A Review of Maxillofacial Rehabilitation Using Osseointegrated Implants in Oncological Patients: Buttress Implant Concept

Cancer leaves important consequences in the shape, function and esthetics of the patient, especially when it is cancer of the oral cavity or upper aero-digestive tract. Although reconstruction with local and microvascular flaps is sometimes a viable option, maxillofacial rehabilitation with osseointegrated implants is a well-reported treatment alternative with a high success rate. The main advantages in this modality of rehabilitation are the decrease in biological and economic costs, simplifying the management of these defects by reducing surgical intervention, hospitalization time, postoperative morbidity and treatment time. There are several classification systems; however, there is no classification system that has accurately described the maxillofacial defect under a surgical, prosthetic and reconstructive approach with osseointegrated implants. The purpose of this study is to guide pro-fessionals in decision-making for maxillofacial rehabilitation using osseointegrated implants located in the anatomical buttresses of the maxillofacial region.


Introduction
In 2018, The Global Cancer Statistics reported just over 18 million new cases worldwide. Lip and oral cavity cancer is ranked 18th with 354,864 new cases (2.0%) and 177,384 deaths (1.9%). However, if we analyze the main cancers that affect the upper digestive tract (salivary glands, hypopharynx, oropharynx, nasopharynx, larynx, lip, and oral cavity), we obtain approximately 887,659 new cases (4.9%) of cancers that could leave important sequelae in the maxillofacial region [1].
Maxillectomy is defined as the partial or total surgical removal of the maxilla and was designed as a surgical treatment for tumors that affect the middle third of the face. During oncological ablative surgery complex maxillofacial defects result that involve the loss of anatomical structures such as dental elements, facial cavities, sense organs, bone tissue and facial soft tissue. Although reconstruction with local and microvascular flaps is sometimes a viable alternative, maxillofacial rehabilitation with osseointegrated implants has been a well-reported treatment with a high success rate [2].
Various techniques and anatomical locations have been described in the facial region for the placement of osseointegrated implants and its reason is based on the bone buttress where a skeletal anchorage is possible to support the functional load of the implants. In general, maxillofacial rehabilitation with osseointegrated implants uses anchors in the zygomatic, pterygoid, nasomaxillary and alveolar zones [3][4][5].

Buttress implant concept and classification
Based on the anatomical zones of the bony buttresses of the facial middle third, we designed a classification system that allows determining the therapeutic options where the placement and functional loading of osseointegrated implants is feasible: zone I or alveolar, zone II or nasomaxillary, zone III or zygomatic, zone IV or pterygoid (Figure 1).

Tilted and axially positioned implants
The use of endosseous osseointegrated implants was introduced to North America in 1982. At that conference, Branemark presented the data from 15 years of work, which was highly evidence-based with long-term clinical follow-up findings setting the guidelines for contemporary implantology [6].
Currently implantology has improved considerably; dental implants have incorporated advances in their anatomy, surface and types of connections, achieving a higher success rate and predictable results [7,8].
Implants angled between 30 and 45° were described as an alternative surgical technique in order to avoid nearby anatomical structures or to achieve an adjacent bone position. Thus, we avoid advanced bone regeneration procedures and minimize the cantilever of prosthetic rehabilitation [9].

Nasopalatine implants
In 1994 Scher. describes the use of the nasopalatine canal as a receptor site for a dental implant, since then controlled studies have been carried out that confirm its viability for the functional load of osseointegrated implants [10].
Peñarrocha et al. treated 13 patients with 78 osseointegrated implants in the rehabilitation of the atrophic maxilla. Of 78 implants, 13 were implanted in the nasopalatine canal, 6 patients had reversible sensory alteration, and 2 implants failed in healthy patients, yielding a success rate of 84.6% [11].
Occasionally, when performing a partial maxillectomy, bone is available throughout the Maxillary alveolar buttress. In these cases, it is possible to use conventional dental implants anchored even in contralateral anatomical structures.

