Tendon tissue engineering: progress, challenges, and translation to the clinic.

The tissue engineering field has made great strides in understanding how different aspects of tissue engineered constructs (TECs) and the culture process affect final tendon repair. However, there remain significant challenges in developing strategies that will lead to a clinically effective and commercially successful product. In an effort to increase repair quality, a better understanding of normal development, and how it differs from adult tendon healing, may provide strategies to improve tissue engineering. As tendon tissue engineering continues to improve, the field needs to employ more clinically relevant models of tendon injury such as degenerative tendons. We need to translate successes to larger animal models to begin exploring the clinical implications of our treatments. By advancing the models used to validate our TECs, we can help convince our toughest customer, the surgeon, that our products will be clinically efficacious. As we address these challenges in musculoskeletal tissue engineering, the field still needs to address the commercialization of products developed in the laboratory. TEC commercialization faces numerous challenges because each injury and patient is unique. This review aims to provide tissue engineers with a summary of important issues related to engineering tendon repairs and potential strategies for producing clinically successful products.

Experimental study on low pulse energy processing with femtosecond lasers for glaucoma treatment.

The feasibility of low energy processing in ocular tissues with femtosecond laser sources was investigated in this research. One laser source was a femtosecond amplifier, and the other was a femtosecond oscillator. The amplifier used in this experiment was a CPA-2001 (Clark-MXR, Inc), with 150 fs pulse duration and 1 kHz repetition rate. The femtosecond oscillator (model 900-B Mira) produced a 200 fs pulse duration and a 76 MHz repetition rate. Both these two laser systems operated at 800 nm wavelengths. Firstly, the pulse intensity thresholds in water produced by the two laser sources were compared. The optical breakdown probability analysis shows that the pulse energy threshold achieved by the oscillator was less than 10% of that achieved by the amplifier. Then, the non-linear propagation of the femtosecond pulses in the ocular tissues was studied with the femtosecond oscillator. The results showed a potential for pulse energy processing at the nanojoule level with a femtosecond oscillator in glaucoma treatment.

Age-related changes in the biomechanics of healing patellar tendon.

By 2030, there will be 70 million people in the United States over the age of 65, and by 2050, 22% of the US population will be considered elderly. It is generally believed that injuries in the elderly heal slower and less completely than in adolescents or young adults. To evaluate aging effects on tissue repair a surgical injury was created in the middle third of one patellar tendon in 1- and 4-5-year-old New Zealand White rabbits. The biomechanical properties of the isolated repair tissues and contralateral normal tendon tissues were compared at 6, 12 and 26 weeks post-injury. We hypothesized that repair tissues would exhibit age-related reductions in biomechanical properties at all time intervals of healing, both based on raw data and when normalized to values from contralateral tendons. Repairs from both age groups were similar, with no significant increase in maximum stress, strain at maximum stress, or modulus between 6 and 12 weeks. At 26 weeks, the repairs in the 4-year-old rabbits had higher maximum stress values than repairs in the 1-year-old rabbits (p=0.03). There were no significant differences in the strain at maximum stress or modulus. When repair tissue properties were normalized to values in the contralateral normal tendon, the maximum stress of the patellar tendon repair tissue from the 4 year old was significantly greater than the corresponding value from the 1 year old at the 26 week time point (p=0.04). In conclusion, these findings do not support the presence of age-related declines in the biomechanics of healing tendon.

The effects of orientation, temperature, and displacement magnitude changes on the sonometrics system accuracy.

The accuracy and reliability of a sonomicrometry system (Sonometrics Corporation, Ontario, Canada) was evaluated for its potential use in measuring 3-D in vivo joint kinematics. Distances between different sets of piezoelectric crystals were measured through a salt solution using ultrasound technology. We evaluated crystal-to-crystal distance under simulated in vivo conditions of changing crystal orientation and displacement magnitude. Crystal-to-crystal distance was also evaluated under changing solution temperature, since the crystals may be used at different temperatures. The 2 mm round and peg crystals were accurate to within 0.5mm for 0 through 180 degrees rotations, but the 2mm round suture loop crystals were only reliable at 0 degrees rotation. The speed of sound through a salt solution (and hence the distance between crystals) versus temperature was fit using a second order polynomial, C=1421.1+3.9808T-3.09x10(-2)T2, with an R2 value of 0.9998. The translational error was less than 0.072 mm for crystal displacements of 0.012, 0.2, 1.0, and 5.0 mm. The system was also accurate under dynamic conditions with translational errors that were less than 0.045 mm under 0.65 Hz motion. These results suggest that the Sonometrics crystals possess attributes (translational accuracy and rotational independence) that could provide the basis for a system capable of measuring joint kinematics.

