Method for Promoting Soft Connective Tissue Growth and Repair in Mammals

This invention concerns materials with chemically induced surface charges for use in augmentation, repair and replacement of both hard and soft mammalian connective tissues. The invention could have direct application to a variety of clinical problems encountered in a number of medical fields, e.g., orthopaedic surgery and maxillofacial surgery.

In the realm of bone surgery there are many osseous defects of diverse etiology, e.g., non-union fractures, bone loss consequent to trauma, malignancy, or infection-induced sequestration, and bone deficits or abnormalities as a result of malformation. Thus, the ability to stimulate formation of bone at specific sites within the skeleton is of considerable clinical importance and a wide array of materials to initiate bone repair and/or to restore or replace missing bone has been examined.

Three major approaches are available to the problem of bone replacement. One is a strictly conformational method whereby defective or missing bone is replaced by an implant (of metal, ceramic or other inorganic material) intended to mimic the form, and optimistically the function, of the missing bone. In the past, this approach has been unsuccessful due to rejection of the material and/or failure of integration of the implant with normal skeletal tissue. More recently, some ceramic materials (hydroxyapatite, tricalcium phosphate) have shown acceptable biocompatability with, and healing in, defect sites with evidence of a direct bond to bone at the interface. These materials, however, lack the mechanical properties of bone, and additionally, bone fails to grow into and become incorporated within the implant. Such materials, then lack the capacity to substitute for natural bone in bone reconstruction.

Alternatively, missing osseous tissue may be substituted for with a matrix which functions as a passive support or scaffold around and into which new bone growth can occur. The matrix attracts, or "turns on", cells that have already been committed to an osteogenic pathway, a process referred to as osteoconduction. Allogeneic bone grafts (banked cadaver bone) succeed exclusively by this mechanism. However, their failure rate, as evidenced by graft loss, sequestration, and delayed healing, is unacceptably high (15-30%). Even when such grafts are well accepted, their healing periods for consolidation and capacity for mechanical stress-bearing are of long duration when compared to autogeneic bone grafting. Further, the present concern about transmissible viral agents, coupled with unfulfilled clinical expectations, has led to limited use of allogeneic grafts at present.

The third approach to bone replacement, involving osteoinduction, occurs when a material or substance induces the ingrowth of new bone from the host's undifferentiated tissues, typically around a temporary matrix. Such material is termed osteoinductive and a number of compounds have been shown to have such a capacity and are enumerated in detail in U.S. Pat. No. 4,440,750 to Glowacki. At present, the most promising of these factors has been "bone morphogenic protein" (BMP) which was extracted from demineralized bone using urea or guanidine hydrochloride and re-precipitated according to the disclosures in U.S. Pat. Nos. 4,294,753 and 4,455,256 to Urist. In addition, U.S. Pat. Nos. 4,434,094 and 4,627,982 to Seyedin and Thomas, respectively, report a substance termed "osteogenic factor" which appears to be a bone generation-stimulating, bone-derived protein. While substances such as these stimulate osteogenesis, the difficulty in using such a protein, or proteins, is that they are normally present at very low concentrations and require large amounts of starting material to obtain sufficient quantities for a few experiments let alone as a routine reagent for bone repair. Until molecular technology identifies the gene or genes for such factors, and recombinant molecules are obtained, the practicality of this approach is of limited clinical value.

Collagen-based solutions, which polymerize after injection or placement (U.S. Pat. Nos. 4,424,208 to Wallace and 4,347,234 to Wahlig), have also been described in U.S. Pat. No. 3,949,073 to Daniels for hard tissue augmentation. In addition, Hollinger, in U.S. Pat. No. 4,578,384, describes a proteolipid incorporated into a biodegradable polymeric matrix comprised of a 50--50 polyacetic acid (PLA) and polyglycolic acid (PGA) for the healing of osseous tissue. The use of collagen-based polymers, however, presents several problems. Collagen, although generally biocompatible, is not completely biodegradable or resorbable and often becomes surrounded, and not absorbed, by host tissues. As such, when placed in a bone bed, where rigidity is ultimately desired, it represents a potential source of long-term mechanical failure. Additionally, collagen, being an organic substance most commonly commercially derived from bovines, allows for the possibility of allergic and immune reactions.

