Users’ needs and expectations and the design of a new custom-made indoor footwear solution for people with diabetes at risk of foot ulceration

Abstract

Purpose

To assess users’ needs and expectations regarding custom-made indoor footwear, and to design such footwear with similar biomechanical efficacy and better usability compared to regular custom-made footwear in people with diabetes at risk for foot ulceration.

Materials and methods

Multidisciplinary systematic design approach. Needs and expectations regarding indoor footwear were evaluated via a questionnaire in 50 high foot ulcer risk people with diabetes using custom-made footwear. We systematically designed indoor footwear, and manufactured this for nine participants. Primary requirement was similar plantar pressure compared to participants’ regular custom-made footwear.

Results

Eighty-two percent of participants expressed a need for custom-made indoor footwear and 66% expected such footwear to increase their adherence. The custom-made indoor footwear had the same bottom construction as participants’ regular custom-made footwear, but with softer and more light-weight upper materials. Peak pressures were similar or lower, while qualitative evaluation showed better usability and lower costs for indoor footwear.

Conclusions

People with diabetes at risk of foot ulceration expressed a clear need for custom-made indoor footwear, and expected such footwear to increase their adherence. Our indoor footwear design provides adequate pressure relief, with better usability, and can be produced at lower costs compared to regular custom-made footwear.

Implications for rehabilitation

• People with diabetes at risk of foot ulceration express a need for special indoor footwear.

• We developed indoor footwear with similar offloading capacity as regular custom-made footwear.

• The indoor footwear is lighter in weight, easier to don/doff and lower in costs.

• At-risk people with diabetes expect this footwear to increase their footwear adherence.

Keywords:

• Diabetes mellitus
• foot ulcer
• prevention
• footwear
• adherence

Introduction

Foot ulceration is a frequent complication of diabetes mellitus and an important precursor to foot infection and amputation [1]. It affects 19–34% of people with diabetes and places a high burden on patients and carers, as well as healthcare systems [2,3]. In the first year after healing of a diabetic foot ulcer, roughly 40% of patients experience a recurrent foot ulcer, while after 3 years this is almost 60% [2]. It is therefore important to focus on the prevention of foot ulceration.

International guidelines recommend custom-made footwear as key preventative intervention [4,5]. The aim of such footwear is reducing ulcer risk by redistributing and lowering peak plantar pressures and providing a proper fit [4]. Adequate protective footwear helps prevent ulcer recurrence, if the footwear is worn consistently [6,7]. However, footwear adherence is often low in people with diabetes who are at risk of ulceration [6,8–10].

To improve adherence, it is important to understand factors associated with non-adherence. However, evidence on associations between footwear adherence and personal, disease or behavioral factors such as gender, diabetes duration, ulcer history and activity levels, is absent, unclear, or conflicting [10]. Predictors for non-adherence are more likely to be found in an individual’s assessment of the footwear itself or individual strategies toward their use [11–14]. For example, perceived benefit of custom-made footwear was the only predictor for its use in multivariate analyses in one study in people with diabetes at high ulcer risk [11], and one of the strongest predictors in a cross-sectional study in people with diabetes with low, moderate, and high ulcer risk [14]. However, this perception can be determined by a variety of factors, such as usability factors weight, ease of use, and aesthetics, but also by costs, clinical expectations, health literacy and acceptance [10,11,15,16]. As weighing of these factors depends on individual preferences [15], no general intervention to improve adherence has yet been successful [17].

Rather than focusing on an intervention to improve the perceived benefit of protective footwear, a contextual approach could prove more beneficial. Adherence to wearing footwear differs between contexts, such as its location (e.g., indoor vs. outdoor) or its purpose (e.g., work vs. leisure) [18,19]. In people with diabetes at high ulcer risk, footwear adherence is especially low indoors, while patients are most active inside their home [12,20,21]. Targeting this context may offer opportunities to improve adherence. The only study that specifically investigated reasons for this low adherence indoors found that participants considered footwear weight and donning/doffing mostly a problem when inside [21]. In addition, footwear that is also worn outside can often be dirty, and people can have a habit to take off their footwear when arriving at home [21]. Following these reasons, it can be expected that custom-made footwear purposely designed for indoor use may improve adherence.

