Leather as an Orthotic Material

By Stephen M. Pribut, D.P.M.

 

 

Leather has been an important commodity for thousands of years. Unlike all of the high tech thermoplastics that are commonly used today, leather is a material with a long history. Primitive societies centered much of their life around the pursuit and use of animals that had useable hides. Stone Age man used untanned hides for clothing and used bone and stone tools to assist in the creation of their wardrobe. The Phoenicians are believed to have traded and manufactured leather goods lending their name to a popular dye, Phoenician Red. The Eskimos, American Indians, Africans and even primitive man used skins and hides for many purposes. As clothing leather was used for skirts, parkas, sandals and moccasins. For shelter it was used to make tents and teepees. In the art of weaponry, leather was used to make shields and weapon sheathes. The first tanners' guilds appeared early in the Roman Empire. They spread throughout Europe by the time of the Renaissance.

Leather is still a hot item in international trade. India, a major exporter of leather, has over 550 tanneries near Calcutta alone. The Vietnamese company, Bitis, has the capacity to manufacture 12 million pairs of shoes per year and plans to enter the markets of Laos, Cambodia, Thailand and Myanmar. Advances in tanning and effluent processing are major priorities of the industry. The refinement of high performance leather has resulted in features that include improved water repellency, low water absorption, fast and soft drying, and durability.

 

Leather comes from hides and skins which are differentiated on the basis of size. Large animals like cattle have hides, while smaller animals like pigs and calves have skins. The technical differentiation occurs at the fifteen pound point. Hides must be processed to produce leather. This processing includes pre-tanning, tanning and finishing. The pre-tanning process involves soaking, cleaning and washing the split hides. Tanning is done by vegetable or chemical means and in some cases a combination of the two methods. The vegetable tanning process is performed by placing the sides in large tanks of oak bark tanning solution for several days. The chemical process involves adding salt to water in large tanks and then adding chromium salts. The pelts are then added and turned for several hours. The finishing process involves washing and rinsing the pelts and then smoothing and pressing them. The tanning process converts the collagen in the hide into leather. The finished leather resists aging, does not putrefy when wet, and does not crack readily when dry.

Characteristics of Leather:

Leather has many desirable characteristics for both clothing and orthotic construction. The fibers are woven in a three dimensional pattern. The chemical properties of collagen, the predominant component of skin, give it a distinctive porosity or breathability. Leather can absorb up to thirty percent of its weight in water vapor without loss of dryness. It is also noted to have a stress-strain performance which consists of an optimal combination of firmness, softness, and elasticity. Leather also retains its shape well. Tanning gives leather the ability to resist the aging process well. The refinement of high performance leather has resulted in features that include improved water repellency, low water absorption, fast and soft drying, and durability.

 

Leather In Foot Orthoses

At the beginning of this century, steel was the only material noted to be useful for foot orthoses. John Joseph Nutt, M.D. in his classic 1913 text "Diseases and Deformities of the Foot" mentions only Whitman and Shaffer devices for the treatment of "weak-foot and flat-foot." Later in the century leather made its appearance with a mention in Otto N. Shuster's 1939 text "Foot Orthopaedics - Second Edition". In the 40ís and 50ís some leather combination devices were used to supplement the steel ones in common use. Later, a full length leather device, the Levi mold became popular. Today acrylic devices dominate the marketplace, but there is still much life in the old fashioned leather material.

The form in which I generally employ leather is as a material in a laminated leather foot orthoses. The laminating increases the rigidity of the leather device.

Pros of Leather Orthotics:

The benefits of leather as an orthotic material are a direct result of its intrinsic characteristics. Patient compliance is significantly enhanced and intolerance is reduced since it is much easier for a patient to adapt to a leather orthotic than a more rigid and non-yielding device. Leather orthotics are readily reshaped, remodeled and otherwise adjustable in the office. It is far simpler to use a grinding wheel to alter the function of leather orthotics than to spot heat or return to a lab their acrylic counterparts. Contours are readily changed and leg length difference adjustments are easily added to the device.

