PCT
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`WO 00/59411
`
`WORLD INTELLECTUAL PROPERTY ORGANIZATION
`International Bureau
`
`(51) International Patent Classification 6 :
`A61F 2/38
`
`(11) International Publication Number:
`
`Al
`
`(43) International Publication Date:
`
`12 October 2000 ( 12.10.00)
`
`(21) International Application Number:
`
`PCT/US99/07309
`
`(22) International Filing Date:
`
`2 April 1999 (02.04.99)
`
`(71)(72) Applicants and Inventors: FELL, Barry, M. [US/US];
`7124 Red Top Road, Hummelstown, PA 17036 (US).
`HALLOCK, Richard, H. [US/US]; 1660 Woodhaven Drive,
`Hummelstown, PA 17036 (US).
`
`(74) Agents: CONGER, William, G. et al.; Brooks & Kushman,
`22nd floor, 1000 Town Center, Southfield, MI 48075 (US).
`
`(81) Designated States: AE, AL, AM, AT, AU, AZ, BA, BB, BG,
`BR, BY, CA, CH, CN, CU, CZ, DE, DK, EE, ES, FI, GB,
`GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG,
`KP, KR, KZ, LC, LK, LR, LS, LT, LU, LY, MD, MG, MK,
`MN, MW, MX, NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI,
`SK, SL, TJ, TM, TR, TI, UA, UG, US, UZ, VN, YU, ZA,
`ZW, ARIPO patent (GH, GM, KE, LS, MW, SD, SL, SZ,
`UG, ZW), Eurasian patent (AM, AZ, BY, KG, KZ, MD,
`RU, TJ, TM), European patent (AT, BE, CH, CY, DE, DK,
`ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE), OAPI
`patent (BF, BJ, CF, CG, CI, CM, GA, GN, GW, ML, MR,
`NE, SN, TD, TG).
`
`Published
`With international search report.
`
`(54) Title: SURGICALLY IMPLANTABLE KNEE PROSTHESIS
`
`(57) Abstract
`
`A self-centering meniscal prosthesis device suitable for minimally invasive, surgical implantation into the cavity between a femoral
`condyle and the corresponding tibial plateau is composed of a hard, high modulus material shaped such that the contour of the device and
`the natural articulation of the knee exerts a restoring force on the free-floating device.
`
`-i-
`
`Smith & Nephew Ex. 1005
`IPR Petition - USP 8,657,827
`
`

`

`FOR THE PURPOSES OF INFORMATION ONLY
`
`Codes used to identify States party to the PCT on the front pages of pamphlets publishing international applications under the PCT.
`
`AL
`AM
`AT
`AU
`AZ
`BA
`BB
`BE
`BF
`BG
`BJ
`BR
`BY
`CA
`CF
`CG
`CH
`CI
`CM
`CN
`cu
`CZ
`DE
`DK
`EE
`
`Albania
`Annenia
`Austria
`Australia
`Azerbaijan
`Bosnia and Herzegovina
`Barbados
`Belgium
`Burkina Faso
`Bulgaria
`Benin
`Brazil
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`Canada
`Central African Republic
`Congo
`Switzerland
`Cote d'Ivoire
`Cameroon
`China
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`Czech Republic
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`Denmark
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`
`ES
`FI
`FR
`GA
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`IE
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`KE
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`KR
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`LC
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`LR
`
`Spain
`Finland
`France
`Gabon
`United Kingdom
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`Ireland
`Israel
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`Japan
`Kenya
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`Democratic People's
`Republic of Korea
`Republic of Korea
`Kazakstan
`Saint Lucia
`Liechtenstein
`Sri Lanka
`Liberia
`
`LS
`LT
`LU
`LV
`MC
`MD
`MG
`MK
`
`ML
`MN
`MR
`MW
`MX
`NE
`NL
`NO
`NZ
`PL
`PT
`RO
`RU
`SD
`SE
`SG
`
`Lesotho
`Lithuania
`Luxembourg
`Latvia
`Monaco
`Republic of Moldova
`Madagascar
`The fonner Yugoslav
`Republic of Macedonia
`Mali
`Mongolia
`Mauritania
`Malawi
`Mexico
`Niger
`Netherlands
`Norway
`New Zealand
`Poland
`Portugal
`Romania
`Russian Federation
`Sudan
`Sweden
`Singapore
`
`SI
`SK
`SN
`sz
`TD
`TG
`TJ
`TM
`TR
`TT
`UA
`UG
`us
`uz
`VN
`YU
`zw
`
`Slovenia
`Slovakia
`Senegal
`Swaziland
`Chad
`Togo
`Tajikistan
`Turkmenistan
`Turkey
`Trinidad and Tobago
`Ukraine
`Uganda
`United States of America
`Uzbekistan
`Viet Nam
`Yugoslavia
`Zimbabwe
`
`-ii-
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`

`

`WO 00/59411
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`PCT /US99/07309
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`SURGICALLY IMPLANT ABLE KNEE PROTHESIS
`
`TECHNICAL FIELD
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`The present invention pertains to prosthetic devices. More
`
`particularly, the invention pertains to self-centering knee joint prostheses which may