Indoor footwear should not only overcome the barriers in indoor usability, it should still have adequate biomechanical offloading capacity as it replaces the regular custom-made footwear of the user [4]. Several orthopedic footwear companies have tried to create such custom-made indoor footwear. However, no systematic design approach and evaluation, integrating users’ and professionals’ perspectives, has been followed. The aim of this study was to follow such an approach to (1) assess users’ needs and expectations regarding custom-made indoor footwear and (2) design custom-made indoor footwear with similar biomechanical efficacy and better usability compared to regular custom-made footwear.

Materials and methods

We used a multidisciplinary systematic design approach and evaluation by means of two studies, integrating perspectives of users (people with diabetes at risk of ulceration in possession of regular custom-made orthopedic footwear), clinicians, pedorthists, medical insurers, and researchers. Both studies were coordinated by the clinical research team (authors of this paper) and took place in three orthopedic footwear companies that work within a multidisciplinary diabetic foot outpatient clinic in a hospital setting. User inclusion criteria were the same for both studies: type 1 or 2 diabetes mellitus, at moderate to high risk for foot ulceration (International Working Group on the Diabetic Foot (IWGDF) risk 2 or 3 [4]), and in possession of regular custom-made footwear (defined as custom-made insoles worn in custom-made shoes [4]). Exclusion criteria for both studies were inability to read Dutch and inability to adhere to study requirements, for evaluating the design the following additional exclusion criteria were applied: a foot ulcer at the time of recruitment, Charcot deformation, amputation at or beyond the tarsometatarsal level, or another condition requiring high-cut footwear (i.e., mid-tibia level or higher) at all times. The requirement for ethical review was waived under the Medical Research Involving Human Subjects Act by the local ethics committee (reference number W17_405 #17.474).

Assessment of users’ needs and expectations regarding custom-made indoor footwear

Study design

Questionnaire.

Questionnaire

A questionnaire was designed to assess users’ needs and expectations regarding custom-made indoor footwear and assessed the following domains: (1) need for custom-made indoor footwear, (2) use and usability of current custom-made footwear, (3) expectations of indoor footwear and willingness of financial contribution, and (4) current living conditions. The questionnaire was based on the Monitor Orthopedic Shoes (MOS), a validated questionnaire that measures the most relevant aspects of footwear usability from a user’s perspective [22] and further customized for this study, with help of a patient representative from the Dutch Diabetes Patient Society, which represents the interests of all persons with diabetes. It consisted of five-point Likert scale questions, multiple choice and open-ended questions. Questions on financial contributions were added because custom-made footwear in the Netherlands costs on average 1500 Euro to produce. These costs are reimbursed by healthcare insurers, with the exception of patients’ “own contribution” of 129 Euros per pair.

Recruitment

Three certified pedorthists (one per company) were instructed to invite a minimum of 15 eligible persons who fulfilled the inclusion criteria. Persons who agreed to participate in the survey were approached by a member from the research team. Depending on their preference, the questionnaire could be completed on paper and returned in a pre-stamped envelope or completed online with Typeform^TM (Barcelona, Spain). If necessary, a postal reminder was sent after one month.

Statistical analysis

We used descriptive statistics to summarize participant characteristics and questionnaire responses.

Design and evaluation of custom-made indoor footwear

Study design

Multidisciplinary two-phase footwear design and cross-sectional within-subject evaluation.

Phase 1: defining and prioritizing requirements

A multidisciplinary team of specialists was formed, and consisted of a rehabilitation specialist, four pedorthists (all with >5 years of experience in the field of diabetic foot disease), three management representatives from three different footwear companies, a healthcare insurer, four human movement scientists, and the patient representative, who advocated the interests of the intended users. Based on discussions within the multidisciplinary team and aligned with the results from the needs assessment, a consensus set of 12 requirements for custom-made indoor footwear was created (Table 1). The key requirement was to have similar peak plantar pressures (±10%) compared to the participants’ regular custom-made footwear.

Table 1. Requirements for custom-made indoor footwear.

