Bone Graft Harvesting

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Chapter 123 Bone Graft Harvesting

Mehmet Zileli, Edward C. Benzel, Gordon R. Bell

Spinal surgeries typically involve neural decompression, fusion, or both. Fusion is performed to stabilize a segment, either because it is believed to be a source of pain or because of concerns regarding possible instability. Potential sources of instability include trauma, infection, tumor, or degeneration (spondylosis). Iatrogenic instability also may occur following previous spinal surgery. When instability is present, either with or without pain, fusion—using bone grafting and possibly instrumentation—often is employed. Because instrumentation provides only a temporary support, solid bony union must be achieved to provide long-term stability.

Bone Graft Specifications

Although a complete description of the myriad substances that are, or have been, used for arthrodesis is beyond the scope of this chapter, some general concepts are presented. An ideal bone graft should be incorporated rapidly, be structurally sound, have antigenic compatibility, be readily available and easily formed, have a low incidence of graft donor site complications, and be cost effective. Unfortunately, none of the grafts currently available today meet all of these requirements.

Each type of bone has its particular advantages: cortical bone graft provides good structural support, and cancellous bone graft provides more rapid incorporation. As the cellular elements in grafts die, they are slowly replaced by “creeping substitution,” as the graft acts as a scaffold for new bone formation. In cortical bone this process is slower than with cancellous bone. Cancellous bone, however, is not as strong as the cortical bone and is therefore less ideally suited for structural support.

Autograft

Autografts are commonly used in spine surgery, and they remain the gold standard for fusion. An ideal autograft should include strong cortical bone for structural support and cancellous bone for more rapid incorporation and fusion.¹ Revascularization of cancellous bone is completed within several weeks, whereas the same process takes several months, or longer, for cortical bone.^2,³ Autografts commonly are used in conjunction with spinal instrumentation, but can also be used for dorsal onlay fusions without instrumentation. A significant advantage of autologous bone is that there is no risk of disease transmission.

Both cortical and cancellous grafts are commonly obtained from the iliac crest. Cortical bone can also be obtained from the fibula. In the early days of spine surgery, even the tibia was used, although the latter rarely is employed today.

Allograft

Allografts are commercially prepared and typically are obtained from cadaver bone. They are characterized by delayed vascularization and incorporation, which is believed to be due to antigenic recognition by the host. Allografts are appropriate in a variety of clinical situations.^4,⁵ They are most commonly used for ventral cervical interbody fusions, where single-level allografts generally lead to solid arthrodesis, similar to the fusion rate with autograft.⁶ However, they incorporate relatively slowly,⁷ and, if used for multilevel fusions, are associated with a pseudarthrosis rate of 63% to 70%.^7,⁸ Fibular allografts are preferred for cervical corpectomy, because the harvesting of a fibular autograft is associated with significant morbidity, including pedal edema, ankle pain, and the risk for peroneal nerve injury.⁵

Xenograft

Xenografts are tissues transplanted from one species to another. Xenografts give less satisfactory results than autograft or allograft, because of histocompatibility differences. Kiel (calf bone) grafts have been used in spine surgery, although their use is of historical significance only.

Additional substances also are used in conjunction with autogenous or allograft bone to augment fusion. These include autologous bone marrow and bone morphogenetic protein (BMP). The latter, when used in the cervical spine or for dorsal spine surgery, typically is used in an off-label manner. The use of BMP for ventral cervical fusions has been associated with significant complications, including airway obstruction.

Bone Graft Substitutes

Nonbiologic materials such as hydroxyapatite,^9,¹⁰ ceramic, and polymethylmethacrylate¹¹ also have been used, either in conjunction with, or in lieu of, of bone graft materials. They have the advantage of being able to be manufactured in a variety of sizes, shapes, and quantities. Polymethylmethacrylate is an inert substance that is rarely used, except in cases where life expectancy is severely limited, as in providing structural support in a patient with metastatic disease to the spine and a very short survival.

Bone Graft Types

The advantages and disadvantages of various autologous bone graft donor sites are discussed in this section. Preservation of the periosteum can provide a source of cells to help to form new bone to fill the defect.

