Abstract: Advancements in the technical aspects of tendon repair have significantly improved the treatment of tendon injuries. Arthroscopic techniques, suture material, and improved rehabilitation have all been contributing factors. Biological augmentation and tissue engineering appear to have the potential to improve clinical outcomes as well. After review of the physiology of tendon repair, three critical components of tissue engineering can be discerned: the cellular component, the carrier vehicle (matrix or scaffold), and the bioactive component (growth factors, platelet rich plasma). These three components are discussed with regard to each of three tendon types: Intra-synovial (flexor tendon), extra-synovial (Achilles tendon), and extra-synovial tendon under compression (rotator cuff). Scaffolds, biologically enhanced scaffolds, growth factors, platelet rich plasma, gene therapy, mesenchymal stem cells, and local environment factors in combination or alone may contribute to tendon healing. In the future it may be beneficial to differentiate these modes of healing augmentation with regard to tendon subtype.
Tendon injuries are encountered routinely in orthopedic practice. Treatment of these injuries has improved tremendously over the last two decades (Tang, 2005; Yamaguchi et al., 2003; Beredijiklian, 2003). Even so, the cost of tendon injuries to our society in terms of human suffering and financial burden continues to be significant. Management of tendon injury has been documented as far back as the 2nd century A.D., yet treatment protocols for many of these injuries are still actively debated (Burkhead and Habermeyer, 1996). While the advancements in basic and clinical research often lead to new therapies and treatment adjuncts, these same advancements can initiate even more controversy in the appropriateness and applicability of these new technologies. Despite the strides made in understanding tendon physiology, many of the mechanisms behind its response to injury remain unknown. Unlike bone, which has the ability to restore normal tissue after injury, tendon heals with scar that is never identical to the uninjured tissue. The goal of clinical practice is to capitalize on that scarring while attempting to limit adhesion formation in order to optimize post-injury function. Current tissue engineering techniques seek to accelerate and/or modulate and/or improve the speed and quality of the healing tendon tissue.
Although intrasynovial tendons, extrasynovial tendons, and extrasynovial tendons under compression all possess a similar biochemical composition, injuries to these tendons are generally managed differently because of the unique anatomy, physiology, function, biomechanics, healing, and post-operative rehabilitation that pertain to each. Each tendon type presents its own unique set of challenges.
Comparative Tendon Physiology
All tendons share basic structural components. Key differences exist, which aid each in its specific function. Collagen is the single most abundant structural component of tendon. Whereas water makes up 70% of its total mass, collagen constitutes 60-85% of tendon dry weight (type I - 95%, type III and IV - 5%) (O’Brien, 1997; Piez et al., 1969; Jimenez et al., 1978). Its molecular architecture provides tendon with its high tensile strength. The intrasynovial flexor tendons and the extrasynovial tendons like the Achilles and quadriceps tendon are similar in their collagen profile. Rotator cuff tendon, however, is slightly different in that it demonstrates the presence of some type II collagen, and interdigitation of some of its fibers (Clark and Harryman, 1992). The ground substance of tendon is composed largely of proteoglycans. Proteoglycans make up 1% of the dry weight of tendon, and consist of a protein core attached to many glycosaminoglycan side chains. They are large, water trapping molecules that are found in higher concentration in tendons that act in compression like the rotator cuff. This function of the rotator cuff results in this tendon possessing a higher proportion of fibroblasts which histologically resemble chondrocytes, producing both aggrecan and type II collagen (Vogel and Chevalier, 1997). Smaller proteoglycans, such as decorin, are present in purely tensile tendons, whereas larger proteoglycans are abundant in tendons that combine tension and compression (Berenson et al., 1996).
Like many tissue types in the body, tendons have a universally conserved organization. The endotenon is a thin, loose connective tissue layer that covers individual tendon bundles and fascicles. It carries neurovascular elements through the substance of the tendon, and is continuous with the overlying epitenon (Edwards, 1946). The epitenon is a layer of connective tissue that surrounds the surface of tendons. This outermost layer is specific to tendon type, aiding in its function. Tendons that glide around acute angles are encased in a synovial tendon sheath filled with synovial fluid, like the flexor tendons of the hands and feet. Tendons that travel a straight course and attach to bone with a large amount of excursion are surrounded by an areolar paratenon, like the Achilles and quadriceps tendon. The rotator cuff lacks both a synovial sheath and a paratenon. Instead, it is lined by the shoulder capsule on its articular surface and a bursa on its subacromial surface.
All tendons receive vascular supply from three sources: 1. the tendons - periosteal insertion; 2. the myotendinous junction; and 3. the surrounding soft tissues. The proportion of the blood supply from each source is not uniform across all types of tendons. The intra synovial flexor tendons have a limited direct vascular supply. The perfusion from myotendinous and periosteal vessels is minimal, and the synovial flexor sheath does not have the same rich blood supply as a paratenon. Vincular vessels from the digital arteries provide the most significant, yet still incomplete, direct blood supply to the tendon. Instead, a vital source of flexor tendon nutrition is from the diffusion of nutrients from the synovial fluid within the sheath, much like articular cartilage (Manske and Lesker, 1982; Manske and Bridwell, 1978). Extrasynovial tendons with a paratenon, like the Achilles and quadricepts tendon, receive a rich, anastomosing blood supply originating from neighboring soft tissue vessels. The Achilles tendon receives up to 35% of its supply from the paratenon (Naito and Ogata, 1983; Kvist et al., 1992). Vessels from the myotendinous junction perfuse the proximal third of the tendon, with a smaller contribution from vessels at the insertion into the calcaneus (Carr and Norris, 1989). In contrast, the rotator cuff does not have a synovial sheath or a paratenon. Instead large arterioles on its bursal surface contribute to its vascular supply (Ling et al., 1990). Larger arteries, the anterior and posterior humeral circumflex and suprascapular vessels, are largely responsible for the blood supply to the rotator cuff.
