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Effects of Tibial Varum and Rearfoot Varus on Foot Function

Discussion in 'Biomechanics, Sports and Foot orthoses' started by admin, Nov 30, 2008.

  1. admin

    admin Administrator Staff Member


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    I am grateful to Kevin Kirby and Precision Intricast for permission to reproduce this November 1998 Newsletter (you can buy the 2 books off newsletters off Precision Intricast):

    EFFECTS OF TIBIAL VARUM AND REARFOOT VARUS ON FOOT FUNCTION​


    Tibial varum is a very common structural deformity of the leg. Tibial varum deformity has been defined as being that position of the distal one-third of the tibia where the distal end of the bisection line of the leg is closer to the midsagittal plane than the proximal end (Fig. 1). Tibial varum is measured with the patient standing in their angle and base of gait with the subtalar joint (STJ) in neutral position (Root, Merton L, W.P. Orien, J.H. Weed, and R.J. Hughes: Biomechanical Examination of the Foot, Clinical Biomechanics Corporation, Publishers, Los Angeles, 1971, p. 122).

    Tibial varum deformity, in an otherwise normal foot, will create a rearfoot varus deformity. Over the last thirty years, classical podiatric biomechanics theory has taught that both tibial varum and rearfoot varus deformities are suspected to cause pronation of the STJ (Sgarlato, T.E.(ed): A Compendium of Podiatric Biomechanics. California College of Podiatric Medicine, San Francisco, March 1971, p. 207). When rearfoot varus deformity is present, the plane of the metatarsal heads is in an inverted position in relation to the ground when the STJ is in the neutral position (Fig. 1).

    [​IMG]

    Figure 1. Tibial varum deformity coexists commonly with a rearfoot varus deformity, where the calcaneus is inverted to the ground when the subtalar joint is placed in its neutral position, as shown above.

    Pronation compensation for a rearfoot varus deformity occurs once the foot stands in the relaxed calcaneal stance position (RCSP). A totally compensated rearfoot varus deformity is said to occur if the RCSP is vertical or everted and the plantar forefoot comes to rest on the ground. A partially compensated rearfoot varus deformity is said to occur when the calcaneus is inverted from the ground and the STJ has reached its maximally pronated position. In other words, feet with partially compensated rearfoot varus deformity do not have enough pronation motion available in the STJ to allow the calcaneus to evert to the vertical position. An uncompensated rearfoot varus deformity is said to occur if both the calcaneus and the plantar surface of the forefoot are inverted to the ground while in RCSP. Uncompensated rearfoot varus deformity will only occur when there is no motion available at the STJ to allow pronation, such as in talo-calcaneal coalition or in post-traumatic conditions of the STJ (Sgarlato, pp. 210-212).

    Tibial varum deformity, like rearfoot varus deformity, does tend to cause pronation of the STJ in most feet. However, with increased degrees of tibial varum/rearfoot varus, supination instability of the STJ may result. Since the classical explanations describing the compensations for tibial varum and rearfoot varus deformities are insufficient to accurately describe this paradoxical observation, it is important that the podiatrist have a clear understanding of the interrelationships of the forces acting in the foot and lower extremity with these common deformities.

    The reason for the increase in the pronated position of the STJ with tibial varum and rearfoot varus deformities is not because of the inverted position of the calcaneus, it is because of the inverted position of the plantar forefoot to the ground. In other words, it is not the inverted calcaneus that causes a pronation moment across the STJ axis in the rearfoot varus foot. It is the inverted forefoot which causes the increase in pronation moment on the STJ in the rearfoot varus foot by transmitting the pronation rotational forces from the forefoot to the rearfoot.

