A tenodesis grasp is a passive hand grasp and release movement that occurs in response to wrist motion. It allows people who lack active control over their finger muscles to create a functional grip by using the muscles that move the wrist. This capability can significantly enhance independence by allowing for the performance of daily activities.
The Mechanism Behind the Grasp
The tenodesis grasp operates on a biomechanical principle involving the tendons that connect forearm muscles to the finger bones. These tendons cross multiple joints, including the wrist and knuckles, functioning like a pulley system. This arrangement means the wrist’s position directly influences the tension in the finger tendons.
When a person extends their wrist, the flexor tendons on the palm side of the hand are pulled taut. This passive tension causes the fingers and thumb to curl into a grasping position. The strength of this grasp is related to the degree of wrist extension and the tension within the tendons.
Conversely, when the wrist is flexed, the tension on the flexor tendons is released. This allows the finger extensor tendons on the back of the hand to passively straighten the fingers, causing the hand to open. This mechanism provides a controlled grasp and release cycle powered entirely by the movement of the wrist, without any active finger muscle contraction.
Utilizing and Strengthening the Grasp
The tenodesis grasp is beneficial for individuals with a spinal cord injury, commonly at the C6 and C7 cervical levels. People with these injuries often retain active wrist extension but have lost voluntary finger flexion. Occupational therapy is central to learning how to use this grasp effectively.
Therapists guide individuals in mastering the motion to perform daily activities like holding eating utensils or brushing teeth. Training involves positioning the hand over an object with a flexed wrist to open the fingers, then extending the wrist to secure it. Repetitive practice helps refine control over the grasp.
Strengthening the wrist extensor muscles is a primary focus of rehabilitation. Exercises build power in these muscles for a stronger, more sustained grip. Maintaining the proper length and passive tension in the finger flexor tendons is also necessary. Therapists advise against overstretching these tendons, as excessive slack can weaken the grasp.
Orthoses and Surgical Interventions
When the natural tenodesis effect is not strong enough, external supports called orthoses can be used. A common device is a wrist-driven hand orthosis, or tenodesis splint, which is worn on the forearm and hand. This device uses a system of linkages that mechanically assists in closing the fingers as the wrist extends, enhancing stability and grip strength.
For some individuals, surgery may be an option to create a more powerful grasp. Tendon transfer surgery involves rerouting a tendon from an active muscle to restore a lost movement. A surgeon might detach an active wrist muscle tendon and reattach it to the finger flexor tendons, creating a direct, active-powered grasp that bypasses the passive tenodesis effect.