United States Patent [19]
`Walker et ale
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,822,365
`Apr. 18, 1989
`
`[54] METHOD OF DESIGN OF HUMAN JOINT
`PROSTHESIS
`
`[76]
`
`Inventors: Peter S. Walker, 15 Hallette Hill Rd.;
`Frederick C. Ewald,4 Black Oak Rd.,
`both of Weston, Mass. 02193
`
`[21] Appl. No.: 97,286
`
`[22] Filed:
`
`Sep. 11, 1987
`
`[63]
`
`[51]
`[52]
`
`[58]
`
`Field of Search
`
`Related U.s. Application Data
`Continuation-in-part of Ser. No. 868,609, May 30,
`1986, abandoned.
`Int. Cl.4
`U.S. Cl
`
`A61F 2/38; B23Q 15/14
`623/20; 623/66;
`364/468
`623/18,20-23,
`623/66; 364/468, 474, 475, 512
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,945,053 3/1976 Hillberry et at
`4,470,158 9/1984 Pappas et at
`4,611,288 9/1986 Duret et at
`4,634,444 111987 Noiles
`4,663,720 5/1987 Duret et at
`4,704,686 11/1987 Aldinger
`PrimaryExaminer-Richard J. Apley
`AssistantExaminer-Alan W. Cannon
`
`623/20
`623/20
`364/474
`623/20
`364/474
`364/468
`
`Attorney, Agent, or Firm-Wolf, Greenfield & Sacks
`[57]
`ABSTRACf
`A method of designing a prosthesis having convex male
`and concave female mating portions is provided for a
`human joint comprising a condylar male portion the
`surface of which is generated by the piecewise analysis
`of an anatomical condyle, which can be an average
`condyle or a selected condyle, or a distortion of an
`average condyle to fit the observed general dimensions
`of a specific patient, and the female portion having at
`least flexion and laxity surfaces, the flexion surfaces of
`which are generated by plotting the path of articulation
`of substantial points of contact between said male por(cid:173)
`tion and a corresponding anatomical female component
`for said joint through the full normal extension-flexion
`range plus normal rotation and posterior-anterior dis(cid:173)
`placement for that joint, and the laxity surfaces of said
`female member comprising raised guide-bearing sur(cid:173)
`faces for resisting dislocation of the condylar portion,
`the height and angle, and therefore, the resistance to
`dislocation, of which guide bearing surfaces increases as
`a function of deviation from the central motion path of
`said male portion and as a function of the flexion angle,
`and which at full laxity for any given angle of flexion
`corresponds substantially to the forces of anatomical
`the
`ligamentous restraint of said anatomical joint at
`limits of laxity.
`
`3 Claims, 7 Drawing Sheets
`
`-1-
`
`Smith & Nephew Ex. 1040
`IPR Petition - USP 7,534,263
`
`

`

`us. Patent
`
`Apr. 18, 1989
`
`Sheet 1 of7
`
`4,822,365
`
`48
`
`L . - . - - - - 70
`
`FIG.2
`
`FIG.3
`
`-2-
`
`

`

`u.s. Patent
`
`Apr. 18, 1989
`
`Sheet 2 of7
`
`4,822,365
`
`(
`
`FIG.4
`
`FIG.48
`
`I FIG.12A
`
`-3-
`
`

`

`u.s. Patent
`
`U.S. Patent
`
`Apr. 13, 1989
`Apr. 18, 1989
`
`Sheet 3 of7
`Sheet 3 of 7
`
`4,822,365
`4,822,365
`
`-4-
`
`

`

`u.s. Patent
`
`Apr. 18, 1989
`
`Sheet 4 of7
`
`4,822,365
`
`FIG.5
`
`44
`
`FIG.6
`
`-5-
`
`

`

`u.s. Patent
`FIG.5A
`
`Apr. 18, 1989
`
`Sheet 5 of7
`
`4,822,365
`
`POSTERIOR
`
`\
`
`-
`
`CONDYLAR
`LATERAL
`
`""
`
`SUPERIOR
`
`F1\TELLA
`
`-36
`
`34
`
`16
`
`ANTERIOR
`32
`
`DISTAL
`
`~e~
`
`CONDYLAR
`MEDIAL
`
`G.58
`FI
`
`~ ~
`PATELLAR
`
`10
`
`SPHERICAL
`
`~~~FIG.5C
`
`TOROIDAL
`
`46
`FIG.8
`
`76
`
`76
`
`1~--""IY78
`
`FIG.9
`
`FIG.lO
`
`-6-
`
`

