No discipline is as precise as the design and manufacture of medical devices. No other function is as crucial in its precision. Exact measurements and tests are a critical necessity— from heart valves to knee and hip replacements, a fraction of a millimeter can be the difference between life and death for a patient. The process is a carefully choreographed sequence of matching design specifications by unit, during which each component is tested, then rejected and scrapped, or accepted. Each step in the process affects all the others that follow, and the process itself affects the quality of life for hundreds of thousands of patients who require a medical device.

Fig. 1 – A technician oversees the quality check process of a medical device during the manufacturing process.
Metrology plays a vital role in the process, measuring not just details, but the precision and correctness of the measurement itself. Metrology enters the process after design and instrumentation stages used to monitor and control the manufacturing process have been selected. The purpose of metrology is to juxtapose instruments with the same specifications with which they were selected. This determines whether or not false results have been reached. Instruments found to be within specifications, a conclusion called “in tolerance”, have a high probability for instrument error that is sufficiently small enough not to induce false results in decisions about the medical device product. If the instrument is found outside of control limits, a process called “out of tolerance” is initiated to investigate whether it caused a defect with the medical device product.

This is critical information. Errors at this stage can trigger false results: a false reject, which leads to a scrap or rework, and is costly to the manufacturer, or a false accept, which can—and has—placed faulty products on the medical device market, the cost of which is significantly higher, up to and including the price of human life. It’s happened. In the 1980s, more than 360 people died due to faulty struts in bileaflet mechanical heart valves. More recently, in 2008, a heart valve device was recalled after discovering a faulty ring that held the valve in place. Thanks to improved quality and testing processes, the defect was discovered in time to prevent human or financial casualties. (See Figure 1)

The variables and processes associated with medical device manufacturing, instrumentation monitoring, and control can introduce a number of errors, any of which, though tiny in actual size, can have serious consequences. Selection of the wrong tool, development of quality blind spots, and operator error are a few of hundreds of potentially disastrous scenarios, causing both false acceptance and false rejection. The danger in false results is that they are generally not evident during the process, which leads to uncontrolled risk, appearing as unwarranted out of tolerance nonconformance report investigations, scrapped product (materials and labor waste), reworked product that unnecessarily expends labor, and the recall of faulty product to avoid consumer injuries or fatalities. These and other faulty results will eventually erode profit margins, an undesirable consequence for all involved. How do these errors occur? What mechanisms can be built in to quality processes to prevent them?

What Can Be Done?

Fig. 2 – Precision instrument selection is crucial for accurate measurement. Shown here: a digital caliper, used to measure inside and outside diameter with accuracy 0.0025 in.
Introduce a two-step process to methodically stem the flood of faulty results: first, identify errors, and second, implement steps to prevent them. To identify errors, pinpoint occurrence spots and identify each error by type. For example, instruments must be calibrated against the specifications for which they were originally considered to be suitable for the intended purpose.

This conclusion may seem unlikely, but consider how errors develop. A manufacturing engineer selects an instrument for a step in the process in accordance with the instrument’s original equipment manufacturer (OEM) specifications. Separately, a calibration coordinator collects all instruments in a facility to calibration suppliers, following company procedures and approved suppliers selected by a separate team of supplier quality engineers. The supplier performs the calibration using their quality system procedures. No other information is shared between the four parties, each believing the others’ outcomes are correct.

The calibration supplier, however, used a military calibration procedure, which deviates from the OEM. Military procedure covers an array of instrument manufacturers and models, such as procedures for digital calipers, pressure gauges, and balances, but their general specifications differ from the OEM. The calibration supplier unknowingly altered the client’s need, and the calibration coordinator may not accurately assess the problem, leaving an in tolerance designation in place.

The manufacturing engineer doesn’t become involved unless there is an out of tolerance result, and likely doesn’t check calibration tolerances against OEM specifications. The quality engineers expect that calibration suppliers maintain quality processes and systems. This common example of calibration disconnect results in a quality blind spot. The mistake? Viewing the process as a singular, forward direction from raw material to finished product. At this stage, risk reduction is early identification of vulnerable parts of the process. Looking backward from the finished product, rather than forward and weighing decisions in the manufacturing process by points of disconnect is made simpler with tools such as 5-Why (See ) and Fishbone Diagrams ( ), which identify error sources that can result in false rejection or false acceptance.

Fig. 3 – A coordinate-measuring machine tip taking measurements with a variety of tips to choose from, based upon dimensions of component to be measured.
The major contributors to false test results are: process tolerance errors, inaccurate tool or instrument selection for the process, training errors, environmental factors, mishandling of instruments, calibration errors, lack of documentation, procedural or operator error, and quality blind spots (changes in process or calibration).

