Histological evaluation of tissue regeneration using biodegradable scaffold seeded by autologous cells for tubular/hollow organ applications.

The accurate interpretation of histological outcomes is a critical endpoint in preclinical studies. Thus, the toxicologic pathologist plays a vital role in conducting a comprehensive microscopic evaluation that would ultimately help defining the safety and functionality in Tissue Engineering/Regenerative Medicinal (TERM) products. In spite of many advances in regenerative medicine, there are no specific guidelines for the histological assessment of TERM products (Jayo et al. Toxicol Pathol 36:92-96, 2008). In this chapter, we describe the methodology designed to facilitate the detection of structural and functional changes when conducting a histological assessment including tissue collection (test article extraction), sampling, processing and fixation, special stains, statistical analysis, and morphometry.

A population of selected renal cells augments renal function and extends survival in the ZSF1 model of progressive diabetic nephropathy.

New treatment paradigms that slow or reverse progression of chronic kidney disease (CKD) are needed to relieve significant patient and healthcare burdens. We have shown that a population of selected renal cells (SRCs) stabilized disease progression in a mass reduction model of CKD. Here, we further define the cellular composition of SRCs and apply this novel therapeutic approach to the ZSF1 rat, a model of severe progressive nephropathy secondary to diabetes, obesity, dyslipidemia, and hypertension. Injection of syngeneic SRCs into the ZSF1 renal cortex elicited a regenerative response that significantly improved survival and stabilized disease progression to renal structure and function beyond 1 year posttreatment. Functional improvements included normalization of multiple nephron structures and functions including glomerular filtration, tubular protein handling, electrolyte balance, and the ability to concentrate urine. Improvements to blood pressure, including reduced levels of circulating renin, were also observed. These functional improvements following SRC treatment were accompanied by significant reductions in glomerular sclerosis, tubular degeneration, and interstitial inflammation and fibrosis. Collectively, these data support the utility of a novel renal cell-based approach for slowing renal disease progression associated with diabetic nephropathy in the setting of metabolic syndrome, one of the most common causes of end-stage renal disease.

Extension of bladder-based organ regeneration platform for tissue engineering of esophagus.

Recent successes in regenerative medicine and tissue engineering of bladder and bladder-like neo-organs have leveraged regenerative constructs composed of a biodegradable scaffold seeded with a population of smooth muscle cells. We have shown that such smooth muscle cells are isolatable from adipose and other sources alternate to the primary organ. We hypothesize that this regenerative platform is not limited to regeneration of bladder and bladder-like neo-organs, but rather represents a foundational technology platform broadly applicable for regeneration of laminarly organized hollow organs. Using esophagus as an illustrative example in support of this hypothesis, we demonstrate that patch constructs composed of adipose-derived smooth muscle cells seeded on a biodegradable matrix catalyze complete regeneration of the esophageal wall in a rodent model of esophageal injury. By implication, such regenerative constructs may potentially be used to mediate the regeneration of any laminarly organized tubular organ.

Regeneration of native-like neo-urinary tissue from nonbladder cell sources.

Urinary pathology requiring urinary diversion, partial or full bladder replacement, is a significant clinical problem affecting ~14,000 individuals annually in the United States alone. The use of gastrointestinal tissue for urinary diversion or bladder reconstruction/replacement surgeries is frequently associated with complications. To try and alleviate or reduce the frequency of these complications, tissue engineering and regenerative medicine strategies have been developed using bio-absorbable materials seeded with cells derived from the bladder. However, bladder-sourced cells may not always be suitable for such applications, especially in patients with bladder cancer. In this study, we describe the isolation and characterization of smooth muscle cells (SMCs) from porcine adipose and peripheral blood that are phenotypically and functionally indistinguishable from bladder-derived SMCs. In a preclinical Good Laboratory Practice study, we demonstrate that autologous adipose- and peripheral blood-derived SMCs may be used to seed synthetic, biodegradable tubular scaffold structures and that implantation of these seeded scaffolds into a porcine cystectomy model leads to successful de novo regeneration of a tubular neo-organ composed of urinary-like neo-tissue that is histologically identical to native bladder. The ability to create urologic structures de novo from scaffolds seeded by autologous adipose- or peripheral blood-derived SMCs will greatly facilitate the translation of urologic tissue engineering technologies into clinical practice.

