Examining the natural behaviour of the feet with sophisticated software is at the root of ensuring best footwear design, as Enrique Montiel Parreño explains.
The Spanish Society of Foot Medicine and Surgery states that almost 70% of the population suffers a foot problem. This could be in the shape of pes planus, hallux, valgus, hammer toes or other conditions. While this may justify the use of customised insoles and/or footwear, this is especially relevant to those people who need to use safety/working shoes due to the type of activity they perform in their daily jobs, associated with certain risks.
National and European safety regulations oblige both employers and employees to make all the necessary efforts to avoid working accidents. Among those measures is the use of Personal Protective Equipment (PPE), which aims to minimise or eliminate the risk associated with each workplace. Safety footwear is PPE that protects feet against any potential risks. The objective is to reduce the number of injuries because severe traumatisms, amputations or even death can be avoided by using appropriate safety footwear.
Numerous injuries and morpho-functional and biomechaniccal foot alterations are treated with customised orthopaedic insoles. Problems like pes planus, pes cavus, differences in leg lengths (dismetry), knee injuries, calcaneus prominences, periostytis, aquilean tendinitis or plantar fascitis, are only a few of the problems treated by using orthopaedic insoles. Safety footwear, however, does not permit the incorporation of customised insoles without influencing a shoe’s fit. This can cause a worker who needs one to discard the use of safety footwear, raising the risk of severe accidents and their consequences.
Several efforts are currently underway to make the shoe industry human-centred by developing new concepts for customising or personalising the final products (Boer et al, 2004, Leng and Du 2006, Boer and Dulio 2007, Raffaeli and Germani 2009).
Science behind design
Clinical experience has shown that the most important biomechanical variables to consider for special footwear design are related to the plantar pressure during walking, the shear loading on the forefoot, the dorsal aspect of the forefoot and toes, and the vertical shear load on the posterior aspect of the heel (Van Schie et al, 2000, Cavanagh et al, 2002, Brown et al, 2004).
For the first group of biomechanical variables, experimental tests have identified how the high pressures under the first metatarsophalangeal (MTP) joint lead to ulceration. Thus, the corresponding biomechanical objective would be to minimise pressure under the first MTP joint. Similar considerations can be used to define the objectives connected to the other groups of variables.
Biomechanical objectives must be satisfied during the design phase of the last – the plastic mould used to make shoes on. This is possible if correlations between these objectives and a set of footwear design features are defined.
The design process of a customised shoe for a patient or worker starts from the analysis of medical and clinical factors. One of the key concepts in the last design process is the need to understand the interaction between the individual patient’s biomechanical foot characteristics and the different footwear design features used to specify the design of the shoe, e.g. material, dimensions and physical properties.
On the other hand, very often safety footwear is not fitted to workers’ needs from the morphologic point of view, preventing or inhibiting their use for people suffering from diseases like hallux valgus, aducted foot, hammer toes or diabetes. In most of the cases, users decide to employ footwear several sizes bigger than the one that would correspond to their foot length, causing problems inherent to the use of inadequate sizes.
The Spanish Footwear Technology Institute, INESCOP, a Spanish safety footwear company and a company specialised in the design and production of made to measure footwear and innovative products for the orthopaedic market, has joined forces to provide a solution for workers with specific needs related to safety footwear due to their feet.
Thanks to the integrated use of different CAD/CAM software tools for the design and manufacturingof lasts and 3D digital-optical foot scanners, it is possible to achieve the design and manufacturing of 100% customised safety footwear, providing the required security and comfort to workers using them every day.
The design of a shoe with a preventive function is based on rules that do not lead to an exact geometric match between the foot and last shape, but lead to specific relationships between foot and last features.
A specific design workflow has been formalised to obtain a customised last using worker/patient biomechanical parameters as inputs. In the safety footwear customisation process, two different levels have been defined according to the conditions the workers’ feet have:
- Soft customisation – workers with special needs at the level of functional foot, so their needs are associated mainly to customised insoles and safety footwear with interior volume enough to incorporate the insoles without loosing fitting. This accounts for the vast majority of cases.
- Hard customisation – workers with severe foot alterations and morphologies that prevent them the use of standard safety footwear, so they require 100% customised footwear and insoles.
In both cases it is possible by using digital systems, to offer a solution to workers at relatively low and affordable costs. The procedure followed is as follows.
Data capture of workers’ feet
If the use of a 3D digital scanner is not possible, specialised trained personnel take the following data:
- Foot mould with a polyurethane sock quickly reactivated with water and blue print to determine the plantar overpressure points and zones. The mould is then delivered to the orthotic lab for insole design and manufacturing, or shoe design and manufacturing
- Report problems the worker shows, determining zones and type of problems. The figure above shows a typical prescription form
All this information is used by a specialist – a podiatrist – to define a treatment, a customised insole/footwear.
Through the use of an optical 3D scanner, the worker’s foot geometry is obtained in a digital way (or even from the foot mould) in a few seconds, transforming this geometry in a file that permits further use and manipulation in subsequent analysis and design processes – customised last and insoles. Some of the advantages that this system offers are related to the digital files. The figure below shows a typical foot scanned.