Despite receiving their blood supply from multiple different sources, many tendons have known regions of hypovascularity; these are known as watershed areas. Flexor tendons as a whole are hypovascular relative to their surrounding environments (Brockis, 1953). The Achilles tendon has a watershed area that is 2cm to 7cm proximal to its insertion (Koh et al., 2002). The presence of a watershed region in the rotator cuff is not universally accepted; however, a “critical zone” just at the insertion of the supraspinatus had been labeled as a region of vascular insufficiency (Codman, 1931; Moseley and Goldie, 1963; Rathbun and Macnab, 1970). Later reports claimed decreased perfusion was limited to the insertional surface of the distal tendon, whereas others have reported hypervascularity to the area (Tang, 2005; Ling et al., 1990). Although these regions of vascular insufficiency are commonly affected in tendon pathology, there is still little evidence to claim direct responsibility.
Tendon healing progresses through three phases, similar to bone and other tissues. The inflammatory phase is characterized by increased vascular permeability and an influx of local inflammatory cells including platelets, macrophages, monocytes, and neutrophils that release chemotactic agents to recruit blood vessels, fibroblasts, and intrinsic tenocytes. During the second, proliferative phase, fibroblasts at the injury produce collagen and matrix, and angiogenesis provides an increased local vascular supply. The third phase is the remodeling phase, in which there is a decrease in cellularity and an increase in collagen organization parallel to the axis of the tendon.
Tendon healing is believed to occur via two separate pathways: intrinsic and extrinsic. The intrinsic pathway model involves cells from the endotendon and epitendon that migrate to the area of tendon injury. In the extrinsic pathway, cells not originating from within the tendon are predominantly involved in repair. These include inflammatory cells, stem cells, and cells from other tissue compartments. Biomechanically, the intrinsic pathway of healing, or completely intratendinous healing, results in strong, more elastic tendons. The extrinsic pathway results in a healing callus, and contributes greatly to the formation of adhesions.
Differences in the anatomy and physiology must be considered in the repair and healing of each tendon type. In all tendons, repair occurs by fibroblasts that are both intrinsic to the tendon (tenocytes) and by those that are extrinsic, residing in the surrounding sheath, paratenon, and/or soft tissues. Although both populations of fibroblasts are important in the repair process, it is likely that the contribution from each is different for each of the different tendon types (Chang et al., 1990). The role of extrinsic healing is relatively limited in flexor tendons, because it leads to the development of adhesions and loss of motion (Chang et al., 2000). Furthermore the presence of synovial fluid physically inhibits the ability of extrinsic growth factors to reach the intrinsic cells of a flexor tendon. In extrasynovial tendons, however, the extrinsic elements of the paratenon provide a significant fibroblastic response, by surrounding the injury in a fibrovascular callus. Furthermore, in the rotator cuff, the subacromial bursa may offer an extrinsic contribution to healing (Uhthoff and Sarkar, 1991). The extrinsic system is thought to respond earlier in the repair process; intrinsic cells of the epitenon, endotenon, and lastly the tendon core respond shortly afterwards (Soslowsky et al., 1996; Khan et al., 1998; 2000). However, the exact contribution of each repair mechanism is undetermined, as is the exact sequence of the cellular response.
The anatomy of a tendon’s injury as well as its function must be taken into consideration. In order to achieve acceptable function after injury to intrasynovial tendons, its gliding function must be restored. In these tendons, limitation of adhesion formation is paramount. In extrasynovial tendons, restoring strength is most important to the tendon’s healing due to high level forces they endure. Restoration of the tendon’s collagen organization and tendon to tendon healing may be of greatest importance in these extrasynovial tendons. In the rotator cuff, injury usually occurs at a tendon bone interface. Healing of tendon to bone must be preferentially considered when attempting to repair this tendon. Re-establishment of the specialized enthesis structure at the bone tendon interface is still a clinical challenge and focus of intense research efforts.
Current tissue engineering techniques utilize any combination of three key components: a cellular component, a carrier vehicle/matrix or scaffold, and a bioactive component. The cellular component should consist of cells that are accessible, manipulatable, and nonimmunogenic. They should be responsive to environmental cues while maintaining a certain degree of phenotypic stability. The carrier component can act as both a delivery vehicle and a matrix scaffold. The carrier/scaffold should be biologically compatible to both the cellular component and the recipient host tissue. The carrier should allow for both in vitro and in vivo cellular seeding as well as integration into local host tissue without causing a detrimental inflammatory reaction. It should be biodegradable and biomechanically robust, acting as a temporary structural framework until its role is replaced by naturally synthesized matrix. The bioactive component should act as an inductive factor for tissue repair, augmenting chemotaxis, cell differentiation, proliferation, and/or matrix production. These include endogenous administration of synthetic growth factors and morphogens, as well as platelet rich plasma (PRP). In almost all instances the main components of tissue engineering are combined.
Animal models of tendon subtypes
Animal model systems are frequently employed to study various tissue engineering methods for orthopedic applications. Flexor tendon healing and repair has been studied primarily in rabbits, chickens, and canines. The model requires a gliding, sheathed tendon that is surgically practical to manipulate. A common model for the rotator cuff is the rat, due to their accessibility and low cost, and also because of the remarkable anatomic similarity to the human shoulder (Soslowsky et al., 1996; Derwin et al., 2006). Larger animals such as sheep, goats, and canines have also been used for their greater size and load bearing. The Achilles tendon is readily accessible in most animal models, and therefore has seen experimentation in most of the above mentioned systems.
