A Closer Look at a Novel Device for Foot Offloading

Inspiring adherence to limited weight-bearing restrictions is a challenging and constant frustration for medical providers. Studies have shown up to an 88 percent nonadherence rate with limited weight-bearing restrictions, even after appropriate patient education.¹ The complexities of life including, but not limited to, work, family, and physical capabilities contribute to difficulties in sustaining a limited weight-bearing status for an extended period. Not adhering to prescribed treatment plans may lead to healing complications, increased burden on the medical system, a compensatory gait—which can cause other injuries—frustration between patients and providers, as well as mental and emotional consequences. Although there are numerous offloading devices on the market, we have observed persistent challenges that demonstrate the continued need for an optimal offloading device. 
 
PopSole is a novel anatomic, bubble-air cushion insole with a modifiable arch and metatarsal pad that allows for immediate customization. In developing this device, we sought to find a solution to help minimize patients’ pain, hasten return to ambulation and function, improve adherence with partial weight-bearing instructions, and maximize healing. Key features include the ability to fit the devices into sensible shoes, maintenance of limb symmetry, and waterproof construction. The clinician can mark the exact bubbles that correspond to areas of foot pain or concern by holding the device up to the foot (Figure 1). One then pops the marked bubbles on the underside of the insole (Figure 2), and places a sticker to keep it deflated (Figure 3), thereby “pocketing out” pain and pressure points while providing anatomic support to the rest of the foot. 
 
Initially, the purpose for our innovation was to help limit pressure and expedite the recovery process for our patients who underwent autologous foot fat grafting.^2–4 Prior to our procedure, we spent substantial time crafting and tailoring multiple pads, orthotics, and surgical shoes to try to offload the surgical site while supporting the rest of the foot. In our discussions with patients, we noted a recurring theme of high patient spending on shoe gear, pads and inserts, along with frustration with the current standard of offloading devices. Patients also confessed to decreased adherence to our post-procedure instructions for limited weight-bearing.

Establishing Design Parameters

While envisioning the anatomic design necessary to support different foot types, we needed to optimize the precise bubble height and material stiffness while providing pressure relief. In collaboration with the Department of Bioengineering in the University of Pittsburgh Swanson School of Engineering, we developed a 3-dimensional, finite element model according to the calcaneus and surrounding soft tissue of an average male patient. The finite element model also accounted for measurable design inputs of material properties and loading conditions based on literature-reported values (Figure 4). A study published in 2020 aimed to optimize the insole parameters to maximally offload a target area while concomitantly limiting contact pressure on surrounding soft tissue.⁵ The ideal parameters were noted to be a bubble height of 8.3 mm and a material stiffness of 5 MPa. The bubble height plays a significant role in offloading relief while the material stiffness has a great influence on the surrounding tissue pressure.

What One Study Reveals About Patient Satisfaction

A 1-month validation study assessed improvement in pain, durability and likeability of the device. Ten participants with a history of foot pain for greater than 6 months were enrolled in an IRB-approved trial of the bubble-based insole.⁶ Due to a high level of satisfaction and good durability, the study was extended with a second cohort for 3 months. Patient-reported outcome measures were evaluated in both groups using multiple validated surveys, including the Mayo survey, the Manchester Foot Pain and Disability Index, the American Orthopedic Foot and Ankle Society (AOFAS) Ankle-Hindfoot Scale, The Foot and Ankle Ability Measure (FAAM), and the Pittsburgh Foot Survey. Improvements were noted in pain, function, and quality of life across the surveys: Mayo survey (P = .018), the Manchester Foot Pain and Disability Index (P = .014), AOFAS Ankle-Hindfoot Scale (P = .032), FAAM Activities of Daily Living (P = .009), and the Pittsburgh Foot Survey (P = .01). Notably, the FAAM Sports subscale approached statistical significance (P = .058). Aggressive use by one participant who went on a 6-mile hike resulted in blisters; however, they healed while continuing to use the device on 1.5-mile daily walks. Bubble deflation is common after use over time as they mold to the feet, but participants still reported improvement in pain and activity. The studies confirmed durability and safety for the 2–3-month period typically needed for patient recovery.

Analyzing Available Options

One very effective offloading modality is a cast due to its circular immobilization and irremovability. However, a cast is quite unforgiving and has a higher complication rate of deep vein thrombosis (DVT), muscle atrophy, and joint stiffness, and is therefore reserved for more complex foot issues where the benefit of the cast will outweigh the potential complications.⁷ Removable cast boots are another offloading option; however, they are not customizable, cause gait abnormality due to limb asymmetry, may be expensive, and removability may prove tempting.⁸ There are multiple therapeutic footwear devices on the market, such as rocker-bottom shoes, but research has shown they are significantly less effective than casts.⁸ Our own personal customer discovery on these products yielded comments regarding frustration with bulkiness, compensatory gait consequences, potential toe or heel exposure in cold or wet weather, and unsightly appearance. Crutches are another viable partial weight-bearing modality but may be accompanied with stability issues and secondary upper extremity injury.
 
The Biodynamics Laboratory in the Department of Orthopaedic Surgery at the University of Pittsburgh conducted a comparative pilot study of the Darco PegAssist and PopSole with a neutral running shoe.⁹ The group collected data according to the following criteria: lower extremity kinematics, pressure insole mapping, and patient outcome/device satisfaction. Data analysis of joint angles showed that the kinematics of the ankle was affected by increased frontal plane and decreased transverse plane motion with the bubble-based insole when compared to a running shoe. However, hip and knee ambulatory mechanics were equivalent. Hip extension decreased during the late phase of gait with the peg-based insole compared to the running shoe, suggestive of an asymmetrical gait when the contralateral foot is in a sneaker. 
 
