Abstract
Plastic Surgery restores unique human qualities such as appearance, speech (palate), hands, to improve interaction with others and quality of life. Three-dimensional printing technology can be applied to Plastic Surgery craniomaxillofacial operations to change the bony skeleton of the skull, face, and jaws. Three-dimensional printing for patient-specific applications have four types: Type I contour models, Type II guides, Type III splints, Type IV implants. Plastic Surgery innovation in 3D printing clinical applications are described here and https://www.slucare.edu/newsroom/kmov-science-of-healing-faces-of-childhood.php.
Plastic Surgery and 3D Printing
The word “plastic” is derived from the Greek word for “shape”, and consequently Plastic Surgery is “shape surgery” that improves quality of life. A principal example is in craniofacial anomalies, as one of the main functions of the face is to appear normal, so that an individual can interact with others. Similarly, although speech is not vital, in a patient with a cleft palate, speech can sound hypernasal and abnormal, which is stigmatizing and jeopardizes normal social interactions. Plastic Surgery can be understood as the field that deals with the functions that make us human: how we look, how we speak (cleft palate), and how we use our hands – these are the very features that allow us to communicate, a unique human function.2 This is even more critical in Pediatric Plastic Surgery which deals with these unique human functions in children, as they are still developing socially and therefore the ability to communicate is essential. Plastic Surgery craniofacial surgery includes both the soft tissues of the ears, eyelids, nose, tongue, lips, and face – and also the bones of the craniomaxillofacial skeleton: skull, forehead, orbits, cheeks, maxilla, and mandible. These craniomaxillofacial operations are frequently practiced in the pediatric population, whose compact surgical anatomy can have higher risks that can benefit from the precision of medical three-dimensional printing. Plastic Surgery is an ideal surgical specialty to help define the role of 3D printing in surgery.
Three-dimensional printing (3DP) has increasing medical applications in education and planning, but its specific role in surgery has remained challenging to define. Our group recently published a systematic review in the journal Plastic and Reconstructive Surgery, which helps solve this problem through a taxonomic analysis of patient-specific 3D printed models that were used in craniomaxillofacial procedures.1 This systematic review analysis was unique in that it only included relevant patient-specific intraoperative applications of 3D-printed parts. Therefore, the results have clear translatable clinical applications to patient-specific surgeries.
The insights from this new classification of 3DP objects for surgery, have broad implications in all surgical and procedural methodologies involving 3DP, or additive manufacturing. This paper focuses on understanding the identified four types of 3D-printed objects that can be translated for surgical use, with an emphasis on Plastic Surgery craniofacial and maxillofacial operations, and its high clinical relevance to all potential procedures utilizing 3DP assistance.
Systematic Analysis Revealing Four 3DP Types
The investigation titled “A New Classification of Three-Dimensional Printing Technologies: Systematic Review of Three-Dimensional Printing for Patient-Specific Craniomaxillofacial Surgery”1 systematically analyzed the current state of research by methodically identifying and extracting data from peer-reviewed journal articles for patient-specific 3DP intraoperative use, resulting in 314 unique papers. To keep the search exclusively focused on clinically relevant direct patient care applications of 3DP in surgery, we used strict inclusion and exclusion criteria that specifically required a physical patient-specific 3D-printed object that was actually used for direct surgical application. Generic models not specific to a patient’s anatomy, or not used for surgery, were excluded.
We then concentrated on the specific clinical applications of craniomaxillofacial surgeries, resulting in 335 patients who had 3D-printed patient-specific data that was actually translated for use in surgery, with operations including those in Table 1. We analyzed these craniomaxillofacial operations to find patterns in the usage of 3DP for direct patient-specific surgical application, and discovered four main categories of utilizing 3DP for patient-specific craniomaxillofacial surgeries which they classified into four types:1
Table 1.
Applications for Patient-Specific 3D-Printing in Craniomaxillofacial Operations
| Type I. Contour model | Type II. Guides | Type III. Splints | Type IV. Implants |
|---|---|---|---|
| Cranial vault reconstruction | Craniosynostosis surgery | Nasal reconstruction | Jaw (orthognathic) surgery |
| Frontal sinus reconstruction | Midface osteotomies | Cleft lip-nose-palate | Intermaxillary Fixation: Arch bars, hybrid system, screws |
| Orbit reconstruction | Mandible resection and reconstruction | Dental occlusion | Prosthetics |
| Hypertelorism correction | Bilateral sagittal split osteotomy of mandible | Cranial defects | |
| Midface reconstruction | Bilateral intraoral vertical ramus osteotomy | Orbital fracture repair | |
| Mandible reconstruction | Hardware placement: plates and screws | Condylar head reconstruction | |
| Fracture visualization | Cranioplasty | Mandibular reconstruction | |
| Dental implant placement | Facial fractures | ||
| Ear reconstruction | Nasoalveolar Molding | ||
| Prosthetic teeth base |
Our new classification of 3DP objects utilized in surgery proceeds logically, as each type is more complex than the previous type:
Type I Contour models are the most straightforward application of medical 3DP, printing out patient-specific anatomy available on a patient’s imaging data, such as a computed tomography (CT) scan. Although fairly straightforward to 3D print, creative application elevates this simple engineering to versatile and useful applications intraoperatively.3–12
Type II Guides require more complex computer processing, as the negative space needs to be mathematically characterized so that a 3D-printed guide can conform to a surface such as a bony surface.13–17 This requires precision of both the image processing, and the 3D printer itself.