Thus, many of the criteria for an acceptable material for bone repair remain unsatisfied with present methodology. A replacement compound that is immunologically acceptable, nontoxic, osteoconductive or osteoinductive, readily available, capable of being shaped or molded, and capable of being integrated with existing bone tissues continues to be sought.

As with bone or hard tissue, the promise of alloplastic biomaterials that are effective in promoting or enhancing soft connective tissue repair and/or augmentation has not been fully realized. Silicone (dimethylpolysiloxane), introduced in 1964, was initially widely used because of its purported lack of absorption. Extensive clinical experience has shown, however, that an unacceptable number of complications, including granulomatous inflammatory changes, migration of the implant material, and chronic erythema of the overlying skin occur and, as a result, this material has never received FDA approval. Collagen in the commerical form of Zyderm and Zyplast (manufactured by Collagen Corp., Palo Alto, Calif.), introduced in 1979, has met with more encourageing results. It has proved effective, without significant side effects, in the adjunctive management of the aging face as well as in the primary treatment of small cutaneous defects from acne, trauma, or prior surgery. However, it is not permanent and repeated treatments are usually necessary to maintain correction. Thus, it is of little use for stimulating connective tissue production and the laying down of new tissue. It is, rather, an inert dermal filler which is shortly resorbed. In addition, because the material is of bovine origin, allergic and immune reactions can occur and are not rare phenomena. Lastly, the use of collagen is presently limited to small deficiencies and has no proven efficacy in large defects (requiring more than 5 cc of material). As such, collagen represents an improved, but limited, material for soft tissue repair and augmentation.

More recently, with the introduction of Fibrel (manufactured by Serono Laboratories, Inc., Randolph, Mass.) in 1987, there was a new type of soft tissue material which could stimulate connective tissue production rather than inertly fill space. Fibrel is a commercial form of a combination of collagen, gelatin, and epsilon aminocaproic acid which, most likely, acts like a reservoir for growth factors which are in high concentrations in coagulum around the collagen base. Early studies suggest that volume maintenance is prolonged but permanency is still not achieved. Furthermore, the inherent risks of bovine collagen, as previously mentioned, remain.

Thus, improved methodologies and different biologic approaches continue to be warranted in the search for materials to effect both hard and soft connective tissue repair and augmentation.

A series of observations provide a background for the use of charged beads for their osteogenic potential. First, demineralized bone powder without its surface charge lacks osteoinductive potential. Second, electric fields are helpful in fostering new bone formation. Third, some tested materials, e.g., hydroxyapatite and titanium, are effective in stimulating bone growth when presented in bead form. In the instant invention, it has been demonstrated that biodegradable beads with chemically induced surface charges promote not only bone or hard connective tissue formation, but soft connective tissue formation as well. The term "soft connective tissue" is generally defined as soft tissue found throughout the body, serving to bind together and support other tissues and organs, and includes, but is not limited to, dermis, adipose, and connective, muscular, and tendonous tissue. The result appears to be dependent upon the charge in each respective application as well as on the recipient site.

The beads of the instant invention are not so important for their particular composition as they are for their physical qualities with regard to chemically charged surfaces created by functional groups. The beads of the instant invention generally are biodegradable organic compounds routinely used as ion exchangers common in many biochemical applications.

Beads with a negative surface charge stimulate formation of hard connective tissue, e.g., bone, within long bones and foster bony repair of defects in parietal bones and in mandibular rami. Beads with positively charged surfaces engender formation of large quantities of dense soft connective tissue when implanted into defects in the cranium or when used as an onlay on the nasal bone surface.