A scaffold serves as a supporting framework. Scaffolds used in tissue engineering, and specifically tendon repair are biocompatible, biodegradable structures that can mimic native extracellular matrix in both form and function. Scaffolds must be able to support and hold cells, whether these cells are implanted on them, or they are chemo-attracted to them. Furthermore, ideal scaffolds offer biomechanical properties that can support early range of motion of healing structures. Investigative interest in bioactive scaffolds is very high due to the potential convenience they offer (manufacturing, storage, and clinical application). They have been studied alone, and in conjunction with other tissue engineering components, including cells and growth factors.
The use of scaffolds alone in flexor tendons has not been highly studied, but they have been combined with tenocytes in an effort to engineer an autologous tendon graft. Leghorn hen flexor tendon tenocytes were harvested, seeded onto polyglycolic acid (PGA) scaffolding shaped as a tendon, and cultured. These grafts were then used to repair 3-4cm flexor tendon defects, demonstrating superior histologic and biomechanical repairs when compared to PGA scaffolding alone (Cao et al., 2002; Adams et al., 2006). However, the use of scaffolds in flexor tendon repairs may have a detrimental effect on tendon gliding, due to their size, and the lack of space within a repaired synovial sheath.
Scaffolds for both the Achilles tendon and the rotator cuff have been investigated both as structural supports and as delivery systems for other tissue engineering modalities. Four of the matrix scaffolds that are commercially available for rotator cuff repair (GraftJacketTM, TissueMendTM, RestoreTM, and CuffPatchTM) were compared in their biomechanical properties to canine infraspinatus tendons. Derwin et al. (2006) found the elastic moduli of these scaffolds to be an order of magnitude less than actual canine tendon, suggesting that these scaffolds may not be used as a means of strengthening an immediate repair. Separately, a dermal matrix graft was able to produce a biomechanical repair equal to that of an autologous tendon graft by twelve weeks in full-thickness canine rotator cuff tears (Adams et al., 2006). Another study comparing different commercially available scaffold patches demonstrated the varying ways they failed under tension (Barber et al., 2006). In a clinical trial, rotator cuff tears that were augmented with porcine small intestine submucosa failed to demonstrate improved healing rates or clinical scores when compared to traditional repair (Iannotti et al., 2006). In goat infraspinatus tendons, polylactic acid (PLA) scaffolds used in conjunction with surgical repair failed to demonstrate a biomechanical advantage, although an earlier study looking at initial repair strength in sheep infraspinatus tears suggested a 25% increase over control repairs (MacGillivray et al., 2006; Koh et al., 2002).
In the Achilles tendon, Brodie et al. (2011) were able to improve the biomechanical properties of Achilles tendon repairs using a bio-adhesive coated scaffold. They used a porcine cadaveric model where suture repair alone was used in one group and in a second experimental group the suture repair was augmented with a tissue scaffold of either pericardium or dermal tissue that was augmented with a bioadhesive coating. In biomechanical testing the experimental group showed an improvement in stiffness, load to failure, and energy expenditure to failure.
Growth factors act as mitogens in tissue repair, and their local administration has been the focus of many investigations. Some of the growth factors that have been identified as having an effect in flexor tendon repair are transforming growth factor β (TGF-β), insulin-like growth factor (IGF), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and basic fibroblastic growth factor (bFGF2). Consistent with the potent mitogenic role that IGF has with many other tissues, it has been shown to increase collagen, proteoglycan, and tenocyte proliferation in rabbit flexor tendons, and has improved healing in a flexor tendonitis model (Abrahamsson et al., 1991; Dahlgren et al., 2002). Simultaneous administration of PDGF-β, IGF-1, and FGF-2 show a synergistic effect on tenocyte proliferation in vitro (Costa et al., 2006). TGF-β is a ubiquitous growth factor, with effects that are largely determined by the individual cell type. In vitro studies have shown increased concentration of this growth factor in injured tendon, and increased collagen synthesis in response to TGF-β administration (Thomopoulos et al., 2007; Klein et al., 2002). However, in vitro and in vivo investigations using a rabbit model demonstrated decreased type-I collagen synthesis and increased range of motion after surgical repair when TGF-β was inhibited, suggesting its potential role in adhesion formation (Bidder et al., 2000; Bates et al., 2006). A knockout study that inhibited the effects of TGF-β showed that adequate tendon healing can still occur in its absence (Tsuboneet et al., 2006). PDGF is one of the earliest responding growth factors following tendon injury. Its presence was upregulated in healing canine flexor tendons, and has been associated with increased collagen and proteoglycan synthesis and tenocyte DNA synthesis in vitro and in vivo (Duffy et al., 1995; Yoshikawa and Abrahamsson, 2001; Thomopoulos et al., 2007). Although VEGF has been found in higher concentrations in healing flexor tenocytes than in uninjured tendon, there are no studies that report results of experimental manipulations of flexor tendon repairs with VEGF (Bidder et al., 2000; Boyer et al., 2001).
