Can Foot Orthoses Have An Impact For Knee Osteoarthritis?
Dr. Kevin Kirby, DPM Given the common nature of knee
A forefoot valgus deformity is defined as an everted position of the forefoot relative to a perpendicular bisection of the calcaneus when the subtalar joint is in its neutral position and the midtarsal joint is maximally pronated and locked. Although the classic etiology of this deformity involves an increased valgus frontal plane unwinding of the neck of the talus, there are probably a number of anatomic and functional factors contributing to this condition.
Types of Forefoot Valgus Deformity
In observing the function of the lower extremities, one must be aware not only of the presence of a forefoot valgus deformity, but also how an individual’s foot may compensate for the deformity.
There are three methods of compensation for this pathology. First, there is a flexible forefoot valgus deformity, wherein the lateral aspect of the foot is allowed to make contact with the ground completely through midtarsal joint compensation. This is achieved principally via longitudinal axis frontal plane supination of the midtarsal joint. This is the fashion in which most patients with forefoot valgus are able to compensate.
Second, in some instances, if the degree of forefoot valgus is greater than the amount of available frontal plane motion in the midtarsal joint, then supination of the subtalar joint with calcaneal inversion occurs. This is termed a rigid forefoot valgus.
Third, cases where longitudinal axis midtarsal joint supination combined with subtalar joint supination are inadequate to fully compensate for the deformity, additional compensation will occur at the oblique axis of the midtarsal joint, allowing additional forefoot plantarflexion.
Although the three types of forefoot valgus may app appear similar during the course of a non-weight bearing musculoskeletal examination, they function differently in stance and during gait. One must also note that, although the more rigid forms of forefoot valgus are not extremely common, they, along with the flexible type of forefoot valgus deformities, comprise greater than 80% of the forefoot to rearfoot problems present in our patient population.
Gait and Stance Evaluation
During gait, the individual with a flexible forefoot valgus deformity exhibits a normal heel contact and midstance period of gait. However, when body weight is transferred into the ball of the foot and the heel raises off of the supporting surface as one approaches the propulsive period of gait, rapid subtalar joint pronation occurs.
This late midstance pronation is due to instability of the midtarsal joint, initiated by longitudinal axis midtarsal joint supination, which is compensating for the everted forefoot position. This unlocked midtarsal joint in turn decreases the ability of the peroneus longus to effectively stabilize the first ray against ground reactive forces, which leads to a hypermobile first ray in gait.
On the other hand, a rigid forefoot valgus is likely to function in a significantly different fashion. In this case, the foot not only approaches the ground in an inverted fashion, but it is maintained with the calcaneus and subtalar joints in a markedly inverted and supinated fashion. No abnormal pronation is noted in this foot from the moment of heel contact through midstance into propulsion.
In static stance, patients with a flexible forefoot valgus deformity will stand with their calcaneus in a perpendicular fashion as long as there is no other deforming force present. The late midstance position, which is so clearly evident throughout gait, is a purely functional anomaly not evident during static stance. Only is a patient possesses an additional significant pronatory force (e.g., compensated equinus or transverse plane deformity) would calcaneal eversion be present in stance.
If a rigid forefoot valgus deformity were present, the patient’s calcaneus would be inverted in stance in order to allow the lateral column of the foot to come into contact with the supporting surface. This may be clinically differentiated from a partially compensated or uncompensated rearfoot varus deformity with a very simple maneuver. If one places either one’s fingers or a small wedge under the lateral aspect of the patient’s forefoot, and the calcaneus assumes a vertical position, then a rigid forefoot valgus is responsible for the inverted position. However, if the placement of the fingers or a wedge does not alter the calcaneal position, then the patient is maximally pronated and inverted due to a rearfoot varus problem.
Symptoms
Most symptomatology associated with a flexible forefoot valgus is the same as one could expect in any foot demonstrating any degree of marked abnormal subtalar joint pronation. Specifically, structural problems- such as heel spur syndrome, flexible hammertoe deformities, plantar tylomas and hallux abducto valgus deformities- are all associated with this mechanical problem. Additionally, soft tissue overuse and postural complaints affecting the extrinsic foot muscles are commonly associated with this type of foot function. Due to the constant frontal plane compensation within the midtarsal joint, those individuals possessing this type of deformity are also prone to chronic lateral instability. This same propensity is present in the rigid type of forefoot valgus disorders. This is often accompanied by a variety of other pathologies, most notably a retrocalcaneal exostosis and/or tyloma associated with the fifth, fourth, or first metatarsal heads.
Treatment
In order to properly treat the more common entity of flexible forefoot valgus, one must first clinically document its presence. This may only be accomplished via a thorough lower extremity musculoskeletal examination wherein care and accuracy are exercised in locating the perpendicular bisection of the calcaneus. The placement of this reference line is critical. Malpositioning of this line is most often responsible for failure to detect the presence of an everted forefoot deformity.
Once the examination is completed and an appropriate neutral position case is obtained, certain prescription items should be considered when ordering the device. Regardless of the type of material ordered, the entire amount of the forefoot valgus deformity present should be balances within the cast via intrinsic posting. As a general rule, both varus and valgus forefoot deformities are best treated via instrinsic posting.
In addition to the forefoot correction, most patients are best controlled when their orthoses possess a rearfoot post. A typical four-degree post with four degrees of motion will assist in placing the calcaneus in a normal inverted position at heel strike while allowing normal pronation to occue over the first 25% of the stance phase of gait.
If a functional orthosis is utilized appropriately in those patients with a flexible forefoot valgus deformity, one may expect a reduction of symptoms associated with previously stated deformities. If a hypermobile first ray can be controlled by locking the midtarsal joint, the bump pain associated with a hallux abducto valgus deformity may be eliminated.
Additionally, if the deformity has not yet progressed to stage three (as categorized by Drs. Root, Weed, and Orien) one might expect to maintain it at its present level.
If the patient possesses a flexible hammertoe deformity affecting the lesser digits, then one might expect these to reduce over a period of several months, since the patients no longer will be gripping for lateral stability because the orthosis has now brought the ground up to the foot. If in conjunction with a digital orthosis, marked reduction of digital contracture can be obtained.
A flexible forefoot valgus deformity is a common cause of abnormal foot function leading to a host of common lower extremity problems. Proper examination and diagnostic techniques will allow the practitioners to reverse most of the symptomatology associated with this deformity with properly prescribed and manufactured orthotics.
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