Use of Computed Tomography and Plantar Pressure Measurement for Management of Neuropathic Ulcers in Patients With Diabetes.

Key Words: Computed tomography, Diabetes, Neuropathy, Pressure, Spiral x-ray, Ulcer.

What are the potential benefits of spiral x-ray computed tomography and plantar pressure measurement for patients with neuropathic ulcers?

Patients with diabetes mellitus (DM) and peripheral neuropathy are at high risk for skin breakdown and subsequent lower-extremity amputation due to unnoticed repeated trauma to the plantar surface of the foot during walking.[1-3] Rehabilitation methods include using orthoses and therapeutic footwear to reduce plantar pressures and prevent skin breakdown. The indications, fabrication, and use (eg, instructions to patients) of orthotic devices and footwear vary greatly among clinicians, and patients experience a high rate (30%-57%) of ulcer recurrence.[4-7] Infected ulcers can lead to amputation.[3,8] The American Diabetes Association estimates that up to 85% of these amputations can be prevented.[9] Programs to prevent lower-extremity amputation have focused on protecting the "at-risk" foot from mechanical stresses that initiate skin breakdown.[3,10,11]

Pecoraro et al[3] found that 72% of amputations include an identifiable causal sequence of minor trauma, skin ulceration, and wound-healing failure. Many factors contribute to skin breakdown on the diabetic foot, but the primary etiology is thought to be excessive, repeated pressure on the insensitive foot. [1-3,10,12] Structural changes and deformities within the foot as a result of complications of DM, particularly peripheral neuropathy, contribute to excessive local pressures.[1,2]

Advances in technology have led to improved methods of measuring plantar pressures during walking. Several reports have described the benefits of in-shoe pressure measurement for managing patients with DM, peripheral neuropathy, and plantar ulcers.[2,13,14] A critical benefit of this technology is the ability to obtain measurements of pressure distribution on the feet of patients who are insensitive to pressure and pain.

Spiral x-ray computed tomography (SXCT) is a recent technology for investigating internal structural characteristics.[15,16] Spiral x-ray computed tomography scanners are widely available, and virtually all new computed tomography scanners incorporate spiral (also known as helical) capability. The spiral scanner is practical and advantageous[15-17] due to improved image quality, minimal x-ray dose, and relatively low cost when compared with other methods for volumetric imaging, especially magnetic resonance methods. Spiral x-ray computed tomography scanning of the extremities avoids exposure of reproductive organs to radiation and is considered a low-risk noninvasive technique.

Although SXCT and plantar pressure measurement have been described separately, we are aware of no reports of simultaneous use of the methods. Images obtained with SXCT have been shown in investigations of patients with transtibial amputation[18-22] to be geometrically accurate and highly reproducible. Spiral x-ray computed tomography provides high resolution and 3-dimensional (3-D) data of the foot, which can be used to gather geometric, tissue composition, mass properties,[23] and bone densitometry information in a single session. Because the data are digital, SXCT offers rapid acquisitions and extensive processing capabilities. The advantage of combining pressure measurement with SXCT imaging is to determine quantitatively how the internal structure of the foot is related to plantar pressures on the foot during walking.

The purposes of this case report are (1) to describe how data from plantar pressure measurement and SXCT were used to treat a patient with recurrent plantar ulcers and (2) to discuss the potential future benefits of this technology in managing patients who are at high risk for skin breakdown and subsequent amputation.

Case Description

The patient was a 62-year-old man with type 1 DM of 34 years' duration. He took 28 units of insulin (8 Regular, 20 NPH) in the morning and 30 units (12 Regular, 18 NPH) in the evening to control his blood glucose levels. He weighed 82.9 kg (185 lb) and was 1.8 m (6 ft) tall. He was self-employed as an optometrist, which he reported required him to be standing or walking 8 to 10 hours a day.