The mechanism(s) by which the charged beads cause bone or soft connective tissue to be generated is at present unclear. It may be that the charged environment alters cell surface moieties so as to permit the expression of osteogenic or fibroblastic potential. Alternatively, it may be that because of their charged surfaces, specific factors bind to the beads and thus effect formation of new bone or fibrous connective tissue.

The ion exchangers, or beads, of the instant invention are readily available and could be linked to a variety of materials. Such materials could be inert where the ion exchanger could exert its effect independently; others could have bone inductive ability (e.g., demineralized bone) where the ion exchanger could augment or facilitate that ability.

Thus, this invention envisions organic, biodegradable substrates, with chemically-induced surface charges, in the form of beads or in different configurations, being used as: 1) material to be implanted into bony and non-bony defects to facilitate and expedite repair; or 2) material of an onlay or inlay to promote augmentation of bone or connective tissue mass; or 3) as material to function as a carrier for growth promoting factors. In general, the material may be used on bony defects caused by trauma, non-union fracture, malignancy and malformation either congenital or caused by aging and on non bony defects or soft connective tissue caused by trauma, malignancy, malformation either congenital or caused by aging.

The above features are objects of this invention. Further objects will appear in the detailed description which follows and will be otherwise apparent to those skilled in the art.

The biodegradable beads in the instant invention are ideally those biodegradable carbohydrate compounds employed as ion exchangers, which are produced by attaching functional groups onto a dextran matrix. A typical ion exchanger bead envisioned in the instant invention is Sephadex, a cross-linked dextran manufactured by Pharmacia Fine Chemicals. These beads with varying positive or negative functional groups are of a spherical gel nature with rigidity and good flow properties. The Sephadex ion exchangers are insoluble in solvents. They are stable in water, salt solutions, organic solvents, alkaline and weakly acidic solutions. In strongly acidic solutions below pH 2 hydrolysis of glycoside linkages may occur. The particle size is in the 40 to 120 micron range (120-325 mesh). They are produced by attaching functional groups to glucose units in the dextran matrix by stable ether linkages. Ion exchangers based on Sephadex G-25, an uncharged dextran, are tightly cross-linked and have minimal swelling and great rigidity. Because Sephadex is bead formed, it is easy to pack and administer to the site.

Different functional groups may be substituted onto the ion exchanger resulting in different degrees of anionic and cationic exchange capability. For example, two functional groups that are substituted onto Sephadex are diethylaminoethyl (DEAE) and carboxymethyl (CM) which result in a weakly basic anion exchanger and weakly acidic cation exchanger, respectively. DEAE Sephadex charged beads, therefore, have a positive surface charge and CM beads have a negative surface charge. The Sephadex beads are desirably moistened in a biologically compatible solution to create an easy to administer paste or slurry. Alternatively, Tyrode's solution, i.e., a standard balanced salt solution, is employed in the amount of 5 ml to 1 cc of UV-sterilized beads to provide a suspension which may be administered through a 22 gauge needle.

Exemplary of other chemically induced charged ion exchangers that are biodegradable are the Sepharose and Sephacel beads of Pharmacia Fine Chemicals. The Sepharose beads may be provided with the diethylaminoethyl (DEAE) and carboxymethyl (CM) functional groups while the Sephacel beads may be provided with the first named functional groups. The Sepharose beads are derived from agarose while the Sephacel beads are derived from cellulose.

(Krukowski, M. et al., J. Bone Min. Res. 2 (Suppl. 1): Abstr. #278, 1987); Krukowski, M. et al., J. Bone Min. Res. 3: 165, 1988) Three types of charged beads (Sephadex: DEAE, positive surface charge; CM, negative surface charge; G-25, uncharged) were injected into femora of sixteen day-old chickens. Beads with a positive surface charge were found: after three days, surrounded by multinucleated giant cells; by four days, with patches of bead-associated new bone along with giant cells; and after one week, with few giant cells but now surrounded by large quantities of new intramedullary bone forming an extensive bead-bone lattice which at times almost obliterated the marrow cavity. No bead-generated new bone was seen with negatively charged or uncharged beads at one week.

