Surgical anatomy of the floor of the oral cavity and the cervical spaces as a rationale for trans-oral, minimal-invasive endoscopic surgical procedures: results of anatomical studies
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Over the past 10 years, several minimally invasive procedures for thyroid surgery have been developed. Because of extensive dissection in the thoracic and neck region, the name “minimal-invasive” is misleading. The aim of this study was to define a new trans-oral access to the cervical spaces especially to the thyroid on the basis of natural orifice surgery. Three embalmed human specimens were dissected for complete review of the anatomical situation in the cervical region. In additional five fresh frozen human specimens after an experimental trans-oral endoscopic minimally invasive thyroidectomy the anatomical structures of the floor of the oral cavity as well as the anterior neck region were evaluated. It was possible to create a working space under the platysma muscle with respect to the surgical planes of the neck and fascial layers. Within this area, the pretracheal region can be reached and the thyroid gland can be visualized and resected. To access the working space, a trocar for endoscopic view is placed medially in the floor of the oral cavity sublingually. The trocar passes the muscles of the floor of the oral cavity easily without relation to relevant anatomical structures. A first exclusively sublingual approach had to be abandoned because triangulation of the instruments could not be reached. Therefore, the approach was modified by positioning the working trocars in the oral vestibule bilaterally. By this way, a road map for accessing all anterior cervical regions directly under the platysma muscle could be established and anatomical landmarks and areas of possible collateral damage could have been defined. This combined sublingual and bi-vestibular trans-oral endoscopic approach enables an easy access to all structures and spaces of the anterior neck region with respect to anatomical preformed layers neck, even to the thyroid as one of the more distant structures.
KeywordsEndoscopic Minimal invasive Thyroidectomy Cadaver study Anatomy Floor of the oral cavity
One of the first attempts to reduce surgical access trauma in thyroidectomy was the unilateral, endoscopic thyroidectomy performed by Hüscher et al.  on the 8 July 1996. As minimally invasive procedures in thyroid surgery evolved over the past 10 years the so-called minimally invasive video-assisted thyroidectomy  has so far become most widespread. This was mainly established by the working groups of Micolli and Ruggieri from Italy [3, 4, 5]. Endoscopic minimally invasive approaches were also established experimentally and clinically for parathyroidectomies by different groups [6, 7, 8]. With respect to thyroid pathologies, techniques were developed to avoid scars in the neck region: such approaches are the chest , the axillary  and the combined axillary bilateral breast approach . However, these techniques do not comply with the term “minimally invasive” and they are associated with an extensive dissection of the skin and subcutaneous layers of the chest and neck region and therefore being rather maximally invasive.
Latest attempts in minimizing surgical trauma lead to an experimental trans-oral access to the thyroid gland using a single-port access via an axilloscope ; this approach have so far not overcome the first experimental state and is in fact a hybrid approach, because additional 3.5-mm skin incisions 15 mm below the level of the larynx were necessary.
Trans-oral resections of the submandibular gland date back to the early sixties of the last century. Downton and Qvist described the trans-oral resection of the submandibular gland by open surgery via the floor of the oral cavity . Other endoscopic surgical procedures were also performed to treat fractures of the mandible, but they were still limited to the narrow spaces surrounding the oral cavity.
The border of the per-oral region to gain access to the cervical spaces by a trans-oral, natural orifice approach was never used successfully before. Therefore, our vision was to develop an endoscopic, minimal-invasive approach for thyroidectomy.
The aim of our study was to define anatomical spaces, surgical planes and related neural and vascular structures to create a safe and reproducible trans-oral access and pathway to the cervical spaces especially to the more distant thyroid gland. This access should guide an easy and safe way to preformed anatomical spaces between different fascial layers of the neck. In these “sliding” layers, a preparation without any bleeding should be possible and neural structures can be spared. The space to be reached is the sub-platysmal layer above the pretracheal strap muscles. Here, the linea alba colli can easily be identified, transsected and the thyroid gland visualized, mobilized and resected. For minimally invasive thyroid resection the principal access to this working space was executed with standard instruments with a diameter of 2.7 and 3.5 mm used in general abdominal surgery for minimally invasive procedures.
Materials and methods
This anatomical study was carried out between 28 January 2008 and 14 May 2008 at the Department of Neuroscience and Anatomy of the Erasmus MC, University Medical Centre Rotterdam (The Netherlands). In total, eight human specimens (5 males, 3 females) with a mean age of 81 years were used for this dissection study.
Three embalmed human specimens were dissected for complete anatomical information of the cervical region. In additional five fresh frozen human specimens after an experimental trans-oral endoscopic minimally invasive thyroidectomy, the anatomical structures and pathways were evaluated.
Anatomical dissections were performed, based on surgical planes: first the skin, subcutaneous tissue and platysma muscle were removed. Beyond the superficial fascia, the vessel architecture and the nerves of the anterior triangle of the neck were separated from the connective tissues and lymph nodes. Then, the different surgical spaces in the submandibular grove were demonstrated and the way through the floor of the oral cavity to the sublingual space was shown.
In the three embalmed human specimen, the vascular system and neural structures of the frontal and lateral neck region were dissected to define landmarks for a pathway through the floor of the oral cavity. After this, we developed the trans-oral access to the pretracheal space in five fresh frozen human cadavers. To qualitatively determine damage to anatomically relevant structures, all five specimens were dissected after performing the surgical procedure.
