Vascularized Bone Grafts in Spine Surgery

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Chapter 124 Vascularized Bone Grafts in Spine Surgery

Bone grafting has had an important role in surgery since Barth first introduced bone-grafting techniques in the late 19th century.¹ Bone grafts typically have been used in the treatment of fracture nonunions, arthrodesis of joints, the filling of bone cavities, replacement of bone lost due to infection, trauma, tumor, augmentation of fracture healing, and spinal fusion. The different types of bone grafts used today include autogenous cancellous, nonvascularized autogenous cortical, vascularized autogenous cortical, allogeneic cancellous, allogeneic cortical, allogeneic demineralized bone matrix, and allogeneic inductive proteins.

Structural bone grafts commonly are used in spine surgery to provide stability in an area where a defect has been created. Currently, the gold standard for bone grafts is the autograft, which has the best biologic compatibility and leads to fewer nonunions. The most common complications associated with its use include pain at the donor site and a lack of incorporation of the graft. The advent of the use of vascularized bone grafts has provided the spine surgeon with a potentially powerful tool to use to treat difficult spine problems. This chapter presents a brief discussion of bone grafts and the basic biology of graft incorporation, along with causes of nonunion. The history of vascularized bone grafts is presented, as is a surgical technique for the donor site. The indications in spine surgery will be discussed along with a review of the results of its use in this field.

Bone Grafts in Spine Surgery

Albee first described utilizing a bone graft for spinal fusion in 1911 as a treatment for Pott disease.² Many advances have been made since that time, and fusion is now the standard treatment for a variety of spinal disorders. Achieving a proper fusion involves two key components: (1) preparation of the site to be fused and (2) stimulation of bone formation with the use of a bone graft. The most effective graft material currently available is autologous cancellous bone. This graft has a large surface area that allows for vascularization of the graft and incorporation with the host bone. In cases in which the fusion must span several segments, the amount of autogenous cancellous bone that is available may not be sufficient.

Autologous cortical bone is another commonly used graft in spine surgery. Unfortunately, this graft has fewer osteoblasts that survive and is associated with a slower rate of revascularization. This slower revascularization results in a slower rate of incorporation of the graft, thus limiting its use.³ The advantage of a cortical graft, however, is that it can provide immediate structural support and that the graft is available in larger sizes. Over time, during a process called creeping substitution, the strength of the graft decreases. During this process, the avascular nature of the graft causes resorption by osteoclasts, while new bone is laid down by osteogenic cells originating from the recipient bed rather than the graft, a phenomenon first observed and described by Phemister in 1914.⁴ This is why a cortical graft (such as a strut graft used in the treatment of kyphosis) may take up to 2 years to incorporate completely. During the process of creeping substitution, the bone graft is found to be weakest at 6 months, increasing the risk of fracture at the graft site.⁵ By retaining its vascular supply and viability of the osteocytes, a vascularized bone graft provides a mechanically stronger support than a nonvascularized graft.

Vascularized Bone Grafts

The use of vascularized bone grafts parallels the developments associated with the history of vascular surgery. The beginnings of vascular surgery can be traced back to Carrel’s classic paper published in 1908, “Results of the Transplantation of Blood Vessels, Organs and Limbs,”⁶ in which he describes a technique whereby blood vessels can be anastomosed. Various tools for anastomosing small vessels were designed and tested in the years following that publication. Androsov designed the first vascular stapling machine,⁷ Jacobsen and Suarez demonstrated the utility of the microscope in the operating room,⁸ and Buncke and Schulz improved microsurgical instrumentation and performed much of the early experimental work in the field.⁹ Strauch et al. used a canine model to transpose a rib to the mandible on its internal mammary pedicle in 1971,¹⁰ and in 1973 a free vascularized rib graft was performed in a dog by McCullough and Fredrickson.¹¹ The first free skin flap using microvascular anastomoses was reported by Taylor in 1973,¹² and in 1975, Taylor transferred a fibula to a tibial defect as the first free vascularized bone graft in a human.¹³

