Bone has a degree of intrinsic repair capacity. Ordinary fractures can usually heal gradually when stable fixation and an adequate blood supply are provided. If a defect is too large, or if severe infection, poor perfusion, or soft-tissue injury is present, bone may be unable to complete repair independently, resulting in non-union or a segmental defect.

Articular cartilage has even less capacity for self-repair. It is almost avascular and contains relatively few cells. Once a full-thickness defect occurs, high-quality repair through intrinsic healing is difficult. Even when new repair tissue forms, its structure and mechanical properties often fall short of those of normal hyaline cartilage.

Autologous bone grafting is an important treatment for bone defects. Bone is commonly harvested from the iliac bone or iliac crest and transplanted into the defect. Because autograft has osteogenic, osteoinductive, and osteoconductive capacity, it has long been regarded as a key standard approach in bone grafting.

Harvesting autograft creates an additional surgical site and may cause donor-site pain, scarring, infection, deformity, or other complications. For extensive bone defects, the volume of bone available from the patient may also be insufficient.

Other options include allogeneic bone grafting, the Masquelet induced-membrane technique, and Ilizarov distraction osteogenesis or bone transport. Each has a defined role, while potential limitations include immune rejection, infection risk, multiple operations, long treatment duration, and prolonged external fixation.

Microfracture is a common cartilage-repair technique. The surgeon perforates the subchondral bone plate, allowing marrow blood and mesenchymal cells to enter the cartilage defect and form repair tissue.

The procedure is relatively simple, but the newly formed tissue is usually fibrocartilage. Fibrocartilage has lower wear resistance and inferior mechanical properties compared with normal articular hyaline cartilage, and may degenerate or be injured again over time.

Autologous osteochondral transplantation can transfer osteochondral plugs from a non-weight-bearing region to the defect, but may injure the donor site and is generally unsuitable for large defects. Osteochondral allografting can treat larger lesions, with continuing concerns about graft infection, storage, and tissue availability.

Tissue-engineering methods such as matrix-induced autologous chondrocyte implantation can also be used for selected patients. They usually require cartilage harvest, ex vivo cell culture, and a second implantation procedure, making the treatment pathway complex and costly.

Black phosphorus is a layered material composed of phosphorus. When it is prepared as nanosheets only a few atomic layers thick, it is called black phosphorus nanosheets.

Compared with white and red phosphorus, black phosphorus has favourable mechanical, electrical, photothermal, drug-loading, and biocompatibility-related properties. It has therefore been widely studied in recent years in electronics, drug delivery, cancer treatment, and tissue engineering.

Black phosphorus can gradually degrade in water and oxygen-containing environments, producing phosphate and other phosphorus-containing substances. Some studies suggest that these degradation products may participate in local bone mineralization and provide a favourable microenvironment for bone regeneration.

Its ability to degrade does not establish complete safety in the human body. The degradation rate, dose, particle size, surface modification, and long-term metabolic fate may all influence its ultimate biological effects.

Promoting local mineralization. The principal inorganic components of bone are closely linked to calcium and phosphorus metabolism. Phosphate and related products released after black phosphorus degradation may participate in local mineralization and promote the formation of bone-like tissue.

Black phosphorus does not directly transform into bone. Bone regeneration is a complex process requiring osteoblasts, stem cells, blood vessels, extracellular matrix, growth factors, and a stable mechanical environment. Phosphorus-containing degradation products are only one factor that may contribute.

Improving the environment for osteogenic cells. In some cell studies, black phosphorus-containing hydrogels, nanofibrous scaffolds, and composite materials promoted adhesion, proliferation, and osteogenic differentiation of bone-marrow mesenchymal stem cells, and increased expression of osteogenesis-related markers such as alkaline phosphatase and BMP-2.

These findings suggest that black phosphorus may support bone formation by improving material-surface properties, releasing phosphate, and modulating the local cellular environment.

Promoting angiogenesis. Adequate blood supply is essential for bone regeneration. Newly formed blood vessels deliver oxygen, nutrients, osteogenic cells, and growth factors to the defect area.

Some black phosphorus-containing composite hydrogels and biomimetic periosteal scaffolds have shown potential in animal experiments to promote endothelial-cell migration, blood-vessel formation, and bone regeneration.

Black phosphorus has a layered structure and a large specific surface area, enabling it to load active agents such as BMP-2, ibuprofen, and strontium ions.

Researchers can combine black phosphorus with hydrogels, microspheres, electrospun scaffolds, and related materials to enable gradual drug release within a defect. Some experimental systems can also use near-infrared light to produce a thermal effect and further control the release rate.

This composite design of material plus drug may increase the local treatment concentration while reducing drug distribution to other parts of the body.

During cartilage injury and osteoarthritis progression, large amounts of reactive oxygen species and reactive nitrogen species may accumulate locally. These oxidative molecules can damage chondrocytes, proteins, and DNA, and accelerate degradation of the cartilage extracellular matrix.

Black phosphorus can scavenge reactive oxygen and nitrogen species. In vitro studies indicate that black phosphorus nanosheets can reduce oxidative damage to chondrocytes caused by hydrogen peroxide, nitric oxide, and related substances.

In rat knee-joint experiments, investigators injected a black phosphorus nanosheet dispersion into the joint cavity and observed reduced reactive oxygen species, suppressed inflammatory responses, and increased synthesis of type II collagen and glycosaminoglycans.

These results suggest that black phosphorus may act by reducing oxidative stress, protecting chondrocytes, and improving the cartilage-repair environment.

Orthopaedic implant-related infection is an important complication of joint replacements, plates, screws, and bone-repair scaffolds.

Black phosphorus has photothermal and sonodynamic response properties. In some experimental systems, near-infrared irradiation produces a local thermal effect, while ultrasound stimulation may induce reactive oxygen species, disrupting bacterial cell membranes and bacterial antioxidant systems.

Researchers have combined black phosphorus with materials such as polydopamine to create titanium implant surface coatings intended to improve biocompatibility, antibacterial activity, and osseointegration at the same time.

This does not yet mean that black phosphorus coatings can routinely prevent infection. Excessive photothermal temperatures may damage normal tissue; ultrasound and light doses also require precise control. Long-term safety and real-world antibacterial effectiveness still require human trials.