Hi Marie,

I always like reading Jeff's comments. Luckily we don't have to contend with
those temperatures normally.
Anyway if you are really determined to use polypropylene then do exactly as
Jeff suggested, but with
those temperatures there will possibly be still a chance of fracture.
Composite devices, i.e. carbon shells may be far less likely to fracture if
there is time to get another pair made and checked.

Cheers

Shane Toohey

----- Original Message -----
From: "Jeff Root" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, September 10, 2004 6:30 AM
Subject: Re: Suitability of orthotic materials in extreme cold


> > I have an orthotic  patient who is planning to walk to the North Pole
next
> > year.
> > I need to know the most suitable material to use for his Arctic boot
> > orthoses.
> > The likely air temperature will be -40C most of the time. Will
> > Polypropylene
> > become
> > brittle at these low temperatures?
>
>
> Marie,
>
> Polypropylene becomes brittle as it approaches its glass transition point.
> If you freeze a piece of polypropylene and strike it or drop it, it can
> shatter in a somewhat glass like manner (but less fragmented).  I don't
> remember the exact glass transition temperature but it is slightly above
the
> freezing point of water.  The warmer the polypro, the more flexible it
> becomes and the cooler it is, the more stiff and brittle it becomes.  For
> example, if a patient is working in a factory in a warm climate and
wearing
> a boot that retains heat, their orthosis will be more flexible.  The more
> force and the more heat, the greater the tendency there is for material
> creep or shape change, but the less tendency there is for fracture.  I had
> one case of an individual who worked inside a large oven in a carbon plant
> and his polypro orthoses actually had melted on the lateral, plantar edge
> due to the heat.  Fortunately he had Spenco like covers on them.
>
> Freezing temperatures will make this patient's orthoses less flexible and
> potentially more susceptible to breakage if the temperature within the
boot
> becomes to cold.  If his boots are off and his orthoses become exposed to
> the cold, then they could fracture when he first starts to walk with them.
> I would advise him to make sure that the devices get warmed up inside his
> boots before he bears any weight on them.  Any type of insulating top
cover
> might allow the orthoses to become colder within the boot than they would
if
> they had no top cover or just a thin, vinyl or leather cover.  In other
> words, there will be better heat conduction between the feet and orthoses
if
> the orthoses don't have any type of insulating top cover on them.  You
might
> want to bottom fill the entire plantar surface of his orthoses with EVA.
> This would act as an insulating layer between the ground (snow or ice) and
> the orthosis and would help retain body heat within the polypro shell.  It
> would also act to support the shell against the boot and help resist
> breakage by reducing orthotic deflection, which might prevent breakage if
> the devices do become very cold.
>
> Respectfully,
> Jeff Root
>
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