Bone Graft Harvesting Techniques

Overview

The success of many spinal procedures is determined by successful bone graft fusion. Although instrumentation can provide immediate rigid fixation, ultimately, bone fusion must occur to prevent long-term failure. Whether the person’s own bone is harvested (autograft) or cadaveric allograft is used, the goal is the formation of a living, bony arthrodesis.

Selecting a Bone Graft

The type of bone graft used depends on the surgical procedure, the surgeon’s preference, and occasionally the patient’s preference. Autograft may be either cortical, cancellous, or mixed cortical and cancellous. Cortical bone is the strongest form of autograft and is typically used when strong structural support is required, such as fusion after anterior corpectomies. Compared with cancellous bone, cortical bone has fewer living cells and less surface area. Autologous cancellous bone provides only 60% of the compressive strength of cortical bone but has very high rates of fusion in appropriate cases. Cancellous bone provides the ideal combination of osteogenic, osteoinductive, and osteoconductive properties based on its composition of living cells and bone matrix proteins and its inherent architecture. Typically, pure cancellous bone is used for posterior spinal fusions, which do not require the graft to withstand compressive forces. Cortical-cancellous autografts are composed of both types of bone and offer the advantages of each. The grafts are stronger than cancellous bone, and they retain many of its advantages. A common example of cortical–cancellous bone is a tricortical iliac crest graft.

Although autograft remains the gold standard for successful formation of long-term arthrodesis, there are associated drawbacks. Complications associated with graft harvest can range from additional postoperative pain to more significant problems. The quality and quantity of autograft are sometimes inadequate. One alternative is cadaveric allograft. Unlike autograft—which is live, nonreactive, and genetically identical to the host—allograft is nonliving bone. Compared with autograft, allograft becomes vascularized more slowly. The rate of bone fusion is also slower, and the risks of bone resorption, rejection, and infection are higher. However, the distinct advantage of allograft over autograft is the lack of complications associated with harvesting. Similarly, a relatively new category of bone graft extenders may lead to successful fusion, despite not relying primarily on autograft or allograft.

Techniques of Bone Graft Harvest

Regardless of the type of bone graft, the chances of successful arthrodesis can be improved with meticulous surgical technique and adequate preparation of the bone graft and surfaces for spinal fusion. Combination of this care and preparation with rigid internal fixation optimizes the likelihood of long-term fusion. In general, local trauma to tissue should be minimized to ensure maximum vascularity of the fusion site. Avoidance of monopolar coagulation and use of copious irrigation during drilling can minimize the risk of thermal injury to the bone. Periosteum and other soft tissue should be meticulously removed from the bone graft and fusion bed, because it can lead to a fibrous interface and nonunion. Typically, the fusion site should be decorticated to improve the chances of successful fusion. The bone graft should be shaped to fit precisely into the fusion site to maximize the surface area of bone-to-bone contact. In addition, space within the fusion bed should be eliminated, and all antiinflammatory medications should be avoided during the perioperative and postoperative period.

Anterior Iliac Crest Grafts

Historically, bone graft harvested from the anterior iliac crest was commonly used for anterior cervical spine procedures that required cortical–cancellous bone. This particular indication is less common because of the recognition that fusion rates are high with allograft. However, anterior iliac crest bone may still have a role in anterior cervical fusions in select patients at high risk for nonfusion, and it is still used with some frequency for anterior lumbar fusion procedures.

Anterior iliac crest bone is obtained through a linear incision made parallel to the iliac crest and directly over the harvest site. The bone should be harvested from at least 3 cm behind the anterior superior iliac spine to avoid disrupting the ilioinguinal ligament or creating an avulsion fracture ( Fig. 72-1 ). A sandbag can be placed under the ipsilateral buttocks to assist with access to the anterolateral iliac spine.

Dissection proceeds through the subcutaneous tissue to the fascial layer. After retractors have been placed, the fascia is opened directly over the iliac crest, and subperiosteal dissection is performed. A fascial cuff and periosteum are left intact for secure closure. To expose a tricortical graft, medial and lateral subperiosteal dissection continues with a periosteal elevator, until adequate bone has been exposed. Dissection along the medial iliac crest must be done with great care. The dissection must remain subperiosteal to avoid inadvertent peritoneal entry or injury to the iliohypogastric, ilioinguinal, or lateral femoral cutaneous nerves ( Fig. 72-2 ). Cauterization should be used sparingly to prevent the possibility of nerve injury.

