12.05.07
Applying Reliability Concepts to Torque-Limiting Instruments
Successful testing and validation programs distinguish leading suppliers from others.
Chad Ryshkus
Teleflex Medical OEM
In the medical device manufacturing industry, providing a reliable product is paramount to the efficacy of the surgical procedure, the outcome of the patient and the reputation of the manufacturer. To provide the most effective products to the market, many OEMs will regularly leverage the expertise of their suppliers. Whether for a specific product or process, this reliance necessitates that the burden of reliability extends throughout the supply chain.
One area in which OEMs frequently leverage the expertise of their suppliers is in the design and maintenance of instrumentation used for orthopedic surgery. The development efforts of most OEMs typically have been focused on the implant and procedure and less on the instrumentation—and rightly so. Surgical instrumentation itself has been a highly specialized field for centuries dating back to the Ancient Greeks and, more recently, to the skilled German craftsmen who refined the use of stainless steel in the 20th century.
Torque-Limiting Instrumentation
Today’s technology has brought much advancement to instrumentation, including the field of orthopedic surgery. Of the numerous instruments used by surgeons during an orthopedic procedure, torque-limiting instruments are the result of applying a simple technology to a specific need. Torque-limiting instruments use a mechanism that limits the torque applied during the tightening of fasteners, such as implant screws. The mechanism, typically housed within the handle of the instrument, is designed to slip after the predetermined torque load is reached. The slip action of the mechanism is similar to that of other clutch mechanisms, such as impact wrenches and electric screwdrivers.
The ability to incorporate a positive limit on the applied torque load takes much of the guesswork out of the hands of the surgeon. Because of its effectiveness, controlling the torque load actually is a straightforward approach for most applications. Engineers can specify the tightening torque of threaded fasteners instead of the actual clamping force generated by the screw, which can be difficult to measure in the operating room. By relying on this method, torque-limiting instrumentation must demonstrate an elevated level of reliability. Performance anywhere outside of the narrow range of acceptable variability may compromise the outcome of the patient’s surgery.
For example, in a typical spinal rod fixation procedure, the efficacy of the implant hinges on the ability of a set screw to maintain its “bite” onto the rod to secure it into the saddle of the pedicle screw. If the set screw is too loose, implant failure is almost inevitable. If the set screw is too tight, the potential for implant breakage becomes a concern. Therefore, the performance limits require that implant designers test and validate an acceptable range of torque settings. This validation not only should incorporate the threaded connection, but also should account for the possible limitations of the driving instrument applying the torque load on the screw.
Reliability Assessment From Life Testing
The data analysis gained from a well-planned, long-term validation test study helps establish the performance limits of a product. Such validation testing ensures that the right product can be built by the supplier to match the performance specifications of the OEM. Testing and analysis are best done in the development stage of the product; however, mature products should not be overlooked as candidates for such testing. Significant product performance can be achieved through an understanding of their design limitations. The challenge for suppliers and manufacturers of orthopedic surgical instrumentation is to ensure reliability over a finite period of time. One way to assess the consistency of a torque-limiting instrument is to examine the accuracy and duty interval, or the useful “calibrated” life, of the instrument.
Accuracy
Duty Interval
Most torque instruments operate accurately for extended periods when no external factors are involved. Duty interval takes into account external factors, such as the field-use conditions an instrument most likely will experience. The result is a measurement that takes accuracy and turns it into a useful indication of performance throughout the lifecycle of the instrument. Loosely defined, duty interval simply is the time in which the instrument will operate under field-use conditions while functioning accurately within a predetermined set of performance constraints. These are the constraints for which the instrument will be validated.
The field-use conditions that place the most dramatic impact on the device should be included in the test protocol or the detailed plan for executing the test. For example, autoclave steam sterilization, which introduces pressurized (30 psi) steam at an elevated temperature of approximately 270°F, is one of the most influential contributors to performance degradation and almost always should be included in the protocol. In addition to this, device use patterns and other aspects of the procedure should be taken into consideration.
After testing has been conducted to simulate the life of the instrument, the resultant values of each test can be plotted on a graph to display the characteristic behavior of the test population. From this plot, the useful calibrated life of the instrument can be readily determined. The behavior of a torque-limiting instrument population throughout each instrument’s operating life can be characterized by a graph similar to that in Figure 2, which, due to its shape, reliability engineers describe as the “Bathtub Curve.”