Chick femora have been examined (two birds with each bead type: positive surface charge, negative surface charge, or uncharged) one month after intraosseous injection.

The uncharged beads when found lodged near the wall of the bone were occasionally seen with small quantities of new intramedullary bone. In contrast, beads away from bone were found in patches of condensed marrow with no evidence of any cell reaction, similar to observations with these beads after one week.

In the case of positively charged beads (the bead type responsible for the impressive osteogenic response at one week), after one month, and after serially sectioning completely through all specimens, no evidence of intramedullary bone could be found nor were any beads or bead fragments visible anywhere in any section. Since in all other injected bones, at all other time points, beads, in greater or lesser amounts, could be visualized within the marrow cavity, it seemed unlikely that, of all the treated birds, only two at one month after injection of the positively charged beads would represent unsuccessful injections. It was concluded that the observations with positively charged beads at one week were transitory and that after a period of one month the advantages were not present.

It is suggested that the positively charged beads invoke a remodeling phenomenon beginning with resorption (large numbers of giant cells) at three days, new bone and giant cells at four days, and evidence only of the formative phase at day seven. It was speculated as to: whether another resorptive phase would follow; whether the beads would remain intact; and whether the intramedullary bone would persist. The findings in birds at one month, coupled with the speculations, along with observations in rats (see below) where positively charged beads were enveloped by multinucleated giant cells which appeared to be ingesting the beads, support a wave of resorption which degraded such beads and simultaneously resorbed the positively charged bead-associated intramedullary bone.

In contrast, beads with negative surface charges, after one month, were found sequestered in limited areas of the marrow cavity. Each bead, individually, was seen surrounded by bone, and the resulting bead-bone mass was encapsulated by a solid bony perimeter.

The three differently charged bead types were injected intrafemorally into rats (two with each bead type) and examined one month later (Krukowski, M. et al., Trans. O. R. S. 13:49, 1988). With uncharged beads, a bead-tissue lattice occupied large portions of the marrow cavity. Giant cells were not evident, the beads appeared intact, and the tissue component was dense, soft connective tissue. With negatively charged (CM) beads, giant cells were lacking, the beads, like the uncharged ones, were intact, but were seen enveloped by large quantities of bone resulting in a bead-bone mass that practically filled the entire marrow space. The bead-associated bone was viable, well-vascularized, and rich in osteocytes.

In contrast, the positively charged beads, those responsible for the osteogenic response in birds at one week, were seen in rats, at one month, to be associated with multinucleated giant cells. Some of the beads had irregular contours and bead fragments were seen in the cytoplasm of some of the giant cells, suggesting the multinucleated cells were actively ingesting the beads. Such ongoing destruction of this bead type in rats, at one month, is in line with their absence in one-month bird specimens. Such destruction or biodegradation by multinucleated giant cells is akin to removal of unwanted foreign materials by foreign body giant cells.

Findings following intrafemoral injection of beads in birds and rats are summarized in Table I. What is apparent is the agreement in both at one month. For positively charged beads, the beads are either gone or in the process of being degraded. With negatively charged or uncharged beads, the beads persist intact and only with the negatively charged beads is there associated bone in both vertebrates.

In preliminary trials, cranial and mandibular defects were packed with positively charged, negatively charged, and uncharged beads. Contralateral defects were left unfilled. One month later, the animals were sacrificed and bones with defects and/or beads were photographed, fixed and processed for histology.

Grossly, beadless defects, and those with uncharged beads, showed limited bone growth at the perimeter but otherwise were occupied by non-mineralized connective tissue. Defects with positively charged beads also contained soft connective tissue but lacked any new peripheral bone. In contrast, defects with negatively charged beads achieved partial bony closure in the cranium and full closure in the mandible.