Regeneration of rodent small intestine tissue following implantation of scaffolds seeded with a novel source of smooth muscle cells.

AIMS: To apply an organ regeneration platform technology of autologous smooth muscle cell/biomaterial combination products, previously demonstrated to be successful for urinary tissue regeneration, to the regeneration of the small intestine. MATERIALS & METHODS: Patch and tubular constructs were implanted in rodent small intestines and histologically evaluated over a time course for evidence of regeneration of the laminarly organized neo-mucosa and muscle layers. RESULTS: Laminarly organized neo-mucosa and muscle layer bundles are demonstrated as early as 8 weeks postimplantation. CONCLUSION: An organ regeneration technology platform of autologous smooth muscle cell/biomaterial combination products can be extended to the regeneration of the small intestine.

Functional evaluation of primary renal cell/biomaterial neo-kidney augment prototypes for renal tissue engineering.

Development of a tissue-engineered neo-kidney augment (NKA) requires evaluation of defined, therapeutically relevant cell and cell/biomaterial composites (NKA constructs) for regenerative potential in mammalian kidney. Previous work identified primary renal cell populations that extended survival and improved renal function in a rodent model of chronic kidney disease (CKD). This study extends that work toward the goal of developing NKA by (i) screening in vivo inflammatory and fibrotic responses to acellular biomaterials delivered to healthy rodent renal parenchyma, (ii) evaluating the functionality of renal cell/biomaterial combinations in vitro, (iii) generating NKA constructs by combining therapeutically relevant cell populations with biocompatible biomaterial, and (iv) evaluating in vivo neokidney tissue development in response to NKA constructs delivered to healthy rodent renal parenchyma. Gelatin and hyaluronic acid (HA)-based hydrogels elicited the least inflammatory and fibrotic responses in renal parenchyma relative to polycaprolactone (PCL) and poly(lactic-co-glycolic acid) (PLGA) beads or particles and were associated with neovascularization and cellular infiltration by 4 weeks postimplantation. Renal cell populations seeded onto gelatin or HA-based hydrogels were viable and maintained a tubular epithelial functional phenotype during an in vitro maturation of 3 days as measured by transcriptomic, proteomic, secretomic, and confocal immunofluorescence assays. In vivo delivery of cell-seeded NKA constructs (bioactive renal cells + gelatin hydrogels) to healthy rodent renal parenchyma elicited neokidney tissue formation at 1 week postimplantation. To investigate a potential mechanism by which NKA constructs could impact a disease state, the effect of conditioned media on TGF-ß signaling pathways related to tubulo-interstitial fibrosis associated with CKD progression was evaluated. Conditioned medium was observed to attenuate TGF-ß-induced epithelial-mesenchymal transition (EMT) in vitro in a human proximal tubular cell line (HK2).

Isolation, characterization, and expansion methods for defined primary renal cell populations from rodent, canine, and human normal and diseased kidneys.

Chronic kidney disease (CKD) is a global health problem; the growing gap between the number of patients awaiting transplant and organs actually transplanted highlights the need for new treatments to restore renal function. Regenerative medicine is a promising approach from which treatments for organ-level disorders (e.g., neurogenic bladder) have emerged and translated to clinics. Regenerative templates, composed of biodegradable material and autologous cells, isolated and expanded ex vivo, stimulate native-like organ tissue regeneration after implantation. A critical step for extending this strategy from bladder to kidney is the ability to isolate, characterize, and expand functional renal cells with therapeutic potential from diseased tissue. In this study, we developed methods that yield distinct subpopulations of primary kidney cells that are compatible with process development and scale-up. These methods were translated to rodent, large mammal, and human kidneys, and then to rodent and human tissues with advanced CKD. Comparative in vitro studies demonstrated that phenotype and key functional attributes were retained consistently in ex vivo cultures regardless of species or disease state, suggesting that autologous sourcing of cells that contribute to in situ kidney regeneration after injury is feasible, even with biopsies from patients with advanced CKD.

Tubular cell-enriched subpopulation of primary renal cells improves survival and augments kidney function in rodent model of chronic kidney disease.

