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Extrinsic + Intrinsic Rearfoot Posting

Discussion in 'Biomechanics, Sports and Foot orthoses' started by stacer, Mar 30, 2010.

  1. stacer

    stacer Member

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    I would be grateful for an explanation of the rational behind the prescription of both intrinsic (Kirby Skive) and extrinsic rearfoot posting in the same orthoses - is the effect cumulative? I have recently taken on a patient whose existing prescription calls for an 18 degree "Kirby" together with a 6 degree extrinsic rearfoot post.

    Many thanks

    Richard Stacey
  2. Richard, the effect is likely cumulative, but probably not in a simple additive fashion. Whoever prescribed the original device was clearly trying increase supination moment about the subtalar joint axis. The medial heel skive will change the shape of heel cup, adding an extrinsic rearfoot stabiliser/ post will effectively increase the angle of the skive (without changing it's depth) and also reduce the span length of the orthosis and in so doing relatively stiffen the medial longitudinal arch section of the foot orthoses.
  3. P.S. the extrinsic post will also influence the frontal plane angulation of the entire shell.
  4. Simon and others,

    The device that Richard discribes ( I understand that I have not seen the foot) sounds extreme to me and the patient picture I get in a pes plano- navicular on the ground with 150kg of body weight to require this much ORF STJ supination moment. I would also be concerned with lateral ankle and peroneal overuse in the future.

    Would you agree with me or not ? as much as you can without seeing the foot of course
  5. stacer

    stacer Member

    Hi Simon,

    Many thanks for that - the patient concerned is a 13 year old boy who initially presented about a year ago to a previous podiatrist at the practice due mainly to a concerned mother rather than any painful symptoms. They have returned due to recent discomfort from the 1st MPJ cut out which I suspect is caused by him outgrowing the orthotic. Other than that there have been no problems and in the view of the mother (an NHS Physio) the orthotic has been very successful (but given the lack of symptoms at initial presentation I'm not sure how that judgement was made)

    I have only seen this patient briefly and I am scheduled to carry out a full examination at the end of April and I am keen not to try and fix it if it ain't broke!!
    The foot appears to be a textbook congenital pes plano valgus type and apart from the orthotic related discomfort remains asymptomatic so the original prescription with the large rearfoot posting appears to have done little harm.

    Nice hearing from you again after all these years



  6. Richard, Could you get any further away from the South West, without leaving the mainland?;)

    As my colleague said, that's a pretty aggressive device. Remember that there is always more than one way to skin a cat... But like you, if it ain't broke....
  7. stacer

    stacer Member

    Hi Simon,

    Looking out of my window at the b........y snow I wish I had stayed in Cornwall but Hey Ho life here has other compensations - the malt whisky isn't too bad and the golf is cheap what more do you want!!

    It's the extreme level of posting which causes me some concern particularly its possible long term effects. I'll have another whisky and think about it some more!!


  8. I'd assess the patient as if YOU were the first one to make him a pair of devices and then decide on the prescription YOU would make. It's not unusual to employ a deep medial heel skive in association with an extrinsic rearfoot post and an inverted pour to the positive cast.
  9. stacer

    stacer Member

    Hi Simon,

    Many thanks for your help, I'll let you know the outcome once I've assessed the patient and you can be assured that any prescription I write will be based on my findings rather than the existing device.

    Best wishes

  10. Richard:

    In addition to Simon's many fine comments, the ultimate test of any orthosis modification for the treatment of a flatfoot deformity is gait function and subjective "feel" of the orthosis. In my practice, orthoses with a large medial heel skive, deep heel cup and inverted balancing position are routine for treatment of the more significant pediatric flatfoot deformities. If the patient is comfortable, gait function if improved, symptoms are improving and there are no new complaints, then you can be confident that what you are doing is therapeutically beneficial for the patient.

    However, if you are not that confident yet with such a orthosis in your own practice, then schedule the patient back for a recheck every 6-8 weeks to monitor their progress after the orthosis has been dispensed. When I first started using the medial heel skive 20 years ago on pediatric flatfoot patients, I saw these patients more often to make sure they were still doing well after they were dispensed the orthosis. Now, that I have seen success so frequently with these techniques over the last two decades, I worry less about them and see them less often.

