Section: CME Category: Biomechanics

Using the Biomechanical Examination to Make Decisions About Flatfoot Surgery

Robert Phillips, DPM

Robert D Phillips, DPM discusses the biomechanical examination and significant steps during the exam that will determine treatment.

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Goals and Objectives
  1. Apply biomechanical measurements as an essential part of a pre-operative evaluation
  2. List important areas that must be evaluated in order to determine whether surgery is appropriate
  3. Understand non-foot forces that contribute to the creation of flatfoot
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  • Lecture Transcript
  • TAPE STARTS – [00:00]

    Male Speaker: This lecture will discuss flat foot surgery from the standpoint of making decisions about important things to document before you select and perform a flat foot surgical procedure.

    Flat foot surgery is a life-changing decision by your patient. The question is whether it will be a positive life-changing decision or a negative life-changing decision. The more information you can get before the surgery, the more likely you are to select the procedure or procedures that will make the patient grateful the remainder of their life that they selected you to do the surgery.

    At the end of this short presentation, I want each resident to make a resolution that they will make the biomechanical exam and an essential part of the pre-op exam. Can you justify that surgery is the best option? Can you justify that the procedure you choose is indeed the best procedure? I don’t believe that you can without the biomechanical exam.

    There are lots of articles about flat foot surgery. I picked one that I consider to be very typical. This is not to cast dispersion on the authors. I don’t know any of them and I commend them for at least going back and then looking at their results. But it shows that little seems to be recorded about the pre-operative criteria nor is there any record that the authors really had and analyzed the etiology of the patient’s pronation.

    As a result of this lecture, I hope to get the ball swinging away from this type of approach to our patients’ foot problems and get us thinking more about custom tailoring our patients’ specific needs.

    This letter to the editor published in podiatric management online in 2013 caught my attention. I don’t know who wrote the letter but I felt that it represented a lack of understating about what flat foot really is. It also demonstrated lack of biomechanical examination before surgery.

    I replied to the author in a letter to the podiatric management news online by listing 14 things that I felt were missing in the clinician’s report. I felt and still do that if this clinician had done a good biomechanical examination, he or she would understand what the real etiology of this patient’s pronation problems were and then make a decision about what surgical procedures would be best.

    I will only have time to cover just a few of these points. Merton L. Root was a pioneer of biomechanical thinking in our profession. This one podiatrist has been more referenced in the literature than any other podiatrist.

    One of the beauties of the Root influence on biomechanics is that Root helped us quit treating flatfeet. The Root idea of biomechanics is that each and every person has their own normal arch height.

    Much of the research that is tried to correlate actual arch height with various types of symptoms has been very disappointing. Indeed, many people walk their whole lives with medial arches making full contact with the ground and never complain of a single symptom. Many people have feet as flat as pancakes and never develop a bunion or callus or any other type of pain.

    So what did Mert Root recommend that we treat? His was a recommendation that we treat abnormal pronation. Note, I did not say pronation. There has to be a normal amount of pronation during the gait cycle. And failure to undergo that pronation will result in just as many symptoms, if not more, than if too much pronation occurs. While the ICD 10 code books still lists acquired pes planus as the diagnosis. I believe that our notes need to reflect that the patient doesn’t just meet that diagnostic criteria but meets our criteria of a higher standard. That is they have abnormal pronation.

    As I mentioned before, I believe that the biomechanical exam before surgery is as important as the pre-surgical radiographs. What are the advantages of this examination? One, 1st and foremost, the examination should identify the etiology or etiologies of the abnormal pronation. The etiology maybe an alignment problem. It may be a muscular imbalance, it may be an arthritis or it may be a neurological problem. If your examination does not reveal the etiology, I would be very hesitant to do surgery.

    Number two, as I noted before, many people go through with their entire life with very flat feet that would never give them a problem. As doctors Lovett and Cotton in 1898 pointed out, "it is those patients who have used their total reserve of pronation that will develop symptoms." I’d be very careful about doing flat foot surgery on a patient who in static stance had not used their entire reserve of pronation.

    [05:05]

    And I believe it’s important to document before surgery that the patient has no reserve of pronation when they stand.

    Number three. Your biomechanical exam should not only determine the surgery that would best address the etiology but also identify problems that could cause other compensations. For example, if you do flat foot surgery on a patient who has a high internal tibial torsion, then you will create a pigeon-toed gait after surgery.

