Abstract:
A physically stabilized biodegradable osteochondral implant includes a porous matrix element of a resilient material and blood coagulated in vitro in open pores of the element. Also disclosed is a method of manufacture of the implant and a method of restoring a damaged articular surface by use of the implant.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a biodegradable osteochondral implant, more specifically to a physically stabilized osteochondral implant comprising a biodegradable porous resilient matrix element, and methods for its manufacture and implantation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Damaged articular surfaces with or without damage in the underlying bone, such as an articular surface of the knee, can be restored by transfer of an osteochondral plug from a neighbouring region that bears no or little weight (for a review, see: Cartilage Restoration, Part 2. Techniques, Outcomes, and Future Directions. J Winslow Alford and B J Cole, Am J Sports Med 33:443-460 (2005). A problem with this method is the limited availability of suitable autografts and also donor site morbidity. Another method for restoration is autologous chondrocyte implantation. In this method normal hyaline cartilage is harvested by biopsy and expanded in vitro. Cartilage remaining at the damaged area is removed so as to leave healthy surrounding hyaline cartilage to form stable vertical walls around a preferably circular cartilage-free area. A periosteal patch of a size fitting into the cartilage-free area of the defect is removed from a suitable non-weight bearing site of the bone and then sewn onto the cartilage so as to cover the damaged articular surface. The expanded chondrocytes are then implanted into the defect by means of a syringe. Another option is an osteochondral allograft transplantation from a suitable donor. More recently various matrix tissue scaffolds have been proposed for the substitution of periostal patches, allowing the in-growth of chondrocytes from neighbouring cartilage and also to transfer cultured cells into the defect. 
         [0003]    U.S. Pat. No. 5,876,452 (Athanasiou et al.) discloses a cylindrical biodegradable, porous bioerodible implant device of a synthetic material, such as poly(DL-lactide-co-glycolide), consisting of a bone phase that abuts against the underlying bone for anchoring and a cartilage phase that interfaces with the adjacent layer of articular cartilage. For improved in-growth of cartilage cells the cartilage portion of the matrix contains transforming growth factor-β (TGF-β). 
         [0004]    U.S. Patent Appln. No. 2005/0043813 (Kusanagi et al.) discloses an acellular matrix implant for implantation into a cartilage lesion comprising a collageneous, gel-gel, polymer of an aromatic organic acid or a thermo-reversible hydrogel fabricated as a sponge or porous honeycomb scaffold. Also disclosed is a biodegradable sealant of a top or bottom cartilage or bone lesion. 
         [0005]    U.S. Patent Appln. No. 2003/0229400 (Masuda et al.) discloses a transplantable osteochondral implant comprising cartilage tissue derived from chondrogenic cells cultured in vitro and having a cell associated matrix, the cartilage tissue being attached to a porous biocompatible support scaffold selected from natural bone, demineralised natural bone, collagen, and bone substitute material. 
         [0006]    A polyvinyl alcohol-hydrogel implant for replacing worn-out cartilage surfaces is available on the market under the trade name SaluCartilage™ (SaluMedica, Atlanta, Ga.; www.salumedical.com). 
         [0007]    In spite of the various devices and methods for restoration of damaged articular surfaces disclosed in the art there is room for substantial improvement. 
       OBJECTS OF THE INVENTION 
       [0008]    It is an object of the invention to provide an osteochondral implant in form of a physically stabilized resilient porous biodegradable matrix element intended for restoring a damaged articular surface and, if there be need, also subchondral bone. 
         [0009]    It is another object of the invention to provide a method of manufacture of the implant. 
         [0010]    A further object of the invention is to provide a method of restoring a damaged articular or other bone surface by means of said osteochondral implant. 
         [0011]    Further objects of the invention will become apparent from the following summary of the invention, the description of preferred embodiments illustrated in a drawing, and the appended claims. 
