Great strides ahead - advances with leg orthotics promise sound future

TWI Bulletin, July/August 2001

Paul Burling obtained his Higher National Certificate in Production Engineering from Cambridge College of Art and Technology.

He has extensive experience in the project management of large commercial projects worldwide. These include in depth knowledge of composite material for military and commercial applications.

Orthoses, which are applied to the lower limbs, perform a variety of functions, which include full stabilisation of paralysed limbs during walking, or weight bearing to relieve loading on lower limb joints. As Paul Burling reports, whilst beneficial advances in design have been made in recent years, they have had to be achieved by empirical iteration because of a lack of information on the forces transmitted to ambulatory orthoses.

The now mandatory EC Medical Devices Directive (EC MDD) makes such an approach untenable in the future since it demands a technical file for new devices which incorporates design data. It is clear that complex combinations of forces and moments do occur in lower limb orthoses, but no comprehensive programme of monitoring has previously been undertaken because of the fundamental difficulties which have to be overcome. The commercial partners in the project were anxious to remedy the situation so as to make possible formal compliance with the EC MDD. They therefore sought collaboration with the research groups - all of whom have an established track record in lower limb biomechanics research.

Objectives

• Devise a means of monitoring forces and moments on lower limb orthoses which does not alter the characteristics of the device in its relationship to the patient, or interfere with the ability of severely disabled patients to walk.
• Ensure a compatible approach to data collection in the different centres monitoring patients for the project.
• Collect appropriate data from patients using a range of lower limb orthoses
• Establish the range of forces and moments which are applied to lower limb orthoses during ambulation by patients who have been routinely prescribed with these devices to enable or improve walking function.
• Publish this data in a form which is readily accessible to designers of new orthoses so that maximum benefit can be taken from modern materials and manufacturing technology.
• Promote use of the data for generation of more effective International Standards for orthotic devices, so as to reduce the mechanical failure rate and increase inherent safety.

Protocol

Data monitoring

The four Data Collection Centres (each consists of a clinical centre, orthosis manufacturer and gait laboratory) measured, fitted and reviewed a range of patients for whom one of the following generic types of orthosis had been prescribed:

- Ankle foot orthoses (AFO)
- Knee orthoses (KO)
- Knee ankle foot orthoses (KAFO)
- Hip knee ankle foot orthoses (HKAFO)

For each of these patients they subsequently implemented protocols agreed within the project for the collection of kinematics and kinetic data during ambulation. This involved three-dimensional gait analysis using appropriate instrumentation, specific retro-reflective marker sets devised for each category of orthosis and a six component force platform ( Fig.1). Each patient was monitored over four separate walks. In addition information relating to the orthosis specification and patient clinical data was recorded.

Data analysis

Results

Orthoses monitored

The number of patient reviews and orthoses monitored in each category were as shown in Table 1.

Table 1

A total of 205 assessments were successfully completed. The number within each orthosis category was a reflection of the referral pattern in the clinical centres associated with the gait laboratories. There was a variation of pathological groups in each centre, though all were specialists in treating severe disablement.

The difference in the number of patient reviews and orthosis assessments was because some patients were either unable to attend the assessment or could not complete the gait laboratory protocol.

Results presentation

Anatomical joints for which results are provided for each generic type of orthosis are shown in Table 2.

Table 2: Anatomical joints for which results are provided

The six data sets for the overall spectrum of ranges of the median of the maximum force and moment (in three planes) for the four walks of each patient for ankle foot orthoses, non-dimensionalised against patient weight and height are, by way of example, shown in Fig.2. For KO there are also six data sets ( ie one joint) for KAFO 12 data sets (two joints) and for HKAFO 18 data sets (three joints).

Implications for specific applications of data

Medio-lateral offset is of no consequence for flexion/extension moments and can be specified at some reasonable value consistent with the anatomy. However, since this offset does influence the valgus/varus, moment a limit of ±75mm has been applied.

Exploitation

• Application of the results by the SME Partners
• Wider dissemination for the benefit of all patients and clinicians

At the completion of the Project all data was made available to the SME Partners, which provides them with the opportunity to respond immediately to the EC MDD.

To satisfy the wider social objectives of dissemination a strategy for public access to the data has been established. The overall anatomical median maximum force and moment data for all patients using each generic type of orthosis will be permanently displayed as histograms of the ranges within which they fall on the following web site: www.ncl.ac.uk/crest/ORLOAD/

Individual or collective correlation of force and moment data against:

- Specific orthotic function,
- Patient pathology,
- Orthosis structural properties,
- Patient age range
- Others which may not yet be foreseeable

may yield important lessons regarding design requirements for particular applications. Since the potential combinations are very large it is impractical to post those on a web site without making it so unwieldy as to inhibit its ready and more general application.

Other analyses of the data, which might rarely be required include:

• Availability of maximum moments in orthoses with structures or hinges outside the 25mm sagittal and 50mm coronal plane data validity limitations.
• Median maximum moments specifically in the flexion or extension ranges of an anatomical joint.

To support these and other potential requirements all the data collected will be held centrally, with the non-dimensionalised force and moment data posted publicly on a web site. Each of the participating centres will offer an additional service for filtering the whole range of data produced within the project to a particular specification of correlating parameters, or to undertake further analysis to provide data which contributes to the design of orthoses which have structures outside the permitted offsets.

The detailed data is to be presented to EC MDD Competent Authorities so that they can consider the implications for incorporation in new Standards.

Key achievements

• First comprehensive database of forces and moments in lower limb orthoses
• First multiple gait laboratory collaborative project for collection of three dimensional kinematics and kinetic data in lower limb orthopædics
• Achievement of all project objectives to budget and on schedule
• Establishment of a practical means of public and professional accesses to the fundamental data
• Establishment of a European network of clinical centres, biomechanical research units and orthopædic commercial organisations which has a common objective for further research and development
• A clear strategy, via a collaboration agreement, for peer reviewed publication of the innovative methodology and fundamental data.