Biomechanical stress [Mechanical engineering]

Biomechanical stress

what leads to excessive tissue stress and injury?

Injury risk profile examplesWelcome back to Trevor Prior for another great post on biomechanics. Trevor is a fantastic London based podiatrist, working at Premier Podiatry with more than 30 years clinical experience. Some great discussion of biomechanical principles and concept of different biomechanical factors building the puzzle for individual injury risk profile. Over to you Trevor ……………..

We have recently discussed the relevance of static measures in clinical practice and how this helps us to determine the functional capacity of an individual. However, it is clear that static assessment at best only predicts a percentage of dynamic function.

Tissue-stress model

In 1995, McPoil and Hunt1 reviewed the literature available at the time and proposed a tissue stress model for assessment and management. They noted that the evidence available indicated that the traditional approach to assessing foot function and subsequent management proposed by Root, Orien & Weed2, 3 may lack validity because of the following:

  1. Poor inter tester reliability of subtalar neutral
  2. The normal foot alignment proposed by Root, Orien and Weed2, 3 was not representative of the general population
  3. The subtalar joint did not function around the neutral position as defined by Root, Orien and Weed2, 3

As a result, McPoil and Hunt1 proposed a tissue stress model based on concepts already established in clinical practice. Specifically, they described a load deformation curve which consisted of:

  1. An elastic region which represents the normal range of stress applied to a tissue during activity
  2. A plastic region which represents excessive load and injury to the tissues
  3. A zone of micro failure which separates the two regions rather like a transition zone

Treatment aims to maintain or return the tissue to the elastic region although clearly, if there has been permanent damage to the tissue (i.e. tendon attenuation or tear), this tissue will now be less resistant to load.

Part of the aim of this approach was to allow flexibility in the assessment and management of foot disorders.

In a roundtable discussion article, Simon Spooner4 noted how the rotational equilibrium theory described by Kirby5 provided insight into how mechanical modelling of forces acting on structures can be applied to the foot and lower limb. By combining this with the tissue stress approach, it provides the clinician with a model by which to manage patients.

Zone of Optimal Stress (ZOOS)

Spooner introduced the concept of a zone of optimal stress (ZOOS) such that, for any tissue or structure, there would be a range of loading within which that tissue could function and remain healthy. Tissue dysfunction can occur when either the load is excessive (i.e. out of the ZOOS) or the properties of the tissue have reduced in relation to normal load (or both).

The natural progression is that management should aim to alter the loading of the tissue such that the tissue is functioning within its ZOOS to allow it to heal.



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