Key requirement
Peak plantar pressure is comparable to regular custom-made footwear
Other requirements
Easy donning and doffing
Lighter in weight than regular custom-made footwear
Regulating heat and breathable
Appearance satisfactory for indoor use
More comfortable than regular custom-made footwear
Safe to use
Good durability
No, or minimal, increase of shear forces compared to regular custom-made footwear
To be worn alternately with regular custom-made footwear
Improves adherence to indoor footwear use
Cheaper to produce than regular custom-made footwear

Phase 2: designing the indoor footwear

In the next phase, the multidisciplinary team held four meetings to evaluate the design, manufacturing, and testing of the indoor footwear. During the first meeting, a pedorthist from each company presented a preliminary model. Its quality was discussed using the set of requirements (Table 1). One model best fitted the key requirements of pressure distribution and other characteristics, and was unanimously chosen as the leading type. This model was subsequently adapted by each pedorthist and discussed at the second meeting.

Phase 3: selection of participants; manufacturing and evaluating the indoor footwear

When all agreed on its properties, the indoor footwear was custom-made for nine participants (three per company, selected by the pedorthist). Nine participants were considered a sufficiently large convenience sample to obtain adequate first impressions when manufacturing the footwear and to provide participants with an opportunity to wear it in real-life. The indoor footwear was made on the shoe last of the participants’ regular custom-made footwear. In-shoe plantar pressure measurements were taken in both indoor and regular custom-made footwear (see next section). If peak pressure in the indoor or regular footwear exceeded 200 kPa or peak pressure in the indoor footwear was >10% higher than in the regular footwear, the pedorthist modified the footwear until pressure requirements were satisfactory [23,24]. Participants were asked for qualitative feedback on usability aspects of the indoor footwear. During the third and fourth meeting of the multidisciplinary team, peak pressure outcomes were presented and discussed, along with the other requirements. Consensus on the final design, including all materials to be used, was reached.

In-shoe plantar pressure measurements

Dynamic in-shoe peak plantar pressures were measured with the Pedar-X in-shoe pressure measurement system (Novel GmbH, Munich, Germany) at a 50-Hz sampling frequency. Participants walked undisturbed over a flat surface in the regular clinical evaluation rooms at the three participating companies. The length of the walkway varied from 4 to 8 m. Participants were asked to walk at a comfortable speed. At one of the companies, walking speed was quantitatively measured and controlled, where speed had to be within 5% of the first attempt; at the other two companies, similarity of walking speed between trials and conditions was judged qualitatively. The first and last steps of each walk were discarded. The walk was repeated until a minimum of 12 midgait steps per foot were collected [25]. Novel multimask software (version 13.3.65) was used for pressure analysis. The mean peak pressures at eight anatomical foot regions were calculated for the left and right foot separately: the toes (hallux, dig 2–3, and dig 4–5), forefoot (metatarsal head 1, metatarsal head 2–3, and metatarsal head 4–5), midfoot and heel.

Statistical analysis

Participant characteristics and in-shoe peak pressures were summarized with descriptive statistics. Wilcoxon’s signed rank tests were used to compare differences in in-shoe peak pressure between indoor and regular custom-made footwear for the eight anatomical regions of both the left and right foot, with alpha at 0.05 and (with a total 16 comparisons) the Holm–Bonferroni method to correct for multiple testing (i.e., sorting the p values from lowest to highest; comparing the p values to nominal alpha levels with alpha = alpha/16 = 0.05/16 = 0.003125 for the smallest p value, etc.). Statistical analyses were performed using SPSS 26.0 (SPSS Inc., Chicago, IL).

Results

Assessment of end-users’ needs and expectations regarding custom-made indoor footwear

Participants

A total 58 participants were provided with the questionnaire to assess their needs and expectations regarding custom-made indoor footwear. The response rate was 90% (n = 52). Two responders were excluded because they did not have diabetes mellitus and were erroneously invited to participate, 50 responders were analyzed. Mean (SD) age of the responders was 69 (13) years and 48% were male (n = 24); this was 67 (6) years and 83% male (n = 5) for the non-responders.

Custom-made footwear use and usability

Of the responders, 92% reported to wear their custom-made footwear for 6–7 days per week. A total of 50% wore their custom-made footwear inside their house >8 h per day, 22% 4–8 h per day, 14% 0–4 h per day, and another 14% never wore their custom-made footwear inside the house – they walked barefoot or in prefabricated footwear. The most frequently reported reason for not or hardly wearing their custom-made footwear inside the house was that the footwear was too heavy (43%). Almost all responders (92%) left their beds at least once during the night, with 24% doing so three times or more. Only 4% wore their custom-made footwear when getting out of bed during the night, most (58%) reported to walk barefoot or on socks during the night as donning/doffing custom-made footwear would require too much effort. Concerning usability, most responders (76%) indicated they were satisfied with their custom-made footwear. Most frequently reported negative usability characteristics concerned the footwear being too heavy or difficulty with donning and doffing (Table 2).

