Custom laser-cut Silastic^® implants for type I thyroplasty in a preclinical model

Abstract

This study aimed to produce customized silicone elastomer implants of varied size and shape for optimization of type I thyroplasty procedures in a rabbit model. Computer-aided design (CAD) models of different implant designs were designed and used to program laser-cutting of a medical grade Silastic^® sheet. Laser-cut implants were produced rapidly and cost-efficiently. Surgical implantation demonstrated vocal fold medialization and phonation in five test subjects. This technique may provide a low-cost alternative or adjunct method to hand-carving or commercial implants.

Introduction

Unilateral vocal fold paralysis (UVFP) is a condition characterized by impaired nerve function of the muscles of the vocal folds (VF). This impairment frequently causes a gap between the VF, which results in dysphonia and can negatively impact quality of life, employment, and productivity [1]. UVFP can cause dysphagia, which puts individuals at risk for aspiration pneumonia, malnutrition, or dehydration [1]. The standard of care for persistent UVFP is type I thyroplasty. Type I thyroplasty has been proven safe and effective for improving voice outcomes as well as reducing aspiration and improving deglutition [2,3]. The procedure is reversible, and implants are customizable. Type I thyroplasty has demonstrated significant long-term improvements on vocal outcome measures such as stability of habitual intensity and maximum phonation time [4] and improves acoustic, aerodynamic, and perceptual voice outcomes [5].

Each patient’s laryngeal anatomy and tissue properties are unique and require a patient-specific approach to type I thyroplasty to attain optimal outcomes. Current best practice for individualized implants involves on-the-spot manual adjustments based on perceptual and qualitative assessments of the patient’s glottal space and voice quality [6]. Computational modeling to plan the surgical approach to UVFP will streamline the process of anatomically personalized implants with the aim to reduce the potential for early failures and complications.

The preclinical stage of research to develop a type I thyroplasty planning tool requires a method of producing implants of a precise size and shape made from medical-grade, bio-tolerable materials. Therefore, we used an iterative process to develop a standardized implant for this stage of the research process. Individualized implants may be further tailored to specifications informed by a computational model in the future.

Methods/Design

3-D modeling software (Tinkercad, 2019) was used to develop computer-aided design (CAD) models of implants in two shapes and two sizes, for a combined total of four implant types. We designed two implant shapes: S1, rectangular, and S2, tetrahedral with greater depth oriented toward the posterior portion of the implant. Each shape was designed in two sizes: D1, 1 × 1 × 2 mm and D2, 1 × 2 × 2 mm (Fig. 1. a.). Each implant had a shelf on the portion of the implant distal to VF placement to facilitate fit within the thyroplasty window. Implant size was scaled based on relative size differences between rabbit and human larynges. These parameters were input to a laser-printer with a 50W CO2 laser (Epilog Zing 24) at the University of Pittsburgh. Twelve implants of each size and shape (N=48) were produced to determine production speed. The precision was set to 500 dpi. Implants were vector-cut from a single sheet of medical-grade silicone elastomer (6 × 8 × .080in non-reinforced sheeting, Bentec Medical, Woodland, CA) at 50% power, 40% speed, and 5000 Hz.

Fig. 1. Development and Implementation Laser-cut silicone elastomer implants.

a. CAD representation of implants created using Tinkercad to create 4 distinct implant shapes and sizes. S1D1 (light blue): rectangular 1 mm × 1 mm × 2 mm; S1D2 (dark blue): rectangular 1 mm × 2 mm × 2 mm; S2D1 (light yellow): tetrahedral 1 mm × 1 mm × 2 mm; S2D2 (dark yellow): tetrahedral 1 mm × 2 mm × 2 mm. An Epilog Zing 24 laser-cutting used parameters from these CAD models to create implants from medical-grade silicone elastomer non-reinforced sheeting. b. Calipers were used to confirm dimensions of laser-cut implants and compare with CAD models for accuracy. Specific example demonstrates accuracy of width in a S1D1 rectangular 1 × 1 × 2 mm implant. c. ex vivo MRI image in axial view of unilaterally implanted rabbit vocal fold with S1D1 rectangular 1 × 1 × 2 mm implant. d. in vivo laryngoscopic image of unilateral implanted rabbit vocal fold with S1D1 rectangular 1 × 1 × 2 mm implant.

