steve laslovich

05-16-2011, 07:30 PM

The literature often refers to rolling of the convex surface of a diarthodial joint when describing motions involving convex concave articular surfaces. Rolling is defined as a combination of pure rotation and pure translation. The ratio of pure rotation to pure translation that occurs during individual movements is reported to vary depending on joint geometry as well as via passive and active mechanisms involving muscles, ligaments, and capsules, etc.

1. My first question is that if no translation is occuring (rolling with slipping) of a convex member's articular surface on its mated articular surface should we always refer to this as spin and therefore not ever consider this as rolling. My problem is with understanding the terms when considering the location of the purported mechanical axis for rotation. If pure rolling occurs, and depending on my reference plane, the axis for rolling would occur at the contacting articular surfaces. Most of the literature refers to the mechanical axes lying within the convex members of most human diarthrodial convex concave joints some distance from the articular surface. If the axis is not at the surface and no translation occurs why do we still call this rolling? For example, in spite of the research which shows very little translation of the humeral head during frontal plane motion of the humerus (abduction) we still say the humeral head rolls upwards and slides downward (based on lever system mechanics). It would seem that this is just describing rotation. If the mechanica axis occurs at the joint surface then lever laws to create obligatory opposite translation to the direction of a convex's member's roll do not apply. Is there a mathematical relationship between how far the mechanical axis (lets say within the humeral head during abduction) lies from the surface and the degree of rolling (rotation with translation) that potentially occurs on a surface where friction between the two is present?

2. My second question involves more about rolling. Since the articular surfaces generally exhibit an extremely low coefficient of friction how could true rolling occur unless there is a force that creates translation. Is it absolutely necessary to have static friction to initiate rolling of a convex surface in human joints? If a force is applied to a bone with a convex joint surface and friction is essentially minimal would the convex member simply spin. It seems to me that to create a rolling motion on a nearly frictionless surface I need both a linear and rotation force to create a roll. If an upward roll occured during humeral abduction I need a linear force that creates an upward translation if friction is minimal or absent. Can someone please explain what I am missing in my understanding?

3. I have been told repeatedly that for rolling to occur in a joint with a very low coefficient of friction that the joint surfaces must be incongruent. I do not see the mechanics of how incongruent surfaces create rolling. Can someone please explain this?

Thanks so much in advance.

Steve Laslovich

University of St. Augustine for Health Sciences

San Diego, California

1. My first question is that if no translation is occuring (rolling with slipping) of a convex member's articular surface on its mated articular surface should we always refer to this as spin and therefore not ever consider this as rolling. My problem is with understanding the terms when considering the location of the purported mechanical axis for rotation. If pure rolling occurs, and depending on my reference plane, the axis for rolling would occur at the contacting articular surfaces. Most of the literature refers to the mechanical axes lying within the convex members of most human diarthrodial convex concave joints some distance from the articular surface. If the axis is not at the surface and no translation occurs why do we still call this rolling? For example, in spite of the research which shows very little translation of the humeral head during frontal plane motion of the humerus (abduction) we still say the humeral head rolls upwards and slides downward (based on lever system mechanics). It would seem that this is just describing rotation. If the mechanica axis occurs at the joint surface then lever laws to create obligatory opposite translation to the direction of a convex's member's roll do not apply. Is there a mathematical relationship between how far the mechanical axis (lets say within the humeral head during abduction) lies from the surface and the degree of rolling (rotation with translation) that potentially occurs on a surface where friction between the two is present?

2. My second question involves more about rolling. Since the articular surfaces generally exhibit an extremely low coefficient of friction how could true rolling occur unless there is a force that creates translation. Is it absolutely necessary to have static friction to initiate rolling of a convex surface in human joints? If a force is applied to a bone with a convex joint surface and friction is essentially minimal would the convex member simply spin. It seems to me that to create a rolling motion on a nearly frictionless surface I need both a linear and rotation force to create a roll. If an upward roll occured during humeral abduction I need a linear force that creates an upward translation if friction is minimal or absent. Can someone please explain what I am missing in my understanding?

3. I have been told repeatedly that for rolling to occur in a joint with a very low coefficient of friction that the joint surfaces must be incongruent. I do not see the mechanics of how incongruent surfaces create rolling. Can someone please explain this?

Thanks so much in advance.

Steve Laslovich

University of St. Augustine for Health Sciences

San Diego, California