`

`u.s. Patent
`U.S. Patent
`
`Apr. 13, 1939
`Apr. 18,1989
`
`Sheet 6 of 7
`Sheet 6 of7
`
`4,822,365
`4,822,365
`
`o
`
`.
`
`F|G.|IC
`
`(9
`LL
`
`F|G.||B
`
`(9
`LL
`
`FlG.||A
`
`(9
`LL
`
`-7-
`
`

`

`u.s. Patent
`
`Apr. 18, 1989
`
`Sheet 7 of7
`
`4,822,365
`
`48
`
`86
`
`88
`
`II I
`
`I
`
`J
`
`I
`L
`
`92
`
`90
`
`FIG.' 3
`
`-8-
`
`

`

`METHOD OF DESIGN OF HUMAN JOINT
`PROSTHESIS
`
`The present application is a continuation in part of 5
`application Ser. No. 868,609, filed on May 30, 1986 by
`the same inventors, now abandoned.
`
`45
`
`1
`
`4,822,365
`
`2
`Instructional Course Lectures (1970), pp.
`
`A.A.O.S.
`64-65.
`However, although the Ewald prosthesis provided a
`more natural movement than the prior art knee joints,
`the Ewald knee was not designed to allow for the natu(cid:173)
`ral knee movement known as laxity. Laxity can be de(cid:173)
`fined as the partially restrained motion or free play in a
`specified direction before substantial ligamentous re(cid:173)
`BACKGROUND OF THE INVENTION
`straint takes place at the extremes of motion (see gener-
`The present invention relates prostheses for human 10 ally, Markolf, K. L., et al, Journal of Bone & Joint
`joints, and more particularly to prostheses for knee,
`Surgery, 63-A; 570-585, 1981; Walker, P. S. Ch. 4, p.
`elbow, or other joints of the body, and more particu-
`202-204, Human Joints & Their Artificial Replace-
`larly to a method for designing the same. In particular,
`ments, pub. C. C. Thomas, Springfield, Ill., 1977). Since
`the present invention has a preferred application as a
`laxity is limited by certain ligaments that are resected
`knee prosthesis. The movement of the knee joint in 15 during implantation of a prosthesis, primarily the cruci-
`flexion and extension does not take place in a simple
`ate ligaments, prior to the present invention, there was
`a need in the art for a prosthesis capable of duplicating
`hinge-like manner, as in some other joints, but in a com-
`plicated movement,
`that
`includes displacements and
`the laxity characteristics of a natural knee joint.
`rotations, so that the same part of one articular surface
`Several designs allowed freedom of motion, by pro-
`is not always applied to the same part of the other artie- 20 viding partial conformity between the femoral and tibial
`ular surface, and the axis of motion is not fixed. Further-
`surfaces, using geometrical radii of curvature. These
`more,
`the knee joint is formed between the longest
`designs included the DuoPatella, Townley, Total Con-
`dylar, and Anametric. However, the shape of the sur-
`bones of the body, consequently with high lever arms
`relative to the foot and to the hip, and therefore, the
`faces is not related to the actual motion and laxity of the
`forces and moments across the joint at the interface 25 normal knee joint.
`between the articulating surfaces exceeds that of any
`The femoral condyles of a knee prosthesis can be
`other joint in the body.
`joined to the femur in several different ways. One
`Attempts to implant knee prostheses employing a
`method is to provide two pegs which insert into the
`metal hinge and intramedullary stems for anchoring the
`trabecular bone of the medial and lateral femoral con-
`hinge to the femur and tibia date back to the 1930's. 30 dyles. Another method is to provide a stem that
`Because of the complexity of the knee joint action, and
`projects from the center of the condyles and is inserted
`into and locked within the medullary canal of the fe-
`the forces and moments across the joint, these prosthe-
`ses were unsuccessful. The elements were subject to
