Custom orthotic devices are classically used for common maladies of the foot. There are two basic types of orthotics: accommodative and functional. Accommodative orthotics are used to distribute the patient's body weight away from a painful area. The role of functional orthotics is to prevent pain during ambulation and more importantly to prevent pathologic range of motion in the joints of the foot. The functional orthotic is casted to the foot in a corrected position and sent to a laboratory where a hard plastic is used to make the device. The development of new, lighter, stronger, materials has increased the variety of devices and computerization has improved the overall quality of these products.
When the foot is subjected to persistent abnormal patterns of gait, a particular set of symptoms will follow. The principles of biomechanics in the lower extremity must be understood before attempting to prescribe these devices.
Human gait has been described by Steindler as " a series of movements where the body moves forward and the legs are placed in front of the trunk to prevent us from falling on our faces1." This point is clearly demonstrated by watching babies take their first steps with arms flailing, body swaying from side to side, and finally falling into mommy or daddy's outstretched arms. This explanation is misleading as to the simplicity of the process. There are 26 bones in the foot alone with 17 functional joints and many more articulations. There are numerous intrinsic and extrinsic muscles of the foot innervated by 6 nerves acting on those joints. In addition, there are precise applications of all the lower extremity and postural muscles. Therefore, each has a specific function and firing order in the sequence of movements that make up gait. Therefore the act of walking must be a well coordinated psychoneural and musculoskeletal process that is both repeatable step after step and accommodates to changes in conditions1. There are many variations of this pathway that may be considered ambulation but not necessarily normal gait. Variants to normal gait can be due to congenital anomaly, anatomic variation and loss of function due to traumatic event. It is when these variants of normal gait are multiplied by years of walking that most foot pathology occurs. Each year of walking consists of over 2.5 million steps and each walking step translates to a force of 2.5 times our body weight.
The goal of podiatric medicine is to identify and eliminate variants to normal gait. Identification of these variations, as one may imagine, is occasionally inexact. The use of shoe inserts was documented in 1845 by Durlacher who used built up leather inlays to treat mechanical foot problems2. In 1888 Whitman used steel plates with a large medial and lateral flange to press into the navicular and cause the foot to invert. If force alone did not cause inversion, the pain elicited from the device caused contraction of the posterior tibial muscle causing inversion3. Since these early devices, many other attempts were made to control foot function with the application of a device in the shoe 4,5,6. The problem was there had been no uniform or widely accepted theory of foot biomechanics to guide therapy.
It was the work of Merton Root et al begun over 30 years ago that has guided much of the theory behind the application of orthotics. Root et al considered any variation from the definition of normal foot alignment to cause abnormal foot function7. They further studied the patterns of these variations and found a particular set of signs and symptoms to correlate with particular variations. They also worked to produce a systematic clinical approach to measure these variations8 which can then be manually reduced if the deformity is non-rigid and casted in that position9. A functional foot orthosis may then be constructed attempting to restore what is considered normal function. This theory has been debated since its introduction and has been met by some skepticism. On the whole, however, Root's work has been viewed with wide acceptance and is closely incorporated into clinical practice.
What exactly is "normal foot function?" Normal function is not easily defined since it may vary depending on several factors It is clear, however, that subjective complaints and objective findings confirm that foot pathology does exist and that orthotics can be applied with good clinical response10,11,12,13. There remains a need for long term, prospective, randomized, controlled trials comparing functional foot orthotics to other devices or treatments14. The best measurement of orthotic efficacy remains subjective, and the direction of current research is toward computerized models which may lead to a better understanding of the normal foot15. Since previous studies have prospectively evaluated the use of functional orthotics with two having positive and one negative outcome16,17,18, the successful widespread use of orthotic devices will have to be evaluated by clinical experience and subjective relief in the patients.
Biomechanics is defined as " the application of mechanical laws to living structures, as to a locomotor system19." Merton Root, one of the most important contributors to the field of lower extremity biomechanics, recently wrote that biomechanics is a necessary basic science for the field of podiatric medicine. No specialty in the field of medicine is more intimately involved, on an everyday basis, with the clinical application of biomechanics. The understanding of basic mechanics and biomechanics of the lower extremity can provide the podiatrist with an invaluable diagnostic ability that cannot be otherwise matched20. The relationship between biomechanics and orthotic devices is the attempt to change bone and soft tissue alignment of the foot extrinsically, correcting biomechanics which may have led to the functional foot problem. The changes made by surgical correction are permanent and may lead to more problems post surgery. Patients are strongly encouraged, therefore, to attempt using a functional orthotic before surgery if the patient will likely benefit. The feature of using orthotics that is most appealing, as compared to surgical reconstruction, is that their effects are reversible. The cost of orthotics is a fraction of the cost of surgical reconstruction, but the cost of orthotics is usually absorbed by the patient as few insurance companies cover the cost of orthotic care. All types of ortho-tics can be grouped into two categories: accommodative and functional. The characteristics of each category can be shared by certain devices.