Previous Treatment and Outcomes

An orthopedic surgeon referred the patient, with a diagnosis of "diabetic plantar ulcer," to our physical therapy clinic for evaluation and treatment with total contact casting. A summary of the patient's ulcer status and treatment is shown in the Table. Initially, the plantar ulcer was located under his forefoot, measured 2 x 2 cm (1 mm deep), and showed no signs of infection (ie, no purulent drainage, redness, warmth, or edema). He had a severe hammertoe deformity of the second toe. The patient reported that the ulcer had been present "off and on" for 27 years, but the current ulcer had been open continually for at least 2 years. He reported that his second metatarsal head was resected 27 years previously in an attempt to remove the bony deformity under the ulcer and allow the wound to heal, but this procedure only increased his hammertoe deformity and associated nonhealing ulcer problems. He reported seeing a podiatrist twice a month to debride callus and trim his toenails.

The patient had absent sensation to pain and pressure distal to the midcalf level, bilaterally, as evidenced by his inability to sense a 6.10 Semmes-Weinstein monofilament.[24-25] An inability to sense a 5.07 Semmes-Weinstein monofilament (10 g) has been operationally defined as a loss of protective sensation, and an inability to sense a 6.10 Semmes-Weinstein monofilament (75 g) has been operationally defined as absent sensation.[24,25] He wore tennis shoes with a soft accommodative insert made of a combination of PPT(*) and molded pink (#1) Plastazote([dagger]) measuring approximately 1 cm thick.

The patient was treated with total contact casting during his initial visit using methods that have been described in detail previously.[26,27] The ulcer was covered with one thin layer of gauze. Stockinette was applied to the lower leg (from the knee to the toes), with 0.32-cm (1/8-in) felt pads applied to the malleoli and anterior tibia and a foam pad placed around the toes. A total contact plaster shell then was molded around the lower leg. The shell was reinforced with plaster splints, and a walking heel was attached to the plantar surface. A fiberglass roll was applied around the plaster for extra durability and to allow weight bearing sooner than would be allowed with plaster alone. The patient was provided with crutches, given a written list of precautions (ie, keep cast dry, call therapist if any problem is noticed), and instructed to limit ambulation to one third of his usual amount.

Seven days later, when the cast was changed, the wound was healed, as defined by complete skin coverage and lack of drainage. He was provided with instructions regarding foot care (eg, visually inspect skin daily) and advised to obtain extra-depth shoes, custom-made inserts, and a rigid rocker-bottom sole. This footwear has been shown to reduce forefoot pressures during walking and is often used for patients with a history of forefoot ulcer.[28] The patient, however, refused to obtain the recommended footwear, expressing a concern over the appearance of the shoes. Although he was advised to use an assistive device to help protect the foot from trauma during walking as the wound continued to mature,[2] the patient stated that he had a busy week scheduled at his clinic.

Six days after the wound healed, the patient walked into the clinic without assistive devices. His foot had an ulcer in the same location. We speculated that high pressures encountered during walking and standing caused the wound to reopen. Due to his busy work schedule, the patient chose to delay casting and to treat the ulcer using dressing changes. The ulcer did not heal, however, and the patient was referred back to the physical therapy clinic 4 months later (Table) for treatment with total contact casting. A cast was applied, and 11 days later at the scheduled cast change, the ulcer was healed. This time, the patient agreed to obtain extra-depth shoes and a custom-molded pink (#1) Plastazote and PPT insert, but he refused the rigid rocker-bottom sole, still expressing objections about the appearance of the rocker soles. The benefits of pressure distribution from the extra-depth shoe with an accommodative insert without a rocker-bottom sole is comparable to walking in athletic shoes with an insert.[29,30]

Table
Treatment and Ulcer Recurrence

Time From
Initial Referral   Status of Ulcer

Referral           2 x 2 cm, 1 mm deep
                   2 yr since onset
1 wk               Healed
2 wk               1st ulcer recurrence
18 wk              0.5 x 0.5 cm, 1 mm deep
19.5 wk            Healed
30 wk              2nd ulcer recurrence (reported)
31 wk              2 x 2 mm, 1 mm deep
1 yr 17 wk         1.5 x 1.5 cm, 0.5 cm deep
                   Open since 30 wk after referral
1 yr 19 wk         0.5 x 0.5 cm, 1 mm deep
1 yr 20 wk         0.2 x 0.2 cm, 1 mm deep
1 yr 20.5 wk       1 x 1 mm, 1 mm deep
1 yr 21 wk         2 x 2 mm, 1 mm deep
1 yr 25 wk         Healed
1 yr 41 wk         3rd ulcer recurrence (reported)
1 yr 49 wk         0.5 x 0.5 cm, 1 mm deep
2 yr 14 wk         0.5 x 0.5 cm, 1 mm deep
2 yr 16 wk         0.5 x 0.5 cm, 1 mm deep
2 yr 23 wk         Healed
2 yr 28 wk         Remained healed
2 yr 35 wk         Remained healed