Evaluation of an electromagnetic position tracking device for measuring in vivo, dynamic joint kinematics.

An electromagnetic position tracking device was evaluated to determine its static and dynamic accuracy and reliability for applications related to measuring in vivo joint kinematics. The device detected the position and orientation of small coiled sensors, maintained in an electromagnetic field. System output was measured against known translations or rotations throughout the measurement volume. Average translational errors during static testing were 0.1 +/- 0.04, 0.2 +/- 0.17, and 0.8 +/- 0.81 mm (mean+/-SD) for sensors 50, 300, and 550 mm away from the field generator, respectively. Average rotational errors were 0.4 +/- 0.31 degrees, 0.4 +/- 0.21 degrees, and 0.9 +/- 0.85 degrees (mean +/- SD) for sensors located at the same distances. Since we intended to use this system in an animal walking on a treadmill, we incrementally moved the sensors under various treadmill conditions. The effects of treadmill operation on translational accuracy were found to be negligible. The effects of dynamic motions on sensor-to-sensor distance were also assessed for future data collection in the animal. Sensor-to-sensor distance showed standard deviations of 2.6 mm and a range of 13 mm for the highest frequency tested (0.23 Hz). We conclude that this system is useful for static or slow dynamic motions, but is of limited use for obtaining gait kinematics at higher speeds.

Effects of age on the repair ability of mesenchymal stem cells in rabbit tendon.

Successful tissue engineered repair in the aging adult requires an abundant source of autologous, multipotent mesenchymal stem cells (MSCs). Although the number of bone marrow-derived MSCs declines dramatically with aging, their effectiveness in repair with increasing age has not been studied. We tested the hypothesis that MSCs harvested from geriatric rabbits would not repair patellar tendon defects as well as MSCs harvested from younger adult rabbits. In a novel within-subjects experiment, autologous MSCs were isolated from 1-year old rabbits, culture expanded, and cryogenically preserved. After housing the rabbits for 3 years, MSCs were re-harvested from the 4-year old rabbits and expanded. Five hundred thousand thawed and fresh MSCs were each separately mixed with type I collagen gel (333.3 x 10(3) cells/mg collagen) 24 h before surgery, and the resulting constructs implanted in bilateral full-length central third tendon defects. Twelve weeks post-surgery, the bone-tendon repair-bone units were failed in tension. Intra-animal (paired) comparisons between repair tissue treated with 1-year old MSCs and repair tissue treated with 4-year old MSCs resulted in no significant differences (alpha=0.05) in material properties including maximum stress (10.8 MPa vs. 9.9 MPa; p=0.762), modulus (139.8 MPa vs. 146.2 MPa; p=0.914), and strain energy density (0.52 N mm/mm(3) vs. 0.53 N mm/mm(3); p=0.966). Despite an age-related trend, there were also no significant differences in structural properties including maximum force (62.9 N vs. 27.0 N; p=0.070), stiffness (24.9 N/mm vs. 12.0 N/mm; p=0.111), and strain energy (87.2 N mm vs. 31.4 N mm; p=0.061). A subset of the rabbits (n=4 1 yrMSC, n=2 4 yrMSC) showed the presence of ectopic bone in the repair region and were not included in the mechanical analyses. We conclude that in the rabbit model MSCs do not lose their benefit as a tendon repair therapy with aging and that MSCs can be cryogenically stored for 3 years and still effectively repair soft tissue injuries.

Mesenchymal stem cells used for rabbit tendon repair can form ectopic bone and express alkaline phosphatase activity in constructs.

