# Patent

## Title
Implant for articular cartilage repair

## Abstract

An implant for articular cartilage repair includes (1) a three-dimensional body formed of cancellous bone having a demineralized section that contains bone morphogenetic proteins (BMP's) that are released by the demineralization but retained in the body, and (2) a cartilage layer formed on a surface of the demineralized section. The cartilage layer is formed by a method including the steps of (a) isolating chondrocytes from articular cartilage of a donor; (b) cultivating the isolated chondrocytes in a medium; (c) suspending the cultivated chondrocytes in agarose; (d) adding the cultivated chondrocytes to the demineralized section of the body, whereby the cultivated chondrocytes are stimulated by the BMP's retained in the body; and (e) incubating the cultivated chondrocytes to form a plurality of layers of chondrocytes on the demineralized section, wherein the plurality of layers of chondrocytes forms the cartilage layer.

## Background
RELATED APPLICATIONS
This application claims the benefit of Provisional Application No. 60/697,563, filed Jul. 11, 2005.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO SEQUENCE LISTING, A TABLE OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX
None.
FIELD OF INVENTION
The present invention is generally directed towards the treatment of articular cartilage defects using an allograft bone scaffold or carrier. In particular, the allograft bone is treated with allograft chondrocytes to form a cartilage surface on the allograft bone.
BACKGROUND OF THE INVENTION
Chondrocytes are cells specific to articular cartilage. The isolation and cultivation of chondrocytes is a standard procedure, which has been undertaken for more than 10 years. Under two-dimensional culture conditions in cell culture vessels, chondrocytes dedifferentiate to fibroblast-like cells. During the dedifferentiation they lose their typical properties (grade of differentiation, cell form, synthesis of cartilage-specific matrix components), which are essential for physiology and biomechanics of cartilage.
For many years there have been attempts to transplant dedifferentiated chondrocytes grown under two-dimensional culture conditions into human joint defects. Transplantation of cells grown in culture provides another method of introducing a new cell population into chondral and osteochondral defects. The procedure uses arthroscopy to take a biopsy from a healthy, less loaded area of articular cartilage. Enzymatic digestion of the harvested tissue releases the cells that are sent to a laboratory where they are grown. Once cultivated, they are injected during a more open and extensive knee procedure into areas of defective cartilage in an attempt to facilitate the repair of damaged tissue.
The technique of autologous chondrocyte transplantation is very complex and requires postoperative immobilization of the patient. The surgical site cannot bear load post surgery because the transplanted chondrocytes have to reach their original biological state (redifferentiation of in vitro dedifferentiated cells), then have to anchor to the defect, synthesize the cartilage-specific matrix and rebuild a new cartilage.
In the procedure of mosaicplasty (autologous cartilage transplantation), cartilage-bone cylinders are removed from non-load-bearing joint areas of the patient and transplanted into the defect. This method shows good clinical results including a good load capacity of the operated knee-joint, but is only applicable to small cartilage defects due to the limited availability of autologous osteochondral cylinders. Classical operation techniques, which do not use cartilage or chondrocyte transplantation (abrasion, debridement, Pridie-drilling), result only in a defect repair with fibrous, less load-bearing tissue unfortunately. Additionally, repeated treatment is necessary as this tissue degenerates over time.
Osteochondral transplantation or mosaicplasty involves excising all injured or unstable tissue from the articular defect and creating cylindrical holes in the base of the defect and underlying bone. These holes are filled with autologous cylindrical plugs of healthy cartilage and bone in a mosaic fashion. The osteochondral plugs are harvested from a lower weight-bearing area of lesser importance in the same joint. Reports of results of osteochondral plug autografts in a small number of patients indicate that they decrease pain and improve joint function, however, long-term results have not been reported. Factors that can compromise the results include donor site morbidity, effects of joint incongruity on the opposing surface of the donor site, damage to the chondrocytes at the articular margins of the donor and recipient sites during preparation and implantation, and collapse or settling of the graft over time. The limited availability of sites for harvest of osteochondral autografts restricts the use of this approach to treatment of relatively small articular defects and the healing of the chondral portion of the autograft to the adjacent articular cartilage remains a concern.