Microscopically, unfilled defects, or those packed with uncharged beads, contained relatively loose connective tissue with some evidence of peripheral new bone ingrowth. In contrast, defects with positively charged beads were filled with dense, highly cellular soft connective tissue and had sharply defined margins devoid of new bone suggesting the beads had an inhibitory effect on bone ingrowth but had a stimulatory effect on fibroblastic activity. In addition, numbers of giant cells were visible with these positively charged beads.

Histological examination of nasal bone onlays gave results similar to those with cranial defects. Only with negatively charged beads was there evidence of new bone formation and beads that had been put in contact with the nasal bone surface were found, at one month, enveloped by new bone resulting in a sizeable bead-bone layer above the original bone. Also, onlays of positively charged beads resulted, at one month, in an impressive composite layer of dense connective tissue and beads.

The formation of large quantities of connective tissue in the craniofacial skeleton or in soft tissue sites, led to examination of surface charge in an animal model of wound repair.

Paired dorsal skin incisions were created in rats and 0.1 ml suspension of positively charged, negatively charged or uncharged sterile Sephadex beads was applied to one wound; the contralateral wound served as control. Ten days later breaking strengths of three strips from each wound were measured with a tensometer. Data (means.+-.SEM) were analyzed by Student's paired t test and ANOVA. Three additional strips from each wound were fixed and prepared for histology.

Wounds with positively charged beads were significantly stronger than control wounds (p<0.001). These wounds were also significantly stronger than other bead-treated wounds (p<0.01). Repeat experiments with positively charged beads showed a 153% increase in strength compared to controls. Histologically, wounds treated with positively charged beads had a collagen-rich connective tissue matrix with more fibroblasts present. These wounds also contained prominent bead-associated multinucleated giant cells. It was concluded that beads with positive surface charges significantly enhance healing and that the presence of bead-associated giant cells suggest a role for the positive charge in macrophage recruitment, an early event in wound repair.

Fifteen male Spraque-Dawley rats, weighing greater than 300 gms., were used as the experimental model. All animals were weighed and recorded prior to surgery. Anesthesia was obtained with IM nembutal (7 mg/kg). Under aseptic conditions, tissue was harvested from the right inguinal fat pad and reduced to a particulate consistency by sectioning into small pieces with a scissors and then being passed repeatedly through a 3 cc syringe attached to a 16 gauge needle until a gelatinous consistency was obtained. The facial recipient site (chosen due to a lack of adipose tissue between the dermis and masseter muscle) was then exposed bilaterally through cutaneous incisions at the inferior border of the mandible. A skin flap was elevated superiorly exposing the underlying muscle.

The harvested fat tissue was then separated into two equal grafts as determined by their weights on an electronic scale (to 0.01 mgs.) A free fat graft was then placed in the right-sided facial pocket. The left-sided facial recipient site received the other half of the free fat tissue (bioactive fat graft) which has been altered by the addition of positively-charged beads (Sephadex-DEAE, 50-100 um in size) which had been prosoaked for 1 minute in a prepared 1 cc solution of bFGF, i.e. basic fibroblast growth factor (10 ug in 1 cc normal saline, mouse pituitary-derived). The amount of beads added to the bioactive grafts was 10% of their original weight by electronic weight determination. The facial wounds were then closed with resorbable suture. No intra- or postoperative antibiotic was given.

After 120 days, the rats were then euthenized by CO.sub.2 asphyxiation and then weighed. The facial sites were reopened and the grafts were easily separated from the overlying skin and underlying muscle. They were then weighed and compared to that at the time of their placement. The % weight change of the grafts was then recorded and compared to the % weight gain of the animal during the study period. The grafts were then placed in formalin, embedded in paraffin, sectioned at 100 um, and stained with trichrome. They were then examined under light microscopy for adipocyte morphology.

All animals tolerated the surgical procedures without incidence. No postoperative infections or facial recipient site complications occurred. Preoperative animal weights averaged 364 gms. compared to a postoperative average weight of 539 gms., representing a 48.1% weight gain of the animal group over the 4 month study period.

