Intro to Allograft Tissue

What is Allograft? 

Allograft is tissue from another person or donor that has been cleaned and processed for use in surgery to aid in the healing and repair process for injuries or bone fractures, as well as the fusion of adjoining bones to help treat chronic pain. This type of graft provides the framework and environment for new bone tissue to form and grow. Allograft is a great alternative for patients with poor bone health, or when obtaining autograft (tissue from one’s own body) is not an option.

Before we take a deeper dive into the different types of allograft available on the market today, it is important to understand a little more about what is needed for successful bone fusion, and the natural process of bone formation and remodeling.

There are 3 components that must be present for successful bone fusion:

• Cells: The presence of specialized cells that have the ability to form new bone (osteogenesis).
• Signal: The presence of growth factors and chemicals that signal the specialized cells to begin the bone-forming process (osteoinductivity).
• Scaffold: A framework for the bone-forming cells to attach and lay down new bone (osteoconductivity).

There are also 3 important steps in the fusion process:

1. Bone resorption: Osteoclast cells are initially responsible for the breakdown of old bone tissue forming cavities and crevices.
2. Bone formation: Osteoblast cells are deposited into the bone cavities and start to create layers of new bone.
3. Bone remodeling: Over time, as new bone is continuously deposited, the entire graft is resorbed and replaced.

The type of graft and health of the host environment impact and determine the success of these steps.¹  Autograft satisfies all the necessary criteria and is known as the “gold standard” graft material in spinal fusion.²  However, the amount of bone tissue that can be harvested is limited, it requires a secondary surgery site, and sometimes isn’t the best option due to patient comorbidities. This is where allograft comes into play.

There are 3 main types of Allograft:

Mineralized bone allografts are osteoconductive, meaning they do not stimulate bone to grow, but rather provide a framework or scaffold, which is necessary for cells to attach, grow along, and fill in during the fusion process.^3,4 They are radio-opaque (can be seen on x-ray imaging) and are compression resistant, providing volume within the defect or fusion space. These allografts come in a wide variety of forms such as chips, particulate, fibers, and cubes, as well as grafts designed to provide support and structure such as interbody spine spacers, struts, shafts, and wedges. 

Demineralized bone matrices (DBM) are prepared using acid extraction to remove the mineralized portion of the allograft bone, leaving behind a collagen matrix. DBMs also act as a scaffold but are radiolucent and less volume-stable compared to mineralized grafts. With that said, however, the demineralization process delivers a scaffold and signaling proteins that promote the differentiation of cells into osteoblasts, which are bone cells responsible for forming the mineral matrix that solidifies bone.⁴ DBM also has a more flexible and soft consistency allowing it to be easily manipulated and molded. It is commercially available in a variety of forms, shapes, and sizes, and can be placed into small defects, or even injected when combined with a carrier.⁵ DBMs are preferred over mineralized bone grafts when more biological help is needed to encourage bone remodeling and fusion.

Cellular bone matrices (CBM) are allogenic bone grafts that contain live mesenchymal stem cells (MSCs) and provide all three of the components (cell, signal, scaffold) essential to new bone formation and healing, making them similar to healthy autograft.⁶ CBM is a combination of bone chips, providing the osteoconductive scaffold and viable cells, along with demineralized bone, providing the signaling proteins. They are available in both a particulate and putty-based form, and are typically utilized for more challenging constructs and for patients with a variety of risk factors that could compromise the potential for new bone growth and fusion.⁷

With a wide range of options available in different forms, shapes, size, and function, allograft is an effective, versatile and readily available alternative to autograft for a variety of surgical procedures, including extensive and complex repairs. Moreover, with no need for an additional surgical site, the use of allograft reduces pain and time to recovery for the patient, while still providing the healing modalities necessary for new bone growth and formation.

References:

1. https://www.spine-health.com/treatment/spinal-fusion/bone-graft-spine-fusion
2. Reisener MJ, Pumberger M, Shue J, Girardi FP, Hughes AP. Trends in lumbar spinal fusion-a literature review.J Spine Surg. 2020;6(4):752-761. doi:21037/jss-20-492
3. Oliveira OR, Martins SP, Lima WG, Gomes MM. The use of bone morphogenetic proteins (BMP) and pseudarthrosis, a literature review. Rev Bras Ortop. 2016;52(2):124-140. Published 2016 Jun 22. doi:1016/j.rboe.2016.03.005
4. Baldwin P, Li DJ, Auston DA, Mir HS, Yoon RS, Koval KJ. Autograft, Allograft, and Bone Graft Substitutes: Clinical Evidence and Indications for Use in the Setting of Orthopaedic Trauma Surgery. J Orthop Trauma. 2019 Apr;33(4):203-213. doi: 10.1097/BOT.0000000000001420. PMID: 30633080.
5. Wang W, Yeung KWK. Bone grafts and biomaterials substitutes for bone defect repair: A  Bioactive Materials/ 2017; 2(4); 224-247. ISSN 2452-199X. https://doi.org/10.1016/j.bioactmat.2017.05.007  
6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7323463/
7. https://allograftacademy.org/learn-about-allografts/bone-grafts/allograft/cellular