Mesenchymal stem cells (MSCs) have been used to repair connective tissue defects in several animal models. Compared to "natural healing" controls (no added cells), MSC-collagen gel constructs in rabbit tendon defects significantly improve repair biomechanics. However, ectopic bone forms in 28% of MSC-treated rabbit tendons. To understand the source of bone formation, three studies were performed. In the first study, the hypothesis was tested that MSCs delivered during surgery contribute to bone formation in the in vivo repair site. Adjacent histological sections in the MSC-treated repair tissue were examined for pre-labeled MSCs and for cells showing positive alkaline phosphatase (ALP) activity. Both cells were observed in serial sections in regions of ectopic bone. Contralateral "natural healing" tendons lacked both markers. In the other two studies, the effects of osteogenic supplements and construct geometry (monolayer vs. 3-D) on ALP activity were studied to test three hypotheses: that rabbit MSCs increase ALP activity over time in monolayer culture conditions; that adding osteogenic inducing supplements to the culture medium increases cellular protein in monolayer culture; and that rabbit MSCs increase ALP activity both in monolayer and in 3-D constructs, with and without media supplements. Culture in monolayer under similar conditions to in vivo (as in the first study) did not increase ALP at 2 or 4 weeks. Medium designed to increase osteogenic activity significantly increased cell numbers (cellular protein increased by 260%) but did not affect ALP activity either in monolayer or 3-D constructs (p>0.12). However, MSCs in 3-D constructs exhibited higher ALP activity than cells in monolayer, both in the presence (p<0.045) and absence of supplement (p<0.005). These results suggest that in vitro conditions may critically influence cell differentiation and protein expression. Mechanisms responsible for these effects are currently under investigation.

In vitro activity of gemifloxacin and contemporary oral antimicrobial agents against 27247 Gram-positive and Gram-negative aerobic isolates: a global surveillance study.

This study was a multi-centre, multi-country surveillance of 27247 Gram-positive and Gram-negative isolates collected from 131 study centres in 44 countries from 1997 to 2000. MICs of gemifloxacin were compared with penicillin, amoxicillin-clavulanic acid, cefuroxime, azithromycin, clarithromycin, trimethoprim-sulphamethoxazole, ciprofloxacin, grepafloxacin and levofloxacin by broth microdilution. Penicillin resistance in Streptococcus pneumoniae was extremely high in the Middle East (65.6%), Africa (64.0%) and Asia (60.4%) and lower in North America (40.3%), Europe (36.9%) and the South Pacific (31.8%). Macrolide resistance in S. pneumoniae was highest in Asia (51.7%) but varied widely between laboratories in Europe (26.0%), North America (21.6%), the Middle East (13.7%), the South Pacific (10.6%) and Africa (10.0%). All the study quinolones were highly active against penicillin-resistant and macrolide-resistant S. pneumoniae. Overall, gemifloxacin had the lowest MIC(90) at 0.06 mg/l with MICs 4-64-fold lower than ciprofloxacin, levofloxacin and grepafloxacin against S. pneumoniae. Gemifloxacin MICs were more potent than grepafloxacin > levoflaxacin > ciproflaxin against the Gram-positive aerobes and shared comparable Gram-negative activity with ciprofloxacin and levofloxacin.

Comparative in vitro potency of amoxycillin-clavulanic acid and four oral agents against recent North American clinical isolates from a global surveillance study.

The in vitro activity of amoxycillin-clavulanic acid was compared with four comparator oral antimicrobial agents; ampicillin, azithromycin, cefuroxime and trimethoprim-sulphamethoxazole against 4536 recent clinical isolates covering 29 species isolated in the US and Canada between 1997 and 1999. Based upon Minimum inhibitory concentrations (MICs), amoxycillin-clavulanic acid was the most active agent against many Gram-positive species and phenotypes including methicillin susceptible Staphylococcus aureus (MSSA) Staphylococcus epidermidis, Enterococcus faecalis, Streptococcus pyogenes, Streptococcus pneumoniae including penicillin intermediate and macrolide resistant strains and was as active as ampicillin against Streptococcus agalactiae, penicillin resistant S. pneumoniae and viridans streptococci. Against Enterobacteriaceae amoxycillin-clavulanic acid in general, displayed weak activity with only Proteus mirabilis and Proteus vulgaris displaying levels of susceptibility above the 90th percentile. Amoxycillin-clavulanic acid had significant activity against many species of Gram-negative non-Enterobacteriaceae including Haemophilus influenzae, Haemophilus parainfluenzae and Moraxella catarrhalis but negligible activity against Burkholderia cepacia, Pseudomonas aeruginosa and Stenotrophomonas maltophilia. Amoxycillin-clavulanic acid continues to retain excellent activity against the majority of targeted pathogens despite 20 years of clinical use.

Comparative in vitro surveillance of amoxicillin-clavulanic acid and four oral comparators against 21232 clinical isolates from europe.

