Biomechanical Evaluation of Tracheal Needle Puncture Forces: Comparative Analysis of Annular Ligaments and Tracheal Cartilage

Tracheostomies are one of the most commonly performed medical interventions in the U.S. due to their crucial role in facilitating respiration [1]. The procedure involves creating an opening in the anterior trachea as an alternative airway passage [2]. Although tracheotomies are one of the earliest recorded surgical procedures [3], complications associated with tracheostomies are still poorly understood. Recent studies have indicated that up to 31% of tracheostomies performed in emergencies resulted in late complications [4] with some studies suggesting a complication rate as high as above 50% [5]. Although the incidence of serious complication is low, due to the large volume of procedure performed annually, post-tracheostomies complications represent a significant burden on patients and on the healthcare system [6], highlighting the need for enhanced comprehension and optimization of the tracheotomy procedure. In this context, airway biomechanical data has proven crucial. Clinically relevant uses for airway biomechanical tissue data today are diverse, ranging from airway and disease modeling to clinical and surgical applications such as airway reconstruction and access. These informative data aid in medical device development [7], surgical simulation [8], procedural teaching/optimization [9,10], and bio-engineering tracheal replacements [11]. Specific to needle injection into the trachea, needles are routinely passed transcervically for serial steroid injections of the trachea/subglottic, for transtracheal anesthesia, and as part of procedures such as percutaneous tracheotomy [12,13]. Unlike when entering the airway during an open surgical procedure, these procedures are performed without direct visualization of the trachea to ensure that the needle passes between tracheal rings. In addition, the angle of approach of the needle is frequently not perpendicular to the tracheal wall. Tracheal puncture data is especially relevant in regard to percutaneous tracheotomy (PCT), which has grown in popularity and frequency over the past few decades [14]. PCT can readily be performed at the bedside by an intensivist (obviating the need for a surgeon and/or taking valuable and limited operating room time) has been shown to be as effective as open tracheostomy in many patients [15], and is further improved by the use of ultrasound guidance [16]. One complication of tracheostomy is tracheal stenosis at the site or above the tracheostomy, which may result from deformity of the tracheal rings from the procedure [17–19].

Much of the biomechanical tissue data currently available focuses on the viscoelastic or pseudo-elastic properties of different types of airway tissues in either human or animal models, including the stiffness of human tracheal cartilage [20], the material behavior of human trachealis muscle, mucosa and submucosa membrane and adventitia membrane [21], porcine [22], and human [23] trachea compliance. Previous investigations regarding puncture forces include the forces required to pierce an excised human cricothyroid membrane (scalpel) and tracheal annular ligaments (needle) [24] and puncture forces for handheld percutaneous tracheostomy devices when used to place a cricothyroid cannula in human cadavers and anesthetized canines [25]. However, no studies have looked at the puncture force for tracheal cartilage rings themselves, as prior work focused solely on the force to puncture the annular ligament between the tracheal rings. Additionally, there have been no studies which examine whether the puncture force changes depending on the angle at which the needle is inserted. These factors are relevant for PCT, as even under ultrasound guidance it can be difficult to place a needle in the tracheal annular ligament without hitting a tracheal cartilage ring, and the puncture angle often is not at 90 deg when PCT is performed.

The purpose of this study is to determine the puncture force for both tracheal cartilage rings and annular ligaments at clinically relevant angles of insertion. This data will be relevant for airway access interventions such as PCT (especially under ultrasound guidance) but may also be useful for in office injection procedures and the development of new medical devices and simulators.

Excised adult porcine airways (larynx and trachea) were obtained from a local slaughterhouse, a total of 15 tracheas were employed in this study. Porcine tracheas were selected as the experimental model due to their exceptional similarities in terms of biomechanical characteristics and properties to the human trachea [26–28]. Each airway was inspected visually for structural damage. Inclusion criteria for the study were macroscopically undamaged airways, presenting no macroscopic defects and no sign of disease. The airways were stored at −20 °C until test day. Prior to testing, the tracheas were first thawed at 4 °C for 2 h and then allowed to come to room temperature. Each sample was prepared for testing by separating the larynx from the trachea at the cricoid using sharp dissection (see Fig. 1). Next, the tracheas were cleaned of adventitia and excess tissue using a clean surgical blade and a combination of blunt and sharp dissection to expose the tracheal rings and annular ligaments, and then trimmed distally to a length of approximately 10 cm to fit on the stage of the testing device (Fig. 1(c)). Two sutures were placed at the end of each trachea, one around the anterior portion of the most proximal tracheal ring (either the first or second tracheal cartilage ring) and the second around the anterior portion of the most distal remaining tracheal cartilage ring to stimulate the trachea tension in vivo. The trachea was placed posterior side down on a round plastic tray mounted to the stage of the testing device. The sutures tensions were adjusted to put the anterior of the trachea in mild extension in order to mimic in vivo conditions for percutaneous tracheostomy and enhance accessibility of annular ligaments. Instant Adhesive (Henkel, Loctite 4013) was used to secure the posterior trachea to the plastic tray to prevent it from rolling during testing. We three-dimensional printed (Ultimarker 3S) ramps to hold the trachea at either 15 or 30 deg for the angled trials.

