Perspective Chapter: Orbital Reconstruction and Orbital Volume

Abstract

Orbital fractures are common presentation in the head and neck trauma centers. They can result in functional and esthetic problems. The primary goal in the repair of the orbital fractures is to restore the orbital shape and volume, free the incarcerated or prolapsed orbital tissue from the fracture defect, and span the bony defect with reconstructive implant material. Titanium mesh was very appropriate reconstructive material for anatomic reconstruction. The orbit has a special complex geometry which makes perfect anatomic reconstruction very difficult. The manual process of fitting and adapting the implant within the orbit is time consuming and operator dependent. The advanced techniques in maxillofacial imaging and computer assisted techniques resulted in improvement in the implant design for management of orbital fractures. The current study was made to review the accuracy of adapting the titanium mesh using STL model versus conventional technique for restoring the orbital volume in management of orbital floor fracture.

1. Introduction

Orbital fractures are one of the most common fractures of the midface and result in significant complications such as enopthalmos, diplopia, restriction of gaze, and dystopia [1]. Orbital reconstruction aims to restore the normal orbital volume and architecture and reduce the herniated orbital tissues to prevent the compications [2]. The choice of the implant material depends on many factors such as: size of the defect, involvement of many walls, adaptation to internal contours, restoration of accurate volume, presence of adjacent sinus cavity, prevention of displacement, restriction of ocular motility, risk of further trauma, and early versus late repair [3].

Avashia et al. reviewed the materials used for orbital reconstruction and classified it into biological materials and manufactured materials. Biologic materials include autografts, allogarfts, xenografts. Manufactured materials include resorbable materials as polymers and nonresorbable materials as porous polyethylene, bioactive glass, silastic rubber, titanium, teflon, nylon, and other materials. Avashia et al. reported no consensus for any material as the optimal choice for orbital floor reconstruction [4].

Many surgical techniques have been adopted and evaluated for correction of orbital volume. Topography of the orbital floor (S shape in sagittal plane) is one of the difficult factors during insertion and manipulation of reconstructive material. However there is a continuous search and study for the best method to achieve accuracy, feasibility, and reliability for restoration of the orbit [5].

2. Titanium implant

Literature review revealed the use of many surgical approaches and many implant materials for orbital reconstruction. Titanium mesh was very appropriate reconstructive material for anatomic reconstruction [6]. Titanium implants are considered as an established implant material to reconstruct the orbital and craniofacial skeleton. There are many forms and shapes of titanium mesh and different thickness and sizes [7].

Titanium mesh is the most commonly used reconstructive material for orbital reconstruction [8]. Intra-operative manual bending and adaptation to the titanium mesh after exposure and reduction of fractured segments is a traditional or conventional technique. However, this leads to more dissection, multiple trials, and longer operative time especially in comminuted fractures. This technique makes the reconstruction process a subject of interpersonal variation because the manipulation and bending of the implant material depends on the operator experience [9].

Titanium meshes have high biocompatible properties. They are easily molded to fit simple and complex orbital defects. They can provide a strong support without change in the shapes or locations over time. They can be fixed to adjacent bone. They have many good characters such as: availability, easy sterilization, and a well-recognized osseointegration. Titanium has a high corrosion resistance because of the spontaneously forming thin oxide layers on the surface. This guarantees its passive behavior to avoid toxic or allergic reactions [4].

In 2009, Lee and Nunery revealed that the use of titanium mesh in the orbital floor can lead to fibrous adhesion around the implant resulting in diplopia or restricted eye movement after orbital floor repair by 2 months. It is a rare complication which requires replacing the titanium mesh with another implant. They considered the titanium itself caused fibrous adhesion and named it “orbital adherence syndrome” [10]. However, in 2013, Kersey et al. revealed in their study that the fracture can result in rupture or splitting through the periorbita, causing fibrous adhesion due to inadequate separation between the orbital contents and the bone or the implant and this complication can ocuur occur with or without the titanium implant [11].

3. Computer assisted surgery

The orbit has a special complex geometry, so the perfect anatomic reconstruction is very difficult. The process of fitting and adapting the implant in the orbit is time consuming and operator dependent. The narrow field in addition to the complex anatomy of the orbit make the orbital reconstruction difficult and almost impossible to achieve “true-to-original” 3D shape [12].

The management of the orbital fractures has changed over the years. The advanced techniques for maxillofacial imaging and computer assisted techniques lead to an improvement in the implant design for orbital reconstruction [13].

Over the last 2 decades, there was great and rapid improvement in the computed tomography which has added a third dimension to the imaging of complex craniomaxillofacial deformity. This development has significantly decreased the degree of inaccuracy that is inherent in any clinical assessment. Three dimensional (3D) reconstruction from volume data collected from helical CT is now an established technique in craniomaxillofacial surgery to provide animated screen images and accurate reproduction of the hard and/or soft tissues with models [14].

