As an increasing number of patients enter the operating room, more and more orthopedic surgeons are becoming orthopedic patients themselves. According to a survey entitled “Occupational Hazards Facing Orthopedic Surgeons,” featured in the March 2012 issue of The American Journal of Orthopedics, orthopedic surgeons are subjected to a multitude of occupational hazards during surgeries, including injury to back, neck, shoulders, arms, and hands. In fact, 66 percent of the orthopedic surgeons surveyed had neck and lower back pain, 49 percent experienced shoulder pain, and 26 percent had wrist pain. More specifically, 24 percent had rotator cuff pathology and cervical disc herniation, 11 percent had carpal tunnel syndrome, and 20 percent had lumbar disc herniation.
For decades, the customary strategy to help orthopedic surgeons alleviate surgery- related strain (and extend their careers) has been to add motorized power to instruments used in conventional orthopedic procedures, such as hip and knee replacements. However, adding power to instruments used in spine surgeries, for example, thoracolumbar, cervical, etc., has been notably less common. Because the bones involved in these surgeries are in close proximity to vital nerves, the concern has been that surgical instruments with too much power or torque could have dire consequences for the patient. Consequently, most spinal surgeons use manual devices to ratchet screws and drill holes into the vertebral column. This technique is not only physically laborious, but it can also precipitate carpal tunnel syndrome and injury to the elbow and rotator cuff.
As surgeon injuries have become an increasing issue, more medical device makers have started committing greater resources to adding power to surgical instruments used in a wide-range of orthopedic procedures. For example, in 2012, Medtronic developed a powered surgical instrument (POWEREASE™) for use in minimally invasive spinal surgery. By adding power to their system, Medtronic has been able to help spinal surgeons spend 51 percent less time tapping the pedicle, 55 percent less time placing screws, and experience 38 percent less “wobble” during reconstructive spinal surgeries. As a result, they are leading the field in helping reduce the physical strain that, over time, can limit the ability of highly skilled orthopedic surgeons to perform these surgeries.
However, adding power to a surgical instrument is not a straightforward, uncomplicated process. There are multifarious factors that must be considered before deciding on what motor technology to incorporate into the system. Making this determination without sufficient understanding of key fundamentals can result in a device that hinders surgeons instead of helping them. The following points should be carefully considered and evaluated in order for medical device makers to select the right motor to support their powered surgical instruments.
Different procedures require different power and torque. Therefore, it is essential that a design engineer understand the powered surgical instrument’s clinical need and application before it is developed. This may require the engineer to investigate how bone mineral density (BMD) can be different depending on the age of the patient and how BMD can vary in different parts of the human body, e.g., upper extremities, oral maxillofacial structure, femoral and tibial bones, etc.
For example, an oral maxillofacial plating system might require 30-40 inch ounces of torque. Whereas, inserting a bone screw into the spine may require more than 100 inch pounds of torque. While the industry has developed standards for measurement of torque requirements for different types of bone screws, the design and development team may need to research consistent lab practices and get actual bone studies in order to truly comprehend what speed and torque is required for a specific application.
Once the power requirements are fully understood, it is also important to discern the actual performance over time for the specific application. If a surgical instrument needs to run for long periods of time or be used over and over, then the corresponding power supply, e.g., a battery pack or an A/C adapter, needs to be matched with the motor.
Motor selection greatly influences the form factor of a surgical instrument. A narrow, pencil grip device that requires 40 inch ounces of torque will need a smaller motor to fit with the smaller design. On the other hand, instruments that demand a higher torque, such as a large screw driver or bone saw, require a larger gearbox to house a bigger motor. In addition, higher torque instruments may require a pistol grip handpiece for better control of the instrument. Shown in Figure 1 is an arthroscopic shaver handpiece.
Total Duty Cycle
Total duty cycle refers to the number of times an instrument is used in a surgical procedure before it fails. Essentially, it is the shelf life of the instrument. Typically, most users (surgeons) expect at least a one-year duty cycle. However, the type of motor that is used can greatly affect a device’s duty cycle. For example, a brush type motor tends to lose its integrity over time as the brushes heat up and become worn.
Conversely, brushless motors tend to have higher durability, more torque per weight, increased efficiency with more torque per watt, reduced noise and electromagnetic interference, and less erosion to the commutator because ionizing sparks, which are often generated by the brush, are eliminated. In addition, the design of brushless motors is such that the motor’s internals can be entirely enclosed and protected from dirt or other foreign matter.
Understanding whether or not the device needs to be autoclaved is also a fundamental element to consider when selecting a motor. If it does, to ensure long-term reliability and performance, it’s critical that the internal design of the system doesn’t affect the motor when it’s exposed to saline, water, solutions, etc. The heat and steam used in the autoclaving process can be quite corrosive to the instruments, as can the detergents used in modern washing and cleaning systems. So, in addition to an in-depth knowledge of the motor design, this requires a familiarity with bearing designs and seal designs.
The more the design team accounts for total duty cycle, the more reliable and cost-effective the instrument can be (less cost associated with repair, loaner instruments, and surgeon downtime).