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`5
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`be surgically implanted between the femoral condyle and tibial plateau of the knee.
`
`BACKGROUND ART
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`Articular cartilage and meniscal cartilage provide the mobile weight
`
`bearing surfaces of the knee joint. Damage to these surfaces is generally due to
`
`genetic predisposition, trauma, and/or aging. The result is usually the development
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`10
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`of chondromalacia, thinning and softening of the articular cartilage, and degenerative
`tearing of the meniscal cartilage. Various methods of treatment are available to treat
`
`these disease processes. Each option usually has specific indications and is
`
`accompanied by a list of benefits and deficiencies that may be compared to other
`
`options. Nonsteroidal anti-inflammatory drugs (NSAIDS), cortisone injections,
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`15
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`arthroscopic debridement, osteotomy, unicompartmental knee replacement, and total
`
`knee replacement have all been used to treat the disease depending on the severity
`
`of the process.
`
`Currently, there is a void in options used to treat the relatively young
`
`patient with moderate to severe chondromalacia involving mainly one compartment
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`20
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`of the knee. Some patients cannot tolerate or do not want the risk of potential side
`
`effects of NSAIDS. Repeated cortisone injections actually weaken articular cartilage
`
`after a long period of time. Arthroscopic debridement alone frequently does not
`
`provide long lasting relief of symptoms. Unicompartmental and bicompartmental
`
`total knee replacements resect significant amounts of bone and may require revision
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`25
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`surgery when mechanical failure occurs. Revision total knee replacement surgery
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`is usually extensive and results in predictably diminished mechanical life expectancy.
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`Therefore, it is best to delay this type of bone resecting surgery as long as possible.
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`DESCRIPTION OF THE RELATED ART
`
`Several approaches have generally been employed in the past to
`
`correct the aforementioned problems. The first approach involves repair of articular
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`5
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`or meniscal cartilage. Repair of the articular cartilage by surgically transplanting
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`autogenous or autologous osteochondral core grafts has had limited success, but is
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`not always indicated. Meniscus repair using barbed "arrows" such as the Bionix
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`"Meniscus Arrow" has been used for "bucket-handle" tears, but is not applicable to
`
`other knee joint problems. Thus, these methods have limited scope and are generally
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`10
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`confined to unique kinds of damage.
`
`In the second approach, a unicompartmental or bicompartmental bone
`
`resection is performed, replacing the bone with a load bearing prosthesis. This
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`resection may be performed only on the femoral condyle, or may include the tibial
`
`plateau. In either case, the resection involves considerable surgical skill, and results
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`15
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`in prosthetic devices physically anchored into the bone structure. Not only is such
`
`reconstruction expensive major surgery, but moreover, the mechanical means of
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`attachment may fail as the patient grows older. Examples of prostheses utilized in
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`such methods are those disclosed in Ries, U.S. Patent 5,549,688; Cloutier, U.S.