The present study was conducted to determine the in vitro activity of amoxicillin-clavulanic acid compared to that of four newer antimicrobial agents (ampicillin, azithromycin, cefuroxime and trimethoprim-sulfamethoxazole). All of the agents were tested against 21232 recent clinical isolates encompassing 37 species submitted from 16 European countries between 1997 and 1999. After 20 years of clinical use, amoxicillin-clavulanic acid continues to retain much of its initial activity against targeted gram-positive organisms, selected gram-negative organisms and major respiratory pathogens.

A potential mechanism for age-related declines in patellar tendon biomechanics.

Injuries to soft tissues such as tendons are becoming ever more frequent among the elderly. While increasing levels of activity likely contribute to these injuries, age-related declines in tendon strength may also be important. Whether these declines in biomechanical properties are associated with changes in fibril diameter or collagen type remains in question. In this study, age-related changes were investigated in patellar tendons from young adult rabbits (1-year old, n = 17) and from rabbits at the onset of senescence (4-year old, n = 33). Patellar tendon biomechanics was correlated with both collagen fibril diameter and with the presence of type V collagen, a known regulator of collagen fibril diameter. We hypothesize that (a) aging from I to 4 years results in significant reductions in patellar tendon biomechanical properties, and (b) these age-related declines are associated with smaller fibril diameters and with the presence of type V collagen. Maximum stress declined 25% between I and 4 years of age (100.7 +/- 5.6 MPa and 74.3 +/- 3.4 MPa, respectively, p < 0.0003) (mean +/- SEM) and strain energy density declined 40% (p < 0.001). The distribution of collagen fibrils from 4-year old rabbits was skewed significantly towards smaller diameters compared to fibrils from 1-year old rabbits (p < 0.001). Type V collagen was observed only in the 4-year old rabbit tendons. These correlations suggest that with increasing age after skeletal maturity, type V collagen may help to regulate the assembly and thus diameter of collagen fibrils and thereby adversely affect patellar tendon strength.

Subject variation in caprine anterior cruciate ligament reconstruction.

We studied the subject and treatment contributions to anterior cruciate ligament (ACL) reconstruction biomechanics by reexaming the results of two bilateral reconstruction studies. Bilateral reconstruction allows a comparison between treatments exposed to the same subject related healing factors. The studies examined the effects of gamma irradiation and the effects of initial graft size and initial graft laxity. In both studies different treatments were applied to contralateral limbs. We found that the subject was the best predictor of outcome, while the surgical treatments had little influence on outcome. There was a large variation between subjects despite similar treatments, and little difference between contralateral limbs despite different surgical treatments. At 26 weeks, the graft cross sectional area and modulus were most strongly influenced (p < 0.002) by the subject. We interpret this as a subject related factor is regulating the quantity and quality of the healing tissue. Potential sources of subject related factors include the subject's pre-operative condition, the activity during the post-operative period, and an intrinsic biologic response. By better understanding the source of subject variation, more successful and consistent ACL reconstructions might be achieved.

Functional tissue engineering: the role of biomechanics in articular cartilage repair.

Articular cartilage shows little or no intrinsic capacity for repair in response to injury or disease, and even minor lesions or injuries may lead to progressive damage and joint degeneration. Tissue engineering is a relatively new but rapidly growing field that has sought to use combinations of implanted cells, biomaterials, and biologically active molecules to repair or regenerate injured or diseased tissues. Despite many advances, tissue engineers have faced significant challenges in repairing or replacing tissues that serve a predominantly biomechanical function, such as articular cartilage. An evolving discipline termed functional tissue engineering seeks to address these challenges by emphasizing and evaluating the role of biomechanical factors in the intrinsic and engineered repair of tissues and organs. In the current study, the authors describe some of the fundamental issues involving the interaction of biomechanical stresses in vivo and in vitro with native and repair articular cartilage and with other biomechanically functional tissues. A more thorough and formal investigation of these issues may provide a basis for developing rational design principles for tissue engineered replacement or repair of load-bearing structures in the body.

Comparative in vitro activity of gemifloxacin, ciprofloxacin, levofloxacin and ofloxacin in a North American surveillance study.