    When a foot with tibial varum and rearfoot varus deformity is standing with the STJ in neutral position, assuming no forefoot deformities are concurrently present, the lateral metatarsal heads will have a greater magnitude of ground reaction force (GRF) against them than the medial metatarsal heads. In many cases of tibial varum and rearfoot varus deformities, in fact, only the lateral aspect of the forefoot contacts the ground while the STJ is in the neutral position (Figs. 1, 2A).
    With the forefoot now relatively inverted to the ground with the STJ in neutral position, the pronation moment on the STJ is increased since the lateral forefoot has a large magnitude of GRF underneath it, and the medial forefoot has little to no GRF underneath it. Since GRF will produce the largest magnitude of pronation moment across the STJ axis when it is acting more lateral to the STJ axis, GRF on the 5th metatarsal head will cause a larger magnitude of pronation moment than GRF on the 4th metatarsal head, which will produce a larger magnitude of pronation moment than GRF on the 3rd metatarsal head, and so on. In other words, with the forefoot inverted from the ground, the relatively large magnitude of pronation moment acting across the STJ axis will cause a strong tendency for the STJ to pronate, unless there is also a strong supination moment resisting this motion.

    The increase in STJ pronation moment will cause eversion motion of both the calcaneus and forefoot during weightbearing activities until the plane of the plantar forefoot is contacting the ground. As the plane of the forefoot starts to evert toward the ground, the GRF under the medial metatarsal heads progressively increases while the GRF under the lateral metatarsal heads progressively decreases (Fig. 2B). Depending on the spatial orientation of the STJ axis in relation to the metatarsal heads at the time of medial forefoot contact with the ground, the progressive increase in GRF on the medial metatarsal heads will have a variable influence on the production of pronation and supination moments across the STJ axis. In other words, in feet which have laterally deviated STJ axes, an increase in GRF plantar to the medial metatarsal heads will cause increased supination moment across the STJ axis when compared to the same increase in GRF on the medial metatarsal heads in feet which have medially deviated STJ axes (Kirby, Kevin A.: "Methods for Determination of Positional Variations in the Subtalar Joint Axis", J. American Podiatric Medical Assoc., 77:228-234, May 1987).
    As the foot with tibial varum and rearfoot varus continues to pronate from the neutral position, in general, the magnitude of pronation moments acting across the STJ axis (which are originating from the effects of GRF acting on the forefoot) tends to decrease since the GRF is gradually transferred from the lateral forefoot to the medial forefoot. If the increase in GRF under the medial metatarsal heads is sufficient and the position of the STJ axis is not medially deviated from normal, then there will be an increase in supination moment acting across the STJ axis as GRF is transferred from the lateral to the medial metatarsal heads. If the increase in supination moment acting across the STJ axis which results from the transfer of GRF from the lateral to the medial metatarsal heads is sufficient to allow a balancing of the supination and pronation moments acting across the STJ axis at that rotational position of the STJ, then the foot will stop pronating and the position of RCSP will result (Fig. 2C). RCSP is then, by definition, a position of rotational equilibrium for the STJ where the supination moments and pronation moments acting across the STJ axis are exactly counterbalanced (Kirby, Kevin A.: "Rotational Equilibrium Across the Subtalar Joint Axis", J. American Podiatric Medical Assoc., 79:1, January 1989).


    If, however, during the pronation motion of the tibial varum/rearfoot varus foot from STJ neutral position to RCSP, the maximally pronated position of the STJ is reached before the transfer of GRF from the lateral to medial forefoot is sufficient to stop pronation motion of the STJ, then a special set of circumstances arise. As the STJ pronates into the maximally pronated position, the lateral process of the talus glides along the posterior facet of the talo-calcaneal joint until it first touches, and then compresses, against the floor of the sinus tarsi of the calcaneus (Kirby, 1989). This stoppage of pronation motion of the STJ, which results from the increase in interosseous compression forces within the sinus tarsi stops the eversion motion of the calcaneus in relation to the ground and rapidly stops the transfer of GRF from the lateral to the medial forefoot.

