158 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. osteoblasts deposit new bone at the site, resulting in repair of the injury. However, when overuse is repeated or continuous, the remodeling process cannot keep up with the damage being done. When this happens, the condition progresses to a stress fracture. Runners are prone to stress fractures, particularly in the tibia and the metatarsals. Consequently, greenstick fractures, or incomplete fractures, are more common in children than in adults. A greenstick fracture is caused when a bone bends or twists but does not break all the way through. Stress fractures are tiny, painful cracks in bone that result from overuse. Under normal circumstances, bone responds to stress-related injury by remodeling. Osteoclasts resorb the damaged tissue, and then Bone Tissue Engineering Fractures and malformations of the skull and face can result from birth defects, infections, cancers, or traumatic injury. These can be dangerous for patients and often require costly healthcare procedures. Current approaches to repair and reconstruction include rigid fixation of an artificial plate, bone grafting, and transfer of new bone tissue to the region. However, these techniques often involve complications, such as poor fitting of the plate or graft failure. Bone tissue engineering, one of the most challenging and exciting new fields for scientists and clinicians, offers some creative new approaches. Engineered therapies for enhanced healing and regeneration of craniofacial bone typically involve a combination of live stem cells and growth factors applied over artificial, biodegradable scaffolds. The success of bone tissue engineering relies on understanding the complex interactions among live stem cells, the regulatory signals, and the platforms used to deliver them. All of these components are collectively known as the tissue engineering triad. The live cells employed are typically mesenchymal stem cells combined with bone marrow. A surgeon harvests a small amount of mesenchymal stem cells from the patient’s iliac crest. These cells are then carefully cultured in a growth medium until they become larger cell colonies that can differentiate into cells that form bone. More recently, scientists have discovered that stem cells can also be extracted from fat. These adipose stem cells can be collected in large quantities with little patient discomfort, in contrast to the invasive and painful extraction techniques used to obtain bone marrow mesenchymal cells. Although promising, more studies are needed to standardize and refine the techniques for harvesting and culturing adipose stem cells. Whatever the source, the live bone cells are cultured and then implanted on a biomaterial scaffold. Many different growth factor delivery techniques and scaffold compositions have been explored, although none has yet emerged as universally recommended. Scientists have discovered that the size of the pores in the scaffolding material can greatly influence the ability of the cells to attach and colonize. Different scaffold materials also exert different biomechanical forces, which can profoundly affect bone cell development. Identifying biomaterials that are optimally capable of responding to physiological and mechanical changes inside the human body remains an important challenge in bone tissue engineering. Once the cells have been implanted on the scaffold, a surgeon inserts the bone cell colony and scaffold material into the site of bone damage. At this point, new challenges arise, including promoting the development of a blood supply to the implant and ensuring the survival of the live cells. Achieving long-term repair requires infusion of antibiotics and growth factors in appropriate amounts and with optimal timing during the healing and repair processes. Utilization of nanoparticles for delivery of drugs and growth factors to the region is one promising approach. Another new approach begins with a CT scan of the bone defect and extraction of a small sample of fat from the patient. The CT scan is used to create a precise, three-dimensional model of the site that requires repair. The model is then placed in a growth chamber along with stem cells from the patient’s fat sample. Ideally, in a few weeks, a perfectly fitting bony replacement part will grow from the patient’s own cells. This promising approach is currently being developed in animals. Clinical Application Jose Luis Calvo/Shutterstock.com Growing new bone tissue is quite a complicated process, as evidenced by this microscopic view of bone structure showing osteons and Haversian canals.