Bone Graft Harvesting




Summary of Key Points





  • Autogenous bone graft is still the gold standard graft substrate for achieving fusion.



  • It is therefore imperative that spine surgeons have an intimate knowledge of the anatomy of various graft donor sites and be familiar with techniques to harvest bone graft.



  • The iliac crest is the most common and versatile source of autogenous bone graft and can be accessed either anteriorly or posteriorly. It can provide both cancellous bone graft as well as structural graft. There are potential complications associated with harvesting iliac crest graft and other grafts, and it is imperative for the surgeon to understand both the anatomy and the potential risks.



Spine surgeries typically involve neural decompression, fusion, or both. Fusion is performed to stabilize a spinal 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 spine surgery, either as a result of inadvertent destabilization of operated segments or because of adjacent level degeneration and instability following prior fusion. When instability is present, either with or without pain, fusion—using bone grafting and possibly instrumentation—often is employed. The key to fusion is achieving solid bony arthrodesis, as instrumentation alone 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 will be 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 autogenous bone has its particular advantages: cortical bone graft provides good structural support, whereas 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 cortical bone and is therefore less ideally suited for structural support.


Autograft


Autograft is commonly used in spine surgery and remains 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. Revascularization of cancellous bone is completed within several weeks, but it takes several months, or longer, for cortical bone. Autografts commonly are used in conjunction with spinal instrumentation but can also be used for dorsal onlay (intertransverse process) fusions without instrumentation. Significant advantages of autologous bone include its ready availability, no risk of disease transmission, and low cost.


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 can be used to augment autograft and are appropriate as the sole graft material in a variety of clinical situations. 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. However, they incorporate relatively slowly, and, if used for multilevel fusions, are associated with a pseudarthrosis rate of 63% to 70%. Fibular allografts are preferred for cervical corpectomy, because of the significant morbidity associated with harvesting of a fibular autograft, including pedal edema, ankle pain, and the risk for peroneal nerve injury.


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, and is generally not recommended.


Xenograft


Xenografts are tissues transplanted from one species to another. Xenografts give less consistent 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.


Bone Graft Substitutes


Nonbiologic materials such as hydroxyapatite, ceramic, and polymethylmethacrylate also have been used, either in conjunction with or in lieu 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 expected survival.




Bone Graft Types


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


Local Bone


Bone that is removed during surgical decompression is commonly used in spinal fusions, particularly posterior lumbar fusions. The laminae consist of both cancellous and cortical bone, which can be morselized manually with a rongeur or can be placed in a bone mill to provide a more consistent substrate for bone grafting. The quality of local bone generally is perceived as being inferior to iliac bone graft because of its cortical content.


Iliac Crest


The most commonly used donor site is the iliac crest. The iliac crest, particularly the posterior iliac crest, is a readily available source of cancellous bone. Its disadvantages include donor site pain, which is a common complaint ; its limited volume, which can be an issue for procedures requiring a significant amount of bone; and its limited utility for procedures requiring a large piece of structural bone for reconstruction.


Ventral Iliac Crest Grafts


Ventral iliac crest grafts commonly are used to provide a tricortical bone graft for various types of ventral cervical fusions ( Fig. 166-1 ). 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 or avulsion fracture of the ASIS. 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 (ACDF), 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. The key to obtaining a good tricortical graft and minimizing harvest site pain and bleeding is to strip the periosteum from both the inner and outer walls of the iliac crest using a Cobb periosteal elevator. An oscillating saw should be used for graft harvest rather than an osteotome to minimize the risk of graft microfracture, which may lead to subsequent 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. The fascia should be closed meticulously to prevent a herniation of the pelvic contents.




Figure 166-1


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.)


Dorsal Iliac Crest Grafts


The dorsal iliac crest is predominantly used to obtain large quantities of dorsal onlay graft material for dorsolateral (intertransverse process) lumbar fusions. More bone is available from the dorsal iliac crest than from the ventral crest. Bone graft may be obtained through the same midline skin incision used to perform the lumbar decompression or through a separate lateral skin incision over the iliac crest. The midline skin incision usually is more commonly 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 superior 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 ( Fig. 166-2 ).




