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Rigid Orthoses for the Insensitive Foot: The "Rigid-Relief" Orthosis

Andrew Novick, MA, PT
James A. Birke, MS, PT
Alicia S. Hoard, OTR
Denise M. Brasseaux, PT
John B. Broussard, PT
Elizabeth S. Hawkins, DPM, MPH

Introduction

The management of neuropathic plantar ulcers can be broadly categorized into two phases: 1) healing of the wound and 2) maintenance of the former lesion in its healed state. Clinical experience has shown that in many cases the latter is the more difficult challenge.

It is well documented that neuropathic plantar ulcers are primarily the result of repetitive mechanical stress to vulnerable areas of the insensitive foot (1-6). Studies have shown that these lesions most often occur at sites of maximum load, usually associated with a bony prominence. Ctercteko et al. found increased vertical force under the 1st metatarsal head (MTH) in diabetic patients, both with and without metatarsal head ulcers, when compared to normals (6). In those patients with ulcers, the lesions occurred at sites of maximum force, and peak forces were significantly greater than those recorded in controls. Similar results showing increased vertical force or pressure under the MTHs in diabetic patients were obtained by several investigators (7-11). Pollard and LeQuesne also recorded maximum horizontal shear forces at the site of neurotrophic ulceration (10). Decreased loading of the toes was also measured, resulting in increased loading of the metatarsal heads due to the reduced surface area onto which forces are distributed (6-9).

Several types of specialized devices have been used to successfully heal plantar ulcers, including total contact casts (TCC), plaster posterior walking splints, molded double-rocker plaster shoes (MDRPS) and patellartendon-bearing (PTB) orthoses (10,12-22). Quantitative studies have examined the effect of these devices on reducing plantar force/pressure. Birke, Sims and Buford, and Pollard, LeQuesne and Tappin showed significant decreases in force/pressure under the metatarsal heads with the TCC (23,25). Novick et al. found similar reductions with a TCC at the MTHs using either a rubber walking heel or cast boot when compared to a standard leather oxford shoe (24). A significant reduction was similarly measured under the heel. The study also examined the posterior walking splint, which was found to be essentially as effective as the TCC in reducing plantar forces under the heel and forefoot. Correspondingly, there was an increase in force at the midfoot, which partially explains the decreased loading at the heel and forefoot. PTB orthoses have also been shown effective in reducing plantar pressure (22,26). Birke and Nawoczenski reported no difference in pressure reduction under the heel and forefoot between the PTB orthosis and a short-leg orthosis (26).

The effectiveness of these healing footwear devices can be explained by several factors. First, the weightbearing forces are evenly distributed throughout the entire plantar surface of the foot due to the total contact fit of the device's footbed. Increasing the surface area that accepts weightbearing forces results in a reduced force being applied to any one specific area of the foot. Secondly, reliefs are built into the footbed by either placing foam padding over the lesion prior to applying plaster for the TCC and walking splint, or by grinding-out the footbed material beneath the lesion before attaching the PTB orthosis. Lastly, all devices, except the MDRPS, transfer weightbearing forces to the lower leg through their proximal extensions. Stress-relieving concepts have also been applied to clinically proven devices such as foam aperture pads (felted-foam), molded Plastazote™ sandals, Plastazote™ boots, Scotchcast™ boots and Unna/fiberglas boots (27-30).

Once the ulcer has healed, the patient can progress to more conventional, permanent footwear. However, for the lesion to remain healed, the same force-/pressure-reducing principles common to the previously described devices must be used in the definitive footwear. Nawoczenski, Birke and Coleman, and Schaff and Cavanagh found significant pressure reduction in the forefoot with the use of a rigid rocker-sole on a leather oxford shoe (31,32). Holmes and Timmerman effectively reduced peak pressures under MTHs by affixing a metatarsal pad directly on the foot's plantar surface just proximal to the site of maximum pressure (33).

Perhaps the most important defense in preventing recurrent ulcers is orthotic intervention. The orthosis is typically one of two types: 1) a flat insole made from a single layer of soft, low-durometer material, such as PPT (The Langer Group, Deer Park, N.Y. 11729) or Spenco (Spenco Medical Corp., Waco, Texas 76710), or 2) a custom-molded orthosis made from a soft or semi-rigid material, such as Plastazote #1 or #2 (Bakelite Sylonite Ltd., Hertfordshire, England), Pelite (Cascade Orthopedics, York, Pa. 17402) or Aliplast (Alimed Inc., Dedham, Mass. 02026). These materials provide total contact and can accommodate biomechanical faults through posting (3,34-36). Reliefs may also be made on their undersurface, corresponding to observed or marked areas of high pressure to further decrease pressure at these sites. The molded orthosis may be covered with a layer of soft insole material for greater shock absorbency.

