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Does high plantar tissue stress delay ulcer healing?

A new study published in Clinical Biomechanics from DFA’s Dr Jaap van Netten’s team has found that cumulative plantar tissue stress is lower in people whose diabetic foot ulcer (DFU) healed compared to those that didn’t heal. But, what is cumulative plantar tissue stress and is that the full story?

This new study used data from a randomized controlled trial (RCT) we summarized last year from global offloading guru Dr Sicco Bus’s team. They found 3 very different offloading devices produced the same DFU healing rates. When they dug deeper, they found each device produced different plantar pressure, daily activity and adherence results; but when taken together they seemed to balance each other out.

They suggested that perhaps this was the reason for all devices giving the same DFU healing rates and recommended future research should look at these measures in combination using a concept called “cumulative plantar tissue stress”. And now with their RCT data, they have taken that step.

What do we know about cumulative plantar tissue stress?

Cumulative plantar tissue stress is not new in diabetes. It was first studied 15 years ago in an award-winning study by Dr Mike Mueller’s team in the USA. They proposed “cumulative plantar tissue stress” as a way to measure the overall weight-bearing stress on the foot and suggested it could be calculated by taking the average plantar pressure of each step multiplied by the steps walked.

They found that people with a DFU history had much lower cumulative plantar tissue stress compared to people without a DFU history. This was opposite to what they expected. They suggested it may be because plantar tissue in those with a DFU history is of poorer quality and requires less cumulative plantar tissue stress to cause a DFU. But, they did measure cumulative plantar tissue stress in the main shoes worn by patients which also may have affected findings; this was typically custom-made shoes to reduce plantar pressure in those with DFU history and off-the-shelf shoes in those without a history.

Since then though, no work has used cumulative plantar tissue stress in people with diabetes. This may be because measuring the factors that make up cumulative plantar tissue stress has historically been impractical or very expensive. But, that was until this new study from Dr van Netten’s team.

What did this new study do then?

In short, they analysed specific data collected as part of the previous RCT. The RCT recruited 60 patients with non-infected neuropathic DFUs and randomized them to wear 3 different removable offloading devices: knee-high cast, cast shoe and prefabricated shoe. In every second patient after 2 weeks, they measured plantar pressure in the device and average daily steps over 2 weeks. Adherence was reported by patients every 2 weeks. Patients were followed to see if their DFU healed after 4 and 12 weeks.

The data for this new study was taken from the 31 patients that had all plantar pressure, daily steps and adherence measures performed, and thus they could calculate their cumulative plantar tissue stress.

So what did they find?

In short, after 12 weeks they found that 21 of these patient’s DFU healed and 10 didn’t heal. The sociodemographic, diabetes history, DFU and offloading device factors baseline characteristics of those that healed and didn’t heal were similar. Surprisingly, the outcomes of cumulative plantar tissue stress, plantar pressures and daily activity were also similar between those that did and didn’t heal.

Was that the end of the story? No, as they decided to look at only those that were adherent to their offloading devices. Dr Bus’s team first did this in their now classic RCT into plantar pressure guided footwear modifications to prevent DFU. Similar to footwear, offloading devices are also only effective when worn and we assume if they aren’t worn the cumulative plantar tissue stress significantly changes.

They then found 27 patients self-reported adhering to wearing their offloading device >50% of the time in the first 4 weeks; but this dropped to 15 after the entire 12 weeks. To keep meaningful numbers, they compared the 27 patients’ adherent at 4 weeks. Also 4 weeks is usually too quick to heal, so they looked at those “healing” after 4 weeks, which they defined as a >75% reduction in DFU size at 4 weeks.

Of the 27 adherent patients, 19 had a “healing” DFU and 8 had a “non-healing” DFU. This time they found that cumulative plantar tissue stress in the healing group was 49% lower compared to the non-healing group, and nearly statistically significant. The authors did some follow-up calculations and found this result would have been significant if they had had 4 more adherent patients.

What was good and not so good about this study?

As per the previous RCT this study had many strengths including: i) randomizing patients with similar demographic and DFU characteristics; ii) objectively measuring a range of different plantar pressures, and activity outcomes; and iii) following patients for 12 weeks.

But it also had limitations, including: i) this was a secondary analysis of data already collected, which means the study was not properly designed or powered to answer the cumulative plantar tissue stress question; ii) they took 10 years to complete the original RCT so other DFU treatment may have changed in that time; iii) they did not report baseline characteristics of the adherent groups; iv) adherence was self-reported which has questionable reliability; v) their “healing” at 4 week outcome was a surrogate measure for DFU’s that healed; and vi) they performed analyses that weren’t planned and still only found near differences in the cumulative plantar tissue stress in adherent patients.

So what does that all mean?

The authors cautiously sum things up nicely, “The results from this explorative study help us improve our understanding of the role of cumulative plantar tissue stress in ulcer healing, and help to inform future larger studies on this topic. When further insight into the role of cumulative plantar tissue stress, plantar pressure and ambulatory activity in ulcer healing becomes available, that may help facilitate clinical decision-making regarding the efficacy of offloading.”

In short, this exploratory study found a possible link between cumulative plantar tissue stress and DFU healing. But, together with the findings from Dr Mueller’s study it does show that concept of measuring cumulative plantar tissue stress seems to have a merit in DFU healing and should be investigated in further larger studies.

In future, this new study may lead us towards measuring cumulative plantar tissue stress to categorise the severity in which our patient’s DFU tissue is under stress; similar to we do now with ankle brachial indexes to categorise the severity of peripheral artery disease. Plus, it may bring us closer to a day when we can tell a patient “we need to reduce your cumulative plantar tissue stress by 22% to make sure your DFU is not stressed and will heal, and to do that you have the following offloading device options”. Now that would be a step towards ending avoidable amputations in a generation … watch this space!

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