`mur. Because of the valgus angle of the femur, it was
`wear resulting in dispersion of metal into the surround-
`necessary to provide both right and left femoral compo-
`ing tissue with consequent complications, and in high 35 nents, which were not interchangeable with each other.
`stresses at the implant-bone interfaces resulting in bone
`In addition, certain situations require a short stem, while
`resorption, pain, and implant failure. However, a hinge
`others require a long stem. Thus, it was necessary to
`does not permit ready access of natural lubrication to
`maintain in stock four different femoral component-
`the joint, and, by its nature, permits rotation only
`s-long stem and short stem versions of both left and
`through a single plane. It cannot duplicate the complex 40 right leg components.
`movements of the knee joint. Thus, a less than satisfac(cid:173)
`tory result is inevitable. See, e.g., D. V. Girzadas et al.,
`"Performance of a Hinged Metal Knee Prosthesis", J.
`Bone and Joint Surgery, Vol. 50-A, No.2, March 1968,
`pp. 355 et seq.
`Since that time, unlinked condylar replacement knee
`replacements have been designed. These allowed some
`freedom of motion between the femoral and tibial re(cid:173)
`placement surfaces. The first example was the polycen(cid:173)
`tric knee by Gunston. (Gunston, J. Bone & Joint Sur- 50
`gery)
`Another early design was by Ewald, in which he
`proposed surfaces representing the anatomical to allow
`normal joint motion. The Ewald prosthesis was an ad(cid:173)
`vancement over the earlier knees in that it permits rota- 55
`tion of the tibia with respect to the femur (i.e., pivoting
`of the medial condyle about the lateral condyle), and
`translation of the femur with respect to the tibia-move(cid:173)
`ments in different planes at once (sagittal and trans(cid:173)
`verse). All of those movements are necessary to dupli- 60
`cate the movement of a natural knee. For a detailed
`description of the control mechanism and the guiding
`components of the knee joint during normal extension
`and more particularly flexion, see O. C. Brantigan et al.,
`"The Mechanics of the Ligaments and Menisci of the 65
`Knee Joint", J. Bone and Joint Surgery, Vol. XXIII,
`No.1, January 1941, pp. 44 et seq.; and A. J. Helfet,
`"Control and Guide Mechanism of the Knee Joint",
`
`OBJECTS AND SUMMARY OF THE PRESENT
`INVENTION
`It is therefore, a general object of this invention, to
`provide a method of designing a prosthesis which over(cid:173)
`comes the foregoing limitations of the known prosthe(cid:173)
`ses.
`A more specific object of the invention is to provide
`a method of designing a prosthesis that provides joint
`motion which accurately simulates the. motion of the
`natural joint.
`It is a further object of the invention to provide a
`method of designing a prosthesis that accounts for laxity
`in the motion of the joint.
`It is yet a further object of the invention to provide a
`knee joint prosthesis that facilitates walking, stair climb(cid:173)
`ing, stair descending, rising from a chair, and other uses
`of the joint.
`It is still a further object of the present invention to
`provide a prosthesis that facilitates the access of the
`natural lubricating synovial fluid to the mating surfaces
`and escape of wear and other abrasive particles within
`and around the prosthesis.
`Yet another object of the present invention is to pro(cid:173)
`vide a prosthesis which acts cooperatively with the
`natural ligaments remaining after the surgery to cause
`the respective components of the prosthesis to move
`substantially as in nature including flexion-extension,
`
`-9-
`
`

