A bone graft transplants bone tissue. Bone grafts to repair and rebuild diseased bones in your hips, knees, spine, and sometimes other bones and joints. Grafts can also repair bone loss caused by some types of fractures or cancers. Once your body accepts the bone graft, it provides a framework for growth of new, living bone.
Bone has the ability to regenerate completely if provided the space into which to grow. As native bone grows, it replaces the graft material completely and results in a fully integrated region of new bone.
Types of Bone Grafts
Depending upon the source where the graft is taken from, the bone grafts can be autograft or allograft.
Also called autologous or autogenous bone graft, this kind of graft is obtained from the same individual receiving the graft. Bone can be harvested from non-essential bones, such as the iliac crest. Iliac crest is most common source of autografts in orthopedic surgery. Other places where graft can be taken from is fibula or metaphyses of long bones.
However very small quantity can be taken from long ends of the bones.
Autogenous bone is the most preferred for the grafting and is deemed as gold standard. There are no risks of graft rejection as the graft originated from the patient’s own body. It is most efficient graft type.
However autografts are associated with an additional surgical procedure and potential for post-operative pain and complications.
Normally a bone graft is either used in blocks or slivers after being harvested. Sometimes grafts are also harvested especially with their suppying vessel and the blood vessel is reanastomesd to the blood vessel at recipient site.
Such a graft is called vascularized graft.
Vascularized fibulae grats have been used to restore skeletal integrity to long bones of limbs in which congenital bone defects exist and to replace segments of bone after trauma or malignant tumor invasion. The periosteum and nutrient artery are generally removed with the piece of bone so that the graft will remain alive and grow when transplanted into the new host site. Once the transplanted bone is secured into its new location it generally restores blood supply to the bone in which it has been attached.
Allograft bone, like autogenous bone, is derived from humans; the difference is that allograft is harvested from an individual other than the one receiving the graft. Allograft bone is taken from cadavers that have donated their bone so that it can be used for living people who are in need of it; it is typically sourced from a bone bank.
There are three types of bone allograft available.
1. Fresh or fresh-frozen bone
2. Freeze-dried bone allograft
3. Demineralized freeze-dried bone allograft
Xenografts are grafts taken fro species other than humans. Till now xenografts have only historical and experimental significance.
Bone substitutes are synthetic materials that are aimed to replace bone grafts. Partial success has been achieved with these and these are discussed separately.
A bone graft is used clinically to provide a bridge of osteogenic tissue, either in a part of the skeleton that is deficient or to establish bony fusion of diseased joint.
Indications of Bone Graft
In 1668 the Dutch surgeon Job Van Meekeren described first bone graft procedure. The graft was derived from a dog’s cranium and was used successfully to repair a traumatic defect in a soldier’s skull.
Duhamel is credited for the work on osteogenesis by his experiments on periosteal new bone formation in 1742.
In 1867, Ollier showed that autografts are viable and separate living bone fragments could live and grow in suitable environment.
Bone grafts are used for various condition
- Delayed and non union of fractures.
- The arthrodesis of joints.
- Filling cavities or defects resulting from cysts, tumors, trauma or other pathologies.
- Fusion of growth palate cartilage.
- Bone and muscle transfer for ligamentous defects.
How Does Bone Graft Act?
Weather transplant or implant, bone graft serves following functions:
Osteogenesis refers to new bone synthesis by graft or host derived cells.
Bone Morphogenic Protein which is present in fresh autografts & modified allografts induce changes is host mesenchymal cells to form osteoblasts .
The graft acts as framework or scaffold into which new capillaries, perivascular tissue and osteoprogenitor cells grow.
Following images show the healing of lesion [aneurysmal bone cyst] in upper tibia.
Functions of Bone Graft
Bone grafts serve two functions, a mechanical function and a biological function. In certain applications one may be important than other but most of time the two functions are intimately intertwined. The net biologic activity of a graft is the sum of its inherent biologic activity (living cells & their products) and its capacity to activate surrounding host tissues to relevant biologic activity (mediated by bioactive factors within the matrix) and its ability to support the ingrowth of host tissue.