    Number four. Finally, the more information we can document, the better we can defend our rationales for our decisions when things don’t work perfectly. There’s always going to be an expert witness who wants to condemn you for any problem that occurs postoperatively. The better you can defend your decision before you do surgery, the harder it will be for someone to find fault after surgery.

    There’re going to be many viewing this presentation who seriously question the need for a biomechanical exam. Some questions are legitimate and others are merely prideful excuses. Number one, certainly, the literature notes that intertester reliability of even experienced clinicians is fair. However, the intertester reliability of experienced clinicians has been shown to be quite good. Reliability something you’re going to have to work on but it is possible.

    Number two, one of the reasons for poor reliability is that the exam techniques have been poorly defined. As an example, the measurement of the subtalar joint range of motion is very dependent on the position of the ankle joint, yet few papers on goniometric examination have addressed this fact. Likewise, the amount of force needed to fully pronate the midtarsal joint has not been well addressed. You may find yourself experimenting with various techniques to improve your ability to get measurements that are reliable.

    Number three, there’s no question that finding goniometers that are well developed is difficult. Those that out there are usually fairly expensive. You may find yourself investing some real dollars to get better instrumentation. I can assure you though it is well worth it.

    Number four, there’s no question that you have to schedule time for a biomechanical exam. You don’t do a good one in three to five minutes. However, I’d like each of you to ask yourself this question, “Would I rather trust my son and daughter’s flat foot surgery to a surgeon who included a biomechanical exam in their pre-op eval or one who didn’t?”

    And number five, don’t be trapped by believing that past experience is a good predictor of success with the next procedure. You may find things that are very different in that next person that will change your mind on the type of procedure you want to do.

    Free goniometer app that I downloaded on my smart phone. There are many such apps to choose from. I use this now for all my stance measurements. We’ll be talking very soon about the necessity of stance measurements.

    As I noted before, if you do nothing else before surgery, determine that the patient has used their reserve of pronation. This requires three things, that you know the non-weight bearing range of motion of subtalar joint, that you measure the relaxed calcaneal stance position and the tibial stance position, and finally that you determine the relaxed heel to leg angle.

    This is a patient of mine who had a very flat foot. With my smart phone goniometer, I measured the relaxed calcaneal stance position which in this case is an inverted angle from the perpendicular. I then measured the relaxed tibial stance position which is even more inverted from perpendicular than the calcaneus. Finally, I subtract the calcaneal from the tibial stance position and get the heel to leg angle.

    In this particular case, we can say that the calcaneus is everted to the leg but not to the perpendicular. The question now is do you want to do flat foot surgery on this patient? Hopefully you have already answered no to that question. Why? Because you will end up with a heel more inverted and the patient will end up walking on the lateral side of the foot, which will cause other problems for the patient.

    After you have done the subtalar joint range of motion and determined the heel to leg angle, you then need to make the determination that either the subtalar joint is not maximally pronated, that it is at the end of its range of motion or that it has even moved beyond its pronation and range of motion. In this particular case, I calculated that the heel is 25 degrees everted to the leg.

    [10:03]

    I don’t remember the non-weight bearing range of motion of the subtalar joint. However, it would be extremely rare to find someone who has more than 25 degrees of eversion motion in the subtalar joint when non-weight bearing. The take home point is, if the patient hasn’t used the reserve pronation, you will have to work a lot harder to make a case that the patient needs flat foot surgery.

    Another deformity that needs to be identified is the laterally displaced calcaneus. You will not see this discussed much in the literature. However, it can have a definite influence on the procedure you choose. In this case, with the patient prone and the subtalar joint neutral, look and see if the center of the bottom of the heel is under the center of the leg. The normal calcaneus may be between 0 to 5 mm lateral to the center of the calcaneus. In this particular case, this is a normal-appearing displacement of the calcaneus to the leg.

    We see here that we’ve got two marks on the heel. One is the heel bisector and one is the projection of the leg bisector down to the heel. I need only a small measuring ruler to measure how many millimeters lateral to the leg the heel is when the subtalar joint is in neutral. If the measurement is more than 5 mm lateral, you may need some orthotic modifications to control the pronation. I’ve listed these above. If it is 10 mm or more, it is doubtful that an in-shoe orthotic can reduce the pronation.

    One of the definite options for this laterally displaced calcaneus is surgery. And here is the surgical option for a laterally displaced calcaneus, the Koutsogiannis procedure to move the plantar calcaneal tubercle under the center of the leg.

