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Mapping the dart thrower’s arc

By Jay D. Lenn

Grant recipients use OREF funding to study wrist motion

Nearly everyone, it seems, has what it takes to play a decent game of darts. Studies have shown that the movement of the wrist needed to throw a dart, generating a certain degree of force and reaching a desired target, is employed in such activities as combing hair, tying a shoe, and pouring from a pitcher.

Although this motion may be common, it’s not well understood. What is known doesn’t significantly inform treatment choices for wrist disorders and treatment outcome assessments.

Scott W. Wolfe, MD, and Howard J. Hillstrom, PhD, received a 2007 Orthopaedic Research and Education Foundation (OREF) Research Grant to document the presence of the dart thrower’s arc in a series of functional activities and to assess differences in patterns of motion associated with the surgical fusion of wrist bones.

Dr. Wolfe is chief of hand surgery at the Hospital for Special Surgery (HSS) and professor of orthopaedic surgery at the Weill Medical College of Cornell University in New York City; Dr. Hillstrom is a biomedical engineer and director of the HSS Leon Root, MD, Motion Analysis Laboratory. The OREF Research Grant provides up to $50,000 in start-up funding annually for up to 2 years.

The kinematics of darts
“Our whole project,” explained Dr. Wolfe, “is built around the idea that few activities in daily life involve a single plane of motion in the wrist—that is to say, only in the plane of flexion-extension or in the plane of radial-ulnar deviation. Instead, most routine motions are coupled motions, involving variable combinations of radio-ulnar deviation and flexion-extension.”

Dart throwing is a coupled motion that moves through an arc from radial deviation and extension to ulnar deviation and flexion. To quantify the coupled effect in the dart thrower’s motion, the researchers are using a three-dimensional motion analysis system that they developed for the upper extremity.

The first phase of the study involves 10 right-handed men as controls. Each participant practices and then performs the following seven tasks: throwing a dart (Fig. 1), shooting a basketball from a free-throw position, throwing a baseball overhand at a target, throwing a football at a target, hammering a nail (Fig. 2), clubbing a stationary target on a table, and pouring from a pitcher.



Fig. 1, A, B. A motion analysis system is used to quantify the movement of the wrist needed to throw a dart, generating a certain degree of force, and reach a desired target. Courtesy of OREF


Fig. 2 The placement of 25 reflective markers attached at points on the hand, wrist, forearm, shoulder, torso, and pelvis enables researchers to assess correlated effects in the shoulder and elbow during functional tasks of the wrist. Courtesy of OREF

Each participant has 25 reflective markers attached at points on the hand, wrist, forearm, shoulder, torso, and pelvis. The placement of markers at sites other than the wrist enables the researchers to assess correlated effects in the shoulder and elbow during functional tasks of the wrist.

An eight-camera motion analysis system records data, enabling the investigators to reconstruct mathematically the movement during these tasks. “When we look at these motions in healthy people,” said Dr. Hillstrom, “we learn what amount of coupling is needed to perform these tasks and whether the coupling is the same for all of the tasks.”

Unrestricted versus constrained motion
Drs. Wolfe and Hillstrom are conducting two additional motion analysis tests with the same 10 control patients and same tasks to evaluate how changes to the architecture of the wrist alter kinematic coupling and functional performance.

In one test, each patient wears a molded plastic splint designed to constrain wrist radio-ulnar deviation. By comparing “uniplanar” motion data from this test with unrestricted motion data, the researchers can judge the degree to which performance of each task depends on kinematic coupling.

The other test compares wrist movement and functional ability between the control subjects and 10 additional right-handed male participants who’ve undergone midcarpal arthrodesis surgery at least 6 months prior to testing and have been discharged from surgical care and rehabilitation.

Bull’s eye: clinical care
Drs. Wolfe and Hillstrom noted that the following clinical practices could eventually be informed by a clearer picture of how the dart thrower’s arc is employed in common tasks and how functional abilities in the corresponding planes of motion are affected by disease and treatment.

  • If research demonstrates that common tasks are performed along the arc of the dart thrower’s motion, rehabilitation exercises to improve range-of-motion and strength could be targeted to that plane.
  • If better data emerges about the functional outcomes of midcarpal arthrodesis—and other surgical treatments—clinicians may be able to help patients make better informed decisions about treatment options and their expected outcomes.
  • Information about the impact of surgical procedures on coupled kinematics may suggest strategies to improve or customize surgical techniques.
  • A better understanding of wrist architecture and its role in coupled motion may influence design criteria for prosthetic devices used in wrist arthroplasty.

Moving forward
Drs. Wolfe and Hillstrom hope that this project will provide the necessary data to apply for a National Institutes of Health (NIH) grant to expand their investigation. To prepare, they have participated in the Orthopaedic Research Society (ORS)/OREF/AAOS Grant Writing Workshop, which provides instruction, a mock review, and mentoring designed to elevate each proposal to the quality needed to compete for NIH funding.

“OREF’s sponsorship of smaller and medium-sized studies enables orthopaedic investigators to cut their teeth on tough questions and develop the knowledge base and pilot data to be competitive and go on to larger-scale clinical trials.”

Jay D. Lenn is a contributing writer for OREF. He can be reached at communications@oref.org

January 2011 Issue