Growth factors have been extensively investigated in Achilles tendon repairs. Injecting vascular endothelial growth factor (VEGF) into healing Achilles tendons has demonstrated increased tensile strength during repair after two weeks, with no difference in the two groups after four weeks (Zhang et al., 2003). Growth and differentiation factor 5 (GDF-5) coated sutures improved histologic quality of repair and biomechanical tensile strength and load to failure in rat Achilles surgical defects during the early stages of tendon healing (3 weeks) (Dines et al., 2007). An earlier study using sutures similarly coated with GDF-5 also described a stronger repair during early healing (2 weeks) in rat Achilles tendons (Rickert et al., 2001). Recent studies, by our group, investigated sutures coated with PDGF to augment rat Achilles tendon repairs. We observed significant improvements in load to failure at earlier time points compared to controls.
The rotator cuff specifically has become a popular model for the study of local growth factor application. The more commonly studied bioactives include members of the transforming growth factor superfamily, such as TGF-β and the several bone morphogenic proteins (BMP); growth factors such as platelet derived growth factor-beta (PDGF-β), insulin-like growth factor-1 (IGF-1) and basic fibroblast growth factor (bFGF) have also been explored. As the number of investigations looking at the local milieu of tendinopathy and tendon repair continues to increase, it is probable that additional bioactive factors will be discovered and will appear in the literature. In a sheep model, a construct that contained a combination of growth factors (BMP 2-7 and TGF-β 1-3) in conjunction with a type I collagen carrier was used to increase load to failure in infraspinatus tendon to bone repair. It was noted, by the author, however, that the improved repair strength could have been due to an observed increased quantity of repair tissue, not an actual improvement in quality of repair tissue (Rodeo, 2007). Cartilage derived morphogenic protein 2 (CDMP-2), also known as growth and differentiation factor 5 (GDF-5) was utilized in a different study to improve biomechanical and histologic quality of repair in rat rotator cuff tears (Murray et al., 2007). Recently studies have been directed towards sustained release of growth factor to the rotator cuff. Manning et al. (2011) used a heparin-fibrin based delivery system. They found that when combining TGFβ-3 to their delivery system and using it in repaired rat rotator cuffs they were able to see both histological and biomechanical improvements in healing up to 56 days post repair.
Platelet Rich Plasma
The use of plasma rich plasma (PRP) by orthopedic surgeons and others has grown rapidly, stimulated in part because of the potential for improved healing that might occur as a result of the extraordinarily high concentration of the various factors within PRP as well as the ease of preparation and low cost. Eppley et al. (2004) found a 6.2-fold increase of VEGF, a 5.1-fold increase PDGF-β, a 3.9-fold increase in EGF, and a 3.6-fold increase in TGFβ-1 in activated PRP. PRP not only contains increased levels of growth factor found in its alpha granules, it also contains elevated amounts of fibrin, fibronectin, and vitronectin. These are important for cell adhesion and epithelial migration (Eppley et al., 2004). In vitro studies using PRP have been promising. Schnabel and his colleagues examined gene expression patterns, cell proliferation, and collagen content of equine flexor digitorum superficialis tendon explants cultured in media consisting of PRP or other blood products (blood, plasma, platelet poor plasma, bone marrow aspirate). The results of this experiment showed that tendons cultured in 100% PRP showed the most significant enhancement of mRNA expression of the matrix molecules COL1A1 (collagen type I), COL3A1 (collagen type 3), and COMP (cartilage oligomeric matrix protein) with no concomitant increase in the catabolic molecules MMP-3 and MMP-13 (Schnabel et al., 2007). Another study found a specific enhancement of the extrinsic healing pathway by PRP. In this study, rat tendon injuries, in which local PRP was injected, were found to have increased levels of circulatory derived cells infiltrating the repair site. These cells included circulating inflammatory cells, as well as cells derived from the bone marrow. This increase in repair site infiltration led to the conclusion that PRP can have an effect on not only intrinsic healing, but also the extrinsic pathway (Kajikawa et al., 2008).
PRP augmentation of repair has been investigated in animal models of flexor tendon injuries. In an equine model, digital flexor tendons were surgically cut and repaired. One group of tendons received a single injection of PRP, while the control group received an injection of saline. The PRP group of tendons had a higher strength to failure and elastic modulus as compared to the controls (Bosch et al., 2010). To date no human trials using PRP in human flexor tendon injuries have been studied.
PRP augmentation of Achilles tendon ruptures has been controversial. Sanchez et al. (2007) looked at Achilles tendon repairs in athletes that were augmented with PRP. He then compared their clinical outcomes with matched cohort that did not have PRP augmentation. They observed that the athletes who received the PRP recovered their range of motion earlier (7 ± 2 weeks vs. 11 ± 3 weeks), showed no wound complication, and took less time to take up gentle running (11 ± 1 week vs. 18 ± 3 weeks) and to resume training activities (14 ± 0.8 weeks vs. 21 ± 3 weeks). In addition, the cross-sectional area of the PRP treated tendon increased less. In a clinical study from the Netherlands comparing the use of PRP injection versus saline injection administered to patients with chronic Achilles tendinopathy, the PRP treated patients did not have any greater improvement in pain and activity than the saline treated patients (de Vos et al., 2010). More recently in a randomized single blinded study PRP was shown to have no effect on Achilles tendon rupture healing in humans (Schepull et al., 2011).
PRP augmented rotator cuff repairs have also been investigated. Recently a randomized controlled trial in arthroscopic rotator cuff repairs using or not using a PRP matrix to augment the repair showed no differences in outcomes between the two groups (Castricini et al., 2011).
Mesenchymal Stem Cells
Mesenchymal stem cells (MSCs) are multipotent cells that are known to be the progenitor cells of fibroblasts in tendon healing sites. Autografting of MSCs to tendon repair sites have been investigated. In vitro studies have shown similar positive results as PRP and growth factor augmentation.
