Meniscal Replacement With a Silk Fibroin Scaffold Reduces Contact Stresses in the Human Knee.

The aim of the current study was to verify if a previously developed silk fibroin scaffold for meniscal replacement is able to restore the physiological distribution of contact pressure (CP) over the articulating surfaces in the human knee joint, thereby reducing peak loads occurring after partial meniscectomy. The pressure distribution on the medial tibial articular surface of seven human cadaveric knee joints was analysed under continuous flexion-extension movements and under physiological loads up to 2,500 N at different flexion angles. Contact area (CA), maximum tibiofemoral CP, maximum pressure under the meniscus and the pressure distribution were analysed for the intact meniscus, after partial meniscectomy as well as after partial medial meniscal replacement using the silk fibroin scaffold. Implantation of the silk fibroin scaffold considerably improved tibiofemoral contact mechanics after partial medial meniscectomy. While the reduced CA after meniscectomy was not fully restored by the silk fibroin scaffold, clinically relevant peak pressures on the articular cartilage surface occurring after partial meniscectomy were significantly reduced. Nevertheless, at high flexion angles static testing demonstrated that normal pressure distribution comparable to the intact meniscus could not be fully achieved. The current study demonstrates that the silk fibroin implant possesses attributes that significantly improve tibiofemoral CPs within the knee joint following partial meniscectomy. However, the failure to fully recapitulate the CAs and pressures observed in the intact meniscus, particularly at high flexion angles, indicates that the implant's biomechanical properties may require further improvement to completely restore tibiofemoral contact mechanics. © 2019 The Authors. Journal of Orthopaedic Research® published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:2583-2592, 2019.

Articular cartilage and meniscus reveal higher friction in swing phase than in stance phase under dynamic gait conditions.

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.

The challenge of implant integration in partial meniscal replacement: an experimental study on a silk fibroin scaffold in sheep.

PURPOSE: To restore meniscal function after excessive tissue damage, a silk fibroin implant for partial meniscal replacement was developed and investigated in an earlier sheep model. After 6 months implantation, it showed promising results in terms of chondroprotection and biocompatibility. To improve surgical fixation, the material was subjected to optimisation and a fibre mesh was integrated into the porous matrix. The aim of the study was the evaluation of this second generation of silk fibroin implants in a sheep model. METHODS: Nine adult merino sheep received subtotal meniscal replacement using the silk fibroin scaffold. In nine additional animals, the defect was left untreated. Sham surgery was performed in another group of nine animals. After 6 months of implantation macroscopic, biomechanical and histological evaluations of the scaffold, meniscus, and articular cartilage were conducted. RESULTS: Macroscopic evaluation revealed no signs of inflammation of the operated knee joint and most implants were located in the defect. However, there was no solid connection to the remaining peripheral meniscal rim and three devices showed a radial rupture at the middle zone. The equilibrium modulus of the scaffold increased after 6 months implantation time as identified by biomechanical testing (before implantation 0.6 ± 0.3 MPa; after implantation: 0.8 ± 0.3 MPa). Macroscopically and histologically visible softening and fibrillation of the articular cartilage in the meniscectomy- and implant group were confirmed biomechanically by indentation testing of the tibial cartilage. CONCLUSIONS: In the current study, biocompatibility of the silk fibroin scaffold was reconfirmed. The initial mechanical properties of the silk fibroin implant resembled native meniscal tissue. However, stiffness of the scaffold increased considerably after implantation. This might have prevented integration of the device and chondroprotection of the underlying cartilage. Furthermore, the increased stiffness of the material is likely responsible for the partial destruction of some implants. Clinically, we learn that an inappropriate replacement device might lead to similar cartilage damage as seen after meniscectomy. Given the poor acceptance of the clinically available partial meniscal replacement devices, it can be speculated that development of a total meniscal replacement device might be the less challenging option.

Biomechanical, structural and biological characterisation of a new silk fibroin scaffold for meniscal repair.

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 ±â€¯0.31 MPa and 0.30 ±â€¯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 ±â€¯10.9 µm) and total porosity (80.1 ±â€¯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.