    Therefore, it is possible, and not uncommon clinically, that a foot with tibial varum/rearfoot varus deformity have a maximally pronated STJ, have an inverted calcaneus and have greater magnitudes of GRF on the lateral forefoot than on the medial forefoot while standing in RCSP. The relatively large magnitude of pronation moment acting across the STJ which results from the increased magnitude of GRF on the lateral forefoot, compared to the medial forefoot, will be counterbalanced by a relatively large interosseous compression force within the sinus tarsi. The effect of the floor of the sinus tarsi of the calcaneus forcefully pushing on the lateral process of the talus in a superior-posterior direction, preventing the STJ from pronating further, not only creates this interosseous compression force within the sinus tarsi but also creates a STJ supination moment which counterbalances the STJ pronation moment created by the increased GRF on the lateral forefoot in this type of foot (Kirby, 1989).

    [​IMG]
    Figure 2. The illustrations above show the posterior aspect of the right foot with tibial varum and rearfoot varus deformity pronating from subtalar joint (STJ) neutral position (A), to relaxed calcaneal stance position (C). When the foot is in STJ neutral position (A), ground reaction force (GRF) on the lateral metatarsal heads causes a strong pronation moment across the STJ axis. The result of the STJ pronation moment is for the STJ to pronate (B). This pronation motion eventually causes the medial forefoot to also contact the ground, lessening the GRF under the lateral metatarsal heads. As the foot continues to pronate until it is in its relaxed calcaneal stance position (RCSP), the transfer of GRF from the lateral to medial forefoot decreases the STJ pronation moment (C). RCSP will then only occur when the STJ pronation moments exactly counterbalance the STJ supination moments so that rotational equilibrium across the STJ axis can be achieved. Note also that as the STJ is pronating from the STJ neutral position (A) to RCSP (C), the spatial orientation of the STJ axis is rotating internally and translating medially in relation to the ground. This relative movement of the STJ axis in relation to the plantar foot greatly affects the STJ pronation and supination moments caused by the actions of GRF on the plantar foot.

    As mentioned earlier, as the degree of tibial varum and rearfoot varus deformity increases, the tendency for supination instability of the STJ likewise increases. This seemingly paradoxical phenomenon is easily explained by the fact that an inverted calcaneus will cause the plantar, or weightbearing aspect, of the calcaneus to be more medial to the STJ axis than normal. Since the part of the plantar rearfoot which is located most inferiorly has been shown to be the medial calcaneal tubercle, then GRF on the plantar calcaneus will tend to be concentrated inferior to the medial calcaneal tubercle. In addition, since the STJ axis is located lateral to the medial calcaneal tubercle in the vast majority of feet, then GRF acting on the plantar calcaneus will produce a significant STJ supination moment (Kirby, Kevin A., Alan J. Loendorf, and Renee Gregorio: "Anterior Axial Projection of the Foot", J. American Podiatric Medical Assoc., 78:159, April 1988).

    By definition, as the degree of rearfoot varus deformity increases, the calcaneus becomes more inverted in relation to the ground when the STJ is in neutral position. The greater the degree of varus tilt of the calcaneus, the more medial will be the position of the medial calcaneal tubercle in relation to the STJ axis. In these feet, the increased moment arm from the medial calcaneal tubercle to the position of the STJ axis increases the STJ supination moment that GRF exerts on the plantar calcaneus. Therefore, if the degree of tibial varum/rearfoot varus deformity is of sufficient degree so that a very large magnitude of STJ supination moment results from the action of GRF on the plantar calcaneus, then supination instability will result. A few of the clinical signs of supination instability are recurrent inversion ankle sprains, peroneus brevis tendinitis or peroneus brevis muscle strain (Kirby, Kevin A. Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997, pp. 177-178, 187-188, 273-274).

    In summary, lesser degrees of tibial varum and rearfoot varus deformities will cause STJ pronation away from the neutral position due to the mechanical necessity for the forefoot to become plantigrade on the ground, not because of some mechanically curious “necessity” for the calcaneus “to come to vertical”, as has been taught in classical podiatric biomechanics theory for the last thirty years. In addition, as the degree of tibial varum and rearfoot varus deformities increases, so does the tendency for supination instability of the STJ to occur during weightbearing activities. Therefore, it is the relatively inverted position of the forefoot to the ground, not the inverted calcaneus, which causes the STJ pronation which is seen in individuals with tibial varum and rearfoot varus deformities.