MSC augmentation of flexor tendon repairs has shown improvements in repair histology in an in vivo horse flexor tendon model; however, there were no improvements in the biomechanics between the two groups (Schnabel et al., 2009).
MSC repair augmentation in an extrasynovial tendon models has been more successful than in intrasynovial tendons. Histologic quality and biomechanical strength of repair were improved in early Achilles tendon healing (3 weeks) in a rabbit model system with the use of marrow derived mesenchymal stem cells in a fibrin carrier system (Chong et al., 2007). Results from separate projects using MSCs to repair 1cm Achilles tendon defects in rabbits also showed an increased biomechanical advantage (Young et al., 1998; Ouyang et al., 2003). Other groups even reported some histological success in using MSCs to create tissue engineered tendon grafts (Long et al., 2005; Calve et al., 2004).
In a rat rotator cuff model, MSCs were added to repair sites. These tendons were then looked at histologically and biomechanically. No improvements in peak stress to failure or stiffness were found between the group of rats in which MSCs were added to the repair sites and those without (Gulotta et al., 2009).
Gene Therapy for Tendon Repair
Gene therapy is the insertion of genetic material into the genome of a cell or groups of cells that can intrinsically change the cells’ function. This therapeutic modality has been studied in tendon repair as a way to modify the cells in injured tendons to heal more efficiently and effectively.
Similar to other aspects of tissue engineering, gene therapy has been utilized to augment the application of growth factors in flexor tendon repair. In a rat flexor tendon model, transduced by plasmids containing PDGF-β, treated tendons produced a significantly greater amount of type I collagen DNA than control tendons in vitro (Wang et al., 2004). Similarly, rat flexor tenocytes transduced with bFGF using a viral vector increased type I and III collagen DNA synthesis in vitro (Wang et al., 2005).
Adenovirus particles carrying growth and differentiation factor-5 that were injected into transected rat Achilles tendons demonstrated a significantly thicker repair site than controls, but only a trend towards greater tensile strength (Rickert et al., 2005). Whereas, local administration of adenovirus expressing BMP-14 into rat Achilles defects demonstrated an increase in both tenocyte proliferation and repair site tensile strength (Bolt et al., 2007).
PDGF-β and IGF-1 transduced fibroblasts combined with a PGA scaffold were shown to improve tenocyte proliferation and collagen synthesis in vitro, and histologic repair in a rat rotator cuff repair model. The same IGF-1 transduced constructs also improved the biomechanical strength of repair in this model system (Dines et al., 2007; Uggen et al., 2005).
Combination of Tissue Engineering Techniques
All of the aforementioned tissue engineering techniques described have been investigated in conjunction with one another, in addition to being investigated alone. Scaffolds have been impregnated with stem cells, and stem cells have been engineered to express greater levels of certain growth factor.
Influence of Local Environment
As the differences in each of the methods of tendon repair augmentation become more apparent, the question arises whether there is an identifiable difference in these tendon types that makes their responses to methods of augmentation more predictable. One aspect that may be important is the local environment of the tendon itself. As outlined in Table 1, the three representative tendon types (flexor, Achilles, and rotator cuff) have different blood supplies, local environments, and primary repair methods. While the synovial based tendons appear to utilize an intrinsic repair method, the non-synovial tendons, which tend to get the majority of their blood supply by direct vascularization from surrounding paratenon and bursa, seem to utilize more of an extrinsic method of repair. This suggests that extra-synovial tendons may be more likely to respond to augmentation techniques that supply the surrounding environment with enhanced growth factors and cellular markers for repair than synovial tendons. These include scaffolds, mesenchymal stem cells, and possibly PRP. Synovial tendons, in contrast, which seem to rely on intrinsic methods of repair, may be more likely to respond to augmentation techniques, such as gene therapy, and locally delivered growth factors that affect the intrinsic healing potential. Furthermore, enhancements of the extrinsic healing pathway in intrasynovial tendons may also be detrimental. Enhanced extrinsic healing may cause increased scarring and adhesions which may inhibit tendon gliding.
Anatomical differences in each tendon type may also contribute to the tissue engineering modality better suited for repair in that tendon. In flexor tendons, the repair is most reliant on tendon to tendon healing, whereas in repairs of rotator cuff injuries bone to tendon healing is most important. The use of growth factors, such as those in the BMP superfamily, have been shown to enhance tendon to bone healing (Rodeo, 2007).
Delivery of the each engineering modality to the three different types of tendons also must be taken into consideration (Table 1). Intra-synovial tendons are bathed in synovial fluids. This fluid turns over frequently. Injecting growth factor or PRP may be problematic. The growth factor may not be present in the local environment long enough to have an effect on healing. Delivery systems are being developed, however, that can keep these growth factors in the local healing milieu long enough to have an effect. These include suture coating and binding systems (Dines et al., 2007; Eppley et al., 2004). Gene therapy can change the healing capacity of a cell after one dose thereby bypassing the necessity for it to have to be present for a long period of time.
While there have been many advances in tissue engineering and tendon repair in experimental settings, translation of this work is required to make this technology practical and applicable in clinical settings. While the currently available methods of tendon repair are encouraging, each has its own limitations that will need to be improved. An understanding of specific tendon anatomy and physiology and the predominant healing pathway found in that tendon aids in directing which tissue engineering modality will be most successful to enhance healing.
The authors report no conflicts of interest.
Daniel A. Grande, Ph.D., Orthopedic Research Laboratory, The Feinstein Institute for Medical Research, Manhasset, New York 11030, USA.