Time From
Initial Referral   Primary Treatment or Measure

Referral           Total contact casting

1 wk               Patient education on foot care
                   Patient refuses to wear recommended footwear
2 wk               Patient chooses to treat wound with dressing
                     changes alone
18 wk              Total contact casting
19.5 wk            Extra-depth shoes, Plastazote insert
                   In-shoe pressure measurement
30 wk
31 wk              Footwear modifications
                   In-shoe pressure measurement
1 yr 17 wk         Total contact casting
1 yr 19 wk         Total contact casting
1 yr 20 wk         Total contact casting
1 yr 20.5 wk       Patient reported pain with cast, decides to
                     discontinue casting and try to allow wound to
                     heal using crutches
                   He agrees to wear extra-depth shoes, Plastazote/
                     PPT insert, and rocker-bottom soles
1 yr 21 wk         In-shoe pressure measurement, crutches, footwear
1 yr 25 wk         Cane, footwear, patient education, in-shoe
                     pressure measurement
1 yr 41 wk
1 yr 49 wk         New footwear, extra-depth, Plastazote/PPT
                     custom-made insert, with modifications, rigid
                     rocker-bottom sole; pressure measurement, seen
                     3 times
2 yr 14 wk         In-shoe pressure measurement (30 psi), patient
                     instructed in walking strategies (slow walking
                     and decreased push-off)
2 yr 16 wk         Experimental testing using SXCT and concurrent
                     pressure measurement, barefoot pressure
                     measurement (>100 psi); patient instructed to
                     never walk barefoot
2 yr 23 wk         Patient wears custom-made footwear always, even
                     to go to bathroom during the night
2 yr 28 wk
2 yr 35 wk

Due to his history of recurrent ulceration, an in-shoe pressure test (F-Scan In-shoe Pressure System([double dagger])) was conducted to determine whether the new footwear reduced plantar pressures at the ulcer site compared with his old shoes. Methods of calibration and testing have been described elsewhere, and measurements obtained with these methods have been shown to be reliable.[31] The sensor was cut to fit inside the shoe and was placed between his sock and the shoe insole. Data were collected at 50 Hz as the patient walked across the room at his normal, self-selected speed.

Plantar pressures were highest at the location of his ulcer and measured approximately 311 kPa, equivalent to 45 psi, while wearing his tennis shoes and old insert. Although good data on in-shoe plantar pressures to identify a threshold of injury are not available, we attempt to keep forefoot pressures evenly distributed, with a peak pressure below 207 kPa (30 psi), in our practice. After several modifications to his new insole, peak pressures were reduced to 207 kPa. The modifications were the addition of a larger metatarsal pad that extended across metatarsal shafts 2 to 5, beginning in the midfoot and continuing immediately proximal to the metatarsal heads. The patient was instructed to use crutches with approximately 30% weight bearing on the right lower extremity for the next 2 weeks and then to resume his normal activity level slowly.

The ulcer recurred a second time 14 weeks later (Table). The patient was seen 1 week later with a referral from his physician to evaluate his footwear for proper fit. Visual inspection indicated the pink Plastazote in his insoles was worn and "bottomed out," meaning that the Plastazote had become compressed, thin, and nonaccommodating in regions of high plantar pressure. The in-shoe pressure system indicated a peak plantar pressure during walking of 332 kPa (48 psi) at the ulcer site. The high pressures appeared to be due to his worn orthoses and shoes. Another layer of pink Plastazote (0.64 cm [1/4 in]) was added to the insole, and the thickness of the metatarsal pad was increased approximately 0.32 cm (1/8 in). A repeated pressure measurement indicated a peak pressure of 262 kPa (38 psi) at the ulcer site. We applied a temporary rigid rocker-bottom sole (20 [degrees] angle) to the patient's shoes, with the apex of the rocker immediately proximal to the metatarsal heads. Repeated testing indicated pressures were reduced to 207 kPa. The patient agreed to wear the rigid rocker-bottom soles in his house, but not in the community.

