Biomechanics: The wrist in motion
Volume 8, Issue 3
Mayo Clinic researchers are using the science of biomechanics to analyze and reproduce the movement of limbs in order to improve their function or help them heal.
Kai-Nan An, Ph.D., is the John and Posy Krehbiel Professor of Orthopedics at Mayo Clinic.
For Kai-Nan An, Ph.D., the wrist is the body's Rubik's cube: a complicated structure of small pieces that move in tandem. It's virtually impossible to solve the Rubik's cube by randomly twisting squares; you need specific algorithms.
Similarly, solving wrist problems requires exact measurement of bone movement.
"You must go beyond the subjective and quantify wrist joint movement for more precise diagnosis and treatment," says Dr. An, co-director of the Biomechanics and Motion Analysis Lab at Mayo Clinic in Rochester, Minn. "Otherwise, a patient might find there's no improvement."
For four decades, scientists in Mayo Clinic's Biomechanics Laboratory have been solving puzzles to improve treatment for patients. The lab's discoveries have shed light on a variety of conditions, including arthritis, scoliosis and myofascial pain. Although this mapping of the muscular-skeletal system is painstaking work, Dr. An makes it sound disarmingly simple.
"We study the motion of the human body. Then we design gadgets so that motion can be quantified," he says.
One recent gadget is a motion simulator that mimics the human wrist. The device helped Mayo Clinic scientists develop a method for imaging a wrist while it's in motion. The four-dimensional computerized tomography (CT) image — the 3-D wrist plus the fourth dimension of time — captures subtle bone movements that aren't visible on conventional CT. One potential application is detecting and surgically correcting wrist instability before the onset of arthritis.
"Seeing the wrist bones in motion is incredibly helpful," says Mayo Clinic orthopedic surgeon Richard A. Berger, M.D., Ph.D., who helped develop 4-D wrist CT. "It's nearly impossible to appreciate how the individual wrist bone motions combine when looking at still images. With 4-D CT, more successful treatments will emerge, leading to improvement in patient outcomes, which is, after all, what it's all about."
The CT innovation is one more example of the fruitful cooperation between physicians and engineers that has long characterized Mayo Clinic research. Dr. An has been on the front lines of that collaboration since he joined Mayo Clinic in 1975 after earning a doctorate in mechanical engineering and applied mechanics. One of the first tasks he was given: Design a surgical implant for the knuckle joint.
"I asked questions based on engineering design criteria: How much force do the knuckle joints expend? How far do they extend? What kind of tissue do they have? Nobody could give me the answers in the '70s," Dr. An says. "We had to do research."
Initially, research in the Biomechanics Laboratory focused on quantifying all human joints from the fingertips to the shoulder. Other studies led to the development of computer models that simulate straightening the spine, to guide surgeons placing rods in patients with scoliosis.
Imaging the moving body
Computer-rendered images of the wrist to show instability after injury. Bottom row injured, top row normal.
Yet another strand of research involves what Dr. An calls "the biology part of biomechanics" — how human cells respond to mechanical stimulation. For example, bone cells actually respond to exercise, which has implications for people with osteoporosis.
Ten years ago, Dr. An began pondering how Mayo Clinic's expertise in medical imaging could be applied to biomechanics. Mayo Clinic researchers had already devised a system for imaging a beating heart. The challenge for the Biomechanics lab was adapting that technology to the musculoskeletal system — specifically, the wrist. "The heart has a regular beat. But in everyday tasks, people do not move their wrists with that regularity," Dr. An explains.
Movement of an injured wrist may look similar to that of a normal wrist, but Mayo Clinic investigators can use these images to precisely measure abnormal movement — and to diagnose and treat wrist pain.
So the researchers built the motion simulator using a deceased donor forearm and wrist that was programmed to move at typical wrist speed. "With the cadaver simulator, we were able to try all kinds of imaging techniques. That helped us develop the algorithms and imaging method for the 4-D wrist CT," says Mayo Clinic radiological physicist Shuai Leng, Ph.D., the first author of a paper published in Medical Physics in 2011 describing the technique.