`

`4,822,365
`
`20
`
`40
`
`45
`
`3
`internal-external rotation, anterior-post(cid:173)
`varus-valgus,
`erior displacement, and laxity.
`Briefly described, the joint prosthesis made by the
`process of the present invention includes a substantially
`smooth male portion and a mating female portion de- 5
`signed to interact. In general, the male portion is similar
`in shape and size to the end of the bone which it re(cid:173)
`places, as determined by sectioning measurements of a
`plurality of actual specimens.
`The female portion is considerably different in shape 10
`from the natural female member since it includes articu(cid:173)
`lar surfaces which are shaped to provide the required
`stability, motion and laxity, previously provided by
`structures such as menisci and ligaments, which are
`necessarily resected at surgery or are made ineffective 15
`by the disease. In other joints such as the elbow, etc.,
`the articular surfaces of the prosthesis likewise function
`as the capsular or the like ligaments which may have
`been destroyed by disease or injury, or necessarily re-
`sected during prosthetic replacement.
`The knee prosthesis of the present invention com(cid:173)
`prises in combination two components: a femoral com(cid:173)
`ponent and a tibial plateau. These components are
`mated, preferably by the method herein described, so
`that they operate in conjunction to permit normal knee- 25
`joint functioning. In particular, the tibial plateau com(cid:173)
`prises articular surfaces, described in greater detail
`hereinafter, which operate in conjunction with the fem(cid:173)
`oral component, to accomplish substantially the same
`result as the combination of the tibial surfaces, the col- 30
`lateral and cruciate ligaments, and menisci in the natural
`knee.
`The femoral component may comprise a pair of con(cid:173)
`dyles and is similar in size and shape to the distal end of
`a normal average femur, and has a range of sizes to span 35
`the normal size ranges of human knee joints. A model
`for the femoral component may be imagined by cutting
`the articular surfaces from the outer surfaces of an ana(cid:173)
`tomical distal femur, while forming medial and lateral
`condyles, substantially smooth and rounded in shape.
`A particular improvement of one form of the present
`invention over the prior art knee joint prostheses is that
`the femoral component bearing geometry is a piecewise
`mathematical analog of the average anatomical femoral
`surface geometry, with a range of sizes.
`The tibial plateau component of the method of the
`present
`invention is considerably different
`from the
`proximal end of a normal tibia. Part of the reason for
`this difference is the objective that the component, in
`conjunction with the ligaments remaining after surgical 50
`intervention and the femoral component, perform the
`function of the cruciate ligaments and the menisci in the
`normal knee joint. Thus,
`the tibial plateau includes
`means to provide for the appropriate amount of stabil(cid:173)
`ity, guide the joint surfaces in an anatomical motion 55
`path, but allow for normal laxity, as the knee moves in
`its full flexion-extension range. Such means may include
`means to guide the lateral condyle of the femoral com(cid:173)
`ponent during flexion in a substantially anterior-post(cid:173)
`erior direction through a curved articular surface of the 60
`tibial plateau while rotating and translating the femur(cid:173)
`tibia in the sagittal plane. These means may be provided
`by extension surfaces in the tibial plateau mated at ex(cid:173)
`tension with the condyles of the femoral component
`with maximal surface contact, flexion surfaces mated 65
`during flexion with the condyles with substantial sur(cid:173)
`face contact, and raised extending upwardly curved
`guiding-bearing laxity surfaces for guiding the move-
`
`4
`ment during flexion and resisting dislocation gradually
`but increasingly as respective components depart from
`the desired path.
`The tibial plateau component is created from a com(cid:173)
`puter generated design originated by sweeping the fem(cid:173)
`oral bearing surface through the prescribed motion,
`including three displacements and two rotations defined
`as a function of flexion in polynomial equations, with
`laxity superimposed. The superimposed laxity consists
`of anterior-posterior displacement, medial-lateral dis(cid:173)
`placement, and internal-external rotation. The laxity
`curves are polynomial equations describing increasing
`resistance as a function of deviation from the central
`motion path, and as a function of flexion angle.
`These and other objects, advantages and features of
`the invention will be more clearly understood by refer(cid:173)
`ence to the following detailed description thereof, the
`appended claims, and to the several views illustrated in
`the attached drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The present invention will be better understood by
`reference to the attached drawings, which illustrate the
`present invention, and wherein:
`FIGS. 1 and 3 are anterior views of a condylar re(cid:173)
`placement total knee;
`FIG. 2 is a side view of FIG. 1;
`FIGS. 4, 4A, and 4B are computer generated views
`ofa femur;
`FIG. 5 is a piecemeal mathematical representation of
`the anatomical femoral surfaces;
`FIGS. 5A-C are geometric analogs of femoral
`FIG. 6 is a mathematical representation of a complete
`femoral component;
`FIG. 7 is a side view ofthe femoral component show(cid:173)
`ing the femoral peg;
`FIG. 8 is a front view of the femoral component
`showing the long stem in place;
`FIG. 9 is a bottom view of the tibial component;
`FIG. 10 is a perspective view of the tibial blades;
`FIG. llA is a tibial surface generated by sweeping
`the femoral component through an average knee mo(cid:173)
`tion path;
`FIG. llB is a computer generated series of curves
`representing laxity motion of a knee;
`FIG. llC is a computer generated superimposition of
`the laxity curves of FIG. llBonto the average knee
`motion of FIG. llA;
`FIGS. 12A and B illustrate a stabilized version of the
`present invention; and
`FIG. 13 illustrates an alternative means of stabilizing
`the tibial plateau in accordance with the present inven(cid:173)
`tion.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`The present invention is directed to a method for
`designing joint prosthesis comprising a male portion
`and a mated female portion having articular surfaces for
`mating with the male portion in the full range of posi(cid:173)
`tions of joint flexion. The female portion preferably has
`flexion surfaces for mating throughout the joint move(cid:173)
`ment during flexion, as well as raised guiding-bearing
`surfaces to guide the joint through flexion and to resist
`dislocation,
`thereby performing or supplementing the
`functions of collateral or cruciate ligaments, and, de(cid:173)
`pending on the joint function, optionally an extension
`surface for mating with the male portion at extension.
`
`-10-
`
`

