Plantar Pressure Analysis: From Research to Frontline Clinical Practice

By Nachiappan (Nachi) Chocklingam

I’ll try and provide you with an overview of the technologies and techniques involved in plantar pressure analysis, how that data is normally reported, and what you can do with the data so it can benefit your clinical management and evidence-based medicine. I’ll also provide examples from the work my colleagues and I have done here at the Centre for Biomechanics & Rehabilitation Technologies at Staffordshire University in the UK regarding clinical management and some aspects of footwear and material in terms of diabetic foot management.

Technology v Technique

This topic is very close to my heart: is technology important or is the technique? Regarding technology, the measurement that we are talking about is the assessment of ground reaction force. The typical equipment that is being used to measure this ground reaction force is a force platform. Force platforms are either strain gauges  or  piezoelectric sensors. These give you the 3 components of the ground reaction force: vertical, anterior/posterior, and medio lateral or side to side. For this discussion, we focus on the vertical component of this ground reaction force because that’s what is involved in plantar pressure measurement: the vertical load  divided by the area covered by the sensor gives you the pressure. 

There also are various types of pressure measurement systems for gait analysis. We normally use walkway-based mats or in-shoe systems. It’s important to understand this load and how it’s distributed for designing, implementing, and evaluating effective clinical management. So it’s not just about the force, it’s about how that force is distributed, and that’s what makes effective clinical management for many of the conditions that we have for the foot at risk or any lower limb gait-related issues, that’s where our plantar pressure measurement system comes into play. 

The scientific and clinical communities have debated if the mat-based system, in-shoe system, or walkway is better. The choice depends on what you want to do with the data and what you really want to get from the system. It is important that any measurement you take does not interfere with the immediate environment. For example, if you want to measure shoe comfort or the function of an orthosis, you need to make sure the in-shoe sensor is not so thick or that the cushioning it provides does not interfere with the testing environment. If the initial sensor is thick, it reduces the volume within the shoe, which then interferes with your measurement and the way that the patient walks (Figure A).

It has also been debated if capacitance-based technology is preferable to resistance-based technology. As all systems have evolved and improved in terms of technology and usability, in my opinion, discussions about these various technologies should relate to power consumption, measurement sensitivity, response to loading time, and cost.

Another important thing that I want to mention is calibration. Whichever system you use, I want to emphasize  that your system should be calibrated properly for each use and what you see on the screen actually makes sense to you. It’s not about the color of the image, it’s about the actual value that you get from that system (Figure A).

It’s also important that people understand the clinical protocols and the techniques to use. Most of the current scientific literature on pressure measurement relates to injury prevention, running injuries, or footwear analysis. The point that I’m trying to get across is that these plantar pressure systems, can give you a lot more information and a lot more data than average pressures and peak pressures. One of the things that’s normally not reported much in many research papers is the central pressure  trajectory, but it can give you a lot of information on the clinical aspects of your management.

The bottom line is, whilst technology and techniques play a role, it’s all about you and the patient or it’s about the research question and how you want to answer it. You will need to think carefully about your approaches to data collection and, more importantly, the reporting because that’s what is important—that other people are able to understand it as well. You need to think about whether it makes sense, and whether you can repeat the same measurement. All this governs the way that you choose the system and the right technique

3D Printing to Address Insufficient Reach of Diabetic Foot Care

Our group here at the Centre for Biomechanics and Rehabilitation Technologies have been focusing on diabetic foot complications and its management for the past 10 years. We and our collaborators have published a lot on this space in the European Union-funded program titled DiaBSmart (www.diabsmart.eu), which focuses on the development of assessment techniques, footwear intervention, and insole intervention for patients with diabetic foot complications.

Here in the UK, or worldwide for that matter, 85% of all diabetic foot amputations begin with a single foot ulcer. Once ulcerated, the healing is very slow and the recurrence of these ulcers is very high. Normally, custom orthoses can prevent 65% of these foot ulcers by reducing the peak plantar pressure by 15% to 21%. This data is based on meta-analysis of the literature and there is enough evidence for it. But the issue that we were dealing with is current custom foot orthoses were slow to produce, they were expensive, and they were not produced in the same way, they’re not repeatable. 

In the UK, only 21% – only one-fifth – of these high-risk patients received these custom orthoses, so our work was to think about how we can address that. We used 3D printing technologies to reduce the cost, and also some frontline patient management issues in terms of collecting data, and how effectively we use the data. In terms of diabetic foot care, plantar pressure measurement is very important and can be used to monitor patients effectively. However, I have given you statistics that show that not enough of this patient population is provided with diabetic insoles or custom footwear. 

Case Study: Charcot Foot

I’m going to touch on  things that we have done, both relating to diabetic foot care. The first one relates to the Charcot foot. One of my collaborators in Newcastle-upon-Tyne here in the UK, Jayasree Ramaskandhan, PhD, has set up a specialist diabetic foot clinic and service in their gait lab, where they do a comprehensive assessment. 

In this case study that I am presenting, a 54-year-old female with a desk-based job presented with right foot Charcot arthropathy with arch collapse and a 41-year history of diabetes. She was treated (off loaded) with a plaster of Paris cast to start with, but it didn’t work, so she ended up on the surgery list. The point that I’m trying to make here is Dr. Ramaskandhan effectively uses plantar pressure assessment technique throughout the patient journey. When the patient comes in, Dr. Ramaskandhan uses plantar pressure along with other gait analysis systems for the baseline assessment. 