As previously noted, transplantation of cells grown in culture provides another method of introducing a new cell population into chondral and osteochondral defects. Carticel® is a commercial process to culture the patient's own cartilage cells for use in the repair of cartilage defects in the knee joint marketed by Genzyme Biosurgery in the United States and Europe. The procedure uses arthroscopy to take a biopsy from a healthy, less loaded area of articular cartilage. Enzymatic digestion of the harvested tissue releases the cells that are sent to a laboratory where they are grown for a period ranging from 2-5 weeks to achieve a 10 fold increase in cell mass. Once cultivated, the autologous cells are injected during an open and extensive knee procedure into areas of defective cartilage where it is hoped that they will facilitate the repair of damaged tissue. An autologous periosteal flap with cambium layer facing down is used to seal the transplanted cells in place and act as a mechanical barrier. Fibrin glue is used to seal the edges of the flap. This technique preserves the subchondral bone plate. Proponents of this procedure report that it produces satisfactory results, including the ability to return to demanding physical activities, in more than 80% of patients and that biopsy specimens of the tissue in the graft sites show hyaline-like cartilage repair. However, long term studies of this procedure in rabbits and dogs showed limited success and showed degradation at the implant site. The original study report has been criticized for not being a prospective controlled randomized study and for lack of quantitative or mechanical data. Of interest, a 14 year follow-up of a similar patient group that underwent diagnostic arthroscopy in combination with one of several treatments (removal of bone bodies, shaving, Pridie drilling) had good to excellent knee function in 78% of the patients. Thus, further studies are needed to assess the function and durability of the new tissue to determine whether it improves joint function and delays or prevents joint degeneration.
As with the perichondrial graft, patient/donor age may compromise the success of this procedure as the chondrocyte population decreases with increasing age. Disadvantages to this procedure include the need for two separate surgical procedures, potential damage to surrounding cartilage when the periosteal patch is sutured in place, the requirement of demanding microsurgical techniques, and the expensive cost of the procedure which is currently not covered by insurance.
The use of implants for cartilage defects is much more limited than that for bone defects. Aside from the fresh allograft implants and autologous implants, U.S. Pat. No. 6,110,209 issued Nov. 5, 1998 shows the use of an autologous articular cartilage cancellous bone paste to fill arthritic defects. The surgical technique is arthroscopic and includes debriding (shaving away loose or fragmented articular cartilage), followed by morselizing the base of the arthritic defect with an awl until bleeding occurs. An osteochondral graft is then harvested from the inner rim of the intercondylar notch using a trephine. The graft is then morselized in a bone graft crusher, mixing the articular cartilage with the cancellous bone. The paste is then pushed into the defect and secured by the adhesive properties of the bleeding bone. The paste can also be mixed with a cartilage stimulating factor, a plurality of cells, or a biological glue. All patients are kept non-weight bearing for four weeks and used a continuous passive motion machine for six hours each night. Histologic appearance of the biopsies have mainly shown a mixture of fibrocartilage with hyaline cartilage. Concerns associated with this method are harvest site morbidity and availability, similar to the mosaicplasty method.
U.S. Pat. No. 6,379,367 issued Apr. 30, 2002 discloses a plug with a base membrane, a control plug, and a top membrane which overlies the surface of the cartilage covering the defective area of the joint.
U.S. Pat. No. 6,488,033 issued Dec. 3, 2002 discloses an allograft plug with a cartilage cap which is surface contour matched to the surface of a condyle defect area which is to be replaced. The allograft plug is transplanted in an interference fit within the cavity site which remains after a condylar defect is removed from a patients condyle.
The present implant and method differs from the above prior art in that it is directed to allograft chondrocyte transplantation on an allograft cancellous bone carrier to provide an implant for cartilage transplantation.
SUMMARY OF TH INVENTION
The present method utilizes techniques of autograft and/or allograft chondrocyte transplantation onto a demineralized allograft cancellous bone implant structure to form an implant with a cartilage layer on the bone structure for use in cartilage repair on a patient at a later time.
It is an object of the invention to use allograft cancellous bone as a carrier on which to grow a cartilage layer for use in cartilage repair.
It is an object of the invention to provide an autograft and/or allograft implant for joints which provides pain relief, restores normal function and will postpone or alleviate the need for prosthetic replacement.
It is also an object of the invention to provide a cartilage repair implant which is easily placed in a defect area by the surgeon using an arthroscopic, minimally invasive technique.
It is further an object of the invention to provide an allograft implant procedure which is applicable for both partial and full thickness lesions.
It is yet another object of the invention to provide a cartilage implant which can be uniformly used for any patient.
These and other objects, advantages, and novel features of the present invention will become apparent when considered with the teachings contained in the detailed disclosure along with the accompanying drawings



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a chart showing pH in the supernatants through a 44 day period;
FIG. 2 is a chart showing calcium in the supernatants corrected by the calcium content of pure medium through a 44 day period; and
FIG. 3 is a chart showing phosphate in the supernatant corrected by the phosphate content of pure medium through a 44 day period.



