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Small Parts, Big Difference in Spine Surgery

Thomas Foley
By Thomas Foley General Manager, Vlier Inc.

More than 200 million people are estimated to be living with osteoporosis, a disease that weakens bones. According to the International Osteoporosis Foundation, the most common osteoporotic fracture is a vertebral compression fracture (VCF), affecting about 750,000 people annually. Vertebral augmentation can stabilize VCFs, which include both vertebroplasty or kyphoplasty procedures. In these minimally invasive surgeries, bone cement is injected into the fractured vertebra to create an internal cast to stabilize the fracture while removing build-up and cartilage. For nearly 20 years, this procedure was completed by surgeons based on look and feel alone, until Vlier introduced a small ball plunger.

Replacing Touch with Torque

In 2007, a large medical supply manufacturer set out to create a tool to improve vertebroplasty and kyphoplasty. It contracted Vlier Inc., Hopkinton, Mass., to create a custom ball plunger based on Vlier’s experience manufacturing spring-loaded devices, quick-release devices, Vlier Lock Pins, and leveling devices. With a retractable device installed inside the surgical tool, the surgeon could more precisely perform the procedures, reducing the amount of bone damage by using torque rather than touch and instinct to guide the instrument.

The Vlier ball plunger is customizable and can take side load pressure and provide repeatable force. (All images provided by Vlier)

Vlier’s traditional ball plunger provides repeatable force while also taking a high degree of side load. Ball plungers are ideal for medical applications as they offer maximum protection against rust and corrosion. Typical uses include positioning, locating, indexing and torque limiting, and they are the smallest and lightest spring-loaded device Vlier makes.

This medical tool needed a stainless steel part that was under 1" (25.4 mm), and could create smooth, repeatable movements every time. The medical device engineers spent over three years to get the surgical instrument to work flawlessly. The Vlier part provides the safety mechanism the medical manufacturer was searching for.

One of the biggest design constraints—which also reaps the greatest reward with this particular ball plunger—was the 15 μm Ra interior surface finish that was smoother than anything the engineers had created before. This is needed to prevent galling, which inhibits the effectiveness of the tool over time.

In 2011, the Vlier team created the final part of the customized ball plunger to be installed in the tool, which is still used by surgeons 10 years later. The small part is comprised of four separate pieces: the body, inside rivet and spring are all made of stainless steel while the ball is ceramic. The part weighs 0.0126 lb (0.0057 kg) and is under 1" (25.4 mm) long. It rests in a groove in the handle of the surgical instrument, in a positive locking position. As the surgeon applies pressure and the tool rotates, the ball plunger acts as a safety, ensuring the surgeon does not remove healthy bone from the spine. When too much torque is detected, the plunger halts the surgeon from scraping more material, warning they are removing healthy cartilage.

Vlier prides itself on solving problems and making an impact. As medical devices change, we are able to develop our parts and respond to needed changes.

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