The second step in the process is implementing preemptive action steps for identified errors. A major manufacturer of heart-lung bypass machines discovered this the hard way. A calibration supplier did not fully calibrate a temperature instrument. For years, the instrument was found to be in tolerance, until a savvy new calibration coordinator began reviewing all calibration certificates, both those found to be in tolerance and out of tolerance. While examining the calibration data, the coordinator determined the instrument physically contained two channels with four thermocouple types on each, but that the calibration report showed just one channel with two thermocouple types included in the calibration.

Further investigation by the coordinator revealed the instrument was used on both channels in the process of checking the temperature of bovine blood coursing through the heart-lung machine, which meant that there was no measurement traceability in the second channel of the monitoring process. The faulty process produced an incredibly dangerous level of unacceptable risk, without a feedback loop to determine in or out of control product temperature measurements.

The results of the investigation revealed multiple calibration supplier discrepancies that a quality checks-and-balances system would have prevented. The manufacturer, therefore, decided to entrust its quality check systems to a supplier with more tightly controlled processes. The manufacturer also implemented a change in the instrument control process, documenting the requirements of calibration for each instrument and passing them along to the calibration supplier, as well as a thorough evaluation of the calibration results with personnel training on out of tolerance results and non-conformance investigations. This step removed previously unknown but harmful risks from the manufacturing process. (See Figure 2)

Starting with All of the Information

Fig. 4 – Medical device processes are extremely sensitive, as are the instruments used to measure them. Shown here: an outside dial caliper, with graduations at 0.001 in.
Provenance is the all-important starting point. Get there by answering a few simple questions: Where did manufacturing process tolerances originate? Who developed and implemented them? Process origins are not always traceable, but when they are, they often reveal errors at earlier monitoring and control points. Know the acceptance tolerances for the part to be measured or tested; this seemingly small detail lays the foundation for subsequent steps, and the selection of an accurate instrument to measure the part or to test the product relies upon individual tolerances. For example, when measuring the sphericity of a femoral head prosthesis with a tolerance of ±2.5 μm (±0.0001 in.), a caliper with the accuracy of ±25 μm (±0.001 in.) won’t work, but a high precision coordinate measuring machine (CMM) or vision system will. (See Figure 3) Using a caliper for such a precision measurement would be akin to holding a wet finger in the wind to forecast tomorrow’s weather. It’s just not suitable for the intended purpose.

Selecting the right instrument for measurement or control can be tough. Minimize the difficulty by becoming acquainted with select instrument manufacturers or distributors who offer make and model choices for instruments (they can also be helpful in choosing a subset of instruments). Faulty tool selection can lead to one of two negative outcomes: false acceptance or false rejection. Using the exaggerated example above, the following scenarios are possible:

A caliper’s resolution is insufficient to view errors in the femoral head component. The stated accuracy can be translated as: This instrument can wear or drift by as much as 25 μm, above or below the correct value, over the duration of its calibration cycle and still be considered suitable for measurements. Assume the caliper wears the full 25 μm to the lower end of the tolerance midway through its calibration cycle. This won’t be discovered until the next calibration is performed; therefore, it will be used continuously until discovery. What does that do to the measurement of the parts checked during that timeframe?

A change of this magnitude means that the operator could be led to believe the femoral head is at its nominal value of sphericity, when really, it is 10 times more out of round than its allowance. The direction of error in the caliper creates a reverse magnitude product error, which could have irregularities 10 times larger than intended. What does this falsely accepted product mean to the patient who receives this femoral head as part of a total hip arthroplasty? The patient will feel the irregularity as the femoral head rotates within the acetabulum. The severity of discomfort ranges from mild irritation to accelerated wear of prosthetics. The manufacturer is exposed to the possibility of product recall and litigation.

How many times have you seen someone use one of these chase threads in a freshly tapped hole? It’s not a deburring tool; it is a precisely machined thread gage used to measure a threaded hole on a previously tapped, reamed, deburred, and otherwise cleaned product. The chance of false testing results increases with each misuse of the instrument. Reevaluated, recalled, reworked, or scrapped product multiplies with calibration interval and use volume.

The final step in false testing result prevention is thorough training of employees. For this step, instruments must be handled with care and precision at all times to avoid further unknown circumstances. (See Figure 4)

What is the key to risk elimination? Be aware of the multitude of ways—large and small—that risk creeps into the medical device manufacture and test process. False test results are triggered by innumerable scenarios, but each result is unfailingly undesirable. As a result, print, digital, radio, and television outlets are flooded with ads from law firms that promise large settlements for failed medical device products. It’s a lucrative business that promises to change the lives of its clients. Efficient quality control processes can change those same lives in faster, longer, and more fulfilling ways.

This article was written by Howard Zion, Director of Service Application Engineering, Transcat, Inc., Anaheim, CA. For more information, Click Here .