Local Bone

Bone that is removed during surgical decompression commonly is used in spinal fusions. It consists of both cancellous and cortical components. It can be morselized manually with a rongeur or can be placed in a bone mill to provide a more consistent substrate for grafting.¹² The quality of local bone generally is perceived as being not as good as iliac bone grafts because of its cortical content.

Iliac Crest

The most commonly used donor site is the iliac crest. The iliac crest is a readily available source of cancellous bone, which provides rapid incorporation. Its disadvantages include donor site pain, which is a common complaint ¹³; limited volume, which can be a concern for procedures requiring a copious amount of bone; and limited utility for procedures requiring a large piece of structural bone for reconstruction.

Ventral iliac crest grafts commonly are used to provide bone for various types of ventral cervical fusions (Fig. 123-1). The incision should be just caudal to the crest, to minimize discomfort that would be caused if the incision lay directly over the graft site, and should be placed approximately 3 to 4 cm lateral to the anterior superior iliac spine (ASIS) to minimize the risk of inadvertent injury to the lateral femoral cutaneous nerve. This nerve lies lateral to the ASIS in 90% of patients; in 10% it lies medial to the ASIS. When harvesting a tricortical graft for anterior cervical discectomy and fusion, an incision 6 to 8 cm long and a subperiosteal dissection of both the inner and outer wall of the ilium are performed. The iliac crest also can serve as a source of structural graft for cervical corpectomy and can provide an 8- to 10-cm length of tricortical strut graft.¹⁴^–¹⁷ Dissection of the iliacus muscle from the inner wall of the crest should be minimized to reduce the risk of hematoma formation and to reduce the risk of postoperative pain. The fascia should be closed meticulously to prevent a herniation of the pelvic contents.^18,¹⁹

FIGURE 123-1 Ventral iliac crest graft harvesting and ventral cervical fusion techniques. The Smith-Robinson technique is shown at the top, the Cloward technique in the middle, and the Bailey-Badgley technique is illustrated at the bottom.

(Redrawn with permission from White AA, Hirsch C: An experimental study of the immediate load bearing capacity of some commonly used iliac bone grafts. Acta Orthop Scand 42:482–490,1971.)

A saw or bur should be used for graft harvest rather than an osteotome to minimize the risk of graft microfracture, which may lead to graft failure. Care should be taken to keep the ventral saw cut at least 2 cm lateral to the ASIS to minimize the risk of ASIS avulsion. After obtaining the graft, the raw donor site bone surfaces should be covered with bone wax, or thrombin-soaked Gelfoam to minimize hematoma formation. A drain usually is not necessary, except for large defects.

Dorsal Iliac Crest Grafts

The dorsal iliac crest is predominantly used to obtain large quantities of dorsal onlay graft material for dorsolateral lumbar fusions. More bone is available from the dorsal iliac crest than from the ventral crest.²⁰ Bone graft may be obtained through the midline lumbar skin incision used to perform the concomitant decompression or through a separate lateral skin incision over the iliac crest. The midline skin incision usually is used to obtain dorsal iliac graft. The optimal site for the underlying fascial incision is 6 to 8 cm lateral to the midline. If the fascial incision is placed lateral to this point, injury to the cluneal nerves may occur, which can result in numbness or pain over the buttocks. This is more likely if a large graft is required. Laterally, the sacroiliac ligaments and joints must be avoided. Care should be taken to minimize the depth of the osteotomy to avoid the sciatic notch where the superior gluteal artery and nerve could be injured. Injury to these structures is unlikely if the dissection is performed subperiosteally. Cancellous bone can be harvested with a gouge, which is helpful in removing strips of cancellous bone (Figs. 123-2 and 123-3).

When harvesting dorsal iliac crest bone for a cervical or thoracic fusion, the iliac crest graft incision can be made either obliquely just below the iliac crest, as described, or vertically.

