Essential Keys To A Pre-Op Biomechanical Evaluation

Pages: 70 - 78
Robert D. Phillips, DPM

Emphasizing the importance of proper biomechanical assessment as a foundation for successful surgical outcomes, this author discusses key measurements such as the resting calcaneal stance position and offers pertinent evaluation pearls that can make an impact in flatfoot and hallux valgus surgery.

I have spoken many times on the importance of separating the term “biomechanics” from the term “orthotic therapy.” The reason that this thought pattern continues to exist in our profession is that in the traditional departmentalized school of podiatric medicine, surgery and biomechanics were two separate departments. The people who were devoted in trying to measure ranges of motion of joints went in the biomechanics, non-surgical department.

   I hope that we will soon read about this archaic form of thinking the same way that we now read about “bloodletting” practices in historical medical books.

   Biomechanics is a science. It is not a therapy. Biomechanics should be the basis on which we understand why tissue is experiencing stress and why deformities occur. In associating with those I consider to be great clinical biomechanists, I have been impressed with their surgical skills. It makes sense to argue that the more we know about the pathomechanics of any deformity, the more likely we are to do the right surgery to correct and try to prevent that deformity from recurring.

   It is indeed disappointing to attend conferences where many are lecturing about doing surgery without giving any specifics of the indications for that surgery. I also continue to marvel at the number of surgeons who adopt a particular surgery to perform because the literature says that a majority of patients who have the surgery have a good or excellent result. One almost never sees in the literature any inquiry as to which patients have the best results and which have only fair or poor results.

   It is true that there are many factors that play into this categorization of results. These issues include general health issues, adherence issues, operative technique issues, etc. However, researchers seldom report and correlate in-depth preoperative static and dynamic tests with the outcome of the procedure.

   When one thinks about the ramifications of doing surgery, the surgeon must do everything possible to get the best results. Unlike a prosthetic or orthotic device that one can discard after a mistake, surgery cannot be discarded if an error in judgment leads to a less than desirable result. I predict that standards of care in the future will gradually shift and more biomechanical analysis will be expected as part of the preoperative evaluation for many musculoskeletal surgeries.

Biomechanical Considerations For Flatfoot And Abnormal Pronation

A full discussion of preoperative biomechanical indications for surgery is prohibitive in such a brief article as this so I will briefly address a couple of basic surgical categories.

   The first one, flatfoot surgery, may seem to be the most obvious. While we continue to see that “pes planus” is still a code-able ICD-9 diagnosis, podiatric medicine has tried to change the idea that flat feet are a problem. Indeed, there are many flat-footed people who will function well for an entire lifetime and never need any treatment.1

   What podiatry has attempted to change in the last 50 years is the idea that we do not treat flat feet but we do treat abnormal pronation.2 Abnormal pronation becomes a problem when it produces stress and strain on soft tissues, and finally bone deformity and cartilage degeneration.3,4 Unfortunately, we have not yet developed the sophistication to accurately predict when those symptoms or deformities, or degenerative changes will occur.5,6

   For most cases of abnormal pronation, clinicians can adequately treat symptoms with non-surgical appliances and in-shoe custom foot orthotics are the most common. However, sometimes surgery may be the most desirable biomechanical approach.

   Over the years, authors have proposed a myriad of surgical procedures and as one reviews any foot surgery textbook, it appears that all types of tendon procedures, osteotomies and fusions are available. Whenever one sees a wide variety of optional treatments for any problem, one can assume there are either multiple causes of that problem or that there is very little known about the etiology of that problem.

   In the case of abnormal pronation, podiatrists have been teaching for many decades that there are multiple etiologies of the problem. The better the surgeon can identify the etiology, the better the chance of picking the correct surgical procedure.

Why One Should Determine The Heel To Leg Angle And Whether There Is Calcaneal Bisector Displacement

The first biomechanical measurement that should be part of the evaluation is the resting calcaneal stance position (RCSP).7,8 Is the calcaneus everted, vertical or inverted? It is important that the clinician not automatically equate the amount of calcaneal eversion relative to the vertical with whether the patient is pronated or supinated.

   The heel is most stable when it is vertical and centered under the center of the leg. However, if the patient has a fully compensated rearfoot varus, the subtalar joint will be pronated from neutral and the heel will still be vertical. If the patient has a partially compensated rearfoot varus, the heel will be inverted in stance and the subtalar joint will be maximally pronated.

   For people with both fully and partially compensated rearfoot varus, it would usually be a mistake to try to do a surgical procedure that would make the rearfoot stand more inverted.

   After measuring the RCSP, the clinician also should measure the relaxed tibial stance position (RTSP).9 By subtracting the RTSP from the RCSP, the clinician can calculate the heel to leg angle. Then compare this angle with the maximum inversion and eversion range of motion of the subtalar joint when the patient is non-weightbearing.10

   Does the heel to leg angle show the subtalar joint to be maximally pronated? Does the angle show the calcaneus to be further everted to the leg than the maximum pronated position during the range of motion exam of the subtalar joint? It is my opinion that one of the criteria for flatfoot surgery is that the patient should be standing with the subtalar joint maximally pronated or subluxed.