    Hope this helps.
  11. stacer

    stacer Member


    Thank you for that, I was introduced to your paper and the concept of the medially deviated STJ axis by Simon Spooner as a student and have been incorporating "The Kirby" medial heel skive in my orthotics for around 10 years now with consistently good results both in the paediatric flat foot and older age groups typically with dysfunctional Tibialis Posterior. My problem on this occasion was its use in conjunction with an extrinsic rearfoot posting, something I had not encountered previously particularly the large angles involved - 24 degrees in total. As an aside, I have always determined the size of the skive by its depth which is simple to apply to the level of deformity rather than an angular value which implies an accurate measurement!!
    In this case it is difficult to assess the impact of the current orthotic since I wasn't involved in the original assessment and I cannot say whether gait function is improved but the patient is comfortable. I am however reassured that that a large amount of supinatory correction is probably appropriate for this patient and I shall bear this in mind when I see this patient for a full assessment at the end of April.

    Many thanks for taking the time out to comment.


  12. efuller

    efuller MVP

    Hi all,
    I agree with Kevin's and Simon's points.

    Kevin, your original medial heel skive paper mentioned that you would use 2, 4, 6 & 8 mm skvies. I recall you lecturing and saying that you rarely use, or don't use the 8 mm skive. I rarely go over 4mm, but I do modify the lateral expansion so that it goes on the plantar surface of the cast to effectively inrease the varus wedge effect. The reason that I don't go higher than 6 is the amount that it changes the shape of the arch and heel. At 8 mm in some feet you will start grinding away the entire plantar surface of the heel of the cast and reduce the arch height. (Or you can change the angle of the skive away from the original paper's suggestion of 15 degrees.)

    Richard, it is important to realize that the patient has to keep their heel on top of the finished device. A high lateral heel cup is important in this regard so that the patient doesn't slide off of the device. One way to figure out if there is too much wedge effect is to look at the patient as they stand on their device out of the shoe. If they are strugglling to keep their foot on the heel of the device then there is too much angulation. Also, the may develop a callus where the lateral heel meets the top of the orthotic. If your patient stands comfortably on the device then there probably is not a problem.

    On the other hand, experimenting on my own foot, there was a point at which more wedging started to cause peroneal fatigue. There may be some feet that have STJ axes so far medial that this may never happen no matter how big the wedge is. If you reach a point where they start to get peroneal fatigue, you can always grind away some of the medial side of the extrinsic rearfoot post.


  13. Richard:

    You seem to have a good appreciation of the concepts of the mechanical effects of the varus heel cup shape that the medial heel skive creates in an orthosis. Of course, you did have the good Dr. Spooner as a professor....was he still considered a heretic back then when he taught you since he didn't want to teach that all orthoses should be balanced to vertical with a 4/4 degree rearfoot post and intrinsic forefoot balancing?:rolleyes:

    That's probably another story for another time.

    As Eric mentioned, I normally do a 2-6 mm medial heel skive and haven't used an 8 mm medial heel skive for at least a few years since it seems to cause more problems. The angle of skive can be varied of course but, through trial and error, I found that a 15 degree varus angle worked pretty well so I would suggest that this is the best angle to start out with.

    Honestly, I never thought the medial heel skive would become that popular of an orthosis modification when I first started using it 20 years ago. I believe it is used worldwide as an effective orthosis modification to add extra supination moment effect to a foot orthosis. I do remember, however, that I was worried of possible medial heel irritation and oversupination problems with a medial heel skive added to an orthosis when I first created it, but found that, often times orthoses were much better tolerated and comfortable for patients with the correct amount of medial heel skive added to the orthosis.

    If you look back at the medical literature, Percy Willard Roberts first described an inverted heel orthosis nearly a century ago in 1916 that uses similar principles as that of the medial heel skive, Blake inverted orthosis, and other inverted heel cup orthoses. I have attached the paper by Roberts for your reading pleasure.

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