Nasomaxillary implants
Although the term "Nasomaxillary implant" has not been defined in the Glossary of Oral and Maxillofacial Implants (GOMI) [12]. We define it as "Implant placement through the alveolar process and into the nasomaxillary buttress".
The nasomaxillary buttress has been well described in facial trauma as it is a key anatomic zone in the reconstruction of fractures of the middle third. This area offers a cortical bone suitable for the anchorage and functional load of osseointegrated implants. However, there are few implant-oriented anatomical studies and some are anthropometric studies [13][14][15][16].
Some authors have described the placement of posterior tilted implants between 30 and 45° that reach an apical anchorage in the nasomaxillary buttress [9,17,18]. However, this type of implant is not frankly a nasomaxillary implant because they are retained from an apical portion and are not strictly into the buttress as in the case of a long implant with an axial orientation.
Nasomaxillary implants can be used as an anchorage point in a location anterior to the prosthetic arch. With this, we achieve anterior stability and the reduction of work forces in posterior implants.

Zygomatic implants
In 1998 Dr. P. I Branemark described zygomatic implants as a bone anchorage alternative with a design between 30 and 52.5 mm long that are inserted into the body of the malar bone [19]. Since then, Branemark and other authors have described various surgical techniques and approaches for zygomatic implant placement that could be used to rehabilitate atrophic jaws or in patients with partial or total maxillectomy [20][21][22].
Scott et al. rehabilitated 28 patients after undergoing rhinectomy for malignant pathological processes, a total of 56 zygomatic implants were used as retainers of maxillofacial prostheses, 1 failed after radiotherapy having a success rate of 98% in 15 years [4].
In general, most authors agree that zygomatic implants are more than 95% successful [23] and in radiated patients the success rate of implants is highly variable with a range between 72 and 98% [2].
Zygomatic implants offer adequate bone quality and quantity. Sometimes when the maxillectomy is extensive, it compromises the malar bone. Performing regenerative procedures with autografts or xenografts are a viable option to improve site conditions.

Pterygomaxillary implants
Tulasne and Tessier in 1989 were the first to describe the technique for the placement of pterygoid implants [24]. This technique wanted to resolve the difficulties caused by the presence of the maxillary sinus and the poor characteristics of the bone in the maxillary tuberosity.
Pterygoid implants are implants between 15 and 20 mm long that allow a bone anchorage of up to 9 mm in the pterygoid process [25]. However, in the maxilectomized patient this length may vary because the posterior segment of the maxilla is not found.
Araujo et al. in 2019 performed a systematic review from January 1995 to January 2018. A total of 634 patients received 1893 pterygoid implants, with a 10-year survival rate of 94.85%.
Pterygoid implants are a viable option and if their main advantage is to decrease the prosthetic distal cantilever [26].

Buttress implant concept: advantages and disadvantages
Maxillofacial reconstruction with osseointegrated implants placed in bony buttresses is indicated in patients who have suffered loss of mid-facial anatomical structures due to benign and malignant pathological processes, trauma, and severe maxillary atrophy.
There are few contraindications to the use of osseointegrated implants and they are mostly relative. Their reason is that they decrease the success rate compared to implantation in healthy patients. The most important contraindications suggest bone healing disorders such as bisphosphonate treatment, radiotherapy, chemotherapy and active infection. However, for each situation there are protocols that help maintain a high success rate and some of them are discussed in this chapter [20,23,27].
The main advantages in this type of rehabilitation are the decrease in the number of reconstructive surgeries, avoiding the use of donor sites and a shorter hospital stay. By performing a single-phase reconstruction, the patient recovers the phonation, swallowing and chewing lost due to the oncological defect. in addition, the patients present a timely and less morbid treatment compared to local and microvascular grafts. To achieve these results, prosthetically guided reconstruction is key [28].

Virtual prosthetic and surgical planning
Three-dimensional (3D) planning in oral and maxillofacial surgery has become a standard in the treatment of multiple conditions of the facial region. Multiple Programs have been designed to perform 3D virtual planning and surgery in the reconstruction with osseointegrated implants (DTX Studio Implant®, SIMPLANT®, DDS-pro®, 3Shape Dental System®). Most of them are equipped with numerous software complements that allow the preparation of surgical guides and prosthetic reconstructions with different attachments through CAD/CAM [29].
Some of the multiple advantages offered by 3D virtual surgery are: simulating different approaches and types of procedures, avoiding damage to neurovascular and anatomical structures, reducing operating time and improving postoperative recovery, reducing complications and obtaining more predictable results. For preoperative evaluation and planning, a CT scan of the head and neck is the Gold standard [3,23]. The planning of the placement of osseointegrated implants must be prosthetically guided. This means that planning must precede the surgical act of implant placement and these must be located where it best suits prosthetic rehabilitation and biomechanical demands. The objective should be a surgical and prosthetic planning with at least four osseointegrated implants with their distributed emergencies in a polygonal manner over the prosthetic arch. In this way, we were able to tripodize and stabilize the prosthetic reconstruction in the face of the functional demands of chewing (Figure 2) [28,30].