The graft cannot exert its biological activity in isolation. It is dependent on surrounding environment for cells to respond to its signals. The mechanical environment of the graft site is also important. Bone grafts are remodeled in response to mechanical load to which they are subjected.
Bone grafts incorporation is a complex multifaceted process and multiple variables influence its rate, pattern and completeness. Incorporation of a bone graft begins with homeostasis with intact bone. Unlike other tissues, bone heals by regeneration and replacement by additional bone.
How Does Bone Graft Enhance Bone Healing?
A bone graft either stimulates the recipient bone itself or causes change in bone to enhance bone healing. A bone graft is incorporated by following processes.
Osteogenesis is the synthesis of new bone with no indications of cellular origin i.e. either by cells of grafted bone or the host bone. Properly handled fresh cortical and cancellous autograft are able to produce new bone. Cancellous bone with its larger surface area covered with quiescent lining cells or active osteoblastic cells has a greater potential for producing new bone of graft origin than the cortical bone.
This early bone formation either by graft cells of cortical or cancellous bone is critical in callous formation during 4 to 8 weeks after surgery and accelerates the bone incorporation.
Osteoinduction is the recruitment of the mesenchymal cells of the host bed surrounding the graft to be differentiated into bone forming cells and cartilage forming cells.
The mesenchymal cells generally arise from perivascular cells associated with proliferating blood vessels, reticular elements of bone marrow, trabecular surfaces and the cambium layer of the periosteum.
These cells then can specialize as osteoblasts, osteoclasts, chondroblasts, chondroclasts, fibroblasts and occasionally lipoblasts. The recruitment and differentiation of these cells is probably modulated by low molecular weight polypeptides such as glycoprotein bone morphogenic protein (BMP).
BMP is a hydrophobic, non species specific protein whose activity does not require viable graft cells and is present in fresh autografts as well as in modified allografts. The other graft derived factors besides BMP are transforming growth factor , Insulin like growth factor I & II, Acidic and Basic fibroblast growth factors, Platelet derived growth factors, Interleukins, Granulocyte macrophage colony stimulating factors etc.
The osteoinductivity of mineralized graft is thought to be minimal except as living graft cells may be producing osteoinductive factors but the osteoinductivity of demineralized bone matrix has been repeatedly shown.
The health of the host bed is critical in the process of osteoinduction because new osteoprogenitor cells are recruited by induction of residual mesenchymal cells in marrow reticulum, endosteum, periosteum and connective tissue.
In addition to above mentioned biologic functions the graft may provide structural support until the recipient tissue can bear weight. In addition to providing mechanical supporting, they function as trellis or scaffold for the ingrowth of new host bone2.
The three dimensional process of ingrowth of sprouting capillaries, tissue and osteoprogenitor cells from the recipient bed into the structure of the graft is termed osteoconduction. Osteoconduction may result from active bone formation and osteoinduction e.g. in a fresh corticocancellous autograft or it may occur passively without the active participation of the graft, as is the case with most cortical allografts. The graft is resorbed gradually and is completely replaced by new host bone a process termed as creeping substitution by Phemister.
Sequence of Events in the Incorporation of Bone Grafts
- Hematoma formation (Host dependent)
- Organisation of hematoma by local inflammatory response (Host dependent)
- Neovascularisation i.e. penetration of neocapillaries with perivascular tissues into the graft (Host dependent)
- Cytodifferentiation by interaction between the host derived cells and the grafted material (chemomodulation and environmental interplay)
- Creeping substitution (Resorption by osteoclasts and neoosteogenesis from osteogenic cells)
- Remodeling (continued resorption and neoosteogenesis under influence of functional stress and strain) to the trabeculated bone.
The initial event is formation of a hematoma rich in platelet derived growth factor, other growth factors and cytokines. A local inflammatory response is stimulated by the implantation procedure and by the presence of the graft.
Clinically, a bone graft has been incorporated successfully when the host graft host bone interface unites and the affected area tolerates physiological weight bearing without fracture or pain.
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