**3.2. "A layer of thymus, blood vessels and inferior parathyroid gland" in CND**

Because of the application of "meticulous capsular dissection," how to preserve the parathyroid gland in thyroidectomy has become a minor problem. Because IPTGs enjoy a more variable position in the adult neck and locate in the area of central neck lymph node dissection, how to preserve IPTG during CND is a major problem.

Some methods are recommended to identify and preserve the IPTG [13, 14]. For example, IPTG is superficial to RLN coronal plane and does not dissect the triangular region in order to protect laterally based blood supply of the IPTG (**Figure 6**). However, actually it is not easy to practice.

**Figure 5.** "Meticulous capsular dissection." The distal branches of the inferior thyroid artery medial to the parathyroid at the level of the thyroid capsule, are identified and controlled. Attie's drawing (left); intraoperative view (right); white arrow: Inferior parathyroid gland; green star: thyroid.

(in the second instance of dissection). Therefore, the inferior parathyroid gland can be actually classified into exposure type and unexposure type according to whether the IPTG is identified and preserved during the thyroidectomy (at the first instance). As to the unexposure type, the

**Figure 4.** Intraoperative views of inferior parathyroid gland according Zhu's classification. Type A1 (left upper), A2 (right upper), A3 (left down) and B2 (right down) are included. White arrow: Inferior parathyroid gland; green star:

**Figure 3.** Classification of inferior parathyroid gland (by JQ Zhu). Type A (close contact) includes A1 (planar attachment), A2 (embedded attachment) and A3 (intra-thyroid); type B (non-close contact) includes B1 (around thyroid), B2 (intra-

thymus) and B3 (blood supply from thymus or mediastinum).

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thyroid; blue star: thymus.

**Figure 6.** Some methods recommended to identify and preserve the inferior parathyroid gland (IPTG). IPTG superficial to recurrent laryngeal nerve coronal plane (left); triangular region left undissected to prevent disruption of laterally based vascular supply of IPTG (right).

**Figure 7.** "A layer of thymus-blood vessel-inferior parathyroid gland (TBP)." The thymus, the inferior parathyroid gland, and the blood vessels connecting them are located in one layer. This layer covers the common carotid artery (innominate artery), the trachea, and the area of paratracheal lymph nodes between them. Diagram of this concept (left); intraoperative views of "a TBP layer" preserved after central neck dissection. SPTG, superior parathyroid gland; ITA, inferior thyroid artery; MTV, middle thyroid vein; IPTG, inferior parathyroid gland; TM, thymus; ITV, inferior thyroid vein; 1, trachea; 2, common carotid artery; 3, recurrent laryngeal nerve; 4, Thymus; 5, inferior parathyroid gland; 6,

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**Figure 8.** Schematic view of primitive pharynx of an 8- to 10-mm embryo. The inferior parathyroid glands are derived from the dorsal part of the third pharyngeal pouch, and the thymus arises from the ventral part of the third pharyngeal pouch.

branches of inferior thyroid vein; 7, a branch of inferior thyroid artery.

### *3.2.1. The meanings of "a layer of TBP"*

To solve this main problem, "a layer of TBP" was firstly put forward by Lei Xie in 2014 [15]. This new concept has two meanings: (1) the thymus, IPTG and blood vessels connecting them are located in one layer; (2) the layer covers the common carotid artery (innominate artery), the trachea, and the area of paratracheal lymph nodes between them (**Figure 7**).

### *3.2.2. Theoretical basis of "a layer of TBP"*

Embryologically, the IPTGs are derived from the dorsal part of the third pharyngeal pouch, and the thymus arises from the ventral part of the third pharyngeal pouch. As the IPTGs and the thymus migrate together toward the mediastinum, they eventually separate. In most cases, the inferior parathyroid glands become localized near the inferior poles of the thyroid, and the thymus continues to migrate toward the mediastinum (**Figure 8**) [16]. In an anatomical study of the adult thymus, Di Marino et al. [8] described the true sheath of the thymus and its relative structures in detail. In the cervical region, adhesion between the thymic sheaths and thyroid is via the thyrothymic ligaments, in which the superior vascular pedicle of the thymus is contained. The superior vascular pedicle mainly includes the superior thymic artery arising from the ITA as well as the inferior and median thyroid veins, which also supply blood to the IPTG. In addition, from the midline cervicothoracic sagittal section, the thymus, blood vessels within the thyrothymic ligament and the posterior layer of thyroid sheath are in the same plane (**Figure 9**), which could be regarded as an anatomical basis for the TBP layer.

