**4. Allograft**

The second category of natural bone grafts is allograft, where tissue from another individual (donor) has to be taken for grafting. The term allograft ('allo' (other)) means harvesting tissue from different individuals to transplant in the subject. Bone harvested from cadavers in different bone banks is the major source of different allografts. Apart from the above-mentioned category, freeze dried (lyophilized) and frozen bones are also used in spine graft surgeries. Allografts are considered more advantageous as they are considered less stressful to the patient. However, they are also plagued with problems of availability and immune rejection. The first report of allograft usage can be dated back to 1800s, when Macewen reported the grafting of tibial fragments to a child [23]. Experimentation reported by Bauer in the beginning of twentieth century marks the starting of the bone bank concept, where he stored bone tissues for weeks in refrigerated condition and then used them for implantation in dogs. It was also established that chilling and boiling of bone tissues before using them as allografts led to destruction of their endogenous proteins and other factors resulting in poor and slow recovery [24]. The necessities posed by the devastation of World War II resulted in advancement and growth in the bone banking approach, with refinement of methods like autoclaving, freeze-drying, irradiating, demineralizing, and chemical treatment of the bone [25]. The advancement made during the war period continued with focus on civilian need and further refinement of the banking approach [23]. Pros and cons of allografts can be summarized in **Table 2**.


#### **Table 2.**

*Common advantages and disadvantages associated with an allograft.* 

#### **4.1 Types of allografting commonly used are**

#### *4.1.1 Cancellous bone graft*

This is primarily an osteoconductive graft, devoid of growth factors (not osteoinductive), and like cancellous autograft, it also provides less mechanical support [26]. It is prepared in small cubicle form, thus sometimes called 'croutons' and is

now marketed in varied sizes as chips or crushed form. It involves an initial cascade of inflammatory events leading to formation of a fibrous tissue around the graft, preventing complete integration of the graft. The survival and efficiency depend a lot on processing and storage conditions of the grafts. Freeze drying is the most common method of preservation of cancellous graft, which may result in destruction of osteoprogenitor cells and osteoinductive factors [27, 28].
