September 2016

Clinical use of plantar pressure measurement

9foot-pressure_v2Originally conceived as research tools, plantar pressure measurement systems are becoming less intimidating, less expensive, and more popular with lower extremity practitioners for treatment planning, patient education, and documenting the effects of therapeutic interventions.

By Shalmali Pal

As the longtime team podiatrist for the Washington Wizards of the National Basketball Association, Howard Osterman, DPM, has experienced both the benefits and the limitations of pressure measurement technology.

One big plus, he has found, is the ability to show patients graphically the effectiveness of an intervention. Players don’t always take to the orthoses he prescribes to help reduce stress on their feet during practice and play; their main complaint is that the devices are too hard.

“They want a softer insole, but that doesn’t really help with controlling their [foot and ankle] motion,” explained Osterman, a partner at Foot and Ankle Specialists of the Mid-Atlantic in Washington, DC.

But taking pressure measurements with and without the orthoses, and then showing those data to the athletes, can help them see the light.

“They can then visually see what’s going on with the stressors, and how we are working to modify those with the orthotics,” he said.

On the other hand, having pressure data on hand isn’t a guarantee of a satisfactory clinical outcome. Case in point: One of the team’s players has suffered from repetitive stress fractures of the fibula, despite the fact that there’s no “true injury” on the affected side, Osterman said. Plantar pressure measurements have been done as part of the workup to see what’s causing these fractures.

Plantar pressure measurement assesses vertical force, just one of many clinically relevant force variables, but practitioners say the data can still be valuable.

“Despite all of the data we’ve gathered using pressure measurement technology and other analytics, we haven’t been able to determine what the underlying cause [of the stress fractures] is, and he’s gone through four seasons [as a player],” Osterman noted. “So we’ve got the data, yet it’s been limited in helping us with this situation.”

These are just some of the issues that can affect the utility of pressure measurement data in a clinical, nonacademic, nonresearch setting. LER: Foot Health took a closer look at the reality of incorporating pressure analysis into everyday clinical practice.

Studies, strengths, and shortcomings

Research on pressure measurements has highlighted both the pros and cons of the technology. A 2008 study1 by Georgeanne Botek, DPM, on staff at the Cleveland Clinics Orthopaedic Research Center in Ohio, and colleagues evaluated whether custom insoles, “tailored to contours of the barefoot pressure distribution and shape of a [diabetic] patient’s foot,” would reduce plantar pressures in the metatarsal head region better than conventional custom insoles (see “Diabetes: Plantar pressures guide offloading efforts,” LER September 2009, page 25).

They reported that the “shape-plus-pressure-based insole” outperformed versus shape-based insoles for offloading pressure from high-risk areas of the forefoot to the midfoot. However, Botek’s group cautioned that the study could not assess whether the improved offloading would necessarily reduce the chance of ulceration, or reulceration, in patients with diabetes.1

A 2014 study2 took a closer look at the efficacy of in-shoe orthoses, designed based on shape and barefoot plantar pressure, for reducing the incidence of submetatarsal head plantar ulcers in patients with diabetes, peripheral neuropathy, and a history of prior ulceration.

The authors, led by Peter Cavanagh, PhD, DSc, Endowed Chair in Women’s Sports Medicine at the University of Washington in Seattle, reported a trend toward statistical significance in favor of the experimental orthoses (compared with standard-of-care orthoses) for the composite primary endpoint–ulcers and nonulcerative lesions–during a 15-month follow-up period.

The hazard ratio was 3.4 for the occurrence of a submetatarsal head plantar ulcer in the control arm versus the experimental arm over the duration of the study. However, although the experimental orthoses were better than the control orthoses for reducing submetatarsal head plantar ulcer recurrence, there was no difference between the devices for incidence of nonulcerative lesions—possibly because sites that ulcerated in patients wearing the control insoles might have resulted in nonulcerative lesions in patients wearing the experimental devices, the authors hypothesized.

In a 2010 study, researchers in England articulated one of the most often cited limitations of pressure measurement systems—they are able to assess vertical force, but not mediolateral shear forces.3

And that limitation may make it difficult to translate pressure data into a clinically relevant finding, said Christopher F. Hovorka, PhD, MS, CPO, LPO, clinical director of the Master of Science program in Prosthetics and Orthotics at the Georgia Institute of Technology in Atlanta.

“Pressure is just a normal or vertical force that’s being indirectly characterized,” Hovorka explained. “I’m using the term ‘characterized’ rather than ‘quantified’ to make a point. A lot of the pressure measurement technology that’s available shows … a pattern or an area of force with a user-friendly, graphical, color-coded interface or map. But in its empirical sense, it’s a tool to characterize a force that is normal to the foot and the ground. I want to make that distinction, because what a lot of foot disorders present as, is not just due to the normal force.”

