Crankl: from the bike shed to the operating theatre, how CDP created a novel surgical tool

It is important to start each innovation journey with the broadest possible mindset because this opens the door to all sorts of different solutions that might exist outside one team’s experiences. A great example of this philosophy in action was the design of a surgical torque wrench that we developed recently. 

Crankl bicycle torque wrench tool concept

The story starts with one of our engineers who was a keen cyclist. He saw an unmet need amongst cyclists who owned ‘high end’ carbon fibre bikes for a simple torque wrench to stop them over tightening screws and damaging their bike frame. So, in his spare time, he came up with a simple, plastic, single piece wrench design that indicates the right torque every time. He publicized his idea on a trends website, but unexpectedly he received messages from surgeons who were also bike enthusiasts.

It turned out that orthopaedic surgeons were frustrated by the tools they had to make sure that screws used to fix fractures and implants were not over tightened. Plates are commonly used to support fractured bones and over tightening the screws can break the screw, damage the bone or make the screws difficult to remove, under tightening can allow the fixation to become loose.

A torque wrench is a tool that indicates or limits the torque applied to a screw. They are used across all engineering sectors and surgeons have similar devices adapted for the operating theatre. However, the inherent cost and complexity of these tools mean that they must be reused, which in turn creates additional processing complexity and cost for the hospital. Before each procedure, they must be thoroughly sterilised and tested, which is time consuming and open to error.

Single use medical devices have seen increasing adoption since their early introduction in the 1960s, initially for their ability to displace durable devices with their requirement for costly reprocessing, calibration, adjustment etc. Over time their potential to deliver when sterility and performance are paramount has become increasingly prominent, as they can be manufactured to tight quality standards and tested, packed and sterilised in controlled factory conditions where economies of scale make this cost-effective. Following pressures towards sustainability and ever-reducing costs the trend is swinging back again – with devices leveraging the benefits of single-use style designs, but with more robust materials and designs to allow a limited number of reprocessing and re-use cycles (multiple-use devices).

What was needed here was a single or multi-use wrench, that was accurate, easy to use and did not require maintenance.

The plastic construction of the bike wrench showed us that a single, low cost, plastic moulding could be used as the active element in a basic torque wrench. However, the surgical version would need to be more accurate & repeatable, have different settings for different screws, and be more usable in the surgical environment – with a form factor that allows single handed use, and haptic feedback indicating when the correct torque is reached.

The design team reviewed the original bike torque wrench design and analysed where it could be improved and adapted for surgery. The original bike wrench had a beam that buckled when the right torque was achieved but this phenomenon was influenced by several parameters that could result in lower accuracy.  Together with a more compact form factor that better suited surgery, a new design was envisaged that only relied on bending, so should be more repeatable. It also provided better haptic feedback to the surgeon.

The design was modelled in 3D CAD and underwent FEA simulation to better understand how it would perform. This allowed the first round of optimisation to get as close as possible to the desired performance. To allow the ergonomics and ‘feel’ to be evaluated, a first model was made using 3D printing. The Crankl surgical torque wrench was born. 

When the wrench was assembled, the team were pleased that they had got close to a design that would meet the surgeon’s requirements. But 3D printed materials perform differently to the injection moulded plastic that would be used in the final design, so another step was needed to verify the system would work. The team needed to have real injection moulded parts in the correct material to test.

Moving to ‘production intent’ manufactured parts is a big step in all medical device developments. Medical devices have to meet strict standards to be placed on the market and CDP’s experienced device development and quality engineering teams ensure that this happens, in line with our ISO13485 certified quality system and device development process. This means that at the end of development all the correct processes and documentation will have been completed to support a submission under the EU Medical Devices Regulations and / or to the FDA, as appropriate to market need.

Obtaining moulded parts is usually an expensive and time-consuming step because mould tools have to be designed and manufactured. These are complex and take time to make, and errors can occur that can affect performance and the validity of the test results – requiring a further iteration. For this reason, CDP has developed a “rapid digital moulding” approach which uses 3D printing to very quickly make mould tools into which target polymers can be injected – rapidly creating production-intent parts.

This process is described in detail in the white paper linked here: http://insights.cambridge-design.com/digital-tooling-whitepaper

So critical-to-function parts were made using digital tooling and moulded plastic components tested. This resulted in the design being refined for a third time and new tools and components manufactured, a process completed in a few days using digital tooling, where it would have taken weeks and months using conventional processes.

This allowed the final prototype of the surgical torque wrench to emerge from the design and testing process, demonstrating that a single or multi-use design is achievable at a fraction of the cost of reusable alternatives. Based on this feasibility work we’re now in discussions with device manufacturers about taking Crankl into a full device development. This project demonstrates that even in mature markets like orthopaedic surgery, where the same basic tools and techniques have been tried and tested over decades, there is still an opportunity to innovate by understanding unmet user needs and taking a different perspective on how to meet them. We believe this is best achieved by a multidisciplinary team with experience from widely differing sectors because often similar problems have found solutions and technologies that only exist in their specific markets. Even if these cannot be directly applied, they always inspire new thinking and new problem-solving approaches that can lead to better, faster and more cost-effective innovation.


Matt Brady

Partner & Head of medical therapy
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