The vascularized bone graft traditionally has been used in refractory nonunions or in areas where there is a large segmental defect. The need to use a more structurally supportive bone graft for kyphosis surgery resulted in the use of the first vascularized bone graft in spine surgery. Surgery for severe kyphosis secondary to infection, trauma, or deformity requires spanning multiple levels. When a nonvascular rib or fibula was used to span such a defect, the length of the graft and the slow rate of incorporation resulted in a high rate of nonunion. Bradford encountered fatigue fractures in 4 of 23 patients when a nonvascularized fibula was used for spinal kyphosis surgery.¹⁴ These results encouraged spine surgeons to seek out alternatives to traditional bone grafts. In two separate reports, Rose et al. and Bradford described successful techniques of using a rib graft with a vascular pedicle.^15,¹⁶ Because the cross-sectional area of the rib was too small and could not provide the structural support needed for certain areas of spinal fusion, surgeons began to explore the use of the fibula as a vascularized graft.^17,¹⁸ Currently, the rib, fibula, and iliac crest are used as primary donor sites for a vascularized graft.

Indications and Principles

Based on previous studies, a vascularized bone graft is indicated for use in the following situations:

• Bone graft greater than 5 cm needed ¹⁹

• Strut graft that will be more than 4 cm from the anterior border of the spine and thus more prone to fracture

• A pseudarthrosis after a nonvascularized bone graft

• An area of the spine that will require radiation postoperatively, as in the setting of a malignancy

• Cases of infection where placing instrumentation or avascular bone would propagate the infection

• Surgeries in which fusion is expected to be difficult to achieve, as with operations for neurofibromatosis

Surgical Technique

The three most common locations from which a vascularized bone graft can be harvested are the rib, the iliac crest, and the fibula. In spine surgery, the rib is the easiest location to harvest. Due to its thin cylindrical structure, the rib may not provide the mechanical stability necessary to fill large segmental defects. The fibula is a larger cylindrical structure that has strong mechanical properties. It can be used to span multiple levels. The disadvantage of this graft is the donor site morbidity associated with harvesting the fibula.

Preoperative Planning

Harvesting of a vascularized bone graft requires that the surgeon be well trained in microvascular techniques. Multiple types of anastomosis may be required in vessels that could be potentially scarred or traumatized. Preoperative angiography should be performed to elucidate the vasculature of the donor and recipient sites. It is important to understand that a normal arteriogram may be misleading, as scarred blood vessels may appear normal. The surgeon often will need to make an intraoperative decision regarding the viability of a blood vessel.

Fibula

The fibula is a long bone that is triangular in cross section and has a high cortical-to-cancellous bone ratio. Up to 25 cm of length can be harvested safely for long grafts. The medullary vascular supply to the fibula arises as a branch of the peroneal artery, and it enters the fibula at the junction of the proximal and middle thirds of the bone. The venous system is similar to the arterial system, with drainage occurring through the venae comitantes of the peroneal artery and the medullary sinusoidal system.

The procedure that follows is described by Vail and Urbaniak using an extraperiosteal dissection.²⁰ This procedure also has been described by Gore et al. in a subperiosteal plane.²¹ Vail and Urbaniak report that the extraperiosteal dissection leads to decreased complaints of pain.

To obtain the fibula graft, the patient is placed supine on the operating table. The leg should be prepped from the hip to the toes and a tourniquet applied to the thigh. After the tourniquet has been inflated, the limb is exsanguinated with an elastic bandage.

A straight lateral incision is made directly over the fibula, with further dissection being performed between the posterior and lateral compartments of the calf. The peroneal muscle is separated from the anterior aspect of the fibula down to the intramuscular septum. Elevating the muscles of the anterior compartment reveals the intraosseous membrane. The muscles of the posterior compartment also are dissected extraperiosteally. The superficial peroneal nerve and a portion of the peroneal artery deep to the fibula are protected, and the fibula is divided with a Gigli saw.

The flexor hallucis longus, the posterior tibial muscle, and the remaining muscles of the anterior compartment are separated from the fibula extraperiosteally. The fibula is elevated from the wound from caudal to rostral, while the pedicle remains intact. Vascular branches entering the soleus muscle are clipped and divided. The peroneal vessels are dissected proximally to their bifurcation from the tibial vessels. The fibular diaphyseal segment, along with approximately 4 to 6 cm of the peroneal vessels, are ligated and dissected from the wound.

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