DETAILED DESCRIPTION OF TH INVENTION
The present invention is susceptible of embodiment in various forms as will hereinafter be described with the understanding that the present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the invention to the specific embodiments disclosed herein.
Sterile cancellous bone replacement structures were utilized for the in vitro grown cartilage replacements, which allow the fabrication of load-bearing constructs. Bone morphogenetic proteins (“BMP's”) from the cancellous bone plugs have a positive effect on chondrocyte differentiation in vitro by stimulating the formation of a native, chondrocyte-phenotype and proper matrix production by the cells. The highest stimulation effect of BMP's on chondrocytes can be observed, if BMP's are immobilized onto a carrier or retained in a biological matrix. In these carriers the natural BMP's of the bone are released by the demineralization but retained in the carrier matrix. For evaluating the effect of the biological carrier structure of cancellous bone on chondrocyte growth, different synthetic carrier materials were used, among them GB 14 (Ca2KNa(PO4)2), ceramics and hydroxyapatite.
In order to develop the in vitro manufactured cartilage constructs, chondrocytes were isolated from cartilage samples taken from donors by enzymatic digestion with collagenase, DNAse and hyaluronidase. Since the total number of chondrocytes and cell proliferation decrease with donor age, only donors younger than 50 years were used. Furthermore, bones having joint diseases and bone fractures were excluded from donor selection. After the enzymatic digestion, a suspension of chondrocytes was obtained and tested by trypan blue exclusion to determine the viability of the cell suspension. The suspension was seeded to standard cell culture vessels in order to expand the cells. Since serum is necessary for a good chondrocyte growth and maturation, the medium used contained 10% fetal calf serum. Ascorbic acid was added to stimulate the collagen production by the cells.
The redifferentiation potential of two-dimensionally expanded cells was tested by transferring them into a three-dimensional agarose gel. During expansion, the cells lost their original phenotype, forming fibroblast-like cells. After transferring them into the agarose gel, the normal, round chondrocyte phenotype could be observed, indicating that the cells were able to redifferentiate. The chondrocytes were resuspended in 0.5% low melting agarose reaching a final density of 2 million cells/ml in order to increase the attachment on the carriers and was then seeded onto the following different carrier materials.
I. Evaluation of Different Carrier Materials for Chondrocyte Cultures
The following synthetic carriers (bone replacement materials) were investigated:
a1) CaCO3/CaSO4 
a2) Ap40 (Apatite), slurry 6.89 μm
a3) GB 14
a4) 46 CaO*23TiO2*31P2O5 
The following biologic carriers were evaluated (human cancellous bone, frozen, sterilized with peracetic acid, provided by the tissue bank of DIZG)
b1) B0/1, donor 432/98, 54 y., cancellous bone from tibia plateau not demineralized
b2) B1/1 totally demineralized
b3) B2/1 superficially(surface) demineralized
b4) B0/2 donor 432/98, 54 y., cancellous bone from femur epiphysis not demineralized
b5) B1/2 totally demineralized
b6) B2/2 superficially(surface) demineralized
b7) C0/2 donor 430/98, 60 y., cancellous bone from vertebral column not demineralized
b8) C1/2 totally demineralized
b9) C2/2 superficially(surface) demineralized
The demineralization process was done according to standard operations for demineralization of cancellous bone. The cancellous cubes were washed in order to remove the cells and then dried at 37° C. for 36 hours. The cubes (1 cm×1 cm×1 cm) were demineralized in 1 N HCl for 24 hours at room temperature. For the totally demineralized cubes, the whole cancellous bone cube was incubated in this solution. In the case of the partially demineralized bone only the front surface of the cube was immersed to 2 mm depth in the solution. Approximately 2 mm of the surface of the cube was totally demineralized with a boundary layer between the fully demineralized section and the mineralized section being about 50 μm thick. Afterwards the cubes were washed with sterile water for injection until the pH was neutral.
II. Macroscopic and Microscopic Evaluation of the Carriers
The CaCO3/CaSO4 (a1) carrier showed a rough surface that could be easily destroyed if touched with forceps. All other materials Ap40, GB 14 and 46 CaO*23TiO2*31P2O5 had a plain and stiff surface, which was not destroyed during manipulation. All synthetic carriers were sterilized prior to use at 186° C./2.5 hours.
The totally and superficially demineralized cancellous bones cubes (b1-9) also showed no differences in material resistance (resistance investigated by test with forceps, qualitative evaluation). Both materials had a soft and spongy appearance.
Large pores in the range of 10-100 μm were observed in all cancellous samples (b1-9) during microscopic evaluation. The pore size represented a multiple of the cell volume of a chondrocyte. Samples obtained from vertebral column had a lower pore size than all other preparations. However, no particular bone tissue; namely, tibia, femur, vertebral column appeared to have superior or inferior chondrocyte growth capacities to the other.
III. Long-Term Incubation of the Carrier Materials
Both synthetic and allograft cancellous carriers were incubated under cell culture conditions in order to evaluate their long-term stability (incubation in Ham's F12 medium at 37° C., 6 weeks, no addition of cells). The pH as well as the calcium and phosphate concentrations were measured in the supernatants every third day.
No structural changes were detected after the 6 week incubation of Ap40 (a2) and 46 CaO*23TiO2*31P2O5 (a4). White, crystalline sediments were observed in the medium after 44 days of incubation from the carrier GB 14. The CaCO3/CaSO4 carrier exhibited dramatic structural changes and volume reduction under incubation at cell culture conditions. The process of material destruction started after 3 days of incubation and was accompanied by a large amount of particle release into the medium.
