16/08/2024
Troubleshooting Orthoses-Shoe Sole Construction Variables
When troubleshooting problems with custom foot orthoses, many orthosis design variables must be considered. The method of casting/scanning, positive cast modifications, types and thicknesses of orthosis shell material, orthosis shell modifications, forefoot extensions, and thickness and type of topcover material will all play important roles in determining whether a patient is comfortable and receives therapeutic benefit from their orthoses. However, regardless of how perfectly the orthoses have been made for the patient’s feet, if the orthoses do not fit well into the shoes and the shoes and orthoses do not mesh well together as a unit, then the custom foot orthoses will fail to attain their therapeutic potential for the patient.
One of the primary considerations regarding the construction of shoes for patients is the geometry of the shoe sole and materials used within the soles. In general, if the shoe has a less compressible (i.e., higher durometer) sole, then the shoe will be better at working with the orthosis to increase the STJ supination moments from the orthosis in feet suffering from pronation-related pathologies (e.g., posterior tibial tendon dysfunction), and will also be better at increasing the STJ pronation moments from the orthosis in patients with supination-related pathologies (e.g., peroneal tendinopathy, lateral dorsal midfoot interosseous compression syndrome). However, higher durometer soles will also feel less cushioned and less “springy” to the patient which may influence their foot and lower extremity comfort and/or shoe-buying decisions.
Many walking and running shoes are now being made with very cushiony and low durometer midsoles which are perceived as being comfortable and highly-cushioned in the shoe store, but after 1-2 months of shoe wear, will undergo "compression-set" which will cause deformation of the shoe sole and may also cause a return of symptoms. Compression-set is a characteristic of midsole foams in shoes which is caused by the large magnitudes of ground reaction force which act on the midsole foam and which results in the midsole becoming thinner over time. Compression-set not only causes the midsole to compress and lose thickness over time, but also reduces the ability of the shoe midsole to absorb the shock of weightbearing activities, such as during walking, running and other sports.
Another very important facet of shoe design which affects foot orthosis function is the heel height differential (HHD) of the shoe. The HHD of the shoe is the difference in thickness of the shoe sole under the heel compared to the thickness of the sole under the metatarsal heads. Within the running shoe industry, the HHD of the shoe is now commonly called “heel drop”. Most running shoes have an HHD ranging from 0 mm (i.e. “zero-drop” shoes) to about 12-14 mm. As a comparison, women’s dress style shoes, or “high-heeled pumps”, may have an HHD of well over 75 mm. Such large variations in HHD from one shoe to another, especially within the shoe wardrobes of many female patients, creates numerous issues for orthosis fit and comfort, including the ability of the orthosis to fit well inside the shoe to help heal the injuries of the patient.
Shoes with increased HHDs are more likely to cause the foot orthosis to rock over the shank of the shoe due to the increased sagittal plane curvature within the shank of these higher-heeled shoes (see my illustration below). For example, an orthosis shell which fits well into an athletic shoe with a 12 mm HHD may rock over the shank of a shoe with a 50 mm HHD (i.e., a 2-inch heel). The easiest way to determine whether the orthosis will rock inside the shoe is to place manual pressure on both the heel and anterior edge of the orthosis inside the shoe and then alternate manual pressure posteriorly and then anteriorly onto the dorsal orthosis. If the anterior edge of the orthosis pops off the insole board of the shoe with manual heel cup pressure, or if the heel cup of the orthosis pops off the insole board of the shoe with manual pressure on the dorsal distal edge of the orthosis plate, then this indicates that the plantar surface of the orthosis is resting on a high spot on the shank of the shoe, somewhere between its heel cup area and its anterior edge. The solution for abnormal sagittal plane rocking of the orthosis is to either modify the orthosis or have the patient use a lower HHD shoe.
One relatively easy method to reduce sagittal plane rocking of orthoses inside shoes is to grind the plantar midfoot region of the orthosis. Another method to remedy rocking of the orthosis over the shoe shank is to add a small heel lift to the plantar heel of the orthosis. Of course, adding even a small heel lift may also increase heel slippage inside the shoe during gait. Finally, the orthosis may also be spot-heated into a higher arch shape to prevent the orthosis from rocking inside the shoe. It must be remembered, however, that spot-heating of orthoses should be saved as a last resort since improper spot-heating will permanently deform the orthosis into a shape which may become intolerable and possibly harmful for the patient.
The HHD of the shoe will also affect the overall function of the foot on the orthosis. Shoes with a higher HHD will reduce the tension within the Achilles tendon, will cause an increased dorsiflexed position of the digits and engage the windlass mechanism of the plantar fascia, which will, in turn, increase the height of the medial longitudinal arch (MLA) of the foot. Alternately, flatter shoes (i.e., "zero-drop" shoes) will increase Achilles tendon tension, increase plantar fascia tension and will decrease MLA height.
I have often found that patients report more plantar MLA irritation to their foot when they are wearing their orthoses in "zero-drop" or low-HHD shoes, whereas they might be completely comfortable in running shoes that have a higher HHD. Popular running shoes such as the Altra, which are “zero-drop”, and Hoka One One, which typically have a 4-5 mm HHD, may cause MLA irritation when custom foot orthoses are worn inside these low-heeled shoes. My favorite solution for this specific problem is to simply add a 3-6 mm thick Korex heel lift under the rearfoot portion of the orthosis to effectively increase the HHD of the shoe. This simple modification often relieves the patient’s MLA irritation from their orthoses and avoids having to lower the MLA of the orthosis just so they can wear their orthoses comfortably in their lower-heeled shoes.
[Reprinted with permission from: Kirby KA: Precision Intricast Newsletter, Precision Intricast, Inc., Payson, AZ, January 2020.]