Table 2. Usability characteristics of participants’ regular custom-made footwear and expected presence or absence thereof in custom-made indoor footwear.

Negative usability characteristics	Not or hardly present	No opinion	Present or very much present
Regular custom-made footwear
Skin irritation	66 (33)	16 (8)	18 (9)
Ulceration	82 (41)	12 (6)	6 (3)
Sweating	82 (41)	16 (8)	2 (1)
Cold feet	72 (36)	10 (5)	18 (9)
Too much weight	46 (23)	32 (16)	22 (11)
Difficulty donning and doffing	48 (24)	30 (15)	22 (11)
Fit (too tight)	70 (35)	26 (13)	4 (2)
Custom-made indoor footwear
Ulceration	88 (44)	10 (5)	2 (1)
Inadequate fit	84 (42)	14 (7)	2 (1)
Skin irritation	74 (37)	24 (12)	2 (1)
Difficulty donning and doffing	74 (37)	18 (9)	8 (4)
Too much weight	70 (35)	28 (14)	2 (1)
Sweating	66 (33)	34 (17)	–
Cold feet	60 (30)	34 (17)	6 (3)
Positive usability characteristics
Good durability	–	22 (11)	78 (39)
Easy maintenance	–	34 (17)	66 (33)
Appealing appearance	6 (3)	64 (32)	30 (15)

Data expressed as % (n) of responders.

Needs, expectations, and priorities for custom-made indoor footwear

Of the responders, 64% were unfamiliar with the concept of custom-made indoor footwear. After explanation, 82% indicated they felt a need for such footwear. The majority of responders (66%) expected to wear indoor footwear more frequently inside their home than their regular custom-made footwear, if it would be provided to them. Most responders expected for indoor footwear that negative usability characteristics (e.g., ulceration, difficulties donning and doffing) would not or hardly be present (Table 2). Positive usability characteristics like easy maintenance and good durability were more frequently expected than an appealing appearance (Table 2). Prevention of ulceration was seen as most important feature of such footwear, while prevention of skin irritation, easy donning and doffing and good fit were also considered important (Table 3). Participants varied in what they were willing to contribute financially, with 20% indicating they would contribute 0 Euros, 26% 1–50 Euros, 32% 51–100 Euros, and 22% 101–200 Euros.

Table 3. Importance of usability characteristics of custom-made indoor footwear.

	Important or very important	No opinion	Not or hardly important	Item ranked as #1 priority
Prevention of ulceration	100 (50)	0	0	54 (27)
Prevention of skin irritation	96 (48)	4 (2)	0	25 (12)
Good fit	96 (48)	4 (2)	0	6 (3)
Easy donning and doffing	92 (46)	4 (2)	4 (2)	4 (2)
Flexibility of the material	86 (43)	14 (7)	0	4 (2)
Prevention of cold feet	70 (35)	26 (13)	4 (2)	4 (2)
Weight	74 (37)	20 (10)	6 (3)	2 (1)
Durability	78 (39)	16 (8)	6 (3)	–
Maintenance	70 (35)	28 (14)	2 (1)	–
Appearance	42 (21)	40 (20)	18 (9)	–
Prevention of sweating	40 (20)	50 (25)	10 (5)	–

Data expressed as % (n) of responders. One responder did not answer the priority question.

Design and evaluation of custom-made indoor footwear

Indoor footwear design

The final design of the indoor footwear included two types, to provide patients with a choice concerning the vamp material (Figure 1). The vamp of type A was made from Alcantara^®, and therefore also included a strengthened toecap and collar. The vamp of type B was made from felt. All materials of both types of the final design are presented in Figure 1 and Table 4. The bottom of the shoe was designed such that it had the same biomechanical properties as the regular custom-made footwear, i.e., the same heel, outsole and rocker profile, and the same custom-made insole.

