Effect of implanting a soft tissue autograft in a central-third patellar tendon defect: biomechanical and histological comparisons

Previous studies by our laboratory have demonstrated that implanting a stiffer tissue engineered construct at surgery is positively correlated with repair tissue stiffness at 12 weeks. The objective of this study was to test this correlation by implanting a construct that matches normal tissue biomechanical properties. To do this, we utilized a soft tissue patellar tendon autograft to repair a central-third patellar tendon defect. Patellar tendon autograft repairs were contrasted against an unfilled defect repaired by natural healing (NH). We hypothesized that after 12 weeks, patellar tendon autograft repairs would have biomechanical properties superior to NH. Bilateral defects were established in the central-third patellar tendon of skeletally mature (one year old), female New Zealand White rabbits (n?=?10). In one limb, the excised tissue, the patellar tendon autograft, was sutured into the defect site. In the contralateral limb, the defect was left empty (natural healing). After 12 weeks of recovery, the animals were euthanized and their limbs were dedicated to biomechanical (n?=?7) or histological (n?=?3) evaluations. Only stiffness was improved by treatment with patellar tendon autograft relative to natural healing (p?=?0.009). Additionally, neither the patellar tendon autograft nor natural healing repairs regenerated a normal zonal insertion site between the tendon and bone. Immunohistochemical staining for collagen type II demonstrated that fibrocartilage-like tissue was regenerated at the tendon-bone interface for both repairs. However, the tissue was disorganized. Insufficient tissue integration at the tendon-to-bone junction led to repair tissue failure at the insertion site during testing. It is important to re-establish the tendon-to-bone insertion site because it provides joint stability and enables force transmission from muscle to tendon and subsequent loading of the tendon. Without loading, tendon mechanical properties deteriorate. Future studies by our laboratory will investigate potential strategies to improve patellar tendon autograft integration into bone using this model.     

Biomimetic approaches to tendon repair

The linear organization of collagen fibers in tendons results in optimal stiffness and strength at low strains under tensile load. However, this organization makes repairing ruptured or lacerated tendons extremely difficult. Current suturing techniques to join split ends of tendons, while providing sufficient mechanical strength to prevent gapping, are inadequate to carry normal loads. Immobilization protocols necessary to restore tendon congruity result in scar formation at the repair site and peripheral adhesions that limit excursion. These problems are reviewed to emphasize the need for novel approaches to tendon repair, one of which is the development of biomimetic tendons. The objective of the empirical work described here was to produce biologically-based, biocompatible tendon replacements with appropriate mechanical properties to enable immediate mobilization following surgical repair. Nor-dihydroguaiaretic acid (NDGA), a di-catechol from creosote bush, caused a dose dependent increase in the material properties of reconstituted collagen fibers, achieving a 100-fold increase in strength and stiffness over untreated fibers. The maximum tensile strength of the optimized NDGA treated fibers averaged 90 MPa; the elastic modulus of these fibers averaged 580 MPa. These properties were independent of strain rates ranging from 0.60 to 600 mm/min. Fatigue tests established that neither strength nor stiffness were affected after 80 k cycles at 5% strain. Treated fibers were not cytotoxic to tendon fibroblasts. Fibroblasts attached and proliferated on NDGA treated collagen normally. NDGA-fibers did not elicit a foreign body response nor did they stimulate an immune reaction during six weeks in vivo. The fibers survived 6 weeks with little evidence of fragmentation or degradation. The polymerization scheme described here produces a fiber-reinforced NDGA-polymer with mechanical properties approaching an elastic solid. The strength, stiffness and fatigue properties of the NDGA-treated fibers are comparable to those of tendon. These fibers are biocompatible with tendon fibroblasts and elicit little rejection or antigenic response in vivo. These results indicate that NDGA polymerization may provide a viable approach for producing collagenous materials that can be used to bridge gaps in ruptured or lacerated tendons. The tendon-like properties of the NDGA-fiber would allow early mobilization after surgical repair. We predict that timely loading of parted tendons joined by this novel biomaterial will enhance mechanically driven production of neo-tendon by the colonizing fibroblasts and result in superior repair and rapid return to normal properties.     

