Effect of wrist position on thumb flexor and adductor torques in paralysed hands of people with tetraplegia

Abstract

Background: People with tetraplegia often have extensive paralysis of the hand yet retain crude hand function. Their hand function is dependent on manipulating wrist position with the neurally-intact wrist extensor muscles to change the passive tension in paralysed thumb muscles. This moves the thumb in relation to the paralysed index finger enabling basic grasp. The aim of this study was to quantify the effect of wrist position on thumb flexor and adductor torques generated in paralysed hands of people with tetraplegia.

Methods: Thumb flexor and adductor torques were measured as the wrist was passively moved from a fully flexed to a fully extended position in 10 people with tetraplegia who had paralysis of all thumb muscles. The relationships between thumb torques and wrist angles were quantified with torque-angle curves.

Findings: There was a consistent curvilinear relationship between wrist angle and both thumb flexor and thumb adductor torques. Thumb flexor torques were greatest and thumb adductor torques were smallest when the wrist was fully extended.

Interpretation: Wrist position influences the thumb flexor and adductor torques generated in the paralysed hand. This is probably due to the effect of wrist position on the passive tension of the thumb muscles spanning the wrist. These findings have implications for people with C6 and C7 tetraplegia who rely on the passive torques generated by the paralysed thumb muscles for hand function.