Figure 1. Schematic drawing with features and two images of the two types of custom-made indoor footwear design (see Table 4 for explanation of the numbers and material details).

Table 4. Features of the indoor footwear design.

Feature	Specification
(1) Vamp	Type A: Alcantara^® (4 mm microfiber); type B: felt (tweed extra 953)
(2) Toe part	Type A: leather; type B: no toe part
(3) Heel cap	Leather
(4) Upperpart	A combination of leather, Velcro fastener and a zipper. The Velcro fastener was used for optimal fitting and allows individual adjustment, with two fixations on both the lateral and medial side. The zipper had a metal ring to support opening and closure.
(5) Collar	Type A: leather with 8 mm sponge inside; type B: no collar.
(6) Contrefort	Thermoplastic reinforcement material (Rhenoflex^® 1.2 mm or Teefe^® 15).
(7) Protective welt	A: none. B: leather
(8) Heel	Wedged ethylene vinyl acetate, shore 60. Heel height was similar to the regular custom-made footwear. Heel rounding was similar to regular custom-made footwear.
(9) Outsole	Ethylene vinyl acetate, 4–6 mm, shore 40, to enhance foot progression and create more stiffness.
Rocker profile	The pivot point and angle of the pivot point were identical to the regular custom-made footwear. Adjustment was permitted, dependent on the insole type that is used (a more flexible insole might need more rocker profile).
Insole	The insole was identical to the insole of the regular custom-made footwear and made on the same shoe last. A 2.2 mm-thick full-length rigid insole was added under the normal insole for additional stiffness (Redoma^® Texon T97^®).
Height	Ankle-high (i.e., approximately 2–5 cm above the ankle).
Stitching	40/3 (Gunze count)

Participants

Nine participants (mean age (SD) 63 (14) years; 78% male (n = 7)) were provided with indoor footwear as part of their regular foot care.

Peak plantar pressures

The indoor footwear showed similar peak plantar pressures in all regions compared with the regular custom-made footwear (Table 5). After one or two rounds of modifications, some participants still presented with peak pressures >200 kPa, but further modifications were not required in the judgment of the pedorthist; peak pressures >200 kPa were less frequently present in indoor footwear (Table 5).

Table 5. Peak plantar pressures of the indoor and regular footwear per anatomical region.

Foot region	Indoor footwear^a	Regular custom-made footwear^a	Median difference^a,b	p Value^c	PP > 200 kPa Indoor footwear^d	PP > 200 kPa Regular custom-made footwear^d
Right foot
Hallux	162 (26–254)	156 (63–213)	–2 (–70 to 115)	0.594	2	2
Digits 2–3	109 (43–153)	114 (69–194)	–6 (–41 to 20)	0.110	0	0
Digits 4–5	99 (41–134)	78 (54–161)	–5 (–27 to 46)	0.953	0	0
Metatarsal 1	140 (38–257)	163 (63–257)	–22 (–33 to 5)	0.049	2	3
Metatarsal 2–3	193 (56–260)	189 (74–280)	–17 (–63 to 30)	0.025^c	2	4
Metatarsal 4–5	114 (44–187)	108 (66–198)	0 (–27 to 46)	0.314	0	0
Midfoot	89 (62–135)	102 (71–133)	0 (–31 to 6)	0.889	0	0
Heel	197 (93–239)	214 (110–313)	–10 (–105 to 16)	0.249	4	5
Left foot
Hallux	106 (60–398)	104 (63–514)	1 (–122 to 30)	0.678	1	3
Digits 2–3	112 (63–209)	99 (53–223)	21 (–61 to 87)	0.260	1	1
Digits 4–5	87 (76–260)	98 (54–209)	0 (–21 to 51)	0.575	1	1
Metatarsal 1	126 (71–232)	157 (63–226)	–3 (–49 to 30)	0.678	2	2
Metatarsal 2–3	147 (100–245)	155 (74–283)	1 (–66 to 27)	0.678	2	2
Metatarsal 4–5	130 (95–260)	135 (71–215)	2 (–29 to 45)	0.859	1	1
Midfoot	132 (99–165)	132 (75–174)	4 (–68 to 39)	0.343	0	0
Heel	176 (124–274)	193 (110–271)	3 (–69 to 38)	0.859	3	4

PP: peak pressure.