Established chronic kidney disease (CKD) may be identified by severely impaired renal filtration that ultimately leads to the need for dialysis or kidney transplant. Dialysis addresses only some of the sequelae of CKD, and a significant gap persists between patients needing transplant and available organs, providing impetus for development of new CKD treatment modalities. Some postulate that CKD develops from a progressive imbalance between tissue damage and the kidney's intrinsic repair and regeneration processes. In this study we evaluated the effect of kidney cells, delivered orthotopically by intraparenchymal injection to rodents 4-7 wk after CKD was established by two-step 5/6 renal mass reduction (NX), on the regeneration of kidney function and architecture as assessed by physiological, tissue, and molecular markers. A proof of concept for the model, cell delivery, and systemic effect was demonstrated with a heterogeneous population of renal cells (UNFX) that contained cells from all major compartments of the kidney. Tubular cells are known contributors to kidney regeneration in situ following acute injury. Initially tested as a control, a tubular cell-enriched subpopulation of UNFX (B2) surprisingly outperformed UNFX. Two independent studies (3 and 6 mo in duration) with B2 confirmed that B2 significantly extended survival and improved renal filtration (serum creatinine and blood urea nitrogen). The specificity of B2 effects was verified by direct comparison to cell-free vehicle controls and an equivalent dose of non-B2 cells. Quantitative histological evaluation of kidneys at 6 mo after treatment confirmed that B2 treatment reduced severity of kidney tissue pathology. Treatment-associated reduction of transforming growth factor (TGF)-ß1, plasminogen activator inhibitor (PAI)-1, and fibronectin (FN) provided evidence that B2 cells attenuated canonical pathways of profibrotic extracellular matrix production.

Black cohosh increases metastatic mammary cancer in transgenic mice expressing c-erbB2.

Black cohosh is an herbal extract that is often used as an alternative to estrogen-based replacement therapies to treat hot flushes that frequently accompany the transition to menopause. Although cancer-free women as well as breast cancer patients and survivors use black cohosh to relieve vasomotor symptoms, there is limited information on its potential to influence breast cancer development or progression. Therefore, in this study, the effects of black cohosh on mammary tumorigenesis were investigated in the MMTV-neu mouse model due to its similarities to HER2(+) breast cancer, including stochastic development of mammary tumors, which frequently progress to metastatic disease. Using an adjusted dose for the mice to correlate to the recommended dose in women (40 mg/d), no differences were detected in the incidence or onset of mammary tumors in black cohosh-treated versus control females. The lack of effect on mammary tumor development suggests that black cohosh would not influence breast cancer risk if given to women before tumor formation. In contrast, black cohosh significantly increased the incidence of lung metastases in tumor-bearing animals compared with mice fed the isoflavone-free control diet. Additional studies will be needed to correlate these findings to women taking different black cohosh products at various times during breast cancer development; however, these results suggest caution for women using black cohosh, especially for extended periods of time. As metastatic progression is linked to patient survival, these data stress the importance of investigating how women's therapies influence all stages of mammary tumorigenesis, particularly for assessing their safety.

Long-term durability, tissue regeneration and neo-organ growth during skeletal maturation with a neo-bladder augmentation construct.

AIMS: To comparatively evaluate bladder regeneration following 80% cystectomy and augmentation using a synthetic biopolymer with autologous urothelial and smooth muscle cells (autologous neo-bladder augmentation construct [construct]) or autotransplantation of native bladder (reimplanted native urinary bladder [reimplant]) in canines. MATERIALS & METHODS: Voiding function, urodynamic assessment and neo-organ capacity-to-body-weight ratio (C:BW) were assessed longitudinally for a total of 24 months following trigone-sparing augmentation cystoplasty in juvenile canines. RESULTS: Within 30 days postimplantation, hematology and urinalysis returned to baseline. Constructs and reimplants yielded neo-organs with statistically equivalent urodynamics and histology. Linear regression analysis of C:BW showed that constructs regained baseline slope and continued to adapt with animal growth. CONCLUSIONS: Constructs and reimplants regained and maintained native bladder histology by 3 months, capacity at 3-6 months and compliance by 12-24 months. Furthermore, construct C:BW demonstrated the ability of regenerated bladder to respond to growth regulation.

Early cellular and stromal responses in regeneration versus repair of a mammalian bladder using autologous cell and biodegradable scaffold technologies.

