Extrinsic flexor muscles generate concurrent flexion of all three finger joints
Introduction
Proper understanding of finger biomechanics could greatly facilitate hand rehabilitation, whether through guiding reconstructive surgeries (Keith et al., 1996), neuromuscular electrical stimulation protocols (Lauer et al., 1999), or other modalities. Finger biomechanics, however, has proven difficult to comprehend, due to the seeming redundancy in muscle actuation of the joints, and to the potential for each finger muscle to affect multiple joints.
One mechanism not clearly understood is the control of flexion of the metacarpophalangeal (MCP) joints. A number of muscle tendons cross this joint on its palmar side, such as the extrinsic flexor muscles [flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS)], and the intrinsic hand muscles [the lumbricals and interossei]. As FDS and FDP do not attach to the proximal phalanx, traditionally the extrinsic flexors have been thought to have only tertiary involvement in MCP flexion, either by acting on the MCP joint only after completing flexion of the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints, respectively, or by acting only indirectly through the connection between FDP and the lumbrical (Long, 1968; Zancolli, 1979). While current thinking suggests a greater role for the extrinsic muscles in assisting MCP flexion, the intrinsic muscles are still seen as the primary MCP flexors, especially in regard to initiation of MCP flexion (Moore and Dalley, 1999; Snell, 2000).
Yet, in voluntary all-joint finger flexion movements, FDS and FDP were observed to be the primary agonists (Darling et al., 1994). In addition, MCP flexion moment arms for FDS and FDP, determined experimentally from cadaver studies, are much greater than those of the lumbricals and interossei (An et al., 1983). These discoveries imply that FDS and FDP participate in initiation of MCP flexion.
Two mechanisms exist by which the actions of the extrinsic flexors might be communicated to the MCP joint. First, the extrinsic flexor tendons pass through annular and cruciform pulleys attached to the finger phalanges (Bejjani and Landsmeer, 1989). These pulleys could translate force from the flexor tendons to the proximal phalanx. Second, the inherent passive resistance of the joints to rotation might allow moments created about the DIP and PIP joints to be propagated to the MCP joint.
Thus, the purpose of this research was to test whether it is biomechanically feasible for the extrinsic flexors to initiate concurrent flexion at all three finger joints, through the use of a biomechanical model of the index finger. The relative contributions of the passive joint torques and the pulley mechanisms to concurrent flexion were examined as well through the model.
Section snippets
Methods
A computer model was created to simulate FDS and FDP shortening in the index finger. Model parameters, such as segment length and diameter and passive static and dynamic resistance to joint rotation, were obtained experimentally. Model results were validated through comparison with index finger motion produced by electrical stimulation of FDS and FDP.
Results
Our biomechanical model showed that shortening of FDS and FDP muscles could initiate concurrent flexion at all three joints, though the inclusion of passive joint torques was necessary to achieve this. Inclusion of the pulley mechanisms in the model with segment mass and inertia (Table 1), but without passive joint torques, resulted primarily in flexion of only the PIP joint, which increased by 75° from its starting posture. The MCP and DIP joints experienced slight extension (Fig. 5).
Inclusion
Discussion
Planning of interventions to correct impairment requires a sound knowledge of the underlying physiological infrastructure. In the case of the finger, understanding the roles of different muscles in controlling MCP flexion could help in deciding which muscles to stimulate or use for tendon transfer. The roles of the extrinsic finger muscles, especially, remain a matter of some debate.
To examine the feasibility of FDS and FDP initiating and generating MCP flexion, a biomechanical model of the
Acknowledgements
This research was supported by the Richard C. and Marion Falk Research Trust and the Whittaker Foundation. The authors would like to acknowledge the work of Dr. David Lin and the late Mr. Andrew Krylow in developing the finger actuator. The authors would also like to thank Dr. Andrew Fuglevand for his advice in regard to the intramuscular stimulation.
References (28)
- et al.
Tendon excursion and moment arm of index finger muscles
Journal of Biomechanics
(1983) - et al.
A dynamic model for finger interphalangeal coordination
Journal of Biomechanics
(1988) - et al.
Coordination of index finger movements
Journal of Biomechanics
(1994) - et al.
An experimentally based nonlinear viscoelastic model joint passive moment
Journal of Biomechanics
(1996) - et al.
Dynamics of human ankle stiffness variation with mean ankle torque
Journal of Biomechanics
(1982) - et al.
Tendon transfers and functional electrical stimulation for restoration of hand function in spinal cord injury
The Journal of Hand Surgery, American Volume
(1996) - et al.
Contribution of the extrinsic and intrinsic hand muscles to the moments in finger joints
Clinical Biomechanics
(2000) - et al.
Architecture of selected muscles of the arm and forearmanatomy and implications for tendon transfer
The Journal of Hand Surgery, American Volume
(1992) - et al.
A 3-D dynamic model of human finger for studying free movements
Journal of Biomechanics
(2001) - et al.
Biomechanics of the hand
Extrinsic muscles of the hand signal fingertip location more precisely than they signal the angles of individual finger joints
Experimental Brain Research
Tendon and pulleys at the metacarpophalangeal joint of a finger
Journal of Bone and Joint Surgery, American Volume
Fine-wire electromyographic recording during force generation
American Journal of Physical Medicine and Rehabilitation
The mechanical role of the digital fibrous sheathapplication to reconstructive surgery of the flexor tendons
Anatomia Clinica
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