Normal radiological anatomy of thyroid cartilage in 600 Chinese individuals: implications for anterior cervical spine surgery

Anterior cervical (AC) surgery is associated with numerous and frequent complications that can be attributed to inaccurate positioning, inadequate exposure, and the use of excessive traction [1‐3]. Attempts have been made to reduce the risk of complications in recent years. This has generally been achieved by improving body-surface positioning [1] and by evaluating the impact of the thyroid cartilage on intraoperative retraction [4, 5], as well as exploring hypopharynx/esophagus protective mechanisms.

Preoperative positioning in anterior cervical approach surgery generally utilizes anatomical structures such as the angle of the mandible, hyoid bone, thyroid cartilage, cricoid cartilage, and carotid artery nodule [1, 6]. For example, approximately 70% of the mandibular angle is located at the interval between C2 and C3 [1], while the majority of cervical decompression surgeries are located below the C4 level [7]. Therefore, the angle of the mandible is not suitable for the majority of positions in anterior cervical approach surgery. The thyroid cartilage is closely associated with the cervical vertebra in terms of anatomy. It is located at the C4–C5 level, serves as a prominent anatomical landmark, and is therefore convenient for pre-surgical positioning in the anterior cervical approach.

Additionally, to expose the vertical vertebral level for surgery, significant retraction pressure may be exerted on the thyroid cartilage and posterior pharyngeal wall or esophagus. Wire retractor tension should thus be increased during surgery due to the great thyroid cartilage transverse diameter [4]. However, Han et al. [5] found that tension outside the thyroid cartilage would not increase the intra-esophageal pressure. Additionally, the posterior border of thyroid cartilage (PBTC) at the time of traction could protect the pharyngoesophageal (PE) wall [4]. However, such a standpoint requires further support from thyroid cartilage related-anatomical data.

Few studies in the literature have summarized the distribution of the transverse diameter of the superior and inferior border planes of the thyroid cartilage in addition to the relative cervical vertebra level [8, 9]. This study aimed to measure the neck computed tomography (CT) images of healthy adults to obtain thyroid cartilage or cricoid cartilage imaging parameters to assist in preoperative positioning, to predict the difficulty of intraoperative exposure and retractor pulling, and to verify that the thyroid cartilage protected the PE wall of the anterior cervical approach.

In this study, we have presented normative values for D1, D2, and D3 as well as V1, V2, and V3. To the best of our knowledge, these values have not been reported in any previous studies. In addition, we found that SBTC is most often at C4 in women and C4/5 in men, IBTC is most often at C5 in women and men, and IBCC is most often at C6 in women and C6 or C6/7 in men in Chinese (Table 6). The results of our work are generally in good agreement with previous studies [1, 9, 11]. Contemporary anatomy texts indicate that the upper border of the thyroid cartilage is at C3/4 or C4 in the USA [11]. A more recent CT study of over 52 asymptomatic New Zealanders found that the vertebral level corresponding to the upper border of the thyroid cartilage was sexually dimorphic and was most often at C4 in women and C5 in men. The inferior border of the cricoid cartilage was also significantly different in women and men (C6 and C7, respectively) [9]. Additionally, we identified changes in these parameters with age, weight, height, and BMI (Table 5). We found that V1, V2, and V3 all increased with age. This may be due to the development and calcification of thyroid cartilage [12, 13]. However, it is curious that a higher BMI was correlated to an increase in D1, D2 and D3 but not correlated to V1, V2, and V3. We were also able to show that weight and height were correlated to increasing D1, D2, D3 and V1, V2, V3. This finding is not surprising given the increase in bone size with height and weight in the normal BMI range. However, the correlation coefficient between D1, D2, D3, and weight and height is higher than that between V1, V2, V3, and weight and height.

There is general agreement on the vertebral levels of key palpable landmarks in anterior cervical spine surgery. However, inaccurate preoperative skin positioning may lead to an enlarged intraoperative incision and range of exposure, resulting in an increase in both the operation time and tension of the intraoperative soft tissue traction [1]. Therefore, an appropriate skin incision can maximize the surgical field and avoid expanding the incision, which may otherwise affect the esthetics and increase the extent of the stripping, thus leading to an increased risk of postoperative anterior cervical hematoma and dysphagia due to elevated hemorrhage and continuous traction [3]. Kirschner wire and other fine metals are extensively used as a reference prior to skin cutting and decompression in anterior cervical approach surgery followed by C-arm perspective positioning. However, an acquaintance with the related anatomical positioning markers close to the cervical level contributes to a reduction in repeated perspective adjustments, thus saving a great deal of time and avoiding an increase in radiation exposure.