Figure 166-2


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


If corticocancellous graft is needed, strips of bone can be harvested of either unicortical or bicortical thickness ( Figs. 166-3 and 166-4 ). If only cancellous bone is required for the fusion, the superior cap of the ilium can be removed, keeping the inner and outer walls of the ilium intact, and gouges or curettes can be used to scoop out strips of cancellous bone. The advantage of this method of obtaining purely cancellous graft is that the inner and outer layers of the ilium are not disrupted, and subperiosteal stripping is not required, thereby eliminating pain that would emanate from disrupted periosteum. Alternatively, an osteotome can be used to create a subcrestal window in the outer wall of the ilium and curettes used to scoop out cancellous bone (see Fig. 166-4 ), but this method creates a defect in the cortical wall and requires stripping of the periosteum, thereby resulting in more dissection and pain.




Figure 166-3


Dorsal iliac crest graft harvesting.



Figure 166-4


A, Unicortical graft harvesting from the dorsal iliac crest (subcrestal technique) using an osteotome. B, Tricortical graft harvesting from the dorsal iliac crest using an oscillating saw.


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 it is relatively easily harvested. Being cortical bone, however, it has a slower rate of incorporation than cancellous bone and is associated with 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. 166-5 ). During fibula graft harvesting, the peroneal nerve must be protected. The peroneal nerve can generally be palpated as it passes just distal to the fibular head and should be well away from the site of fibular resection, because the fibula is generally harvested at the junction of its middle and distal thirds. 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 (see Fig. 166-5 ). After fibula harvest, a few days of compressive leg wrapping with elevation of the leg will minimize swelling and discomfort.




Figure 166-5


Fibula graft harvesting. The midportion of the fibula is removed with an oscillating saw. Note the proximal location of the common peroneal nerve passing just below the fibular neck.


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.


Vascularized fibular grafts have been utilized in the past. 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. It is rarely, if ever, used currently because 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 after bone harvest to prevent fracture.


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. Its use is now almost exclusively relegated to thoracic or thoracolumbar fusions.




Complications of Bone Graft Harvesting


Graft harvesting complications are common, and pain from a bone graft harvest site sometimes is more debilitating and severe than the pain from the actual surgical procedure. 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 from the neck incision site.


Chronic Pain


Graft donor site pain is nearly universal in the early postoperative period but may be persistent in up to one third of patients and may continue throughout the first 3 months postoperatively in up to 15% of patients. One study reported that donor site pain was present for more than 10 years following surgery in more than one third of patients. 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 biomechanical 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 associated with avulsion of the ASIS), and heterotopic bone formation.


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, if only cancellous bone is required, it 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. 166-2A ).


The lateral femoral cutaneous nerve can be injured during ventral iliac crest harvest procedures, especially if the incision is very close to the ASIS. Injury to this nerve can result in meralgia paresthetica and has been reported in 1% to 14% of cases. It is characterized by numbness or dysesthesia on the ventrolateral thigh. 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. To avoid this complication, the incision must be kept at least 2 cm dorsal to the ASIS.


The superior cluneal nerves are the most commonly injured cutaneous nerves following dorsal iliac crest bone grafting. 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 in avoiding this problem. Exposing the iliac crest under the deep fascia also may avoid injuring these nerves. Another technique recommended to avoid injury to the cluneal nerves is to make a separate vertical, rather than horizontal, skin incision medial to the cluneal nerves.


Injury to the sciatic nerve or superior gluteal nerve may occur during posterior iliac bone grafting if the dissection is too deep. Injury to the superior gluteal nerve can result in ipsilateral hip abductor weakness. Other cutaneous nerves, such as the ilioinguinal, iliohypogastric, genitofemoral, superior gluteal, and femoral nerves may, rarely, be injured.


Vascular and Other Visceral Organ Injuries


The superior gluteal artery can be injured during dorsal iliac crest graft harvesting. This can cause severe hemorrhage. It lies between the gluteus medius and minimus muscles and is avoided by careful subperiosteal 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. In addition to vascular structures, the ureter and other visceral organs also may be injured.


Hematoma Formation


Hematoma formation has been reported in 9% of iliac crest graft cases. The bleeding typically comes from the adjacent muscles and bone surfaces and can, therefore, be minimized by careful surgical technique and the use of bone wax prior to closure. When a bicortical or tricortical graft is taken, care must be exercised to perform a meticulous subperiosteal dissection to minimize muscle bleeding. 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. 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 pelvic 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. 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.


Cosmetic Deformity


Cosmetic deformity may occur with ventral iliac crest full-thickness tricortical graft harvests. Several techniques, such as longitudinal crest splitting, use of a subcrestal window technique, removal and replacement of iliac crest cap following cancellous graft removal, and reconstruction with synthetic material, may diminish its incidence.

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Feb 12, 2019 | Posted by in NEUROSURGERY | Comments Off on Bone Graft Harvesting

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