Contribution of biomechanics to management of ligament and tendon injuries

The contribution of biomechanics to the advancement of management of ligament and tendon injuries has been significant. Thanks to Professor Y.C. Fung's writing and guidance, our field of research has done fundamental work on anatomy and biology of ligaments and tendons, developed methods to accurately determine mechanical properties, identified various experimental factors which could change the outcome measurements as well as examined biological factors that change tissue properties in-vivo. Professor Fung also gave us his quasi-linear viscoelastic theory for soft tissues so that the time and history dependent properties of ligaments and tendons could be properly described. We have further adopted Professor Fung's eight steps on methods of approach for biomechanical investigation to understand as well as enhance the treatment of ligament and tendon injuries during work or sports related activities. Examples on how to better treat the tears of the medial collateral ligament of the knee, as well as how to improve reconstruction procedures for the anterior cruciate ligament are presented in detail. Currently the use of functional tissue engineering for ligament and tendon healing is a topic of great interest. Here the use of biological scaffolds, such as porcine small intestinal submucosa, has shown promise. For the last 35 to 40 years, the field of biomechanics has made great strides in the treatment of ligament and tendon injuries, and many patients have benefited. The future is even brighter because of what has been done properly in the past. Exciting advances can be made in the field of tissue engineering through novel in-vitro culture and bioscaffold fabrication techniques. Recent technology can also allow the collection of in-vivo data so that ligament and tendon injuries can be better understood. Yet, solving new and more complex problems must still follow the stepwise methods of approach as taught by Professor Fung.     

Determination of biomechanical characteristics of restrictive adhesions and of functional impairment after flexor tendon surgery: a methodological study of rabbits.

Formation of restrictive adhesions is one of the main obstacles in rehabilitation following hand surgery. Most experimental work, however, involves only a macroscopic and/or histologic evaluation of the amount of adhesions, and their functional characteristics are poorly described. The aim of this study was to develop an experimental technique for characterization of the biomechanical properties of the finger-tendon unit. An instrument was developed for continuous and simultaneous recording of tensile load, tendon excursion and angular rotation in the distal interphalangeal joint of rabbit digits. Utilizing this instrument, it was revealed that the first 50 degrees of flexion required virtually no tensile load either in unoperated digits or immediately after tenorrhaphy. Thereafter, the load required to obtain further flexion was progressively increased. The strength of adhesions, determined 2 weeks after tenorrhaphy, was best expressed as the maximum tensile load recorded before 50 degrees of flexion was reached. This measurement could also be used to register the strength of the tendon repair and to detect partial tendon rupture during the measurement. The technique allows both adequate measurements of the strength of the adhesions and of the tendon gliding ability after flexor tendon surgery.     

Quadriceps tendon allografts as an alternative to Achilles tendon allografts: a biomechanical comparison

Quadriceps tendon with a patellar bone block may be a viable alternative to Achilles tendon for anterior cruciate ligament reconstruction (ACL-R) if it is, at a minimum, a biomechanically equivalent graft. The objective of this study was to directly compare the biomechanical properties of quadriceps tendon and Achilles tendon allografts. Quadriceps and Achilles tendon pairs from nine research-consented donors were tested. All specimens were processed to reduce bioburden and terminally sterilized by gamma irradiation. Specimens were subjected to a three phase uniaxial tension test performed in a custom environmental chamber to maintain the specimens at a physiologic temperature (37 ± 2 °C) and misted with a 0.9 % NaCl solution. There were no statistical differences in seven of eight structural and mechanical between the two tendon types. Quadriceps tendons exhibited a significantly higher displacement at maximum load and significantly lower stiffness than Achilles tendons. The results of this study indicated a biomechanical equivalence of aseptically processed, terminally sterilized quadriceps tendon grafts with bone block to Achilles tendon grafts with bone block. The significantly higher displacement at maximum load, and lower stiffness observed for quadriceps tendons may be related to the failure mode. Achilles tendons had a higher bone avulsion rate than quadriceps tendons (86 % compared to 12 %, respectively). This was likely due to observed differences in bone block density between the two tendon types. This research supports the use of quadriceps tendon allografts in lieu of Achilles tendon allografts for ACL-R.     

