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Abstract '99 del Seminario "Tecnologie al servizio dell'uomo"

Introduction

Generally in prosthetics and orthotics we assume component failure to be the complete fracture or destruction of a particular component, as in the case of this SACH foot, usually after some period of use.

Figure 1 - Structural Failure of prosthetic SACH foot

Failures of components however can be categorised into two forms.

1. Structural failure - which involves the fracture or destruction of a component.
2. Functional failure - although there may be little or no evidence of any structural problem , the component no longer performs the function that it should. Interestingly the dictionary definition of failure is non-performance.

Ideally a regular prediction, check and review process can monitor and help predict failures.

Regular inspection and servicing can also increase components like. Unfortunately in many cases this review can only be conducted when the patient themselves contacts a clinic having detected a problem. Most of us would be familiar with this situation.

PREDICTION

Fatigue is the most common cause of prosthesis failure and whilst this is difficult to predict, it is not impossible to plan for. The International Standards covering the components used dictate that the device should be able to withstand between 1-3 million cycles. This can be extrapolated to the number of steps a patient would take in a given time period and thus dictate a simple prediction of when the device should be reviewed, inspected or replaced. Large scale fatigue studies are being carried out by REHAB Tech, and these will form the basis of recommended inspection intervals through the TechGUIDE.

Brief Overview of Component Failures

A review of prosthetic component failures reported to REHABTech has revealed some interesting features. Failures include both functional and structural failures. Components like cars will eventually fail. These failures include predominantly prosthetic and orthotic failures, however mobility aids and wheelchair failures have also been investigated.

Figure 2, Reason for device failure

The main causes for failure are inherent design problems, poor assembly or misuse of the product or a combination of the two.

The main causes for failure are inherent design problems, poor assembly or misuse of the product or a combination of the two. The large number of design problems is amplified if the same component is reported several times ( generally the item has been taken off or modified to rectify the inherent problem by the manufacture). Design faults also include using inappropriate items being used for the device.

Assembly or misuse problems include poor clamping, grinding pylons, over torquing components and infrequent inspection of the component. Several of the components were simply worn out. While manufacturing faults were small, reflecting the quality of the products for sale fatigue was apparent in most failures whether started from assembly, manufacturing or design faults. In nearly all fatigue cases the item failed prior to being replaced, indicating there is little routine inspection of devices taking place.

Figure 3- Main prosthetic component failures

Feet-ankle components and knees are the most common components to fail. This would be expected as they carry the highest loads and movement. They also are the more complicated components. The proportion of silicon liners is very high considering their low number in actual use.

For prosthetic components 43 % of reported failures occurred on components not included in the TechGUIDE. As the vast majority of components used in Australia are included, this indicates that there is a far higher chance of a component failing in use if it is not in the TechGUIDE

Patient weight does not seem to be a dramatic factor in the component failure. For patients over 60 Kg's the number of failures represented is probably more an indication of the relative populations in these groups.

Figure 4, Patient weight at failure

Failure Reporting

All component failures should be reported. To assist this REHAB Tech has created a service fault and breakdown report is available our website http://www.monash.edu.au/rehabtech

The advantages of informing REHAB Tech of component faults include:

1. Notification to manufacturer of common faults - allowing for better designs in the future.
2. Prevention of possible accidents at other prosthetic centres.
3. Receipt or acknowledgment to reporting centre.
4. Failure investigation by REHAB Tech.

5. Results are reported with recommendations to the clinic, service providers, suppliers and manufacturers and maybe included in TechLINK an industry newsletter.
6. Information data base of components and their structural suitability for different conditions.
7. All failure information is regarded as confidential and only recommendations describing generic components will be made public. The exception is were a component has been recalled by Therapeutic Goods Administration, Australia.
8. Component failures can be avoided before they actually break. Using the correct component with correct assembly will help, but the most vital parameter in safe device usage is regular inspection.

EXAMPLES

1. Fracturing of this plate which is the top of a kneecage. This component would not have passed full a flexion test to international standards.

2. The failure in this component shows a fatigue crack of almost 70% of the cross section. This fracture meant that the leg would totally disarticulated from the socket. This would easily have been noticed if the knee had been serviced at some stage. The fatigue failure of this crack indicates that the crack had been growing for a considerable amount of time prior to failure and should have been evident by the size of the cracks involved. 
3. This 'al' pylon failure shows tell tale signs of being clamped in a vice and ground up the top, probably to fit into an adaptor. This would seriously compromise the surface temper of the pylon increasing the chance of failure.