   Another biomechanical measurement to assess is the displacement of the calcaneal bisector relative to the lower leg bisector.11 One can do this with the patient standing or laying prone. The clinician moves the subtalar joint until the heel bisector is parallel to the lower leg bisector. Radiographic methods can also accomplish this.12

   Measure the number of millimeters that the calcaneal bisector is medial or lateral to the leg. Normal should be 5 mm or less. The more lateral the calcaneal bisector is, the more that an eversion couple is set up for the subtalar joint to pronate when the patient places an axial load on the lower limb.

   When it comes to the design of orthotics and shoes, it is desirable for the center of the shoe to be under the center of the leg. Once the heel gets more than 10 mm lateral to the leg, it is almost impossible to utilize an in-shoe orthotic to equalize the inversion-eversion moments.

   For these cases, either a shoe has to have modifications to bring the center of the sole under the center of the leg or surgery has to bring the center of the calcaneus under the leg.13

What You Should Know About Examining The Subtalar Joint Axis

The plotting of the subtalar joint axis on the plantar foot is a very important part of the preoperative exam. I am fond of the following quote by Close:

   “The position of this (subtalar joint) axis in the foot is thus of great importance in the study of the action of muscles whose tendons pass in relation to it. The problem of exactly where to place the insertion of a tendon to be transferred is solved by knowledge of the position of the axis and the state of other muscles passing it.”14

   The plotting of the subtalar joint axis should occur with the subtalar in the “forefoot flat position,” which is defined as that subtalar joint position in which the forefoot is parallel to the ground when the midtarsal joint is fully pronated. The normal subtalar joint axis should lie lateral to the first metatarsal head.15 If it is medial, then any surgery should try to bring it more lateral or else the foot may still pronate to the end of its range of motion after surgery. One of the most effective procedures that moves the subtalar joint axis more laterally is the Evans osteotomy to lengthen the lateral side of the os calcis.

Evaluating The Forefoot To Rearfoot Relationship

Assessing the forefoot to rearfoot relationship also needs to be an important part of the pre-op evaluation.16 If the patient has a forefoot varus, one must address this in any surgery.17 Evaluate the forefoot with the subtalar joint in a position in which the calcaneus is perpendicular to the ground. While the traditional method of measuring has been with the patient supine and non-weightbearing, this may also occur with the patient standing.18

   With the heel in the desired post-op position, if the first metatarsal is off the ground, the clinician should gently push down on the first metatarsal head to see if it can make contact with the ground. If it cannot, then the clinician must determine whether the patient has a true forefoot varus or a metatarsus primus elevatus. The recording of the forefoot to rearfoot relationship needs to be in the evaluation as well as the range of motion of the first metatarsal.

Pertinent Pearls On Evaluating The Midtarsal Joint

The clinician should also assess the range of motion of the midtarsal joint, both the oblique axis as well as the longitudinal axis. While there are no published methodologies for goniometric recording of these measurements, the clinician can at least make some qualitative observations.

   One can grade the longitudinal axis as having a low amount of motion, which I consider to be less than 5 degrees; a moderate amount of motion, which I consider to be between 5 and 20 degrees; or a high amount of motion, which I consider to be more than 20 degrees of motion. The reason for this is that during the contact period of gait, the anterior tibialis fully supinates the long axis of the midtarsal joint.

   Then as the forefoot makes contact with the ground from lateral to medial, the anterior tibialis relaxes and the forefoot starts its eversion motion relative to the rearfoot. If the forefoot excessively inverts to the rearfoot during contact, then the subtalar joint may function as if the patient has a forefoot varus. Most of the time, a functional orthotic can be effective for these patients.

   The oblique axis direction of motion is also important to observe. Does the patient have a transverse plane dominance of motion or sagittal plane dominance? Again, no quantitative methods exist for this observation yet there are differences in the type of pronated feet that will result.19

   Transverse plane dominance will produce a foot that exhibits the “too many toes” sign whereas the foot with sagittal plane dominance will exhibit more of an effect of fallen arches. The foot with transverse plane dominance will show a high degree of cuboid abduction on a dorsal-plantar view of the foot whereas the foot with sagittal plane dominance will show a much higher degree of talar declination and loss of calcaneal inclination angle.

   The foot with the transverse plane dominance will function much more like a foot that has a medially displaced subtalar joint axis. The foot with the high degree of sagittal plane mobility may exhibit an “equinus-like” function of the foot, in which it takes a lot less tension in the Achilles tendon to produce a pronated foot. For these types of patients, 10 degrees of ankle joint dorsiflexion may not be enough and it may be desirable for the patient to exhibit at least 15 degrees of ankle joint dorsiflexion.