Hovorka gave as an example the pressure ulcer, a term that he said implies the ulcer was generated by normal or vertical force. But the cause of superficial or intermediate level ulcers is more likely to be tangential (mediolateral) force in addition to or instead of normal force, and that’s where Hovorka said he can see limitations of using pressure analysis alone to predict or prevent ulcers.

Plantar pressure measurement can be helpful for making pre- vs post-treatment comparisons, particularly when it is paired with other function analysis technologies.

For David J. Levine, DPM, CPed, a podiatrist with Foot and Ankle Specialists of the Mid-Atlantic in Frederick, MD, the fact that mediolateral shear forces aren’t within the purview of pressure measurement analysis isn’t necessarily a huge drawback.

“There are mechanical issues in the foot that may lead to ulcers that aren’t picked up by pressure mapping, but I think pressure analysis works well for what it is,” Levine said. “There’s always something clinically that you think you know, but it helps to see the data to confirm that. But you are always going to have to rely on your clinical expertise given the limitations of pressure analysis.”

Clinical benefits remain

Despite its limitations, the experts said they still see clinical value in pressure measurement. For Hovorka, this technology can be helpful for pre- and post-treatment comparisons, particularly if it’s paired with other assessment tools—videography, for example.

For instance, gait analysis with videography will offer a kinematic profile of the patient as he moves through the gait cycle, Hovorka explained. The movement analysis data can be reviewed along with the pattern of force from the pressure measurement map.

“Now you are adding other outputs into the mix to provide a suite of data. These movement trends can give you a better idea as to whether the intervention has been effective,” he said.

Levine offered an example of a patient with a diabetic ulcer that he recently treated. The patient underwent pressure analysis while wearing a cast boot, a surgical shoe, and a regular shoe.

“We thought we were effectively offloading the ulcerated area with the cast boot; that seemed the most effective way to do that. But the pressure analysis indicated that wasn’t the case,” he said, adding that the treatment modality selected ended up being a commercially available running shoe, modified with rocker soles and a plug of softer material directly under the ulcer.

It is patients with diabetes who can gain the most from data generated by plantar pressure analysis, experts agree. Levine pointed out that that patients with diabetes and neuropathy aren’t always able to provide accurate feedback regarding the fit or feel of an orthosis or other intervention, so a pressure analysis can do that on their behalf.

Osterman suggested data from pressure analysis may help determine if patients, with or without diabetes, who have structural foot deformities will benefit from preventive surgery.

A study done in Israel used plantar pressure analysis to see if the pressure distribution normalized in patients with symptomatic tarsal coalition who underwent resection of a symptomatic tarsal coalition. They found that normal plantar pressures during running were not recreated after surgery, but that normal walking pressure distribution was consistent with a favorable clinical outcome.4

In patients with diabetes, motor neuropathy may foster deformities, such as hammertoes and tarsal coalition, that become areas of high pressure and could become preulcerative sites, when combined with limited joint mobility5,6 (see “Deformity and diabetes dictate surgical strategy,” LER, February 2011, page 31).

“The premise is that we can address conditions before they lead to an ulcer or abscess, and that goes for anything from a bunion to a hammer toe to a collapse arch,” Osterman said. “We may look at a patient and say, ‘Your diabetes is under control and your neuropathy is under control, but here is a peak area of pressure. Can we address this surgically now?’”

Hovorka cautioned against putting too much faith in pressure analysis data for clinical decisions, given its limitations.

“I think that people may assume that pressure measurement technology can examine the entire stance phase of gait,” he said. “They forget about the dynamic of tangential forces. If the goal is to use it for diagnosis…it should be used for output in that vertical force. If a person has a pathology caused by a condition other than in the freestanding phase of gait, you increase the potential for error.”

The nondiabetic foot

Levine cited leg length discrepancy as a condition for which pressure analysis may show more information than meets the eye.

“We want the feet to function as symmetrically as possible, of course,” Levine said. “If someone has a leg length discrepancy, you may be able to pick up more subtle data on that discrepancy from the pressure analysis than you would simply by doing the clinical exam. You can then use those data to design a heel lift or some other device to address the issue.”

Osterman suggested pressure analysis might be particularly well suited for assessing the arches of the feet, particularly in patients with chronic plantar fasciitis.

“Take a high-arched foot that’s rigid … during stance as well as the entire gait process. The impact [during stance or gait] causes straight, direct force and not a shear force. That’s hard to visualize when someone is wearing a shoe and walking down the hall, but it’s something you should be able to quantify with a pressure analysis,” he said.

Plantar pressure analysis data could be useful in patients with Charcot-Marie Tooth disease, Osterman said.

“These patients get atrophy of the musculature, so the bones really do take a lot of stress,” he said. “Having data on the vertical force can help determine the design of an insole or the best type of shoe that’s going to help manage those stressors.”