All relevant steps of the dissections were documented by a digital SLR camera (Canon EOS 400D), image processing was performed with Adobe Photoshop CS4 Extended Version©.
Our hypothesis was that a working space under the platysma muscle in the anterior neck region (level VI) can be created with respect to the surgical planes and fascial layers of the neck and without significant damage to anatomical structures, such as vessels and nerves. Within this area, it should be possible to reach the vessels and lymph nodes under the sternocleidomastoid muscle as well as the pretracheal region. In this compartment, the thyroid can be visualized and resected.
The submandibular triangle
Superficial to the submandibular gland, the facial vein crosses the superficial fascia to reach the anterior border of the mandible. The facial artery enters the triangle under the posterior belly of the digastric and stylohyoid muscle; it ascends to emerge above or through the upper border of the gland (Fig. 2). The marginal mandibular branch of the facial nerve courses through the triangle under the platysma muscle and under the superficial fascia but outside the submandibular “bag”. It courses over the facial vessels as it travels upward to supply the peri-oral muscles.
The hypoglossal nerve enters the triangle deep to the posterior belly of the digastric muscle. It lies on the surface of the hypoglossus muscle and courses deep to the mylohyoid muscle to supply motor function to the tongue. The lingual nerve, a branch of the mandibular nerve, is found under the border of the mandible on the hypoglossus muscle superficial to the hypoglossal nerve. It is attached to the submandibular gland by the submandibular ganglion and courses deep to the mylohyoid muscle to provide sensation to the anterior tongue and floor of the oral cavity (Fig. 2).
Exclusively sublingual versus sublingual bi-vestibular access
With this method, we performed the first successful endoscopic thyroidectomy utilizing a tree-point exclusively trans-oral access on the 14 May 2008.
The concept of natural orifice surgery (NOS) is based on the principle of reaching the pathology to be treated by passing through natural entries of the human body (oral cavity, vagina, anus or urethra). This should minimize access trauma, spare working time and therefore be gentler than standard surgical procedures. NOS principles in patient’s procedures were followed in resections of tumors of the rectum and in endoscopic transvaginal appendectomies as well as cholecystectomies [14, 15, 16, 17, 18]. Natural orifice translumenal endoscopic surgery utilizes also a natural orifice as the access point but is limited to truly endoscopic procedures: these techniques were established by Kalloo and his team in 2000 [19, 20] and meanwhile different surgical procedures have been performed by this technique [14, 15].
Regarding neck surgery endoscopic approaches have been tested experimentally especially in the harvesting of sentinel nodes in the recent years [21, 22, 23] as well as for endoscopic submandibular gland resection [24, 25, 26]. All those techniques as well as the minimally invasive approaches for thyroidectomy are no real NOS procedures because they always depend on skin incisions to reach the working area by endoscopic view and instruments.
The trans-oral endoscopic approach developed in this anatomical study is a feasible access to the anterior neck region (level VI): the thyroid gland as well as the central lymph node stations surrounding the trachea could be reached, a working space can be created by the insufflation of carbon dioxide and a dissection of the relevant structures is possible by endoscopic instruments. The major vessels of the neck (common carotid artery, internal jugular vein) and neural structures (recurrent laryngeal nerve and vagus nerve) can be visualized. The technique itself is limited to the anterior neck as an approach to the lateral lymph node stations (levels II–IV) requires a passing of these major vascular structures. This issue must be addressed in further anatomical proof-of-concept studies.
In the anatomical dissection studies, we could show that a direct subplatysmal access through the oral cavity passes no regions of risk, especially vessels and nerves. The only structure under risk is the mental nerve (third branch of the trigeminal nerve) when the vestibular working trocars are brought in place: this nerve usually is located directly under the submucosa of the oral vestibule and can be identified easily. Therefore, dissection and preservation should be practicable.
no history of previous neck surgery in the field of action;
the targeted specimen volume should not extend 30 ml and
singular nodules should not exceed 20 mm in diameter to allow a harvesting of the resected specimen.
Principle limitations are associated with the use of an endoscopic technique itself: bleeding and suboptimal view are typical problems of endoscopic surgeries and might perhaps complicate a clinical application for thyroid surgery.
A study on feasibility and safety in an animal experiment with short time survival will address and clarify these questions.
Nevertheless, the presented endoscopic minimally invasive approach uses an exclusively trans-oral approach and therefore meets the criteria of NOS. Major advantages of the technique in our view are time-saving access respecting the anatomically given surgical planes which probably avoid postoperative scar formation and swallowing disorders. Not at least it is a minimally invasive procedure without any visible scars.
Therefore, in conclusion by this way a minimally invasive approach for thyroidectomy could have been established in anatomical studies. The intimate knowledge of the anatomical structures of the floor of the oral cavity and a dissection according to surgical planes enables a trans-oral access to all structures and spaces of the anterior region of the neck. As has been proven in this study, even the thyroid gland as one of the more distant structures can be reached.
The project was supported by Karl Storz GmbH, Tuttlingen, Germany and the New European Surgical Academy (NESA), Berlin, Germany.
Conflict of interest statement
There is no financial relationship with the organizations that sponsored the research. The authors declare that there is no conflict of interest.
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