`
`Patent 4,207,627; and Shah, U.S. Patent 5,263,987.
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`20
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`The third approach has been to replace the meniscal cartilage
`
`("meniscus") with a soft, compliant material. In theory, such devices cushion the
`
`femoral and tibial bearings surfaces and distribute loads uniformly over a large
`
`portion of the knee joint due to the ability of these devices to elastically deform.
`
`This ability to deform can also be a detriment, however, when it is desired to isolate
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`25
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`portions of the articular cartilage or bone surfaces from loads. Moreover, such
`
`devices are prone to tearing or disintegration under repeated stress due to their low
`
`tensile strength and modulus. Being flexible, they may be ejected from the
`
`meniscular cavity if not anchored in place. Anchoring devices may create an area
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`susceptible to fatigue fracture, causing dislocation of the prosthesis and further
`
`damage to the knee joint.
`
`Thus, for example, Kenny, in U.S. Patent 4,344, 193, discloses a
`
`meniscus prosthetic device of a rubbery material such as silicone rubber, having two
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`5
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`prominences, which interact with a space defined by the geometry of the femoral
`
`condyles. This interaction involving the prominences, together with surgical sutures
`
`secured to surrounding soft tissue, are said to maintain the meniscus fixed in the
`
`proper location. A porous border, into which fibrous tissue ingrowth is theorized
`
`to occur, may also assist in performing the locating function. A similar approach
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`10
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`is disclosed in Stone, U.S. Patents 4,880,429; 5,007 ,934; and 5, 158,574, where the
`
`meniscus comprises a porous matrix of biocompatible fibers or fibers of natural
`
`origin to act as a "scaffold" for regrowth of native meniscal tissue. The device is
`
`manufactured with an outer contour substantially the same as that of a native
`
`meniscus.
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`15
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`In Kenny, U.S. Patent 5,171,322, a meniscus prosthetic device is
`
`composed of a biocompatible, deformable, flexible and resilient material having the
`
`shape of a natural meniscus, but having a tail which may extend through holes bored
`
`in the bone to anchor the device.
`
`In similar fashion, Wall, in U.S. Patent
`
`4,502,161, discloses an extra-articular extension attached to the bone outside the
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`20
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`joint; while Richmond, U.S. Patent 4,919,667 employs natural fibrous growth to
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`positively anchor his device, again shaped like a natural meniscus. Schwartz, U.S.
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`Patent 5,344,459 utilizes a soft device of rings that are inflatable with air, liquid, or
`
`semisolid to provide a gel cushion between joint surfaces.
`
`The previously described devices of the prior art second approach all
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`25
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`utilize soft, cushiony materials which are anchored in place by mechanical means or
`
`through tissue regrowth to prevent movement of the device or its extrusion (spitting)
`
`from the compartments. One device which differs from those previously described,
`
`and which has been used in knee reconstruction, is the so-called "Macintosh knee,"
`
`where a hard prosthesis is located by means of protruding ridges, generally in the
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`30
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`form of a cross, which nest into corresponding grooves cut into the tibial plateau to
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`5
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`10
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`15
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`prevent movement of the device. These devices have been found to cause pain in the
`
`knee joint. This type of prosthetic device and the so-called "McKeever" device
`
`require very invasive surgical procedures, require large arthrotomy, require bone
`
`and tissue resection, and are irreversible processes.
`
`SUMMARY OF THE INVENTION
`
`The present invention pertains to a meniscal device suitable for
`
`surgical implantation into a knee compartment defined by the space between a
`
`femoral condyle and the respective tibial plateau. The device is a hard, self(cid:173)
`centering meniscal device devoid of physical means that fix its location. The device
`does not have the natural shape of the meniscus, but rather is designed such that
`
`articulation of the knee results in a modest amount of lateral/medial and
`anterior/posterior translation, relative to the tibial plateau, of the device. The
`
`present invention also pertains to a process for the manufacture of suitable meniscal
`devices.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGURE 1 depicts the relationship between the radius (RFC) and the
`femoral condyle.
