Biology of Bone Healing and Current Bone Regeneration Strategies

There is an increasing amount of bone disease and trauma as a result of trauma, cancers and degenerative conditions. As a result, various bone replacement and repair techniques including biomaterial substitutes cell-based therapies and nanomedicine have been developed to address this need.

Bone plays a major role in physical and muscular support, organ protection as well as mineral storage and production of blood cells. Musculoskeletal diseases such as rheumatoid arthritis, osteoporosis as well as bone fractures are commonly occurring and are often painful. The current gold standard for treating bone defects is by using bone autografts, where bone from one part of the patient’s body is removed and used to replace the damaged tissue. Bone autografts often result in chronic pain at the donor or acceptor sites, nerve injury and infection. In some instances, allografts- bone from other donors- are used, but possess the added risk of disease transmission and implant rejection. To mitigate these risks and side effects, the research is being conducted into the use of natural and synthetic biomaterials for bone grafting and tissue regeneration. This article looks at the recent approaches in bone tissue engineering, while shedding light on the basic biology of bones and the mechanism of bone repair.

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Biology of Bone and Bone Repair

Bone is a multi-functional connective tissue with a complex structure comprising of calcium phosphate, parallelly-assembled type I collagen and living bone cells. The most important bone cells include osteoclasts, osteogenic, osteoblasts and osteocytes, which work collectively to maintain bone health and structure. Osteoblasts, derived from the osteoprogenitor cells of mesenchymal origin, are responsible for bone growth and remodelling, and do this through the synthesis, deposition and mineralization of bone matrix. Osteocytes are inactive osteoblasts, which have a distinctive star shape, and are vital for communication within bone tissue. Osteoclasts work to break down bone through mineral dissolution and resorption. Imbalances in the osteoblast-osteoclast activity is what leads to conditions such as osteoporosis (higher osteoclast activity) and osteopetrosis (higher osteoblast activity).

In the event of a fracture, bone regeneration follows a highly organised multi-stage process that starts with inflammation, followed by bone production and then remodelling. Immediately the bone breaks, there is an increased amount of blood flow to the site of injury, which promotes inflammation and the formation of a hematoma. Similar to wounds, the hematoma provides a base matrix that guides cells in new bone formation. The hematoma is then replaced with fibrous tissue and cartilage, much like granulation tissue formation in wounds. Osteoblasts then deposit new bone material throughout this soft callus, which then slowly becomes hard bone. Remodelling takes place after this, in a slow process where osteoclasts work to restore the newly formed bone to its original form, while creating pathways for blood vessels and nerves to path through.

Stages of bone fracture repair and remodelling

When this highly regulated healing process cannot take place in a proper and timely manner, for example in patients with degenerative bone diseases or extensive fracturing, other methods are employed to facilitate healing, with the conventional approach being autografts and at times allografts, whose risks we have previously discussed. Other strategies that have been recently developed include;

1.??????Demineralized Bone Matrix

Demineralized bone matrix (DBM) is a scaffold matrix obtained by decalcifying cortical bone derived from long bones such as the femur. DBM is less immunogenic than allograft bone graft and contains an abundance of osteoinductive factors that prompt bone regeneration. DBM implantation has been successful for appendicular, axial and craniofacial bone repair and is used in approximately 20% of bone grafting cases in the United States.

Because DBM is a human-derived tissue product, several factors such as the bone donor age and gender, method of preparation, processing and sterilization protocols can all play a part in the clinical outcome and performance of the product. This variability makes the comparison of DBM products difficult when assessing the clinical effectiveness and how it can best be used.

2.??????Synthetic scaffolds

Various synthetic materials have also been assessed as potential acellular tissue regeneration matrices. Recently, poly-L-lactide (PLLA) films have been used to successfully regenerate the radius bone in mature rabbits, with new cortical bone developing over the defect where the film was placed. Similarly, Poly-ε-caprolactone-co-lactide (PCLA) has been used as a filler material for femoral bone defects in rats, which exhibited good biocompatibility and retained their form for over 12 weeks in vitro.

To increase the bioactivity of these scaffolds, active biomolecules can be added during manufacture to enhance the cellular activity of native cells and boost bone regeneration. Bioactivity can be implemented in a number of ways, including: (a) applying recombinant growth factors such as BMP-2, BMP-7 and rhPGDF-BB, which all trigger a cascade of bone repair and regenerative activities, (b) applying proteins that target specific cell receptors, and (c) applying small molecules that trigger specific cell signalling pathways which are associated with mineral deposition and remodelling.

3.??????Nanomedicine

Infection is always a concern when treating open fractures and microbial presence within the injury slows down the regenerative capacity of native cells as they try to fight off the infection. As a result, numerous categories of antibiotic-carrying nanoparticles have been developed to deliver antibacterial drugs within fractures, with the ability to enter the membranes of microorganisms and destroy them from within. Other nanoparticles disinfect the injury site by producing massive oxidative stress to microorganisms through free radical construction to destroy their contamination hazard.

Silver is the most commonly used nanoparticle material due to its highly extensive antibacterial properties that has been effectively demonstrated in S. Aureus, Bacillus subtilis, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Escherichia coli.

4.??????Cell-based Products and Therapies

Bone cells such as osteoblasts, as well as mesenchymal stem cells (MSCs) and dental pulp-derived stem cells (DPSCs), may also be used as the bioactive component in bone transplants. These cells can be incorporated in bone grafts to stimulate mineralization and regeneration when implanted. Various studies involving autologous MSCs transplanted into bone defects in rabbit and mice skulls have exhibited new bone mineralization after several weeks. These cells can also be cultured into synthetic scaffolds prior to implantation to achieve a similar effect in big bone defects.

5.??????Biomaterial-based 3D cell printing substitutes

This approach uses 3D images of the bone defect site, often obtained through multiple MRI and CT scan images, and computationally processed to create a 3D model of a bone replacement that is specific to that patient. This personalized bone graft model is then 3D printed with a biocompatible material such as titanium which is commonly employed in jaw replacement surgery. Resulting Ti6Al4V implants using this technique have presented outstanding physiochemical and biological properties and support the bone regeneration across their surface after implantation. Bioceramics and biopolymers such as polyetheretherketone (PEEK) are also being explored for their use in custom bone implants, with research now in pre-clinical and clinical phases.

Source: M. Ansari, “Bone tissue regeneration: biology, strategies and interface studies,” Progress in Biomaterials, vol. 8, pp. 223–237, Nov. 2019, doi: 10.1007/s40204-019-00125-z