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Abstract: The rate of rerupture of repaired Achilles tendon in young and athletic populations remains high despite improvement in surgical techniques, suture design, and postsurgical management. Acellular biological matrices can be used to enhance the immediate strength of repaired tendons and to serve as scaffolds for cell in-growth and constructive tissue remodeling. A number of commercially available matrices have been used clinically, albeit with varying degrees of success and failure. The disparity is likely attributable to the different physical and biochemical properties of individual matrices. In this study, we investigated the biomechanical characteristics of 2 different acellular collagen matrices, namely TissueMend® and GraftJacket®, using a sheep Achilles tendon repair model. Static and cyclic creep, cyclic and linear construct stiffness, maximum load to failure, and displacement at maximum load were determined at time zero. We found that the maximum load to failure, displacement, and ultimate failure mode were similar between tendons augmented with either acellular collagen matrix; however, TissueMend augmentation yielded lower creep and smaller construct elongation than did GraftJacket. The results indicated that the strength of TissueMend-augmented tendons and GraftJacket-augmented tendons was not statistically significantly different, although tendons augmented with TissueMend displayed greater stiffness, which may be clinically advantageous in the restoration of ruptured tendons. [ABSTRACT FROM AUTHOR]

Biological fixation of soft tissue-based grafts for anterior cruciate ligament (ACL) reconstruction poses a major clinical challenge. The ACL integrates with subchondral bone through a fibrocartilage enthesis, which serves to minimize stress concentrations and enables load transfer between two distinct tissue types. Functional integration thus requires the reestablishment of this fibrocartilage interface on reconstructed ACL grafts. We designed and characterized a novel mechanoactive scaffold based on a composite of poly-alpha-hydroxyester nanofibers and sintered microspheres; we then used the scaffold to test the hypothesis that scaffold-induced compression of tendon grafts would result in matrix remodeling and the expression of fibrocartilage interface-related markers. Histology coupled with confocal microscopy and biochemical assays were used to evaluate the effects of scaffold-induced compression on tendon matrix collagen distribution, cellularity, proteoglycan content, and gene expression over a 2-week period. Scaffold contraction resulted in over 15% compression of the patellar tendon graft and upregulated the expression of fibrocartilage-related markers such as Type II collagen, aggrecan, and transforming growth factor-beta3 (TGF-beta3). Additionally, proteoglycan content was higher in the compressed tendon group after 1 day. The data suggest the potential of a mechanoactive scaffold to promote the formation of an anatomic fibrocartilage enthesis on tendon-based ACL reconstruction grafts. [ABSTRACT FROM AUTHOR]

A significant challenge to orthopaedic soft tissue repair is the biological fixation of autologous or allogeneic grafts with bone, whereby the lack of functional integration between such grafts and host bone has limited the clinical success of anterior cruciate ligament (ACL) and other common soft tissue-based reconstructive grafts. The inability of current surgical reconstruction to restore the native fibrocartilaginous insertion between the ACL and the femur or tibia, which minimizes stress concentration and facilitates load transfer between the soft and hard tissues, compromises the long-term clinical functionality of these grafts. To enable integration, a stratified scaffold design that mimics the multiple tissue regions of the ACL interface (ligament-fibrocartilage-bone) represents a promising strategy for composite tissue formation. Moreover, distinct cellular organization and phase-specific matrix heterogeneity achieved through co- or tri-culture within the scaffold system can promote biomimetic multi-tissue regeneration. Here, we describe the methods for fabricating a tri-phasic scaffold intended for ligament-bone integration, as well as the tri-culture of fibroblasts, chondrocytes, and osteoblasts on the stratified scaffold for the formation of structurally contiguous and compositionally distinct regions of ligament, fibrocartilage and bone. The primary advantage of the tri-phasic scaffold is the recapitulation of the multi-tissue organization across the native interface through the layered design. Moreover, in addition to ease of fabrication, each scaffold phase is similar in polymer composition and therefore can be joined together by sintering, enabling the seamless integration of each region and avoiding delamination between scaffold layers., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Soft tissue-to-bone transitions, such as the osteochondral interface, are complex junctions that connect multiple tissue types and are critical for musculoskeletal function. The osteochondral interface enables pressurization of articular cartilage, facilitates load transfer between cartilage and bone, and serves as a barrier between these two distinct tissues. Presently, there is a lack of quantitative understanding of the matrix and mineral distribution across this multitissue transition. Moreover, age-related changes at the interface with the onset of skeletal maturity are also not well understood. Therefore, the objective of this study is to characterize the cartilage-to-bone transition as a function of age, using Fourier transform infrared spectroscopic imaging (FTIR-I) analysis to map region-dependent changes in collagen, proteoglycan, and mineral distribution, as well as collagen organization. Both tissue-dependent and age-related changes were observed, underscoring the role of postnatal physiological loading in matrix remodeling. It was observed that the relative collagen content increased continuously from cartilage to bone, whereas proteoglycan peaked within the deep zone of cartilage. With age, collagen content across the interface increased, accompanied by a higher degree of collagen alignment in both the surface and deep zone cartilage. Interestingly, regardless of age, mineral content increased exponentially across the calcified cartilage interface. These observations reveal new insights into both region- and age-dependent changes across the cartilage-to-bone junction and will serve as critical benchmark parameters for current efforts in integrative cartilage repair., (© 2014 American Society for Bone and Mineral Research.)