`
`FIGURE 2 illustrates the shape of the femoral condyle in cross
`
`section.
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`20
`
`FIGURE 3 illustrates certain spatial relationships with respect to an
`embodiment of the subject invention device.
`
`FIGURE 4 illustrates the distorted elliptical (kidney bean) shape of
`
`a device.
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`25
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`planes.
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`FIGURE 5 and 6 illustrate cross-sections of a device in orthogonal
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`FIGURE 7 illustrates a device contour and its relationship with the
`
`femoral and tibial base planes.
`
`FIGURE 8 illustrates the axes and planes which may be used to
`
`generate the shape of a meniscal device in one embodiment of the subject invention.
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`5
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`FIGURE 9 illustrates the relationship of various coordinates and axes
`
`of a device viewed perpendicular to the plane of the device.
`
`FIGURE 10 illustrates one embodiment of a device viewed in plan.
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`FIGURE 11 illustrates the device of Figure 10 viewed from the side.
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`DESCRIPTION OF THE PREFERRED EMBODIMENTS
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`10
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`The prosthetic meniscal devices of the subject invention are
`
`unicompartmental devices suitable for minimally invasive, surgical implantation. By
`
`the term 11meniscal devices 11
`
`is meant that the devices are positioned within a
`
`compartment in which a portion of the natural meniscus is ordinarily located. The
`
`natural meniscus may be maintained in position or may be wholly or partially
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`15
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`removed, depending upon its condition. Under ordinary circumstances, pieces of
`
`the natural meniscus which have been torn away are removed, and damaged areas
`
`may be trimmed as necessary. In somewhat rarer instances, the entire portion of the
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`meniscus residing in the meniscal cavity may be removed. Thus the term 11meniscal
`
`device" is descriptive of the location of the device rather than implying that it is a
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`20
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`replacement for, or has the shape of, the natural meniscus. Actually, as described
`
`hereinafter, the shape of the meniscal device is not the same as the natural meniscus,
`
`and in most cases, will not entirely replace the meniscus.
`
`By the term 11unicompartmental 11 is meant that each device is suitable
`
`for implantation into but one compartment defined by the space between a femoral
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`25
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`condyle and its associated tibial plateau.
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`In other words, the device is not a
`
`11bicompartmental 11 device which, in one rigid device, could be inserted into both of
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`the two femoral condyle/tibial plateau compartments. In many, if not most cases,
`a device will be inserted into one compartment only, generally the medial compart(cid:173)
`
`ment, as the meniscus and associated articular surfaces in these compartments (left
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`5
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`knee medial and right knee medial compartments) are most subject to wear and
`damage. However, it is possible to insert two separate devices into the medial and
`lateral compartments of the same knee, or to use two such devices that are mechani(cid:173)
`
`cally but non-rigidly linked.
`
`The meniscal devices are translatable but self-centering. By
`
`"translatable" is meant that during natural articulation of the knee joint, the device
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`10
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`is allowed to move, or change its position. Thus, the device is devoid of means of
`
`physical attachment which limit its movement, for example, screws, mating ridges
`and depressions, porous areas to accommodate tissue regrowth, and the like.
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`By the term "self-centering" is meant that upon translation from a first
`
`position to a second position during knee articulation, the device will return to
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`15
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`substantially its original position as the articulation of the knee joint is reversed and
`the original knee position is reached. Thus, the device will not progressively
`
`"creep" towards one side of the compartment in which it is located. Rather, the
`angle of attack of the femoral condyle and/or tibial plateau bearing surfaces against
`
`the meniscal device will ensure that the device reversibly translates during articula-
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`20
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`tion, maintaining the meniscal device, on average, in the same location for any given
`
`degree of knee articulation.
