FINGER JOINT STIFFNESS
Zong-Ming Li, Gregg Davis, Shouchen Dun
Hand Research Laboratory, Department of Orthopaedic Surgery
University of Pittsburgh, PA, USA. zmli@pitt.edu
INTRODUCTION
Finger joint stiffness, a manifestation of many joint diseases,
severely compromises hand function. One of the causes of finger joint
stiffness is the tightness of the intrinsic muscles. Clinicians routinely
evaluate finger joint stiffness to determine intrinsic tightness by
performing the Bunnell-Littler test [1]. This test involves an evaluation
of proximal interphalangeal (PIP) joint resistance against a manually
applied force at both full extension and full flexion of the
metacarphalangeal (MCP) joint. However, the Bunnell-Littler test is
subjective and largely depends on the experience and skill of the tester.
The objective of this study was to quantitatively study finger joint
stiffness with the MCP joint at different position.
METHODS
A testing apparatus (Figure 1) was constructed to collect passive PIP
joint torque-angle data of the index finger with the MCP joint at
extended (0°) or flexed (60°) positions. A low payload (3 kg) industrial
robotic arm (KR 3, KUKA Robotics Corporation, Augsburg, Germany)
was used to induce cyclic motion about the PIP joint and a miniature 6DOF force/torque transducer (Nano17, ATI Industrial Automation, NC,
USA) was attached to the end-effecter of the robot arm to measure the
passive PIP joint torque.
A thermoplastic splint maintained a
functionally neutral wrist angle of 15° extension. A lockable linkage
with a finger clamp was mounted on the splint to prescribe the desired
MCP joint angle (0° or 60°).
Ten young male subjects without diagnosed intrinsic muscle
tightness participated in this study. Each subject was tested at MCP
extension and flexion in a random order. The angular motion was
applied to the PIP joint in a cyclic manner between full extension (0°)
and 90° of flexion at 7° per second. Each trial consisted of twenty
flexion/extension cycles. Throughout testing, the subject was instructed
to remain relaxed and offer no voluntary resistance to the induced
motion. For each trial, the first ten cycles of torque-angle data were
considered as preconditioning, and the last ten cycles were averaged.
The equilibrium angle of the PIP joint was determined by the zero
torque intercept of the flexion curve. PIP joint stiffness was calculated
as the slope of the induced flexion curve in a 20° window starting from
the equilibrium angle. Paired t-tests were performed for the statistical
analyses with a significance level of α = 0.05.
decrease in equilibrium angle, (b) an decrease in flexion torque at a
given PIP joint position, and (c) an increase in extension torque at a
given PIP joint position. For the induced flexion, the average
equilibrium angle changed significantly from 47.6° ± 15.0° with the
MCP joint extended at 0° to 21.9° ± 11.4° with the MCP joint flexed at
60° (p < 0.001). This may be due to passive tightening of the PIP joint
extensors as the MCP joint assumes a flexed position. The PIP joint
showed a significant increase in stiffness from 0.44 ± 0.24 N-mm/degree
with the MCP joint flexed at 60° to 0.69 ± 0.22 N-mm/degree with the
MCP joint extended at 0° (p < 0.01). This increase is most likely due to
a passive tightening of the intrinsic muscles as the MCP assumes an
extended position, because passive tightening of the intrinsic muscles
provides an additional PIP extension torque due to their attachment to
the extensor hood via the lateral bands.
Joint stiffness is commonly evaluated based on a subjective feeling
of joint resistance against applied force, or the observation of a
limitation of range of motion. We have successfully developed a
quantitative measure of PIP joint stiffness. PIP joint stiffness is
dependant on the MCP joint angle, supporting the regulation role of the
intrinsic muscles on PIP joint stiffness, and the use of Bunnell-Littler
test for the evaluation of intrinsic muscle tightness. In future studies, we
will compare PIP joint stiffness between asymptomatic hands and hands
with intrinsic muscle tightness. We expect to find a much higher
elevation in PIP joint stiffness when the MCP joint changes from flexion
to extension for hands with intrinsic tightness. Other applications
include the examination of the effectiveness of surgical procedures of
intrinsic muscle release to reduce finger joint stiffness.
Figure 2: Representative ten cycles of torque-angle data of the PIP
joint at 60° MCP joint position.
Figure 1. Schematic of experimental setup to measure passive
stiffness of the PIP joint.
RESULTS AND DISCUSSION
A reproducible torque-angle relationship was demonstrated across
multiple cycles (Figure 2). The induced flexion and extension torqueangle curves showed distinct hysteresis. As the MCP joint changed
from an extended position to a flexion position (see Figure 3), the
torque-angle curves shifted to the left (Figure 3), leading to (a) an
Figure 3. Representative PIP joint torque-angle data of the induced
flexion portion with MCP joint at 0° and 60º.
REFERENCES
[1] Bunnell, 1953. J Bone Joint Surg Am 35: 88-101
ACKNOWLEDGEMENTS
The Pittsburgh Foundation
51st Annual Meeting of the Orthopaedic Research Society
Paper No: 0120