The patient was not seen again for physical therapy until 9 months later. The ulcer had increased in size (1.5 x 1.5 cm, 0.5 cm deep). A total contact cast was applied and changed twice, with progressive healing (Table). Four weeks later, the patient called to report "a vague pain" at the anterior mid tibia, and the cast was removed that day. Redness was observed at the anterior mid tibia, but there was no skin breakdown or drainage in this area. The ulcer size was unchanged, and the patient chose to discontinue use of the cast.

The patient was seen 1 and 2 weeks later to monitor his progress and to suggest alternative treatments to total contact casting, with the goal of lowering plantar pressures, protecting the wound site, and allowing the wound to heal. In-shoe pressure measurement was conducted and showed good distribution of pressures over the forefoot ([is less than] 207 kPa) while the patient walked without an assistive device in his extra-depth shoes with Plastazote/PPT inserts and rocker-bottom soles. He also was instructed to use crutches and allow only partial weight bearing (about 30%) on the right foot.

The patient was seen in the clinic 2 weeks later, and the ulcer was healed. Although the patient reported wearing his shoes and using crutches "all the time," wearing time when getting up during the night to go to the bathroom was not addressed specifically. In-shoe pressure analysis revealed good pressure distribution (ie, no sharp peaks at the second metatarsal head and pressures less than 210 kPa) during walking. He was instructed to continue to use a cane for the next 2 weeks and to increase his activity gradually.

The patient was not seen again for 5 months, when he was again referred by his orthopedist for evaluation of his footwear using the in-shoe pressure measurement. He reported that the ulcer had recurred for the third time about 2 months previously. He reported concerns about the fit of his shoes and requested help in identifying anything else that could be done to help heal the ulcer. Visual inspection of his shoes showed the heel counter was soft and distorted into a varus position. The lateral posterior aspect of the sole was worn away in a wedge shape by about 1.5 cm. The in-shoe pressure measurement revealed a moderately high peak of pressure (~242 kPa [35 psi]) at the ulcer site.

Because the old shoes were worn and he showed high pressures at the forefoot, new shoes of the same design were prescribed. Two weeks later, he wore his new shoes and insoles to the clinic and repeated in-shoe pressure testing. After several minor modifications to the metatarsal pad as described before, the peak plantar pressure was 186 kPa (27 psi) at the ulcer site. He was seen a final time 3 months later, and the ulcer showed no change in size. The patient was instructed to walk slower and decrease his push-off during walking. Such gait changes have been shown to reduce forefoot pressures.[32] Whether the patient was able to permanently adjust his walking pattern is not known.

At his final clinic visit, the patient was discharged from physical therapy because there did not appear to be other treatment options to heal the ulcer. His orthopedic surgeon had suggested additional surgery to correct the severe foot deformity (ie, prominent metatarsal and hammertoe deformity), but because of the poor outcome of his previous foot surgery, the patient refused additional surgery. Total contact casting was considered, but the patient and the therapist believed that although the ulcer might heal inside the cast, it would recur when he resumed his normal activity schedule. Walking with crutches also was considered, but the patient did not want to use crutches permanently. We speculated some sort of severe structural problem was related to the recurrent ulcer, but we had exhausted all possible traditional evaluation and treatment techniques that we were aware of.

SXCT and Concurrent Plantar Pressure Measurement

Two weeks later, the patient signed a consent form approved by the internal human subjects committee to participate in an experimental protocol combining SXCT and plantar pressure measurement to assess his foot deformities further. We speculated that the combination of SXCT and concurrent pressure measurement in a weight-bearing position might provide insight as to the mechanism of his recurrent skin breakdown.