    [Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 123-126.]
     
  2. Admin2

    Admin2 Administrator Staff Member

  3. This newsletter from over 10 years ago is one of the first times where I tackled the task of trying to explain Root biomechanics with Subtalar Joint Axis Location/Rotational Equilibrium (SALRE) Theory. Root and coworkers taught that an inverted calcaneus would naturally try to "pronate to vertical" implying that, for some unknown biomechanical reason, that the calcaneus vertical position was the most natural or stable position of the calcaneus. This newsletter also describes one of the paradoxes of Root theory, where Root and coworkers taught that less rearfoot varus deformity would cause subtalar joint (STJ) pronation and more rearfoot varus deformity would cause STJ supination. I remember, as a podiatry student hearing these lectures and thinking that this didn't make good mechanical sense to me why a lesser deformity would cause STJ motion in one direction and why a greater deformity would cause STJ motion in the other direction!

    The ability of SALRE theory to explain such clinical observations as I have noted above, that Root and coworkers had difficulty with using their premise that "the calcaneus would seek vertical position", I believe, is one of many benefits of SALRE theory for the clinician that is trying to understand why the foot reacts the way it does when certain structural "deformities" exist within the foot and lower extremity. SALRE theory predicts that the inverted calcaneus of rearfoot varus does not, by itself cause pronation as I was taught in podiatry school. SALRE theory predicts that it is the forefoot which is inverted to the ground in a rearfoot varus deformity in the STJ neutral position that will cause the STJ pronation moment that will tend to force it into pronation since the effect of ground reaction force plantar to the calcaneus is to cause a STJ supination moment that tends to supinate the STJ, not pronate the STJ. The foot will seek its position of relaxed calcaneal stance position only when rotational equilibrium across the STJ (i.e. summation of STJ pronation moments = summation of STJ supination moments) has been achieved.

    I just wish that I was taught such simple physics concepts when I was either a podiatry student or a Biomechanics Fellow. Understanding "podiatric biomechanics" would have been much easier.
     
  4. Lawrence Bevan

    Lawrence Bevan Active Member

    Kevin

    I know you are phenomally busy but..........

    When will the "Newsletters vol 3" be out?????????????!!!!!!!!!!!

    Cheers
    Lawrence
     
  5. Steve The Footman

    Steve The Footman Active Member

    I think a lot of people are hanging out for it.
     
  6. Griff

    Griff Moderator

    I am also hoping that Precision Intricast use the same 'marbled' effect for the cover as the previous 2 books so that when it goes on my shelf next to Vol 1 and 2 my mild obsessive compulsive disorder is sated.

    Any chance you could put in a word for me Kevin...
     
  7. Lawrence, Steve and Ian:

    I am currently compiling the third Precision Intricast Newsletter book that contains 74 newsletters from November 2002 to December 2008. Yes, the "marbled" cover is still planned to be in place. I just organized the newsletters into chapters this weekend and have the chronology page finished. Now for the finishing touches: contents, preface, acknowledgements, index, title pages and, finally, find a printer that will do a nice job for us.

    It is very encouraging to hear that so many of you find these books helpful. That was what the original idea was behind these newsletters that first started in October 1986 and has now grown to a total of 258 monthly newsletters I have written over the past 22+ years. The late Paul Rasmussen, owner of Precision Intricast, and I wanted to provide knowledge to clinicians on foot and lower extremity biomechanics and orthosis therapy that otherwise would have been unavailable from any other source.

    We will see whether I have any more newsletters left in me after I finish this project.:morning:
     
    Last edited: Dec 1, 2008
  8. CEM

    CEM Active Member

    good to hear of the 3rd book coming out, i will be in the queue to get that when it is available
     
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