The interface between bone and connective tissues such as the Anterior Cruciate Ligament (ACL) constitutes a complex transition traversing multiple tissue regions, including non-calcified and calcified fibrocartilage, which integrates and enables load transfer between otherwise structurally and functionally distinct tissue types. The objective of this study was to investigate region-dependent changes in collagen, proteoglycan and mineral distribution, as well as collagen orientation, across the ligament-to-bone insertion site using Fourier transform infrared spectroscopic imaging (FTIR-I). Insertion site-related differences in matrix content were also evaluated by comparing tibial and femoral entheses. Both region- and site-related changes were observed. Collagen content was higher in the ligament and bone regions, while decreasing across the fibrocartilage interface. Moreover, interfacial collagen fibrils were aligned parallel to the ligament-bone interface near the ligament region, assuming a more random orientation through the bulk of the interface. Proteoglycan content was uniform on average across the insertion, while its distribution was relatively less variable at the tibial compared to the femoral insertion. Mineral was only detected in the calcified interface region, and its content increased exponentially across the mineralized fibrocartilage region toward bone. In addition to new insights into matrix composition and organization across the complex multi-tissue junction, findings from this study provide critical benchmarks for the regeneration of soft tissue-to-bone interfaces and integrative soft tissue repair.

The purpose of this study was to compare the gap formation during cyclic loading, maximum repair strength, and failure mode of single-row full-thickness supraspinatus repairs performed using 2 knotless suture anchors with differing internal suture-retention mechanisms in a human cadaver model. Nine matched pairs of cadaver shoulders were used. Full-thickness tears were induced by detaching the supraspinatus tendon from the greater tuberosity. Single-row repairs were performed with either type I (Opus Magnum PI; ArthroCare, Austin, Texas) or type II (ReelX STT; Stryker, Mahwah, New Jersey) knotless suture anchors. The repaired tendon was cycled from 10 to 90 N for 500 cycles, followed by load to failure. Gap formation was measured at 5, 100, 200, 300, 400, and 500 cycles with a video digitizing system. Anchor type or location (anterior or posterior) had no effect on gap formation during cyclic loading regardless of position (anterior, P=.385; posterior, P=.389). Maximum load to failure was significantly greater (P=.018) for repairs performed with type II anchors (288±62 N) compared with type I anchors (179±39 N). Primary failure modes were anchor pullout and tendon tearing for type II anchors and suture slippage through the anchor for type I anchors. The internal ratcheting suture-retention mechanism of type II anchors may have helped this anchor outperform the suture-cinching mechanism of type I anchors by supporting significantly higher loads before failure and minimizing suture slippage, potentially leading to stronger repairs clinically., (Copyright 2013, SLACK Incorporated.)

The emphasis in the field of orthopaedic tissue engineering is on imparting biomimetic functionality to tissue engineered bone or soft tissue grafts and enabling their translation to the clinic. A significant challenge in achieving extended graft functionality is engineering the biological fixation of these grafts with each other as well as with the host environment. Biological fixation will require re-establishment of the structure-function relationship inherent at the native soft tissue-to-bone interface on these tissue engineered grafts. To this end, strategic biomimicry must be incorporated into advanced scaffold design. To facilitate integration between distinct tissue types (e.g., bone with soft tissues such as cartilage, ligament, or tendon), a stratified or multi-phasic scaffold with distinct yet continuous tissue regions is required to pre-engineer the interface between bone and soft tissues. Using the ACL-to-bone interface as a model system, this review outlines the strategies for stratified scaffold design for interface tissue engineering, focusing on identifying the relevant design parameters derived from an understanding of the structure-function relationship inherent at the soft-to-hard tissue interface. The design approach centers on first addressing the challenge of soft tissue-to-bone integration ex vivo, and then subsequently focusing on the relatively less difficult task of bone-to-bone integration in vivo. In addition, we will review stratified scaffold design aimed at exercising spatial control over heterotypic cellular interactions, which are critical for facilitating the formation and maintenance of distinct yet continuous multi-tissue regions. Finally, potential challenges and future directions in this emerging area of advanced scaffold design will be discussed.