Accommodative orthotic devices fit in the shoe to accommodate foot deformity. They function to transfer weight away from a painful area and place increased pressure where the foot has enough intrinsic padding to allow for better ambulation. There is no attempt to rigidly control motion around a specific joint but rather to control ground reactive forces around a specific location. Accommodative devices are applied at the location of a rigid deformity, arthritic joint or near a painful chronic skin lesion. They are particularly valuable for at risk patients with diabetes, peripheral vascular disease and neuropathy who are prone to pedal ulceration. Accommodative orthotics decrease vertical and shear force which leads to ulceration21. An example of a specific, prescription accommodative device allows pressure to be dispersed around a painful 4th metatarsal head to the adjacent areas:
1. Two 1/8 inch layers of closed cell foam, (foam product that will not allow complete deformation and rebounds after pressure is removed), placed under the 5th metatarsal and at the periphery of any skin lesion
2. The rest of the device made from 1/8 inch open cell foam. (Foam product that is easily deformed and will not rebound) and 1/8 closed cell foam.
3. Metatarsal pad for placement behind the metatarsal heads (places pressure under the metatarsal shafts where there is padding from the intrinsic muscles)
4. SpencoŽ top-cover ( neoprene product which has great shock absorption and reduces shear forces which cause callous formation).
This is fairly simple device is made for a specific pathologic process. Obviously the scope of orthotics is broad and can be applied to many functional variations of the foot.
As discussed earlier in the paper, Merton Root et al framed much of what physicians for many years knew to be true: shoe inserts make certain patients feel better. The foot operates best around a single position. Excessive deviation from that position will cause certain deformities, and Root regarded this position to be the point where the subtalar joint is neither pronated nor supinated9. Most functional orthotics attempt to capture the foot in this neutral position and then rigidly hold the foot in this position. The position must allow for both pronation and supination but not excessive end range of motion. During heel contact, pronation is wanted because the foot has to adapt to changing terrain and dissipate shock. Without pronation at heel contact, every bump or incline would cause us to fall down. Later in the gait cycle, when the weight is being sent from the rearfoot to the forefoot in preparation for propulsion, the foot should be supinating. During propulsion, the foot would like to act like one single, stable bone. The body will compensate for the floppy foot by contracting the powerful long flexors and extensors to stabilize the foot. Many forefoot problems can be directed to this mechanism, for example bunions and hammertoes.
As an example, the foot that has a pathologic increase in 5th ray range of motion allows weight transfer from the rearfoot to the forefoot exerting excessive pressure under the 4th metatarsal head during forefoot loading and, therefor, causing pain. Such a problem can be treated using a functional device to restore function to the 5th ray. The patient is given a full biomechanical examination consisting of gait analysis, measurement of ranges of motion from the hip to the toes, and possibly weight bearing foot radiographs. After analyzing the above information, the foot can be casted in the desired position. One common technique is the neutral suspension cast. The cast is made using extra fast drying plaster fashioned into a moccasin around the foot. The cast is allowed to dry and then is carefully peeled from the patient's foot taking care not to disturb the impression. The cast is then evaluated and sent to an orthotic/prosthetic company. Orthotic prescriptions may range from flexible polypropylene materials to rigid acrylic or carbon/graphite composite devices. The method for making the device can range from hand constructed in the lab to a precise computer honed device. The correct device for the patient depends on the activities, pathology and preference of the individual patient. For the patient in our example, the most appropriate orthotic may be a semi-rigid polypropylene device with a full length SpencoŽ topcover with lesion accommodation under the 4th metatarsal head. This device will control excessive instability and distribute weight evenly during ambulation and limit pain.
Further scientific advances are necessary to produce more reliable measurement of functional pathology and lead to a better understanding of the foot in its normal state. Study of the biomechanics of the lower extremity must be considered an integral part of any treatment plan aimed at certain foot pathology. The use of both accommodative and functional orthotics provide a non-invasive, inexpensive treatment that does reduce pain in many foot deformities. The use of accommodative orthotics is a good preventive measure for those at risk, such as diabetics, neuropathic patients, or patients with vascular disease. Advances in the fields of biomaterials and computerized biomechanical assessment/manufacturing techniques will improve the reliability, reproducibility, and overall quality of custom orthoses.
REFERENCES
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