Despite the use of insoles and custom-molded orthoses, often in conjunction with rigid rocker-soled shoes or other shoe modifications, many patients are prone to reulceration. For this reason, an alternative type of orthosis was sought. It was reasoned that an orthosis combining the shock-absorbing features of soft insole material with the TCC's rigid, custom-molded, pressure-relieving design might yield a more favorable outcome. The use of rigid material has also been supported by Rubin, Cohen and Rzonca, and Reigler (35,37). In addition, the shape of the foot appears to change between the non-weightbearing (NWB) and full-weightbearing (FWB) positions (38). Historically, the foot is casted NWB in the subtalar neutral position, and the molded orthosis formed over this positive model. However, when the weightbearing foot is placed on this NWB orthosis, the lowering of the medial longitudinal arch, and change in both length and width of the foot, creates an imperfect fit. This mismatch could be critical in the success or failure of an orthosis made from a rigid material. To accommodate this, it was decided the foot should be casted in FWB to obtain the foot's exact contour in its functional position when the greatest forces are applied and the resultant osseous and soft tissue reactions have occurred. Reigler also advocated casting the foot in the weightbearing position to obtain its more physiologic position (37).

This article describes a method of fabricating a rigid device vacuum-formed over a full-weightbearing plaster model. Neoprene crepe (Southern Leather Co., New Orleans, La. 70113) is added to the plaster model at the healed lesion sites prior to molding to create the relief between the orthosis and the healed lesion site. This rigid-relief orthosis is then covered with a soft, shock-absorbing material and placed in a shoe having adequate height in the toe box, such as an extra-depth shoe or leather tennis shoe.

Fabrication Methods

Casting Procedures

The healed lesion site is marked on the patient's foot with a felt-tipped pen for later identification on the positive plaster model. The patient is placed in FWB stance position with weight evenly distributed between both feet. Two sets of six-inch-wide plaster of paris splints, each three layers thick, are overlapped in the center and cut long enough to cover the front of the toes and the posterior heel. The plaster is moistened then placed on a piece of one-inch-thick foam rubber that is covered with a thin sheet of plastic for protection from moisture. After smoothing, it is placed under the patient's foot. The plaster is molded and allowed to dry while the foot is maintained in its optimal position for maximum function (see Figure 1 ). This may be subtalar neutral position or the position deemed most suitable based upon existing deformities or limitations of mobility.

Positive Model Preparation

After drying, the site of the healed lesion is identified in the shell and remarked with the felt-tipped pen (see Figure 2a and Figure 2b ). Plaster is poured into the shell and the hardened positive model is removed and smoothed. The sulcus between the forefoot and toes is eliminated with additional plaster, built-up even with the plane of the heel and MTHs (see Figure 3 ). This effectively lowers the distal end of the orthosis and prevents edge pressure against the anterior forefoot.

The healed lesion site is identified on the positive model. A piece of 1/4-inch-thick neoprene crepe is cut so it extends beyond the edges of the site by approximately 1/4-inch on all borders (see Figure 4a and Figure 4b ). This allows a slope to be ground into the crepe, removing its sharp edge while maintaining full thickness under the healed lesion site. It also reduces the risk of edge pressure from side-to-side movement of the site over the orthosis while supporting the bone just proximal to the healed lesion site, much like a metatarsal pad. When the site is under an MTH, the crepe is extended at least one inch so the orthosis will allow for distal migration of the MTH. The crepe relief pad is briefly heated in the oven then affixed onto the model with contact cement.

An additional piece of 1/4-inch crepe is buffed then similarly added on top of the first to obtain proper depth of the relief (see Figure 5 ). Usually some minor buffing is required to achieve the final shape of the crepe relief so the final thickness measures between 3/8- and 1/2-inch (see Figure 6 ). Two 1/4inch rather than one 1/2-inch crepe were used since it was easier to mold the thinner material. Other materials, including plaster, can be used to create the relief.

