Plastic Welding Innovation Seals Novel Endodontic Cannula

An endodontic specialty company, Produits Dentaire SA (PD-Dental) of Vevey, Switzerland, envisioned a flexible plastic cannula that could outperform, and ultimately replace, typical metal irrigation cannulas used in root canal surgery. The company reasoned that a flexible plastic cannula with side-facing irrigation vents near the tip would be safer and more effective since it would slide more easily through the complex curves of root canals while enabling more complete irrigation and disinfection.  

After preliminary designs showed promise, the next challenge was to produce the cannula on a commercial scale. After considerable effort to develop a workable injection molding process, the manufacturer settled on a cannula comprised of two, injection molded components, both made of a sterilizable, medical-grade polypropylene. The first component would be the hollow, conical cannula stalk, which incorporated dual side-facing irrigation ports, while the second was a cannula base. One end of the base would be a luer taper for connecting the cannula to a hypodermic irrigation syringe, while the other end would hermetically seal to the cannula stalk using an ultrasonic plastic weld. PD-Dental planned to name the new, flexible irrigation cannula product IrriFlex (Figure 1).

Ordinarily, making an ultrasonic weld between two polypropylene parts is not difficult. However, the unusual shape and lightweight structure of the cannula and base proved very difficult to weld, prompting some to declare the weld “impossible” to make on a production scale.

To understand the welding problem, it is necessary to first understand the process. Every ultrasonic weld requires that the upper component (in this case, the cannula stalk) be held by a sonotrode, which transmits ultrasonic energy through that part to the base component. The combination of ultrasonic vibration and pressure from the sonotrode to the base melts the plastic at the part interfaces and completes the weld. 

In this case, when a typical sonotrode design was used to weld the cannula stalk, the high-frequency vibration required for welding created an unwanted side effect, called diaphragming. Similar to the ripple effect of a pebble hitting water, diaphragming means that the vibratory energy required for welding causes the fragile cannula stalks to flex rapidly during the welding process, frequently causing deformation or cracking (Figure 2).  

To enable good welds without cannula breakage, Emerson created a welding sonotrode that incorporated a simple system of acoustic “dampeners.” The dampeners served to cushion and isolate the fragile cannula stalk within the sonotrode without changing the vibratory energy needed to complete the cannula stalk/base weld.

After several iterations, the final sonotrode design was augmented with a vacuum port to enable automatic part loading/unloading of the welder (Figure 3). The time from initial failures to completion of a commercially viable weld process was less than six weeks. The new sonotrode was paired with an ultrasonic welding platform designed for fully automated welding of the very small IrriFlex cannula assemblies (Figure 4). The same system supports medical product production by collecting and storing weld production records that comply with the U.S. Food and Drug Administration’s 21 CFR, Part 11 regulations for electronic records and signatures. 

To date, two automated ultrasonic welders have supported the production of more than six million IrriFlex cannula assemblies. The unique design of the flexible cannula, which includes dual side-facing outlets near the tip, ensures that irrigation fluids are dispensed evenly throughout complex root structures. This ensures more complete removal and elimination of debris, smear layer, and biofilm and helps to ensure more positive endodontic outcomes. Buoyed by positive customer feedback and growing demand, PD-Dental is now marketing the IrriFlex product worldwide. 

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Thierry Maire is technical supervisor, Branson Ultrasonics, Emerson in Geneva. For 12 years, he has led the development of ultrasonic technology solutions for customers. He holds a master’s degree in mechanical engineering from Joseph Fourier University in Grenoble, France. 
 
As sales and operations manager, Emerson, Franck Didierjean provides sales engineering, innovative technical solutions, and technical support to customers in France, the Netherlands and Switzerland. He holds a master’s degree in mechanical engineering from the National Institute of Applied Sciences in Lyon, France.