Fibula

The fibula has the advantages of providing strength and length, and being relatively easily harvested. Its disadvantages include a slow rate of incorporation and harvest site complications. A fibular strut graft should be harvested from the middle third of the fibular shaft through a long skin incision on the lateral side of the leg, extending through the lateral intermuscular septum, preserving the periosteum (Fig. 123-4). During fibula graft harvesting, the peroneal nerve must be protected. Distally, the fibula should be harvested no more than 10 cm proximal to the ankle joint to minimize the risk of injury to the ankle syndesmosis, which is important for the stability of the ankle joint. The peroneal muscles should also be preserved. The middle third of the fibula should be osteotomized using an oscillating saw rather than an osteotome, which could cause fracture of the graft (Fig. 123-5). After fibula harvest, a few days of compressive leg wrapping with elevation of the leg will minimize swelling and discomfort.

FIGURE 123-4 A, Unicortical graft harvesting from the dorsal iliac crest using an osteotome. B, Tricortical graft harvesting from the dorsal iliac crest using an oscillating saw.

FIGURE 123-5 Fibula graft harvesting. The midportion of the fibula is removed with an oscillating saw.

Care must be exercised when harvesting a long segment of fibula to avoid proximal extension of the graft to the region of the neck of the fibula, where the common peroneal nerve is in jeopardy of injury. Injury to this nerve may result in pain and weakness in the foot and ankle.²¹

The advantage of a vascularized fibular graft is its more rapid incorporation. The technique of harvest and vascular anastomosis is technically demanding, and is no longer commonly employed in routine spine surgery.

Tibia

The subcutaneous ventromedial aspect of the tibia historically has been a site used occasionally for bone grafting. Currently, it is rarely used, because other, more suitable, autogenous or allograft alternatives exist. The potential morbidity associated with tibial grafts is significant, with tibial fracture being its main disadvantage. The tibia must be protected for several months to prevent fracture.²²

Rib

Rib can be easily harvested, especially during thoracic spine operations. It is, however, a weak, poorly vascularized graft. Biomechanically it is inferior to the fibula. If taken with its artery, it is suitable for use as a vascularized strut graft.²³^–²⁵ It is used almost exclusively with thoracic or thoracolumbar fusions.

Complications of Graft Harvesting

Graft harvesting complications are common, and pain from a bone graft harvest site sometimes is more severe than the pain from the actual surgical procedure.^18,²⁶^–³³ Although the complications usually are minor, a review of 1244 cases from multiple series demonstrated that their occurrence is about 20%, whereas only a 0.2% complication rate was reported at the neck incision site.³⁴

Chronic Pain

Graft donor site pain is nearly universal in the early postoperative period ¹³ but may be persistent in up to one third of patients^35,³⁶ and may continue throughout the first 3 months postoperatively in up to 15% of patients.²⁹ One study reported that donor site pain was present for more than 10 years following surgery in more than one third of patients.³⁶ The reason for this chronic pain is not well understood, but it often is associated with the patient’s overall pain syndrome. In addition, there are specific reasons for chronic donor site pain, including sacroiliac joint disruption, hernia through the graft site, fracture at the graft donor site (predominantly on the ventral ilium), and heterotopic bone formation.³⁷

The magnitude of acute pain depends to a large extent on the size of the graft and the breadth and depth of the donor site wound. Therefore, one should consider harvesting unicortical or small tricortical grafts to minimize soft tissue injury. In addition, cancellous graft can be harvested from the dorsal iliac crest by removing the cap of the iliac crest and harvesting the cancellous bone from between the two tables of the ilium without disrupting either its inner or outer wall. In addition, the cap can be replaced to minimize the defect created to obtain the cancellous bone.

Nerve Injury and Pain

Incisions for bone grafts may injure nerves or may cause entrapment of nerves due to scar formation (see Fig. 123-2A).

FIGURE 123-2 Possible nerve injury sites during dorsal (A) and ventral (B) iliac crest bone graft harvesting.

The lateral femoral cutaneous nerve can be injured during ventral iliac crest harvest procedures, especially if the incision is very close to the ASIS.³⁸ Injury to this nerve can result in meralgia paresthetica and has been reported in 1% to 14% of cases.^32,^37,³⁸ It is characterized by numbness or dysesthesia on the ventrolateral thigh.^30,³² The nerve usually passes beneath the inguinal ligament, approximately 1 cm medial to the ASIS. However, in about 10% of cases it may pass above the inguinal ligament and just lateral to the ASIS.³⁹

To avoid this complication, the incision must be kept at least 2 cm dorsal to the ASIS.³⁸

