The Reliability of the GNRB® Knee Arthrometer in Measuring ACL Stiffness and Laxity: Implications for Clinical Use and Clinical Trial Design.
Kayla Smith, Nicole Miller, Steve Laslovich
PMID 36237656WHAT IT FOUND
The robotic knee device gave repeatable measurements of front-to-back knee movement and tissue stiffness in healthy young adults, but measurement error was large enough that small changes should not be treated as real.
Key findings
01The device gave similar results when the same examiner retested, when two examiners compared, and when testing was repeated two weeks later.
02A change in anterior tibial displacement of 2.11 mm and a change in stiffness of 9.6 mm/N were the minimum differences that could be considered beyond measurement error.
03For a parallel clinical trial, 19 participants would be needed to detect a 1 mm change in anterior tibial displacement at 134 N.
STILL TO COME
How it was doneWhat they foundWhat it means for PTs
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What it does not show
The study included only 12 healthy university student and staff volunteers with no current knee pain or ACL history, so it does not show reliability in patients with knee injury or after ACL reconstruction. The reliability confidence intervals were wide, from 0.54 to 0.91, so the true reliability could be moderate rather than good. Testing forces were 134 N and 200 N, well below anterior shear forces reported up to 1070 N during activities such as jumping, so it may not reflect functional knee loads. Female hormone levels were not measured, and the paper notes sex hormone effects on laxity are uncertain. The authors observed small drops in patellar pad force during testing and noted soft tissue thickness can make strap tension difficult, but they did not test how these factors affect results. The study did not test patients with ACL injury or use the device to guide rehabilitation, so it cannot prove the measurements change patient outcomes.
Declared interests
The authors reported no conflicts of interest.
The easy way to misread this
Do not read this as proof that the device can safely guide ACL rehabilitation decisions. It was tested only in healthy young adults with no knee injury, and changes smaller than the minimum detectable change may be measurement noise rather than real tissue change.