#### *3.2.3. How to practice "a layer of TBP" during CND*

According to the ATA guidelines, bilateral CND involves removal of the prelaryngeal, pretracheal, and both the right and left paratracheal nodal basins; and unilateral CND involves Inferior Parathyroid Gland Preservation In Situ during Central Neck Dissection for Thyroid… http://dx.doi.org/10.5772/intechopen.78636 23

**Figure 7.** "A layer of thymus-blood vessel-inferior parathyroid gland (TBP)." The thymus, the inferior parathyroid gland, and the blood vessels connecting them are located in one layer. This layer covers the common carotid artery (innominate artery), the trachea, and the area of paratracheal lymph nodes between them. Diagram of this concept (left); intraoperative views of "a TBP layer" preserved after central neck dissection. SPTG, superior parathyroid gland; ITA, inferior thyroid artery; MTV, middle thyroid vein; IPTG, inferior parathyroid gland; TM, thymus; ITV, inferior thyroid vein; 1, trachea; 2, common carotid artery; 3, recurrent laryngeal nerve; 4, Thymus; 5, inferior parathyroid gland; 6, branches of inferior thyroid vein; 7, a branch of inferior thyroid artery.

*3.2.1. The meanings of "a layer of TBP"*

based vascular supply of IPTG (right).

22 Thyroid Disorders

*3.2.2. Theoretical basis of "a layer of TBP"*

*3.2.3. How to practice "a layer of TBP" during CND*

To solve this main problem, "a layer of TBP" was firstly put forward by Lei Xie in 2014 [15]. This new concept has two meanings: (1) the thymus, IPTG and blood vessels connecting them are located in one layer; (2) the layer covers the common carotid artery (innominate artery),

**Figure 6.** Some methods recommended to identify and preserve the inferior parathyroid gland (IPTG). IPTG superficial to recurrent laryngeal nerve coronal plane (left); triangular region left undissected to prevent disruption of laterally

Embryologically, the IPTGs are derived from the dorsal part of the third pharyngeal pouch, and the thymus arises from the ventral part of the third pharyngeal pouch. As the IPTGs and the thymus migrate together toward the mediastinum, they eventually separate. In most cases, the inferior parathyroid glands become localized near the inferior poles of the thyroid, and the thymus continues to migrate toward the mediastinum (**Figure 8**) [16]. In an anatomical study of the adult thymus, Di Marino et al. [8] described the true sheath of the thymus and its relative structures in detail. In the cervical region, adhesion between the thymic sheaths and thyroid is via the thyrothymic ligaments, in which the superior vascular pedicle of the thymus is contained. The superior vascular pedicle mainly includes the superior thymic artery arising from the ITA as well as the inferior and median thyroid veins, which also supply blood to the IPTG. In addition, from the midline cervicothoracic sagittal section, the thymus, blood vessels within the thyrothymic ligament and the posterior layer of thyroid sheath are in the same plane (**Figure 9**), which could be regarded as an anatomical basis for the TBP layer.

According to the ATA guidelines, bilateral CND involves removal of the prelaryngeal, pretracheal, and both the right and left paratracheal nodal basins; and unilateral CND involves

the trachea, and the area of paratracheal lymph nodes between them (**Figure 7**).

**Figure 8.** Schematic view of primitive pharynx of an 8- to 10-mm embryo. The inferior parathyroid glands are derived from the dorsal part of the third pharyngeal pouch, and the thymus arises from the ventral part of the third pharyngeal pouch.

**Figure 10.** Lateral margin dissection approaches in paratracheal lymph node dissection. The blue line shows the traditional approach to directly expose the common carotid artery (CCA), which could lead to the inferior parathyroid gland injury due to the destruction of the TBP layer. The blue line shows the new dissection approach based on the TBP layer (layer of thymus-blood vessel-inferior parathyroid gland) concept. The thymus (TM), inferior thyroid blood vessels and inferior parathyroid gland (IPTG) are lifted upwards and laterally, exposing the CCA (innominate artery)

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**Figure 11.** The relationship between the TBP layer, inferior thyroid artery (ITA) and recurrent laryngeal nerve (RLN). A is ITA branches lateral to RLN; B is ITA branches medial to RLN. MTV, middle thyroid vein; IPTG, inferior parathyroid gland; TM, thymus; ITV, inferior thyroid vein. Red cycle means that the dissection in this region (between the cricoid

cartilage and ITA level) should be emphasized, especially when the concept of "a layer of TBP" is performed.

underneath. TR, trachea; ITV, inferior thyroid vein.

**Figure 9.** Midline cervicothoracic sagittal section. The thymus, blood vessels within the thyrothymic ligament and the posterior layer of thyroid sheath are in the same plane, which could be regarded as an anatomic basis of the "TBP layer". 1, thyroid isthmus; 2, superficial layer of cervical fascia; 3, pretracheal cervical fascia; 4, brachiocephalic trunk; 5, pretracheal space; 6, left brachiocephalic vein; 7, sternothyroid muscle; 8, anterior wall of thymic sheath; 9, thyropericardial layer; 10, serous pericardium; 11, anterior interpleural ligament; 12, thymus; 13, subthymic fatty tissue.

removal of the prelaryngeal, pretracheal and one paratracheal nodal basins. In addition, "a layer of TBP" is mainly applied in lateral margin dissection of paratracheal nodal basin.