`

`4,822,365
`
`6
`5
`faces, a computer model was created which is useful for
`An important aspect of the present invention is that
`prosthetic design.
`the femoral and tibial components are totally designed
`Using the computerized average, it was determined
`by computer programs.
`that regions along the posterior femoral condyles, criti-
`The invention in a preferred embodiment comprises a
`method for designing a knee prosthesis consisting of 5 cal to the mechanical function of the knee, could best be
`two components, a femoral component and a tibial
`described as spheroidal sections. Additional geometri-
`plateau, which are mated and operable in conjunction
`cal analogs, including toroidal and conical surfaces,
`along with the natural members remaining after resec-
`were used to describe the bearing surfaces. This led to a
`tion to simulate the natural movement of a knee joint.
`mathematical model, an important feature being that the
`However, the concepts of the present invention can be 10 three bearing surfaces, the lateral condyle, the medial
`applied to other joints as well.
`condyle, and the patella groove, are each substantially
`FIGS. 1-3 illustrate the femoral component 10 (re-
`parallel to one another in differing sagittal planes.
`ferred to as the femoral component) of the present in-
`The femoral component is preferably constructed of
`vention. FIG. 1 is an anterior view of a condylar re-
`an inert metal alloy-stainless steel, cobalt-chromium
`placement total knee, fitted into a knee joint. FIG. 2 is 15 alloy, for example, those sold under the trademarks
`a side view, and FIG. 3 is an anterior view with the
`VITALLIUM or ZIMALLOY, or a titanium alloy
`patella removed to show the posterior cruciate liament.
`being suitable and preferred. The component may be
`The femoral component 10 comprises a lateral condyle
`formed by molding molten, softened or powdered alloy
`12, medial condyle 14, intramedullary stem 16 (see FIG.
`metal, or by machining or otherwise shaping the metal
`12A), and a trochlear surface 18. As best shown in FIG. 20 or other material, e.g., using computer numerical con-
`:~~~~~)~~ ~~~f~~~~ ~~:~;e~1~~~~in~0;:~
`I, the femoral component 10 comprises extension sur-
`faces (lateral) 20 and (medial) 22, as well as flexion
`using CNC from which the knee can be made from
`surfaces (lateral) 24 and (medial) 26.
`injection molding.
`Descriptions of femoral geometry have been pro- 25
`FIG. 5 represents a piecewise mathematical represen-
`vided by several authors. See, e.g., Mensch, J. S. et al.,
`tation of the anatomical femoral surfaces. This is the
`"Knee Morphology As A Guide To Knee Replace-
`basis for the femoral component surface. The surface is
`ment", Clin. Orth. No. 112,October 1975, pp. 231-241.
`contained in a computer program, which will generate
`Profiles of the femoral condyles in the sagittal plane
`a surface of required size and shape, by expanding,
`were shown in 1972 (Seedhom, B. B., et al., "Dimen- 30 contracting, or distorting the average surface.
`sions of the Knee", Annals of Rheumatic Disease, Vol.
`One method contemplated is to take the overall di-
`31, pp. 54-58 (1972», and modelled by various mathe-
`mensions of a patient's natural joint, and then to distort
`matical curves (Langa, G. S., "ExperimentalObserva-
`the average surface which has been stored, in three
`tions and Interpretations on the Relationship Between
`dimensions so as to approximate the three dimensional
`the Morphology and Function of the Human Knee 35 size of the patient's joint, and then to make the prosthe-
`Joint", Acta Anat., Vol 55, pp. 16-38 (1963); Erkman,