Orthotic Fabrication

A piece of Subortholen (three- or four-mm thick) [JMS Berkshire Resource Inc., Garfield, N.J. 07026] is heated and molded over the positive model in a vacuum chamber. When cooled, the material is cut from the model with a cast saw. Rough edges are smoothed, and the device is placed under the FWB patient to identify final trimlines. The length of the finished rigid-relief orthosis extends to the junction of the toes with the forefoot (approximately one inch beyond the MTHs) to prevent edge pressure (see Figure 7 ). This reduces MT joint extension, but the rocker design of the shoe can substitute for this motion. The inner edges of the orthosis are beveled from the inside out to reduce edge pressure (see Figures 8a, b, c and d ). A piece of firm orthotic material is secured under the Subortholen shell and ground level to the bottom of the relief (see Figure 9 ). When the relief is under the 1st MTH, it can be used as a medial post, if necessary. The orthosis is lined with Spenco or PPT to provide a soft, shock-absorbent interface (see Figure 10 ).

Case Studies

Two case studies describing the application of the rigid-relief orthosis in the management of patients with chronic reulceration follow. In both cases the lesions were healed with either a TCC or walking splint, but reulcerated while wearing shoes with a semi-rigid orthosis or molded sandals.

Case 1

Patient GE is a 60-year-old female who has had diabetes for 24 years and Type I diabetes for the last 17 years (see Figure 11 ,and Figures 12a and b ). Examination showed no sensation on the dorsal or plantar aspects of either foot. An ischemic index of greater than .85 was measured on the left, while the right vessels were unable to be occluded. Her initial foot problem was an ulcer under the right 4th toe in 1983 that subsequently developed an infection and was amputated. She had repeated episodes of stress fractures in her right foot during 1984-85 and surgery to the tendons of her right lesser toes in 1986. In November 1987, a large blistered area developed into an ulcer under the right 4th MTH following a full day of walking in bedroom slippers. This lesion was successfully healed with casting and a molded sandal relieved under the lesion.

In September 1988, she was progressed to a semi-rigid, molded orthosis covered with PPT and relieved under the right 4th MTH. The orthosis was placed in a rigid rockersoled shoe. She required maintenance callus trimming and minor orthotic modifications, noting at times discoloration due to subcutaneous hematoma, but no lesions to the 4th MTH. Reulceration of the right 4th MTH occurred in May 1989 after walking all day in molded sandals at a shopping mall. A plaster walking splint was made, and the wound healed in three months.

An examination on July 20, 1990, revealed the initial lesion at the right 1st MTH, for which a walking splint was made. The patient returned for four follow-up visits -which included wound care and callus trimming-until the ulcer healed in seven weeks. At this time she was instructed to wear the splint for one more week then alternate the splint with the molded sandals. She also was allowed to begin gradual wearing of extra-depth shoes and semi-rigid orthoses, beginning at one hour/day and adding one hour each day. The 1st MTH was found to have reulcerated Oct. 1, 1990, prompting a return to the splint. The lesion healed in two weeks, but again reulcerated by Nov. 5, 1990, after wearing the shoes/semi-rigid orthosis for periods of up to four hours/day instead of the prescribed one hour/day. This lesion healed with a cast, but she experienced three more episodes of 1st MTH reulceration over the next five months.

When the last lesion healed by April 12, 1991, the rigid-relief orthosis was issued. She was instructed in a gradual break-in period for the shoes/rigid-relief orthosis to be worn for only one hour during both a morning and afternoon session for a three-day time period, followed by an additional hour each session for each three-day period. At all other times she was to wear the splint. This allowed complete knowledge of the effects of the shoe/rigid-relief orthosis since the splint was known to permit healing at the 1st MTH.

When she returned April 26, 1991, she had been wearing the shoe/rigid-relief orthosis for five hours in both the morning and afternoon sessions and developed only a small callus. The callus was trimmed, and she was instructed to continue increasing her time with the rigid-relief orthosis until she was wearing it full time. She returned May 10, 1991, with a small hematoma under the 1st MTH that she stated was not present until she wore her molded sandal for two hours. Wearing of the rigid-relief orthosis had increased to seven hours for each time period the previous week ending May 4, 1991, and she reported wearing the rigid-relief orthosis for 16 hours May 4, 1991, and 10 hours May 5, 1991, each time without any problems. The patient then went on vacation for over three weeks, wearing the rigid-relief orthosis part-time for up to 6 hours/day and remained ulcer-free at both the 1st and 4th MTHs for a total of more than two months.