Preoperative positioning in anterior cervical approach surgery generally utilizes anatomical structures such as the angle of the mandible, hyoid bone, thyroid cartilage, cricoid cartilage, and carotid artery nodule [1, 6]. The mandibular angle is located at the interval between C2 and C3 [1], while the majority of cervical decompression surgeries are located below the C4 level [7]. Therefore, the angle of the mandible is not suitable, although the distance from it to the surgical level can be measured on preoperative X-ray films. The thyroid cartilage, however, is mostly located at the C4–C5 level according to the present study and is more or less fixed in front of the cervical vertebrae apart from when swallowing or intonation occurs. The cartilage does not easily move with body position but is easily accessible compared with the hyoid bone and carotid artery nodule. Additionally, the anatomical location of the thyroid cartilage angle is more obvious in men due to the presence of the Adam’s apple. Therefore, preoperative X-rays allow for the location of almost all anterior cervical approach surgical levels. Therefore, the corresponding surgical level can be judged rapidly based on the position of the SBTC, IBTC, and IBCC prior to skin cutting and decompression.

The thyroid cartilage or trachea and the posterior pharyngeal wall or esophagus in front of the surgical level should be pulled to one side during surgery. Therefore, a wide thyroid cartilage will often add to the difficulties incurred in exposing the surgical level. The thyroid cartilage is an important barrier in anterior cervical approach surgery. Chio et al. [4] discovered through intraoperative ultrasound positioning that the thyroid cartilage rarely rotated along with the traction of the retractor, while translation of the thyroid cartilage with the posterior pharyngoesophagus was common. The traction of the retractor should be increased when pulling a larger thyroid cartilage to sufficiently expose the surgical level. Nevertheless, increasing intraoperative traction may lead to cervical hematoma and soft tissue swelling. However, Han et al. [5] conducted a study in 2015 in which the tension of the retractor at the time of pulling and the pressure in the esophagus were measured. Moreover, it was verified that increasing the tension outside the thyroid cartilage would not increase the pressure in the esophagus. In contrast, translation of a larger thyroid cartilage with the posterior pharyngoesophagus can be seen at the time of traction, which can protect the PE wall [4].

The exact pathogenesis of cervical esophageal injury remains unknown [7]. Although the incidence of PE injury has steadily increased [14‐16], PE injury is unfortunately hard to diagnose even with advancements in radiography [17]. Thus, the prevention of injury is most important during anterior cervical spine surgery. Direct injury with an instrument or retractor during dissection is a possible cause of PE injury [15, 18, 19]. Our results (Tables 3 and 4) suggest that the average transverse diameter of the SBTC and IBTC or IVCC gradually decreased in healthy adults from top to bottom, while that of the corresponding cervical vertebral body or intervertebral disc gradually increased. Therefore, the traction of the retractor should be increased when conducting high-level anterior cervical approach surgery to sufficiently expose the surgical level, which will therefore lead to cervical hematoma formation and soft-tissue swelling [3]. In contrast, the required traction of the retractor when performing lower cervical surgery is small since the coverage width of the thyroid cartilage gradually decreases; however, this may lead to direct pressure being placed on the esophagus. According to previous studies [15, 16], PE wall injury most often appears in lower cervical surgery (C5–C7), which may be related to a susceptibility to direct intraoperative traction [18, 19]. The anatomical location of the thyroid cartilage was further identified in this study, and it was found that the thyroid cartilage was mostly located above the C5 level (66.8%), while the trachea was mostly located below the C5 level. Such an anatomical feature provides a basis for the view that the thyroid cartilage does not protect the PE wall in lower cervical surgery (C5–C7). Therefore, when operating below the C5, special care should be taken during retraction to prevent injury of the esophagus wall.

This study does, however, acknowledge some weaknesses. Since this is a retrospective study, it is not possible to evaluate the patient face to face. Our sample size is considerable compared to other similar studies [8, 9]. The selection of individuals was only based on patient complaints and history records, and it is possible for these data to be skewed by outliers. A second limitation of our study is the relatively high proportion of elderly individuals enrolled. This occurred because age was not used as a screening criterion when cases were selected. While the aged-predominant nature of the population might limit our ability to draw conclusions about age variability, we do not believe it will limit the relevance of our work to anterior cervical spine surgery. Standardized head positions in CT scans are also different among different studies [9], but we have excluded patients with CT scan planes that are not consistent with the measurement plane because of incorrect head positions. Finally, it is important to acknowledge that the population in this study is drawn from an urban, university hospital in East China. Surgeons whose practice setting is significantly different from ours must be aware of the potential for ethnic- and race-based differences in normative values.

In summary, we present normative values for the thyroid cartilage and cricoid cartilage at three planes of SBTC, IBTC, and IBCC in Chinese individuals. The individual and gender differences in the location of the thyroid cartilage and the size of the thyroid cartilage and the cricoid cartilage provided an anatomical basis to help localize the skin incision, to predict the difficulty of intraoperative exposure and retractor pulling, and to verify that the thyroid cartilage protects the PE wall.