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Better Than Eyeballing: Sensors Aid in Joint-Replacement Surgeries

Knee and hip replacement surgeries are increasing along with the growing population of those aged 60 and older. ADI sensors enhance devices that aid in surgical navigation for less than the cost of traditional computer-assisted surgical systems and more accurately than manual methods.

in partnership withADI

As the ranks of those aged 60 and older increase, so does the need for joint-replacement surgeries. According to the U.S. Centers for Disease Control, approximately one million knee- and hip-replacement surgeries are performed each year in the United States alone.

As people live longer, the resulting age-related deterioration of their joints frequently requires medical intervention. Research shows that the number of such surgeries, already on the upswing, is expected to continue rising dramatically: The number of hip replacements is expected to [almost triple] in the 20-year period from 2005 to 2025, with the number of knee replacements increasing more than 650 percent over that same period, according to a study presented at a recent annual meeting of the American Academy of Orthopaedic Surgeons.

Bottom line: The need for replacement surgeries that are accurate, efficient, and cost-effective has grown exponentially as well. To meet that demand, OrthAlign, Inc., a medical-technology company, has developed a palm-sized, single-use device equipped with sensors from Analog Devices, Inc. (ADI). Surgeons can use the device to help them navigate during surgery and more quickly and accurately align hip and knee joints in these increasingly common, yet still tricky, operations.

The OrthAlign devices provide surgical teams with real-time data and feedback about joint positioning and realignment, without the teams needing extra time or having to make an extra effort. They help improve surgical precision while lowering costs, and that translates into substantial benefits for both the fast-growing population of patients needing hip or knee replacements and the health-care system overall.

“The idea was to develop something that can maintain the accuracy of [surgical] navigation, but be fast, simple, and easy to use,” says orthopedic surgeon David J. Mayman, who uses the technology in his work at the New York City-based Hospital for Special Surgery.

Improving Surgical Outcomes

Typically, such replacements involve surgeons realigning the knee or hip by inserting a rod and manually eyeballing its alignment with the help of mechanical guides. Another method involves using computer assisted surgery (CAS), in which surgeons use computer-enabled tracking systems or robotic devices to enhance their view and increase the accuracy of the procedure. The challenge: Traditional CAS systems, while quite accurate, can take up a lot of operating-room space. Typically, they’re also expensive, with multimillion-dollar price tags.

What sets the OrthAlign devices apart from traditional CAS systems is the relatively compact size of the devices, which contain the ADI iSensor microelectromechanical system (MEMS) sensing technology. The company’s OrthAlign Plus and KneeAlign technology families deliver advanced navigation and sensing capabilities from these devices, seamlessly integrating them into the surgeon’s workflow. In a field where precision is fundamental to successful outcomes, OrthAlign’s devices meet, and even exceed, all alignment benchmarks, and not only surpass conventional mechanical guides in performance, but also match the precision of CAS systems—and all at lower cost, according to the company.

Rather than incorporating camera-based navigation into their solutions—which would add significant expense and size to a surgical navigation system—OrthAlign's design team employs the ADI iSensor MEMS inertial measurement unit (IMU) technology. In the case of a knee replacement, for instance, IMU allows an orthopedic surgeon to determine within seconds the center of rotation of a patient's femur. As the patient's knee is put through a full sweep of motion, MEMS IMU to quickly make decisions and operate more precisely.

The ADI sensors used in the devices are the same IMUs used in a variety of other products, ranging from guided missiles to drones.

 “For the OrthAlign products, we are using some technologies that are familiar to people from their smartphones, or even from military technology like drone and missile guidance,” says Jonathan Nielsen, director of product development at OrthAlign. “We use accelerometers and gyroscopes, both a type of inertial sensors, which when of significantly high performance and combined with sensor processing, are truly enabling to applications like ours.”

The ADI technologies that are part of the OrthAlign surgical devices were first used in the automotive industry some 30 years ago, originally to detect the conditions necessary to trigger airbags. About a decade ago, ADI began to reposition its inertial technology for wider use in other industries, says Bob Scannell, a business development manager for ADI’s Inertial MEMS Products.

“Because of the capabilities originally driven by the automotive world, we were able to develop a rather unique technology base with high-performance sensors at relatively low cost,” Scannell says. “When we focused on the industrial application base, the value of our technology to this emerging need to detect precise motion in complex and safety-critical conditions quickly became clear and also drove even more rapid advancements in high-performance inertial sensing.”

A Collaboration Leading to Precision and Innovation

ADI worked closely with OrthAlign to provide products that not only would enable the company’s design vision, but also would get to market more quickly. The ADI iSensor MEMS IMU design team was actively involved during OrthAlign’s development phase, analyzing their data and providing insights on tuning and interpreting the sensor data, as well as performing additional application-specific qualification tests. That allowed OrthAlign to focus all its efforts on back-end processing and overall system integration.

The ADI iSensor MEMS IMU designed into the OrthAlign solution provides six-degrees-of-freedom measurement via three accelerometers and three gyroscopes that sense linear and angular rate motion, respectively. The ADI iSensor MEMS IMU is capable of precisely tracking an instrument's motion on all axes, even during demanding operating conditions, due to its proprietary sensing techniques, which are also efficient at rejecting unwanted motion such as  from vibration, shock, and gravitational influences.

The compact ADI iSensor MEMS IMU device provides access to temperature-calibrated and dynamically compensated sensor data over a standard digital serial peripheral interface (SPI)interface, and makes it possible for the user to digitally tune the sensor filtering and processing to adapt to multiple application scenarios.

 “As surgeons, we don’t like surprises. It’s really nice to be in the operating room and having numbers in front of you” from the device, Mayman says. “There’s nothing worse, as a surgeon, than seeing your post-op X-ray and thinking ‘Ooh, that’s not what I thought I was doing.’ So, it’s [helpful] having that confidence to be able to move on and say, ‘I know that’s right.’”


To learn more about Analog Devices, visit To learn more about OrthAlign, visit

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