Computer-assisted planning congenital differences and anomalies: Craniopagus twin separation

Introduction

Craniofacial surgery commonly involves carefully orchestrated surgery between plastic surgeons, neurosurgeons and other specialists. In some cases, the neurosurgeons provide a craniotomy for access to the midface. For example, correction of orbital hypertelorism or monobloc frontofacial advancement. In other cases, neurosurgeons are performing craniectomies that the plastic surgeons will remodel and reassemble. Commonly, plastic surgeons and neurosurgeons will work side by side to repair fractures involving the skull base, frontal sinus, and the orbital roof. A common theme is that one team leads and the other follows. It is always important for each discipline to keep in mind that the way that they proceed will have an impact on the other discipline. This requires advanced experience and exemplifies what we call interdisciplinary care as opposed the multidisciplinary care, or simply working together. This is particularly important when we discuss the care of craniopagus twins.

Amongst the congenital craniofacial disorders that are faced by such a team, craniopagus twins are remarkably rare. The risks are doubled because two children are being operated on at once and interventions on one are likely to impact the other until they are effectively separated. It is crucial to recognize that in such conjoined twinning the anatomy is quite variable, beyond the anomalies of other more common craniofacial disorders.

Textbooks, surgical atlases, and anatomical dissection, whether on cadavers or done virtually, are necessary to teach surgeons at every stage of their training and career. Imaging the craniofacial skeleton has allowed planning since radiographs were first available. Advances in imaging have allowed surgeons to understand the pertinent anatomy of their specific patients in greater detail over the years. In the early days two-dimensional (2D) radiographs allowed diagnosis and surgical planning that enabled our mentors and those before to achieve remarkable success, although this seems rather primitive today. Imaging technology was translated to the operating room indirectly. In other words, the images were transferred to the patient through the additional knowledge, experience, and imagination of the surgeon. This continued to be the case with the age of computer tomography. With the further development of software that was able to reconstruct a three-dimensional (3D) image from the computed tomography (CT) scan, teaching, diagnosis, and treatment were elevated to a new level. Although the quality of imaging was clear enough for a layperson to appreciate, the transfer of this information was still a leap to the surgical field.

Historically, the information provided in radiographic imaging provide the surgical team information about the normal and abnormal anatomy of the craniopagus. This allowed the team to assess the feasibility of separation. The individual steps required to actually separate them remained in the surgeon’s imagination. The advent of virtual planning revolutionized the way in which the anatomy could be visualized, manipulated and communicated. This can be seen in many ways throughout the chapters of this textbook. This allows the key decisions to be made before going to the operating room. This provides the team the luxury of time to assess options without wasting time under anesthesia. Additionally, the virtual planning provides the family images that can be reviewed with the surgical team prior to surgery. This valuable addition to informed consent is readily apparent. After being purely visual, imaging became tangible when it was possible to manufacture stereolithographic models through 3D printing. This allowed surgeons and their students to perform osteotomies on the stereolithographic models, a step up from practicing these procedures on models of a normal craniofacial skeleton. The capability for a surgeon to hold the anomalous anatomy in their hands is very powerful indeed. A 3D model is indispensable when demonstrating complex surgery to patients and other surgical team members. These stereolithographic models can be “printed” from different materials, including those that can be drawn on with pencils. This allows skin flaps and osteotomies to be planned while sitting at a desk, at any time, while options are considered, demonstrated, and discussed with other team members. Planning for complex cases, such as craniopagus separation surgery, may take several weeks or months.

As well as being used to create stereolithographic models, 3D information can also be transferred to a workstation in the operating room. Sterilized wands can be used in the operative field showing the surgeon immediately where the tip of the wand is in the CT and/or magnetic resonance imaging (MRI), and most powerfully, what adjacent structures lie close in the patient’s anatomy but are not yet visible to the naked, even magnified eye.

Additional benefits include hardware, such as plates and mesh, that can be contoured directly on the model and then sterilized for surgery. This saves time during surgery and may also confirm correct placement according to the fit of the contoured hardware. Ultimately this technology allows custom implants and hardware to be produced with precision, also saving time in surgery. Note that despite this, training, skill, and judgment are still required for the safe planning and conduct of these operations. Implant materials continue to be developed and refined, and their compatibility and durability must be taken into account. Experience, training, and skill remain quintessentially important.

Classification system

Craniopagus twinning presents a unique challenge because complex anatomy required for survival and quality of life is shared between the twins, and successful separation depends on correctly partitioning these structures, making surgical planning critical in such cases. Craniopagus twins are classified into a four-category system that assesses the twins for a vertical or angular configuration, and the presence or absence of significantly shared dural venous sinuses. In the angular subtype, the cerebral hemispheres are arranged horizontally within a shared cranium, whereas in the vertical subtype, a longitudinal arrangement with one continuous cranium housing the cerebral hemispheres is present. The surgical approach to separation of each type will be slightly different; however, this framework allows for prioritization and planning of early surgical procedures that will occur prior to final separation (see Figs. 17.1 and 17.2 ).