`sis by CNC (computer numerical control) machining
`M. J. et al., "A Study of Knee Geometry Applied to the
`techniques to arrive at a prosthesis which will fit in the
`Design of Condylar Prosthesis", Biomedical Engineer-
`patient's system precisely as his natural joint, but in
`ing, Vol. 9, pp. 14-17 (1974); and Rehder, D., "Morpho-
`which the articulating surfaces will conform operation-
`metrical Studies on the Symmetry of the Human Knee 40 ally to the surfaces generated by the computer synthesis
`Joint: Femoral Condyles", Journal of Biomechanics,
`of the average surfaces. Another method is to develop a
`Vol. 16, pp. 351-361 (1983». However, the above data
`number of average surfaces covering different size
`is limited in its ability to describe the ~hr.ee-dimensional
`ranges, and then to select the nearest one in size to the
`geometry of the surfaces. Such description could have
`joint of the patient. FIG. 5A shows the geometrical
`application in computer modelling of the knee joint in 45 analog of femoral condyles, including the posterior 28,
`order to study joint mechanics. Another area of applica-
`distal 30, anterior 32, patella 34, and superior 36 regions.
`tion is in knee prosthesis design and evaluation.
`FIG. 5B is a geometrical analog of condylar surfaces
`To design the femoral component of one form of the
`showing the lateral condylar, medial condylar, and
`present invention, piecewise mathematical analog of an
`patellar bearing surfaces 12, 14, 18. FIG. 5C is a third
`average anatomical femoral surface geometry was used. 50 geometric analog of the condylar surfaces showing the
`The preferred method of calculating the femoral sur-
`spherical, toroidal, and conical surface analogs, referred
`face comprises the sectioning of embedded cadaver
`to by reference numerals 38,40, and 42, respectively.
`knees into twenty-five sections. The sections were cop-
`The tibial plateau component 48 of the present inven-
`ied and digitized into a computer, using thirty to forty
`tion is also shown in FIGS. 1-3. The tibial plateau 48
`points per section, with a greater point density around 55 includes lateral articular surface 50, medial articular
`the condylar surfaces. A typical set of sections viewed
`surface 52, and therebetween raised surface 54. As used
`from the anterior and lateral sides are shown in FIGS. 4,
`herein, "articular surface" refers to the part of the sur-
`4A and 4B.
`face of a component of the prosthesis which during
`FIG. 4 shows a view of the average femur as seen
`normal joint movement is in pressure receiving relation
`from the anterior-lateral. In that computer model, 60 (either by direct contact or by near contact with a lubri-
`there are twenty-five parallel sagittal sections, each
`eating medium therebetween) with another component.
`section having forty points. FIG. 4A shows front and
`Thus in the case of the tibial plateau 48, its articular
`side views (note that front view seen on the right side of
`surfaces 50, 52 are those portions which contact (usu-
`FIG. 4A has only seventeen sections), and FIG. 4B
`ally with a fluid film therebetween) the femoral compo-
`shows a sketch of the sections from the front. The infor- 65 nent. Articular surfaces 50, 52 include upwardly raised
`mation on average profiles and shapes of the bearing
`guiding-bearing lateral surfaces 56, and upwardly raised
`surfaces was used to develop geometrical analog. By
`guiding-bearing medial surfaces 58. Within guiding-
`parametrising the various regions of the bearing sur-
`bearing surfaces 56 and 58 are extension surfaces 60 and
`
`-11-
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`