       SUMMARY OF THE INVENTION 
       [0012]    In this application “biodegredable” comprises all kinds of degradation of an implant or a portion thereof in the living body, such as enzymatic, oxidative, and hydrolytic degradation. Furthermore, in this application, “of the same (polymer) material” relates to the chemical nature of the material but not to its form. “Top”, “bottom”, “lateral” faces or sections etc. relate to their disposition in respect of the bottom of a recess prepared by the surgeon in the bone of a joint for implantation. In this application “physically stabilized” refers to an implant matrix element of a resilient material with open pores, which has been made substantially rigid by filling the pores with blood and allowing the blood to coagulate in the pores. 
         [0013]    According to the invention is disclosed an osteochondral implant in form of a physically stabilized porous resilient biodegradable osteochondral implant matrix element. The invention is based on the insight that such stabilization can be achieved by soaking the implant matrix element with blood and making the blood coagulate in vitro in the pores of the matrix. The implant of the invention retains substantially the form and the dimensions of the matrix element from which it is made. 
         [0014]    The implant of the invention has preferably cylindrical, conical or other rotationally symmetric form. Other shapes, such as cubes or parallelepipeds tailored specific requirements, are also within the scope of the invention. The implant of the invention may be made in any suitable form and size. Most preferred are cylindrical implants. Cylindrical implants ending in a cone, a frustum of a cone or a hemisphere, possibly with rounded bottom face edges, are also preferred. Their diameter is selected so as to make them fit into recesses, in particular bores, in the bone having a diameter of from 3 or 5 mm to 20 mm or more, in particular of from 8 mm to 12 mm. By cutting (milling) rather than drilling the depression into which the implant is to be inserted into the bone, an implant of any desired form with parallel top and bottom faces can be obtained, substantially in form of a parallelepiped. Such an implant is advantageously cut out from a corresponding sheet material According to the present invention the cutting (milling) of the recess in the bone and of the implant matrix element can be controlled by the same or similar software used in dedicated computer-controlled cutting (milling) apparatus. 
         [0015]    The matrix element from which the physically stabilized biodegradable osteochondral implant of the invention is made is porous and of a resilient polymer material. Its porosity is of a kind allowing it to be soaked with blood. The material of the implant thus has a sponge-like nature with open pores. A preferred porosity is one of 50% or more, more preferred of 75% or more, even more preferred of 85% or more, most preferred about 90% or more. The porosity of the implant is such that the pores are open pores or comprise open pores. Open pores are pores in communication with the surface of the implant. It is preferred that 50% or more of the pore volume is comprised by the volume of open pores, more preferred 75% or more, even more preferred 85% or more, most preferred 90% or more. A preferred diameter of an open pore is one that allows blood to pass through it, such as a pore diameter of 4 μm or more, in particular of 10 μm or more. It is preferred that at least 50% of the open pore volume is accessible by pores of a pore diameter of 4 μm or more, in particular of 10 μm or more, more preferred 75% or more, even more preferred 85% or more, most preferred 90% or more. 
         [0016]    The biodegradable resilient porous osteochondral implant matrix element of the invention is preferably of a substantially homogeneous polyurethane urea material. It is important to use a polymer material that, while porous, resilient and biodegradable, will preserve its physical structure for extended periods of time so as to provide physical support for in-growing bone and cartilage cells and for expanded chondrocytes with which it may have been seeded, such as for a year and preferably for two years and even three years or more. It is also possible to use in the invention other biocompatible polymer materials with the proviso that they must meet these requirements. Useful materials can be selected, for instance, from the group consisting of (L-lactic acid) and its co-polymers and D-lactic acid and/or glycolic acid (Y S Nam et al., Polymer 20, 1783-1790 (1999); polyglycolide, poly(L-lactic acid), poly(D,L-lactic acid), poly(D,L-lactide-co-glycolide; poly(ε-carprolactone), (DL-lactide-co-caprolactone), poly(glycolide-co-trimethylene carbonate), poly(dioxanone) (S L Ishaug-Riley et al., Biomaterials 20, 2245-2256 (1999); tyrosine-PEG-derived poly(ether carbonate) (C YU et al., Biomaterials 20, 253-264 (1999); poly(ortho esters), copolymers of β-hydroxybutyric acid and hydrovaleric acid, poly(anhydrides), poly(trimethylene carbonate), poly(iminocarbonates) (J Kohn et al., Biomaterials 12, 292-304 (1991); tyrosine-derived polycarbonate (V Tangpasuthadol et al., Biomaterials 21, 2371-2378 (2001); poly(trimethylene carbonate-ε-caprolactone)-block-poly(p-dioxanone) (J-T Hong et al., J Polym Sci: Polym Chem 43(A), 2790-2799 (2005). 