PURPOSE: Internal organ regeneration holds promise for changing medical technology and decreasing organ shortages. Current medical treatment for internal organ failure is largely limited to organ transplantation. A construct composed of synthetic biopolymer with autologous cells has shown long-term clinical benefit in patients undergoing augmentation cystoplasty. However, to our knowledge early cellular and stromal events during bladder regeneration have not been elucidated. MATERIALS AND METHODS: In situ cellular responses to 2 biopolymer implants, including a poly(lactic-co-glycolic acid) (Sigma-Aldrich) based biodegradable mesh scaffold with autologous urothelial and smooth muscle cells (construct) and a poly(lactic-co-glycolic acid) based biodegradable mesh scaffold alone without cells (scaffold), were compared in a canine model of augmentation cystoplasty. Healing events were correlated with urodynamic assessments. RESULTS: Construct implants regenerated baseline urodynamics as early as 4 months after implantation. Urodynamics following scaffold implantation failed to return to baseline by study termination at 9 months. Functional differences elicited by construct and scaffold implants correlated with structural differences in the neotissues. Construct stroma had greater vascularization with gently folded, interwoven connective tissue elements. Scaffold stroma was dense, haphazardly organized connective tissue. Urothelium regenerated in response to construct and scaffold implantation. However, only construct had normal stroma, well developed detrusor and abundant alpha-smooth muscle actin (Vector Laboratories, Burlingame, California) cell staining at early time points, leading to a structurally and functionally complete regenerated bladder wall at 9 months. CONCLUSIONS: Early cellular and stromal events distinguish healing processes that lead to bladder wall regeneration or repair. Construct implants containing cells elicit early healing processes that culminate with the regeneration of complete mucosal and muscular components, whereas the response to scaffold implantation is consistent with reparative healing, that is with mucosal growth but incomplete tissue layer development.

Tissue engineering and regenerative medicine: role of toxicologic pathologists for an emerging medical technology.

Tissue Engineering Regenerative Medical (TERM) products are a new technology currently in human clinical testing for a variety of unmet medical needs involving tissue and organ dysfunction and failure. Safety evaluation of TERM products overlaps 3 established product paradigms: pharmaceuticals (biologically active substances), transplantation (cells or tissue), and devices (biomaterials). As TERM products recapitulate organ or tissue structure and function with unique biological activity and characteristics, they require new preclinical paradigms to bring TERM products through to clinical trials. Establishing TERM-product safety programs requires broad-based knowledge of tissue and organ homeostasis, regenerative biology, and translational medicine to design new preclinical paradigms. Therefore, toxicologic pathologists have a compelling scientific role in evaluating TERM products, characterizing tissue responses, and helping distinguish optimal (regeneration) from deficient or incomplete outcomes indicative of substandard functionality (repair). As new-tissue engineering and regenerative medical technologies develop for tissue and organ regeneration, the toxicologic pathologist will be asked to develop novel testing, reevaluate established toxicologic diagnostic criteria, and reinterpret tissue responses that may extend beyond current standards.

In vitro and in vivo comparison of five biomaterials used for orthopedic soft tissue augmentation.

OBJECTIVE: To compare biomaterials used in orthopedics with respect to in vitro cell viability and cell retention and to in vivo tissue healing and regeneration. ANIMALS: 65 adult female Sprague-Dawley rats and synovium, tendon, meniscus, and bone marrow specimens obtained from 4 adult canine cadavers. PROCEDURES: Synovium, tendon, meniscus, and bone marrow specimens were used to obtain synovial fibroblasts, tendon fibroblasts, meniscal fibrochondrocytes, and bone marrow-derived connective tissue progenitor cells for culture on 5 biomaterials as follows: cross-linked porcine small intestine (CLPSI), non-cross-linked human dermis, cross-linked porcine dermis, non-cross-linked porcine small intestine (NCLPSI), and non-cross-linked fetal bovine dermis. After 1 week of culture, samples were evaluated for cell viability, cell density, and extracellular matrix production. Biomaterials were evaluated in a 1-cm(2) abdominal wall defect in rats. Each biomaterial was subjectively evaluated for handling, suturing, defect fit, and ease of creating the implant at the time of surgery, then grossly and histologically 6 and 12 weeks after surgery. RESULTS: All biomaterials allowed for retention of viable cells in culture; however, CLPSI and NCLPSI were consistently superior in terms of cell viability and cell retention. Cell infiltration for NCLPSI was superior to other biomaterials. The NCLPSI appeared to be replaced with regenerative tissue most rapidly in vivo and scored highest in all subjective evaluations of ease of use. CONCLUSIONS AND CLINICAL RELEVANCE: These data suggested that NCLPSI and CLPSI have favorable properties for further investigation of clinical application in orthopedic tissue engineering.

