Purpose Due to the negative effects of meniscectomy, there is a need for an adequate material to replace damaged meniscal tissue. To date, no material tested has been able to replace the meniscus sufficiently. Therefore, a new silk fibroin scaffold was investigated in an in vivo sheep model.MethodsPartial meniscectomy was carried out to the medial meniscus of 28 sheep, and a scaffold was implanted in 19 menisci (3-month scaffold group, n = 9; 6-month scaffold group, n = 10). In 9 sheep, the defect remained empty (partial meniscectomy group). Sham operation was performed in 9 animals.ResultsThe silk scaffold was able to withstand the loads experienced during the implantation period. It caused no inflammatory reaction in the joint 6 months postoperatively, and there were no significant differences in cartilage degeneration between the scaffold and sham groups. The compressive properties of the scaffold approached those of meniscal tissue. However, the scaffolds were not always stably fixed in the defect, leading to gapping between implant and host tissue or to total loss of the implant in 3 of 9 cases in each scaffold group. Hence, the fixation technique needs to be improved to achieve a better integration into the host tissue, and the long-term performance of the scaffolds should be further investigated.ConclusionThese first in vivo results on a new silk fibroin scaffold provide the basis for further meniscal implant development. Whilst more data are required, there is preliminary evidence of chondroprotective properties, and the compressive properties and biocompatibility are promising.

The relative paucity of techniques currently available to repair bone tissue necessitates the development of innovative and more effective clinical strategies. [1,2] Of these, the combined integration of macroporous scaffolds, primed human-cell populations, and growth factors to organize and promote tissue formation is a particularly attractive approach. [3,4] However, bone tissue engineering is currently compromised by its inability to produce load-bearing scaffolds. For example, collagen scaffolds are used for a range of osteogenic applications, [5][6][7] but exhibit a compressive strength of ca. 0.034 MPa, [8] approximately three orders of magnitude lower than that of cancellous bone with values between 10 and 50 MPa. On the other hand, pure calcium phosphate mineral-based scaffolds, which currently dominate the commercial bone substitute materials market, lack a fibrillar protein component and are correspondingly brittle, typically failing catastrophically at compressive loads of less than 5 MPa.Recent studies have explored the possibility of replacing collagen with silk-based resorbable implants. [9][10][11][12] Silk is a fibrillar protein with excellent biocompatibility and mechanical strength, [13] and methodologies exist to convert silk fibers into regenerated silk fibroin solutions that can be subsequently reconstituted into macroporous 3D architectures with b-sheet secondary structure by salt leaching, gas foaming, extrusion layering or freeze drying. [9,[13][14][15][16][17][18] Although these scaffolds are potentially suitable for tissue engineering, similarly to collagenbased biomaterials the reconstituted silks are compromised by low mechanical strength, due in part to partial degradation of the native protein structure during fibroin dissolution. Moreover, attempts to impart significant mechanical reinforcement by calcium phosphate mineralization have not been very successful, due to poor adhesion and integration at the protein/mineral interface. [19,20] As a consequence, more advanced uses of silk are currently focused on novel delivery devices for morphogens, cytokines, and cell populations in models of bone defects. [21][22][23][24][25] In contrast, we present herein the first example of a silk/calcium phosphate macroporous scaffold that is load-bearing with mechanical properties comparable to cancellous bone. The mechanical strength is far in excess of other materials previously produced, and is achieved through the use of high-quality silk fibroins [26] and an integrated procedure for gelation, freezing, and mineralization. Moreover, we demonstrate the effectiveness of these load-bearing materials as nonpyrogenic osteoregenerative scaffolds by in vitro and in vivo testing, and suggest that such materials represent a new class of potentially implantable alternatives to the use of allograft and autograft procedures, for example in surgical applications, where immediate load bearing is required.Silk/calcium phosphate macroporous scaffolds were prepared by freezing phosphate-containing aqueous gels...

Most previous studies investigated the remarkably low and complex friction properties of meniscus and cartilage under constant loading and motion conditions. However, both load and relative velocity within the knee joint vary considerably during physiological activities. Hence, the question arises how friction of both tissues is affected by physiological testing conditions occurring during gait. As friction properties are of major importance for meniscal replacement devices, the influence of these simulated physiological testing conditions was additionally tested for a potential meniscal implant biomaterial. Using a dynamic friction testing device, three different friction tests were conducted to investigate the influence of either just varying the motion conditions or the normal load and also to replicate the physiological gait conditions. It could be shown for the first time that the friction coefficient during swing phase was statistically higher than during stance phase when varying both loading and motion conditions according to the physiological gait pattern. Further, the friction properties of the exemplary biomaterial were also higher, when tested under dynamic gait parameters compared to static conditions, which may suggest that static conditions can underestimate the friction coefficient rather than reflecting the in vivo performance.

Meniscal injury is typically treated surgically via partial meniscectomy, which has been shown to cause cartilage degeneration in the long-term. Consequently, research has focused on meniscal prevention and replacement. However, none of the materials or implants developed for meniscal replacement have yet achieved widespread acceptance or demonstrated conclusive chondroprotective efficacy.A redesigned silk fibroin scaffold, which already displayed promising results regarding biocompatibility and cartilage protection in a previous study, was characterised in terms of its biomechanical, structural and biological functionality to serve as a potential material for permanent partial meniscal replacement. Therefore, different quasi-static but also dynamic compression tests were performed. However, the determined compressive stiffness (0.56 ± 0.31 MPa and 0.30 ± 0.12 MPa in relaxation and creep configuration, respectively) was higher in comparison to the native meniscal tissue, which could potentially disturb permanent integration into the host tissue. Nevertheless, µ-CT analysis met the postulated requirements for partial meniscal replacement materials in terms of the microstructural parameters, like mean pore size (215.6 ± 10.9 µm) and total porosity (80.1 ± 4.3%). Additionally, the biocompatibility was reconfirmed during cell culture experiments. The current study provides comprehensive mechanical and biological data for the characterisation of this potential replacement material. Although some further optimisation of the silk fibroin scaffold may be advantageous, the silk fibroin scaffold showed sufficient biomechanical competence to support loads already in the early postoperative phase.

The menisci protect the articular cartilage by reducing contact pressure in the knee. To restore their function after injury, a new silk fibroin replacement scaffold was developed. To elucidate its tribological properties, friction of the implant was tested against cartilage and glass, where the latter is typically used in tribological cartilage studies. The silk scaffold exhibited a friction coefficient against cartilage of 0.056, which is higher than meniscus against cartilage but in range of the requirements for meniscal replacements. Further, meniscus friction against glass was lower than cartilage against glass, which correlated with the surface lubricin content. Concluding, the tribological properties of the new material suggest a possible long-term chondroprotective function. In contrast, glass always produced high, non-physiological friction coefficients.