The superior cluneal nerves are the most commonly injured cutaneous nerves following dorsal iliac crest bone grafting.⁴⁰ They arise from the superficial fascia, 6 to 8 cm lateral to the posterior superior iliac spine (PSIS). Injury to these nerves during dorsal iliac crest graft harvesting may cause analgesia over the buttock or painful neuromas. A hockey-stick or longitudinal incision may be helpful for avoiding this problem. Exposing the iliac crest under the deep fascia also may avoid injuring these nerves. Another technique recommended to avoid their injury is to make a separate vertical skin incision medial to the cluneal nerves.⁴¹

Other cutaneous nerves, such as the ilioinguinal, iliohypogastric, genitofemoral, superior gluteal, and femoral nerves may, rarely, be injured.³⁷

Vascular and Other Visceral Organ Injuries

The superior gluteal artery can be injured during dorsal iliac crest graft harvesting.^42,⁴³ This can cause severe hemorrhage. It lies between the gluteus medius and minimus muscles, and is avoided by careful subperisoteal dissection. If the artery is transected, it may retract into the pelvis, and controlling the bleeding may be difficult. Dissection of the gluteal muscles from the pelvis distally may be necessary.⁴³ In addition to vascular structures, the ureter and other visceral organs also may be injured.⁴⁴

Hematoma Formation

Hematoma formation has been reported in 9% of iliac crest graft cases.³⁵ The bleeding typically comes from the adjacent muscles and bone surfaces and can, therefore, be minimized by the use of bone wax prior to closure. For larger bone graft defects, use of a drain may be helpful.

Pelvic Fracture or Instability

Pelvic fracture or instability is rare, but may occur during either ventral or dorsal iliac graft harvesting procedures. Bone graft harvest performed too close to the ASIS can result in avulsion of the ASIS. Staying 2 cm behind the ASIS during bone resection will reduce the likelihood of this occurrence and also will reduce the risk of injury to the lateral femoral cutaneous nerve.⁴⁵ In addition, aggressive dorsal iliac crest grafting can result in injury to the sacroiliac joint or can produce a stress fracture to the ilium. Either of these can result in instability and can cause severe pain with ambulation.

Local Infection

Infections from the bone graft site are uncommon and have been reported in fewer than 1% of cases.²⁹ The risk of dorsal iliac graft site infection can be minimized by taking the bone graft through a separate facial incision, rather than from the laminectomy/fusion incision. A separate deep facial incision reduces the risk of cross-contamination between the laminectomy/fusion site and the bone graft site.

Herniation through the Graft Site

Herniation of the abdominal contents following full-thickness iliac crest grafting has been reported but is rare.^46,⁴⁷

Cosmetic Deformity

Cosmetic deformity may occur with ventral iliac crest full-thickness tricortical graft harvests. Several techniques, such as longitudinal crest splitting,⁴⁸ use of a subcrestal window technique,²⁹ and reconstruction with synthetic material⁴⁹ may diminish its incidence.

Where and When to Use a Bone Graft

Ventral Cervical Operations

Discectomy

Following cervical discectomy, the depth of the disc space is measured, and the graft is sized accordingly. The cartilaginous surfaces of the end plates are prepared, but the underlying cortical bone is preserved to minimize the risk of graft subsidence. A rasp or high-speed bur is used to create two parallel surfaces to optimize contact between the end plates and graft surface. A tricortical iliac crest graft of the appropriate size is harvested.

To facilitate insertion of the graft, distraction of the vertebral bodies is performed, either by cervical traction or by distracting across pins placed in the adjacent vertebral bodies (Caspar Cervical Distractor; Aesculap, Inc., Center Valley, PA).⁵⁰ Care must be exercised in gently tapping the bone into the distracted disc space to minimize risk of neural injury and to reduce the likelihood of cracking the graft. If ventral plating is not used, the graft should be recessed approximately 1 to 2 mm to lock the graft in the interspace.

After graft insertion, the cervical traction or distraction is released. The use of a plate is optional, although ventral plating currently is used in most single-level and nearly all multilevel ventral fusions. The advantage of the plate is that it facilitates fusion, reduces the likelihood of graft extrusion, and shortens the length of postoperative immobilization in a cervical orthosis.