Comparison of the two types of fat grafts revealed marked differences in their macroscopic appearance. The free fat grafts (right-side) were smaller in size and more adherent to the overlying dermis. their color was predominantly white with few scattered areas of yellowish fat, suggestive of significant fibrosis of the graft. As a group, they averaged preoperative weights of 0.375 gms. compared to 0.303 gms. postoperatively, a decline of 19.2% in graft weight over the study period. When compared to the increase in the body mass of the animals (+48.1%), the free fat graft loss was actually greater, averaging -45.4%.

The bioactive fat grafts (left-side) appeared larger in size with a greater amount of yellow color. Their exterior surface appeared rough or dimpled in spots, indicating those areas of the graft where the cellulose beads predominated. As a group, they averaged preoperative weight of 0.392 gms. compared to 0.559 gms. postoperatively, an average increase of 42.3%. When compared to the weight gain of the animals, the bioactive fat graft weight gain was very comparable.

Histologically, graft differences were likewise seen. The free fat grafts were marked by considerable alteration of adipocyte morphology with frequent areas of cell wall disruption and large `empty` areas. Most vessels were located at the periphery of the graft. This histological appearance contrasted sharply with that of the normal inguinal fat pad. The bioactive fat grafts exhibited mostly intact adipocyte cells throughout the graft, particularly in areas near the cellulose beads. The few areas of disrupted cells observed were always considerably removed from the beads. Adipocyte cellular size varied considerably, from very small cells to more normally-sized ones. These smaller adipocyte cells, interspersed amongst the larger ones, were predominantly seen in close proximity to the beaded areas. Collagen formation was also noted in many areas between the fat cells as evidenced by trichrome staining.

The utilization of bFGF, a know potent mitogen for mesodermally-derived cells, is a logical choice for a potential biochemical stimulus to the graft due to the embryologic origin of fat tissue. However, a delivery vehicle for the agent was felt to be mandatory to allow a sufficient concentration over time to be retained within the graft. Positively-charged cellulose beads were chosen due to their hydrophilicity and profound connective tissue response to their positive surface charge. Whether either one of these additions to the bioactive fat grafts, or both synergistically, accounts for their improved results is presently unknown. The histologic finding of many small adipocytes or adipocyte "buds" amongst the larger adipocytes in the bioactive grafts may provide some support for preadipocyte cell differentiation.

In summary, negatively charged beads stimulate bone or hard connective tissue formation when injected into marrow cavities of long bones or when used as an onlay on the nasal bone surface. Similarly, the negatively charged beads foster bony repair of defects in the cranium and in mandibular rami. In contrast, positively charged beads stimulate production of dense fibrous connective or soft tissue evident in cranial defects and prominent in nasal bone onlays.

There may be employed a flat sheet containing the beads or various materials in different physical forms, i.e., a mesh, coated with functional groups such as those coating the beads. Such material may be cut into appropriate shapes and sizes to overlay or fill defects to provide a significant clinical value for augmentation and repair of bone and fibrous connective tissues. In addition, such materials may also serve as a vehicle to introduce growth factors or other mitogenic proteins to such hard and soft tissue sites.

The functional groups providing the chemically induced positive or negative charges may be incorporated as a surface coating in alloplastic devices or materials to provide the hard or soft tissue growth formation capability and may be by way of ingrowth into the aforementioned alloplastic devices, prosthesis or materials. Such alloplastic devices and materials may be joint replacements, hip sockets and bone plate fixation material, for example, using biocompatible materials exemplified by metals, bioceramics and plastics. Such materials may be biocompatible artificial materials that replace or augment natural hard or soft connective tissue exemplified by titanium, vitallium (an alloy of titanium and stainless steel), hydroxy apatite and plastics such as methyl methacrylate and Proplast and Cilastic.

Various changes and modifications may be made within this invention as will be apparent to those skilled in the art. Such changes and modifications are within the scope and teaching of this invention as defined in the claims appended hereto.