The flexible, but non-compressible nature of leather allow it to provide both shock absorption and control. I find leather orthotics most helpful for patients with an intrinsic foot problem that requires both mechanical control of abnormal foot motion and the reduction of vertical forces in segments of the foot. Most leather orthotics have a deeper heel seat than their acrylic counterparts. This assists in giving more stability and rear foot control to the device. The depth of the heel cup in combination with the posting provide the device with frontal plane motion control. If I find that there is excess motion control, rather than lower the heel cup, I usually reduce the posting or skive the medial aspect of the orthotic to conform better to the counter of the shoe. If the midfoot control is excessive, the device may be adjusted on a grinding wheel to reduce the slope below the midtarsal area. By virtue of extending the device to the digital sulcus, pronatory control continues for a much longer time in the gait cycle than for that of orthotic devices that stop before the metatarsal heads. This provides for significant reduction of abnormal pronation during the midstance period of gait and continuing after the point of heel off.

Drawbacks of leather:

Leather devices are more bulky than thermoplastics for a given degree of motion control. In most cases these devices will not readily fit into ski boots, skates, bicycling shoes or even loafers. While they can withstand a limited amount of moisture they do not hold up well under repetitive soaking.

Uses:

Leather orthotics are useful in both controlling excess pronatory forces and in limiting excessive vertical stress. The material also affords significant shock absorption. The clinical entities I most often use leather for include:

 

In the treatment of sesamoiditis leather is an ideal material to use to accommodate vertical stress in the region of the sesamoid bones. Leather laminated orthotics with a sesamoid accommodation also work excellently for the treatment of Turf Toe. Unfortunately most of those individuals treating this injury do the opposite and place a "Morton's extension" or platform beneath the first metatarsal. This is the opposite of what I have found to be most effective for the treatment of this condition. As in all of the conditions we treat, limiting abnormal foot motion is also helpful. In the treatment of metatarsal stress fractures much of the abnormal forces can be reduced by accommodation of the ray. Forces through the metatarsals are further limited by control of excessive rearfoot and midtarsal joint pronation. For the treatment of plantar fasciitis and heel spur syndrome, control of abnormal pronation is of paramount importance. Success in the treatment of heel pain is further enhanced by using a deep heel cup which holds the soft tissue plantar to the calcaneus in a better position to cushion it, and by the shock absorbing character of the leather material. In resistant cases it is helpful to add an accommodative indentation in the orthotic at the level of the medial calcaneal tuberosity. This is easy to accomplish in your office. Leather orthotics can be used for a variety of other conditions including achilles tendonitis. The combination of a heel lift and the control of abnormal motions of the rearfoot combine to assist in alleviating this condition.

 

Orthotic Production:

 

The production process is a labor intensive one. The leather is cut, wet and strapped over the cast and then allowed to dry for at least twelve hours before the next layer is applied. Approximately 4 or 5 layers of leather are used and then several layers of a latex, wood and cork dust conglomeration are applied. The orthotic is then ground by hand into the finished product. In today's high tech environment it is difficult to find laboratories with extensive experience in the production of leather orthotics. The laboratory I use is one of the longest operating orthotic laboratories specializing in the production of leather foot orthoses, Schuster and Richards Laboratory (800-272-9770), College Point, N.Y. This laboratory has been in existence for several generations.

 

References and Web Resources:

  1. Biomechanics April 1997, Volume IV, Number 4 (http://www.biomech.com/archive/1997/apr97/) Materials: Leather as an Orthotic Material.by Stephen M. Pribut, D.P.M. A print format, peer reviewed, article which is the basis for and substantially similar to this current web text.
  2. Leathernet (http://www.leathernet.com/ ) A thorough Internet resource on leather.
  3. Orthotic Materials: An Overview by Jim Willis and Edwin Black (Biomechanics, February 1996 )
  4. The Mechanics of Foot Orthoses for Runners by Joe Hamill, PhD, and Timothy R. Derrick Biomechanics, February 1996
  5. O'Flaherty, Fred, and others, eds. The chemistry and technology of leather. New York. Reinhold. (American Chemical Society monograph series, no. 134. 1956-65. 4 vol.
  6. Central Leather Research Institute http://sunsite.sut.ac.jp/asia/india/jitnet/india/csir/clri.html

 

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