Type III Splints require significantly more computing and engineering to be able to accurately perform virtual surgery to virtually cut the anatomy and move the pieces to the final position.19–23
Type IV Implants require the most computing, and materials science engineering, to be able to create biocompatible implants that are placed in the body usually indefinitely polyethylene.7, 10, 17, 25 There are also regulatory hurdles for implantable materials in the human body.
Table 1 demonstrates the applications of 3DP in craniomaxillofacial surgery and with representative procedures applied in each of the four categories.
Benefits of the Four Types of 3DP Surgical Applications
The benefits of the four types of 3DP objects for surgical applications are increasing in scope. In Plastic Surgery, craniomaxillofacial operations have their foundations in bone reconstructive surgery that changes the overlying soft tissues, so they are an ideal application for surgical 3DP as current 3DP technology still most often uses solid printing material that simulates hard tissues very accurately. Therefore, craniomaxillofacial operations involve bone surgery (osteotomies, recontouring, plating), provide an excellent opportunity to detect and employ emerging 3DP technologies. Another significant benefit is that craniomaxillofacial surgeries usually require significant symmetry and functionality, both of which can be simulated and virtually modeled with advanced image processing and computer software. Plastic Surgery craniomaxillofacial surgeries frequently involves children with cleft-craniofacial-maxillofacial birth defects and therefore have an even greater stringent requirement of precision in the small pediatric anatomy.
One of the keys for the most benefit for all these types is to understand the concept of mirroring, to find normal tissue to mimic, in order to design the most accurate replica specific to the patient. This involves analyzing the contralateral side that is normal, and using computer software to take its mirror image, thus creating something close to what the ipsilateral anomaly should look like. The contralateral body is therefore being mirrored and then used as a template. This new mirrored piece can be 3D-printed for contour models, guides, splints, and implants. In a patient without a contralateral normal side, other methods are needed for estimating a normative, patient-specific anatomy. In our systematic review, we found that 62% of patients had the mirror technique applied to their 3DP surgical object,1 making this a very common technique that has significant clinical applications.
Clinical Applications and Techniques of the Four 3DP Types
Our Plastic Surgery center has been utilizing 3D-printed patient-specific objects for patient-specific craniomaxillofacial surgeries for the past 19 years (since the year 2000), as of the writing of this article. These were done at high cost with commercial 3D printers. However, now we have brought 3DP in-house at our academic pediatric hospital since 2017, where we now have faster turn-around, immediate customization, and decreased individual case costs after original investment.
Our in-house 3DP center follows a similar workflow identified by Jacobs and Lin.1 The most common imaging file we use for 3DP for patient-specific surgery is from the patient’s CT scan. Typically, the CT scan is part of the standard of care by the primary treating medical team for diagnostic reasons, and our hospital’s radiology department uses as low as possible dosages. In addition, some of our patients have other imaging modalities ordered by their primary treating medical team, including MRIs and ultrasounds, and we are actively testing different methods of utilizing this imaging data. The most common format for imaging is the DICOM format, which we can then utilize multiple commercial software to segmentalize the anatomic structures into the 3DP digital file, which is in the format known as STL. Segmentation can be tedious depending on the area of detail needed, the amount of interference or scatter, and the complexity of the anatomy to be delineated. For contour models, once the segmentalization is done, it is up to surgeon preference the area that has the greatest utilization to be 3D-printed, as the constraints of the print area may prevent the entire anatomic region from being printed as one piece. For guides, which harness the negative space, the software can be used to create a 3DP based on the negative space around an object that needs guidance on the entry or incision point, or the negative space of a defect that needs reconstruction. For splints, additional software is necessary that can perform virtual surgery and simulate cuts and movements on the CT data, to create a virtual final position that a 3DP splint can be printed to represent this final position. Finally for implants, at this point in time hospital-based 3DP hospital centers will not have the manufacturing facilities that can achieve the regulatory requirements of printing an object that can be placed in a human indefinitely, so most 3DP hospital centers will still depend on commercial medical 3DP companies for implants.
