The biomechanics of the hand with focus on the intrinsic muscles and the paralyzed hand.
The intrinsics consist of 18 muscles distal to the wrist crease. They are divided into groups: Thenar, hypothenar, lumbrical, interosseous and the Adductor pollicis. They are the precision instruments of the upper limb – power resides more proximally but the intrinsics direct and refine this power in order to achieve maximum, strong, synchronous performance.
This performance is achieved through the evolved biomechanics and interaction of the various groups with the skeletal and ligamentous elements. The main themes of this performance are opposition, stabilisation of the MCP joints, abduction and adduction, and the synchronous flexion and extension of the digital joints.
In addition, the intrinsics give the hand its characteristic shape, tone, posture and language as well as stability of the joints – for instance the stabilisation of the thumb MCP joint by the thenar musculature and the Add Poll.
Paralyses, particularly of the median and ulnar nerves, impairs one or more of these themes of performance. The loss of function sheds light on the essentials of biomechanics and also guides reconstruction. The major paralyses will be considered with particular reference to the impairment of performance. The principles of reconstructive surgery will be discussed.