The development of computer assisted surgeries represents a new technology and a turn point in the field of craniofacial reconstruction. Some of the computer assisted techniques are used for virtual reconstruction of the fractured orbit and rapid prototyping. STL model was used for adapting and contouring the orbital mesh to allow accurate orbital volume, decrease operative time, decrease hospital costs, strengthen the surgical skills, and improve patient outcomes [15, 16, 17].

Lim et al. found that the direct intraoperative trimming and adaptation may take long operative time depending on the extension of the defect and experience of the surgeon when compared with STL models [18]. On the other hand, preoperative rapid prototyping reduces the intraoperative time, risk of orbital mesh malposition, poor anatomical contour, and trauma to soft tissue because of multiple insertions during trimming and adaptation of the titanium mesh [19].

Rapid prototyping is a new technique characterized by rapidly and accurately preparing solid bodies with complex shapes, so it has a promising and extensive application in the medical fields [20]. Since its introduction into craniomaxillofacial surgery in the 1990s, it has been used for the treatment of various medical problems, such as orbital hypertelorism, craniosynostosis, facial asymmetry, craniomaxillofacial defects, maxillofacial implants, orthognathic surgery, tumor surgery [21].

Rapid prototyping is a three dimensional (3D) printing process which involves an additive manufacturing technology which offers an expedient and accurate reproduction of an osseous anatomy. The intact orbit was mirrored onto the fractured one to create virtual model. The virtual model data were converted to STL (Standard Triangulation Language or Standard Tessellation Language) format to form a solid physical orbital model using a 3D printer and computer-aided manufacturing machines. STL model was used to adapt and contour the orbital mesh to allow accurate orbital volume, decrease risks and time consuming, and help improve postoperative results [18, 21].

In 1986, Hull introduced the stereolithography apparatus (SLA) technique to create an accurate, hardened, three dimensiona; acrylic models from CT data [22, 23]. In 1990, the first stereolithographic patient model was built. It represented an actual three- dimensional model to reproduce the anatomy of a patient based on CT images taken during that patient’s examination [24].

In 1998, Perry et al. reported that 3D models of the facial skeleton differ in their accuracy, reproducibility and cost. The early attempts to build and form models from CT scans were stacked polystyrene slices where each of which represented a corresponding slice from an axial scan. Since then model building has developed into 2 distinct processes: computer aided manufacture (CAM) and stereolithography. In the computer aided manufacture (CAM) technique, the models are milled by computer guidance from different materials as soft expanded polyurethane and titanium alloy. It is considered as a removal process by an expensive milling machine. Stereolithography is a computer controlled construction process including 0.25 mm layer by layer polymerization of LASER curable liquid resin which are built on top of each other. It is accurate, slow and expensive technique [14].

In 1999, Holck et al. described the benefits of the sterolithography (SLA) modeling system for planning the surgery for bony orbital pathology. They reported that the SLA models were beneficial preoperatively for evaluating the dimensions of the bony defects and surgical planning. Intraoperatively, SLA models facilitated the surgical rehabilitation of the orbit leading to postoperative satisfactory results [24].

In 2006, Metzger et al. measured the accuracy of a technique for making individual preformed titanium meshes for orbital fractures. The study included 5 patients with unilateral orbital fractures and the patients underwent preoperative CT scans of 1 mm thickness followed by surgical planning using surgical planning software, then stereolithographic models were built after using the mirroring tool from unaffected side on affected side. Titanium meshes were then adapted manually on the STL models and sterilized. Intra-operatively, the meshes were positioned with the aid of navigation tools to ensure correct placement of the mesh as on the template by using reference points. Postoperative CT scans were obtained to compare the actual position of the titanium mesh implant to the planned position of the orbital floor. They reported accurate reproduction of the planned surgery [25].

In 2006, Schon et al. used individually preformed implants to reconstruct an extensive orbital floor fractures in 19 patients. The orbital floor and walls were studied by preoperative diagnostic CT scan data. The form of the virtual reconstructed orbit was printed into a model for the orbital cavity by a template machine. They reported that the orbital reconstruction using a preformed implants is less time consuming, more accurate, and less invasive in comparison to free hand efforts for the restoration of the orbital fractures using titanium mesh and calvarial grafts [26].

In 2010, Zhang et al. studied 21 patients with delayed treatment of unilateral impure fracture of the orbit and post-traumatic enophthalmos. They used anatomically adaptive titanium mesh depending on computer-aided design and computer-aided manufacturing techniques (CAD/CAM). After exposure to orbital floor defect and reduction to the herniated soft tissue, the titanium mesh was placed to restore the internal orbit. Measurements were taken to evaluate the change in the degree of enophthalmos and orbital volume before and after surgery. They found that this method was useful to some degree to decrease the expanded orbital volume and correct post-traumatic enophthalmos [27].

In 2012, He et al. made a retrospective review of a consecutive clinical case series. 64 patients from 2008 to 2010 were diagnosed with delayed orbitozygomatic fractures with enophthalmos. Traditional surgery and computer-assisted treatment (navigation and 3D models) were used for zygoma reduction. They found that computer-assisted surgery can improve the treatment results [28].

