Joint surgery is undoubtedly one of the most demanding chapters in the veterinary medicine of companion animals, less because of any shortage of techniques than because of the intrinsic biological limit of the tissues one is called upon to repair. Ligaments, cartilage and meniscal fibrocartilage share a very poor capacity for spontaneous regeneration, a modest or absent vascularization and a hostile mechanical environment, subjected to cyclic loads counted in the millions of repetitions over the animal's lifetime. Within this context, silk fibroin stands out as one of the most promising protein biomaterials, because it offers a combination that is difficult to replicate with traditional synthetic polymers: mechanical strength and toughness comparable to those of native connective tissues, slow and programmable degradation, high biocompatibility once the sericin has been removed, and a processing versatility that allows fibres, porous matrices, hydrogels and biphasic constructs to be obtained from the same raw material.
The orthopedic burden in companion animals
To understand the interest in fibroin, one must start from epidemiology. In the dog, rupture of the cranial cruciate ligament is by far the most frequent cause of hindlimb lameness and represents one of the orthopedic conditions with the greatest clinical and economic impact, with a progressive degenerative component that in most cases leads to osteoarthritis of the stifle regardless of the treatment adopted. The ligamentous lesion is frequently accompanied by meniscal damage and, over time, by cartilaginous remodelling of the entire joint. Alongside this picture coexist the osteochondritis dissecans of large breeds during growth, focal chondral lesions and the degenerative arthropathies of the cat, the latter historically underdiagnosed but far from rare.
Established surgical approaches intervene for the most part on joint biomechanics rather than on true tissue reconstruction. Tibial plateau levelling osteotomies and tibial tuberosity advancement neutralize cranial tibial translation by modifying the geometry of the stifle, but they do not restore a functional ligament; extracapsular stabilizations with suture material entrust holding power to a passive structure destined either to fail or to stabilize only thanks to periarticular fibrosis. The synthetic ligaments tested over the decades have shown recurring limitations, from fatigue failure of the material to the production of wear debris, through to foreign-body reactions. It is in this space, where mechanical stabilization does not coincide with biological regeneration, that fibroin-based tissue engineering proposes a paradigm shift: not to replace the tissue with an inert artefact, but to provide a scaffold that guides the reconstruction of a functional autologous tissue.
Why fibroin meets the requirements of joint surgery
The properties that make fibroin interesting in this field derive directly from its structural organization. Its richness in crystalline beta-sheet domains confers on the fibres a tensile strength and a fatigue resistance that few other structural proteins can offer, a decisive characteristic in an anatomical site where the construct must bear cyclic loads for the time required for tissue neoformation. This robustness is accompanied by a toughness that allows the material to absorb energy without fracturing in a brittle manner, a behaviour far closer to that of a native ligament than that of many rigid polymers.
On the biological side, the removal of sericin substantially reduces the immunogenic component and restores a substrate well tolerated by joint tissues, capable of supporting the adhesion, proliferation and differentiation of the cells of orthopedic interest, from ligamentous fibroblasts to chondrocytes and mesenchymal stem cells. The degradation of fibroin, moreover, is governed by a slow enzymatic process that can be modulated through the degree of crystallinity and the geometry of the construct: one can obtain a material that persists for months or years, a precious span of time when the structure must accompany the maturation of a neoligament or the integration of an osteochondral graft. Completing the picture is the extraordinary processing plasticity, which allows the same protein solution to be transformed into twisted threads that mimic the architecture of a ligament, into porous matrices for cell colonization, into injectable hydrogels or into gradient scaffolds for the repair of the osteochondral unit.
Cruciate ligament reconstruction: anisotropic architecture and osteoligamentous integration
Ligament reconstruction is the application in which fibroin best expresses its potential, and it is no coincidence that it is the one on which the most mature research has concentrated. The cruciate ligament is a strongly anisotropic structure, with bundles of collagen fibres oriented along the direction of load, and any credible substitute must replicate not only its overall strength but also its hierarchical organization. Twisted-rope geometries, in which fibroin filaments are assembled into ordered matrices resembling a cable, make it possible to approach the mechanical properties of the native ligament and to offer the cells a topographic track along which to align, a condition essential for the newly formed tissue to acquire the correct fibrous architecture rather than depositing a disorganized scar.