^a Data expressed as median (range) peak pressure, in kPa.

^b Difference is peak pressure in indoor footwear minus custom-made footwear; a negative score means lower pressures in the indoor footwear.

^c Cut-off for p values to be considered statistically significant was corrected for multiple testing with the Holm–Bonferroni method; with the smallest p value (p = 0.025) being larger than its corrected cut-off (alpha/16 = 0.05/16 = 0.003125), all differences are considered statistically not significant.

^d Data expressed as n.

Qualitative evaluation of requirements

All participants reported the indoor footwear to be easy to don and doff, lighter in weight, acceptable in cosmetic appearance, and comfortable to wear. Pedorthists in the multidisciplinary project group estimated that the indoor footwear could be fabricated for around 350 Euros per pair, excluding any overhead costs, and provided the indoor footwear could be manufactured with the existing last of the regular footwear. Detailed breakdown of all costs associated with producing the indoor footwear was outside the scope of the project. Other requirements (heat regulation, safety, shear, durability, and adherence) could not be assessed in this study.

Discussion

To improve adherence to wearing custom-made footwear, and ultimately prevent foot ulcers in people with diabetes, we developed custom-made indoor footwear via a multidisciplinary systematic design and evaluation approach. We integrated perspectives of users (people with diabetes at moderate-to-high risk of ulceration in possession of custom-made orthopedic footwear), clinicians, pedorthists, researchers, and insurers, and incorporated their needs and expectations in a set of 12 criteria the indoor footwear needed to fulfill. The primary criterion (i.e., similar offloading capacity as regular custom-made footwear [4]) was assessed quantitatively in a group of nine users, and we found this criterion to be met. This indoor footwear is thereby a new offloading device in the armamentarium for diabetic foot ulcer prevention.

We found the need for indoor footwear to be high, and participants expected such footwear to increase their use of prescribed footwear. Participants indicated ulcer prevention to be the most and appearance the least important requirement for indoor footwear. Other factors discriminating indoor from regular custom-made footwear, according to user needs, were the importance given to easy donning and doffing, flexibility of the materials, and prevention of cold feet. Finding appearance to be the least important characteristic was unexpected, as this is often considered important by people with diabetes assessing their prescribed protective footwear [11,13,26]. Apparently, people impose different requirements on footwear that is made specifically for indoor use. This can be explained by indoor footwear not being visible for other people, and thereby not being seen as a visible representation of their disease in social situations [27]. Imposing different requirements on indoor footwear was also seen in the importance given to easy donning and doffing, a frequent complaint in relation to low indoor use of regular custom-made footwear, and flexibility of materials. The finding that the majority of participants indicated prevention of ulceration or skin irritation to be the most important factor is supported by the results in a recent multi-ethnic population in Singapore [28], but different from a study in a similar Dutch population, where only 5% of participants assessed this as a priority [11]. However, that study was done more than 10 years ago, and has led to increased education and attention for communication and explaining the need for orthopedic footwear in this population [11,16]. Despite the importance given to the ulcer protective characteristics of the footwear, we found that most participants leave their bed at night with their feet unprotected, and some never use their regular footwear at home, which is in line with earlier research [12,20,21]. As every step without protection imposes a risk of ulcer development, unprotected walking during the night is undesirable.

We used a set of 12 predefined requirements to design custom-made indoor footwear, based on these results and additional multidisciplinary input. The key requirement was for the indoor footwear to have similar offloading capacity as a person’s regular footwear, as plantar pressure reduction is the most important criterion for ulcer prevention footwear, and the indoor footwear is to replace the regular custom-made footwear inside the home [4]. We would have accepted up to 10% higher peak plantar pressures in comparison to regular footwear because of lower walking speed inside one’s house than either outside or in the gait lab, with subsequent lower pressures [29,30], but found peak plantar pressures to be similar in the indoor footwear. While it remains important to confirm this in a larger group of participants, we conclude that biomechanically and from a user’s perspective our design is fit for use in everyday practice.

