As always, you have some good points to make. I do not disagree with the ideas about moments around the subtalar joint axis. I likewise utilize such thinking in many of my patients.
As I approach the patient, though, my thinking always goes like this: 1) what anatomical structure(s) are painful? 2) What type of mechanism could produce such pain? (i.e. tension, compression, etc.) 3) Why is the mostly likely mechanism occurring? (pronation of the STJ, weakness of the peronei, etc.) 4) What are the goals of the orthotic therapy? The more specific I can identify the goals of therapy, the more likely I am to achieve those goals and have a happy patient.
While orthotics always change moments around at least one joint of the foot (actually any pad or rock in the shoe will change the moments around at least one joint of the foot), sometimes my only goal is a redistribution of pressure. You have probably noted how "rigid" orthotics alleviate pressure under the calcaneal tubercle. Maybe that's all I need to do, and I don't even worry in that case too much about how much the moments around the STJ axis are being changed. Sometimes the orthotic only decreases the plantarflexion moment around the midfoot joints being created by the plantar intrinsic musculature in early propulsion. Again in this case no attention is being paid to the STJ axis moments. As you have probably already noticed, not everyone with heel pain is a pronator, yet orthotic therapy seems to help a great many that seem to have little if any abnormal rearfoot pronation.
My comments about division of feet were intended not to be a precise suggestion, but only to demonstrate how little attention is paid to specific foot types or etiologies of the abnormal pronation or supination in most of the orthotic studies. Usually what we get is a statement like this, "XX number of subjects were chosen. All were healthy with no symptoms in their feet and no history of injury..." There is no indication of what types of feet were examined, whether they looked only at feet with laterally displaced calcanei, medially displaced STJ axes, forefoot varus feet, feet with greater than 20 degrees of frontal plane movement around the MTJ, or any other of a number of proposed foot types. Therefore, with such a broad spectrum of foot types in the study it may not be possible to statistically prove or disprove the hypothesis of the study. The possibility of type 2 errors seems to be much higher than the power studies would predict.
Again thank you for your important comments. :) I'm pretty much in the same track you're in about moments and I appreciate the tremendous work being done by you and the many others out there to put more mechanics into biomechanics.
I trust that you and yours had a great holiday season. I will try to make a few more postings in the future than I have done the past year (if I can get time away from printing so many pictures of the granddaughter - now I know why research productivity decreases after the grandchildren come).
Best of wishes for 2005
P.S. - I would like to propose a couple of questions:
1) Is it ever possible to stand with all the plantar muscles of the foot relaxed, and have any of the joints of the midfoot (MTJ, CNJ, MCJ) not dorsiflexed to their end range of motion? If so, where are the plantarflexion moments coming from that would equalize the dorsiflexion moments being placed around these joint by the GRF?
2) With a Kirby skive, is the calcaneal fat pad directly above the skive displaced medially or laterally? How could we prove or disprove any statements about movement of the calcaneal fat pad?