Case 2

Patient MG is a 63-year-old male with Hansen's disease (see Figure 13a , Figure 13b , and Figure 14 ) with lack of sensation on the plantar aspects of both feet and a resultant long history of bilateral plantar foot lesions. More recent involvement to the left foot included an ulcer at the 1st MTH in January 1988 that measured 15 mm in diameter and 15 mm in depth. The lesion was healed with the use of a plaster walking splint. The next recorded ulcer on Sept. 2, 1988, was of two weeks' duration under the left 1st and 3rd MTHs. These were treated with a Plastazote boot with reliefs cut out under the lesions. Three weeks later, a small spicule of necrotic bone was noted and removed. A splint was applied Oct. 12, 1988, promoting closure of the 3rd MTH ulcer by Jan. 5, 1989, and the 1st MTH lesion approximately two months later. Custom shoes with a rigid rocker-sole and semi-rigid orthoses were issued in June 1989. Reulceration of the 3rd MTH, 1 cm in depth, was noted one month later. Additional PPT was placed on the left orthosis and the rocker repositioned more posteriorly. The lesion was healed by Aug. 25, 1989, but reopened by Oct. 2, 1989. Notes indicate the 1st MTH ulcer recurred twice more, measuring 3 mm in depth and 2 mm in diameter on July 31, 1990. The Plastazote boot, followed by a short course with the splint, promoted healing by Sept. 17, 1990.

The patient returned to his custom shoes with semi-rigid orthosis, alternating with the molded sandal, and remained healed until the 1st MTH lesion recurred Oct. 30, 1990. Having made multiple modifications to the shoes and semi-rigid orthosis, he was casted for a rigid-relief orthosis. It was delivered Nov. 28, 1990, and the patient was closely monitored while wearing only the shoe/rigid-relief orthosis until his departure for a three-week vacation Dec. 18, 1990. At that time the lesion had improved but remained open. He returned Jan. 9, 1991, and was examined one week later. The 1st MTH ulcer had healed while wearing the rigid-relief orthosis in a standard low-top tennis shoe with a rigid rocker-sole and occasionally a molded sandal. The lesion has remained healed for more than eight months while the patient continues to wear the rigid-relief orthosis in the tennis shoe, with only occasional use of the molded sandal.

Summary

The rigid-relief orthosis has been clinically shown to be an effective, adjuvant modality in preventing recurrent plantar ulcers. lts benefits can be summarized as follows:

  1. the rigid relief reduces or totally eliminates pressure on the vulnerable, healed lesion site;
  2. weightbearing casting results in more even distribution of weightbearing forces throughout the remaining plantar foot;
  3. the rigid material resists deformation, thereby providing original protection for the life of the device; and
  4. the rigid material does not bottom-out, which extends longevity and requires fewer replacements.

The following precautions must be heeded to minimize complications:

  1. the orthosis must be carefully broken in, following the schedule described in the first case study, and
  2. properly fitting shoes are necessary to prevent sliding of the foot over the contoured, rigid orthosis.

Minor modifications can easily be made to the orthosis, if necessary, through grinding or spot heating and remolding. Rigid-relief orthoses should ideally be used in conjunction with a rigid rocker-soled shoe. They can also be incorporated into an AFO by extending the material proximally behind the posterior shank for those patients requiring rearfoot stability or dorsiflexion assist. The rigid- relief orthosis is only part of a total program that must include comprehensive patient education, with emphasis on careful inspection to detect problems early.

Acknowledgments

Sincere thanks are extended to Carol Langlois, Jerry Simmons and Val Coor at the Paul W. Brand Biomechanics Lab, Rehabilitation Research Department, Gillis W. Long Hansen's Disease Center, for providing artwork, photography and technical assistance.


Andrew Novick, MA, PT, is a research physical therapist at the Gillis W. Long Hansen's Disease Center, Carville, La. 70721.

James A. Birke, MS, PT, is director of physical therapy at the Gillis W. Long Hansen's Disease Center, Carville, La. 70721.

Alicia S. Hoard, OTR, is deputy director of physical therapy at the Gillis W. Long Hansen's Disease Center, Carville, La. 70721.

Denise M. Brasseaux, PT, is a staff physical therapist at the Gillis W. Long Hansen's Disease Center, Carville, La. 70721.

John B. Broussard, PT, is a staff physical therapist at the Gillis W. Long Hansen's Disease Center, Carville, La. 70721.

Elizabeth S. Hawkins, DPM, MPH, is a research podiatrist at the Gillis W. Long Hansen's Disease Center, Carville, La. 70721.

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