of curvature oriented substantially in the transverse plane. The posterior concavities 80, 82 curve inwardly toward the sagittal central axis and sweep posteriorly and toward the central sagittal axis. The anterior 76, 78 and posterior 80, 82 surfaces have the same constant radius of curvature in the coronal plane and share the same tangent intersection in the coronal plane. The coronal curvature of the posterior surface is maintained as it turns toward the central sagittal axis. This construction allows the tibia to rotate about its longitudinal axis and translate posteriorly as the knee flexes.
The anterior and posterior concavities may have a raised periphery at the anterior 86 and posterior 88 ends to contain and prevent dislocation of the femur from the tibia. The raised periphery also provides stability to the knee during flexion. The anterior concavities may have lateral elevations 89, which contain the condyles 22, 24 so that the tibial component has little laxity during initial flexion and prevent tibial axial rotation. In contrast, the posterior concavities are designed without constraining lateral elevations and are designed to enable tibial axial rotation.
In the embodiment shown in
The relative motion path of the femoral and tibial components is illustrated in
In full extension, the central post 90 is positioned in the anterior end portion of the guide slot as best seen in
As the femoral component rotates to an intermediate position show in
As the femoral components rotates to a position in full flexion shown in
In a similar manner, as the femoral component rotates from full flexion, or any intermediate position in flexion, back to full extension, valgus-varus restraint and medial-lateral restraint gradually increase. As the femoral component returns to full extension, the gap between the central post 90 and the guide slot 110 reduces to a minimum, thereby further restraining the knee.
As described above, the unique design of the prosthesis enables desirable maximum valgus-varus and medial-lateral rotational restraint in full flexion. The unique design also provides progressively decreasing and increasing restraint as the knee flexes. Because of this unique design, the curved bearing surface 49 of the cam 47 and the geometry of the bearing surfaces cause the tibia to rotate axially as the knee flexes. In a preferred embodiment, tibial rotation is enabled up to at least about 5 degrees, preferably up to at least about 6 degrees, more preferably up to about 7 degrees. However, the cam and articulating surfaces could be designed to enable greater tibial axial rotation if desired.
It should be appreciate by those skilled in the art that the prosthesis 10 can be provided in a variety of sizes to accommodate patients of varying sizes. Therefore, the dimensions described above with respect to certain components will obviously vary.
Furthermore, the prosthesis can be provided in a wide variety of performance characteristics without departing from the scope of the invention. For example, the prosthesis 10 can be custom designed with high restraint in full flexion and a small δR or large δR. Similarly, the prosthesis 10 can be designed with less restraint in full flexion and a small δR or large δR. In each case, however, the relative level of restraint will be change over the range of flexion of the prosthesis 10.
In the embodiments shown in
Furthermore, to create even more complex 6R profiles, the inner surfaces of the medial and lateral webs could be fitted with separate cam surfaces 405, 406 such as shown in
Even more complex cams can be fitted to the inner surfaces of the medial and lateral webs, which would create multiple, discrete δR profiles over multiple, discrete ranges of flexion. Many other useful surface profiles should be readily apparent to those skilled in the art.
The femoral component and tibial component may have various surface profiles and may be constructed in various manners and from various materials such as those described in U.S. patent application Ser. No. 12/388,125 entitled Total Knee Replacement Prosthesis with High order NURBS Surfaces, filed Feb. 18, 2009, and U.S. patent application Ser. No. 12/388,182 entitled Total Knee Replacement Prosthesis, filed Feb. 18, 2009, both of which are incorporated herein by reference. For example, the femoral component 20 and the tibial platform 54 may be machined, cast, forged or otherwise constructed as a one-piece integral unit from a medical grade, physiologically acceptable metal such as cobalt chromium alloy, stainless steel, titanium, titanium alloy or nickel cobalt alloy.
The tibial liner may also be constructed in various manners and from various materials. For example, the tibial liner may be machined, molded or otherwise constructed as a one-piece, integral unit out of a medical grade, physiologically acceptable polymeric materials such as any polyolefin, including high-density polyethylene, low-density polyethylene, linear-low-density polyethylene, ultra-high molecular weight polyethylene (UHMWPE), or mixtures thereof. Polymeric materials, as used herein, also include polyethylene of various forms, for example, resin powder, flakes, particles, powder, or a mixture thereof, or a consolidated form derived from any of the above. Ultra-high molecular weight polyethylene (UHMWPE) refers to linear, non-branched chains of ethylene having initial average molecular weights in excess of about 500,000, preferably above about 1,000,000, and more preferably above about 2,000,000. Often the molecular weights can reach about 8,000,000 or more. The material can be treated, for example, by radiation, chemistry, or other technology to alter its wear properties and/or strength or hardness. Initial average molecular weight means the average molecular weight of the UHMWPE starting material, prior to any irradiation.
It is to be understood that the description, specific examples and data, while indicating exemplary embodiments, are given by way of illustration and are not intended to limit the present invention. Various changes and modifications within the present invention will become apparent to the skilled artisan from the discussion, disclosure and data contained herein, and thus are considered part of the invention.
