Abstract
Posttraumatic secondary reconstruction of the craniofacial skeleton is challenging. Many times the surgeon is faced with malunion and asymmetries that can be very difficult to correct using osteotomies and repositioning of bony segments. The chapter discusses the use of custom craniofacial implants as an alternative to posttraumatic secondary reconstruction.
Keywords
custom implants, secondary reconstruction, CAD/CAM, craniofacial trauma
Background
Craniofacial injuries are ideally treated in the acute setting. In certain circumstances, it is appropriate for treatment to be delayed. Resuscitation and treatment of life-threatening injuries in multiple trauma victims may preclude facial trauma treatment. For example, the treatment for patients with intracranial hemorrhage involves removal of portions of the skull for surgical access. The resulting brain and dural injuries are addressed acutely, but the bony reduction is sometimes suboptimal or delayed, resulting in deformity. By necessity, resultant deformities are generally treated in a delayed fashion.
For delayed reconstructions, computer-aided design (CAD) and computer-aided manufacturing (CAM) implants can be a valuable adjunct. They can provide near anatomical replication of the cranial vault and facial skeleton. If one side of the face or vault is uninjured it can be “mirrored” to reconstruct the injured side. If that is not possible, the surgeon can design implants appropriate for the situation.
The use of alloplastic materials in the reconstruction of craniofacial deformities has both inherent advantages and limitations. The use of alloplasts avoids the morbidity of autogenous bone harvest. The use of CAD/CAM implants avoids intraoperative carpentry and therefore significantly reduces operative time. However, because alloplastic materials do not integrate into the bony skeleton, they are only used as non-load-bearing onlays except for the cranial vault, where they replace missing skull to restore contour and protect the brain.
Background
Craniofacial injuries are ideally treated in the acute setting. In certain circumstances, it is appropriate for treatment to be delayed. Resuscitation and treatment of life-threatening injuries in multiple trauma victims may preclude facial trauma treatment. For example, the treatment for patients with intracranial hemorrhage involves removal of portions of the skull for surgical access. The resulting brain and dural injuries are addressed acutely, but the bony reduction is sometimes suboptimal or delayed, resulting in deformity. By necessity, resultant deformities are generally treated in a delayed fashion.
For delayed reconstructions, computer-aided design (CAD) and computer-aided manufacturing (CAM) implants can be a valuable adjunct. They can provide near anatomical replication of the cranial vault and facial skeleton. If one side of the face or vault is uninjured it can be “mirrored” to reconstruct the injured side. If that is not possible, the surgeon can design implants appropriate for the situation.
The use of alloplastic materials in the reconstruction of craniofacial deformities has both inherent advantages and limitations. The use of alloplasts avoids the morbidity of autogenous bone harvest. The use of CAD/CAM implants avoids intraoperative carpentry and therefore significantly reduces operative time. However, because alloplastic materials do not integrate into the bony skeleton, they are only used as non-load-bearing onlays except for the cranial vault, where they replace missing skull to restore contour and protect the brain.
Clinical Presentation
Calvarial Deformity
These patients are typically ones who have undergone significant head trauma, often resulting in intracranial hemorrhage and emergent craniectomy. After an extended hospital course, they are often discharged to a rehabilitation facility and may regain some or most of their neurological function depending on their age. By the time they present for secondary reconstruction, they may have a noticeable depression at their craniectomy site. Some patients also undergo neurological deterioration that is related to the “Syndrome of the Trephined.” This syndrome, also known as “sunken skin flap syndrome,” is a poorly understood entity. It is characterized largely by neurological deficits after craniectomy and subsequent improvement after cranioplasty. Although the spectrum of deficits varies widely, most of the reported cases involve motor weakness, cognitive deficits, language deficits, or a combination of the three. In a recent literature review, the only consistent characteristic of those affected by the syndrome of the trephined was that greater than 90% exhibited a visibly sunken skin flap. Most patients have an improvement in their deficits within days after cranioplasty. Unfortunately, the pathophysiology is unknown. One of the leading theories is that the change in pressure seen by the brain without the structural support of the calvarium changes brain physiology, thereby causing neurological deficits.
Midface Deformity
These patients have typically undergone blunt facial trauma with subsequent facial fractures. Depending on the trauma burden, their facial fractures may or may not be treated operatively in the acute setting. For example, there may be a displaced zygoma fracture in a patient with concomitant head trauma and extremely labile vital signs which preclude operative repair. If the patient does not become stable within the first 4 weeks, repair may be deferred entirely. Alternatively, if a patient does have operative repair of facial fractures but the reduction is not perfect, they may still end up with facial asymmetries. In either case, patients present with facial deformities in a delayed fashion and have facial asymmetries such as brow malposition, lack of malar projection, enophthalmos, and/or lack of infraorbital rim definition.
Mandible Deformity
These patients have typically sustained a Le Fort pattern midface fracture as well as a mandible fracture. Operative treatment is aimed at restoring normal occlusion and involves open reduction, internal fixation, and sometimes maxillomandibular fixation. These are severe injury patterns and perfect bony reduction and facial symmetry are sometimes compromised in favor of achieving perfect occlusion. Depending on the location of the mandibular fractures, various parts of the mandible may be displaced to create a mandibular asymmetry, deformity, or bony step-offs that can be bothersome and warrant reconstruction.