Evaluation of four methods of flexor tendon repair for postoperative active mobilization

Active mobilization of repaired flexor tendons requires sufficient suture strength. This study was designed to investigate the suitability of four newly developed and comparatively strong tendon sutures for flexor tendon repair with active digital mobilization. Fifty fresh flexor digitorum profundus tendons were randomly assigned to five groups and repaired using the Tang, cruciate, Robertson, Silfverskiold, and modified Kessler suture methods. The repaired tendons were subjected to mechanical testing in an Instron tensile machine to determine the 2-mm gap formation force, ultimate strength, elastic modulus, and energy to failure of the sutures. The 2-mm gap formation forces of the sutures were 43.0 N for the Tang, 37.4 N for the cruciate, 25.0 N for the Robertson, 32.3 N for the Silfverskiold, and 21.2 N for the modified Kessler methods. The ultimate strength of the sutures was 53.6 N for the Tang, 46.3 N for the cruciate, 41.6 N for the Robertson, 41.0 N for the Silfverskiold, and 24.7 N for the modified Kessler methods. Statistically, the gap formation force and ultimate strength were the highest in the Tang, higher in the cruciate, and the lowest for the Robertson and the modified Kessler methods. The elastic modulus of the repaired tendons, as represented by the linear slope of the force-displacement curve, was also statistically the largest in the Tang, larger in the cruciate, and lowest for the Robertson and modified Kessler methods. Energy to failure was statistically the largest in the Tang, higher in the cruciate, lower in the Silfverskiold and the Robertson, and the lowest for the modified Kessler methods. It was concluded that significant differences exist in mechanical properties of the newly developed tendon suture methods. Among the methods for tendon repair that were tested, the Tang and the cruciate sutures were the best candidates for flexor tendon repair in the hand with postoperative active mobilization because of their superior tensile strength, elastic properties, energy to failure, and reasonable operation time.     

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 absence of oestrogen receptor beta disturbs collagen I type deposition during Achilles tendon healing by regulating the IRF5‐CCL3 axis

Achilles tendon healing (ATH) remains an unanswered question in the field of sports medicine because it does not produce tissue with homology to the previously uninjured tissue. Oestrogen receptor β (ERβ) is involved in the injury and repair processes of tendons. Our previous study confirmed that ERβ plays a role in the early stage of ATH by affecting adipogenesis, but its role in extracellular matrix (ECM) remodelling is unknown. We established a 4‐week Achilles tendon repair model to investigate the mechanism through which ERβ affects ATH at the very beginning of ECM remodelling phase. In vitro studies were performed using tendon‐derived stem cells (TDSCs) due to their promising role in tendon healing. Behavioural and biomechanical tests revealed that ERβ‐deficient mice exhibit weaker mobility and inferior biomechanical properties, and immunofluorescence staining and qRT‐PCR showed that these mice exhibited an erroneous ECM composition, as mainly characterized by decreased collagen type I (Col I) deposition. The changes in gene expression profiles between ERβ‐knockout and WT mice at 1 week were analysed by RNA sequencing to identify factors affecting Col I deposition. The results highlighted the IRF5‐CCL3 axis, and this finding was verified with CCL3‐treated TDSCs. These findings revealed that ERβ regulates Col I deposition during ATH via the IRF5‐CCL3 axis.     

Bone marrow-derived matrix metalloproteinase-9 is associated with fibrous adhesion formation after murine flexor tendon injury

The pathogenesis of adhesions following primary tendon repair is poorly understood, but is thought to involve dysregulation of matrix metalloproteinases (Mmps). We have previously demonstrated that Mmp9 gene expression is increased during the inflammatory phase following murine flexor digitorum (FDL) tendon repair in association with increased adhesions. To further investigate the role of Mmp9, the cellular, molecular, and biomechanical features of healing were examined in WT and Mmp9(-/-) mice using the FDL tendon repair model. Adhesions persisted in WT, but were reduced in Mmp9(-/-) mice by 21 days without any decrease in strength. Deletion of Mmp9 resulted in accelerated expression of neo-tendon associated genes, Gdf5 and Smad8, and delayed expression of collagen I and collagen III. Furthermore, WT bone marrow cells (GFP(+)) migrated specifically to the tendon repair site. Transplanting myeloablated Mmp9(-/-) mice with WT marrow cells resulted in greater adhesions than observed in Mmp9(-/-) mice and similar to those seen in WT mice. These studies show that Mmp9 is primarily derived from bone marrow cells that migrate to the repair site, and mediates adhesion formation in injured tendons. Mmp9 is a potential target to limit adhesion formation in tendon healing.     