         [0017]    The physically stabilized biodegradable osteochondral implant of the invention is intended for implantation into a surgically provided recess in a joint surface extending through the cartilage layer into the subchondral bone. The recess has preferably a substantially flat bottom. The sidewalls of the recess are preferably of a substantially perpendicular extension in respect of the bottom and comprise a lower subchondral bone section and an upper cartilage section. Upon implantation the bottom face of the implant is in abutment with the recess bottom while the side walls of the implant are in abutment with the side walls of the recess. The height of the implant corresponds substantially to the height of the recess. The implant thus has a form so as to substantially fill the recess while not extending from it. 
         [0018]    A preferred height of the implant is from 1 mm to 6 mm and even 10 mm, in particular from 2 mm to 4 mm. The width of the implant may vary over a wide range such as from 3 or 5 mm to 20 mm and more, in particular of from 8 mm to 12 mm. 
         [0019]    The rigidity of the implant matrix element is substantially increased prior to implantation by soaking the implant matrix element with blood of the patient or animal selected for implantation and coagulating the soaked blood in the pores of the matrix. The so formed rigidified implant is then implanted. By “substantially increased rigidity” is understood a resistance to compression by a factor of 2 or more, in particular of 4 or more. Up to a compression of 25% of a non-stabilized matrix element a load causing such compression, when placed on the physically stabilized implant of the invention, will produce a compression of 50% or less and even of 25% or less of that effected on the non-stabilized matrix element. 
         [0020]    According to another important aspect of the invention, the implant matrix element is humidified prior to soaking with blood, such as by contacting it with water or saline or by storing it in a humid atmosphere. 
         [0021]    According to a further important aspect of the invention, the humidified implant matrix element is soaked with blood by compressing it, contacting it with blood in a compressed state, and allowing it to expand in contact with blood. 
         [0022]    Prior to implantation the blood in the implant is allowed to coagulate. According to a still further important aspect of the invention coagulation is accelerated. Acceleration may be provided by, for instance, supplying energy to the implant, such as in form of radiation, in particular microwave radiation. Acceleration of coagulation may also be provided by contacting the matrix element soaked with blood with a coagulation enhancer, such as human coagulation factor VII. 
         [0023]    Thus, according to the invention is provided a method of physically stabilizing a resilient porous biodegradable osteochondral implant matrix element, comprising providing a biocompatible aqueous media; humidifying the implant matrix element by contacting it with the aqueous media; compressing the implant; contacting it with blood obtained from a patient or animal into which the implant is intended to be implanted or blood obtained from a serologically compatible person or animal; allowing the implant to expand in contact with the blood so as to be soaked with it; coagulating the blood in the implant in vitro. In a hydrated porous resilient matrix the coagulation rate of blood drawn into its pores can be controlled by adding a coagulation delaying agent such as heparin to the aqueous humidifying media. 
         [0024]    It is preferred for the non-stabilized resilient porous implant matrix element to be contacted with the aqueous media for a given period of time and at a given temperature, such as a time sufficient for obtaining equilibrium hydration at the contact temperature. A preferred temperature is from about 10° C. to about 45° C., more preferred at from about 18° C. to about 40° C. A preferred contact period is from 10 min to 6 h. 
