Laterally deviated foot types; 1st & 5th MPJ ulcerations - explanation & management

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Good afternoon.

Ulcerations due to medial column biomechanics are generally easier to both understand (for me) & successfully manage. Rarely do I struggle to get, and keep, these ulcerations healed. Therefore a lot of my thoughts the last few months has been geared towards the 'high arched' 'pes cavus' 'laterally deviated STJ' foot types with ulcerations.

I have found that these feet with ulcerations under the 1st MPJ respond very well to a heel raise, lateral forefoot post and occasionally a lateral skive. After being in insoles for some time, I have observed less tension in the peroneal tendons and musculature and a general relaxation of the soft tissue contracture/relaxation of hallux retraction improving medial slip of the fascias ability to be used appropriately instead of being in maximal tension. This makes logical biomechanical sense and 1st MPJ ulcerations are, in my opinion, predominantly sheer bought about by hip external rotation when there is no available saggital plane motion available at the 1st MPJ.

I have found that those with ulcerations under the 5th MPJ do not respond either; as well, or not at all to a similar prescription. I really do struggle to rationalise how I can get pressure off of the lateral column. These patients do have eversion range available in standing. I naively assumed that increasing STJ pronation would move pressure medially to the forefoot through propulsion.

My questions are as follows: why do these similarly structured feet appear to load either the 5th or the 1st MPJ? Is this related to 1st & 5th metatarsal length with those loading the 5th MPJ having a shorter 1st met??

What can I do, or what prescription/assessment variables have I not considered, to be able to better help my patients with 5th MPJ pressure & ulcerations??

Any recommended reading would also be appreciated.

Thanks in advance.

 

Use a valgus wedge from heel to just behind the 5th met head. Possibly add a valgus wedge under the metatarsal heads with a hole under the fifth met head. If this does not work you may have to add pronation moment from a brace, because you cannot increase the pronation moment more from the ground.

The key concept is center of pressure relative to the STJ axis. The center of pressure is the average point of force on the bottom of the foot or more precisely the point about which there is no moment. If you look at a pressure map of the foot. Each point that has force applied to it will have a magnitude for that force and a distance from the center of pressure. When you take all the force times distance from one side of the center of pressure it should equal the force times distance from the other side of the center of pressure.

To balance the foot, without any muscle contraction, the center of pressure has to be under the subtalar joint axis. When the subtalar joint axis is more lateral the center of pressure will have to be more lateral for balance to happen. This means the forces will have to be higher laterally, because, the distance to the axis is smaller.

The axis location is determined by the joint surfaces of the bones. So, if you move the bones the axis will move. If you are able to pronate the STJ then you will cause internal rotation of the talus and the axis will internally rotate and shift more medially. So having the foot in a more pronated position will tend to reduce the lateral forces.

In some feet the axis is so far lateral that there is not enough foot on the lateral side of the axis to achieve balance. In these feet there will have to be constant contraction of the peroneal muscles to keep the foot flat on the floor. Constant contraction of peroneus longus will tend to plantar flex the first metatarsal and increase load under the first metatarsal head.

I'm guessing the difference between the feet that you describe is that peroneus longus pulls the first down more and is more relaxed/less needed with the valgus wedge.

 

Cheers Eric.

To be honest I note a STJ axis is laterally deviated with associated pathology and then go about trying to fix it. I can't say I really note the level of the deviation and then collate that to whether the issue is the 1st/5th MPJ... although I will do going forwards.

I always assumed that those with the most plantarflexed 1st ray were those with the most lateral axis due to the increased activity of p.longus. I am always therefore baffled when it is the lateral Met head that is so challenging to offload. I suppose individual anatomy plays a part but there are a few patients that really have me scratching my head.

Thanks for the response as you always get me thinking. Hope you're well