Murine patellar tendon biomechanical properties and regional strain patterns during natural tendon-to-bone healing after acute injury

Tendon-to-bone healing following acute injury is generally poor and often fails to restore normal tendon biomechanical properties. In recent years, the murine patellar tendon (PT) has become an important model system for studying tendon healing and repair due to its genetic tractability and accessible location within the knee. However, the mechanical properties of native murine PT, specifically the regional differences in tissue strains during loading, and the biomechanical outcomes of natural PT-to-bone healing have not been well characterized. Thus, in this study, we analyzed the global biomechanical properties and regional strain patterns of both normal and naturally healing murine PT at three time points (2, 5, and 8 weeks) following acute surgical rupture of the tibial enthesis. Normal murine PT exhibited distinct regional variations in tissue strain, with the insertion region experiencing approximately 2.5 times greater strain than the midsubstance at failure (10.80±2.52% vs. 4.11±1.40%; mean±SEM). Injured tendons showed reduced structural (ultimate load and linear stiffness) and material (ultimate stress and linear modulus) properties compared to both normal and contralateral sham-operated tendons at all healing time points. Injured tendons also displayed increased local strain in the insertion region compared to contralateral shams at both physiologic and failure load levels. 93.3% of injured tendons failed at the tibial insertion, compared to only 60% and 66.7% of normal and sham tendons, respectively. These results indicate that 8 weeks of natural tendon-to-bone healing does not restore normal biomechanical function to the murine PT following injury.     

Biomechanical testing of various suture techniques for Achilles tendon repair with and without augmentation by using synthetic polyester grafts

Following surgical Achilles tendon reconstruction surgery, there is a distinct trend towards an early and faster rehabilitation protocol to avoid muscle atrophy. However, this procedure involves the risk of a higher complication rate. In order to reduce the occurrence of re-ruptures and pathological tendon extensions, a tendon reconstruction with the highest possible primary stability is desirable. Therefore, the aim of this study was to determine if augmentation using synthetic polyester tapes (QuadsTape™) could provide greater primary stability in case of different tendon suture techniques.90 tendons of the superficial toe flexor of pigs were divided into 9 groups. The reconstruction method was combined using the factors suture technique (Kessler and Bunnell), augmentation (non-augmented and augmented with QuadsTape™) and defect type (end-to-end and 10 mm gap). The biomechanical measurements were performed on a material testing machine and consisted of a creep test, a cyclic test and a tear-off test. This study compared creep strain, ultimate load failure, maximum stress and stiffness.Irrespective of the type of defect involved, augmentation of the tendon sutures led to a significant increase of the maximum force (not augmented: 82.30 ± 25.48 N, augmented: 135.73 ± 30.69 N, p < 0.001) and the maximum stress (not augmented: 2.26 ± 0.83 MPa, augmented: 4.13 ± 1.79 MPa, p < 0.001). Furthermore, there was a non-significant increase in stiffness and no significant differences were observed with respect to creep strain.Augmentation of Achilles tendon reconstruction using QuadsTape™ increases composite strength and stiffness in the in vitro model, thus potentially contributing to the feasibility of early rehabilitation programs. Biological factors still need to be investigated in order to formulate appropriate indications.     

Method and strain rate dependence of Achilles tendon stiffness

Tendon stiffness is calculated by dividing changes in tendon force by tendon elongation. For this purpose, participants are commonly asked to perform a maximal muscle contraction (“active” method). Alternatively tendon elongation can be achieved by means of a passive joint rotation (“passive” method). The purpose of this study was to compare Achilles tendon stiffness obtained from both methods across different tendon strain rates. Twenty adults performed a series of ramped maximum isometric plantarflexions of different durations. Passive ankle rotations of different angular velocities were also performed. Achilles tendon stiffness was obtained from a combination of motion analysis, isokinetic dynamometry and ultrasonography and compared across methods at three strain rates. At all strain rates, tendon stiffness obtained from the active method was 6% greater compared to the passive method. In spite of this systematic bias, there was good agreement between the methods. Intraclass correlation coefficients were greater than 0.98, and more than 95% of data points fell into the 95% confidence intervals. This agreement will be acceptable in many research contexts. We also found a linear increase in tendon stiffness with increasing strain rate, which must be taken into consideration when interpreting or reporting tendon stiffness.     

Gene therapy and tissue engineering for sports medicine

Sports injuries usually involve tissues that display a limited capacity for healing. The treatment of sports injuries has improved over the past 10 to 20 years through sophisticated rehabilitation programs, novel operative techniques, and advances in the field of biomechanical research. Despite this considerable progress, no optimal solution has been found for treatment of various sports-related injuries, including muscle injuries, ligament and tendon ruptures, central meniscal tears, cartilage lesions, and delayed bone fracture healing. New biological approaches focus on the treatment of these injuries with growth factors to stimulate and hasten the healing process. Gene therapy using the transfer of defined genes encoding therapeutic proteins represents a promising way to efficiently deliver suitable growth factors into the injured tissue. Tissue engineering, which may eventually be combined with gene therapy, may potentially result in the creation of tissues or scaffolds for regeneration of tissue defects following trauma. In this article we will discuss why gene therapy and tissue engineering are becoming increasingly important in modern orthopaedic sports medicine practice. We then will review recent research achievements in the area of gene therapy and tissue engineering for sports-related injuries, and highlight the potential clinical applications of this technology in the treatment of patients with musculoskeletal problems following sports-related injuries.     