`

`4,822,365
`4,822,365
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`l0
`
`15
`
`20
`
`25
`
`30
`
`35
`
`7
`62. Between guiding-bearing surfaces 56 and 58 and
`extension surfaces 60 and 62, respectively, are flexion
`surfaces (lateral) 64 or (medial) 66. The flexion, exten-
`sion, and guiding surfaces 56-66 overlap and coincide at
`certain points.
`Preferably, the tibial plateau is constructed of an inert
`molded, high density plastic, such as high density poly-
`ethylene. The CNC machining advantages discussed
`above with respect to the femoral component also apply
`to the manufacture of the tibial plateau component 48.
`Tibial plateau 48 and femoral component 10 are
`shown together in a flexion position in FIG. 2. In this
`view, the leg bones, namely the femur 68, the tibia 70,
`and the fibula 72, are also shown. The cement, e.g.,
`methyl methacrylate 74, used to seat the tibial plateau
`48, is also shown. Femoral component 10 contacts the
`flexion surfaces 64, 66 of the tibial plateau 48 at two or
`more lateral-medial spaced substantial points, which
`lend stability to the joint during flexion. FIG. 3 illus-
`trates the two components in the position of extension,
`in which position three or more lateral, medial, and
`anterior-posterior spaced substantial points of contact
`exist between the opposed surfaces, whereby the joint is
`effectively locked in extension.
`,
`The term “substantial points of contact” are used
`herein to describe the contact betweeen two mating
`curved surfaces, the male member of which is slightly
`smaller in curvature. In theory a point or line contact is
`made, but in practice, when at least one member is
`resilient, more than a point or line of contact results
`under pressure.
`With the exception of the design of the femoral com-
`ponent, as decribed above, and the details set forth
`hereinbelow, the prosthesis of the present invention is
`similar to the Ewald prosthesis described in U.S. Pat.
`No. 3,798,679, the disclosure of which is hereby incor-
`porated herein by reference.
`An important distinction and advancement over the
`prior art is the incorporation of laxity concepts within
`the design parameters of the tibial plateau. Laxity is
`generally defined as the partially restrained motion or
`free play of a joint in a specified direction, before sub-
`stantial ligamentous restraint takes place at the extremes
`of motion. Laxity can include linear or rotational trans-
`lation in any of the three mutually perpendicular coor-
`dinate axes. For purposes of the present invention, lax-
`ity is only considered in anterior-posterior displace-
`ment, medial-lateral displacement, and internal-extemal
`rotation, these being the most significant.
`The term “total laxity” is the total amount of laxity
`movement or rotation which occurs between the limits
`of the applied force or torque. In the natural joint, the
`limits of laxity are determined by the interaction of the '
`menisci, the ligaments, the femoral component, and the
`tibial plateau. However, in a prostheses, the menisci and
`several ligaments are removed, and the limits of laxity
`are determined by the natural components remaining
`after resection acting cooperatively with forces intro-
`duced by the contour of the prosthetic interface and the
`user’s weight.
`A simple tibial surface can be generated by the above-
`mentioned femoral surface by simply moving the femo-
`ral surface about a fixed axis, producing a cylindrical
`type of surface. However, to produce a surface which
`incorporates features of anatomical knee motion, laxity
`characteristics, and _stability, the tibial plateau of the
`prosthesis must be contoured in an non-anatomical way.
`Therefore, in accordance with the present invention,
`
`45
`
`50
`
`55
`
`65’
`
`8
`the tibial plateau can be designed in different ways, for
`example by incorporating the average three-dimen-
`sional femoral motions along each of the three coordi-
`nate axes, plus intemal-extemal rotation and anterior-
`posterior displacement. The 3-dimensional motion of
`the femur on the tibia is mathematically described by
`using data of anatomical knee motion such as published
`by Kurosawa, et al (Kurosawa, H.; Walker, P. S.; Abe,
`S.; Garg, A.; Hunter, T.; Journal of Biomechanics,
`18:4-87-499, 1985). The mathematical equations enable
`the femur to be positioned correctly on the tibia as a
`function of knee flexion angle. A tibial surface gener-
`ated by sweeping the aforementioned femoral surface
`through an anatomical knee motion path is shown in
`FIG. 11A.
`Next, laxity, otherwise thought of as flexibility of the
`knee, was considered. The laxity behaviour for the
`anatomical knee has been reported by several authors
`including Markolf, et al (Markolf, K. L.; Bargar, W. L.;
`Shoemaker, S. C.; Amstutz, H. C.; -Journal of Bone &
`Joint Surgery, 63-A:570—585, 1981). The laxity curves
`show that the resistance to displacement from the neu-
`tral position, steadily increase with the displacement,
`the more so when the knee is weight-bearing. Cubic
`equations to express this laxity behaviour were deter-
`mined, and then the theory of Walker (Walker, P. S.;
`Ch. 4, p. 202-204, Human Joints & Their Artificial
`Replacements, publ. C. C. Thomas, Springfield, Ill.,
`1977) was used to express this in terms of horizontal,
`rotational and vertical movements of the femur on the
`tibia. A tibial surface generated by sweeping the femo-
`ral surface through laxity curves at different flexion
`angles is shown in FIG. 11B Different motions can be
`superimposed, for example average knee motion and
`average laxity, as shown in FIG. 11C. The tibial sur-
`faces of the present
`invention differ from hitherto
`known prostheses, because they have built into them
`surface geometries which produce precise and mathe-
`matically defined motion, laxity, and stability. These
`tibial surfaces do not have simple geometries which can
`be defined by simple radii, but have continuously
`changing radii all over the surface. These surfaces of the
`present
`invention differ additionally from hitherto
`known prostheses. These surfaces of the present inven-
`tion, especially when they include characteristics of
`normal knee motion such as rollback, are especially
`advantageous. Motion along different paths will be
`smooth with no sudden stops in any direction but gradu-
`ally increasing resistance. Such surfaces will readily
`accomodate different motion paths for different activi-
`ties and individuals, and will allow a degree of latitude
`in surgical placement.
`Further, the addition of laxity parameters as specifi-
`cally defmed above, provides provide a narrow gap
`between the femoral component and the tibial plateau
`laterally of the primary articular surface. This is advan-
`tageous because it facilitates access to the articular sur-
`face of the synovial fluid around the prosthesis, and also
`enables debris to be removed, thereby improving the
`life and operation of the prosthesis.
`The tibial plateau can be similarly made more versa-
`tile by replacing the conventional stem blade with a
`design shown in FIGS. 9-10. The stem blades of the ~
`present invention comprise transverse blades 76 and 78.
`A specially deigned gap 80 between blades 76 allow a
`longer stem to be press fit within, for situations calling
`for a long stem. FIG. 9 shows a bottom view of the
`tibial component 48. The location of the blade fixation is
`
`-12-
`
`