`
`Contrary to most devices which are composed of soft, compliant
`material designed to assume the function of the natural meniscus which they replace,
`
`the present device is composed of relatively hard, relatively high modulus material.
`Suitable materials are, for example, steel, ceramics, and reinforced and non(cid:173)
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`25
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`reinforced thermoset or thermoplastic polymers. The device need not be made of
`a single material, but composite structures of steel/thermoplastic, steel/ceramic,
`ceramic/polymer, etc., may be used. Alternatively, composites of above materials
`with biologically active surfaces or components may be used. Biologically active
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`30
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`components include surfaces that may contain pharmaceutical agents to stimulate
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`cartilage growth or retard cartilage degeneration that may be delivered at once or in
`
`a timed- release manner.
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`Generally, portions of the devices expected to have the most wear due
`
`to either greater movement relative to the mating surface, i.e., relative to the femoral
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`5
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`condyle or tibial plateau; or high stress, may be made of stronger, more abrasion
`resistant material than the remainder when composite structures are used. This
`
`method may be ideal for use in conjunction with cultured chondrocyte implantation
`(cartilage cells used as seeds) or osteochondral transplantation or mosaicplasty.
`Moreover, when the locus of damage to the articular cartilage or to portions of the
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`10
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`bone structure are known, the relatively constant radius of the surface of the
`meniscal device will bridge the defective areas at these loci, thus redistributing load
`
`to healthy tissue and allowing inflamed, diseased, or other damaged areas to
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`regenerate.
`
`For example, a portion of the femoral condyle, tibial plateau, articular
`cartilage, etc., may have been damaged or may experience tissue degeneration. The
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`15
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`continued load experienced at such points and the wear experienced as the knee
`If suitable
`
`flexes will substantially hinder the regeneration of healthy tissue.
`
`biologically active materials, chondrocytes, etc. are applied to the damages or
`degenerated surface to assist in tissue regeneration, these will, under ordinary
`circumstances, be rapidly dissipated. If a flexible, cushiony material is inserted
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`20
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`within the knee compartment, the damaged area will still experience intimate contact
`with the damaged area under static loads, and will also experience continued wear
`
`and abrasion under non-static conditions. Under such circumstances, active
`substances will be rapidly dissipated. However, more importantly, newly
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`25
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`regenerated articular cartilage not having the necessary density or cohesiveness to
`withstand wear, will be rapidly eroded away.
`
`The subject invention meniscal load distributing devices may be
`supplied with a contour which allows the devices to act as a surface which distributes
`
`the loads evenly over regions of healthy articular cartilage, in general, abutting and
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`30
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`bridging surfaces where articular cartilage degeneration or damage has occurred.
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`Active substances may be applied at once or in a timed-release manner to the
`degenerated or damaged articular cartilage surface by means of, or in conjunction
`
`with, the meniscal device. Because the recess or shape of the meniscal device
`protects the damaged area from loads and wear, tissue regeneration may occur
`without disturbance. The regenerating tissue will have time to mature and crosslink
`into a fully developed matrix. Moreover, as regeneration proceeds, the regenerating
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`5
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`tissue will assume a shape dictated by the shape of the meniscal load-distributing
`device. Growth under these circumstances has the greatest potential for dense,
`
`ordered growth most closely replicating the original surface.
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`10
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`The hardness of the meniscular devices is preferably higher than
`Shore 60 D. The shore hardness may range from that common for engineering
`
`grade plastics to hardened steel and titanium, and preferably on the portion of the
`Rockwell hardness scale typical of steels, hard plastics and ceramic materials. From
`
`the high hardness desired of the meniscal device, it is readily apparent that the
`devices function in a manner completely different from those of the prior art such
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`15
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`as Stone, Dedo, Schwartz, Richmond, and Kenny. The purpose of the devices of
`the subject invention is to achieve a span-like effect to bridge the defective areas.