Abrahamsson SO, Lundborg G, Lohmander LS. Recombinant human insulin-like growth factor-I stimulates in vitro matrix synthesis and cell proliferation in rabbit flexor tendon. J Orthop Res 9:495-502, 1991.
Adams JE, Zobitz ME, Reach JS, Jr, An KN, Steinmann SP. Rotator cuff repair using an acellular dermal matrix graft: an in vivo study in a canine model. Arthroscopy 22:700-709, 2006.
Barber FA, Herbert MA, Coons DA. Tendon augmentation grafts: biomechanical failure loads and failure patterns. Arthroscopy 22:534-538, 2006.
Bates SJ, Morrow E, Zhang AY, Pham H, Longaker MT, Chang J. Mannose-6-phosphate, an inhibitor of transforming growth factor-beta, improves range of motion after flexor tendon repair. J Bone Joint Surg 88:2465-2472, 2006.
Beredjiklian PK. Biologic aspects of flexor tendon laceration and repair. J Bone Joint Surg 85-A(3):539-550, 2003.
Berenson MC, Blevins FT, Plaas AH, Vogel KG. Proteoglycans of human rotator cuff tendons. J Orthop Res 14:518-525, 1996.
Bidder M, Towler DA, Gelberman RH, Boyer MI. Expression of mRNA for vascular endothelial growth factor at the repair site of healing canine flexor tendon. J Orthop Res 18:247-252, 2000.
Bolt P, Clerk AN, Luu HH, Kang Q, Kummer JL, Deng ZL, Olson K, Primus F, Montag AG, He TC, Haydon RC, Toolan BC. BMP-14 gene therapy increases tendon tensile strength in a rat model of Achilles tendon injury. J Bone Joint Surg 89:1315-1320, 2007.
Bosch G, van Schie HT, de Groot MW, Cadby JA, van de Lest CH, Barneveld A, van Weeren PR. Effects of platelet-rich plasma on the quality of repair of mechanically induced core lesions in equine superficial digital flexor tendons: A placebo-controlled experimental study. J Orthop Res 28(2):211-217, 2010.
Boyer MI, Watson JT, Lou J, Manske PR, Gelberman RH, Cai SR. Quantitative variation in vascular endothelial growth factor mRNA expression during early flexor tendon healing: an investigation in a canine model. J Orthop Res 19:869-872, 2001.
Brockis JG. The blood supply of the flexor and extensor tendons of the fingers in man. J Bone Joint Surg 35-B:131-138, 1953.
Brodie M, Vollenweider L, Murphy JL, Xu F, Lyman A, Lew WD, Lee BP. Biomechanical properties of Achilles tendon repair augmented with a bioadhesive-coated scaffold. J Biomed Mater Res 6(1):015014, 2011.
Burkhead WZ, Jr, Habermeyer P. The rotator cuff: a historical review of our understanding. In: Rotator Cuff Disorders. pp. 3-20. Burkhead WZ, Jr, (ed.). Williams and Wilkins, Philadelphia, Pennsylvania, USA, 1996.
Calve S, Dennis RG, Kosnik PE, 2nd, Baar K, Grosh K, Arruda EM. Engineering of functional tendon. Tissue Eng 10:755-761, 2004.
Cao Y, Liu Y, Liu W, Shan Q, Buonocore SD, Cui L. Bridging tendon defects using autologous tenocyte engineered tendon in a hen model. Plast Reconstr Surg 110:1280-1289, 2002.
Carr AJ, Norris SH. The blood supply of the calcaneal tendon. J Bone Joint Surg 71:100-101, 1989.
Castricini R, Longo UG, De Benedetto M, Panfoli N, Pirani P, Zini R, Maffulli N, Denaro V. Platelet-rich plasma augmentation for arthroscopic rotator cuff repair: a randomized controlled trial. Am J Sports Med 39(2):258-265, 2011.
Chang J, Most D, Stelnicki E, Siebert JW, Longaker MT, Hui K, Lineaweaver WC Gene expression of transforming growth factor beta-1 in rabbit zone II flexor tendon wound healing: evidence for dual mechanisms of repair. Plast Reconstr Surg 100:937-944, 1997.
Chang J, Thunder R, Most D, Longaker MT, Lineaweaver WC. Studies in flexor tendon wound healing: neutralizing antibody to TGF-beta1 increases postoperative range of motion. Plast Reconstr Surg 105:148-155, 2000.
Chong AK, Ang AD, Goh JC, Hui JH, Lim AY, Lee EH, Lim BH. Bone marrow-derived mesenchymal stem cells influence early tendon-healing in a rabbit Achilles tendon model. J Bone Joint Surg 89:74-81, 2007.
Clark JM, Harryman DT, 2nd. Tendons, ligaments, and capsule of the rotator cuff. Gross and microscopic anatomy. J Bone Joint Surg74:713-725, 1992.
Codman E, IB A. The pathology associated with rupture of the supraspinatus tendon. Ann Surg 93:348-359, 1931.
Costa MA, Wu C, Pham BV, Chong AK, Pham HM, Chang J. Tissue engineering of flexor tendons: optimization of tenocyte proliferation using growth factor supplementation. Tissue Eng 12:1937-1943, 2006.
Dahlgren LA, van der Meulen MC, Bertram JE, Starrak GS, Nixon AJ. Insulin-like growth factor-I improves cellular and molecular aspects of healing in a collagenase-induced model of flexor tendinitis. J Orthop Res 20:910-919, 2002.
de Vos RJ, Weir A, van Schie HTM, Bierma-Zeinstra SMA, Verhaar JAN, Weinans H, Tol JL. Platelet-rich plasma injection for chronic Achilles tendinopathy: a randomized controlled trial. JAMA 303(2):144-149, 2010.