Finally, the choice of transferring this STL file to the printing software, is an important team decision with the engineering portion of the team. Careful consideration needs to be given to the type of material, the degree of resolution needed. Typically, the surgeon would prefer a sterilizable material to use the 3DP object intraoperatively, and this will limit the number of choices available. However, for procedures that are not sterile, or for presurgical simulations, a wider variety of materials can be used.
We will go through several typical Plastic Surgery craniofacial and maxillofacial surgeries utilizing this new classification of the four types of patient-specific 3DP objects for surgical application:
Type I. Contour model
Type I models are our most common models used in surgery. The technology is straightforward, and the segmentalization with an engineer and the surgeon is enough to isolate the structures that are needed for surgical visualization. As in-house 3DP becomes more affordable, these models can be utilized in almost any surgery where greater visualization is needed, precision is necessary for appropriate alignment, and where having an intraoperative model reveals hidden anatomy of the patient. This is especially useful in the pediatric population with their finer anatomy and limited incisions. A prime example for our center is in facial fractures, as the visualization is difficult because incisions are small to avoid scars in the face, the reduction of the fragments needs to be precise, and the urgent nature of pediatric fractures (that can heal in the incorrect position more quickly than adult fractures due to faster nature of pediatric healing) demands the quicker turnaround that an in-house 3D printer can achieve.
Figures 1–3 demonstrate a patient with a severely comminuted mandible fracture from a gunshot wound. Our in-house 3D printer was used to print a patient-specific model of the mandible that could be used to pre-bend reconstruction plates. The pre-bent hardware was then sterilized and brought in the operating room to be used to internally fixate the mandible fracture. The 3D-printed mandible (white) in figure 1, is the patient’s mandible after the gunshot injury – note the free-floating butterfly segment with multiple smaller fragments. Using our software, the uninjured image of the (left) side of the mandible was used to make a mirror image to replace the injured fractured (right) mandible. This theoretically perfect mandible can now have a titanium plate bent to match it before or during surgery (Figures 2–3), and intraoperatively the comminuted pieces can be screwed directly to the plate, resulting in faster and more precise repair of the mandible fracture.
Figure 1.
3D-printed models from our in-house 3D-printer. The white mandible is the patient’s injured mandible from the gunshot. The blue mandible was made by taking the less injured left side of the white mandible, and mirroring it to create a theoretically perfect mandible. This predicted preinjury mandible can be used as our contour model. The patient’s maxilla is printed as the blue skull to demonstrate that the theoretical blue mandible’s condyles were seated within the TMJ joints, indicating the blue mandible has normal functional size and shape.
Figures 2 and 3.
These images demonstrate the reconstruction plate being pre-bent to the 3D-printed predicted preinjury model.
Type II. Guide
Our most common application of guides is in pediatric craniofacial reconstruction, especially bone defects such as those found in skulls, orbits, zygomas. Once the image capture is performed, the DICOM file is converted to an STL file, and then software allows us to capture the negative space to make a guide. For example, in the clinical situation of a child with a skull defect (post-traumatic, post-reconstruction, post-congenital), these are usually irregular skull defects that take significant intraoperative time trying to find an area of skull to use as a donor site, and then fashioning an irregular donor to match the skull defect, then finding out it does not fit well and re-doing it – a time-consuming process. By printing a guide for the skull defect (the negative space representing the defect), we can use this elsewhere on the skull to quickly trace out a perfectly-shaped donor piece of bone, that matches the skull defect precisely. This saves time and bleeding morbidity on pediatric patients, which is important for improving outcomes in pediatric craniofacial surgery.
Figures 4 and 5 show a patient’s skull with a large midline cranial skull defect. The skull and the guide were printed using our in-house 3D printer. The printed guide in the figure below (blue) sits perfectly in the cranial defect, demonstrating the precise dimensions of the printed guide. We print the guide by using a type of resin that can withstand deformation temperatures during the sterilization process. Figure 6 shows a complex fronto-orbital craniofacial defect, with a 3DP yellow guide that allows harvest of autologous bone that matches the patient’s specific three-dimensional foreheadorbital contour. These guides are used real-time on the surgical field, to harvest a split-cranial bone graft of the exact shape needed, which saves time compared to the traditional method carving and checking until it fits.
Figure 4.
Left, aerial view of a patient’s cranium with a significant skull defect, with the 3D-printed guide (blue) fitting perfectly in the cranial defect. This blue printed guide can be used during surgery to harvest a precisely shaped bone graft.
Figure 5.
Right, posterior to anterior view of the cranium and cranial defects with a 3DP-guide in one of the two large cranial defects. A separate 3DP-guide was used for this second, distinct skull defect.
Figure 6.