Plantar pressure data were collected during barefoot walking and during SXGT data acquisition using the F-Scan In-shoe Pressure System (software version 3.847).([double dagger]) The foot was loaded during SXGT data acquisition to simulate peak plantar pressures during walking. The methods had been developed and tested on a volunteer who did not have diabetes or a foot deformity.[33] Three lead marker's were placed on the forefoot portion of the sensor to enable precise localization and identification of a sensor on the plantar surface of the foot during SXCT. The sensor was attached to the patient's foot with tape, and a thin sock was placed over the foot to secure the sensor. The F-Scan sensor was calibrated using the patient's body weight according to manufacturer recommendations and previous research.[31] After allowing the patient to adjust to the hardware during walking, data during 3 walking trials were collected. Location and surface area of peak plantar pressure were recorded and printed to help train the patient during the next set of simulated loading conditions.

After acquisition of data during walking, the patient was positioned on the loading device to practice loading his forefoot (Fig. 1a). The patient was seated with his hips flexed to about 90 degrees, the knees flexed to about 40 degrees, and the ankles in slight plantar flexion (10 [degrees]). Only the forefoot was in contact with a digital scale placed on the base of the loading device (Fig. 1a). The patient practiced pushing against the loading device with 25% to 75% of his body weight. The amount of weight bearing was confirmed using the digital scale and the F-Scan system located on the base of the loading device. We determined whether peak plantar pressures on the forefoot during walking corresponded to the peak plantar pressure during simulated loading of the forefoot by assessing peak plantar pressure location, magnitude, and contact area.

[Figure 1a ILLUSTRATION OMITTED]

After the patient practiced placing his forefoot on the loading device, pressure data and SXCT data were collected simultaneously (Fig. 1b). The patient was asked to apply a load equivalent to 50% of his body weight, which was monitored with the F-Scan sensor and the digital scale on the loading device.

[Figure 1b ILLUSTRATION OMITTED]

The SXCT raw data and images were archived on an optical disk. The archived data and images were copied to a Siemens satellite computed tomography console([sections]) and reconstructed at 1 mm resolution. The SXCT image data were imported into the ANALYZE software program([parallel]) operating on an interactive workstation for resizing, segmenting, and measuring the 3-D data.

For the purposes of this case report, we focus on a parsimonious number of variables taken from the SXCT data that were most relevant to the patient's problem. Hammertoe deformity was defined as the angle between the metatarsal and the proximal phalanx. Soft tissue under the midshaft of the metatarsal was used as an indication of the intrinsic muscle size. The length of each metatarsal was measured, and the midpoint was calculated. The amount of soft tissue, relative to the plantar skin surface, was measured orthogonal to the metatarsal shaft. Soft tissue thickness also was measured orthogonal to the loading surface under the distal second metatarsal.

To provide a frame of reference, the data obtained from the patient were compared with data obtained from a 40-year-old subject matched for sex, height (1.8 m [6 ft]), weight (80.6 kg [185 1b]), and shoe size (USA size 11). Data collection methods were identical to those used for the subject without impairment, who also signed a consent form.

Outcomes of SXCT and Pressure Testing

Figure 2 shows the peak plantar pressures measured during barefoot walking for a complete stance phase. The peak plantar pressure was 886 kPa (128 psi) under the second metatarsal head, the location of his recurrent ulcer. These values were approximately 4 times higher than those values obtained while the patient walked in therapeutic shoes.

[Figure 2 ILLUSTRATION OMITTED]

Data in Figure 3 verify that the plantar pressures during simulated loading at 50% of body weight, which were collected simultaneously with SXCT data (Fig. 3a), were similar to the plantar pressures during the push-off phase of walking (80% of stance, Fig. 3b). During walking, the contact area was 25 [cm.sup.2] and the peak plantar pressure was 886 kPa at the second metatarsal head. During simulated loading with 50% body weight, the contact area was 27 [cm.sup.2] and the peak plantar pressure was 753 kPa at the second metatarsal head. Figures 3c and 3d show the pressure data superimposed over the SXCT image data and confirm that the greatest pressures occurred at the distal second metatarsal and the ulcer location.