Bathtub Curve
Early Life and Break-in
The first failure type to consider is early life failures—those units whose failure occurs immediately after initial calibration or a short cycle of use. The failure rate in this period quickly decreases, as those units that will exhibit failure will have already done so, and the remaining population should conform to acceptable parameters. However, if not caught, units that fail to stay within the specified calibration limits early in their lifecycle can be problematic. They have the potential to remain out of specifications until they are either removed from field use or sent back to the supplier for recalibration. The most effective incoming inspections at the OEMs are those that mimic or make use of the same procedures as the instrument supplier.
In the electronics industry, to increase reliability, a portion of the life of a device is consumed by a process called “burn-in.” In a comparable example—power supplies—part of the production process is to power cycle the device to “weed out” the units that will fail at the beginning of the life cycle. The result is a more robust product population that should be more reliable.
The same model can be used with torque-limiting devices. In the beginning of the design phase of the torque instrument, extensive testing is performed to estimate the duration of the break-in period. By understanding this duration, a break-in process can be developed to simulate real-world experiences by alternately exercising and autoclaving the torque-limiting devices. These units actually will get conditioned before final calibration. The conditioning cycle will decrease the potential for failure and increase the reliability of the device before it is sent out to the field.
Useful Life
Theoretically, if the break-in period is accurately determined, it can be expected that only random failures would have a significant impact over the course of the useful life of the instrument. The duration of this period is of particular importance to the OEM, as this is the time in which the instrument most likely is in the hands of the surgeon performing the procedure. Therefore, it is essential that OEMs have a tracking program in place to manage the use of the instruments in the field. These programs often have been overlooked as a valuable tool or poorly managed by OEMs in the past. OEMs should look for a supplier that can assist them in the management of the useful life of the torque instruments.
End of Life and Wear-Out
At or near the end of the useful life is where an increase in the rate of failure becomes evident. As the name implies, wear-out failures are those failures in which the torque-limiting mechanism cannot continue to operate accurately because of worn out components. The mechanical nature of torque instrumentation is one of the reasons the useful life of a torque instrument is of finite length. Wear-out failures can be the result of excess wear of the components of the torque mechanism, spring relaxation or lubricant breakdown. Even minor misalignment caused from the expansion and contraction of the precision components during autoclave sterilization can have a dramatic impact on the performance of the instrument.
Many of these issues can be mitigated by proper cycling in the break-in phase. Continuous product and process improvements by the supplier also will have a positive influence on minimizing the random failures and assist in maintaining performance levels throughout the useful life. Most suppliers will recommend a routine maintenance schedule for the OEMs to use as a guideline to keep torque-limiting instruments from reaching the wear-out period and avoiding any complications that may result.
Recalibration or Retirement
A sound recalibration or retirement program is just as critical as the reliable performance of a torque-limiting instrument. It is imperative that OEMs have systems in place through which they can quickly and efficiently retire, replace or recalibrate the torque instrumentation in the sets currently used in the field or distribution centers. With numerous versions of torque instruments employed in just as many diverse procedural sets—each experiencing dissimilar usage rates—designing an instrument management program that encompasses reliability factors can be daunting. Understanding the actual use of each individual unit can be quite difficult. Tracking methods such as barcoding or emerging radio frequency identification technology are making this task progressively easier to handle.
Recalibration processes that require the torque instruments to be out of service for an extended period can pose problems for OEMs, especially if replacement instruments are not readily available. A supplier’s ability to turn around a torque instrument efficiently and in the condition of its original manufacture is vital to the success of the OEM and reduces its exposure to the risk of lost revenue. A supplier’s recalibration program should be comprised of the same methods, procedures and documentation demonstrated in the original manufacture. OEMs that utilize a supplier-engineered torque product should be sure that the supplier has these standard procedures in place for handling such torque instrument recalibrations and refurbishments.
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Reliability is an important aspect of any product, but is an absolute requirement for torque-limiting instrumentation for orthopedic surgery. Unfortunately, some suppliers of instrumentation may view long-term validation testing to be burdensome. However, understanding and applying the concepts of reliability separates leading suppliers of torque instrumentation from the rest of the pack. With the success of an implant dependent on the instruments used during installation, implant manufacturers need to ask more from their suppliers. As industry testing and reliability validation programs become more critical, OEMs will need to seek those suppliers that have established successful testing and validation programs.