Hovorka said the management of any conditions where there is limited compliant tissue next to the affected joint may benefit from pressure data.

“In rheumatoid arthritis, the fat pads—or the skin and other soft tissues that lie just underneath or plantar to the joints of the foot—if those soft tissues have shifted, are atrophied, or are more rigid than a typical healthy foot, then you are going to run into a problem where any kind of force, vertical or tangential, can do damage to the soft tissues structures, joints, and bones,” he explained.

Obesity is another disease state where compliant tissue may be overloaded to the point where the structures of the foot-ankle complex can’t adapt, he noted. Pressure analysis could help clinicians characterize the location, distribution, and magnitude of force in relation to anatomical structures (ie, where the fat pads that should protect the foot may be shifted or even torn from their
anchoring points), and use the pressure distribution data in conjunction with clinical insights to formulate a treatment plan.

“That’s where the pressure measurement technology may be helpful, because you can combine the data output from pressure measurement technology with clinical observations, such as wasting of the compliant tissue and the onset of rigidity of the joint structures that may be in response to excessive stress,” he said. “As the patient is standing, the pressure analysis can record the pattern and magnitude of the perpendicular force distribution in the foot, which may provide additional insights for the clinician.”

Weighing the investment

Even when a technology offers clinical benefits, there’s one significant question that needs to be answered—is it worth the time and money? On balance, yes, but there are caveats, the experts noted.

First, patients need to be sold on the out-of-pocket investment in pressure measurement analysis because it’s not a service that is typically reimbursed by the Centers for Medicare and Medicaid Services (CMS), or by private insurance, said Levine, who has worked with pressure measurement technology off and on since 1995.

“And they have to understand that it’s just a test—like when they see the cardiologist and get an electrocardiogram (EKG)—it’s not a cure for anything,” he stressed.

Although a patient may be willing to pay for a pressure study up front, once she’s been diagnosed and begun treatment, there is often a reluctance to pay again for follow-up pressure studies, Levine warned.

“The patient just wants to get better; they aren’t necessarily interested in the results at follow-up to see why there’s been an improvement,” he said. “As a result, I don’t do a lot of postanalysis because, after the initial pressure analysis and treatment, patients most often see improvement, and that’s enough for them.”

But, looking at the bigger picture, building a database of pre- and post-treatment assessment will be key to getting payers to take plantar pressure analysis seriously as evidence-based medicine, Osterman noted.

“The advantage of reimbursement is that more people who could benefit from a test are likely to undergo it,” he explained. “Now, chances are that any reimbursement of pressure analysis would be at a fairly low rate, so you’d have to make it up in volume, but if you are going to use this technology for pre-assessment, then you’ll need to be able to do post-assessment as well to show the outcomes.”

Then there is the issue of time, a commodity clinicians are generally short on. Levine explained that, in his gait lab, he has a technician who does the pressure analysis, but Levine himself then sifts through the data.

“I think the companies have done their best to make the data easy to interpret, but it still takes time,” he said. “It’s a very interesting tool to use, and I always learn something new when I use it. But then it also generates more data than I really need because I’m a clinician, not a researcher.”

Taking the time to first learn to interpret pressure data correctly, and then documenting pressure changes, may be worthwhile, Hovorka said, if it ultimately means something to other parties involved in the care process—the referring physician, the insurance carrier, and, of course, the patient.

Shalmali Pal is a freelance writer based in Tucson, AZ.

REFERENCES
  1. Owings TM, Woerner JL, Frampton JD, et al. Custom therapeutic insoles based on both foot shape and plantar pressure measurement provide enhanced pressure relief. Diabetes Care 2008;31(5):839-844.
  2. Ulbrecht JS, Mauger DT, Hurley T, et al. Prevention of recurrent foot ulcers with plantar pressure-based in-shoe orthoses: The CareFUL Prevention Multicenter Randomized Controlled Trial. Diabetes Care 2014;37(7):1982-1989.
  3. Spooner SK, Smith DK, Kirby KA. In-shoe pressure measurement and foot orthosis research: a giant leap forward or a step too far? J Am Podiatr Med Assoc 2010;100(6):518-529.
  4. Hetsroni I, Ayalon M, Mann G. Walking and running plantar pressure analysis before and after resection of tarsal coalition. Foot Ankle Int 2007;28(5):575-580.
  5. Boulton AJ, Kirsner RS, Vileikyte L. Neuropathic diabetic foot ulcers. N Engl J Med 2004;351(1):48-55.
  6. Ormeci T, Mahir Mahirogullar M, Aysalc F. Tarsal tunnel syndrome masked by painful diabetic polyneuropathy. Int J Surg Case Rep 2015;15:103-106.

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