During this patient journey, Dr. Ramaskandhan and colleagues at Newcastle do the whole array of preoperative subjective and objective assessments, problems, etc. But plantar pressure analysis forms an important central role in assessing these patients. The surgeons and the multidisciplinary team look at the plantar pressure data and then that drives their clinical decision making on the surgical planning. It resulted in a fantastic 1-year follow-up data set, as shown on the right (Figure B). 

The reason I wanted to showcase this work is because, as far as I know, this is the first time Charcot patients are being fully treated this way anywhere in the world and pressure assessment is used throughout the patient journey. In terms of first assessment to a full follow-up, this is the first time that we’re doing a comprehensive biomechanical analysis, including plantar pressure assessment, in these patients. This is an exciting development, and hopefully the work will be published in the coming months.

Thermal Measurements, Interdigital Pressures Aid Risk Assessment

While the topic of this presentation is plantar pressure assessment, when you’re talking about the diabetic foot and the foot at risk, you must also consider temperature measurement and other things like interdigital pressures. Examining how foot deformity affects interdigital pressures and plantar pressure measurement provides an evidence base for your clinical intervention and how you can improve your clinical intervention. 

The next topic is extremely novel. In a pilot study that I conducted with my collaborators in Malta,  we hypothesized that combining thermal measurements and plantar pressure measurements would offer a more reliable assessment tool for providing risk categorization of the diabetic foot. The correlation between plantar pressure and temperature in patients living with diabetes as compared to plantar pressure and temperature in healthy individuals clearly shows that combining these 2 important outcome measures actually results in improved risk categorization of these patients for better clinical management.

I wanted to raise the subject of thermal measurements because, while it is not the focus of this presentation—the main topic is plantar pressure measurement—when taking on/combining the various roles as a clinician, a scientist, and a researcher in a lab, you are collecting all sorts of data and you’re assessing various outcome measures. In that sense, plantar pressure forms a core element of your overall patient management or overall data set. To answer your research question, it’s important that you understand various aspects and use plantar pressure measurement in an effective way. 

Footwear Interventions

The other thing that I wanted to talk about is the work we have done at Staffordshire through the EU-funded DiaBSmart program, in terms of understanding and providing footwear interventions for patients with diabetes. We  tested various  polyurethane (PU) materials of varying densities. These PU materials have various stiffnesses. In our 2017 paper, “Subject Specific Optimisation of the Stiffness of Footwear Material for Maximum Plantar Pressure Reduction,” we showed that subject-specific optimization of stiffness in footwear material maximizes the plantar pressure reduction and so it effectively helps the patient. So plantar pressure can be used to choose an appropriate material.

When we talk about appropriate material, we are referring to appropriate engineering stiffness. Our work, “A Clinically Applicable Non-invasive Method to Quantitatively Assess the Visco-hyperelastic Properties of Human Heel Pad, Implications for Assessing the Risk of Mechanical Trauma,” also published in 2017, shows that we have developed a method to quantitatively assess the visco-hyperelastic properties of the plantar soft tissues and its relationship with mechanical trauma on the ulcer formation. The program of work showed that if we can match the stiffness of the plantar soft tissues to the stiffness of the material, then we have much better patient outcomes. Once we learned from that work, we moved on to talk about how we can effectively use it in the clinic using newer technologies. That’s where 3D printing technology came in. 

In our 2020 paper in Gait & Posure, “Optimised Cushioning in Diabetic Footwear can Significantly Enhance their Capacity to Reduce Plantar Pressure,” we have shown that by varying the print structures we can change the stiffness to match the patient’s specific required stiffness, which actually improves patient outcomes in terms of diabetic patients (Figure C).

Both of these examples relate to insole or footbed. But it’s a combination of footwear and insole that makes that difference. My colleague Mickey Edison has done a lot of work on footwear tuning. As a part of that work, we’re also looking at the insole footwear combination in terms of diabetic foot and the foot at risk. 

Conclusion

The key messages that I want to give are:  (1) please consider the right technology that suits your need and (2) please use appropriate techniques. I highlighted 2 things: the thickness of the sensor and calibration. The message I would like to impart is please don’t blame the technology or the commercial vendor. Most times it’s inadequate technique that you employ that makes all the difference. Please make sure you calibrate the system and use appropriate technologies. Today’s technology works well.

I also want to emphasize that plantar pressure assessment can contribute to effective clinical management and evidence-based clinical management, so stick with it. Plantar pressure measurement can give a lot more data than just peak pressures. 

Finally, I want to touch on the amount of data that we get from plantar pressures. Imagine if you have 100 sensors and you record the data at 100 Hertz for 7 or 8 seconds. That’s an enormous amount of data, but we reduce it to 1 number, which is the peak pressure or sometimes average pressure. However, we need to think about how effectively we can use the rest of the data set and newer technological developments. How can we use artificial intelligence (AI) to help us? Because in terms of AI and modeling data, the quality of the output depends on the input data. With plantar pressure measurement systems and the amount of data derived from that, we can provide good input and, in that sense, we can come up with systems that would help us with effective clinical management. 

Books for Further Reading

• Richards J. The Comprehensive Textbook of Clinical Biomechanics, 2nd ED [formerly Biomechanics in Clinic and Research]. Elsevier; 2018.
• Burden A, Payton CJ. Biomechanical Evaluation of Movement in Sport and Exercise : The British Association of Sport and Exercise Sciences Guide. 2nd Ed. Taylor & Francis Group; 2017.
• Muller B, Wolf SI (Eds). Handbook of Human Motion. Springer International Publishing AG; 2018.
• Chocklingam N (Ed). Technologies and Techniques in Gait Analysis: Past, Present, and Future. The Institution of Engineering and Technology; 2022.