Back to fundamental biomechanics this month with an overview of the original paper by Dr. Kevin Kirby addressing the medial heel skive technique. While this is an old paper by today’s standards, its principles and explanations are still important to modern day orthotic design and manufacture. Whether you prescribe Kirby skives on a regular basis or you are new to this practice, a recap of the original paper is worthwhile of your CPD time.
The Medial Heel Skive Technique: Improving Pronation Control in Foot Orthoses
Kirby, K. (1992)
Journal of the American Podiatric Medical Association 82(4) pp 177-188
Kirby points out that to reduce the pronated position of the subtalar joint (STJ) during weightbearing, a supinatory moment must be exerted by the orthotic. While medial rearfoot wedging helps to achieve this in many cases, it is often not enough for severely pronated feet or those with a medially deviated subtalar joint axis. The medial heel skive “is based on the idea of inverted heel foot braces used by Whitman and Roberts in the early 1900s and results in an actual increased amount of varus wedging within the heel cup of the orthosis. The increased varus wedging in the orthosis heel cup causes an increased orthosis reactive force on the medial aspect of the plantar heel, which results in increased supination moment across the subtalar joint axis during weightbearing activities”.
What it all means:
Dr. Kirby proposes the medial heel skive technique, commonly referred to as the “Kirby Skive”, as a method for increasing the supination moment of an orthotic.
- Force exerted by the ground on the plantar structures of the foot is called ground reactive force (GRF), which acts vertically upwards
- Forces acting at the interface between the orthotic and the foot are called an orthosis reactive force (ORF)
- Any GRF or ORF which acts medial to the STJ causes a supination moment
- Any GRF or ORF which acts lateral to the STJ causes a pronation moment
- The position of the STJ axis will affect this. For more information CLICK HERE
- The more medially deviated the subtalar joint axis, the greater the magnitude of pronation moment acting upon that foot during weightbearing activities
- The more medially deviated the STJ axis, the harder the foot is to control with an orthotic because there is less surface area to exert a supinatory moment. (See diagram below).
- The medial heel skive technique adds an increased amount of varus wedging to the heel cup area of the orthosis
- On a positive cast, the plantar aspect on the heel is divided into 3. The medial 3rd is then scored to depths of 2mm increments and filed away at an angle of 15° (See images below).
Putting it into practice:
- Dr Kirby, explains that “even though the medial heel skive technique is a useful method of adding extra pronation control to an orthosis, it is often difficult to determine when to use the technique in patients who already have orthoses but are not completely asymptomatic”. Often a trial and error method can help determine how effective a Kirby skive will be by adding layers of mole skin to the inside medial heel cup of an existing orthotic. If your patient has not had orthoses before, a Bio-Skive prefab device is an excellent place to start. CLICK HERE FOR MORE INFORMATION
- The Kirby skive works very well for conditions such as paediatric flexible flat feet and posterior tibial dysfunction. Be cautious when treating patients with plantar fasciitis if it is particularly painful at the point of plantar fascial insertion; sometimes the skive can aggravate the tender area. If in doubt, refer to the point above.
- If you are going to integrate a Kirby skive into your prescription, think about other factors too. For example, deep heel cups will help to control the foot better than shallow heel cups.
- Remember that the size and the width of your patient’s foot may affect the look and control level of your Kirby skive. A 4mm skive will look different on a UK size 20 rugby players orthoses than on a child’s UK size 1.