Iliac crest grafts can be used in a three- or two-cortex construct. Three well-known graft configurations are used for ventral cervical spine surgery (see Fig. 123-1):

1. The Smith-Robinson–type graft is a tricortical horseshoe-type graft. Smith and Robinson have described the harvesting of the graft from the ventral iliac crest at a depth of approximately 1 cm and a width of 5 to 6 mm.⁵¹ This graft is inserted into the intervertebral space, so that its cancellous portion is directed dorsally and the cortical surface ventrally. To prevent collapse, extrusion, or nonunion, Bloom and Raney have modified the graft position, and the cortical portion is inserted directly dorsally.⁵²

2. The Cloward-type graft is a dowel graft. After harvesting the graft from iliac crest with a cylindrical bur, it is impacted in the intended graft site with a 1-mm, narrower bur. A special instrument set is used for graft harvesting, graft bed preparation, and graft impaction. If a Cloward type of graft is used in multilevel fusions, avascular necrosis of the vertebra may be encountered as a complication ⁵³ if two adjacent levels are fused. The use of allograft bone rather than autograft is more common currently.⁴ For multilevel fusions, a Smith-Robinson type of graft may be inserted at one disc level with a dowel graft at the adjacent space. Other alternatives for multilevel fusions include multilevel tricortical grafts (i.e., the Smith-Robinson technique) or cervical corpectomy.

3. The Bailey-Badgley–type graft is an iliac crest strut graft in which a trough is prepared in the ventral aspect of the vertebral body at a limited depth (0.5 cm).¹⁹ The trough is cut to the full vertical height of the vertebra, and the discs are cleaned with a rongeur to a depth of approximately 1.2 cm.^19,³⁴ A cortical cancellous iliac graft is then placed into the trough.

The strongest constructs are provided by the Smith-Robinson type of graft.⁵⁴ Tricortical grafts are stronger than bicortical grafts,⁵⁵ although graft breakage is a relatively uncommon complication. The optimal autologous graft has a substantial cancellous surface to facilitate bone incorporation.

Corpectomy

Reconstruction following cervical corpectomy typically involves use of either an iliac crest autograft or a fibular allograft. Iliac crest graft can be used for shorter segmental defects, but will not withstand axial stress following reconstruction for multilevel corpectomies.⁵ In addition, the curved nature of the iliac crest sometimes makes it difficult to insert the graft when reconstructing a multilevel defect. The use of iliac crest autograft to reconstruct a multilevel corpectomy also leaves a significant defect in the iliac crest. A fibular allograft is used, therefore, for more than a two-level corpectomy and usually is the preferred choice for even a single-level corpectomy. We prefer to pack the medullary canal of the fibula with autogenous bone from the corpectomy to increase the healing potential of the fibular allograft.

Several methods are available for inserting a fibular graft into a corpectomy site (Fig. 123-6). In general, it is preferable to preserve the cortical end plates to minimize the risk of graft subsidence into the vertebral bodies. In the “keystone method” (see Fig. 123-6C) a defect is made in the midportion of both the rostral and caudal vertebral end plates.^5,⁵⁶ In the “dovetail method” (see Fig. 123-6B) the bone is keyed into the ventral cortex of the vertebral body,⁵⁷ and the graft is inserted first into the rostral end, and then gently tapped into the caudal vertebra. In this instance, manual or skeletal traction may be helpful.⁵⁸ The iliac crest graft also can be tailored in a T-shaped form in which the cortical portion of the graft is faced dorsally (see Fig. 123-6A).

FIGURE 123-6 Cervical graft insertion after central corpectomy. A, T-shaped iliac crest graft. B, Notched fibula graft (dovetail method). C, Straight fibula graft (“keystone” method). The advantage of the grafts shown in A and B is that the bone graft sits on cortical bone of the end plates, thereby reducing the likelihood of graft subsidence. In C, the graft is countersunk through the cortical end plates into the vertebral body, thereby increasing the likelihood of graft subsidence.

(Redrawn with permission from Whitecloud TS: Cervical spondylosis: the anterior approach. In Frymoyer JW, editor: The adult spine: principles and practice, New York, 1991, Raven, pp 1165-1185.)