Simulated surgery in stereolithographic biomodels
Stereolithography is a solid three-dimensional prototype obtained through the processing of data obtained from computed tomography or magnetic resonance imaging. In recent years, stereolithographic manufacturing has made great strides in the quality, resolution, and precision of manufactured parts and is becoming increasingly important in medicine and surgery [31].  Three-dimensional printing has been used to produce anatomical models, surgical guides and templates, implants, and molds. The main advantages include: the possibility of preoperative planning, the precision of the process used and the time saved in the operating room. However, other studies report inconsistency in precision and additional costs in treatment [32].
The maxillofacial reconstruction with osseointegrated implants simulated in a biomodel, allows to determine the lengths and final positions of the implants, prepare the surgeon for the surgical procedure and minimize the possibilities of errors favoring the results of the treatment (Figures 3 and 4).

Prosthetic and facial reconstruction
Oral and facial deformity can cause functional and psychological deterioration in oncological patients. The aims of prosthodontic reconstruction are the rehabilitation of the shape, function and esthetics of the lost anatomical structures by means of artificial substitutes. The main facial subunits that require reconstruction due to malignant pathological processes involve the ear, forehead, eyes and brow, nose, check, lips and chin [33].
Currently, there are multiple workflows where they combine conventional prosthetic preparation with the use of facial scanners and custom 3D impressions  A Review of Maxillofacial Rehabilitation Using Osseointegrated Implants in Oncological… DOI: http://dx.doi.org /10.5772/intechopen.93224 that allow the direct or indirect manufacture of definitive maxillofacial prostheses from conventional silicone [34,35].
The main advantage with dental prosthodontic reconstruction is the adaptation of a retained implant obturator that allows a posterior palatal seal. With this, most patients successfully recover phonation, swallowing, and chewing function. Artificial facial prostheses allow us to better characterize the demanding anatomy of the face. In most cases, the retention of facial prostheses occurs when copying the anatomical defect, on other occasions magnetized attachments can be used in prosthetic reconstruction or protective glasses that also improve the characterization of the face (Figure 5).

Radiotherapy treatment and implants
In early clinical stages, surgery is the first decision. However, adjuvant radiotherapy is sometimes indicated in cases of close excision margins (<5 mm), involved (<1 mm), and in suspicion or confirmation of lymph node metastasis with or without extracapsular extension [36]. In general, radiation therapy includes conventional radiation therapy or intensity modulated radiation therapy (IMRT). The latter is more convenient by precisely targeting radiation to a specific area and reducing the dose to nearby anatomical structures such as malar bone, grafts, implants, salivary glands, eyes, and spinal cord [37].
Schiegnitz's study suggests that radiation negatively affects implant survival, but there is no statistically significant difference in survival when implants are inserted 12 months before or after radiation therapy [38]. Other authors have shown favorable results. With implantation at least 6 weeks before radiotherapy since there is a surgical area with less hypoxia, hypovascularity and hypocellularity [27,39] (Figure 6).

Chemotherapy treatment and implants
Systemic antineoplastic therapy includes induction, neoadjuvant, adjuvant, and palliative therapy [40]. The survival rate of osseointegrated implants in the patient with antineoplastic and antiangiogenic therapy has not yet been elucidated. Controlled animal studies have shown negative effects on the osseointegration process with chemotherapeutic regimens with cisplatin, bevacizumab and sunitinib [41][42][43]. However, retrospective studies in humans have reported that chemotherapy has no detrimental effects on the osseointegration and functional stability of dental implants. The success rates reported by these studies were 97.6 and 99.1% [44][45][46].

Conclusion
The placement of osseointegrated implants in anatomical buttresses of the upper jaw is a predictable treatment with a high success rate. By reconstructing the lost anatomical structures with artificial substitutes, a better characterization of the face is achieved and an important esthetic defect is covered with acceptable results.
The main advantages in this modality of reconstruction include the decrease in surgical times, hospital stay and postoperative morbidity. By rehabilitating phonation, swallowing and masticatory function, we improve the quality of life and social reintegration of the oncological patient.
Currently, technological advances point to the development in the design of customized implants; however, controlled studies are required to evaluate the use and behavior of these implants in the different scenarios presented by the oncological patient.