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Vertebral Body Tethering Testing

How does vertebral body tethering testing contribute to ensuring the durability and safety of spinal implants? See the significance of this cutting-edge testing approach and its pivotal role in driving forward new innovations in spinal surgery.

Our team shares a strong focus on advancing the medical device industry through innovative research, design, and testing. We have studied the complex behaviour and performance of vertebral body tethering implant systems through extensive custom test development and analysis. We continue to work diligently on advancing current methods to be a leader in testing novel spine devices.

Figure 1: Graphic image of a spine model with scoliosis, showing the intended use of a VBT device.

What is vertebral body tethering?

Vertebral body tethering (VBT) is a surgical approach to treat scoliosis. Scoliosis is a spinal deformity mainly diagnosed in growing children where the cause is usually unknown (idiopathic). Traditional treatment plans may begin with external bracing and physical therapy, but often it leads to surgical intervention where spinal fusion is required. As a result, the spine curvature is corrected but the young patients are left with a lifetime of rigid fixation hardware which limits their spine mobility and quality of life significantly.

In a fast-growing market, spine companies are constantly researching and developing new ways to treat spinal diseases and stand out against competitors. Vertebral body tethering (VBT) implant systems offer desirable advantages over traditional approaches that use rigid hardware such as supporting spinal growth and sustaining spine mobility. Vertebral body tethering devices work effectively by implanting several anchors (screws) into the vertebrae along the lateral convex side of the spine and connecting a flexible cord (tether), as shown in Figure 1. Thereby, tension is applied naturally to the VBT system as the child grows and spinal alignment becomes corrected all while preserving spinal movement.

Why do vertebral body tethering devices need testing?

Evaluating implant performance is critical to determine the safety and effectiveness of any medical device, especially for novel designs such as VBT systems where clinical and predicate data are limited. In other words, there lacks a complete understanding of the failure modes, wear patterns, and overall performance of VBT devices.

The challenge with testing VBT devices stems from its flexible construct nature designed to work in tension. When you combine those key features with a poly woven cord component subject to multi-axial physiological loading endured from spinal movement, understanding how the device will perform becomes complex and variable. Wear patterns and failure modes are dependent on many factors such as direction and degree of loading, device design, patient growth rate and activity level, number of vertebral levels implanted, and surgical technique. Understanding these elements can be measured through a series of in vitro mechanical testing to determine where, when, how, and why the VBT device shall fail. In turn, providing extremely valuable data for assessing implant performance and proving the safety and effectiveness for pre-market approval.

Figure 2: Photographic image of OIC’s hip simulator adapted to conduct a VBT wear test.

How have we modified existing test methods for vertebral body tethering devices?

Testing VBT devices using appropriate and relevant test methods is the pivot point for obtaining accurate and reliable results. Unfortunately, there is no recognized test standard specific for VBT devices. However, we have studied and developed a series of applicable test methods adapted and combined from recognized test methods for spine implants such as ASTM F1717, ASTM F1798, ASTM F543, and ISO 18192-1.

We have found that it is important to evaluate the performance of the VBT device through testing the mechanical behaviour of individual components and subassemblies to get a better understanding of multi-level constructs. The attachment points between the cord and the anchor are tested through static loading to determine the yield load and failure modes of this subassembly. Furthermore, dynamic single-level or multi-level subassemblies are tested to simulate long-term tensile loading and evaluate wear patterns and fatigue life. Additionally, isolated material testing is conducted on the cord to consider creep and stress relaxation behaviour. Anchor fixation is measured by pull-out and torsion testing. To simulate physiological conditions, we highly recommend testing in a heated wet environment when applicable.

Although static, dynamic, and material testing is important, the most rigorous (and clinically relevant) type of testing for a VBT is wear testing (Figure 2). We have designed and developed a unique way to apply clinically relevant complex multi-axial loading on a subassembly of a VBT device. We have created custom fixtures adapted from standard hip simulator wear testing methods, to apply a tensile load when cycled to replicate spinal motion. As the cord articulates with its connecting parts, wear on the cord is forced to develop under the worst-case conditions. Particulates may be generated and can be collected and characterized through qualitative and quantitative analysis. Wear testing allows implant durability and performance to be pushed to its limits, allowing wear patterns and failure modes to be evaluated carefully.

Our expert testing support and services

Our experienced team of specialized engineers have developed, trialed, and mastered several custom test methods for VBT devices including static, dynamic, and multi-axial wear testing. While we have carefully created these methods from recognized test standards because none currently exist, our senior biomedical engineer and technical lead for spine and osteosynthesis, Sara Gustafson P.Eng, M.Sc., is actively supporting a specialized Working Group of ASTM on the development of a new ASTM test method for VBT devices. We are excited to share our technical knowledge and experience with you to support you from the planning and protocol development through to conducting all test methods required for VBT devices in our well-equipped lab.

We have a vast understanding of the complexity of VBT implant systems and how they function, perform, and wear through in vitro testing and post-test analysis. Our goal is to provide our clients with accurate pre-market approval data by applying relevant and reliable test methods, ultimately to help bring novel spinal devices to market quickly and safely.

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