Getting measurements and foot last matching
Throughout the use of several CAD systems which make the treatment of the digitised foot image, more than 27 foot measurements are obtained automatically to provide parameters to match foot data with existing lasts, and decide whether a hard or soft customisation is necessary.
The design workflow proceeds to a software tool used for retrieving lasts from a dedicated database and selecting the one to be used for the customer.
The database contains:
- The geometry of the last, i.e. a mesh in .stl format
- The characteristic measurements – last length, instep girth, or ball girth
- The footwear design features – heel height or apex angle
By comparing mould digitised/foot scans with specifically digitally designed lasts, which permit you to incorporate a customised insole without loosing fitting (mainly by adding extra volume in the bottom part), the first objective is to determine whether a worker’s foot dimensions can be placed inside one of the existing lasts (usually in a database), so the adequate size is selected and a customised insole is designed and milled starting from foot digital data obtained in the 3D scanner.
The pictures below show a typical digital foot last matching process. For the matching foot last, specific tools have been developed to permit correct alignment and positioning of the foot and last, as the two objects have to be precisely situated in respect of one another to make matching possible and useful.
Practicalities and purpose
The purpose of the software is to extract standard measurements from the geometrical shapes of digitised 3D feet or last models. The geometries are initially oriented in a canonical reference system through a repeatable process. The canonical system requires the sole of the foot to lie on the X-Y plane with the long axis of the foot aligned with the X-axis, and the leg aligned with the Z-axis. The proposed procedure accepts three types of geometries: shoe last, foot digitised on a plane and foot digitised raising the heel with a support.
Four main groups of functionalities are implemented in the software:
- Pre-alignment of foot and last, analysing geometric properties
- Positioning and alignment of foot and last according to a protocol
- Calculation of meaningful points which are then used to measure foot and last, and export of measurements and geometries defined during the measurement process
- To implement the alignment and measurement phase, characteristic points on the lasts and feet
In some cases, the analysis of workers’ feet leads to the decision of designing and making a unique and specific last for each person, considering specific geometry and injury/disease description, and prescription by a specialised podiatrist. This implies higher costs, as it means a specific last for each foot has to be created and the footwear model also has to be designed, in many cases a different piece for the left and right feet.
A specific case for the use of this procedure and technology is the case of adduct foot. This is a very common disease which affects one out of 1,000 people. The metatarsus adductus is a morphologic alteration affecting people from birth, generally due to their fetal position, and it consists of the adduction (deviation to the foot medial line) of the forefoot with respect to the rear foot. In a mild case the forefoot is deviated smoothly to the medial line. In the severe cases, the forefoot is deviated completely to the inner side, it is not possible to reduce it and there is a ‘groove’ separating forefoot from rear foot.
In this instance, the worker will have pain all over the foot, most specifically in the toes and the inner part of the foot. A high foot curvature prevents the worker from comfortably using safety shoes, which in general are quite stiff. This data led to the decision of designing and making a fully customised shoe, one for the left and one for the right foot.
Reconciling function with design
The new designed last respects the worker’s foot curvature and its morphology, also permitting the use of a 100% customised insole that improves the support of this specific foot type, which generates calluses in the external lateral part of foot.
The result is a shoe adapted to the specific needs and morphology of the worker providing all the necessary protective elements to perform his or her tasks.
The specific features of this footwear are (Type I):
- Objects drop with maximum impact energy of 196 Joules Toe blasting resistance: 1,500 Kgf static load • Resistance to folding • Resistance to corrosion – steel toe cap
- Toe blasting resistance: 1,500 Kgf static load
- Resistance to folding
- Resistance to corrosion – steel toe cap
To evaluate the time saving benefits for each pair of shoes, the proposed last design approach was compared with a traditional approach.
Significant improvements are found in all phases, from the footwear design feature evaluation to the final foot last fitting verification. When the approach proposed in this paper is effectively deployed in a footwear company and its customer chain, the effect on the final product will be a cost reduction. A quantitative evaluation is difficult to provide because the selling price depends not only on management processes but also on other factors such as material and delivery costs.
A general approach to manage shoe last development that innovates the entire process of customising shoes for people with diabetes has been proposed, describing the approach, the definition of the design framework and the results from the implementation of the design platform.
Analysis showed that:
- Matching foot last software allows technicians to orient a foot and a last using a protocol which goes beyond a simple geometric analysis
- The software allows feet and shoe lasts to be measured to evaluate their fitting, according to a set of measurements defined by experienced technicians. Using this tool, the time taken for foot/last fitting is drastically reduced
- The technology integrates measurement software, allowing the lasts to be rapidly modelled according to the foot pathologies. Specific modelling commands are available to design the last without using the base commands provided by the CAD system
Development of more integrated safety footwear design tools requires further research on the definition of outsole and insole measurements. By expanding the set of measurements, a new concept of foot/last/sole/insole fitting will be formulated.
Published: 04th Sep 2013 in Health and Safety International