Five New Zealand white rabbits weighing between 2.5kg-4.0kg were used in this study. Humane animal care and use in accordance with the Animal Welfare Act and the NIH Guide for the Care and Use of Laboratory Animals was reviewed, approved, and monitored by the University of Pittsburgh’s Institutional Animal Care and Use Committee (protocol #21120467). A non-stimulated in vivo rabbit phonation model as previously described [7–9] was used to determine safety and physiological efficacy. Briefly, the cricoid and thyroid cartilages were drawn together with fixed sutures to narrow the glottal space in a phonatory configuration, and an incision was made in the trachea inferior to the larynx to pass humidified airflow through the glottis [10]. We performed type I thyroplasty using the Isshiki method [4,11] and placed randomly selected implant shapes and sizes, specifically, S1D1 (n=1), S1D2 (n=1), S2D1 (n=2), and S2D2 (n=1). Stroboscopic laryngoscopy was used to monitor perioperative laryngeal geometry. Larynges were harvested and 3D magnetic resonance imaging (MRI) was performed at 11.7 Tesla with an isotropic resolution set at 60 μm.

Results

Forty-eight implants were laser-cut from Silastic^® sheeting in less than one minute. Assessment of size and shape using calipers demonstrates reliability to the CAD model (Fig. 1. b.) The approximate cost per implant is $0.30. Effective VF medialization was confirmed in vivo using laryngoscopic evaluation and ex vivo using MRI (Fig. 1. c–d.)

Model reconstructions of the larynx from MRI were performed to evaluate VF medialization. The larynx and VF cover were reconstructed before and after unilateral right-sided insertion of the implant (implanted condition, Fig. 2. d–f). Comparison of the VF edge in the rest condition, highlighted in blue, with the VF edge in the implanted condition, highlighted in red, demonstrates successful implant induced medialization in the right VF.

Fig. 2. Vocal fold medialization from implant using one sample as an example.

Model reconstructions in axial orientation with anterior at the bottom of the field and posterior at the top of the field. a-b. Reconstruction of the larynx at paramedian before implantation; c. Reconstruction of the vocal fold cover layer at paramedian before implantation; d-e. model reconstruction of the larynx after implantation; f. model reconstruction of the vocal fold cover layer after implantation; g. vocal fold cover comparisons before and after implantation; h. definitions of quantitative parameters to evaluate vocal fold adduction.

To quantify VF medialization, we compared VFs in rest and implanted conditions at the narrowest plane in the larynx (Fig. 2. g). We defined the following parameters to evaluate the VF adduction qualitatively (Fig. 2. h.); detailed results are listed in Table 1 for a selected sample. A₀ is the glottal area at rest condition; overall VF adduction is calculated by (A₀−A)/A₀, where A is glottal area in the implanted configuration. D₀ is initial distance between VF medial surface and the middle plane at Z, and D is distance after implant insertion. Maximum displacement is calculated by max(D₀−D) with the location denoted as Z₁/L; Maximum relative displacement is calculated by max((D₀−D)/D₀) with the ratio denoted as Z₂/L. In the sample shown in Figure 2, the overall VF adduction ratio from the implant is 39%, indicating substantial medialization of the VF toward the midline. Medialization was observed across all samples, with variations observed in medialization between subjects consistent with differences in laryngeal anatomy.

Table 1.

Quantitative parameters to evaluate vocal fold adduction

Implant Side	A0 (mm2)	A (mm2)	(A0−A)/A0	Max(D0−D) (mm)	Z1/L	Max((D0−D)/D0)	Z2/L
Right	4.51	2.76	39%	0.48	73%	47%	73%

Discussion/Conclusion

We developed a precise, custom laser-cut silicone laryngoplasty implant which is easily reproducible. Producing implants in-house and reduced cost of labor may offset the initial investment of obtaining a laser cutter as the reduced price point represents significant savings compared to hand-carved implants or commercially available preformed implants ranging from $38–$228 [12,13]. The implants were designed with Tinkercad, a free and easy to use web app designed with the public in mind.

Limitations of this approach include laser cutter access and need to sterilize implants after cutting and prior to insertion. Subjects were randomized to shape/size of implant rather than using an imaging/expert guided selection, which would likely result in improved medialization. Further, we do not evaluate phonatory parameters in this study.

This simple technology has the potential to create a library of implant shapes and sizes that can be produced within seconds at minimal cost, increasing the customizability of implants while reducing the time-cost of creating them. Laser-cutting of implants is readily scalable and may present a future opportunity for clinical investigation. For our future work, we plan to produce custom laser-cut implants informed by volumetric and structural data from magnetic resonance imaging of the larynx to study how precise implant dimensions affect vocal fold vibration.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. Final datasets will be publicly available when complete, per the resource sharing plan described in NIDCD award #5R01DC016236.