According to the U.S. Food and Drug Administration (FDA), biologics are usually considered living entities (eg, cells or tissues) but can also be nonviable compounds such as growth factors. Biologics have been used for the treatment of a variety of musculoskeletal conditions in orthopaedics, with target sites including bone, cartilage, and soft tissues (ligaments, tendons, and muscle). Biologics may be autogenous or may be transplanted from humans (allografts) or from other species (xenografts).

Delivery of the biologic to the site of pathology may be challenging. Traditionally, open surgery has been required to deliver a biologic agent such as autogenous bone graft or bone graft substitute to a specific anatomic location. Newer biologics, such as growth factors, may not require surgery for delivery, but are often delivered in supraphysiologic amounts to achieve therapeutic benefit. However, this introduces risks associated with high concentrations of exogenous growth factors. These limitations highlight the need for better understanding and optimization of the in vivo environment in which biologics function.

Investigational efforts have sought to incorporate and apply nanotechnology into the effective delivery and integration of biologics. These materials can be vehicles for delivery as well as anchors at the target site, presenting biologics to the surrounding environment. The site of interaction and, more importantly, the local environment play a role in how biologics exert their ultimate effect.

Ideally, nanotechnology may provide a means to track vehicles destined for specific anatomic locations and deliver complex packages that may only be opened in a particular environment. These unique features will require a better understanding of complex biologic processes and enable enhanced and novel treatment of patients.

The nanoscale
Nanoscale materials are less than 100 nm but greater than 1 nm in size. They can include crystalline solids with nanoscale grain sizes, solid materials with nanoscale surface coatings, particles with nanoscale particle size, or fibers with nanoscale diameters.

Nanoscale materials may vary greatly in their characteristics depending on their composition, and this is a focus of many bioengineers worldwide. Interestingly, some of the classic laws of physics do not apply at the nanolevel compared to larger scale. These differences in behavior are important because many of the cellular and molecular interactions that govern in vivo phenomena occur on a nanoscale. Clinical, translational, and basic science research efforts at the nanoscale level have occurred in many fields of medicine—including orthopaedic surgery—to treat damaged tissues or organs.

Drug delivery
Laboratories have been using nanotechnology to deliver a diverse variety of drugs, including chemotherapeutics and microRNA inhibitors. These delivery modalities can contain imaging agents for real-time biologic imaging. A variety of medical disciplines, as well as physics and engineering are collaborating to advance targeted drug delivery.

Several varieties of nanoparticles, including multifunctional nanoparticles, localize the biodistribution of conventional therapies to the target site. For example, nanoparticles have been loaded with chemotherapeutic drugs to overcome drug resistance in cancer cells. Gold-containing nanoparticles may even be used to kill tumor cells and, in certain preparations, may even prevent inflammation in chronic conditions.

Surface modulation
Non-oncologic orthopaedic surgery may have an even broader use for nanotechnology. One area of focus has been the alteration of orthopaedic implant surfaces to improve host bone-implant interactions. Covering surfaces with nanoparticles may alter the recipient site and form a more interactive surface.

For example, in vivo study has shown that tantalum implants coated with nanoscale hydroxyapatite crystals show improved new bone growth compared to those with micron-scaled coatings. Implants with nanoscale surface features, even in the absence of coating, may enhance surrounding osteoblastic function by mimicking the nanoscale features of native bone. The relatively recent discoveries in stem cells and regenerative medicine along with nanotechnology will ultimately affect bone–implant interactions, thereby greatly improving the osseous integration of orthopaedic implants.

Bone graft and bone graft substitutes
Bone graft, which is used in many orthopaedic procedures, is delivered through surgical means or by image-guided, less invasive techniques. The surgeon considers tradeoffs between autograft donor site morbidity and lesser biologic potential of nonautograft options. Nano-engineered constructs seek the therapeutic properties of autograft, without requiring donor site morbidity, by mimicking the relevant nanoscale architecture of autologous bone graft.

Composite bone graft scaffolds consisting of nanohydroxyapatite (nHAP) and collagen have been created in attempts to reproduce naturally occurring scaffold. The nHAP affords enhanced osteoconduction while the collagen provides improved biodegradative and osteoinductive properties. The hydroxyapatite crystals in nHAP are in the 50-nm to 1000-nm range; the nanoscale crystals provide increased surface area and opportunity for interaction with surrounding cells and molecules. Both in vivo experiments and clinical case series support the benefit of these constructs. Further clinical trials are needed to establish their role as effective bone graft substitutes.