Correction to: In vivo performance of a novel silk fibroin scaffold for partial meniscal replacement in a sheep model.

The author would like to correct the errors in the publication of the original article. The corrected details are given below for your reading.

In vivo performance of a novel silk fibroin scaffold for partial meniscal replacement in a sheep model.

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. METHODS: Partial 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. RESULTS: The 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. CONCLUSION: These 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.

Transcriptional heterochrony of scute and changes in bristle pattern between two closely related species of blowfly.

Temporal shifts in the expression of regulatory genes, relative to other events taking place during development, can result in changes in morphology. Such transcriptional heterochrony can introduce dramatic morphological changes that involve rather few genetic events and so has the potential to cause rapid changes during evolution. We have shown previously that stereotyped species-specific bristle patterns on the notum of higher Diptera correlate with changes in the spatial regulation of scute expression. scute encodes a proneural gene required for the development of sensory bristle precursors and is expressed before pupation in discrete domains on the presumptive notum at sites where the macrochaete precursors arise. Thus, for Ceratitis capitata and Calliphora vicina, species separated from Drosophila melanogaster by about 80 and 100 million years respectively, the domains of sc expression differ. In all three species, a second phase of ubiquitous sc expression, after pupation, precedes formation of the microchaete precursors. Here, we describe sc expression in Phormia terranovae, a species belonging to the family Calliphoridae that is closely related to C. vicina. We find that spatial regulation is almost identical between P. terranovae and C. vicina, in spite of their different bristle patterns. The timing of sc expression differs, however, between the two. The first spatially restricted phase of expression is slightly delayed and the second ubiquitous phase remarkably accelerated, such that there is a period of overlap. As a result, the last precursors from the first phase of expression arise at the same time as the first precursors from the second phase of expression and are morphologically indistinguishable from the late-arising microchaetes. These observations illustrate the power of developmental heterochrony in bringing about rapid morphological change.

Gene duplication at the achaete-scute complex and morphological complexity of the peripheral nervous system in Diptera.

The number of achaete-scute genes increased during insect evolution, particularly in the Diptera lineage. Sequence comparison indicates that the four achaete-scute genes of Drosophila result from three independent duplication events. After duplication, the new genes acquired individual expression patterns but, in Drosophila, their products can compensate for one another, which raises the question: why retain all four genes? The complexity of the spatial expression of these genes on the notum increased in the lineage leading to the higher Diptera, allowing the development of stereotyped bristle patterns. This probably coincided in time with gene duplication events, raising the possibility that an increase in gene copy number might have provided the flexibility necessary for more complex transcriptional regulation.

scute expression in Calliphora vicina reveals an ancestral pattern of longitudinal stripes on the thorax of higher Diptera.

In Drosophila the stereotyped arrangement of sensory bristles on the notum is determined by the tightly regulated control of transcription of the achaete-scute (ac-sc) genes which are expressed in small proneural clusters of cells at the sites of each future bristle. Expression relies on a series of discrete cis-regulatory elements present in the ac-sc gene complex that are the target of the transcriptional activators pannier (pnr) and the genes of the iroquois complex. Stereotyped bristle patterns are common among species of acalyptrate Schizophora such as Drosophila, and are thought to have derived from an ancestral pattern of four longitudinal rows extending the length of the scutum, through secondary loss of bristles. To investigate evolutionary changes in bristle patterns and ac-sc regulation by pnr, we have isolated homologues of these genes from Calliphora vicina, a species of calyptrate Schizophora separated from Drosophila by at least 100 million years. Calliphora vicina displays a pattern of four rows of bristles on the scutum resembling the postulated ancestral one. We find that sc in Calliphora is expressed in two longitudinal stripes on the medial scutum that prefigure the development of the rows of acrostichal and dorsocentral bristles. This result suggests that a stripe-like expression pattern of sc may be an ancestral feature and may have preceded the evolution of proneural clusters. The implications for the evolution of the cis-regulatory elements responsible for sc expression in the proneural clusters of Drosophila, and function of Pnr are discussed.