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

08-04-2010, 06:43 AM
Paolo and Dave,
My experience with this question is from an engineering and not clinical perspective so please forgive any na´vetÚ in that area. Classical control methods would consider an unstable system to be similar to one that is a ball sitting atop a sphere. Any slight perturbation will cause the ball to roll off in any direction with nothing to resist or control it. Likewise, human joints, though not as simple as this example, can exhibit similar characteristics. This, I assume, is the instability you are talking about.

In the ball on the sphere example I could plot displacement (y-axis) vs. force (x-axis) and get a plot that shows any force can cause an infinite number of displacements (vertical line). Now suppose I changed to the top of the sphere to add a little dished out portion before the ball could roll down the side. Then, there would be some displacement/force curve that would become a vertical line once the force got too high. The slope of this curve would be considered compliance and eventually the compliance would be infinite (vertical line). Infinite compliance is instability.

On a practical level with human joints the curves are not so simple to create or interpret, but for your specific joint you will need to come up with some way to plot compliance. Then you will need to determine some sort of threshold of where you consider instability to occur for that joint. You may consider plotting the 1st or 2nd derivatives of these curves to assist in this determination.

I hope that helps.

Robb Colbrunn
Director, BioRobotics and Mechanical Testing Core
Cleveland Clinic
Cleveland OH USA

-----Original Message-----
From: * Biomechanics and Movement Science listserver [mailto:BIOMCH-L@NIC.SURFNET.NL] On Behalf Of Dave Smith
Sent: Wednesday, August 04, 2010 1:22 PM
Subject: Re: [BIOMCH-L] Joint stability, any standard definition?

Dear Paolo

It might first be worth precisely defining what you mean by unstable. It may be like trying to define the height of a 'low table' or its usefulness in terms of its height.

Is the/a joint unstable when it exhibits an unusualy large range of motion?
Is it unstable when the joint has a large range in a plane that is not the usual primary plane of motion? Or is it unstable when the joint cannot maintain a fixed position under a certain load? or is it when the joint cannot maintain the expected supporting role during a dynamic or static action. Is it unstable just when the patient / subject says it is or is it when they experiences symptoms or show clinical signs that could be attributed to one or some of the above.

If you have a fixed definition of an unstable joint, e.g. a certain joint is unstable when it exhibits 'x' range of motion under 'y' load during an activity of interest, is it then determined to be stable if it does not fulfil those criteria even tho the patient describes the joint as unstable because it gives way during the activity of interest. This is the problem with defining such a parameter, how do you apply it to your patient if they dont fit in the box?

Without those pre set criteria the clinician has the freedom to determine what the problem is with the joint (or any other part) based on his own skill, judgement and education.

It might be useful to classify joint stability in an attempt to achieve some standardised comparative assessment data. However if the patient says it is better, then it is better.

If the patient or the clinician thinks that the joint is problematic because of its instability, then make it more stable. Why would you want to precisely define how stable it is or unstable it was before, or after, the patient said it was better?

Not critisizing, just some thoughts

Regards Dave Smith
Podiatrist, MSc App Biomech
FootHouse Podiatry
----- Original Message -----
From: "Caravaggi, Paolo"
Sent: Wednesday, August 04, 2010 4:27 PM
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

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

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