Evaluation of the carcinogenic potential of clofibrate in the FVB/Tg.AC mouse after oral administration--part I.

This study was conducted as part of the International Life Sciences Institute (ILSI) program to evaluate the carcinogenic potential of clofibrate, a nongenotoxic, peroxisome proliferator-activated receptor (PPAR) alpha agonist following oral administration to Tg.AC (transgenic) and wild-type FVB (nontransgenic) mice for a minimum for 6 months. Clofibrate was well tolerated at doses up to 500 (males) and 650 (females) mg/kg/day. Oral administration of clofibrate to Tg.AC or FVB (wild-type) male and female mice for 6 months did not result in the increased formation of neoplastic lesions. Epithelial hyperplasia in the urinary bladder (Tg.AC and FVB) and prostate gland (Tg.AC only), and interstitial-cell hyperplasia in the testes (Tg.AC) were noted at 500 mg/kg/day. Non-neoplastic nonproliferative findings included hepatic hypertrophy and hematopoietic changes (myeloid hyperplasia, myelodysplasia, lymphoid depletion, and erythropoiesis) in Tg.AC and FVB mice of both sexes; reproductive (cystic degeneration and dilatation, hypospermia, spermatocele, dilated inspissated protein) and urogenital (tubular-cell hypertrophy, degenerative/regenerative nephropathy, necrosis/fibrosis) changes in Tg.AC and FVB male mice; congestion in the lung in male Tg.AC mice; gall bladder dilatation in female Tg.AC mice; and adrenal (intracellular lipofuscinosis and atrophy) and heart (eosinophillic myofibers) findings in Tg.AC mice of both sexes and in female FVB mice. The results of this study indicate that the clofibrate is not carcinogenic when administered to Tg.AC mice by oral gavage for 6 months at doses up to 500 (males) and 650 (females) mg/kg/day, which did produce liver hypertrophy.

Evaluation of the carcinogenic potential of clofibrate in the FVB/Tg.AC mouse after dermal application--part II.

This study was conducted as part of the International Life Sciences Institute (ILSI) Alternatives to Carcinogenicity Testing program and evaluated the carcinogenic potential of clofibrate, a nongenotoxic, peroxisome proliferator-activated receptor (PPAR) alpha agonist following dermal application to transgenic Tg.AC and nontransgenic FVB mice for a minimum of 26 weeks. Clofibrate doses of 12, 28, or 36 mg/200 microl/day were used. Positive controls for papilloma formation were benzene (174.8 mg/200 microl), and 12-o-tetradecanoylphorbol-13-acetate (TPA [0.00250 mg/200 microl]). Clofibrate was tolerated at doses up to 36 mg/200 microl. In Tg.AC mice, clofibrate produced a dose-related increase in the incidence of mice with cutaneous papillomas; and dose-related decreases in mean time to first tumor, mean multiplicity of tumors per mouse, and mean weeks to maximal yield, as well as numerous nonneoplastic microscopic lesions in the liver, kidney, spleen, and skin. Benzene and TPA induced both neoplastic and/or non-neoplastic proliferative lesions in Tg.AC mice. Clofibrate did not increase the incidence or multiplicity of papillomas, or any other tumors in FVB mice. These data show that the Tg.AC dermal model has increased sensitivity in detecting skin papillomas caused by the nongenotoxic rodent carcinogen, clofibrate, compared to wild type FVB mice, at systemic exposures that are 3x higher than the systemic exposure observed in humans taking clofibrate (AUC = 1100 microg.h/ml) at the recommended maximum therapeutic dose of 500 mg. In addition, this study supports the proposed concept that Tg.AC model may detect compounds with nongenotoxic carcinogenic potential in a shorter timeframe than conventional mouse carcinogenicity bioassays.

An Uncaria tomentosa (cat's claw) extract protects mice against ozone-induced lung inflammation.

