Applying Wrist Biomechanics to Address Instability
Whether you are an OT who specializes in hand therapy, a PT who sees the occasional patient with wrist pain, or an AT rehabilitating an injured athlete, you know one thing is true: the wrist is a complex joint. Working knowledge of the intricate biomechanics of the wrist is critical to not only understanding your patient’s impairments but determining the best plan of care. Here, we’ll explore how an appreciation for the importance of midcarpal motion in the treatment of proximal row instabilities is indispensable for certified hand therapists or those seeking to expand their training in this area.
Looking at Wrist Anatomy and Motion
Wrist motion is composed of contributions from both the radiocarpal and midcarpal joints:
- The mechanics of the radiocarpal joint (RCJ) involve a mobile proximal row, which consists of the scaphoid, lunate, triquetrum, and pisiform moving on a stable distal radius and the articular disc. RCJ motion is biased toward motion in the orthogonal plane, such as flexion and extension, as well as radial and ulnar deviation.
- At the midcarpal joint, the distal carpal row, which is composed of the trapezium, trapezoid, capitate, and hamate, articulates with the proximal carpal row.
When you consider that the plane of motion for the midcarpal joint lies along a 45-degree angle from the frontal plane, the “S” shaped configuration of the midcarpal joint from a dorsal view of the wrist that we are accustomed to observing suddenly changes.
The Role of the Dart Thrower’s Motion
Let’s look at how these come together in one movement. The simple act of grasping the handle of your coffee cup and bringing it to your mouth to take a sip isn’t an action that we think too deeply about, yet this daily action involves an intricate function of the wrist that we perform repetitively and regularly known as the Dart Thrower’s Motion or DTM.
DTM is a coupled motion produced by radial wrist extension and ulnar flexion, which allows us to perform many functional tasks. The trapezium, trapezoid, and capitate move together as a functional unit along the distal pole of the scaphoid.1 The interfacet ridge, located at the distal pole of the scaphoid, lies between the trapezium and trapezoid facet and guides the oblique plane of motion in the dart thrower’s plane.
Midcarpal Motion and Rehabilitation
From a rehabilitation standpoint, midcarpal motion is important to utilize for several reasons. One instance is the case of scapholunate (SL) or lunotriquetral (LT) instability, in which improved midcarpal joint motion will minimize movement within the proximal row and provide a protective effect for the injured segment.2 Therefore, in the early stages of rehabilitation, it’s beneficial to have the patient begin with mid-range motion within the dart thrower’s plane.3
Wrist Recovery in Real Time
Patient Synopsis
Several months ago, a 12-year-old right-hand-dominant female presented in my practice with progressive onset of right wrist pain over seven months. Her pediatrician referred her to hand therapy with a diagnosis of “right wrist pain and stiffness.” The patient’s history included involvement in gymnastics for three years between the ages of five and eight years old. She was very active in athletics and, at the time, was playing basketball, which exacerbated her wrist pain.
- Symptoms: The patient noted she experienced up to a 7/10 pain at worst, with global pain along the dorsal and volar wrist. Her wrist bothered her when applying pressure, such as cutting a watermelon, and when performing any type of gripping-with-twisting activity like brushing her teeth. Her primary complaint was an inability to write for more than three minutes before the onset of pain, which significantly limited her ability to take notes in school.
- Evaluation: The patient’s active and passive wrist range of motion was equivalent to the uninvolved side. Resisted testing of the wrist flexors and extensors did not provoke any symptoms. With stability testing, she presented with a negative midcarpal shift test (ruling out Midcarpal Instability); however, she presented with positive Lunotriquetral (LT) Ballottement, Shear, and Derby tests, indicating laxity at the LT interval.
Findings
The patient presented with significant hypomobility at the trapezoid-on-scaphoid articulation, indicating midcarpal stiffness. Because the trapezium, trapezoid, and capitate move together as a functional unit on the distal pole of the scaphoid, manual testing needed only to focus on moving the trapezoid on the distal pole of the scaphoid to assess midcarpal mobility.
It was helpful to see the difference between the patient’s grip strength with and without a brace for support:
- In the standard grip testing position over three trials, she presented with an average of 41.3 pounds with 7/10 pain (versus 47 pounds on the uninvolved left).