The failure has cut straight through the pylon because these components were used in conjunction with a torque absorber.

FAILURE IMPLICATIONS

This brings us to the implications of these failures.

The implications are two fold

The first technical/safety implication involves aspects such as

• Economy, - how to ensure products have long life,

• Safety for the patient and avoidance of inconvenience and harm.

The second implication involves the legal legislation such as the product liability act.

this act places the responsibility of failure on the practitioners, distributors and manufacturers of the prostheses or products in our case the distributors, prescribers and prosthetist/orthotists.

INVESTIGATIONS

A fatigue tester has been deigned and built at REHAB Tech which mimics gait. A prosthesis can be put into the machine and is 'walked' with a certain load. This introduces a complex loading pattern rather a single load direction. (This is set up in a conventional load tester, whether testing static or sinusoidal fatigue load.)

Figure 5 -Fatigue Tester

Figure 6 - Conventional load tester

A graphic illustration of the requirement of service intervals was briefly conducted recently at the REHAB Tech.

If serviced every 50,000 cycles by simply relubricating re-torquing bolts and loctite all the relevant bolts, a knee being evaluated showed minimal wear and thus could continue to be used. However, if the same knee was allowed as is , after only 100,000 cycles, twice as much as the service intervals , it had worn almost beyond being serviceable.

Destructive testing can give us an insight into product's compliance to international structural standards and the service and maintenance intervals that a component might require. This compliance to standard is of increasing importance and is really the responsibility of the users of the components.

This does not however, help up to investigate components already in use.

Non-destructive testing is the ability to investigate a component without structurally altering the component.

Figure 7- Dye penetrant test, UV lamp box

In routine inspections at the REHAB Tech, dye penetrant crack-detection method is used to assess components. This technique means that small cracks which may be otherwise undetected will glow because they will fill with fluorescent dye.

Cracks have been detected and components replaced prior to any further problem being caused to the prosthesis or patient.

RESEARCH

STEP-COUNTER FOR PROSTHESIS

1. Simple event counter
2. Load Sensitive counter

Simple event counter. (Figure Figure 8)

Load Sensitive monitor – using a Load cell (on display)

Figure 8 - Simple event Counter

Figure 9 - Load cell Monitor

Results

Clinical and mechanical considerations are dominated by knowing the usage profile of a particular patient or prosthesis. It is only on this type of basic information that we are able to make professional decisions.

Future- Why measure load?

With Technologies such as Osseointegration where the prosthesis is fixed directly to the bone, the ability to monitor use of the device and measure the loads that are exerted will become critical.

Figure 10 - Osseintagrated Transfemoral prosthesis (courtesy Branemark Osseointegration Cntre, Sweden)

Fatigue

Commonly used components are being analysed and inspection intervals based on fatigue life calculations are being determined. This is a large project involving a number of different components and cross correlation with both fatigue tests and reports from the field.

Details On Maximising Component Service Life

When deciding the service life of a component we need to know the history of the component, i.e: how many times it was loaded and unloaded, not just how old in terms of weeks, months or years. This does not necessarily depend on whether it was used on the same patient. A detailed process has been established by REHAB Tech as part of its education of the Industry.

CONCLUSION

In conclusion there has been a steadily increasing demand on the REHAB Tech for investigation of failures both from prosthetic/orthotic facilities and legal representatives of claimants against prosthetic/orthotic facilities within Australia.

In most cases there has been little evidence of any form of inspection or investigation techniques. As the prosthetics/orthotic industry moves towards a more self regulating environment knowledge in this area will have increasing importance.

It is only though the adherence to international standards and regular inspections as part of industry codes of practice that component failures and their implications will be minimised in the future.

Generally industry codes which call for established inspection intervals ,compliance to recognised standards and establishment industry investigation techniques are just as applicable in the area of prosthetics and orthotics in relation to component use, manufacture and service and review of patients.

REHAB Tech is assisting by establishing an information and education network for the inspection and investigation of components.

Bill Contoyannis

Manager/ Rehabilitation Engineer

REHAB Tech

c/o C.G.M.C.

260-294 Kooyong Road

Caulfield 3162

Melbourne AUSTRALIA

bill.contoyannis@eng.monash.edu.au

Contattaci tramite e-mail: ilitec@inge-ns.ing.uniroma1.it