The debilitating effects of rotator cuff tears and the high incidence of failure associated with current grafts underscore the clinical demand for functional solutions for tendon repair and augmentation. To address this challenge, we have designed a poly(lactide-co-glycolide) (PLGA) nanofiber-based scaffold for rotator cuff tendon tissue engineering. In addition to scaffold design and characterization, the objective of this study was to evaluate the attachment, alignment, gene expression, and matrix elaboration of human rotator cuff fibroblasts on aligned and unaligned PLGA nanofiber scaffolds. Additionally, the effects of in vitro culture on scaffold mechanical properties were determined over time. It has been hypothesized that nanofiber organization regulates cellular response and scaffold properties. It was observed that rotator cuff fibroblasts cultured on the aligned scaffolds attached along the nanofiber long axis, whereas the cells on the unaligned scaffold were polygonal and randomly oriented. Moreover, distinct integrin expression profiles on these two substrates were observed. Quantitative analysis revealed that cell alignment, distribution, and matrix deposition conformed to nanofiber organization and that the observed differences were maintained over time. Mechanical properties of the aligned nanofiber scaffolds were significantly higher than those of the unaligned, and although the scaffolds degraded in vitro, physiologically relevant mechanical properties were maintained. These observations demonstrate the potential of the PLGA nanofiber-based scaffold system for functional rotator cuff repair. Moreover, nanofiber organization has a profound effect on cellular response and matrix properties, and it is a critical parameter for scaffold design.

Achieving functional graft integration with subchondral bone poses a significant challenge for orthopaedic soft tissue repair and reconstruction. Soft tissues such as the anterior cruciate ligament (ACL) integrate with bone through a fibrocartilage interface, which minimizes stress concentrations and mediates load transfer between soft and hard tissues. We propose that biological fixation can be achieved by regenerating this fibrocartilage interface on biological or synthetic ACL grafts. This study focuses on the in vivo evaluation of a stratified scaffold predesigned to mimic the multitissue transition found at the ACL-to-bone interface. Specifically, the scaffold consists of three distinct yet continuous phases: Phase A for ligament formation, Phase B for the interface, and Phase C for the bone region. Interface-relevant cell types, specifically fibroblasts, chondrocytes, and osteoblasts, will be tri-cultured on this scaffold, and the formation of cell type- and phase-specific matrix heterogeneity as well as fibrocartilage formation will be evaluated over 8 weeks in a subcutaneous athymic rat model. Acellular scaffolds as well as scaffolds co-cultured with fibroblasts and osteoblasts will serve as controls. It was found that the triphasic scaffold supported multilineage cellular interactions as well as tissue infiltration and abundant matrix production in vivo. In addition, controlled phase-specific matrix heterogeneity was induced on the scaffold, with distinct mineral and fibrocartilage-like tissue regions formed in the tri-cultured group. Cell seeding had a positive effect on both host infiltration and matrix elaboration, which also translated into increased mechanical properties in the seeded groups compared to the acellular controls. In summary, the biomimetic and multiphasic design coupled with spatial control of cell distribution enables multitissue regeneration on the stratified scaffold, and demonstrates the potential for regenerating the interface between soft tissue grafts and bone., ((c) 2008 Wiley Periodicals, Inc.)

Biological fixation of orthopedic soft tissue grafts to bone poses a significant clinical challenge. The clinical success of soft tissue-based grafts for anterior cruciate ligament (ACL) reconstruction is limited by the lack of functional graft integration with subchondral bone. Soft tissues such as the ACL connect to subchondral bone via a complex interface whereby three distinct tissue regions (ligament, fibrocartilage, and bone) work in concert to facilitate load transfer from soft to hard tissue while minimizing stress concentration at the interface. Although a fibrovascular tissue forms at the graft-to-bone interface following surgery, this tissue is nonphysiologic and represents a weak link between the graft and bone. We propose that the re-establishment of the native multi-tissue interface is essential for biological graft fixation. In vivo observations and our in vitro monolayer co-culture results suggest that osteoblast-fibroblast interaction is important for interface regeneration. This study focuses on the design of a triphasic scaffold system mimicking the multi-tissue organization of the native ACL-to-bone interface and the evaluation of osteoblast-fibroblast interactions during three-dimensional co-culture on the triphasic scaffold. We found that the triphasic scaffold supported cell proliferation, migration and phenotypic matrix production while maintaining distinct cellular regions and phase-specific extracellular matrix deposition over time. This triphasic scaffold is designed to guide the eventual reestablishment of an anatomically oriented and mechanically functional fibrocartilage interfacial region directly on biological and synthetic soft tissue grafts. The results of this study demonstrate the feasibility of multi-tissue regeneration on a single scaffold, and the potential of interface tissue engineering to enable the biological fixation of soft tissue grafts to bone.