[Figure 3 ILLUSTRATION OMITTED]

Figure 4 shows the SXCT images of an oblique slice of the right forefoot along the midshaft of the second metatarsal for the patient (Fig. 4a) and the comparison subject (Fig. 4b) using similar methods. Obvious differences between the 2 feet included the patient with DM showing a hammertoe deformity (45 [degrees] versus 25 [degrees]), decreased contact area under the toes and metatarsal heads (27 versus 49 [cm.sup.2]), decreased thickness of intrinsic muscles and soft tissue under the midshaft of the metatarsal (14 versus 28 mm), and resection of the second metatarsal head. The soft tissue thickness under the distal second metatarsal was slightly greater for the patient with DM than

for the comparison subject (7.5 versus 5.5 mm).

[Figure 4 ILLUSTRATION OMITTED]

Implications for Treatment of This Patient

The SXCT and pressure data highlighted the magnitude of the patient's foot deformities. We thought that the most important finding was the extremely high plantar pressure (886 kPa [128 psi]) under the distal second metatarsal during barefoot walking, which corresponded with the location of the recurrent ulcer. The data from the SXCT illustrate the structural deformities that may have contributed to this extremely high pressure. A striking structural difference in the patient's foot compared with the comparison subject's foot was the loss of intrinsic muscle size, as evidenced by the decreased soft tissue thickness under the midshaft of the second metatarsal (14 versus 28 mm; Fig. 4). Because the density of muscle is different from the density of fat and bone, SXCT data can be used to identify muscle tissue easily. The severe atrophy of the intrinsic muscles in the patient's foot likely is a result of the patient's peripheral neuropathy. This atrophy appeared to contribute to the patient's instability at the metatarsophalangeal joint (MTPJ) and severe hammertoe deformity (Fig. 4). This lack of MTPJ stability and the hammertoe deformity appeared to result in the decreased contact area under the toes and metatarsal head (Fig. 4). We did not give the patient exercises to increase mass or performance of the intrinsic muscles of the foot because he had only a few degrees of active motion at the MTPJs and we believed that his ability to contract these muscles was limited by peripheral neuropathy, a condition that is not reversible with exercise. More research is needed, however, to determine whether any of the weakness associated with DM and peripheral neuropathy in the lower extremities, especially the foot and ankle, is reversible with exercise.

The SXCT images also illustrated the resected second metatarsal head and confirmed that the greatest pressures were localized at this location (Figs. 3 and 4). Although other authors[33] have reported decreased soft tissue thickness under the metatarsal heads in patients with DM and peripheral neuropathy, we did not observe decreased soft tissue thickness under the second metatarsal head in this patient compared with the foot of the comparison subject. The lack of difference in this patient may be due to the resection of the metatarsal head.

Repeated loading on the plantar surface of the foot during walking with this magnitude of pressure (886 kPa) likely would cause extreme pain to an individual with intact sensation, but this patient with peripheral sensory neuropathy was able to walk painlessly. The pressure of 886 kPa was reduced to 207 kPa when the patient wore his therapeutic footwear (described previously). When asked whether he ever walked barefoot at home, the patient said that he awoke 3 or 4 times a night and walked barefoot to the bathroom, a distance of about 12 m (40 ft).

We speculated that the extremely high plantar pressure while walking barefoot to the bathroom contributed to the patient's skin breakdown. Although Brand[2] described a moderately high pressure (275 kPa) repeated hundreds or thousands of times during walking as the primary mechanism of injury, an extremely high pressure (886 kPa) may contribute to skin breakdown with fewer repetitions, for instance, walking 12 m to the bathroom 3 or 4 times a night. We instructed the patient to always wear his therapeutic shoes, even when he awoke during the night to go to the bathroom. We showed the patient the results of the barefoot plantar pressure measurement and the SXCT, and he was shown how the pressures were reduced when wearing his therapeutic shoes.

We also discussed with the patient how his current orthoses might be altered, given the pressure measurement and SXCT findings. The primary suggestion was to change his orthoses to increase the size of the metatarsal pad, especially under the second metatarsal. We speculated that a larger metatarsal pad would help to further stabilize the second metatarsal and shift weight bearing away from the area of the distal metatarsal. Because we believed the primary problem contributing to the patient's ulcer was his barefoot walking at night, we delayed changing his orthoses until we could evaluate the effect of wearing shoes at all times. No other change in treatment was initiated at that time.

