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Grafts in Spinal Surgery

Terminology

Donor Site

The area from which the bone graft is taken (harvested) e.g iliac crest.

Graft

A graft is a tissue or substance that is placed at a site different to its site of origin. In the spine a graft is generally the patient’s own bone (autogenous graft), bone from another person (allograft) or substances that are used to extend or enhance bony fusion. Other surgical grafts include skin, liver, cornea, kidneys, etc.

Graft Substitute

A material capable of achieving fusion rates equal to or greater than autogenous bone without the need for autograft.

Graft Extender

This is a substance that, when added to autograft, allows for the fusion of more levels or the use of less autograft and results in a rate of fusion equal to that of autogenous bone.

Graft Enhancer

A graft enhancer, when added to autogenous bone, yields a higher rate of fusion than autogenous bone alone.

Osteoconductive

The ability of a material to act as a scaffold for attachment of osteogenic cells.

Osteoinductive

Biologic stimuli from growth factors and cytokines (such as BMP’s (bone morphogenic protein), TGF-b (transforming growth factor), IGF (insulin-like growth factor), PDGF (platelet derived growth factor)) are used to promote osteoblastic progenitor cells to proliferate and differentiate into bone.

Osteogenic

The presence of osteogenic cells must be present either within the graft (autograft), transplanted into the graft at surgery (bone marrow aspirate) or able to migrate into the graft from surrounding tissue (adjacent bone, periosteum, muscle, fat or connective tissue). 

What sort of Grafts, Graft Substitutes and Graft Enhancers are there?

Biological

Cortical

This is often allograft (such as fibula) that is used for its mechanical strength in cervical spine and lumbar spine when trauma, tumour, and degenerative conditions require anterior column support

Cancellous

This is the less strong, central part of a living bone that is used as a graft when structural support is not as important as osteoinductive and osteoconductive potential, such as in posterolateral fusion in posterior deformity (scoliosis) cases. Normally it is taken from the patients own iliac crest (anterior or posterior).

Autologous

This is graft originating from the skeleton of the same individual into whom it is to be placed. 

Allograft

By definition this originates from another being of the same species and is commonly cadaveric. It is prepared by the processes of freeze drying or fresh frozen. Freeze drying allows a long storage life and decreased antigenicity at the expense of mechanical strength. Fresh frozen graft maintains its strength. Cadaveric allograft is sterilised (low dose irradiation, ethanol, etc) which renders it primarily osteoconductive as most of its osteoinductive capabilities are lost with the sterilisation and preparation procedures.

Xenograft

This is graft that is taken from a different species.

Non-Biological

Ceramics

Ceramics are formed by heating and pressurising calcium phosphates. Tricalcium phosphate and hydroxyapatite are the most commonly used ceramics in spinal fusion. They are safe and biocompatible. However they are brittle and have low fracture resistance. Therefore they are not generally used in load-bearing situations. The porous structure of the ceramics allows vascularisation and a large surface area to which osteogenic cells can adhere.

Demineralised Bone Matrix (DBM)

DBM is what is left when the bone mineral is extracted from bone by acids. It is made up of collagen, various (non-collagenous) proteins, and growth factors. Amongst these growth factors is bone morphogenic protein (BMP, also called bone morphogenetic protein). Numerous types of DBM are available including gel, putty and sheet fibres. It is generally used as a graft extender.

Bone Morphogenic Proteins (BMP)

BMP’s are a family of at least 24 related growth factors within the TGF (transforming growth factor) superfamily. They are able to stimulate bone formation by causing pluripotent precursor cells to differentiate into bone-forming cells (osteoblasts and chondroblasts)

Applications of Grafts in Spine Surgery

Cervical

  • Posterior/posterolateral – normally autologous cancellous bone with occasional extenders
  • Anterior – often tricortical corticocancellous autograft from the iliac crest. Also metal/mesh cages can be used with or without plates.

Thoracic

  • Posterior/posterolateral – most commonly used in deformity surgery in children. Again autologous cancellous graft is used but extenders are not always required in children due to their ability to fuse so readily.
  • Anterior – mesh cages filled with autologous graft with extender can be used.

Lumbar

Problems with Grafts

Donor Site Complications

The incidence of donor site complications in spinal surgery is not insignificant. Complication rates have been noted to range from 6-25%. Noted complications are painful scarring, increased blood loss, donor site pain, prolonged rehabilitation time, pelvic fracture, neuropathy and infection.

Volume

In spine surgery there is often a disparity between the amount of graft required and the amount that can be harvested from the local bone/iliac crests (pelvis). This is why graft extenders are used.

Physical Problems

Autologous graft, allograft and ceramics  can collapse, resorb or change position after implantation. Outside of the spine this may be little more than an annoyance but in the spine this may result in nerve root or cord compression with serious consequences. It is for these reasons that metal cages have been designed to act as spacers /containers and allow fusion to progress around or through them while at the same time avoiding graft spillage into the spinal canal.

Non-Union

Fusion is not guaranteed with autologous graft. The more spinal levels involved in the fusion, the more the chance of successful fusion decreases. It is for this reason that other technologies have been utilised to extend and enhance the chance of fusion.