The remaining requirements concerned a variety of usability, durability and outcome characteristics, and costs. While some of these requirements need to be assessed in studies with a longer follow-up and more participants (e.g., temperature regulation, safety, shear forces, durability, and – most importantly – adherence), all other requirements were satisfactorily met. Participants were positive about the indoor footwear’s usability, which may contribute to increased adherence [10,13,26]. However, investigating the effects of providing indoor footwear on adherence is beyond the scope of this study and will be assessed in another study. Also, adherence is a multidimensional phenomenon [31], and the importance attached to the various requirements of indoor footwear varied between individuals in our needs assessment. It remains important to consider individual preferences when discussing the provision of indoor footwear and whether such a footwear solution matches their needs.

The companies involved estimated that the indoor footwear can be produced for 25–35% of the cost of regular custom-made footwear, provided it is made on an existing shoe last, and in addition to regular custom-made footwear. Another reason for these lower costs comes from using less durable materials compared to regular custom-made footwear. This is justified as indoor footwear will experience less “wear and tear”. However, while most participants were willing to contribute financially to its provision, coverage of most production costs through healthcare insurance or systems alike, remains a requirement for implementation.

A limitation of this study was its potential for selection bias, with pedorthists inviting their patients for participation. However, participants were picked arbitrarily, without using a specific procedure, characteristics of participants and their usability assessments were similar to other studies [11,13], there was adequate variation in answers provided, and in the plantar pressure evaluation participants were their own controls. Second, while “not increasing shear forces” was included as one of the requirements, with no reliable measurement system available we were limited by not being able to assess this requirement [32]. Third, we were not able to quantitatively control walking speed during plantar pressure measurements at two locations. However, visual observations showed no difference in walking speed between regular and indoor custom-made footwear, and this was confirmed quantitatively at the one location where speed could be measured.

In daily clinical practice in the Netherlands, custom-made indoor footwear is occasionally already prescribed by specialists in rehabilitation medicine and manufactured by orthopedic footwear companies. However, large variation is present in design and production, and likely efficacy, as these are all made based on clinical experience and without following a systematic design approach. With the current study, we were the first to follow such an approach, integrating perspectives of users and professionals, and providing evidence for important biomechanical and usability requirements. The resultant indoor footwear is described within this paper with sufficient detail for others to also start implementing the prescription and production of indoor footwear that meets all requirements.

In future research, it is needed to investigate various clinical, behavioral, and product outcomes. Most importantly, this concerns investigating if indoor footwear indeed improves adherence to wearing prescription preventative footwear, as well as its effect on foot ulcer prevention. Factors such as climate, culture and religious beliefs might also play a role in wearing of and satisfaction with indoor footwear [18,19], and it remains to be investigated if these and other individual preferences can be satisfactorily dealt with in the current design.

Conclusions

People with diabetes at moderate-to-high risk of foot ulceration and in possession of regular custom-made footwear express a clear need for special custom-made footwear for indoor use, and expect such footwear to increase their adherence. Following a multidisciplinary systematic design approach, we designed custom-made indoor footwear with adequate offloading properties, better usability and at lower costs than participant’s regular custom-made footwear. This indoor footwear can be made in daily clinical practice, while its effect on footwear adherence and ulcer prevention needs to be evaluated in further studies.

Acknowledgements

We would like to thank the orthopedic footwear companies, their pedorthists and their management representatives for their time and energy dedicated to the project: Livit Orthopedie (Jan Pulles, Carlijn Renee, Esther Mik), Buchrnhornen BV (Frans Buchrnhornen, Bart van Heerebeek) and OIM Orthopedie (Sjakko Lieben, Yvette Kerkum, Cock Vergeer). We would also like to thank Diabetes Vereniging Nederland (Diabetes Association Netherlands) (Dayline Coffie, Olof King) and CZ Zorgverzekering (CZ Health Insurance) (Marjan Peelen, Tanja Bastiaansen, Dion van Bommel) for their participation in the project group, and MyOrthopedics (Vincent Bergmans) for providing all details on the materials of the indoor footwear.

Disclosure statement

The authors report no conflicts of interest. The companies who participated in the research manufactured the custom-made indoor footwear according to the design described in this paper. However, there is no protected intellectual property to any of the ideas presented, there are no ideas or outcomes withheld from publication because of commercial interest, and none of the companies or researchers has any protected financial interests in relation to the design presented.

Data availability statement

Data can be made available upon reasonable request.

Additional information

Funding

This study was supported by an unrestricted grant from CZ Fonds.