The patient was seen 7 weeks later, and the ulcer was healed. He reported that he always wore his therapeutic shoes, even when going to the bathroom during the night. He ambulated without an assistive device and reported that he was "busier than ever" at work. The patient was seen 2 months later, and the ulcer remained healed.

Discussion

This patient's ulcer healed when pressures were reduced at the ulcer site, either using total contact casting or with consistent use of crutches or footwear. Total contact casting has been shown in a controlled clinical trial[27] to be effective in healing neuropathic ulcers, and it has been shown in other research to reduce pressures at the forefoot? We believe that this patient's recurrent ulcer healed because he wore his therapeutic shoes whenever he was weight bearing, even when going to the bathroom during the night.

Although we typically tell patients to always wear their shoes, adherence to this guideline can be difficult because patients lack sensory feedback.[2] In addition, practitioners may not specify that shoes should be worn even if getting up during the night. The results of the SXCT and pressure testing provided the visible "proof" to this patient that he had extremely high plantar pressures that were related to his severe foot deformity. Although he was unable to sense these high pressures, the pressure and SXCT data allowed him to see the high pressures related to his foot deformity and ulcer when walking barefoot and how these pressures could be reduced by wearing his therapeutic shoes. Although we have not previously measured plantar pressures during barefoot walking because we usually want to measure pressures when patients wear footwear, perhaps measurements should be taken during barefoot walking to better demonstrate to patients the contribution of plantar pressures to ulceration and how therapeutic footwear reduces these high pressures. Research is needed to determine optimal methods for patient education to compensate for sensory neuropathy.

A number of questions and limitations arise regarding the evaluation and treatment for this patient. It is possible that the ulcer will reoccur.[4-7] He continues to have the underlying risk factors of severe foot deformity and peripheral neuropathy. The wound, however, has been closed for greater than 3 months, and even this amount of time is important in terms of preventing infection and possible subsequent amputation.

Spiral x-ray computed tomography and pressure testing are not part of standard care for patients with neuropathic ulcers. We have used in-shoe pressure testing for several years to help optimize the pressure distribution of orthoses for patients with DM and peripheral neuropathy as described in this case report and as described elsewhere.[13,14] A threshold of injury is unknown and likely is different from patient to patient, depending on a number of variables. Although we believe that in-shoe pressure testing is helpful to quantify pressure distribution, research is needed to define guidelines for use and to determine whether this technology can help to prevent skin breakdown.

We are aware of no reports using simultaneous SXCT and pressure testing. These evaluations were part of ongoing research to determine whether these procedures are useful to help heal or prevent plantar ulcers and prevent lower-extremity amputation. The combined results of the SXCT and pressure testing seemed to help convince this patient of the need to wear his shoes at all times. We do not know whether barefoot testing alone would have had the same results in this patient. We know that the SXCT data helped to confirm the location of high pressures to the location of the ulcer and to clearly identify this patient's structural deformities. Insights for orthosis modification were generated but were not pursued because the ulcer healed. We speculate that this testing may be helpful in the management of patients with recurrent plantar ulcers who are at high risk for infection and amputation,[3,5] but additional research is needed. The results of this report further document the need to be aware of the mechanical factors that can contribute to skin breakdown.[1-3,10,12-14] We believe that clinicians should question patients and assess footwear carefully for any factors that may contribute to trauma at the ulcer site, even if pressure testing and SXCT are not available.

Cost is another consideration for using the approach and the procedures that we chose. Cost is variable depending on the institution, the type of scan, and the patient's insurance carrier. Currently, the charge tot a lower-extremity computed tomography scan at our institution is $669. We contend that this cost should be considered in comparison with the total cost of care for diabetic foot ulcers. One Swedish study showed the average costs of treating 274 patients with foot ulcers and DM over a 3-year period to be $16,100 if primary healing of the nonischemic wound occurred and $63,100 if a lower-limb amputation was required.[11] Costs for a computed tomography scan could be justified, in our opinion, if the treatment can help to heal a plantar ulcer and prevent infection or an amputation. Additional research is needed to determine whether computed tomography and pressure testing are beneficial and cost-effective in the management of neuropathic foot ulcers.