`

`9
`shown located anteriorly so as to be in line with the
`canal of the bone. Thus, when a long stem is added, it
`goes directly down the canal.
`FIG. 10 is a perspective view ofthe tibial blades. This
`provides fixation for standard components, and gives 5
`resistance to shear forces and bending moments in all
`planes. For added fixation, a long stem is pressed into
`the space 80 between the blades.
`FIG. 13 is another embodiment of the tibial fixation
`means. Instead of using the arrangement shown in 10
`FIGS. 9 and 10, a tapered peg 86 can be attached to the
`base of the tibial component 48. Onto peg 86 can be
`cemented a longer stem 88 or a stem 90 having blades 92
`extending therefrom. Such an arrangement provides
`added flexibility in case of bone loss or other situations. 15
`FIGS. 12A and B illustrate a stabilized version of the
`standard condylar knee. The femoral component has an
`intercondylar box 82 'which receives a raised part or
`post 84 of the tibial component. If the post 84 is about
`half the height shown, then anterior-posterior stability 20
`is obtained. This substitutes for the anterior and poste(cid:173)
`rior cruciate ligaments. If the post is about the height
`shown, varus- valgus stabilityis also obtained, substitut(cid:173)
`ing for the lateral and medial collateral ligaments.
`Although only preferred embodiments of the inven- 25
`tion are specifically illustrated and described