         [0025]    According to the invention is also provided a resilient porous biodegradable osteochondral implant physically stabilized by coagulated blood disposed in pores thereof. The physically stabilized implant of the invention is substantially non-resilient. It can be used for implantation into a patient or animal for restoring a damaged articular surface. The three-dimensional shape of the physically stabilized implant corresponds essentially to that of the non-stabilized implant in an uncompressed state. 
         [0026]    According to the invention is furthermore provided a method of restoring a damaged articular surface. The method comprises providing a recess in the surface extending over substantially the entire area of damage and extending into the subchondral bone over substantially the same area; providing a physically stabilized implant of the invention of form mirroring the form of the recess; optionally securing the implant to the adjacent bone and/or cartilage. Securing may be obtained, for instance, by any of suture, staple, pin, adhesive, such as fibrin glue, hook means, and combinations thereof. 
         [0027]    In essentially the same manner as for restoration of cartilage and bone of a load bearing site in a joint the present invention can also be applied to donor site augmentation, that is, to restore cartilage and bone of a non-load bearing site, such as one from which a osteochondral plug has been removed. 
         [0028]    The invention will now be explained in greater detail by reference to a number of preferred embodiments illustrated in a rough drawing. 
     
    
     
       SHORT DESCRIPTION OF THE FIGURES 
         [0029]      FIG. 1  is a perspective view of a prior art cylindrical implant matrix element; 
           [0030]      FIG. 1   a  is a corresponding axial sectional view; 
           [0031]      FIG. 1   b  is an enlarged portion of  FIG. 1   a  showing open pores; 
           [0032]      FIGS. 2   a - 2   e  illustrate the hydration of a cylindrical implant matrix element of the invention; 
           [0033]      FIGS. 3   a - 3   e  illustrate the loading of a hydrated cylindrical implant matrix element with blood and the formation of the physically stabilized implant of the invention by coagulation of the blood-loaded matrix element; 
           [0034]      FIGS. 4-6  are sectional views illustrating the implantation of the physically stabilized implant of the invention in a joint with a damaged articular surface; 
           [0035]      FIGS. 7   a  and  7   b  illustrate the effect of hydration of a cylindrical implant matrix element on the penetration of blood into the matrix by soaking ( FIG. 7   a , hydrated matrix;  FIG. 7   b , dry matrix). 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0036]    A resilient porous biodegradable cylindrical osteochondral implant matrix element  1   a  illustrated in  FIGS. 1 ,  1   a  comprises a flat top face  2 , a flat bottom face  3 , and a cylindrical lateral face  4  equidistant from rotational axis A-A. The implant matrix element  1   a  is of a polyurethane urea material (Artelon®, Artimplant AB, Vastra Frolunda, Sweden) with pores  5  opening at the faces  2 ,  3 ,  4  (enlarged partial view,  FIG. 1   b ). Prior to implantation the implant matrix element  1   a  is hydrated by soaking it with water or saline. For soaking the implant matrix element  1   a  is compressed to remove air from the open pores  5 , then immersed into the soaking media, and allowed to there to expand. After reaching a state of hydration equilibrium, excess water or saline, respectively, is removed by compressing the implant matrix element  1   a  so as to form a hydrated resilient biodegradable cylindrical osteochondral implant matrix element  1   b.    
         [0037]    The manufacture of a stabilized resilient biodegradable porous cylindrical osteochondral implant of the invention  1  from the matrix element  1   a  is illustrated in  FIGS. 2 and 3 . 
