isolation of periosteal culture : periosteal tissue is isolated from the distal femora and proximal tibia of nzw rabbits . in a sterile laminar flow hood , the periostea are placed into a 100 mm . petri dish and with a no . 15 scalpel and forceps are diced into small ( 1 mm 2 ) pieces . the pieces are then transferred into a 60 mm petri dish as explants and prepared for three dimensional culture . isolation of marrow mesenchymal stem cells : after periostea have been isolated from the bone surface , the diaphysis of the femora are resected and the marrow cavity is flushed from one end with an injection of tyrodes balanced salt solution . the resulting marrow plug is then prepared into a single cell suspension by successively drawing the tissue through increasingly narrower syringe gores , starting with an 18 gauge syringe and ultimately ending with a 22 gauge . the final cell suspension is then passed through a 20 μm nylon filter . cells are then counted , tested for viability , and then prepared for suspension culture . chondrocyte isolation : articular cartilage samples obtained from donor rabbits are treated as follows to isolate free chondrocytes . the articular cartilage specimens are transferred to a sterile 60 mm petri dish and diced into pieces of approximately 1 mm 3 and then transferred to a glass spinner flask . to this flask 15 ml of rpm - 1640 medium with 25 mm hepes buffer ( ma bioproducts ) containing clostridial collagenase ( sigma , type 1a 0 . 37 mg / ml ), deoxyribo - nuclease ( dnase ) ( worthington , activity : 133u / mg , 15 mg / ml ), testicular hyaluronidase ( sigma , type is ; 10 mg / m l ), and penicillin - streptomycin ( 5 , 000u / 5000u ; 1 % v / v ) is added . this enzyme - medium mixture is filter - sterilized ( 0 . 45 micrometers , nylon membrane ) and the samples are spun on a magnetic stirrer at 370c for 4 - 6 hours . to harvest the cells , extracellular matrix debris is removed by passing the entire medium - cell mixture through a sterile 120 μm nylon mesh ( nitex corp .). this is followed by centrifuging at 1500 rmp for 5 minutes . the cell pellet is resuspended with phosphate buffered saline and centrifuged . the washing procedure is repeated twice . the cells are counted and viability tested by the trypan - blue exclusion method . chondrocytes are then spun into a pellet for later suspension in agarose . tissue culture conditions : to prepare a three dimensional culture system , a 1 % solution ( w / v ) of a low melting point agarose is prepared in phosphate buffered saline ( pbs ), and autoclaved . upon cooling , chondrocytes or marrow cells are mixed into the still liquid solution , stirred well and then overlaid into 60 mm petri dishes . this results in an even spatial distribution of cells in a three dimensional matrix . for periosteal cultures , cooled agarose is overlaid into 60 mm petri dishes and explants of periosteal tissue are placed into the soft agarose . after six hours to allow adequate gelation , cultures are overlaid with a layer of medium . primary cultures of periosteal cells and chondrocytes are cultured in medium &# 34 ; a &# 34 ; as described by osborn et al . this is a low serum formulation ( 50 / 50 dmem / hams f - 12 ) which will minimize extraneous effects of calf serum . cultures are fed fresh medium every other day , and examined daily for growth using an inverted phase contrast microscope . three dimensional cultures are prepared from both tissue types as previously described . after 48 hours to allow equilibration of cultures , medium is replaced with medium containing the following supplements : basic fibroblast growth factor ( bfgf ) at 0 . 1 , 1 . 0 , and 10 ng / ml ; transforming growth factor beta ( tgfb ) 3 ng / ml . control samples are fed med . a alone . replicate culture samples are harvested at days 3 , 7 , and 14 and treated as described below . cultures are incubated with sulfate 35 for 4 hours ( 10 uci / ml ). at the end of incubation , labeled medium is aspirated off and the samples are rinsed five times with phosphate buffered saline . samples are then extracted with 4m guhcl , ph 5 . 