We all know what functional hallux limitus is. It is that condition in which a full range of dorsiflexion is available at the first MPJ, but for some reason, during gait, the joint just stops moving. This results in compensations that can be pathologic if the forces are great enough. The diagnosis of FHL is not easy and has led to some controversy around the very existence of the condition.
We have also done some work on windlass function and have built a device that measures the force to dorsiflex the hallux to any number of degrees. So we get a force degree curve for the windlass (at least that is what I thought we were doing!). It was obvious that many people have a high force to establish the windlass mechanism and some have a low force. Some people have quite a delay during dorsiflexion before resistance (ie the windlass) is felt. Some people have an immediate resistance to dorsiflexion from the windlass. We have done a lot of testing and experiments with this device and the effects of various interventions.
In my thought processes, the water was starting to get murky between just what is FHL and how is that related to the various windlass dysfunctions (ie high force to establish or a delayed onset) and how the different interventions for FHL and windlass functions work.
Then I realised, that the device we were using to measure windlass function, was not really measuring windlass function. It was really measuring dorsiflexion stiffness of the first MPJ (of which the windlass probably contributes almost all of the stiffness!). This lead me to rethink some of what is happening at the first MPJ.
Think this through:
- Imagine a force degree curve, with degrees of dorsiflexion along the x axis and forces in Newtons along the Y axis. Everyone will have a different shape to that curve
- Some people will have a steep slope. Lets call this high dorsiflexion stiffness (previously called a high force to get the windlass established)
- Some people will have a flatter slope to that curve. Lets call this low dorsiflexion stiffness (previously called a low force to get the windlass established)
- Some people will have a very flat slope, then a steep slope (previously called a delayed onset to the windlass).
Also, think this through:
- A hallus rigidus will have a vertical stiffness curve. No amount of force will move it from 0 degrees
- A structural hallux limitus, will have a slope to the curve (may be high or low), but after x degrees of dorsiflexion, the stiffness curve will go vertical (no amount of force can move it beyond that x degrees.
So what really is functional hallux limitus:
All functional hallux limitus is: its a temporary increase in stiffness (or slope of the curve) at the first metatarsophalangeal joint. That temporary increase in stiffness may be so small, that the body does not notice. Or that temporary increase in stiffness may be so great that the body has to get around it to continue moving forward (ie the traditional compensations for FHL).
We really need to reconceptualise hallux rigidus, structural hallux limitus, functional hallux limitus and the windlass mechanism dysfunctions in terms of the force/degree curve (ie stiffness curve) of the first MPJ.
Theoretically, the aims of foot orthoses are to lower the stiffness curve (make it easier for the windlass to get established), steepen the initial part of the curve if its flat (get the windlass to come on earlier) and smooth the curve (get that temporary increase in stiffness out of the curve, ie functional hallux limitus).
See Podiatry Arena for more on this.