A medium pH of 7.32 was measured during long-term incubation of CaCO3/CaSO4 carrier. The long term incubation only lasted 16 days as no further incubation was possible because of degradation of carrier. In supernatants of GB 14, the carriers had a relatively high pH of more than 7.5 (7.52) detected after 44 days. In the case of 46 CaO*23TiO2*31P2O5-ceramic a pH of 7.5 was determined at days 4 and 7 followed thereafter by only physiological pH-values (pH 7.4). Supernatants of Ap40 showed a physiological pH during the entire investigation increasing to 7.5 after 44 days.
A release of calcium into the medium was observed during incubation of CaCO3/CaSO4, Ap40 and GB 14 carriers (calcium release of CaCO3/CaSO4>Ap40>GB 14). Additionally, high amounts of phosphate were detected in supernatants from GB 14 carriers with an increase in phosphate in 46 CaO*23TiO2*31P2O5 carriers.
Carriers that release particles are not suited for transplantation into the knee joint because of the potential induction of cartilage damage and joint dysfunction. Also, carriers with high disposal of calcium and phosphate are not useful because they may induce undesired chondrocyte calcification. Consequently, only Ap40 and 46 CaO*23TiO2*31P2O5 were used as non-biological carriers for establishing three-dimensional chondrocyte cultures.
After 6 weeks of incubation of the biological carriers (cancellous bone tissue forms), there was no evidence of structural changes, volume reduction or release of calcium or phosphate. Only physiological pH values (pH 7.4) were measured in the supernatants during long-term incubation of the cancellous bone samples. All biological carriers did not release significant amounts of calcium or phosphate to the medium. There were no significant pH changes observed if the bone cubes were submitted to along term incubation in medium and the final pH reached physiological values of approximately pH 7.4 after 44 days of incubation.
IV. Isolation and Cultivation of Human Chondrocytes
Human chondrocytes were isolated by enzymatic digestion of human articular cartilage (donor age <50 years, no degenerative defects of the knee joint, no fractures of the underlying bone). A high cell viability of >90%, in many cases 100% was measured after the isolation.
The cells were cultivated in medium without further supplementation with growth factors. The medium was changed every third day. A good cell adhesion on the cultures vessels (cell culture flasks) was observed after 7 days of incubation at 37° C. and 5% CO2. Growth kinetics showed that the population doubling time as well as the lag time were increased with increasing passage of the cells. The chondrocytes showed a dedifferentiated, fibroblast-like phenotype in two-dimensional culture.
The redifferentiation potential of the dedifferentiated chondrocytes was evaluated in 3D agarose cultures where the typical round cell form was observed (original chondrocyte phenotype, sign of redifferention of dedifferentiated cells).
V. Development of Three-Dimensional Chondrocyte Cultures
The different carriers were fixed to the culture vessels (24 well plates) by sealing with 4% low melting agarose. The surfaces of the carriers were prepared for cell seeding by coating with poly-L-lysine (incubation 1 hour, non-bound lysine removed by washing). The chondrocytes taken from both live and deceased donors were resuspended in 2% low melting agarose and incubated in the refrigerator for 7 minutes in order to obtain a more viscous suspension. The top of each carrier was overlaid with this viscous cell suspension. The seeded carriers were then transferred into the refrigerator for 15 minutes to allow for gelation. This was followed by a 1-hour incubation in the incubator (37° C., 5% CO2). Afterwards, each vessel was supplied with culture medium. All cultures were incubated for 40 days and the medium was changed every third day.
Because of the low material stability of all other synthetic materials only Ap40 and 46 CaO*23TiO2*31P2O5-ceramic were used for the establishment of three-dimensional chondrocyte cultures. Ap40 showed a pH increase as well as increases in calcium in the supernatent. 46 CaO*23TiO2*31P2O5 showed an increase in the phosphate in the supernatant. Fibroblast-like, dedifferentiated cells were observed at the surfaces of these materials after 15 days incubation, however, the majority of the surface was not covered by cells. The cells formed only a single layer and were not surrounded by significant amounts of extracellular matrix. The amount of cells did not increase in the following culture period and there was no macroscopic sign of neocartilage formation on these carriers.
There was a significant colonization of the osteobiological carriers (cancellous bone) with chondrocytes detectable after 40 days incubation. The range of cell density (i.e., cells per ml, or cells per cm3) will provide attachment on the osteobiological carrier in a range from between 0.2 to 10 million cells per ml, or cells per cm3. The cells showed a chondrocyte-like, round appearance. The chondrocytes covered the entire carrier surface and were surrounded by significant amounts of extracellular matrix. A multi-layer growth and formation of cell clusters could be detected. The multilayer chondrocyte growth was about 4 layers high forming a cartilage layer approximately 2.0 mm thick with a cell density of approximately 10 million cells per cm3. This compares to the thickness of articular cartilage which is 2-5 mm. In articular cartilage, the chondrocytes are not arranged in multilayers, but in stacks of 1 to 8 cells, and the cellularity per amount of tissue is lower. In the case of partially demineralized cancellous carriers, a neo-cartilage formation could be observed macroscopically by appearance of a white opalescent capsule which was stable if impressed with forceps. In all other carriers this capsule formation was not noticeable macroscopically.
The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. However, the invention should not be construed as limited to the particular embodiments which have been described above. Instead, the embodiments described here should be regarded as illustrative rather than restrictive. Variations and changes may be made by others without departing from the scope of the present invention as defined by the following claims:

## Claims

1. An implant for articular cartilage repair, comprising a three-dimensional body formed of cancellous bone and having a demineralized section formed by demineralization, the demineralized section containing bone morphogenetic proteins (BMP's) that are released by the demineralization but retained in said body, and a cartilage layer formed on a surface of the demineralized section, said cartilage layer being formed by a method including the steps of:
(a) isolating chondrocytes from articular cartilage of a donor;
(b) cultivating the isolated chondrocytes in a medium;
(c) suspending the cultivated chondrocytes in agarose;
(d) adding the cultivated chondrocytes to the demineralized section of said body, whereby the cultivated chondrocytes are stimulated by the BMPs retained in said body; and
(e) incubating the cultivated chondrocytes to form a plurality of layers of chondrocytes on the demineralized section, wherein the plurality of layers of chondrocytes forms said cartilage layer.

2. The implant as claimed in claim 1, wherein the chondrocytes are allograft chondrocytes.

3. The implant as claimed in claim 2, wherein the donor is less than 50 years of age.

4. The implant as claimed in claim 2, wherein the donor is alive.

5. The implant as claimed in claim 2, wherein the donor is deceased.

6. The implant as claimed in claim 1, wherein the demineralized section has a depth of approximately 2 mm.

7. The implant as claimed in claim 6, wherein said body includes a mineralized section and a boundary layer formed between the demineralized section and the mineralized section, the boundary layer having a thickness of about 50 microns.

8. The implant as claimed in claim 1, wherein said body includes pores having a size that ranges from 10 to 100 microns.

9. The implant as claimed in claim 1, wherein step (b) includes expanding the chondrocytes in a two-dimensional culture until the chondrocytes are dedifferentiated.

10. The implant as claimed in claim 1, wherein step (e) is performed until said cartilage layer has a thickness of approximately 2.0 mm.

11. The implant as claimed in claim 1, wherein the plurality of layers of chondrocytes includes four layers.

12. The implant as claimed in claim 1, wherein the chondrocytes are autograft chondrocytes.

13. The implant as claimed in claim 1, wherein said body is formed of allograft cancellous bone.

14. The implant as claimed in claim 1, wherein step (b) includes placing the surface of the demineralized section in a culture vessel and preparing the surface of the demineralized section for cell seeding by coating the surface with poly-L-lysine.

15. The implant as claimed in claim 1, wherein step (e) is performed until said cartilage layer has a thickness in a range of from 2 mm to 5 mm.

16. The implant as claimed in claim 1, wherein said body includes pores having a size that represents a multiple of the cell volume of a chondrocyte.

17. The implant as claimed in claim 1, wherein step (e) is performed for a period of at least 40 days.

18. The implant as claimed in claim 1, wherein the cell density of the chondrocytes in said cartilage layer is about 10 million cells per cm3.

19. The implant as claimed in claim 1, wherein all of said body is demineralized.