6 at 4 ° c . for 24 hours . the extracted sample is then run on a sephadex g - 25 column and the high molecular weight fractions are pooled and scintillation counted . cultures are incubated with tritiated proline for 4 hours , washed extensively and extracted with tca . samples are then scintillation counted . cultures are incubated with tritiated thymidine for 24 hours prior to sample harvest , extracted with tca and scintillation counted . all data is expressed in cpm and normalized by dna content using the fluorometric dye binding assay with hoechst 33258 . this example is designed to critically evaluate the ability of ferromagnetic dipoles to increase the rate of protein synthesis and proliferation in this culture system . primary cultures of periosteum tissue , marrow cells and chondrocytes are prepared and cultured as previously described . experimental groups are prepared with ferromagnetic dipoles placed in a matrix grid pattern every 2 mm glued to the bottom of the dishes so as to be oriented perpendicular to the dish bottom . the agarose / cell suspension is then overlaid on top of the dipole array . the study is conducted in an analogous fashion to example 1 with the following differences : growth factor containing mediums are specially tagged to magnetic beads . control sample are treated in a similar fashion but lack the ferromagnetic dipole array in the bottom of the petri dish . this example is conducted in an identical fashion as example 1 with the exception that medium supplemented with fgf alone ( tbfb ) is added for the first 5 days in culture . cultures are then fed the medium containing the tgfb ( 3 ng / ml ) and fgf is no longer used . the medium containing the tgfb is continued for a total culture time of two weeks and samples are treated as described in example 1 . the dosage of fgf used is determined from the optimal response found from the results of example 1 . control cultures are fed med . a alone . this example tests the reverse temporal sequence for growth factor addition as stated in example 2 . in this experiment tgfb ( 3 ng / ml ) alone is added initially followed on a 5 day fgf . again , samples are treated as described above with respect to dosages and controls . this example critically evaluates the ability of ferromagnetic dipoles to increase the rates of protein synthesis and proliferation in this culture system . primary cultures of periosteum tissue , marrow cells and chondrocytes are prepared and cultured as previously described . experimental groups are prepared with ferromagnetic dipoles placed in a metric grid pattern every 2 mm glued to the bottom of the dishes so as to be oriented perpendicular to the dish bottom . the agarose / cell suspension is then overlaid on top of the dipole array . the study is conducted in a fashion analogous to example 1 with the following differences : growth factor containing mediums are specially tagged to magnetic beads . control samples are treated in a similar fashion but lack the ferromagnetic dipole array in the bottom of the petri dish . clinically , a surgical procedure for the sequential application of growth factor chondrogenesis utilizing magnetically targeted particles is carried out as follows . referring to fig1 a portion of a joint 10 is diagrammatically shown , comprising a cartilage layer 12 , having a cartilage defect 14 . below the cartilage layer 12 is a subchondral bone layer 16 , and a mesenchymal cell depot 18 . referring to fig2 a series of holes 20 are drilled at the site of the cartilage defect extending into the subchondral bone , and femorally into the mesenchymal cell depot . referring to fig3 an amalgam containing a metabolically - porous scaffolding material , such as collagen or synthetic alternatives , is added to a magnetically active pellet . this cylindrical mass 22 is then injected into the drill holes . as shown in fig4 the entire holes may be filled with the porous scaffolding material and magnetic material 22 . a series of growth factors 32 attached to magnetic particles 26 are then injected into the joint in a timed sequence . continuous passive motion or controlled range of motion exercises and limited weight bearing are applied to the surface , the scaffolding serving to protect the chondrogenesis . sequential injections of various growth factors or chondrocytes which are magnetically tagged may be made at various time intervals . referring to fig5 a joint 34 is diagrammatically shown having cartilage defects which have been implanted with magnetic particle - containing scaffolding 36 , to which an external magnetic field 38 is applied . the magnetic field 38 magnetizes the magnetic particles and causes them to attract the growth factors attached to the magnetically tagged particles which are subsequently injected into the joint in a timed sequence . alternatively , the external magnetic field may be utilized to direct the magnetically tagged growth factor to the proper geometric position in the desired sequence in place of the magnetically active pellet which is incorporated in the scaffolding material . the magnetic particles implanted at the site of the cartilage defect may be energized by an external magnetic field with a differential effect on the selected growth factors . the implantation of the magnetically tagged implant can be accomplished arthroscopically , utilizing instrumentation specifically designed for this task . among the