    Now, this particular drawing, you see the heel also inverted and I would say that this procedure you can do both a translational as well as angular correction. For most patients, I would not recommend inverting the heel when you move it medially. The great thing is that you have done an examination pre-op that justifies that you have chosen the right procedure.

    The next step of the biomechanical exam is to look at the pronation moments around the subtalar joint axis. The subtalar joint axis is extremely important in understanding why the foot pronates or supinates. This quote by the famous orthopedic surgeon JR Close [phonetics] should emphasize the importance of locating the subtalar joint axis. He said, “Nobody should be doing surgery to move any muscle around the ankle until he knew where the subtalar joint axis was for any particular person.”

    You’ve all seen the drawing of the axis of the subtalar joint in countless books. But it was 1st drawn by Manter in 1941 and it has been reproduced by various authors and illustrators. Then in 1989, Kirby said that the Manter drawing was incorrect, and he redrew the axis as shown on the right.

    So in 1989, I listened to Dr. Kirby, and I did my own independent study, and published the results in 1992. And this is what I found to be the projection of the subtalar joint axis on to the bottom of the foot. With the subtalar joint axis in this location, normal forces pushing up on the bottom of the foot, combined with normal Achilles tendon tension, produces equal pronation and supination torques. In other words, the person can stand with the subtalar joint in neutral position with no tension on any of the other tendons. To plot the subtalar joint axis, place the patient in a supine or sitting position and place the subtalar joint in its neutral position. Hold a mild dorsiflexion force against the 4th and 6th metatarsal heads. With the thumb of the free hand start pushing directly superiorly, against the plantar surface of the foot, you’ll feel one of three things happen; you will feel either the rear foot pronate, or you will feel the rear foot supinate, or you will feel the subtalar joint not moving. If you feel the rear foot pronate, move your thumb medially and push up again. If you feel the rear foot supinate, move your thumb laterally and push up again. If you feel no motion in the rear foot, mark the center of your thumb print with an X on the bottom of the foot.

    You should find at least three inpreferably four to five points on the bottom of the foot where there is no motion of the rear foot when you push upward. You will quickly see the X marks you made will line up in a straight line. Connect all the Xs and extend your line distally and proximally, so that it runs all the way from the posterior heel to the anterior forefoot.

    [0:15:00]

    If the subtalar joint axis is medial to the center of the 1st metatarsal head, you can diagnose the patient with a medially displaced subtalar joint axis. There are two types of medially displaced subtalar joint axis.

    One type is where the axis has a normal position under the rear foot, but has greater than 16 degrees of angulation with the sagittal plane, so that it goes medial to the 1st metatarsal head. The other type is where the axis has a 16 degrees less angulation with the sagittal plane, but it is medially located under both the rear foot and the forefoot.

    I will call these type 1 and type 2 medially displaced subtalar joint axes. When you see this condition, with the plantar bulge of the 1st cuneiform, you know the patient has a severe medially displaced subtalar joint axis.

    In this case, the axis is so far medial, that the peroneus longus can no longer planter flex the 1st ray, but instead, subluxes the 1st metatarsal cuneiform joint downward.

    It is important that you plot the subtalar joint axis with the subtalar joint in neutral position. If the subtalar joint is pronated when you plot the axis, you will erroneously conclude that the axis is too far medially. If the subtalar joint is supinated, you will erroneously conclude that the subtalar joint axis is too far laterally. This is because the subtalar joint axis moves with the talus. So if the talus adducts, so does the subtalar joint axis. So the more the rear foot pronates, the more the subtalar joint axis medially, which accentuates the amount of pronation of the subtalar joint.

    If the rear foot supinates, the axis moves laterally and the stronger will be the supination torques on the rear foot in stance. Again, a more detailed explanation is in my 1992 paper. If you have a type 1 medially displaced subtalar joint axis, the Evans osteotomy is the procedure to fix the problem. When you put the bone in the anterior calcaneus, you push more the forefoot medially to the subtalar joint axis and it markedly increases the supination torque on the rear foot produced by ground pushing up on the bottom of the foot.

    The type 2 medially displaced subtalar joint axis is a little more difficult to deal with. The Evans may push the forefoot more medial to the subtalar joint axis but you may also need to push the rear foot more medial to the axis. The Silver procedure may be a better choice than the Evans as it will be reorienting the plantar calcaneal tubercle more medial to the subtalar joint axis.

