MANUFACTURING
Contact techniques
There is now a wide range of medical implants and an important requirement for their success is their form accuracy. The variety of techniques employed for the form measurement of medical implants can be divided into two broad platform groups: Coordinate measuring machines (CMMs) and dedicated form instruments. Historically, CMMs have been used for dimensional and positional control on components, however, they are increasingly finding applications in form characterisation.
A classical CMM would have a gantry structure with a discrete point probe. As the probe contacts a surface it would monitor the X,Y, Z position of that point in space using linear scales on each of the three axes. There are a number of potential problems when trying to monitor circular form using this type of CMM. They include
• no dedicated (circular) reference datum
• limited number of data points taken
• the degrees of freedom involved in attaining each data point.
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Figure 1 [6] shows the problem of acquiring too few data points when measuring form using a CMM system.
Despite this limitation, the current ISO standard, ISO 7206, Implants for Surgery, Partial and Total Hip Joint Prosthesis, Parts I to VIII, specifies this technique using 21 data points for monitoring “sphericity” on hip implants. However, manufacturers are increasingly aware that this is not an accurate method to employ for form measurement and they are moving to alternative techniques.
A recent enhancement of CMMs is the introduction of scanning probe heads. These systems are more suitable for form measurement because the scanning probe heads allow the probe to stay in continual contact with the surface and they can acquire approximately 500 data points/s. Some CMMs can move at speeds of up to 500 mm/s, however, they typically measure using the scan mode at less than 10 mm/s. This is because higher speeds can generate dynamic forces in the structure of the CMM and the probe itself, which can affect measurement accuracy.
Requirements for measurement
Before examining dedicated form measuring systems, it is worthwhile to review some of the form accuracies that are required on medical implants. In a classical hip implant made of cobalt chrome, the form would typically be controlled with a roundness (deviation) measurement around the circumference in the vertical and horizontal direction. Tolerance could be typically in the region of less than 5-µm deviation, however, manufacturers can achieve less than 1 µm. Given these rather exacting tolerances, some of the features required for an accurate form measurement system include
• an accurate datum
• the ability to accurately set up a component relative to a datum
• sufficient data points
• a gauge with good frequency response
• a gauge with good resolution
• low system noise.
Dedicated form instruments
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Figure 2. Typical form (roundness) measurement system.
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There are a number of dedicated form (roundness) measurement systems available with fixed single point inductive gauges and high precision rotational spindles that act as a datum for roundness measurement (Figure 2). These systems measure form deviation by bringing an inductive gauge into contact with a component and rotating it at speed. As the component rotates, the gauge monitors the deviation in form relative to the spindle datum.
Noncontact gauging
The techniques discussed so far are all contact measurement technologies. However, for medical implants, there is often a desire to use noncontact techniques because the substrates can be delicate (optical moulds) and even minor scratches would mean scrapping the product (orthopaedic implants). There are a variety of noncontact gauges available. Technologies include
• capacitance systems
• interferometry
• confocal gauges
• laser triangulation.
A confocal gauge is a single point measuring probe, which means it is an easier replacement to make on a roundness platform than, for example, an interferometric (areal measurement) gauge. They also offer good lateral and vertical resolution characteristics, which are typically in line with the form measurement requirements in the medical device industry.
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Figure 3. (click to enlarge) [7] Schematic of a confocal gauge.
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Confocal gauges work on a relatively simple principle (Figure 3 [7] and 4). White light is shone onto a beam splitter and directed down a fibre-optic cable onto a spectral aberration lens. This lens divides the light into its different wavelengths (red to violet). At a certain point on the surface only one wavelength will be in focus. All the light is reflected back through the gauge head onto an optical pinhole, which only allows the light in focus to pass through. The light then passes through a spectrometer grating, which deflects different wavelengths by different amounts to a charged coupled device. This indicates the height of the surface.
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Figure 4. Confocal gauge head.
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Latest developments
A recent development to assist medical implant manufacturers has been the introduction of confocal gauges onto roundness platforms. The combination of a precision mechanical platform, with a precise and fast noncontact gauge offers them a unique solution for form characterisation. Roundness platforms can offer spindle datums with form deviations less than 0.2 µm, and confocal gauges offer vertical and lateral resolutions of 10 nm and 2 µm respectively, with data logging rates of 5000 points/s.
Typically, dedicated form measurement systems offer the best solution for form (roundness) measurement on hip implants. Recent noncontact developments on these platforms help meet the requirements of medical component manufacturers.
Ciarán C. Murphy is Medical Business Manager at Taylor Hobson Ltd, 2 New Star Road, Leicester LE4 9JQ, UK, tel. +44 116 276 3771, e-mail: Ciaran.Murphy@ametek.co.uk [8], www.taylor-hobson.com [9]