Nanotubes
Nanotubes are hollow, nanosized cylinders with high tensile and shear strength, favorable microstructure for bony in-growth, and a high capacity to hold antibiotics or growth factors such as bone morphogenetic proteins (BMPs). The current local delivery limitations of BMPs via collagen sponges may be overcome with a nanotechnology-based approach. The size, internal structure, and shape of nanotubes loaded with growth factors can be manipulated to control the release of these growth factors, and certain nanotubes can be dual-layered, allowing for release of multiple growth factors at once or in succession.

Carbon-based nanotubes can even be categorized as biomedical implants secondary to their mechanical properties and nonbiodegradable nature. Their strength and ability to be augmented with osteogenic materials have made them an attractive area of research as alternative implant surfaces and stand-alone implants.

Nanotubes (Fig. 1) may be useful in addressing infection, one of the worst complications in joint replacement surgery, which frequently results in loss of the implant. As with growth factors, nanotubes may enable the controlled release of antibiotics to treat periprosthetic infections. Using antibiotics locally at the surgical site may reduce the need for systemic antibiotic use and hence limit the further development of bacterial resistance.

Silver in particular has been used on wounds as an antibacterial agent and now is being applied to metal surfaces to combat infection. Nanophase silver incorporated onto the surface of titanium orthopaedic implants in the form of titanium nanotubes has bactericidal potential. These nanoparticle-antibiotic constructs may allow a more targeted and localized treatment strategy for periprosthetic infection.

Nanofiber scaffolds
Another nanoengineered vehicle for use in biologics delivery is the nanofiber scaffold. Owing to their continuous nature, nanofibers can achieve a close morphologic approximation of natural extracellular matrix when used in a three-dimensional (3D) network.

Nanofiber scaffolds are highly porous with a range of pore sizes and an extremely high surface area to volume ratio compared to conventional materials. These scaffolds are commonly fabricated via electrospinning, a process in which electric potential is used to create fibers from a liquid material reservoir.

Various materials have been used for electrospun scaffolds, including nonbiodegradable polymers and naturally occurring materials such as collagen or silk. Composite electrospun scaffolds have been created that include nHAP and collagen components. Growth factors, often in the form of platelet-rich plasma, have also been added to nanofiber scaffolds. Nanofiber scaffolds have found success when utilized in animal experiments; their clinical use requires further testing.

Nanofibers created using biodegradable substrates such as poly(lactic-co-glycolic acid) and chitosan have been extensively studied for their chondrogenic, osteogenic, or antibacterial properties, and have been evaluated in both in vitro and in vivo studies with promising results. They remain an exciting area of research in tissue engineering, and have been accepted as an adjunct in tendon-repair treatments and local bone regeneration.

Nanofiber scaffolds have also been used for delivery of cell-based therapies, such as mesenchymal stem cells (MSCs). MSCs have more robust growth patterns when seeded on scaffolds with nanoscale fibers. In addition, the properties of the scaffold can be "tuned" to guide the MSC toward the desired osteogenic or chondrogenic differentiation pathway.

Ligands that guide differentiation toward the desired phenotype may also be incorporated. Because chondrocytic phenotypes are easily lost to fibroblastic phenotypes if cells are expanded in monolayer culture, 3D scaffolds may be especially useful. In vitro studies have found that chondrocytes produce more type II collagen in nanofiber scaffolds compared to conventional scaffolds. Long-term replacements of segmental bone defects may incorporate the use of biologics and stem cell-laden scaffolds.

Biologics are a rapidly advancing field of orthopaedic surgery, and nanotechnology is an intriguing and novel means to deliver biologics and facilitate their function. New biologics coupled with advances in regenerative medicine will be delivered in host tissues to treat an increasing variety of complex orthopaedic conditions. The next decade will be filled with advances in nanotechnology that make the future exciting in the field of orthopaedics. Diverse nanomaterials will continue to be developed by engineers and used by researchers and clinicians to improve the outcome for various diseases.

Adam I. Edelstein, MD, and Francis J. Hornicek, MD, PhD, are member of the AAOS Biological Implants Committee.

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