View Full Version : Re: Joint stability, any standard definition?

08-08-2010, 11:14 PM
Paolo and Biomech-l,

Let me start my comments with an example to illustrate the ideas of
stability and control for the sake of subscribers who aren't sure.

Many people believe the Wright Brothers invented the airplane in 1903.
They did not. In 1901, Wilbur Wright noted there were plenty of flying
machines and lots of engine designs to go with them. The ability to
control these components, he explained, would usher in the age of
heavier-than-air flight. The Wright Brothers own patents on devices to
control the motion of their flying machine.

In biomechanics, we know the skeletal system and the muscles that affect
joint position as the Wrights understood their systems. Controlling the
dynamic processes in gait to prevent falls, negotiate obstacles, carry
loads, adjusting to terrain are analogous to maintaining altitude and
course, maneuvering, take off and landing. Instability in any area of
gait (or flight) may result in system failure or loss of function.

A biological system has a functional envelope: homeostasis. In
engineering terms, this is steady state. In the uncontrolled condition,
Newton's Laws are appreciated. A body in motion or at rest keeps doing
that until acted on by a force changing position and speed setting a new
steady state. A feedback control system responds to the outside effect
to restore the steady state. In the airplane, the pilot is a crucial
part of the feedback control although electronics and computerized
systems are part of this system as well. When system failure occurs
(crash), the structural and mechanical parts are examined but so are the
command and control components. In joint motion, stability comes from
the structural (skeletal) and mechanical (muscles, tendons, ligaments)
parts and from the command and control components (central and
peripheral nervous system).

As an example of joint stability, consider the ankle. In the case where
a clinically, excessive passive range of motion is noted and the subject
is asymptomatic, a dynamic control system is at work, the ankle joint
axis functions where it should and proximal and distal joints work
within normal limits. Another ankle with normal range of motion
subjected to a severe sprain which tore the lateral joint capsule has a
disturbed proprioceptive feedback: functional instability. Adjacent
joints are affected by the uncertainty in placing the ankle and the
involved muscles function outside their normal phasic activities.

I suspect you are correct in finding no stability criteria. The
clinical assessment is too subjective and not dynamic. An investigation
into the airworthiness of a bee or a stealth fighter based on form alone
would reach a conclusion that neither could fly. I would recommend you
continue to investigate dynamic challenges to your system. The Wrights
performed extensive wind tunnel experiments (in vitro) before they were
able to get their system operational.

I think you'll find tracking the instantaneous axis of motion an
important component of stability in general and that different criteria
are appropriate based on joint size, type and function.

At the PIPJ in particular, motion in the joint is affected by insertions
distal to the joint and the extensor sling apparatus so it's no surprise
you can find significant differences in flexion or extension indifferent
configurations. What results would you expect (as compared to the in
vivo system) if you kept each separate muscle connection intact and
tested the range of motion it produces when other soft tissue is
disconnected? Is the muscle still able to perform its function? Which
conditions allow hyperextension or cause medial or lateral deviation?

As a podiatrist, let me point out: the feet also have PIPJs and their
functional requirements may be quite different from those in the hand.
Hammer toes, claw toes, etc., may be the result of instabilities at
these joints due to failure of neuromuscular components. When you know
something about finger instability, maybe that can help understand toe

James A. Furmato, DPM, PhD
Chief Engineer, Gait Study Center
Assistant Professor, Department of Orthopedics and Medicine
Temple University School of Podiatric Medicine
TUSPM Gait Study Center
148 N. 8th Street
Philadelphia, PA 19711
Phone 215-625-5370
Cellular Phone 609-933-2017

-----Original Message-----
From: * Biomechanics and Movement Science listserver
[mailto:BIOMCH-L@NIC.SURFNET.NL] On Behalf Of Caravaggi, Paolo
Sent: Wednesday, August 04, 2010 11:28 AM
Subject: [BIOMCH-L] Joint stability, any standard definition?

Dear Biomech-l subscribers, I was wondering if any of you is aware of an
objective test to assess joint stability. According to my literature
research neither a standard definition of joint stability nor standard
evaluation tests to determine the degree of instability at joints have
been established. In most cases the level of instability is subjectively
assessed by clinicians by applying dislocating forces to the joint. When
more scientifically-objective approaches were taken, joint rotations to
triplanar joint displacements (or rotations) are normally shown.
However, although differences to the normal/stable joint are graphically
presented, when/if the joint can objectively be considered unstable is
not reported.

As far as our specific case, we are trying to quantify the level of
instability at the proximal interphalangeal joint of the finger in-vitro
through active flexion/extension of the joint following the release of
supporting ligaments and the disruption of the joint by systematic
resection of bone at its proximal aspect. We are indeed finding
significant differences in the flex/ext rotation to joint displacement
curves across different configurations (intact, disrupted..) but we are
now facing the issue of establishing some kind of objective index for
joint-stability. In other words, which variable is more relevant here
and how far from the normal/non-pathological configuration a joint can
still be considered to be stable?

Any suggestions and/or further comments on this matter are welcome.


Paolo Caravaggi, PhD
Joint Biomechanics Lab, Orthopedics dep.
University of Medicine and Dentistry of New Jersey
185 South Orange Avenue, Newark, NJ 07103
Tel. +1 973 972 1426