The puncture force tests were performed using a MACH-1 Biomechanical System (Biomomentum cat# V500CSS), consisting of a load frame with one vertical and two horizontal stages with stepping motors. A 100 N single-axis load cell (Biomomentum cat# MA999) was attached to the vertical stage and calibrated before each testing session. A needle holder (Biomomentum cat# MA690) loaded with a new, unused 18-gauge needle (BD, cat# 305196) was then attached to the load cell for each puncture test (see Fig. 1(c)). The Mach-1 Motion and Mach-1 Analysis software were used to control and record the movements of the stages and puncture forces. A Basler camera (acA1440-220uc) with a lens (Fujinon, HF12.5HA-1S) was used to record videos.

For each trachea, two series of data were collected. Starting at the proximal end of the trachea, each annular ligament was punctured, and the puncture force data collected. After the most distal annular ligament was punctured, this process was then repeated with the tracheal cartilage rings, again starting with the most proximal ring (excluding the one with suture around it) and continuing to the most distal ring. The first set of tracheas were punctured while placed flat (at 0 deg), and the perforating needles were setup to approach the tissue at 90 deg. The second and third sets of tracheas were place on a custom three-dimensional printed ramp (Fig. 1(c)), resulting in an increase in incidence angle (respectively of 15 deg and 30 deg). This adjustment resulted in a needle insertion direction away from the head, meaning that the needle was angled caudally, away from the face, as commonly practiced (as depicted in Fig. 1(a)). The trachea was kept hydrated during this process using saline. For each puncture, a new 18-gauge needle was placed on the needle holder attachment under the load cell. To ensure proper alignment, the needle was manually positioned directly over the center of the annular ligament or tracheal cartilage ring, following the standard practice observed in tracheostomy procedures. Subsequently, a programed sequence initiated by Mach-1 Motion was executed. This sequence involved the gradual advancement of the needle until it made contact with the surface of the specimen. To maintain consistency and enable meaningful comparisons across all groups, the needle was then retracted 5 mm from the tissue surface. Next, the needle was swiftly inserted into the airway, with a speed of 4 mm per second, reaching a depth of 15 mm. A pause of 5 s followed before the needle was removed from the airway, with a speed of 2 mm per second (see Fig. 2(d)). The force on the load cell and stage motion was simultaneously recorded continuously during this sequence at 100 Hz. For each puncture, the system yielded a TXT file containing the values of the force register by the load cell (in the Z axis), the position of the needle (in the Z axis) and the time coordinate. Notes were taken as to whether the needle punctured the intended target (annular ligament versus tracheal cartilage ring) and whether the needle successfully punctured completely into the airway or caused tracheal deformation without complete puncture. All results of all punctures tests were imported into R (version 4.2.1) for statistical analysis and data visualization.

Each puncture test was analyzed individually and parsed for the maximum puncture force. Using the extracted maximum puncture force, we determined the average maximum puncture force for each group and conditions and then we calculated the significance of a difference between the cartilage and annular ligament of each group using an ANOVA followed by a Tuckey's Honestly Significant Difference (Tuckey's HSD) posthoc test for pairwise comparisons. Next, we computed the Kruskal–Wallis rank sum test to determine whether there are significant variations in the maximum puncture force across different angle of puncture within the cartilage annular ligament. Finally, we compared the average maximum puncture force in cartilage to the maximum puncture force in annular ligament.

Representative graph depicting force versus time curves of a tracheal annular ligament and a cartilage puncture are shown in Fig. 2(a). We evaluated a total of 15 Porcine tracheas divided into three different experimental groups. Because of change in tissue properties in proximal-distal axis [29] and the location of the typical tracheostomy tube [2] (in part due to the fear of hemorrhage from the isthmus), thus we only considered the top three proximal annular ligaments and cartilage in our analysis. The summary statistics of the puncture test are shown in Table 1.

The tracheal annular ligaments mean puncture forces were 1.81 ± 1.55 N (range 0.38–4.7) flat 0 deg, 1.09 ± 1.06 N (range 0.34–4.56) at 15 deg and 1.76 ± 1.62 N (range 0.42–5.14) at 30 deg. We found no significant difference in the observed mean tracheal annular ligaments amongst groups, in other words the angle of puncture appears to not significantly affect the maximum puncture force in our dataset available in the Supplementary Materials on the ASME Digital Collection. Interestingly, we do report a high standard deviation relative (SD) to the mean (Table 1). High standard deviation observed is in line with previous studies [24]. The tracheal cartilage ring mean puncture force was evaluated to be 4.55 ± 1.17 N (range 3.11–6.48, 5.36 ± 0.62 N (range 4.01–6.22) and 5.33 ± 0.92 N (range 4.13–7.33), respectively, for flat 0 deg, 15 deg, and 30 deg. Once again we found no significant difference in the average mean puncture at different angle; however, we observe a much smaller SD in cartilage punctures (Fig. 3).

