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Barker, Dan
11-12-1997, 04:12 AM
Dear Colleagues

As part of the design process of a new MCP joint we are reviewing
biomechanical models of the finger which provide a solution for the joint
reaction force at the MCP joint.

Many papers have tackled this problem, notably An, Chao and colleagues. The
methodology in all these papers was to solve equilibrium equations at the
MCP and IP joints of the fingers considering the application of an external
load.

To create the moment equilibrium equations, the moment arms of the tendons
were required. Moment arms, defined as the perpendicular distance from the
centre of joint rotation to the line of the tendon, were determined at each
joint based on techniques such as biplanar xray. These moment arms at each
joint were then used in the equilibrium equations. This implies that the
tendon has a line of action, at the joint under consideration, originating
from the bone distal to the joint under consideration. This to my mind does
not seem to be the case. For example the flexor profundus tendon does not
attach to the proximal phalanx, however in the models a moment arm is
assumed at the MCP joint. Surely the only action on the proximal phalanx is
via the digital sheath, completely altering the assumed line of action.

This has been noted by Delattre and colleagues "The mechanical role of the
digital fibrous sheath: Application to reconstructive surgery of the flexor
tendons. JF Delattre et al. Anat Clin (1983) 5: 187-197." These workers
pointed out the dual mechanical function of the extrinsic flexor tendons ie
the phalanges are stabilised both by direct insertion AND action of the
tendon on the tendon sheath.

Is anyone aware of force analyses which have considered this arrangement.
Have I missed some assumptions used in the previous models. My feeling is
that indirect load transfer via the pulleys of the finger must alter the
joint reaction force at the MCP joint.

As customary I will post all replies

Regards

Dan Barker (sbarkds@rgh.sa.gov.au)
Research Engineer
Repatriation General Hospital
Division of Orthopaedic Surgery
Daws Rd. Daw Park 5041 S.A.
Australia
Fax: 61 8 8374 0712
Tel: 61 8 8275 1107

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