Biomechanical effect of rapid mucoperiosteal palatal tissue expansion with the use of osmotic expanders

The comparative study was performed to investigate the biomechanical properties (maximum tangential stiffness, maximum tangential modulus and tensile strength) of expanded mucoperiosteal palatal tissue after rapid expansion regimen correlated with histological findings. Rabbit palatal model was used to correlate the non-operated control group, sham-operated control (subperiosteal tissue dissection) groups and 24- and 48-hour tissue expansion groups. There was no observed damage of tissue collagen network in both tissue expansion groups analyzed immediately after expansion, and biomechanical profile was not significantly different from the profile of control groups. However, rapid tissue expansion activates remodeling of mucoperiosteal tissue structure that revealed significant changes in mechanical properties during the 4-week follow-up. The 24-hour expansion induced transient increase of resilience observed 2 weeks after surgery in comparison to the control groups. As a result of maturation of newly created collagen fibers and mucoperiosteum rebuilding, there were no significant differences between any of the analyzed tensile parameters 4 weeks after the 24-hour expansion. Increased and elongated inflammatory response and connective matrix synthesis observed during healing of 48-hour expanded tissue led to a significant decrease of tensile strength value in comparison to the control groups. Even though 4 weeks after surgery, the resilience of 48-hour expanded tissue was similar to the control groups, tissue healing was not completed and limited scar formation might considerably change the final biomechanical tissue profile. These findings provide new information about tensile properties to rapid mucoperiosteal palatal tissue expansion with the use of osmotic expanders for cleft palate repair by tissue augmentation.     

Effects of exercise on the biomechanical,biochemical and structural properties of tendons

Tendon has been shown to undergo remodeling in response to strength or endurance training, however, compared to muscle, studies of the effects of exercise on tendon are limited and the information is inconsistent. Exercise may influence the structure, chemical composition and/or mechanical properties of tendon. Studies that have examined mechanical changes of tendon in response to endurance training suggest that ultimate failure strength and stiffness increase with training. Available reports indicate that increases in tensile strength and stiffness are probably not associated with increases in collagen concentration or with tendon hypertrophy. The paucity of data renders it impossible to evaluate the response of other structural, chemical and mechanical parameters to training. Furthermore, few investigators have included discrete measures of structural, biomechanical and biochemical variables within a single study. The lack of integrative studies makes it difficult to definitively associate changes in the mechanical properties of tendon with chemical composition and structure.     

Tissue engineering: chondrocytes and cartilage

Tissue engineering offers new strategies for developing treatments for the repair and regeneration of damaged and diseased tissues. These treatments, using living cells, will exploit new developments in understanding the principles in cell biology that control and direct cell function. Arthritic diseases that affect so many people and have a major impact on the quality of life provide an important target for tissue engineering. Initial approaches are in cartilage repair; in our own programme we are elucidating the signals required by chondrocytes to promote new matrix assembly. These principles will extend to other tissues of the musculoskeletal system, including the repair of bone, ligament and tendon.     

Enthalpy efficiency of the soleus muscle contributes to improvements in running economy

During human running, the soleus, as the main plantar flexor muscle, generates the majority of the mechanical work through active shortening. The fraction of chemical energy that is converted into muscular work (enthalpy efficiency) depends on the muscle shortening velocity. Here, we investigated the soleus muscle fascicle behaviour during running with respect to the enthalpy efficiency as a mechanism that could contribute to improvements in running economy after exercise-induced increases of plantar flexor strength and Achilles tendon (AT) stiffness. Using a controlled longitudinal study design (n = 23) featuring a specific 14-week muscle–tendon training, increases in muscle strength (10%) and tendon stiffness (31%) and reduced metabolic cost of running (4%) were found only in the intervention group (n = 13, p < 0.05). Following training, the soleus fascicles operated at higher enthalpy efficiency during the phase of muscle–tendon unit (MTU) lengthening (15%) and in average over stance (7%, p < 0.05). Thus, improvements in energetic cost following increases in plantar flexor strength and AT stiffness seem attributed to increased enthalpy efficiency of the operating soleus muscle. The results further imply that the soleus energy production in the first part of stance, when the MTU is lengthening, may be crucial for the overall metabolic energy cost of running.     