In 2016, Oh et al. made a study to use individualized prebent titanium-Medpor mesh implants and stereolithographic modeling in a series of patients who underwent orbital wall reconstruction. They obtained good results and concluded that orbital reconstruction can be optimized by using individually manufactured rapid prototype skull model and premolded synthetic scaffold by computer-aid of mirroring-reconstruction of 3-dimensional images and 3-dimensional printing techniques [29].

In 2020, Sigron et al. made a study to compare the efficacy of the intraoperative bending of titanium mesh with the efficacy of pre-contoured “hybrid” patient-specific titanium mesh using 3D-printed anatomical models as bending guides for the surgical repair of isolated orbital floor fractures. They concluded that t the use of 3D-printed orbital models leads to a more accurate reconstruction and a time reduction during surgery [30].

4. Orbital volume measurement

Orbital volume measurement after maxillofacial trauma reveals a significant data for evaluating the severity of the injury and prevention of possible complications caused by enophthalmos [31]. The Volume of the orbit is the space formed by the size and position of the orbital walls [32].

In 1873 in France, Gayat may have been the first to publish orbital volume data [33]. He used lead pellets to fill the orbital cavity of 11 skulls and poured them into a graduated cylinder to determine the orbital volume. He found the average of the orbital volume approximately equal to 29 cm³. After Gayat, some authors used alternative methods for measurement on corpses using silicone, water, glass beads or sand were used [34].

During the 20th century, OVM for living patients became possible with the development of medical imaging techniques. The first OVM for a living patient was performed in the 1960’s with manual evaluation of roentgenographic images [33].

With the advent of tomodensitometry, volume measurement of irregular objects became possible. In 1985, Forbes et al. transferred the data from CT scan images to special program to calculate the volume of the orbital structures. The software counted the number of pixels to calculate the volume according to the number of slices and special formulas [35].

Manual segmentation for OVM (planimetry methods) is considered to be the most common method for this purpose. It depends on the summation of the manually delineated areas obtained from a CT images [36]. The operator manually delineates the boundaries of the orbital bone cavity on a series of CT slices. The boundaries are defined by the operator and not by standard charts. This can represent a source of errors with low reproducibility. Planimetry method is extremely time-consuming to be accurate but its advantage lies in its availability on all standard medical imaging softwares [37].

Automatic methods for OVM contains automatic segmentation of the orbital cavity which can be undertaken using a function integrated within software. This method relies on atlas segmentation [38]. Semi-automatic method for OVM is defined as a method using volumetric built-in functionality in software, combined with manual adjustments. Various semi-automatic methods are available, depending on the software used. The are some softwares which are considered as different methods described in the literature for OVM [37].

Several studies have shown a correlation between an increased OV and enophthalmos. Using planimetry, Whitehouse et al. showed that enophthalmos increased by 0.8 mm per 1 cm³ of OV expansion [39]. In 1993, Charteris et al. proposed that the amount of increase in the orbital volume may determine the need for surgical intervention [40].

Some authors consider the OV as a predictive of long-term symptoms, while other authors did not find a significant correlation between an increased OV and enophthalmos [41]. Choi et al. showed that the OV cannot be considered as a reliable measure to estimate the size of enophthalmos because of inter-individual variations in the OV [42].

The use of intraoperative navigation during the orbital surgery according to data from CT scans or MRI assisted in implementation of preoperative plan, volume measurement, and protecting the vital structures [38]. In the recently published studies, administration of CT images was recommended as the standard method to determine the volume of the orbital cavity in living patients [34]. However, there is no consensus concerning the gold standard for orbital volume measurement [43].

The difference in the volume of bony orbit can reach 0–8% between the right and left orbit when measured in the same person [35, 44], and up to 22% between subjects [32]. This normal difference can be considered as a protective factor for the surgeon, allowing him some malleability in the work for not insisting to achieve the ideal OV and reflecting the great tolerance in orbital volumetric restoration. A perfect symmetry between the 2 orbits is not necessary, and a variation of around 10–20% of the volume between the 2 orbits may involve no or only minor imperceptible facial irregularity. This minor difference may lead to a satisfactory result for the patient and the surgeon [44].

Stereolithography (SLA) technology developed into many types such as powder bed fusion, fused deposition modeling (FDM), selective laser sintering (SLS) for 3D printing which is known as additive manufacturing or desktop fabrication. Some metals as titanium, silver, gold, steel, and stainless steel can be used as raw materials for 3D printing. However, the plastics are most commonly used such as: acrylonitrile butadiene styrene (ABS) or “Lego” plastic material, polylactic acid (PLA) which is readily available in soft and hard grades, polyvinyl alcohol (PVA), and polycarbonate (PC). ABS materials are sturdy, strong, and providing more structural integrity than PLA which is less expensive, more biocompatible, and providing more precise prints [45, 46, 47, 48].

5. Conclusion

Both techniques conventional technique and STL model for titanium mesh adaptation lead to significant correction of the orbital volume. Conventional technique is still a valid and cheap method among the attractive new techniques. STL technique is helpful in the cases presented with massive orbital disruption and/or malunion. It offers less operative time and less tissue manipulation.