Ozone (O(3)) inhalation has been associated with respiratory tract inflammation and lung functional alterations. To characterize the O(3)-induced lung inflammation in mice, the effective dose and exposure time were determined. Total protein levels of bronchoalveolar lavage fluid (BALF), cytological smears, and lung histopathology and morphometry were used to assess and measure the degree of pulmonary inflammation in the mouse model. Ozone inhalation caused acute pneumonitis that was characterized by a high number of infiltrating neutrophils (PMNs) immediately after exposure and increased levels of protein in BALF in mice killed 8h after O(3) exposure. The anti-inflammatory properties of Uncaria tomentosa (UT) have been documented previously. To evaluate the anti-inflammatory effects of UT, male mice were given an UT extract for 8 days, exposed to O(3), and killed 0 or 8 h after O(3) exposure. When compared to untreated controls, UT-treated mice had significantly (p < 0.05) lower levels of protein in BALF, lower degree of epithelial necrosis, higher number of intact epithelial cell nuclei in bronchial wall, and decreased number of PMNs in the bronchiolar lumen. Therefore, UT extract appeared to prevent O(3)-induced respiratory inflammation in male mice.

Soy phytoestrogens do not prevent bone loss in postmenopausal monkeys.

The putative skeletal effects of dietary soy phytoestrogens (SPE) were examined in comparison with those of conjugated equine estrogens (CEE; Premarin) in a 3-yr longitudinal study in ovariectomized female monkeys. Controls received alcohol-extracted soy protein with low phytoestrogen content, and treatment groups received either CEE (admixed into the control diet) or unextracted soy protein isolate containing SPE. The acknowledged bone protective effect of CEE was reflected by higher bone mass (by dual energy x-ray absorptiometry) and lower bone turnover marker levels. In contrast, control and SPE groups lost significant lumbar spine bone mineral content and density and whole body bone mineral content within the first year, resulting in reduced bone mass for both groups compared with CEE (P < 0.0005). No effect of SPE was observed for any bone mass measure (P > 0.44), although transient, estrogen-like effects of SPE on serum alkaline phosphatase, calcium, and C-terminal cross-link of type I collagen were observed at 3 months (P < 0.02). These results suggest that SPE may be poor substitutes for mammalian estrogens in protecting against bone loss resulting from estrogen deficiency.

Effects of boric acid supplementation on bone histomorphometry, metabolism, and biomechanical properties in aged female F-344 rats.

Postmenopausal women may benefit from dietary interventions in order to increase bone strength and prevent fractures. Dietary boron (B) may be beneficial for optimal calcium metabolism and, as a consequence, optimal bone metabolism. The present study evaluated the effects of boron, in the form of boric acid, with or without 17beta-estradiol (E2) supplementation (via subcutaneous implant), in ovariectomized (OVX) aged 13- mo-old F-344 rats. Boric acid was administered by gavage at a subtoxic dose (8.7 mg B/kg/d) for 40 d. Results indicate that serum level of minerals as well as osteocalcin (a marker of bone resorption) are dependent to a greater extent on the hormonal status of the animals than on boron supplementation. Boron treatment increased the E2-induced elevation of urinary calcium and magnesium. Bone mineral density (BMD) of the L5 vertebra and proximal femur was highest in the E2-treated groups; no increase in BMD was conferred by boron treatment. By histomorphometry of the proximal tibial metaphysis, osteoblastic, osteoid, and eroded surfaces were significantly suppressed by E2 treatment, but not by boron treatment. In biomechanical testing of femur and vertebra, neither E2 nor boron treatment significantly increased bone strength. At the levels given, boron alone provided no protection against OVX-induced osteopenia. In addition, combination therapy (B + E2) provided no additional benefits over those of 17beta-estradiol treatment alone in this aged rat model.

Histologic findings with a bioabsorbable anterior cruciate ligament interference screw explant after 2.5 years in vivo.

We retrieved a high-molecular-weight poly-L-lactic) (PLLA) anterior cruciate ligament (ACL) interference screw (Arthrex, Naples, FL) after 30 months in vivo during revision ACL surgery. Gross, histologic, histomorphometric, and molecular weight measurements were carried out on the implant and the surrounding bone. These studies showed a 75% decrease in the molecular weight of the screw, with implant fragmentation and new bone formation adjacent to the screw and graft. Healing of the graft within the bony tunnel with no significant inflammatory reaction had occurred. The clinical implications of these findings are that this implant dissolves slowly, and it was physically present at 30 months in vivo. It is a safe, nonreactive alternative to traditional metal interference screws used for ACL graft fixation. It will eventually be substituted by bone and will eliminate some of the problems associated with metallic devices.