   Assess the degree of forefoot adductus before surgery. Theoretically, one do this by taking a dorsal-plantar X-ray with the patient in standing position and the subtalar joint in neutral position. The angle is the comparison of the longitudinal bisector of the second metatarsal and the long axis of the rearfoot. If this angle is greater than about 15 degrees, the practitioner should be very careful about trying to do flatfoot surgery as the patient may have a difficult time postoperatively fitting shoes that have enough of a curved last.

   When there is a high degree of forefoot adductus, the clinician needs to determine why the patient is still having such a high degree of pronation that he or she is considering doing surgery. One may also need to address the forefoot adductus as part of the surgical procedure.

   When considering surgery for abnormal pronation, the clinician needs to ensure that he or she has identified all of the pronatory etiologies. For example, examination for internal femoral torsion or internal tibial torsion should be part of the evaluation. If the patient has either of these abnormalities, correcting the pronation may actually cause the patient to walk with a pigeon-toed gait. This may exacerbate clumsiness.

   The clinician should also ensure that the patient does not have extremely short hamstring muscles or contraction of the iliopsoas muscles. Both of these conditions create strong pronation forces on the foot that will continue even after any surgery.

Fundamental Biomechanical Insights For Hallux Valgus Cases

Another common surgical procedure is the correction of hallux valgus. The literature is replete with the concept that most people with hallux valgus have some type of pronation problem during the gait cycle.20,21

   Again, the clinician should identify whether pronation is the etiology of the deformity as well as the etiology of any pronation that is occurring. Most of the patients I evaluate for hallux valgus get one of two options: orthotic therapy or surgery with post-op orthotic therapy.

   It is important to identify the stage of the deformity and convey to the patient what an orthotic can and cannot do.22 Good orthotic therapy can theoretically reverse Stage I bunions. One can theoretically stop Stage II bunions from progressing but they will not reverse. Theoretically, clinicians can slow down but not stop the progression of Stage III bunions. One cannot change Stage IV bunions in their progression with orthotic therapy but orthotic therapy may alleviate some of the secondary symptoms.

   Before performing bunion surgery, assessing first ray range of motion is of prime importance in the biomechanical examination.23,24 Unfortunately, there are no instruments that can measure this.25,26 There is also a tremendous amount of research into how this range of motion changes in reaction to other joint motions. I have noted clinically that when the long axis of the midtarsal joint is supinated, the first ray seems to have more range of motion than when the long axis is pronated. No controlled study has confirmed this observation.

   I recommend that physicians observe the total first ray range of motion with the midtarsal joint fully pronated when the subtalar joint is in its neutral position. It has been traditional to record this total range of motion in the number of millimeters that the first metatarsal head moves above and then below the second metatarsal head. We assume that the neutral position is halfway between the maximal dorsiflexed position and the maximal plantarflexed position.

   When doing hallux valgus surgery, it is important that the surgeon not create an inverted forefoot to ground position. If the patient has a rearfoot varus deformity — which may be due to a subtalar varus or a lower leg varus — the surgeon will want to plantarflex the first ray during surgery.27 Likewise, if the patient has a forefoot varus deformity, the surgeon will also want to plantarflex the first ray when performing surgery.

In Summary

I want to impress on all the importance of analyzing the mechanical function of the foot before and after any musculoskeletal surgery on the foot and the ankle. Surgeons sometimes get caught up in the aesthetics of the foot after their surgery, believing that if it looks good, it must function well.

   However, it is well known that changes in the structure of the foot can have marked effects not only on those structures in close proximity to the surgical site but on structures that are far distant to the surgical site. For example, there is good evidence as to the effects of limited motion of the first MPJ having a negative impact on the function of the spine. Changes in subtalar joint position may produce changes in hip or patellar function.28,29

   Unfortunately, there is much research and development yet to occur in analyzing the structure and function of the foot. There is much to do in understanding how the ligamentous structures interact to create the ranges of motion of the midtarsal joint and the midfoot joints.

   There is still much research and development to be done when it comes to creating instruments that can accurately measure the ranges of motion available in the joints of the foot.30 There has been some development of instrumentation for analyzing the dynamic function but we need more research in order to understand how the availability of motion in the pedal joints translates into dynamic function.

   I hope more surgeons will pay attention to the preoperative conditions before surgery as well as postoperative function.

   As this research and development evolves, better care will result, and we can expect the general population to retain their mobility for ever increasing years.

   Dr. Phillips is affiliated with the Orlando Veterans Affairs Medical Center in Orlando, Fla.


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   For further reading, see “Keys To The Biomechanical Evaluation Of The Symptomatic Adult-Acquired Flatfoot” in the April 2012 issue of Podiatry Today, “Emerging Concepts In Podiatric Biomechanics” in the December 2006 issue or “Emerging Trends In Research With Orthoses And Biomechanics” in the April 2009 issue.

   Also check out the DPM Blog “What Happened To Continuing Education For Podiatric Biomechanics?” at .

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