After the retractors have been deepened to provide excellent exposure to the tricortical graft, the bone may be harvested using either oscillating saws or osteotomes. If the graft will serve a weight-bearing function, oscillating saws are preferred, because osteotomes can cause microfractures that can weaken the graft. Some surgeons temporarily pack the medial and lateral exposure to avoid injury to the muscle or peritoneal cavity.

After the graft has been harvested, bleeding is controlled with bone wax or Gelfoam soaked in thrombin. Drains are rarely needed, and the wound is closed in multiple layers; the periosteal layers and fascial layers are closed with interrupted sutures.

Posterior Iliac Grafts

The posterior iliac region can be used to obtain tricortical grafts, cortical matchstick grafts, cortical-cancellous plates, or cancellous bone strips ( Fig. 72-3 ). Bone can be obtained from the iliac crest or in a subcrestal fashion. When a posterior iliac crest tricortical graft is planned, the graft is harvested from the posterior superior iliac spine (PSIS) or lateral to the PSIS to avoid the sacroiliac joint and sciatic notch. However, the graft should not be taken more than 8 cm from the iliac spine to avoid the risk of injury to the superior cluneal nerves, which can cause buttock numbness or painful neuromas ( Fig. 72-4 ).

Several variations of incision are used. Some surgeons prefer a vertical incision directly over the PSIS. Others prefer a curved skin incision beginning at the PSIS and extending superolaterally. Dissection proceeds through the subcutaneous tissue, and the fascia is opened directly over the iliac crest. Dissection continues medially and laterally in a subperiosteal fashion to avoid injury to the gluteal artery branches, which can cause brisk bleeding. Great care should also be used to avoid dissection in the region of the sciatic notch, where the main trunk of the superior gluteal artery, sciatic nerve, and ureter can be injured. Medial dissection involves stripping off part of the iliacus muscle. Care must be exerted to remain subperiosteal to avoid injury to the ilioinguinal nerve or pelvic contents. Subperiosteal dissection also avoids injury to the ureter, which lies within the retroperitoneal fat pad.

Grafts are obtained using a combination of oscillating saw, osteotomes, and bone gouges or curettes. Hemostasis is obtained with bone wax or Gelfoam soaked in thrombin. Drainage is rarely necessary, and the wound is closed in layers; interrupted sutures are used to approximate the periosteal and fascial layers, and a layered closure is critical to avoid herniation of the abdominal contents.

If tricortical bone is unnecessary, an alternative technique, known as the subcrestal exposure, can be used to harvest unicortical and cancellous bone. In this case, an incision is centered just lateral to the posterior iliac spine, and dissection proceeds along the posterior surface; the gluteal fascia is detached lateral to the PSIS so that an adequate cuff of connective tissue is left for closure. The dissection avoids the sciatic notch, which can easily be palpated. When dissection is adequate, a Taylor retractor can be used to assist with exposure. A window of cortical bone can be removed using straight osteotomes. If additional cancellous bone is required, it can easily be obtained using bone gouges. The inner cortical table should not be breached. Cancellous bone and cortical-cancellous matchsticks are ideal for occipitocervical posterior fusions. After hemostasis has been obtained using bone wax or Gelfoam, the gluteal fascia must be reapproximated to the periosteum to avoid gait disturbances. Again, the wound is closed in layers; meticulous closure of the periosteum and fascial layer prevents abdominal herniation.

Alternative Autologous Sites

In most cases, the iliac crest is the preferred site for autologous bone graft, but bone can also be harvested from the rib, fibula, and calvarium. Ribs have a relatively thin cortex, are mechanically weak in resisting compressive loads, and provide a relatively small volume of bone. However, they are sometimes a useful alternative, if other sites cannot be used. Because of their limited mechanical strength, rib grafts should not be used to reconstruct major spinal deformities without the application of a rigid internal fixation device.

To harvest a rib graft, a linear incision is made in the skin directly over the rib’s surface ( Fig. 72-5 ). The outer surface of the rib is exposed by incising the overlying muscles and periosteum. Blunt dissection with a Doyen rib dissector is used to detach the intercostal muscles and parietal pleura from the undersurface. Care is taken to avoid injury to the neurovascular bundle, which lies just along the inferior surface of each rib. The ends of the rib grafts are dissected sharply using a rib cutter or oscillating saw. In most cases, we prefer the oscillating saw, because the rib cutter can crush, splinter, and weaken the ends of the ribs. The remaining bone edges are smoothed and waxed to prevent pleural puncture and to avoid pneumothorax. After hemostasis is obtained, and the wound has been closed in multiple layers, a routine postoperative chest radiograph is obtained to rule out pneumothorax.