Patient with complex fronto-orbital craniofacial defect, with a 3DP yellow guide that is used to harvest autologous bone graft. This yellow guide not only captures the unpredictable shape of the skull defect, but also captures the complex 3D angulation of the lateral aspects of the human forehead where it sweeps into the temple region. This yellow 3D guide allows harvest of bone that has this congruent angle, ensuring an autologous bone graft that matches the patient’s specific forehead contour.
Type III. Splint
As in the systematic review study, one of the most common areas that we use splints is for jaw surgeries, especially orthognathic surgery to correct underbite or overbite.18–24 This surgery is usually done when children reach skeletal maturity at about 16 to 18 years of age or older. Children born with cleft lips have the most severe form, as their midface contains the cleft disease and does not grow normally, leading to a midface that is much smaller than their mandible (lower jaw) – in other words a severe underbite. Before 3DP splints, custom splints would be fashioned by hand with dental impressions made into dental models, on an articulator to simulate “virtual” surgery movements, and manual splints were made to be used intraoperatively. Nowadays, the dental wet lab can be replaced by dental scanners and 3D printers. To successfully perform this virtual surgery, specialized software is necessary to accurately measure cephalometric angles (for example, sella-nasion-A point and sella-nasion-B point) and recognize landmarks such as anterior nasal spine (ANS), as well as simulate the movement to arrive at a final virtual position. Once the program can simulate this virtually, it is a similar process to print the negative space to create a splint that can be used during jaw surgery.
Type IV. Implant
As discussed in the above referenced systematic review, this is the least common type of 3DP object, in part because of the manufacturing sophistication necessary to 3DP biocompatible implantable materials such as porous polyethylene25 (such as Medpor from Stryker), polyetheretherketone (PEEK from DePuy Synthes), or in the future other biocompatible or even bioengineered constructs. The other significant hurdle is the regulatory concern, which is sensible given that these implants are replacing tissues when there are no other good autologous reconstructive options available, such as a child with small anatomy and therefore limited donor sites. Therefore, currently most hospital 3D printers would not be able to do this. We have applied patient-specific custom implants (some may be additive manufacturing, but there is also another field of subtractive milling that is beyond the scope of this discussion), but only for severe defects. For example, children with large craniofacial defects where there is not enough donor rib or donor bone, will be candidates for implants. Another area is our patients with cleft lip-nose-palate may have such severe maxillary hypoplasia that need a severe advancement and we have used commercial custom titanium plates that are patient-specific for the final virtual position of the maxilla (combining the concept of splints and implants). It is clear that implants will be the subject of significant medical research and advancement.
Conclusions for Plastic Surgery 3DP for Patient Care
Despite the rapid advancement of 3DP in medicine and surgery, 3DP outcomes studies are still in their infancy. Part of this is the lack of a systematic classification of the techniques used, making it more difficult to make comparison studies. Plastic Surgery craniomaxillofacial surgery is an ideal specialty to study patient-specific 3DP surgeries, as the operations are bone-intensive, and 3DP is currently better at simulating hard tissues. Another area that Plastic Surgery offers for studying outcomes, is the priority placed on normal shape and normal symmetry. Both of these attributes are well-suited for computer processing and virtual simulation. The uniqueness of our recent systematic review and classification1 is that its sole focus is on 3DP patient-specific anatomy for surgery, which is a patient-centric approach to advancing 3DP applications. This systematic review and analysis of direct surgical applications of patient-specific 3DP revealed four classifications of 3DP: Type I contour models, Type II guides, Type III splints, and Type IV implants. By understanding this classification and the four types of patient-specific 3DP for surgery, this will stimulate new innovations. Multiple examples of these techniques are described above, and can also be seen on our video at https://www.slucare.edu/newsroom/kmov-science-of-healing-faces-of-childhood.php.
Our experience so far with patient-specific 3DP for surgery is that it can make surgeries faster, more accurate, and less invasive with smaller scars. This is especially beneficial to pediatric patients where anatomy is finer and more compressed, length of surgery can lead to more blood loss, which is even more critical in children. This will further improve Plastic Surgery craniomaxillofacial operations, and other medical and surgical subspecialties, to benefit the patients.
Acknowledgment
The authors thank our 3D-printing technologist Brian Albers who created the 3D-printed surgical objects shown in the Figures, at our 3D Printing Center of Excellence at SSM Health Cardinal Glennon Children’s Hospital at Saint Louis University.
Footnotes
Alexander Y. Lin, MD, (above), and Lauren M. Yarholar, MD are in the Division of Plastic Surgery, Department of Surgery, Saint Louis University School of Medicine, St. Louis, Missouri and at SSM Health Cardinal Glennon Children’s Hospital, St. Louis, Missouri.
Contact: Alexander.Lin@Health.SLU.edu
Disclosures
None reported.
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