Do Cells Contribute to Tendon and Ligament Biomechanics?

Introduction

Acellular scaffolds are increasingly used for the surgical repair of tendon injury and ligament tears. Despite this increased use, very little data exist directly comparing acellular scaffolds and their native counterparts. Such a comparison would help establish the effectiveness of the acellularization procedure of human tissues. Furthermore, such a comparison would help estimate the influence of cells in ligament and tendon stability and give insight into the effects of acellularization on collagen.

Material and Methods

Eighteen human iliotibial tract samples were obtained from nine body donors. Nine samples were acellularized with sodium dodecyl sulphate (SDS), while nine counterparts from the same donors remained in the native condition. The ends of all samples were plastinated to minimize material slippage. Their water content was adjusted to 69%, using the osmotic stress technique to exclude water content-related alterations of the mechanical properties. Uniaxial tensile testing was performed to obtain the elastic modulus, ultimate stress and maximum strain. The effectiveness of the acellularization procedure was histologically verified by means of a DNA assay.

Results

The histology samples showed a complete removal of the cells, an extensive, yet incomplete removal of the DNA content and alterations to the extracellular collagen. Tensile properties of the tract samples such as elastic modulus and ultimate stress were unaffected by acellularization with the exception of maximum strain.

Discussion

The data indicate that cells influence the mechanical properties of ligaments and tendons in vitro to a negligible extent. Moreover, acellularization with SDS alters material properties to a minor extent, indicating that this method provides a biomechanical match in ligament and tendon reconstruction. However, the given protocol insufficiently removes DNA. This may increase the potential for transplant rejection when acellular tract scaffolds are used in soft tissue repair. Further research will help optimize the SDS-protocol for clinical application.     

Bridging tendon defects using autologous tenocyte engineered tendon in a hen model

Tendon defects remain a major concern in plastic surgery because of the limited availability of tendon autografts. Whereas immune rejection prohibits the use of tendon allografts, most prosthetic replacements also fail to achieve a satisfactory long-term result of tendon repair. The tissue engineering technique, however, can generate different tissues using autologous cells and thus may provide an optimal approach to address this concern. The purpose of this study was to test the feasibility of engineering tendon tissues with autologous tenocytes to bridge a tendon defect in either a tendon sheath open model or a partial open model in the hen. In a total of 40 Leghorn hens, flexor tendons were harvested from the left feet and were digested with 0.25% type II collagenase. The isolated tenocytes were expanded in vitro and mixed with unwoven polyglycolic acid fibers to form a cell-scaffold construct in the shape of a tendon. The constructs were wrapped with intestinal submucosa and then cultured in Dulbecco's Modified Eagle Medium plus 10% fetal bovine serum for 1 week before in vivo transplantation. On the feet, a defect of 3 to 4 cm was created at the second flexor digitorum profundus tendon by resecting a tendon fragment. The defects were bridged either with a cell-scaffold construct in the experimental group ( n= 20) or with scaffold material alone in the control group ( n= 20). Specimens were harvested at 8, 12, and 14 weeks postrepair for gross and histologic examination and for biomechanical analysis. In the experimental group, a cordlike tissue bridging the tendon defect was formed at 8 weeks postrepair. At 14 weeks, the engineered tendons resembled the natural tendons grossly in both color and texture. Histologic examination at 8 weeks showed that the neo-tendon contained abundant tenocytes and collagen; most collagen bundles were randomly arranged. The undegraded polyglycolic acid fibers surrounded by inflammatory cells were also observed. At 12 weeks, tenocytes and collagen fibers became longitudinally aligned, with good interface healing to normal tendon. At 14 weeks, the engineered tendons displayed a typical tendon structure hardly distinguishable from that of normal tendons. Biomechanical analysis demonstrated increased breaking strength of the engineered tendons with time, which reached 83 percent of normal tendon strength at 14 weeks. In the control group, polyglycolic acid constructs were mostly degraded at 8 weeks and disappeared at 14 weeks. However, the breaking strength of the scaffold materials accounted for only 9 percent of normal tendon strength. The results of this study indicated that tendon tissue could be engineered in vivo to bridge a tendon defect. The engineered tendons resembled natural tendons not only in gross appearance and histologic structure but also in biomechanical properties.