materials which stimulate repair of cartilage are cartilage derived growth factor ( cdgf ), a cationic polypeptide which stimulates the proliferation of cultured mouse fibroblasts , as well as chondrocytes and endothelial cells , transforming growth factor beta ( tgf - beta ), a multi - functional peptide , which regulates the proliferation and differentiation of many cell types . other growth factors are connective tissue activating peptides ( ctap ), insulin - like growth factor ( igf - 1 ), acidic fibroblast growth factor ( afgf ), and basic fibroblast growth factor ( bfgf ). according to the invention , it has been demonstrated in the laboratory that growth factors can be tagged with a magnetically active particle , and maintain their activity . this is the first time that chondrogenic and osteogenic growth factors have been so tagged and the factors demonstrated to be active . specifically , a partisome particle has been attached to bfgf and to tgf - beta , and it has been experimentally proved that the growth factor remains biochemically active . a polymer has been constructed in the laboratory impregnated with ferromagnetic iron particles . these impregnated polymers are implanted in rabbits in an initial experiment to establish biocompatibility and effectiveness . it has been further demonstrated that one can attach the partisome particle to a chondrocyte itself . the chondrocyte remains viable , and maintains viability indefinitely . furthermore , it has been shown that the chondrocyte can be localized to a desired area by use of an external magnet , ferromagnetic particles embedded in a biodegradable polymer , and an external magnetic field . again , this represents the first time that a chondrocyte has been magnetically tagged and directed to a desired location with the chondrocyte remaining viable . the magnetic particles which are being employed herein are supplied by bioquest , inc . ( formerly molecular bio - quest , inc .) of atkinson , n . h . the material is disclosed in a patent to chagnon et al , 1993 ( u . s . pat . no . 5 , 225 , 282 ) entitled &# 34 ; biodegradable magnetic microcluster comprising non - magnetic metal or metal oxide particles coated with a functionalized polymer &# 34 ;, forms the basis of the ability to tag the growth factors and the chondrocytes . this work builds on a previous patent of same assignee chagnon et al , 1991 ( u . s . pat . no . 5 , 071 , 076 ) &# 34 ; method for producing magnetic microparticles from metallocenes &# 34 ;, chagnon , 1992 ( u . s . pat . no . 5 , 147 , 573 ) &# 34 ; superparamagnetic liquid and colloids &# 34 ;. also related is a patent of daniel grande entitled &# 34 ; technique for healing lesions in cartilage &# 34 ;, 1989 ( u . s . pat . no . 4 , 846 , 835 ). he proposed at that time that the chondrocytes would be seeded in a three dimensional collagen matrix which served as the graft material . the graft would be held in place by a periosteal flap . the disclosures of these patents are hereby incorporated by reference . the swedish researchers referred to above built on this approach , but did not use any scaffolding . it now appears to be the case that the scaffolding , which is present in the actual cartilage defect , interferes with needed chondrogenesis and may not be needed or appropriate . the present scaffolding sits in a series of cylinders below the defect and seeks to attract the growth factors or chondrocytes to the site . it is to be understood that the present invention is not to be limited to the exact details of operation , or to the exact compounds , compositions , methods , procedures , or embodiments shown and described , as various modifications and equivalents will be apparent to one skilled in the art , wherefore the present invention is to be limited only by the full scope which can be legally accorded to the appended claims .
a method for repairing a defect in cartilage , comprising the provision of apertures in the cartilage by drilling holes at the base of the cartilage defect , which holes may enter the mesenchymal depot , introducing a porous scaffold material containing a plurality of magnetic particles into the apertures , and subsequently and sequentially injecting magnetically - tagged cartilage growth promoting materials such as various growth factors or chondrocytes into the area of the defect . the magnetically tagged growth promoting material is then drawn into the apertures by magnetic attraction of the magnetic particles contained in the porous scaffold material , either by virtue of the particles being permanently magnetic , or by the imposition of an external magnetic field . the present application claims the biodegradable porous scaffold material containing the plurality of magnetic particles .