During the paratracheal lymph node dissection, the medial dissection margin is defined using electrocautery along the tracheal lateral wall from Berry's ligament to the brachiocephalic vessels. Lateral margin dissection aims to identify and preserve the TBP layer first (**Figure 10**) rather than directly exposing the common carotid artery. The thymus, inferior thyroid blood vessels and their branch stumps are regarded as reference points, with the fibrofatty tissue removed by electrocautery. During this process, the TBP layer is slowly identified and lifted upwards; the common carotid artery (innominate artery) beneath the layer is exposed. The medial border of the common carotid artery is dissected down to the prevertebral fascia. The TBP layer and the common carotid artery are retracted laterally, whereas the trachea is retracted medially, exposing the paratracheal compartment. The recurrent laryngeal nerve (RLN) is freed from the fibrofatty tissue and retracted laterally. The envelope of level VI lymph nodes is then retracted medially and excised en bloc [17].

The relationship between the TBP layer, inferior thyroid artery (ITA) and RLN can be further clarified. Because the TBP layer is superficial to the common carotid artery, whereas the ITA

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**Figure 10.** Lateral margin dissection approaches in paratracheal lymph node dissection. The blue line shows the traditional approach to directly expose the common carotid artery (CCA), which could lead to the inferior parathyroid gland injury due to the destruction of the TBP layer. The blue line shows the new dissection approach based on the TBP layer (layer of thymus-blood vessel-inferior parathyroid gland) concept. The thymus (TM), inferior thyroid blood vessels and inferior parathyroid gland (IPTG) are lifted upwards and laterally, exposing the CCA (innominate artery) underneath. TR, trachea; ITV, inferior thyroid vein.

**Figure 9.** Midline cervicothoracic sagittal section. The thymus, blood vessels within the thyrothymic ligament and the posterior layer of thyroid sheath are in the same plane, which could be regarded as an anatomic basis of the "TBP layer". 1, thyroid isthmus; 2, superficial layer of cervical fascia; 3, pretracheal cervical fascia; 4, brachiocephalic trunk; 5, pretracheal space; 6, left brachiocephalic vein; 7, sternothyroid muscle; 8, anterior wall of thymic sheath; 9, thyropericardial layer; 10, serous pericardium; 11, anterior interpleural ligament; 12, thymus; 13, subthymic fatty tissue.

removal of the prelaryngeal, pretracheal and one paratracheal nodal basins. In addition, "a layer of TBP" is mainly applied in lateral margin dissection of paratracheal nodal basin.

During the paratracheal lymph node dissection, the medial dissection margin is defined using electrocautery along the tracheal lateral wall from Berry's ligament to the brachiocephalic vessels. Lateral margin dissection aims to identify and preserve the TBP layer first (**Figure 10**) rather than directly exposing the common carotid artery. The thymus, inferior thyroid blood vessels and their branch stumps are regarded as reference points, with the fibrofatty tissue removed by electrocautery. During this process, the TBP layer is slowly identified and lifted upwards; the common carotid artery (innominate artery) beneath the layer is exposed. The medial border of the common carotid artery is dissected down to the prevertebral fascia. The TBP layer and the common carotid artery are retracted laterally, whereas the trachea is retracted medially, exposing the paratracheal compartment. The recurrent laryngeal nerve (RLN) is freed from the fibrofatty tissue and retracted laterally. The envelope of level VI

The relationship between the TBP layer, inferior thyroid artery (ITA) and RLN can be further clarified. Because the TBP layer is superficial to the common carotid artery, whereas the ITA

lymph nodes is then retracted medially and excised en bloc [17].

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**Figure 11.** The relationship between the TBP layer, inferior thyroid artery (ITA) and recurrent laryngeal nerve (RLN). A is ITA branches lateral to RLN; B is ITA branches medial to RLN. MTV, middle thyroid vein; IPTG, inferior parathyroid gland; TM, thymus; ITV, inferior thyroid vein. Red cycle means that the dissection in this region (between the cricoid cartilage and ITA level) should be emphasized, especially when the concept of "a layer of TBP" is performed.

enters the central neck compartment posterior to the carotid sheath, branches of the ITA need to traverse paratracheal fibrofatty tissue anteriorly to the TBP layer. In general, these branches of the ITA abut against the carotid artery medially and run into the TBP layer; therefore, the TBP layer, carotid artery and ITA branches can easily be retracted laterally, allowing en bloc excision of the paratracheal fibrofatty tissue (**Figure 11A**). The alternative situation is that the ITA branches are not very close to the carotid artery, and the RLN traverses between them (**Figure 11B**). For completeness of dissection and RLN preservation, it is suggested that the paratracheal dissection should be divided into two parts according to the level of the ITA: a dissection cranial to the ITA (between the cricoid cartilage and ITA level) and one caudal to the ITA (between the innominate artery and ITA level).
