Quantification of Orthotic Design

History:  Foot control research is well documented over the last 200 years.

Even so, little clinical agreement exists as to how to control the biomechanical pathology of human gait.  For more than two centuries, the “art” of orthotic design has limited the science of human biomechanics.

 In 1987, Nick Grumbine, DPM correctly wrote that “The level of therapy of a functional orthotic is directly proportional to the angle of control afforded to the supported foot,” and that “Such control stems from the effectiveness of the orthotic in holding or supporting the foot and all its components at specific angles.” 

 Problem: No one has defined a way to measure these “specific angles”. When these specific angles of support are not measured, supination of the foot can not be related to orthotic design. Pronation during gait may be implicated in the pathogenesis of many chronic and acute medical conditions, but treatment with a supinatory device can not be studied scoemtofically.

Hypothesis: A functional orthotic changes the horizontal angle of support, and supinates the foot in the frontal plane, during closed kinetic chain function.  “Specific angles” in orthotic design can be defined and measu. These specific angles quantify the angular relationship between orthotic design, and supination of the foot during closed kinetic chain function.

 Method:   

Review orthotic design.

Identify the “Specific Angles” in orthotic design.

Define and measure the “Specific Angles.”

Discuss other related design characteristics.                                                     

Review

 Seventy US patents awarded for orthotics/insoles between 1823 and1998 were reviewed. Most inventions described a cup shape to the proximal portion of their device. The cylindrical shape of the heel and the orthotic, were said to form a “ball and socket” configuration, where the heel could rotate freely.  Deep heel cups, or the addition of medial, and/or lateral posts, do not change the angle of support provided by this circular shape during heel contact. Primary function of the “cupola” is to help position the critical embodiment or wedge, inside a shoe, during later stages of the gait cycle.

Identify the Specific Angles

Angle # 1: Most of the authors described a wedge that follows the curves created by the medial, lateral, and transverse arches of the foot. This contoured wedge starts where the heel ends, gradually increases toward the apex of the medial longitudinal arch (TNJ), and then decreases to nothing just proximal to the metatarsal heads.

The frontal plane angle of this wedge, at any point along the increasing then decreasing sagittal plane curve, is defined mathematically by the angle at its highest point. In the frontal plane, the change in angle of support from the horizontal plane can be defined by two points on the horizontal, and one point on the device surface.

Functionally, this anatomically contoured wedge changes the angle of support directly under the medial arch/column, and results in supination of the STJ and external rotation of the leg from late heel contact throughout all of mid-stance. 

Angle # 2: Some patents described a separate varus wedge, positioned in the sagittal plane, directly under the weight bearing metatarsal heads.  Measurement of this angle in the frontal plane represents the vertical difference in support between the weight-bearing first metatarsal or medial column, and the weight bearing fifth metatarsal or lateral column.

Functionally, a varus support, applied to the plantar first metatarsal, will generate a retrograde, vertically directed component of force through the medial column.  This force elevates the first metatarsal, supinates the talus, and externally rotates the leg during weight bearing gait.

 Angle # 1 and angle # 2: Some of the patents described a varus wedge at both rear foot (1) and forefoot (2) positions. Functionally, these supinatory devices change the angle of support from the horizontal plane, supinate the foot, and externally rotate the leg, beginning just after heel contact and continuing throughout all of propulsion.

Define and measure the “Specific angles.”

The contour at the apex of the plantar arch, in the frontal plane, forms an exponentially increasing parabolic curve as it follows up the medial side of the arch. Measurement of an angle that would represent this contoured wedge, changes depending on the points (P1, P2, P3) used to define the angle. When the correct points are used to define this specific angle, a direct relationship with supination of the foot during mid-stance can be consistently documented.

Specific Angle 1 (Theta 1) is measured at the highest point of the arch, in line with the Talo-Navicular joint. This frontal plane section is divided into 4 parts from medial to lateral. Point A is on the horizontal plane, 25 percent of the distance from medial to lateral. Point B is on the horizontal plane, 75 percent of the distance form medial to lateral. Point C is on the orthotic surface, directly above point A. 

By definition these points include the more planer central portion of the orthotic, and exclude the more cylindrical shaped “medial flange”, from the angular measurement. The parabolic contour of this part of the plantar foot has resulted in controversy, and significantly complicated attempts, to quantify this “specific angle”, in orthotic design.

Specific Angle 2 (Theta 2) is measured in a frontal plane section through the weight bearing first and fourth metatarsals.  Point D is on the horizontal plane directly under the weight bearing first metatarsal. Point B is on the horizontal plane, lateral to the medial column, and medial to the weight bearing surface of the fifth metatarsal.  Point E is located on the superior surface of the wedge, directly under the first metatarsal.

Anatomically, as this specific angle of support increases, the first metatarsal is elevated from the horizontal plane. Functionally, a force is transmitted through the medial column that causes the STJ to supinate during late mid-stance, and during all of propulsion.

Important Design Considerations:

Many different materials, both rigid and flexible are used for orthotic construction. The ability of a device to maintain its “specific angle” of support during weight-bearing is a critical part of functional/angular orthotic design. Materials, that can partially or fully compress, during weight bearing, decrease the “specific angles” of support, and the ability of the device to supinate the foot during function.

Cushion, especially under the weight-bearing heel and metatarsal regions of the foot, was discussed in all of the patents that were reviewed. The benefit of shock absorption is admitted, but it does not change the angle of support from the horizontal, and is not considered part of quantified orthotic design. 

Positioning of the wedge against the foot during function was described, often in detail, as a “critical part” of each invention. Clinically, phase of gait placement of the wedge, is as important as the angle of the wedge itself. Some inventions use straps to secure optimal positioning of the wedge during function. Most orthotic devices rely on the shoe, or an insole modification, to position the angular embodiment.

Conclusions:

Qualitatively, functional orthotics contour the plantar foot. They use varus angled wedges placed at the apex of the arch, and between the weight bearing first and fifth metatarsals. Functionally they change the angle of support in the frontal plane, supinate the foot, and externally rotate both tibial and femoral segments during gait.

Quantitatively, the wedged portions of any foot control device can be identified, measured, and expressed in mathematical terms. The relationship, between quantified orthotic design as defined here, supination of the human foot, and external rotation of the lower extremity can be documented, and studied scientifically.

Theta-Orthotics, PO Box 1574 Riverton, Ut. 84065