The porcine thyroid has been described in the literature for nearly a century,[13] and the porcine thyroid model has been utilized in thyroid research for decades. This animal model is frequently selected given its low cost and wide availability. The anterior neck anatomy of the pig is similar to that of humans, as is the thyroid gland itself. As such, the pig offers a suitable animal model for thyroidectomy.
A variety of simulators have been used for head and neck endocrine surgery including augmented reality, cadaveric, porcine, and silicone models.[14] The porcine thyroid model has been widely utilized in prospective studies to optimize intraoperative neural monitoring technologies and to determine safe practices for avoiding recurrent laryngeal nerve injury in thyroid surgery.[5, 15–20] Similarly, the porcine model has been used to study the effects of neural injuries related to thermal damage, traction, compression, entrapment and transection of the vagus nerve or recurrent laryngeal nerve.[18, 21] The porcine model has been used to evaluate the recovery profiles of laryngeal muscles after administration of neuromuscular blockade agents and response to blockade reversal with Sugammadex.[16, 22] Indeed, significant contributions to our understanding of neural safety are the result of porcine studies.
Another area of research that relies heavily on this model is that of endoscopic and robotic studies. There have been descriptions of minimally invasive neck dissection, thymectomy and thyroidectomy in a porcine model,[10, 23–29] but none of these studies describe the anatomy in detail.
Given the relative frequency of use of this model, it is surprising to note that the most recent literature describing the thyroid anatomy of a pig was published in 1964,[30] and no operative guide for porcine thyroidectomy was encountered in review of the literature. While this model is widely accepted and utilized, there is a paucity of information characterizing the differences between porcine and human anterior neck anatomy. Awareness of these dissimilarities are critical in the planning of surgical studies.
As previously discussed, studies related to neural mechanisms and neural safety frequently utilize the porcine model. Investigations on neural functions via the porcine model have been successful because porcine laryngeal framework, musculature and innervation is similar to that of humans. However, the relationship of the recurrent laryngeal nerve (RLN) to the thyroid represents a crucial variation from human anatomy. The pig does not have a lateral suspensory ligament anchoring the thyroid to the trachea. As such, the RLN is much less likely to be injured when dissecting the thyroid from the trachea. In addition to the absence of ligamentous suspension from the trachea, the porcine thyroid itself appears as a single-lobed structure anterior to the trachea. The human thyroid is bi-lobed and wraps around the trachea with extension of the lateral aspects of each lobe towards the tracheoesophageal groove. The pig thyroid does not reach as far laterally, and thus is not in as close proximity to the tracheoesophageal groove. In this regard, evaluation of minimally invasive approaches to thyroidectomy or of the use of novel thermal or ablative instruments used for thyroidectomy are likely to underestimate possible risk to the RLN in the porcine model.
The superior and inferior thyroid arteries of the human thyroid are relatively well defined and constant; ligation and transection of each artery represent critical steps in the operative sequence. The vascular anatomy of the pig is limited, in most cases, to a single thyroid artery that approaches the dorsal surface of the right caudal border of the thyroid gland. The thyroid artery of the pig arises from the right omocervical (inferior cervical) artery. A second thyroid arterial branch is present in some specimens, arising from the left common carotid artery and joining the thyroid at the dorsal surface of left caudal border. A thyroid vein can be identified on each side, with the larger typically seen on the right side.[30] In this study using cadaveric pigs, the arterial and venous contributions were not clearly identified in all specimens, likely due to their small caliber. While this study was performed using cadaveric specimens, we suspect that the existence of fewer and smaller caliber thyroid vasculature would similarly underestimate the risk of encountering significant bleeding in the porcine model relative to humans.
The pathophysiology and function of the parathyroid glands in the pig are analogous to those of humans and serve a critical role in biomedical research. However, in contrast to humans, pigs possess a single pair of parathyroid glands which are embedded within the cranial aspect of the thymus gland near the carotid bifurcation.[31] In humans, the four parathyroid glands are located at the posterior medial aspect of the thyroid gland, often in close relation to the thyroid capsule. During human thyroidectomy, the parathyroid glands are carefully separated from the thyroid, to protect glands and their accompanying vasculature. Conservation of parathyroid tissue is essential to avoid the risk of lifelong hypocalcemia. In porcine thyroidectomy, the risk to the parathyroid glands is dramatically lower given their location within the thymus gland. This variation from human anatomy represents another crucial distinction, as evaluation of endoscopic, robotic or other novel techniques in the porcine model cannot provide important data on parathyroid safety during thyroidectomy.
While the thymus is rarely visible within the human neck, we identified the thymus glands of the pig as bulky structures in the lateral neck. The pigs used in this study were approximately 8–10 weeks old and likely had larger thymus glands given their relatively young age. The thymus glands remain present in full grown pigs but do decrease in size with age. While the existence of thymus glands in the anterior neck of the pig does not directly affect the ability to perform thyroidectomy in the porcine model, visualization of the carotid sheath and RLN in the tracheoesophageal groove may be obstructed by prominent thymic tissue. In non-survival studies, the thymus can be easily removed for better identification of deep neck structures. In survival studies, it cannot be resected due to risk of hypocalcemia, but instead must be retracted laterally.
In recent years there has been an explosion in the development of innovative surgical technologies and an emphasis on the development of minimally invasive approaches. As a result, there is a demand for reliable and realistic surgical models to appropriately evaluate the safety, feasibility, and functionality of these novel designs. Cadaveric and live animal studies serve a critical purpose in surgical research, particularly in the evaluation of surgical equipment, techniques, and approaches. Live animal models provide realistic tissue handling, bleeding, functional nerve stimulation and movement related to breathing, effectively replicating the surgical environment. In-vivo studies are less constrained than any other experimental model and are critical in preclinical research methods.
The current anatomic study of the porcine thyroid was performed in an open fashion to characterize the anatomic structures and relationships entirely. The anatomy described in this study can be applied to minimally invasive techniques. Understanding the differences between the anterior neck and thyroid anatomy of the porcine model and that of a human is critical to the design and interpretation of studies utilizing this animal model.
The porcine thyroid model described in this work can serve as a model to translate engineering technologies from the lab to a realistic pre-clinical scenario. Defining standard surgical setup and steps may bring uniformity to future research that permits more robust comparison and study. In the future, thyroid surgery could potentially be conducting using semi-autonomous technologies.[13] We anticipate that this realistic porcine surgical model will allow for further development and testing of such technologies.