The in vitro activity of gemifloxacin, a new fluoroquinolone, was compared to three marketed fluoroquinolones; ciprofloxacin, levofloxacin and ofloxacin against over 4,000 recent clinical isolates covering 29 species isolated in the United States and Canada between 1997-1999. Based on MIC(90)s, gemifloxacin was the most potent fluoroquinolone tested against a majority of Gram-positive isolates: Streptococcus pneumoniae, penicillin resistant S. pneumoniae, macrolide resistant S. pneumoniae, ciprofloxacin non-susceptible (MIC > or = 4 microg/mL) S. pneumoniae, S. pyogenes, S. agalactiae, viridans streptococci, Enterococcus faecalis, methicillin susceptible Staphylococcus aureus, methicillin resistant S. aureus, S. epidermidis, S. hemolyticus, and S. saprophyticus. Against Enterobacteriaceae and aerobic non-Enterobacteriaceae Gram-negatives, gemifloxacin was usually comparable to ciprofloxacin and levofloxacin and more potent than ofloxacin for the following species: Citrobacter freundii, Enterobacter aerogenes, E. cloacae, Escherichia coli, Klebsiella oxytoca, Klebsiella pneumoniae, Morganella morganii, Proteus mirabilis, P. vulgaris, Providencia stuartii, Serratia marcescens, Acinetobacter lwoffii, A. baumannii, Burkholderia cepacia, Haemophilus influenzae, H. parainfluenzae, Moraxella catarrhalis, Pseudomonas aeruginosa, and Stenotrophomonas maltophilia. Gemifloxacin was generally 16-64 fold more potent than the other fluoroquinolones tested against Gram-positive organisms and retains excellent activity comparable with ciprofloxacin and levofloxacin against a majority of Gram-negative pathogens.

Comparative in vitro potency of gemifloxacin and fluoroquinolones against recent European clinical isolates from a global surveillance study.

Gemifloxacin, a new fluoroquinolone with enhanced activity against gram-positive aerobes, was compared to ciprofloxacin, levofloxacin and ofloxacin against 21,464 recent isolates from 16 European countries. Gemifloxacin was the most potent fluoroquinolone against streptococci including penicillin-, macrolide- and ciprofloxacin-resistant Streptococcus pneumoniae, Staphylococcus aureus, coagulase-negative staphylococci, Acinetobacter spp., Haemophilus spp. and Moraxella catarrhalis. This drug was more potent than or comparable to ciprofloxacin against members of the family Enterobacteriaceae, Burkholderia cepacia, Pseudomonas aeruginosa and Stenotrophomonas maltophilia. Gemifloxacin is a promising fluoroquinolone with potent in vitro activity.

A comparative In vitro surveillance study of gemifloxacin activities against 2,632 recent Streptococcus pneumoniae isolates from across Europe, North America, and South America. The Gemifloxacin Surveillance Study Research Group.

From 1997 to 1999, 94 study centers in 15 European, 3 North American, and 2 South American countries contributed 2,632 isolates of Streptococcus pneumoniae to an international antimicrobial susceptibility testing study. Only 62.0% of isolates were susceptible to penicillin, while 22.3% were penicillin intermediate and 15.6% were penicillin resistant. Resistance to trimethoprim-sulfamethoxazole (24.4%), azithromycin (26.0%), and clarithromycin (27.1%) was also highly prevalent. For the penicillin-resistant isolates (n = 411), the MICs at which 90% of isolates are inhibited (MIC(90)s) for gemifloxacin, levofloxacin, ofloxacin, clarithromycin, and azithromycin were 0.03, 1, 2, >16, and >64 microgram/ml, respectively. Similarly, for isolates resistant to both azithromycin and clarithromycin (n = 649), gemifloxacin, levofloxacin, ofloxacin, and penicillin MIC(90)s were 0.03, 1, 2, and 4 microgram/ml, respectively. Overall rates of resistance to trovafloxacin (0.3%), levofloxacin (0.3%), grepafloxacin (0.6%), and ofloxacin (0.7%) were low. For ofloxacin-intermediate and -resistant isolates (n = 142), gemifloxacin had the lowest MIC(90) (0.12 microgram/ml) compared to the MIC(90)s of trovafloxacin (0.5 microgram/ml), grepafloxacin (1 microgram/ml), and levofloxacin (2 microgram/ml). For all S. pneumoniae isolates tested, gemifloxacin MICs were

In vitro characterization of mesenchymal stem cell-seeded collagen scaffolds for tendon repair: effects of initial seeding density on contraction kinetics.

