Calvarial Bone Grafts

INTRODUCTION

Calvarial bone grafting (CBG) is a well-established technique in the reconstruction of the skull base, midface, orbital, and nasal defects. The treatment of bone defects in a pediatric population, while uncommon, has had noteworthy outcomes as well. However, with continued evolution of excellent alloplastic implants, including sophisticated computer-generated implants for the upper craniomaxillofacial skeleton, the frequency of autogenous reconstruction has been proportionately reduced. The modern developments of alloplastic implants have also highlighted the benefits of autogenous materials in the reconstruction of the facial skeleton. Among the various choices available to surgeons, CBG remains an important and viable resource in facial plastic and reconstructive surgery.

Bone grafts have been an important component of craniomaxillofacial reconstruction for many decades with grafting from various donor sites including calvarium, iliac crest, and rib. Each donor material has been popular at various points in time. The advantages of calvarial bone over other donor sites include direct donor site proximity to the reconstruction site, minimal donor site morbidity, and the observation that there may be less resorption of calvarial bone in comparison to iliac, iliac crest, and rib bone. Conversely, the disadvantage of calvarial bone is the relative lack of cancellous bone and the inability to bend calvarium except in the pediatric population. In this chapter, I will focus on the technique of harvesting calvarial bone grafts and then provide several examples of how such bone grafts can be used.

HISTORY

Each patient should undergo a complete systems-based examination including cardiac, pulmonary, and neurologic. A complete history of all medical conditions, surgeries, medications, and allergies is required. A focused history with regard to the surgical site of interest is performed in a systematic fashion with regard to bone loss, soft tissue compromise, and impaired function. It is particularly important to assess whether the patient has a history of neurologic surgery, scalp surgery, diffuse scalp burns, or radiation therapy to the head. History of head trauma should also be noted. Be sure to note the patient’s dominant side, as it is often recommended to take a bone graft from the skull over the nondominant hemisphere. All of the patient’s concerns are addressed as well as his or her expectations of surgery.

PHYSICAL EXAMINATION

A complete head and neck examination is performed as many patients have sustained complex injuries. Evaluation and documentation of V1 to V3 and VII is required to consider any deficiencies prior to surgery.
A focus on the size and character of the defect to be reconstructed is documented photographically and in the written record. A careful examination of the scalp is also important to assess for any previous scars, surgical interventions, burns, radiation effects, hair loss, or soft tissue compromise.

INDICATIONS

Indications for calvarial bone grafts include reconstructive cases in which osseous grafting material is essential for recontouring or repair. Anterior skull base trauma, nasal trauma, and defects secondary to extirpative oncologic procedures are several examples. CBG is particularly useful for reconstruction in severe cases of nasal bone and cartilaginous comminution. CBG can also be used in select cases for reconstruction of upper midfacial defects in conjunction with alloplastic material and vascularized tissue. The use of CBG to reconstruct the atrophic maxilla and mandible for facilitating dental implants has also been described.

PREOPERATIVE PLANNING

Depending on the indication for CBG, consideration of the amount of bone that is required may dictate the type of bone graft that is harvested. In particular, if a large solid piece of bone is required as opposed to smaller strips of bone, it might be advantageous to take advantage of a craniotomy bone segment and split this bone to obtain a larger sheet of calvarial grafting material. The most common situation in which this might exist is for reconstruction of the skull base in either trauma or anterior skull base reconstruction after tumor extirpation. Conversely, for reconstruction of the maxilla and periorbital regions, which are the most common indications for the requirement of grafting material, outer table bone grafts are usually adequate.

Consideration of the precise location of the skull where the grafts are harvested is important. One needs to be aware of the differences in the thickness of the skull and the presence of developmental sutures at which point there is fusion of the inner and outer cortex. The two principal developmental sutures of concern are the coronal suture between the frontal and parietal bones as well as the sagittal suture in the midline. The latter is particularly important due to the presence of the sagittal sinus deep to this area. Accordingly, the midline should be avoided. Other issues include the curvature of the bone as in some cases curvature is desired (e.g., in the zygoma, orbit, or frontal bone), whereas in other areas, a flatter bone graft may be desired (e.g., the nasal dorsum). The most common site for outer calvarial graft harvesting is the parietal bone lateral to the sagittal developmental suture, posterior to the coronal developmental suture, and posterior to the temporal line (or attachment of the temporalis muscle).

SURGICAL TECHNIQUE

Harvesting of the Graft

While exposure to this region is often facilitated by the existence of a coronal approach that may have already been performed, occasionally, direct incisions are required over the parietal region for the sole purpose of harvest for a calvarial bone graft. I favor a geometric incision to best camouflage such incisions just as I favor such geometric designs for coronal incisions. The periosteum over the desired parietal harvest area is exposed and adequate retraction is required. While numerous techniques exist for harvesting the outer table graft, I use the combination of drilling the periphery of the desired graft shape with a cutting burr followed by the use of either a sharp curved osteotome or right-angled sagittal saw to enter the diploic space between the outer and inner cortex as well as to separate the outer table graft from the inner table within this space (Figs. 45.1, 45.2, 45.3 and 45.4). Keys to successful harvest of the graft include precise identification of the diploic space with the cutting burr. This is important because on rare occasions, there is very limited or no diploic space and the surgeon must be aware that, in such cases, in an effort to identify this bleeding diploic space, one may actually encounter the dura. Accordingly, it is very important to be observant of the appearance of dura during this process. A similar key is that while cutting horizontally through a diploic space with either an osteotome or saw, the dura may again be encountered. I always make the assumption that when in doubt, the dura may

be exposed and I remain diligent in elevating with either the saw or osteotome in a plane and direction that is truly parallel to the inner cortex and the underlying dura. This is facilitated by the use of very sharp and curved osteotomes as well as efficient right-angled saws. It should be noted that during the harvest of outer table grafts, diploic veins may be encountered and significant bleeding may occur. This can be alarming at times and may be requiring packing of the wound with resorbable hemostatic materials, but I have found it has never been a major problem.

FIGURE 45.1 Top: Cutting burrs are used to identify the diploic space and outline bone grafts. Bottom: Using an osteotome to develop graft between inner and outer calvarial cortices.

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