Sunday, April 27, 2008
What are foot orthoses?
Since the 70s the humble arch support taken on a new life of it own with the introduction of pedal mechanics. The history of arch supports coincides with the fear of flat feet which has been a Judo-Christian pre-occupation for thousands of years. Belief man (not woman) was made in the image of God (this has recently changed in PC versions of the scriptures), meant Christian Art always represented the male form in painting and sculpture with a well formed arch. In truth feet flatten during locomotion but arch feet took on a far more sinister connotation during the 18th and 19th centuries when Europe was pro-occupied with antis-emetic anthropometry. Flat feet were deemed Jewish feet and as a condition was associated with laziness, sloth behaviour and lack of general fitness. The arch support fitted to shows became the saving grace and turned human failing into a god like supremacy. By the 19th and 20th century the world was pre-occupied with muscular Christianity and with games became a healthy pastime. This peaked in the 30s with the cult of physical culture and once again the arch support was there to support Colossus or anyone else for that matter with weak arches. The perceived allopathic action of the arch support was to replace the missing curve under the medial longitudinal arch of the foot. Standing and walking over the arch support gave the foot support and rested strains that might be otherwise are associated with a flat foot. For some people these were very comfortable whereas other found them painful to wear. Like so many other panaceas for the foot quietly most people took them out and forgot about them. By the seventies better understanding of three dimensional movement was possible through kinetic, kinematic and kinesiology analysis. This led directly to a relaunch of interest in foot orthosis (orthosis meaning straight) with the drive mainly coming from podiatrists. Photographic gait analysis provided much needed framed activity with which scientists could analyse foot ground contact and quickly a feasible hypothesis was developed in California which attempted to describe the foot and ground activity during weight bearing. By 1977, the publication of Normal and abnormal function of the foot by Root, O’Rien and Weed saw the establishment of the subtalar joint theorem as a credible description of foot mechanics. This held considerable sway with the foot orthotic industry that began to reinvent the old arch support as a vehicle to control rearfoot motion and balance the foot through propulsion. Unfortunately absence of more sophisticated analytical instrumentation prevented detailed enquiry and subsequently many of the claims for foot orthoses remain without proof. Overall however most people find the devices not uncomfortable. Shoe inlays (aka foot orthoses) can be divided into two groups. Functional Foot Orthoses are capable of controlling functional pathology of the foot and leg by restricting the full range of joint motion. Middle range motion is maintained with the use of wedges or posts and high impact resistant plastics are used for this purpose. The device consists of a shell of heel cup and lateral platform to the ball of the foot. The shell shape mirrors the contours of the weight bearing foot throughout the complete stance phase. Additions of wedges and levers to balance with horizontal ground surface assist in stability reducing patho-mechanical changes (damage to tissues cause by instability). By wedging, tilting and lifting foot segments during propulsion this is thought to help the lever action of the lower limb during closed chain motion. Simply put a series of lifts and levers are used to prevent body compensations for perceived foot and leg deformities. These may be either bony or muscular. Abnormal compensations resulting in patho-mechanical changes are considered to be the cause the majority of painful symptoms associated with foot pronation during the propulsive phase of gait. Accommodative Foot Orthoses provide the weight bearing foot with increased shock absorption and pressure redistribution. A series of semi rigid synthetics are used for this purpose and the devices may be bespoke or bought over the counter. Functional and Accommodative devices appear similar and it requires a good working knowledge of material science to discriminate between them. It is common to prescribe combination foot orthoses which exhibit properties of both functional and accommodative foot orthoses. With the onset of cinematography and particularly video, the ability to observe cyclic activity such as walking has become a reality. Interest in rehabilitation concentrated the efforts of clinicians and bioengineers to develop models of normalcy. The Root Paradigm was one popular method of describing foot function during the walking cycle. What is not in dispute is the foot is considered to have four separate functions during the cycle: decelerator and shock absorber; mobile adapter to ground surface; rigid lever; & accelerator. The total time to complete stance phase is 6/10 of a second in normal walking. This is much less in running. The point where deceleration is overtaken by acceleration corresponds to mid stance (i.e. single stance support when the heels are in apposition). Root et al took this as the subtalar neutral position and defined the hypothetical relationships between forefoot to rear foot and rear foot to leg. Deviations were defined as fixed plane deformities in time and described using standard orthopaedic nomenclature. The purpose of the functional orthosis is to replace the difference between actual foot leg relationships with hypothetical normal, at mid stance. The amount of torque and twist as body weight passes over the shell is considerable at this point. Hence the need to incorporate high impact resistant plastics with rigid properties. Lifts and levers can be made from rigid or semi rigid components. Most over the counter foot orthoses incorporate semi rigid materials simply as a fail safe mechanism. A second and separate management of the foot relates to shock attenuation. During the stance phase there are two peak shocks to the musculo-skeletal system. The first occurs just after heel strike and the second, just prior to the heel lifting off the ground. Synthetic materials have the ability to modify vertical shock forces at the critical stages during the stance phase to prevent jarring to the system. This property is referred to as shock attenuation. The material must be able to react to repeated shock waves without exhibiting permanent set. In releasing strain energy an insole material should be able to return to its original shape, thus exhibiting good elastic memory. Throughout contact phase materials need to be able to adjust continuously to equalise underfoot pressure. This is known as an Isobaric Action. At critical temperatures plastic foams change shape to accommodate the sole of the foot and as a result ground reaction forces are spread over wider surfaces providing a waterbed or isotactic effect. The material next to the skin should present a friction free surface to prevent kinetic energy being converted to heat. Materials with low thermal conductivity insulate the skin surfaces and prevent loss of heat from tissues. This is particularly helpful in people with poor circulation. Clearly not all manufactured insoles provide these properties but many over the counter inlays have the potential to do so. From experience the best combination are cellular or non cellular polyurethane elastomers as the base for the inlays with closed cell cross linked polyethylene foam as a top cover. The torque placed on plastic inlays when a person stands, walks and runs over them would be the equivalent of a medium sized elephant pirouetting. The most innovative new material now incorporated into foot gear is viscoelastic. These materials are liquid and gas and should not exist as a solid. By clever manipulation of the polymer structure viscoelastic now appear as soft solids. Pressures cannot pass through this material and they are used to reduce peak shock during heel strike. For example normal shock at heel strike would be the equivalent force applied to the head resulting in whiplash during a car crash. Often this viscoelastic material is caught in the heel of the sports shoe.