Next, we compared the mean puncture force in each group. In every experimental group, the annular ligament puncture forces were significantly lower than the cartilage puncture force.

Tracheal punctures are common surgical and medical intervention for airway management; however, little is known on the mechanics of tracheal punctures, specifically their long-term consequences on the structural and biomechanical properties of the trachea. Evidences have implicated post-tracheostomy complications [30] as a potential contributing factor to pathologic condition impaired regeneration [31]. The data in this study present a unique in-depth analysis of the differences in puncture force criteria for perforating the cartilage and annular ligaments in porcine tracheas. The primary findings of this study highlight notable observations. First, the investigation revealed that manipulating the angle of penetration by up to 30 deg, deviating from the normal line, did not yield a statistically significant impact on the force required for piercing the porcine tracheal cartilage or the porcine annular ligaments. Second, a discernible disparity was observed in the mean puncture force between the cartilage and the annular ligament, with the cartilage exhibiting significantly higher puncture force (Fig. 4).

Our results are comparable to those of DeSchmidt et al. [24] in human tracheas. Our higher mean puncture force of the Annular Ligaments may be attributed to the difference in needle size (16G versus 18G). These data suggest that needle diameter impacts puncture force [32], though the specifics may remain to be worked out. In addition, our selection of an 18G needle closely reflects the most commonly employed needle for many transtracheal procedures in clinical practice. The large standard deviation in the mean annular ligament puncture forces, as also noted in De-Schmidt et al., may be attributed to cartilage structure variation and the challenges associated with successfully spreading the cartilage rings of the trachea during the puncture test. Due to their shape, the cartilage ring when not sufficiently spread apart, may overlaps with the annular ligaments and during annular ligament puncture the needle could travel through part of the neighboring cartilage. Thus, special care must be taken to ensure that the cartilage rings are sufficiently spread apart. We suspect the reason the angle of puncture did not significantly impact the puncture force is in part because the needle bevel makes it is easier to puncture between the cartilage in high angle of incidence punctures.

Histologically under the microscope, tracheal cartilage and the annular ligaments are distinct and can easily be distinguished; however, macroscopically, even under direct visualization of the tracheal rings ex vivo, we found it challenging to ensure proper puncture of the annular ligament. Our data suggest that based on puncture force alone, we could discriminate between cartilage and annular ligament punctures and thus locate with higher precision and confidence the site of puncture even with limited visual access. This is clinically relevant as cartilage injury is common during PCT [33] and has been proposed to contribute to post-tracheostomy complications such as tracheal stenosis and risk of necrosis of the cricoid cartilage [31]. Due to the large volume of tracheotomies performed, the high resources expenditure associated with the intervention, and outcome variability [34], consideration should be dedicated to the optimization and refinement of the procedures. We believe our study provides important insights for physicians, trainees and medical devise engineers by exploring the foundation for biomechanical feedback during the tracheotomy procedure. Furthermore, the critical difference in needle placement may be further exacerbated in the case of PCT because subsequent dilation is performed over a wire at the site of entrance into the airway. In the case of annular ligament penetration, this is anticipated to cause the cartilaginous rings to be forced apart from each other as the dilation proceeds and a tracheostomy tube is placed. Comparatively, if the initial puncture is through the cartilage, subsequent dilation could force the cartilage to break and be forced into the airway, leading to future airway stenosis.

There were limitations in this study. The study is limited by the fact that the tracheas were studied ex vivo, thus the derived puncture data does not account for the additional resistance due to skin penetration, soft tissue and fat in the neck, and adventitial attachments of the tracheal to surround tissues that may impact resistance of the cartilaginous superstructure to deformation. Additionally, the forces measure in this study were done in a static framework, although in practice great care is taken to position the patient and the physician, the hands of the physician and the neck of the patient may move relative to each other. It would be necessary to follow up this study with a handheld puncture device study to verify that the puncture force difference still significantly diverges. Furthermore, there remains the possibility that the process of freezing tissue resulting in microscopic tissue property changes within the cartilage or annular ligament. In addition, our analysis only focused on the proximal three data point of each trachea, we posit that the first 3 tracheal rings and annular ligaments should have a relatively similar skin and the related structures makeup due to their proximity.

In conclusion, we observed that tracheal cartilage punctures forces are significantly higher than annular ligament puncture in a wide range of angle.

• Funding provided by the United State Department of Defense (DOD) DS21RES34 from the USAF 59th Medical Wing Office of Chief Scientist (No. 100014044; Funder ID: 10.13039/100000005). The views expressed are those of the author(s) and do not reflect the official views or policy of the Department of Defense or its Components.