The anterior cruciate ligament (ACL) functions as a mechanical stabilizer in the tibiofemoral joint, and is the most commonly injured knee ligament. To improve the clinical outcome of tendon grafts used for ACL reconstructions, our long-term goal is to promote graft-bone integration via the regeneration of the native ligament-bone interface. An understanding of strain distribution at this interface is crucial for functional scaffold design and clinical evaluation. Experimental determination, however, has been difficult due to the small length scale of the insertion sites. This study utilizes ultrasound elastography to characterize the response of the ACL and ACL-bone interface under tension. Specifically, bovine tibiofemoral joints were mounted on a material testing system and loaded in tension while radiofrequency (RF) data were acquired at 5 MHz. Axial strain elastograms between RF frames and a reference frame were generated using crosscorrelation and recorrelation techniques. Elastographic analyses revealed that when the joint was loaded in tension, complex strains with both compressive and tensile components occurred at the tibial insertion, with higher strains found at the insertion sites. In addition, the displacement was greatest at the ACL proper and decreased in value gradually from ligament to bone, likely a reflection of the matrix organization at the ligament-bone interface. Our results indicate that elastography is a novel method that can be readily used to characterize the mechanical properties of the ACL and its insertions into bone., (Copyright (c) 2006 Orthopaedic Research Society.)

The widespread clinical implementation of hamstring tendon (HT) autografts for anterior cruciate ligament (ACL) reconstruction is currently limited by the unpredictable integration of the graft with subchondral bone and a lack of devices that are capable of promoting biological fixation of HT grafts to bone. The site of HT graft fixation within the bone tunnel has been identified as the weak point in the reconstructed ACL, likely due to the failure of the graft to reestablish the physiological tendon-bone interface capable of transmitting load from the ligament to bone while minimizing stress concentration at the interface. Although a fibrovascular tissue has been shown to form at the graft-bone interface, this fibrovascular tissue is non-anatomically oriented compared to the native fibrocartilage found at direct ligament to bone insertions. Interface tissue engineering embodies a new approach for graft fixation, focusing on securing tendon grafts to bone via biological fixation wherein the complex functional interface found natively at tendon and ligament junctions with bone are regenerated at the graft insertion site into the bone tunnels. This study focuses on the in vivo evaluation of a novel biomimetic, triphasic scaffold system co-cultured with relevant cell types found at the graft-bone interface, specifically fibroblasts, chondrocytes, and osteoblasts. The scaffold is intended to promote biological fixation of HT grafts to bone by guiding the reestablishment of an anatomically-oriented and mechanically functional fibrocartilage interfacial region. It was found that the cell-seeded triphasic scaffolds supported cellular interactions as well as tissue infiltration and abundant matrix production in vivo. In addition, controlled phase-specific matrix heterogeneity was induced on the scaffold, with distinct mineral and interface-like tissue regions. The results of this study demonstrate the feasibility of multi-tissue regeneration on a single graft, as well as th- e potential of interface tissue engineering to enable the biological fixation of soft tissue grafts to bone.

Biodegradable polymer-ceramic composites are attractive systems for bone tissue engineering applications. These composites have the combined advantages of the component phases, as well as the inherent ease in optimization where desired material properties can be tailored in a well-controlled manner. This study focuses on the optimization of a polylactide-co-glycolide (PLAGA) and 45S5 bioactive glass (BG) composite for bone tissue engineering. The first objective is to examine the effects of composition or overall BG content on the formation of a Ca-P layer on the PLAGA-BG composite. It is expected that with increasing BG content (0%, 10%, 25%, 50% by weight), the required incubation time in a simulated body fluid (SBF) for the composite to form a detectable surface Ca-P layer will decrease. Both the kinetics and the chemistry will be determined using SEM+EDAX, FTIR, and mu-CT methods. Solution phosphorous and calcium concentrations will also be measured. The second objective of the study is to determine the effects of BG content on the maturation of osteoblast-like cells on the PLAGA-BG composite. It is hypothesized that mineralization will increase with increasing BG content, and the composite will support the proliferation and differentiation of osteoblasts. Specifically, cell proliferation, alkaline phosphatase activity and mineralization will be monitored as a function of BG content (0%, 10%, 50% by weight) and culturing time. It was found that the kinetics of Ca-P layer formation and the resulting Ca-P chemistry were dependent on BG content. The response of human osteoblast-like cells to the PLAGA-BG composite was also a function of BG content. The 10% and 25% BG composite supported greater osteoblast growth and differentiation compared to the 50% BG group. The results of this study suggest that there is a threshold BG content which is optimal for osteoblast growth, and the interactions between PLAGA and BG may modulate the kinetics of Ca-P formation and the overall cellular response.