Bone Graft Harvest
Robert M. Galler, DO*
Volker K. H. Sonntag, MD
Division of Neurological Surgery, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona
Department of Neurological Surgery, State University of New York, Stony Brook, NY
Spinal fusion with or without instrumentation requires the use of bone graft. Bone graft may be autogenous or exogenous. It may be acquired from various sites and exists in various forms. The methods of obtaining bone graft as well as the properties of each type are reviewed.
Key Words: bone graft, fusion, spine
The use of transplanted bone for surgical fusion of the spine is a well-established technique. Since first reported in 1911, different methods for acquiring and using bone graft have been developed. The most popular sites for harvesting bone grafts are the region of the spine that has been decompressed, the rib, and the iliac crest.
The ideal bone graft has specific features that allow healing and eventual fusion at the operative site: osteogenesis, osteoconduction, and osteoinduction. Osteogenesis refers to the native capacity to form bone. This process is accomplished by osteoblasts under the influence of many factors such as bone morphogenetic protein (BMP). Cells capable of this task are found in living marrow. Allograft lacks this capacity because the marrow is absent. The influence exerted on these cells is called osteoinduction. Methods of producing agents that effectively induce bone formation, such as BMP, have been the focus of extensive research and are covered fully elsewhere. Osteoconduction is the physical property present in the structural composition of a bone graft that acts as a scaffold for new bone to form. Allograft and autograft can both accomplish this goal. Allograft acts purely as a osteoconductive material while autograft has all three features. Although autograft has a biological advantage, it requires more effort to acquire and its harvest is associated with potential complications. Postoperative pain, wound infection, numbness, and cosmetic deformity can occur at the graft site.
Types of Bone Graft
Cancellous bone graft from an autologous source is an excellent medium for fusion. It has all three properties of an ideal graft and is readily available. When obtained for fusion of the cervical spine, purely cancellous bone is most often used to pack a strut graft or as onlay material for posterolateral arthrodesis. Cancellous bone alone is unable to withstand compressive forces and should not be used as a structural element in a surgical construct.
Pure cortical grafts are used when structural integrity is the paramount concern. They are most often used as strut grafts in vertebral reconstructions in patients who have undergone corpectomy. These struts tend to be allograft bone in the form of fibula. The primary purpose of the strut is support and osteoconduction. The center of the graft should be filled with cancellous bone to add osteogenic and inductive properties to the graft.
Figure 1. Bone grafts harvested from the anterolateral ilium should remain 2 to 3 cm behind the anterior superior iliac spine to avoid an avulsion fracture. Tricortical bone grafts can be harvested for (A) single-level interbody or (B) multisegment vertebral body reconstruction.
The combination of the structural integrity of cortical bone and the osteogenic and inductive features of cancellous bone make corticocancellous grafts a popular choice. This type of graft is most often harvested from the iliac crest because the rib provides inadequate structural support for large reconstructions. Tricortical iliac crest graft has been used extensively for cervical spine arthrodesis for more than 50 years. Harvesting large iliac crest struts is associated with potential morbidity. Furthermore, the morphology of the iliac crest and the area of the spine being reconstructed limit their use.
Bone graft material can be obtained at the time of anterior decompression. The drilled bone that remains after the endplates are prepared should not be wasted. This material is easily collected for later use with a curette or a Penfield No. 1 dissector. The center of a strut or allograft ring can then be filled with this bone graft without the need for a separate incision.
The anterior approach to the iliac crest is used for anterior reconstructive procedures (Fig. 1). Cancellous or corticocancellous grafts can be obtained with this method. The patient should be positioned supine with a sandbag under the ipsilateral gluteal region to accentuate the anterior superior iliac spine. The incision is made parallel to the hip so a wide area should be cleaned and prepared. At least 2 cm of the anterior superior iliac spine needs to be kept intact to avoid injury to the insertion of the sartorius muscle and to the inguinal ligament. The lateral femoral cutaneous nerve may have an anomalous course in this region and should be avoided. The integrity of the anterior superior iliac spine should not be compromised, or a stress fracture can result from the forces of the sartorius and rectus femoris musculature.