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Biomechanical Decision Making
The KK Approach Part 1
Biomechanical Decision Making
The KK Approach Part 1
Anyone who has read this blog even occasionally is likely aware of my views about the importance of biomechanics. Another of my passions is thought process and how we make decisions to treat the complex problems affecting the lower extremity. I’ve become an evangelist for a biomechanical decision-making method I term the kineticokinematic approach, or KK approach. Today’s edition will start a three-part series discussing this method. Today, we’ll define what this approach is, and in the next two parts we’ll apply it to nonsurgical and surgical decisions for treatment of various pathologies of the foot and ankle.
Kinetics and Kinematics
Let’s first take a step back to podiatric medical school and review a few important terms. For our purposes, kinetics focuses on forces that act on the structures of the extremity, while kinematics describes positions of those structures. For example, the position of a foot joint during gait, say, the subtalar joint and its associated axis of motion, is a kinematic parameter, while ground reactive forces directed either medial or lateral to that joint are kinetic in nature.
These two measures are the primary means for us to examine biomechanics of any system, but especially the foot and ankle. Much of the biomechanics lower extremity specialists have used for so many years has been kinematic in nature. Much of the system Root, Orien, and Weed advocated was kinematic. Placing the subtalar joint in an optimal position, describing the foot in terms of “criteria of normalcy,” and using a foot orthosis to “control” a foot are all kinematic methods. The concept of planal dominance in flatfoot deformity is also kinematic (compensation for pathological flatfoot is in one principle plane).
It makes perfect sense that a kinematic approach would be the dominant one for so long. Measuring the positions of things is much easier than determining the amount of force, pressure, or strain on a structure. Radiographs, for example, provide us a simple method to evaluate the positions of bones and joints in the foot. Technologies for studying kinematic parameters have existed for years, including 2-D and 3-D motion capture video and stereophotogrammetry systems, for instance. A simple physical examination can tell us a lot about the kinematics of the foot.
On the converse of the relative ease to study kinematics, is the difficulty in drawing out kinetic information about the lower extremity. Determining the strain on a particular pedal structure is very difficult on a live human, forcing us to study this on cadavers. Imagine inserting a strain gauge into your plantar fascia and then walking to determine how much strain is on that aponeurosis. Not terribly feasible. We have made big strides in plantar pressure mapping systems, which are openly available for those interested. However, whether it’s in-shoe pressure measurement or walking on a treadmill, we’re still limited to the external structures of the foot, ie, the plantar surface. If I want to know the amount of force or where that force is located in an ankle joint after a fracture, for example, I’d have to extrapolate that from prior research. There is no imaging method that allows us to determine this information.
The KK Approach
My goal is to combine, as much as possible, the kinetic and kinematic methods to create one unified approach when making evaluation and treatment decisions. By examining all of the methods available, and using the benefits of each, it is possible to move beyond our current limitations to help our patients. With all that said, let’s now take a look at the KK approach.
- Step 1 - Determine the specific anatomical structure that is damaged or under strain
- Step 2 - Clarify the underlying biomechanical cause.
- Step 3 - Treat the cause by adjusting forces (kinetics) or addressing deformities (kinematics).
For those of you with an interest in biomechanics, you may say this sounds a lot like the tissue stress theory, and you would be correct. However, this method goes a step beyond by recognizing all of the components that need to go into treatment, including not only a focus on strain but also the causes of that strain and an improved understanding of what the treatments we apply actually do to our patients. This method also leads to new research questions because it points out holes in our cumulative knowledge. For example, what kinetic effects do certain foot orthosis modifications have on the foot? How much force does a 12mm medial heel skive exert, and how effective is it to supinate the subtalar joint? What about a first metatarsal head cutout? How much force is eliminated from the first metatarsal head, and how much is placed on the other metatarsal heads?
A KK Example
As a wrap up to this introduction, let’s take a look at a simplified example: a painful big toe callus. Mr Jones is our hypothetical 56-year-old male presenting to you complaining of a painful callus on the plantar medial aspect of his hallux interphalangeal joint.
Step 1 is to determine the specific strained or damaged anatomical structure. The history identifies the complaint, location of pain, and various parameters affecting the complaint. You begin your examination by palpating the callus (patient says, “Ow” so you know that’s really the painful spot). You also palpate and/or put through a range of motion other parts of the big toe, the interphalangeal joint, sesamoids, and 1st MTP joint.
Step 2 is to clarify the underlying biomechanical cause.Your examination now becomes more comprehensive, where you note increased hallux IP joint motion, a valgus rotated hallux, a dorsally positioned 1st metatarsal head (forefoot supinatus), pes planus foot type with slightly valgus heel, and decreased ankle joint dorsiflexion with the knee extended versus flexed. You’ve established a diagnosis of hallux limitus, pes planus with calcaneovalgus, and gastrocnemius equinus. The increased hallux IP joint motion is compensation for the decreased 1st MTP joint motion, and the callus is an end result of the excessive pressure on the plantar medial aspect of the toe, resulting from the previously mentioned diagnoses.
Step 3 is to treat the cause by addressing kinetics and kinematics of the foot. There are several options here, but for simplicity, your goal is to relieve the pressure on the plantar medial part of the great toe. Let’s say you decide to treat with a foot orthosis. The combined KK approach would now have you make decisions on orthosis modifications not based on just reducing the pressure on the callus but rather choosing those modifications for their effects on the foot. Reasonable components of the prescription might be the following: deep heel cup with medial heel skive (to increase the supinatory torque medial to the subtalar joint axis, thereby decreasing medial column load), minimal arch fill (to reposition the medial arch joints and plantar flex the 1st metatarsal),a heel lift (to decrease the plantarflexory force of the gastrocnemius), a valgus forefoot post (to move force from the medial forefoot to the lateral), and a first metatarsal head cut out with poron fill (to decrease the force under the callus).
My apologies for the long description of the treatment. Word economy is challenging with orthosis prescriptions! For the sake of time I did not include a discussion of the biomechanical research supporting these recommendations. Some of the orthosis components have research support while others need validation. The KK approach has changed this problem from a simple callus into constellation of diagnoses that allow the provider to institute a rational, reproducible treatment plan.Instead of simply “making the foot straight,” or “treat the strained tissue in some way,” we’ve now expressed a systematic approach for almost any lower extremity complaint.
This can be a lot to process, so consider rereading if it’s not completely clear, especially those of you who are residents or students. In the next two weeks, we’ll examine this method further and see how it can be applied to other complaints with both nonsurgical and surgical applications
Jarrod Shapiro, DPM
PRESENT Practice Perfect Editor