Potential Benefit of Procedures

We believe that there are a number of other potential benefits of using the methods described in this report. First, these methods could be used in research to measure structural changes that occur in the foot as a result of the complications of diabetes and peripheral neuropathy and to determine how these changes are related to plantar pressures during loading. The vast amounts of data provide an almost limitless number of variables to study. Variables that can be quantified and that appear to be important from this patient include hammertoe deformity, contact area under the metatarsal heads, intrinsic muscle size, and soft tissue thickness under bony deformities (Fig. 4). We chose to scan the patient's foot in a weight-bearing position that simulates the push-off phase of walking because most neuropathic ulcers occur under the metatarsal heads[2,12,27] and appear to be a result of the high pressures that occur during the push-off phase of walking. This patient showed the highest forefoot pressures at 80% of stance, which is consistent with our other pilot work and as reported by others.[2] A better understanding of the relationship between structure and plantar pressures may have direct implications for orthosis fabrication to distribute pressures and prevent skin breakdown.

Another possible benefit of these methods is that they could lead to improved computer models of the foot to help design and fabricate orthoses. The computer model would require detailed, 3-D geometric knowledge of the soft tissue envelope surface location and the underlying skeletal framework. These 3-D geometric data, in addition to the load distribution and material properties, determine the transfer characteristics. Given 3-D geometric description of tissue displacements using SXCT, material properties reported in the literature and determined experimentally, and applied loading forces, the associated pressure distributions can be estimated using finite element analysis. Quantitative measurements relevant to orthosis design obtained using SXCT imaging, plantar pressure measurements, and finite element analysis tools can be modeled to determine which of the measured variables are most important to predictive orthosis evaluation. Contact areas between plantar soft tissues and the shoe or ground, stress, strain, and pressure distributions can be examined. Interventions, including use of orthoses or surgery, could be tested on the model before validation using patient populations. Although work has been reported using finite element analysis and a 2-dimensional model,[35] we are aware of no reports using SXCT and 3-D imaging. Our initial work has indicated that it is possible to generate comprehensive 3-D geometric models that include bone, muscle, and fat from the SXCT data for use in finite element modeling and analysis (Fig. 5). More research is needed to develop these methods and implications.

[Figure 5 ILLUSTRATION OMITTED]

(*) Professional Protective Technology, 21 E Industry Ct, Deer Park, NY 11729.

([dagger]) Bakelite Xylonite Ltd, London, England, distributed by Alimed Inc, 297 High St, Dedham. MA 02026.

([double dagger]) Tekscan Inc, 307 W First St, South Boston, MA 02127.

([sections]) Siemens Medical Systems Inc. 186 Wood Ave S, Iselin, NJ 08830-2770.

([parallel]) Mayo Foundation, Biomedical Imaging Resources, Rochester, MN 55905.

References

[1] Boulton AJM, Betts RP, Franks CI, et al. Abnormalities of foot pressure in early diabetic neuropathy. Diabet Med. 1987;4:225-228.

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MJ Mueller, PhD, PT, is Assistant Professor, Program of Physical Therapy, Washington University School of Medicine, Box 8502, 4444 Forest Park Blvd, St Louis, MO 63108 (USA) ([e-mail]). Address all correspondence to Dr Mueller.

KE Smith, AAS, is Engineer, Mallinckrodt Institute of Radiology, Washington University School of Medicine.

PK Commean, BEE, is Engineer, Mallinckrodt Institute of Radiology, Washington University School of Medicine.

DD Robertson, MD, PhD, is Assistant Professor, Mallinckrodt Institute of Radiology, Washington University School of Medicine.

JE Johnson, MD, is Associate Professor, Department of Orthopaedic Surgery, Washington University School of Medicine.

Concept, research design, writing, data collection and analysis, and facilities and equipment were provided by Mueller, Smith, Commean, and Robertson; project management, fund procurement, and institutional liaisons, by Mueller and Robertson; and subject recruitment, by Mueller.

Subjects signed a consent form approved by the Washington University School of Medicine Human Subjects Committee.

This article was submitted March 10, 1998, and was accepted September 3, 1998.