Radiological Evaluation
Dedicated CT scans of the face with thin cuts are the optimal imaging modality when evaluating a patient with posttraumatic deformity. These can be reformatted to create 3-dimensional reconstructions that can aid in creating molds for surgical planning or virtually designing custom implants.
CAD/CAM Facial Implants
Computerized tomographic (CT) scans are fundamental to CAD/CAM facial implant technology. Implementation of CAD/CAM implants is a multi-step process. It requires a CT scan with sufficient data to reconstruct an accurate 3D image. The resultant image can be used to design an implant virtually or to create a model of the 3D image on which an implant design is fashioned. Implants are subsequently manufactured using milling or 3D printing techniques.
This allows for millimeter precise assessment of the contour deficit: symmetry, height, width, projection, and its relationship to the entire facial skeleton. Applying knowledge of the normal skeletal relationship and aesthetic criteria, virtual surgery can be performed to design and to correct the posttraumatic deformities.
Implant Materials
Various materials are available for CAD/CAM implants. These include silicone rubber (Implantech, Ventura, CA), polyetheretherketone (PEEK, Johnson & Johnson), porous polyethylene (MEDPOR, Stryker Corporation, Kalamazoo, MI; OMNIPORE, Matrix Surgical, Atlanta, GA), and a porous composite material (HTR, Biomet, Jacksonville, FLA).
Each material has its advantages and disadvantages.
Silicone
Solid silicone or the silicone rubber used for facial implants is a vulcanized form of polysiloxane. Solid silicone has the following advantages: it can be sterilized by steam or irradiation, it can be carved with either a scissors or scalpel, and it can be stabilized with a screw or a suture. Because it is smooth, it can be removed quite easily. Disadvantages include the tendency to cause resorption of bone underlying it, particularly when used to augment the chin, the potential to migrate if not fixed, and the potential for its fibrous capsule to be visible when placed under a thin soft tissue cover.
Custom PEEK (Polyetheretherketone)
PEEK implants are extremely hard and inflexible. PEEK is most often used for cranial vault reconstruction where long incisions are possible allowing excellent exposure and access for implant placement. The more limited exposure for midface and mandible onlay reconstruction often requires PEEK implants to be segmented to allow implant placement. This makes reconstruction more tedious and time-consuming. Its rigidity precludes any imperfections in design to be accommodated by intraoperative manipulations of the implant.
Porous Polyethylene
Polyethylene is a simple carbon chain of ethylene monomer. The high density, porous variety – Medpor (Porex, Fairborn, GA) and Omnipore (Matrix Surgical, Atlanta, GA) – is used for facial implants because of its higher tensile strength. Its firm consistency resists material compression while still permitting some flexibility. It has an intramaterial porosity between 125 and 250 µm, which allows some fibrous ingrowth. The porosity of Medpor® and Omnipore has both advantages and disadvantages. The advantages of porous polyethylene include its tendency to allow soft tissue ingrowth, thereby lessening its tendency to migrate and to erode underlying bone. Its firm consistency allows it to be easily fixed with screws and contoured with a scalpel or power equipment without fragmenting. However, its porosity causes soft tissue to adhere to it, making placement more difficult and requiring a larger pocket to be made than with smoother implants. The soft tissue ingrowth also makes implant removal more difficult than with smooth surface implants. This material is the implant material of choice for the senior author [M.J.Y.].
HTR
HTR derives its name from the acronym for “hard tissue replacement” (Biomet, Jacksonville, FL). It is a porous composite of polymethylmethacrylate and polyhydroxyethyl methacrylate which allows some soft tissue ingrowth. A calcium hydroxide coating imparts a negative surface charge to encourage bony ingrowth and deter adhesion of bacteria to the implant. It is inflexible and relatively brittle, making implant placement through limited access and fixation difficult.
Indications
CAD/CAM implants have utility for calvarial, midface, and mandible reconstruction. CAD/CAM implants are most often used to reconstruct areas of skull loss. Defects may result from direct trauma, surgical decompression of intracranial pressure, or infection after craniotomy. Cranioplasty protects the brain from environmental trauma while restoring preinjury appearance facilitating integration into society. Cranioplasty often improves neurological function in those afflicted by the “Syndrome of the Trephined.” CAD/CAM techniques using mirroring techniques of contralateral intact anatomy allow near anatomic preinjury contour. In addition to the aesthetic benefit, a preformed implant significantly decreases operative time.
Requisites for successful cranioplasty include avoiding implant contamination by communication with the frontal sinuses, ethmoid, or mastoid, elimination of dead space for fluid communication between the implant and the brain, appropriate fixation, and adequate soft tissue coverage. Fig. 3.12.1 shows a failed alloplastic cranial implant reconstruction resulting from frontal sinus communication and dead space between the implant and brain. Reconstruction required removal of the contaminated implant, isolation of the frontal sinus, obliteration of the dead space with a free tissue transfer, and tissue expander scalp reconstruction to assure adequate soft tissue coverage of the CAD/CAM porous polyethylene implant.