`However, in a composite variation, any single component (like a bioactive material
`
`component) may be softer than the supporting material. Rather than deforming to
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`20
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`distribute a load relatively equally on the mating surfaces, the meniscal devices of
`the present invention function as rigid, substantially non-deforming, self-centering
`bearings, which do not necessarily spread the load uniformly, but rather may
`concentrate the load upon desired points, spanning areas of imperfection. If a soft
`
`and/or low modulus elastomer or thermoplastic is used for the entire device, not only
`is the load not concentrated on healthy tissue, but moreover, damaged areas due to
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`25
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`wear and/or degeneration will also be subjected to loading, decreasing the
`opportunity for the body's natural regenerative capability to function.
`
`The high modulus of the subject meniscal devices thus allows for the
`provision of recessed or non-contacting areas of the device to encourage articular
`cartilage regeneration. In softer, lower modulus materials, the naturally occurring
`loads, which may exceed 1000 lbs/in2 in certain cases, will cause the softer devices
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`to deform and allow ordinarily non-contacting areas to contact bone or cartilage for
`which contact is desired to be avoided. A flexural modulus of elasticity for load
`
`bearing portions of the meniscal devices should therefore be preferably greater than
`
`2x105 psi, and more preferably greater than 3xl06 psi. Portions of the device not
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`5
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`exposed to the highest loads may be made of lower modulus materials, which may
`be softer as well, e.g., in a non-limiting sense, nylon, polyurethane, polypropylene,
`polyester, and the like, optionally fiber reinforced.
`
`As indicated previously, the meniscal devices of the subject invention
`
`may be manufactured so as to substantially contain or have deposited thereon, a
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`10
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`biologically or pharmaceutically active material. This is particularly suitable when
`the device bridges a defective area of bone or articular cartilage. In such cases, the
`
`meniscal device may be provided with a coating containing a biologically or
`pharmaceutically active material, for example one that promotes tissue regrowth or
`
`one that decreases inflammation. Such materials may also, and more preferably, be
`contained in a portion of the meniscal device. The portion may be filled with
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`15
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`medication, or may be filled with a gel, paste, or soft polymer material that releases
`
`medication over a period of time. Preferably, this medically active portion does not
`
`actually contact, or minimally contacts, the damaged tissue. This freedom from
`contact is made possible by the surrounding bearing surfaces. Coatings may also be
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`20
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`of a gel, paste, or polymer containing time-release medicaments. Biologically and
`pharmaceutically active materials are identified subsequently herein as 11 active
`
`materials. 11
`
`The actual shape of the meniscal devices may be tailored to the
`
`individual. Individuals with high varus (heels in, knees out - typical degenerative
`
`arthritis or valgus (heels out, knees in) deformation due to wear, degeneration, or
`disease, may require meniscal devices which are of considerably greater thickness
`over the portions where wear is most advanced. In youthful patients, where trauma(cid:173)
`
`induced damage rather than severe wear or degeneration has occurred, differences
`in device thickness will be more moderate. In general, the meniscular devices are
`kidney-shaped when viewed from above, and have a negative meniscus shape when
`viewed from the side, i.e.; the thickness along the periphery of the device is greater
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`than the thickness along the center of the device. The kidney-shape in plan (Figure
`
`4) may be described generally as elliptical, the shape resembling a distorted ellipse,
`
`with the distortion (30) (Figure 8) generally determined by the shape and location
`
`of the tibial spine. The device covers not only the peripheral areas of the meniscus
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`5
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`but also the central weight bearing surface of the femoral condyle and tibial plateau.