    The remainder of this lecture will concentrate on the documentation that you should be doing on the forefoot joints in planning flat foot surgery. The relationship of the forefoot to the ground has been of great interest over the last 150 years. The tripod model of foot support is still current, and the three points of contact being the center of the calcaneal tubercle, the 1st metatarsal head and the 5th metatarsal head.

    Anyone who performs pressure studies during gait will tell you that all of the metatarsal heads will contact the ground during midstance. Now that does not mean that all of the metatarsal heads will have equal weight. However, I have adopted as my rule number one for biomechanics that all metatarsal heads must come down to the ground.

    Now Sarrafian popularized the concept that started in 1929 with Steindler and was labeled in 1945 as the twisted plate by MacConaill. Because the metatarsal heads are all in contact with the ground during stance, there is always this twisted plate going on between the forefoot and the rear foot.

    These drawings from Hicks in 1953 also demonstrate that in order for the heel to evert with subtalar joint pronation, the forefoot has to invert to the rear foot. Likewise, in order for the heel to invert with subtalar joint supination, the forefoot has to evert to the rear foot.

    When you look at the back of the heel on the left side, you’ll measure that the heel is 6 degrees everted from the perpendicular. Therefore, you know from twisted plate theory that the forefoot is 6 degrees inverted to the rear foot. On the right, the heel is 7 degrees inverted from perpendicular. Therefore, you know from twisted plate theory that the forefoot is 7 degrees everted to the rear foot.

    A basic rule then for the left foot is if you keep the forefoot from inverting to the rear foot, the rear foot cannot evert from being perpendicular to the ground. On the right, if you keep the forefoot from everting to the rear foot, you can stop the rear foot from inverting to perpendicular.

    So you look at this severe flat foot, you’ve measured the heel is several degrees everted from the perpendicular.

    [20:04]

    And you also know that the forefoot is inverted to the rear foot the same amount that the heel is everted from perpendicular. Now, this inverted state of the forefoot to the rear foot can be because the forefoot is fixed in that position or it has moved into that inverted position. Therefore, the examination of the forefoot to rear foot becomes an essential part of your exam preoperatively.

    If the forefoot is fixed in an inverted state to the rear foot, we call it a forefoot varus. The forefoot to rear foot relationship is measured with the subtalar joint in neutral and with the thumb pushing up gently against the 4th and 5th metatarsals until you feel an end range of motion. It is important that the subtalar joint not be pronated because the forefoot varus may disappear when the subtalar joint pronates due to increased mobility in the midtarsal joint. To determine if that inverted forefoot to rear foot relationship is true forefoot varus, ask the patient to stand up and put the subtalar joint in neutral position. If you let the pressure down against the forefoot, you can push 1st metatarsal down to the ground, then you can say that the patient has a forefoot supinatus.

    For these people, don’t do surgery right away, but instead have them wear orthotics for a year and see if the varus deformity doesn’t go away.

    This was one of the early procedures advocated to reduce forefoot varus. Put the patient under general anesthesia, put one wrench on the heel and one on the forefoot and twist until the deformity is gone. Then put the patient in a cast. I don’t advocate this technique.

    A good procedure to correct true forefoot varus is the Cotton procedure proposed in 1936 as an opening wedge osteotomy of the 1st cuneiform. There have been a few modifications proposed over the years, but we still use this basic procedure when we have true forefoot varus. Unfortunately, you run the risk of having a postoperative plantar flexed 1st metatarsal with two to five varus, and you’ll still need an orthotic to support the two to five varus.

    So what do you do if you find no forefoot varus but the foot still needs flat foot surgery? Then you know that there has been actual inversion motion between the heel and the metatarsal heads.

    Where can that motion occur? It can occur in the midtarsal joint and what we call the long axis. It can occur in the cuneiform navicular joints which share a common joint capsule or it can occur in the metatarsocuneiform joints. Your job now is to find out in which one of these joints you’re going to get the most inversion motion of the forefoot against the rear foot.

    Unfortunately, we don’t have much in the way of clinical tools for quantitating how much motion is occurring in these forefoot joints. This is something I’d like to see developed by some of you out there. However, you can definitely put something in your chart about the qualitative analysis of the amount of motion in these joints. Start by looking at the mobility of the long axis of the midtarsal joint. Hold with one hand the heel with the subtalar joint in neutral position. With the other hand, grasp the forefoot around the cuboid and navicula. Now, gently invert the forefoot against the rear foot and judge whether there is a mild amount of mobility that is up to about 10 degrees of motion, a moderate amount of mobility say 10 to 20 degrees of motion, or a high amount of mobility more than 20 degrees. There are some people who excessively pronate mainly because the long axis of the midtarsal joint supinates too much when the anterior tibial is contracting during swing and contact phase.