Pedal pressure insole mapping (Figure 5) revealed that the bubble-based insole (with forefoot and heel regions popped) reduced peak force in the heel compared to regular shoes during indoor (P = .04) and outdoor walking (P = .02) when analyzed with both the smaller and larger sensor areas. The peg-based insole reduced peak force in the forefoot area compared to regular shoes during indoor (P = .05) and outdoor walking (P < .001) when analyzing the 4-sensor area. The peg-based insole also had reduced peak force in the forefoot region when analyzing the 9-sensor areas compared to bubble-based insole during both indoor (P = .04) and outdoor walking (P < .05). Participants were able to perceive this deviation in hip range of motion, ultimately showing significant preference to the bubble-based insole. The neutral running shoe may not have been the ideal comparison for forefoot pressure offloading due to flexibility in the forefoot of the sneaker. A stability sneaker may have a more comparable sole stiffness to the peg-based insole. We hope to address these parameters more definitively in future studies.⁹
 
In the participant satisfaction survey, 100% stated they prefer the bubble-based insole over the peg-based option, which shows great potential to increase patient adherence.⁹ Increased adherence may translate to hastened recovery compared to current standard of care.9 While the newest peg-based insole comes with an anatomic arch, it is not adjustable for a given foot type.

Considering Indications for Use

Although our initial focus was to offload autologous foot fat grafts, our research in participants with generalized foot pain indicated a need for broader use. We currently recommend the bubble-based insole for conditions including heel pain, pinpoint foot pain, bone bruise pain, calluses, blisters, warts, and forefoot and heel fat pad atrophy. In our experience, it may also serve as a reasonably priced trial device prior to the use of custom orthotics or as a gap product for patients awaiting fabrication/modification of their custom orthotics. If a foot injury is more severe and requires a walking cast or surgical shoe, such an insole may help augment these modalities. We also feel there may be potential to encourage a proper gait with decreased compensation during the rehabilitation process from more complicated foot injuries. We hope to pursue future robust clinical trials with the PopSole in the diabetic, surgical, and traumatic populations.
 
Beth Gusenoff, DPM, FACFAS, is a Clinical Assistant Professor of Plastic Surgery in the Department of Plastic Surgery at the University of Pittsburgh. She is Co-Director of the foot fat grafting program at UPMC and CEO of BRG Innovations LLC. 

Jeffrey Gusenoff, MD, is a Professor of Plastic Surgery at the Department of Plastic Surgery at the University of Pittsburgh with an international reputation for fat grafting and body contouring. He is Director of the foot fat grafting program at UPMC and Co-CEO of BRGInnovations LLC  

Disclosures: Beth Gusenoff, DPM, and Jeffrey Gusenoff, MD, have intellectual property in the PopSole. Both doctors are Co-CEO of BRG Innovations, LLC. Research was supported by the Pitt Innovation Challenge, Clinical and Translational Science Institute, and Coulter Translational Research II Program.

References
1.     Gershkovich G, Arango D, Shaffer GW, Ndu A. Weight bearing compliance after foot and ankle surgery. Foot Ankle Int. 2016;1(1). doi:10.1177/2473011416S00089
2.     Gusenoff JA, Mitchell RT, Jeong K, Wukich DK, Gusenoff BR. Autologous fat grafting for pedal fat pad atrophy: a prospective randomized clinical trial. Plast Reconstr Surg. 2016;138(5):1099-1108. doi:10.1097/PRS.0000000000002667
3.     Minteer DM, Gusenoff BR, Gusenoff JA. Fat grafting for pedal fat pad atrophy in a 2-year, prospective, randomized, crossover, single-center clinical trial. Plast Reconstr Surg. 2018;142(6):862e-871e. doi:10.1097/PRS.0000000000005006
4.     James IB, Gusenoff BR, Wang S, DiBernardo G, Minteer D, Gusenoff JA. A Step in the Right Direction: A Prospective Randomized, Controlled Crossover Trial of Autologous Fat Grafting for Rejuvenation of the Heel. Aesthet Surg J. 2021;41(7):NP959-NP972. doi:10.1093/asj/sjab095
5.     Strayer ST, Moghaddam SRM, Gusenoff B, Gusenoff J, Beschorner KE. Contact pressures between the rearfoot and a novel offloading insole: results from a finite element analysis study. J Appl Biomech. 2020;36(5):326-333. doi:10.1123/jab.2019-0356
6.     Lubarsky R, Gusenoff B, Gusenoff JA. Prospective cohort validation study of a novel foot offloading device. Plast Reconstr Surg Glob Open. 2021;9(11):e3950. doi:10.1097/GOX.0000000000003950
7.     Boyd AS, Benjamin HJ, Asplund CA. Splints and casts: indications and methods. American family physician. Am Fam Physician. 2009;80(5):491-499.
8.     Bus SA. The role of pressure offloading on diabetic foot ulcer healing and prevention of recurrence. Plast Reconstr Surg. 2016;138(3 Suppl):179S-187S. doi:10.1097/PRS.0000000000002686
9.     Frankston N, Zarei M, Munsch M, Gusenoff B, Gusenoff J, Anderst W. A pilot study evaluating the effect of a novel foot offloading device on lower extremity kinematics. Presented at the 45th Meeting of the American Society of Biomechanics. Atlanta GA, Aug 10-13, 2021.