`
`For example, the inside (central) thickness ((17) Figure 7) may range
`
`from about 0.010 inches (0.25mm) to 0.20 inches (5mm) over a somewhat elliptical
`
`area measuring, for a hypothetical average knee, about 1.0 inches (25.4mm) along
`
`the minor axis and 1.40 inches (35.6mm) across the major axis. The meniscal
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`10
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`devices are generally thicker at the posterior portion (11) of the device (the portion
`
`of the periphery nearest the posterior of the knee joint) as compared to the lateral (7)
`
`or medial (6) sides. The medial(6) side of a medial compartment device, (lateral side
`
`of a lateral compartment device) is generally thicker than the lateral (7)(the side
`
`along the tibial spine) side, and the medial (6) and anterior (4) sides are generally
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`15
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`of the same thickness. The outside thickness may range up to 0.40 inches (lOmm)
`
`in some cases.
`
`The edges of the device are rounded rather than presenting the sharp
`
`corners of the devices of U.S. Patent 5, 15 8, 57 4. This rounded periphery is
`
`necessary due to the fact that the device will be allowed to move within the cavity.
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`20
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`Movement of a device having a periphery with sharp corners would result in the
`
`potential for severe damage to the surrounding tissue and articular surfaces, in
`
`addition to causing pain. The "kidney shaped" devices are termed "substantially
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`elliptical" as that term is used herein. The "depression" in the elliptical shape on the
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`part of the device which will be proximate to the tibial spine (30 in Fig. 4) will vary
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`25
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`from patient to patient. It is possible, due to the great range of variability of human
`
`anatomy that this depression might be absent in devices for some patients. However,
`
`the overall shape in plan is substantially elliptical regardless.
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`As shown for the femoral and tibial surfaces of the device in Figure
`
`1 and in Figure 2, the surfaces of the meniscal device generally are convex or
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`30
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`concave in a symmetrical manner, i.e., their radius of curvatures in a given
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`PCT /US99/07309
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`direction, are in general, relatively constant. There are generally four directions of
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`radii need to describe the two surfaces, as illustrated in Figures 1-9, the femoral
`
`anterior to posterior (RFc)(2), the tibial anterior to posterior (RTc)(13), the femoral
`
`medial to lateral (RFcx)(3)and the tibial medial to lateral(RTcx)(14). Typical values
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`5
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`would be RFc from 1.1-2.0 inches (28-51mm), RFcx from 0.5-1.5 inches (12.7-
`
`38mm), RTP from 6-12 inches (15.2-30.5cm) and RTPx from 1.5-3 inches (38-76mm).
`
`An example of a device would have the following values: R Fc = 1. 6 inches ( 40. 6mm),
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`RFcx= 1.2 inches (30.5mm), RTP= 10 inches (25.4cm) and RTPx=2.3 inches
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`(58.4mm). However, it is also necessary to allow for an increasing or decreasing
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`10
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`radius to accommodate a specific patient's needs. For example, the RFc of such a
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`device may have a radius of 1.3 inches (33mm) at the most anterior point of the
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`device but may increase in a geometric manner to a radius of 1. 8 inches ( 45. 7mm)
`
`at its most posterior aspect. Simultaneously, the RFcx may have a radius of 0.8
`
`inches (20.3mm) at the most anterior point of the device but may increase in a
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`15
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`geometric manner to a radius of 1.3 inches (33mm) at its most posterior aspect. Such
`
`transitions of radii would occur in a smooth manner consistent with a bearing
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`surface.
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`The asymmetric shape of the device still allows for a good fit to the
`
`femoral condyle as the femoral condyle has an almost constant radius of curvature,
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`20
`
`as shown in Figure 1, in the area that the tibial plateau moves against. This radius
`
`of curvature, when viewed from above, as in Figure 3, generally describes the
`
`contour angle (or the predominent orientation of the radius of curvature along the
`
`anterior to posterior direction) of the femoral condyle. In addition, the posterior rim
`
`and the large radius of the tibial side of the device prevents the device from
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`25
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`"spitting". Thus, regardless of whether the knee is in extension or flexion, the
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`degree of "tightness" remains the same and the device will not restrain or limit the
`
`motion of the knee. Further, the surface area of the femoral side may be smaller than
`
`its corresponding tibial surface along the anterior, medial (medial side of a medial
`
`device) and posterior aspects of the device. In such a manner, the femoral side of the
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`30
`
`device would be closer in size to the femoral condyle, while the tibial plateau would
`
`remain fully covered thus, giving the device a "sloped" shape along the
`
`aforementioned edges. Such a device shape would be suitable for use with certain
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`anatomical shapes as well as for use with a partially or fully intact meniscus. The
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`term "substantially immune from spitting" means that the device, without any
`
`physical attachment to the knee, will ordinarily remain in place in the knee
`
`compartment over the normal range of activity expected of the knee.