- With a prefabricated Wrist Restore brace using the ulnar strap, which provided a “boost” to the pisiform to re-align the LT interval, the patient’s strength improved by 5 pounds and symptoms reduced from 7/10 to 2/10 pain.
Treatment Plan
Manual therapy techniques promoted mobility of the midcarpal joint, thereby decreasing stress on the LT interval. Additional treatment included the use of a prefabricated Wrist Restore brace to provide an ulnar boost to re-align the LT interval with daily activities, proprioceptive training4,5 and progressive strengthening of the extensor carpal ulnaris (ECU).6 From a biomechanical standpoint, a focus on the restoration of midcarpal motion proved beneficial in reducing the stress on the hypermobile LT interval.
Outcome
After four visits over 12 weeks, the patient presented with the following at discharge: negative LT Ballottement, Shear, and Derby tests. With joint-specific testing of the midcarpal joint, the motion of the trapezoid on the scaphoid was now equal to the uninvolved left. Grip strength improved by 10 pounds (without the support of the Wrist Restore brace), and her pain decreased from 7/10 to 0/10. Since the patient’s grip strength was equal with and without the Wrist Restore brace, she could gradually start weaning from the brace with daily activities, only using it for playing basketball or other heavier tasks.
- Ongoing Care: The “Soda Can Maneuver” is one of the most effective hand therapy home exercises for treating proximal row injuries. This training technique involves the patient pushing down on the proximal pole of the scaphoid while pulling up on the trapezoid and while also moving within the dart thrower’s plane The “Soda Can Maneuver” promotes carry-over with midcarpal motion in daily activities.
To familiarize yourself with manual wrist therapy techniques that you can use in your clinical practice, I offer a comprehensive course that includes step-by-step guidance for addressing stiffness at the radiocarpal and midcarpal joints. This course and others can be found in the new Certified Hand Therapist exam prep program!
- Moritomo, H., Apergis, E. P., Garcia-Elias, M., Werner, F. W., & Wolfe, S. W. (2014). International Federation of Societies for Surgery of the hand 2013 committee's report on Wrist Dart-throwing motion. The Journal of Hand Surgery, 39(7), 1433–1439. https://doi.org/10.1016/j.jhsa.2014.02.035
- Konopka, G., & Chim, H. (2018). Optimal management of scapholunate ligament injuries. Orthopedic Research and Reviews, 10, 41–54. https://doi.org/10.2147/ORR.S129620
- Bergner, J. L., Farrar, J. Q., & Coronado, R. A. (2020). Dart Thrower's motion and the injured scapholunate interosseous ligament: A scoping review of studies examining motion, orthoses, and Rehabilitation. Journal of Hand Therapy, 33(1), 45–59. https://doi.org/10.1016/j.jht.2018.09.005
- Hagert, E. (2010). Proprioception of the wrist joint: A review of current concepts and possible implications on the rehabilitation of the wrist. Journal of Hand Therapy, 23(1), 2–17. https://doi.org/10.1016/j.jht.2009.09.008
- Hincapie, O. L., Elkins, J. S., & Vasquez-Welsh, L. (2016). Proprioception retraining for a patient with chronic wrist pain secondary to ligament injury with no structural instability. Journal of Hand Therapy, 29(2), 183–190. https://doi.org/10.1016/j.jht.2016.03.008
- Esplugas, M., Garcia-Elias, M., Lluch, A., & Llusá Pérez, M. (2016). Role of muscles in the stabilization of ligament-deficient wrists. Journal of Hand Therapy, 29(2), 166–174. https://doi.org/10.1016/j.jht.2016.03.009
- Caggiano, N., & Matullo, K. S. (2014). Carpal instability of the wrist. Orthopedic Clinics of North America, 45(1), 129–140. https://doi.org/10.1016/j.ocl.2013.08.009
- Garcia-Elias, M., Alomar Serrallach, X., & Monill Serra, J. (2013). Dart-throwing motion in patients with scapholunate instability: A dynamic four-dimensional computed tomography study. Journal of Hand Surgery (European Volume), 39(4), 346–352. https://doi.org/10.1177/1753193413484630
- Moritomo, H., Viegas, S. F., Elder, K., Nakamura, K., DaSilva, M. F., & Patterson, R. M. (2000). The scaphotrapezio-trapezoidal joint. part 2: A Kinematic Study. The Journal of Hand Surgery, 25(5), 911–920. https://doi.org/10.1053/jhsu.2000.8637