    One of the solutions for a person whose only etiology of pronation is a high amount of mobility only in the long axis of the midtarsal joint who doesn’t have a significant rear foot deformity and has a normal subtalar joint axis is the subtalar joint arthroereisis. In this case, you block rear foot eversion, which means that the forefoot doesn’t invert to the rear foot in stance. Of course, I would definitely use an orthotic that applies eversion force against only the forefoot after surgery in order to decrease the stress on the implant. Navicular joint and the metatarsal joint is a little more difficult.

    Martin discussed in many of his papers the short 1st metatarsal. However today, we would properly diagnose this as metatarsus primus elevatus. In such a foot, the lesser metatarsals invert so that there is a common plane for metatarsal hits 5 to 1, which makes the foot function like you have a forefoot varus.

    First try to isolate motion of the 1st metatarsocuneiform joint. Place the subtalar joint in neutral. Place your thumb across all four lesser metatarsal heads. Pronate the midtarsal joint.

    [25:00]

    Hold the four lesser metatarsal heads tight and do not allow them to move against each other. With your other thumb, grab the 1st metatarsal head. Now gently dorsiflex the 1st metatarsal to its end range motion. Compare your thumbnails to estimate how many millimeters above or below the 2nd metatarsal head the 1st metatarsal head is. Then gently plantar flex the 1st metatarsal head and estimate the number of millimeters it plantar flexes.

    Now if you release the pressure of your hand holding the 2nd and 3rd metatarsal heads and move the 1st ray up and down, you will see much more motion between your two thumbs. This is because you are allowing motion in the 2nd and 3rd cuneiform navicular joints.

    If you see a much higher amount of motion after you release the pressure into the 2nd and 3rd metatarsals, you should also correlate that you see this motion on a weight-bearing lateral x-ray foot by noting a significant dorsiflexion angle at the 1st cuneiform relative to the navicula.

    If you measure the forefoot valgus angle with the patient prone non-weight bearing, but you see a high amount of motion in both the metatarsocuneiform and the cuneiform navicular joints, then consider a Young’s tenosuspension procedure. This procedure will markedly limit dorsiflexion motion at both of the joints that produce 1st ray up motion. However, the increased tension in the anterior tibialis will also mildly invert the long axis as the midtarsal joint. This is why I would reserve this procedure for those patients with very pronated feet but who have an everted forefoot to rear foot deformity.

    So what else can be done surgically to prevent the forefoot from inverting against the rear foot? Several arthrodesis have been devised over the years to fuse the medial column joints. The Hoke, the Lowman, the Miller are just some of these. Be careful using the Miller procedure as with total elimination of 1st ray motion, you may end up with a hallux limitus.

    It is impossible in this short lecture to describe everything you should evaluate when to determine the correct flat foot surgery. However, I would briefly remind you not to forget to measure the ankle joint dorsiflexion with the both the knee extended and the knee flexed.

    No matter what the ankle joint dorsiflexion is before flat foot surgery, the patient will invariably have less ankle joint dorsiflexion after surgery. If a patient has less than 10 degrees of dorsfilexion before surgery, the Achilles must be lengthened. If a person does have more than 10 degrees of ankle joint dorsiflexion before surgery, you should try to determine how much they will have after surgery. If you are not doing a posterior osteotomy of the os calcis, then the wise idea is to make two different weight-bearing lateral x-rays of the foot. One, in a relaxed stance and one with the patient standing with the subtalar joint neutral. Try to determine how much the calcaneal inclination will increase after surgery. Use cutouts or any mathematical skills you have. It is still not an exact science but the better you try and the more often you do it, the better will be your estimate.

    A final note of advice, treat your patients like you want to be treated. If you make a mistake with an orthotic, you can always remove it from the shoe. You can’t undo flat foot surgery. The results last a lifetime. Therefore I believe much more care needs to be taken in evaluating a patient for flat foot surgery than for making an orthotic.

    This short lecture is only a brief overview of some of the important things that should be documented before flat foot surgery. I leave my best wishes for your success in your conservative and surgical biomechanical treatment of the abnormally pronated feet of the patients that put their trust in you.

    TAPE ENDS - [29:11]