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`5
`
`The ability of the subject meniscal devices to translate yet be self(cid:173)
`
`centering is created by the geometry of the devices in conjunction with the geome(cid:173)
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`tries of the femoral condyles and tibial plateaus. The bearing surface geometries of
`
`the tibial plateaus and the femoral condyles define the axis of joint rotation of the
`
`knee. Figure 2 shows the shape of the femoral condyle in cross section. Figure 3
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`10
`
`shows the angle (8) of the contour of the femoral condyle relative to the tibial plateau
`(5) to be such that the planes of symmetry of the respective condyles are not
`
`orthogonal to the axis of rotation of the joint, but instead are at angles that converge
`
`toward the anterior portion of the particular knee compartment.
`
`The axis of rotation of the tibia on the femur is 90 degrees to the path
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`15
`
`of the tibial plateau against the femoral condyle. The two tibial plateaus (medial and
`lateral) are not in the same plane with each other but do act in a relatively constant
`
`radius to its respective femoral condyle. In other words, although the symmetry of
`
`the device's femoral side may be matched with the femoral condyle while the leg is
`
`in full extension, the rotation of the tibial plateau against the femoral condyle is
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`20
`
`along a constant axis of rotation (90 degrees to the axis of rotation), thus the
`
`angularity of the axis of symmetry of the femoral condyle relative to the axis of
`
`symmetry of the tibial plateau is not parallel but at some acute angle. Also, the axis
`
`of symmetry of the tibial plateau is not parallel to the path of rotation of the tibia
`
`relative to the femur but also at some mildly acute angle. Thus, the true orientation
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`25
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`of the device, regardless of the relative orientations of symmetry of the tibial side
`
`to the femoral side is 90 degrees to the true axis of rotation as described in Hollister
`
`et al., "The Axes of Rotation of the Knee"' CLIN. ORTHOPAEDICS AND REL. RES.,
`290 pp. 259-268, J.B. Lippincott Co., © 1993, herein incorporated by reference.
`Any localized positions of higher loads are self-limiting due to the ability of the
`
`30
`
`device to translate both rotationally and laterally which mimics the true motion of
`
`the natural meniscus as described by Hollister.
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`The geometry provided by the meniscal device thus mimics the
`geometry of the tibial plateau with the meniscus intact with respect to the femoral
`
`condyle and mimics the geometry of the tibial plateau with the meniscus removed
`with respect to the tibial plateau, resulting in but little translation relative to the tibia,
`
`5
`
`except for a relatively small rotational and lateral components. With respect to the
`
`femoral condyle, however, the device experiences large relative movement, and a
`
`rotational component brought about by any difference in the contour angle (22) of
`the femoral condyle and the concave meniscal device topmost surface (femoral
`
`surface). This rotational component further ensures that the device is self-centering,
`and cannot be "spit" from the joint.
`In general, the contour angle (22) of the
`
`10
`
`femoral surface of the meniscal device should be within +I- 15 °, and in general, less
`than 20°, of the contour angle of the femoral condyle relative to the tibial plateau.
`Too large an angle will provide too high a centering force, and may accelerate wear
`
`of the femoral condyle articular cartilage or the device itself.
`
`15
`
`In the "rest position," where the knee is in full extension, the outer
`
`contours o