Derwin KA, Baker AR, Spragg RK, Leigh DR, Iannotti JP. Commercial extracellular matrix scaffolds for rotator cuff tendon repair. Biomechanical, biochemical, and cellular properties. J Bone Joint Surg 88:2665-2672, 2006.
Dines JS, Grande DA, Dines DM. Tissue engineering and rotator cuff tendon healing. J Shoulder Elbow Surg 16(5 Suppl):S204-S207, 2007.
Dines JS, Weber L, Razzano P, Prajapati R, Timmer M, Bowman S, Bonasser L, Dines DM, Grande DA. The effect of growth differentiation factor-5-coated sutures on tendon repair in a rat model. J Shoulder Elbow Surg 16(5 Suppl):S215-S221, 2007.
Duffy FJ, Jr, Seiler JG, Gelberman RH, Hergrueter CA. Growth factors and canine flexor tendon healing: initial studies in uninjured and repair models. J Hand Surg 20:645-649, 1995.
Edwards DA. The blood supply and lymphatic drainage of tendons. J Anat 80:147-152, 1946.
Eppley BL, Woodell JE, Higgins J. Platelet qualification and growth factor analysis from platelet-rich plasma: implications for wound healing. Plast Reconstr Surg 114: 1502-1508, 2004.
Gulotta LV, Kovacevic D, Ehteshami JR, Dagher E, Packer JD, Rodeo SA. Application of bone marrow-derived mesenchymal stem cells in a rotator cuff repair model. Am J Sports Med 37(11):2126-2133, 2009.
Iannotti JP, Codsi MJ, Kwon YW, Derwin K, Ciccone J, Brems JJ. Porcine small intestine submucosa augmentation of surgical repair of chronic two-tendon rotator cuff tears. A randomized, controlled trial. J Bone Joint Surg 88:1238-1244, 2006.
Jimenez SA, Yankowski R, Bashey RI. Identification of two new collagen alpha-chains in extracts of lathyritic chick embryo tendons. Biochem Biophys Res Commun 81:1298-1306, 1978.
Kajikawa Y, Morihara T, Sakamoto H, Matsuda K, Oshima Y, Yoshida A, Nagae M, Arai Y, Kawata M, Kubo T. Platelet-rich plasma enhances the initial mobilization of circulation-derived cells for tendon healing. Am J Physiol Cell Physiol 215(3):837-845, 2008.
Khan U, Kakar S, Akali A, Bentley G, McGrouther DA. Modulation of the formation of adhesions during the healing of injured tendons. J Bone Joint Surg 82:1054-1058, 2000.
Khan U, Occleston NL, Khaw PT, McGrouther DA. Differences in proliferative rate and collagen lattice contraction between endotenon and synovial fibroblasts. J Hand Surg 23:266-273, 1998.
Klein MB, Yalamanchi N, Pham H, Longaker MT, Chang J. Flexor tendon healing in vitro: effects of TGF-beta on tendon cell collagen production. J Hand Surg 27:615-620, 2002.
Koh JL, Szomor Z, Murrell GA, Warren RF. Supplementation of rotator cuff repair with a bioresorbable scaffold. Am J Sports Med 30:410-413, 2002.
Kvist M, Jozsa L, Jarvinen M. Vascular changes in the ruptured Achilles tendon and paratenon. Int Orthop 16:377-382, 1992.
Ling SC, Chen CF, Wan RX. A study on the vascular supply of the supraspinatus tendon. Surg Radiol Anat 12:161-165, 1990.
Long JH, Qi M, Huang XY, Lei SR, Ren LC. Repair of rabbit tendon by autologous bone marrow mesenchymal stem cells. Zhong Hua Shao Shang Za Zhi (Chinese Journal of Burns) 21:210-212, 2005.
MacGillivray JD, Fealy S, Terry MA, Koh JL, Nixon AJ, Warren RF. Biomechanical evaluation of a rotator cuff defect model augmented with a bioresorbable scaffold in goats. J Shoulder Elbow Surg 15:639-644, 2006.
Manning CN, Kim HM, Sakiyama-Elbert S, Galatz LM, Havlioglu N, Thomopoulos S. Sustained delivery of transforming growth factor beta three enhances tendon-to-bone healing in a rat model. J Orthop Res, epub ahead of print, Jan. 18, 2011.
Manske PR, Bridwell K, Lesker PA. Nutrient pathways to flexor tendons of chickens using tritiatedproline. J Hand Surg 3:352-357, 1978.
Manske PR, Bridwell K, Whiteside LA, Lesker PA. Nutrition of flexor tendons in monkeys. Clin Orthop Relat Res (136):294-298, 1978.
Manske PR, Lesker PA. Nutrient pathways of flexor tendons in primates. J Hand Surg 7:436-444, 1982.
Moseley HF, Goldie I. The arterial pattern of the rotator cuff of the shoulder. J Bone Joint Surg 45:780-789, 1963.
Murray DH, Kubiak EN, Jazrawi LM, Araghi A, Kummer F, Loebenberg MI, Zuckerman JD. The effect of cartilage-derived morphogenetic protein 2 on initial healing of a rotator cuff defect in a rat model. J Shoulder Elbow Surg 16:251-254, 2007.
Naito M, Ogata K. The blood supply of the tendon with a paratenon. An experimental study using hydrogen washout technique. Hand 15:9-14, 1983.
