Q: Whenever we take our son into see the Physical Therapist, she always uses a funny looking metal device to measure his leg motion. I can't remember what the thing is called but it just seems to me like it would be very easy to get a different measurement every time just based on where you place the tool and how far you move the kid or the device. Am I wrong about this? I don't want to criticize but it just doesn't seem like a very accurate way to measure a moving target.
A: You are most likely referring to a gadget called a goniometer. This two armed measuring stick is used to measure joint range-of-motion. It is actually a fairly reliable measuring tool based on studies that compare how close two (or more) people come using the same tool on the same child (and on other children).
Usually the goniometer is made of solid plastic or metal so there is very little give or flexibility in the tool itself. Each arm of the goniometer is lined up with preset bony landmarks. By using the same physical reference points from person-to-person, it's easy to get a close approximation of the angle.
When the same therapist using the same tool measures the same person, there could be a little intra-rater difference. But for the most part, this will only differ by three to five degrees of motion. In the long-run and in the big picture, any significant changes over time will be easily recognized.
Goniometric measurements can be very useful aids in the initial assessment, determining a diagnosis, and later, making an evaluation of progress made. Evaluating children for problems such as slipped capital femoral epiphyses (SCFE), synovitis or other inflammatory hip problems, and femoroacetabular impingement often requires hip range-of-motion measurements for a clear diagnosis. And the goniometer is the best tool available for this task.
Reference: Wubhav N. Sankar, MD, et al. Hip Range of Motion in Children: What is the Norm? In Journal of Pediatric Orthopaedics. June 2012. Vol. 32. No. 4. Pp. 399-405.