         [0038]    In an exemplary manner, the compressed state of the resilient biodegradable cylindrical osteochondral implant matrix element  1   a  (“matrix element”) is obtained by disposing the matrix element  1   a  between upper  6  and lower  7  faces of upper  8  and lower  9  brackets, respectively, of a matrix compression tool, such as pincers  10  ( FIGS. 2   a ,  2   b ). The matrix element  1   a  loosely held by the pincers  10  is immersed into saline  20  disposed in an open container  21 . The brackets  6 ,  7  are then displaced towards each other until firm resistance by the compressed implant matrix element  1   ac  is met ( FIG. 2   c ). The compressed implant matrix element  1   ac  is allowed to expand in the saline so as to soak saline  20  into its expanding pores  5  ( FIG. 2   d ). The saline soaked implant matrix element  1   a  is kept for one hour in the saline  20  so as to form a hydrated implant matrix element  1   b . The hydrated implant matrix element  1   b  is removed from the container  21 . Excess saline  20  is expelled from the pores  5  of the hydrated matrix element  1   b  by compressing it by means of the pincers  10  (not shown). The hydrated implant matrix element  1   b  devoid of excess saline is allowed to expand by loosening the grip on the pincers  10  ( FIG. 2   e ). It can be kept for a desired period of time in a hydrated state in a humid atmosphere or be used directly. 
         [0039]    The hydrated implant matrix element  1   b  devoid of soaking media  20  is soaked with blood  22  in an open container  23  in essentially the same manner as when soaking the dry implant matrix element  1   a  with saline  10  ( FIGS. 3   a - 3   e ). Blood  22  used for soaking is freshly drawn from the person or animal selected for joint cartilage repair. The compressed hydrated implant matrix element  1   bc  is allowed to expend immersed in blood. Upon full expansion a hydrated implant matrix element  1   bl  soaked with blood  22  is obtained ( FIG. 3   c ), which is removed from the container  23  and may be rinsed shortly with water or saline to remove blood from its surface. Prior to implantation blood  22  in the pores  5  of the loaded implant matrix element lbl is allowed to coagulate to form the physically stabilized resilient porous biodegradable osteochondral implant  1  of the invention ( FIG. 3   e ). Coagulation can be accelerated by, for instance, adducing energy to the implant matrix element lbl loaded with blood, such as by irradiation with IR or microwave radiation ( FIG. 3   d ). 
         [0040]    In the non-hydrated (dry) state of the implant matrix element  1   a  blood soaked into it will start coagulating immediately upon contacting a dry implant matrix element surface. Thereby outer pores, that is, pore sections disposed near the faces  2 ,  3 ,  4 , will be occluded and prevent blood cells, in particular erythrocytes, from passing into inner pores, that is, pore sections disposed at a distance from the faces  2 ,  3 ,  4 . The result of soaking non-hydrated implants matrix element  1   a  specimens and hydrated implant matrix element  1   b  specimens with blood is shown in  FIGS. 7   a  and  7   b , respectively. The white inner zones of the soaked dry implant matrices of  FIG. 7   a  represent inner pore sections not filled with blood due to rapid coagulation of blood in outer pore sections preventing blood cells from penetrating deeper into the matrix. 
         [0041]    The implantation of a physically stabilized resilient porous biodegradable osteochondral implant  1  of the invention into an articular bone is illustrated in  FIGS. 4-6 . 
         [0042]      FIG. 4  illustrates a load-bearing portion of a bone  12  pertaining to a joint comprising a damaged articular area  13  substantially free from hyaline cartilage  14 . The invention aims at restoring the cartilage in this and similar areas of defective bone surface. For this reason a cylindrical bore  15  is cut into the bone  12  ( FIG. 5 ) covering the entire damaged area  13  and radially extending into the surrounding cartilage  14  to form a circumferential cartilage wall section  16  extending, at the one hand, from the subchondral bone  12 /cartilage  14  border in an axial direction towards the joint and, at the other hand from said border in the opposite direction into the bone  12 . The diameter of implant  1  is selected so as to correspond to that of the bore  15 . Next the implant  1  is inserted into the bore  15  with its free end face  3  ahead until the face  3  is abutting the bottom  17  of the bore  15  ( FIG. 6 ). The implant  1  is so dimensioned that, in an implanted state, its top face  5  is substantially flush with the joint surface  19  of the cartilage  14  surrounding the implant  1 . Slow biodegradation of the polyurethane urea scaffold over time allows it to be fully replaced by healthy cartilage and osseous tissue so that the damaged joint area is fully restored.