O’Brien M. Structure and metabolism of tendons. Scand J Med Sci Sports 7:55-61, 1997.
Ouyang HW, Goh JC, Thambyah A, Teoh SH, Lee EH. Knitted poly-lactide-co-glycolide scaffold loaded with bone marrow stromal cells in repair and regeneration of rabbit Achilles tendon. Tissue Eng 9:431-439, 2003.
Piez KA, Miller EJ, Lane JM, Butler WT. The order of the CNBr peptides from the alpha-1 chain of collagen. Biochem Biophys Res Commun 37:801-805, 1969.
Rathbun JB, Macnab I. The microvascular pattern of the rotator cuff. J Bone Joint Surg 52:540-553, 1970.
Rickert M, Jung M, Adiyaman M, Richter W, Simank HG. A growth and differentiation factor-5 (GDF-5)-coated suture stimulates tendon healing in an Achilles tendon model in rats. Growth Factors 19:115-126, 2001.
Rickert M, Wang H, Wieloch P, Lorenz H, Steck E, Sabo D, Richter W. Adenovirus-mediated gene transfer of growth and differentiation factor-5 into tenocytes and the healing rat Achilles tendon. Connect Tissue Res 46:175-183, 2005.
Rodeo SA. Biologic augmentation of rotator cuff tendon repair. J Shoulder Elbow Surg 16(5 Suppl):S191-S197, 2007.
Sanchez M, Anitua E, Azofra J, Andia I, Padilla S, Mujika I. Comparison of surgically repaired Achilles tendon tears using platelet-rich fibrin matrices. Am J Sports Med 35:245-251, 2007.
Schepull T, Kvist J, Norrman H, Trinks M, Berlin G, Aspenberg P. Autologous platelets have no effect on the healing of human Achilles tendon ruptures: a randomized single-blind study. Am J Sports Med 39(1):38-47, 2011.
Schnabel LV, Lynch ME, van der Meulen MC, Yeager AE, Kornatowski MA, Nixon AJ. Mesenchymal stem cells and insulin-like growth factor-I gene-enhanced mesenchymal stem cells improve structural aspects of healing in equine flexor digitorum superficialis tendons. J Orthop Res 27(10):1392-1398, 2009.
Schnabel LV, Mohammed HO, Miller BJ, McDermott WG, Jacobson MS, Santangelo KS, Fortier LA. Platelet rich plasma (PRP) enhances anabolic gene expression patterns in flexor digitorum superficialis tendons. J Orthop Res 25(2):230-240, 2007.
Soslowsky LJ, Carpenter JE, DeBano CM, Banerji I, Moalli MR. Development and use of an animal model for investigations on rotator cuff disease. J Shoulder Elbow Surg 5:383-392, 1996.
Tang JB. Clinical outcomes associated with flexor tendon repair. Hand Clin 21:199-210, 2005.
Thomopoulos S, Zaegel M, Das R, Harwood FL, Silva MJ, Amiel D, Sakiyama-Elbert S, Gelberman RH. PDGF-BB released in tendon repair using a novel delivery system promotes cell proliferation and collagen remodeling. J Orthop Res 25(10):1358-1368, 2007.
Tsubone T, Moran SL, Subramaniam M, Amadio PC, Spelsberg TC, An KN. Effect of TGF-beta inducible early gene deficiency on flexor tendon healing. J Orthop Res 24:569-575, 2006.
Uggen JC, Dines J, Uggen CW, Mason JS, Razzano P, Dines D, Grande DA. Tendon gene therapy modulates the local repair environment in the shoulder. J Am Osteopath Assoc 105:20-21, 2005.
Uhthoff HK, Sarkar K. Surgical repair of rotator cuff ruptures. The importance of the subacromial bursa. J Bone Joint Surg 73:399-401, 1991.
Vogel KG, Chevalier P. Using RT-PCR to measure mRNA levels for aggrecan and biglycan in bovine tendon. 43rd Annual Meeting of Orthopedic Research Society, San Francisco, CA. Trans Orthop Res 22:abstr #111, 1997.
Wang XT, Liu PY, Tang JB. Tendon healing in vitro: genetic modification of tenocytes with exogenous PDGF gene and promotion of collagen gene expression. J Hand Surg 29:884-890, 2004.
Wang XT, Liu PY, Xin KQ, Tang JB. Tendon healing in vitro: bFGF gene transfer to tenocytes by adeno-associated viral vectors promotes expression of collagen genes. J Hand Surg 30:1255-1261, 2005.
Yamaguchi K, Levine WN, Marra G, Galatz LM, Klepps S, Flatow EL. Transitioning to arthroscopic rotator cuff repair: the pros and cons. Instr Course Lect 52:81-92, 2003.
73Yoshikawa Y, Abrahamsson SO. Dose-related cellular effects of platelet-derived growth factor-BB differ in various types of rabbit tendons in vitro. Acta Orthop Scand 72:287-292, 2001.
Young RG, Butler DL, Weber W, Caplan AI, Gordon SL, Fink DJ. Use of mesenchymal stem cells in a collagen matrix for Achilles tendon repair. J Orthop Res 16:406-413, 1998.
Zhang F, Liu H, Stile F, Lei MP, Pang Y, Oswald TM, Beck J, Doresett-Martin W, Lineaweaver WC. Effect of vascular endothelial growth factor on rat Achilles tendon healing. Plast Reconstr Surg 112:1613-1619, 2003.
[Discovery Medicine; ISSN: 1539-6509; Discov Med 12(62):75-84, July 2011. Copyright © Discovery Medicine. All rights reserved.]