The true test, however, is not only the body of the ligament but its integration at the extremities. A ligament graft must anchor firmly to the bone by reconstructing the enthesis, that complex transition zone where the fibrous tissue connects to the bone through a gradient of mineralized fibrocartilage. The most advanced strategies exploit the tunability of fibroin to create gradient constructs, in which the portion destined for the bone tunnel is functionalized to favour osteointegration, while the intra-articular portion retains its ligamentous characteristics. The colonization of the scaffold with mesenchymal stem cells, often of medullary or adipose derivation, and the exposure to controlled mechanical stimuli during maturation help to orient the differentiation and to accelerate the formation of an organized neoligament. In the dog this approach finds ideal clinical ground, because cruciate rupture is so frequent as to fully justify the effort of developing a regenerative substitute capable of overcoming the limits of both biological grafts and synthetic prostheses.
Cartilage and osteochondral repair
Articular cartilage poses a challenge of a different nature. Avascular, aneural and populated by a scant number of chondrocytes embedded in a dense matrix, it lacks an effective repair mechanism, and focal chondral lesions tend to progress towards degeneration of the entire joint surface. Here fibroin is employed above all in the form of hydrogels and porous scaffolds as a vehicle for chondrogenic cells, offering a three-dimensional microenvironment that supports chondrocytic differentiation and the deposition of a matrix rich in glycosaminoglycans and type II collagen. The possibility of obtaining injectable hydrogels further opens the way to less invasive procedures, in which the material is introduced in situ and polymerizes, adapting to the geometry of the defect.
When the lesion also involves the subchondral bone, as occurs in the osteochondritis dissecans typical of large-breed dogs during growth, the problem becomes that of reconstructing a functional unit in which cartilage and bone are correctly integrated. In these cases one resorts to biphasic constructs, in which an upper phase optimized for chondrogenesis continues into a lower phase designed for osteogenesis, often mineralized or enriched with components that favour bone formation. The cohesion between the two phases and the reconstruction of a continuous gradient, rather than a sharp interface destined to delaminate, represent the truly critical element, and the workability of fibroin makes it possible to modulate its composition progressively across the thickness of the construct in a way that is difficult to achieve with other materials.
The meniscus and fibrocartilaginous structures
Meniscal involvement deserves particular attention precisely because of its frequent association with joint instability. Medial meniscal injury often accompanies cruciate rupture in the dog, and meniscectomy, historically performed to remove the damaged tissue, demonstrably accelerates joint degeneration by depriving the joint of a structure essential for load distribution. The preservation and repair of the meniscus have therefore become priority objectives, and fibroin lends itself to the fabrication of fibrocartilaginous scaffolds capable of supporting tissue regeneration in the avascular portions, where spontaneous healing does not occur. The combination of adequate compressive mechanical properties, a porosity that favours cell colonization and a degradation compatible with the long timescales of fibrocartilaginous maturation makes these constructs interesting candidates for a conservative strategy aimed at preserving the biomechanics of the joint rather than sacrificing it.
Strategies of biological functionalization
The value of fibroin as a scaffold multiplies when it is considered not as a passive material but as a platform to be biologically enriched. Its surface chemistry and porous structure allow growth factors relevant to each tissue to be incorporated and released in a controlled manner: molecules of the TGF-beta family and IGF to orient chondrogenesis, bone morphogenetic proteins to stimulate osteogenesis in the portions destined for bone anchorage, angiogenic factors where neovascularization is desirable. The release kinetics can be shaped by acting on the degree of crystallinity and on the architecture of the construct, so as to accompany the various phases of regeneration rather than being exhausted in an initial peak devoid of lasting effect.
Alongside signalling molecules, fibroin acts as a cellular vector, hosting mesenchymal stem cells which, suitably stimulated by the microenvironment and by mechanical loads, differentiate towards the desired phenotype. The convergence of structural support, biochemical signals and cellular component defines the most advanced horizon of these implants, in which the construct does not merely fill a defect but actively instructs the regenerative process, guiding the formation of a tissue that progressively takes over its functions while the material is resorbed.