Mesenchymal stem cells (MSCs) were isolated from bone marrow, culture-expanded, and then seeded at 1, 4, and 8 million cells/mL onto collagen gel constructs designed to augment tendon repair in vivo. To investigate the effects of seeding density on the contraction kinetics and cellular morphology, the contraction of the cell/collagen constructs was monitored over time up to 72 h in culture conditions. Constructs seeded at 4 and 8 million cells/mL showed no significant differences in their gross appearance and dimensions throughout the contraction process. By contrast, constructs seeded at 1 million cells/mL initially contracted more slowly and their diameters at 72 h were 62 to 73% larger than those seeded at higher densities. During contraction, MSCs reoriented and elongated significantly with time. Implants prepared at higher seeding densities showed more well aligned and elongated cell nuclei after 72 h of contraction. Changes in nuclear morphology of the MSCs in response to physical constraints provided by the contracted collagen fibrils may trigger differentiation pathways toward the fibroblastic lineage and influence the cell synthetic activity. Controlling the contraction and organization of the cells and matrix will be critical for successfully creating tissue engineered grafts.

An in vivo model for load-modulated remodeling in the rabbit flexor tendon.

This study tested the hypothesis that eliminating in vivo compression to the wrap-around, fibrocartilage-rich zone of the flexor digitorum profundus tendon results in rapid depletion of fibrocartilage and changes in its mechanical properties, microstructure, extracellular matrix composition, and cellularity. The right flexor digitorum profundus tendons of 2.5-3-year-old rabbits were translocated anteriorly to eliminate in vivo compression and shear to the fibrocartilage zone and, at 4 weeks after surgery, were compared with tendons that had sham surgery and with untreated tendons. The translocated tissue showed a significant increase in equilibrium strain under a compressive creep load (p < 0.05). The thickness and area of the fibrocartilage zone also decreased significantly (p < 0.05). The nuclear density decreased by 40% in the fibrocartilage zone (p < 0.005); however, nuclear shape and orientation were not significantly altered. Glycosaminoglycan content in the fibrocartilage zone was also depleted by 40% (p < 0.02). The tightly woven basket weave-like mesh of collagen fibers in the zone appeared more loosely organized, suggesting matrix reorganization due to translocation. Moreover, immunoreactive type-II collagen and link protein in the fibrocartilage zone also decreased. With use of this unique in vivo model, this research clearly elucidates how changing tissue function (by removing compressive forces) rapidly alters tissue form.

Functional tissue engineering: the role of biomechanics.

"Tissue engineering" uses implanted cells, scaffolds, DNA, protein, and/or protein fragments to replace or repair injured or diseased tissues and organs. Despite its early success, tissue engineers have faced challenges in repairing or replacing tissues that serve a predominantly biomechanical function. An evolving discipline called "functional tissue engineering" (FTE) seeks to address these challenges. In this paper, the authors present principles of functional tissue engineering that should be addressed when engineering repairs and replacements for load-bearing structures. First, in vivo stress/strain histories need to be measured for a variety of activities. These in vivo data provide mechanical thresholds that tissue repairs/replacements will likely encounter after surgery. Second, the mechanical properties of the native tissues must be established for subfailure and failure conditions. These "baseline data" provide parameters within the expected thresholds for different in vivo activities and beyond these levels if safety factors are to be incorporated. Third, a subset of these mechanical properties must be selected and prioritized. This subset is important, given that the mechanical properties of the designs are not expected to completely duplicate the properties of the native tissues. Fourth, standards must be set when evaluating the repairs/replacements after surgery so as to determine, "how good is good enough?" Some aspects of the repair outcome may be inferior, but other mechanical characteristics of the repairs and replacements might be suitable. New and improved methods must also be developed for assessing the function of engineered tissues. Fifth, the effects of physical factors on cellular activity must be determined in engineered tissues. Knowing these signals may shorten the iterations required to replace a tissue successfully and direct cellular activity and phenotype toward a desired end goal. Finally, to effect a better repair outcome, cell-matrix implants may benefit from being mechanically stimulated using in vitro "bioreactors" prior to implantation. Increasing evidence suggests that mechanical stress, as well as other physical factors, may significantly increase the biosynthetic activity of cells in bioartificial matrices. Incorporating each of these principles of functional tissue engineering should result in safer and more efficacious repairs and replacements for the surgeon and patient.