The lab team then used 4-D imaging to quantify bone movement in normal wrist specimens and in specimens with varying degrees of ligament damage.
"Putting a number on that wrist motion is very important," says Mayo Clinic biomedical engineer Kristin D. Zhao. "It helps us assess abnormal motion earlier, so we can intervene and prevent the progression to osteoarthritis."
This state-of-the-art imaging can spare people the pain and frustration of an undiagnosed problem.
"Lots of little bones have to move properly in the wrist, so an abnormality may be visible only during motion," notes Cynthia H. McCollough, Ph.D., director of the Mayo Clinic CT Clinical Innovation Center. "For a patient with pain, the static CT might look fine. But the patient keeps coming back and telling the doctor, 'It hurts.' "
Mayo Clinic-developed imaging method captures subtle bone movements in wrist not visible on conventional CT image.
Measuring myofascial pain
Shuai Leng, Ph.D., and Kristin Zhao
That type of experience is all too familiar for people with myofascial pain — chronic, deep muscle pain that doesn't respond to conventional treatments such as heat and ice. Myofascial pain is controversial because there have been no tests that can quantify the taut bands of stiff and sore muscle that characterize the condition.
Myofascial pain is just the sort of puzzle that intrigues Dr. An.
"I thought, 'Why don't I go in and see if I can identify this taut band area?' " he says. "If doctors can find the source of pain, they can treat it. There will be no subjective argument."
Specifically, Dr. An wondered if magnetic resonance elastography, developed by Mayo Clinic researchers to detect hardening of the liver, could be used to image more complex muscle tissue. Magnetic resonance elastography works by measuring the speed at which mechanical waves pass through tissue. The waves travel faster through stiffer tissue.
To discover if magnetic resonance elastography worked on muscle, Dr. An's lab team first experimented with gels that simulated contracted muscle tissue. Magnetic resonance elastography images of the gels revealed a V-shaped pattern that indicated high wave speed. Next, they imaged muscle identified as possible taut bands in patients. Those images showed a similar V-shaped pattern and faster wavelength, indicating stiffer tissue. In a 2008 study published in Clinical Biomechanics, the Mayo Clinic scientists reported that magnetic resonance elastography consistently identified taut bands where muscle was 50 to 100 percent stiffer than uninvolved muscle or than muscle in people without myofascial pain.
Magnetic resonance elastography could be useful for studies gauging the effectiveness of myofascial pain treatments.
"It's very exciting that we're able to visualize these muscles and measure the tension," says Mayo Clinic rehabilitation specialist Jeffrey R. Basford, M.D., Ph.D., who co-authored the taut band study. "Imaging could be quite valuable in research on the effectiveness of therapies such as injection or massage."
Science and compassion
It is Dr. An's concern for patients that makes such discoveries possible.
"He views the human form with an incredibly high level of respect and reverence. His eye is always on the patient at the heart of the clinical question," Dr. Berger says. "He can take engineering principles and mathematical approaches, blend them with a true understanding of the clinical problem, and derive answers that most others would never dream of. I cannot tell you how many times he has asked me 'Why this?' or 'Have you thought about that?' or said something that has stopped me in my tracks and forced me to reassess what I am doing. He is absolutely one of the smartest people I have ever met."
This ability to engage physicians in the research process provides the "ultimate environment" for translating discoveries from the lab to patient care, Dr. Berger adds. "Dr. An guides the clinician through the research steps in a supportive and collaborative manner, bringing clinical expertise directly into the laboratory."
As an engineer, Dr. An welcomes the opportunity to work with physicians and to translate his findings into patient care.
"At Mayo, the labs are always open," he says. "If the surgeons have a question, they come to the lab. If we can't answer the question, we design an experiment to do that. Mayo is the best place for research translation."