How Autoclavable Brushless DC Motors Can Simplify Surgical Instrument Design

Today, powered surgical hand tools are used in almost all surgical specialties: ENT, Orthopedics, Neurology, Eye, and Plastics. Initially pneumatic powered, surgical tools have now migrated to electric energy. Electric instruments offer greater performances than pneumatic ones and allow better power control, lower noise level, and improved portability. They only require a simple electrical outlet or small battery, whereas pneumatic tools need complex, bulky, and high-maintenance air supply systems.

Fig. 1 – Arthroscopic electrical bone shaver

The conversion from pneumatic to electrical power was made possible due to significant innovation made in the field of electric motors. The main challenges resided in designing a brushless DC motor that would be powerful enough, be extremely compact, and survive the repeated sterilization cycles needed for surgical hand tools.

What is an Autoclave Cycle?

The most common sterilization method used in hospitals is autoclaving, also called steam sterilization. During autoclaving, surgical hand tools are exposed to 100% humidity, 275 °F, and pressure variations for up to 18 minutes. Most autoclaves also have additional vacuum cycles to facilitate steam penetration and kill bacteria, viruses, fungi, and spores that can eventually hide inside the tool. Repeated exposure to moisture is what gives tools and electric motors manufacturers the most problems causing significant electric failures.

Different Approaches

Below are four various approaches taken by surgical hand tool manufacturers for the selection of DC motors for powered surgical hand tools.

The Disposable Tool

Fig. 2 – Pre & post vacuum class B autoclave cycle.

One approach is to use very inexpensive DC motor and plastic components. These single-use tools must be discarded after surgery. Hospitals have been very concerned of increasing their amount of hazardous waste and impacting their green initiatives. In addition, disposable tools are not always the most economical option, especially for surgeries that are performed multiple times per day.

The Non-Autoclavable & Non-Sterile Motor/ Battery Pack

Another approach is to use regular DC motors attached to a non-autoclavable battery pack. This requires the surgical staff to remove the motor/battery pack prior to tool sterilization. The first issue is that the motor and battery are non-sterile components — meaning the medical staff must follow a special process to add the motor/battery pack to a sterile tool, leaving room for user error. Surgeons have also been very concerned about using surgical tools that have non-sterile components inside. The second issue is that it is impossible to ensure the surgical staff will remove the non-autoclavable motor/battery pack prior to sterilization, which will ultimately result in premature electrical failure.

Redundant Seals

Figs. 3 and 4 – Left: Brushless slotted motor cross-section – (1) Winding; (2) Slotted Lamination; (3) Space available for winding protection, ie: coating, molding. Right: Brushless slotless motor cross-section – (1) Winding; (2) Lamination; (3) Air gap – No room for winding protection

Another approach is to use a regular DC motor permanently attached to the tool and try to seal it from the outside environment. In most cases, this results in very bulky designs due to the sealing redundancy needed to achieve satisfactory performance. The tool efficiency is also drastically reduced as dynamic shaft seals are added to the tool or motor shaft. This means higher current draw, which results in shorter battery life and increased tool temperature. Moreover, no sealing system is perfect — all will end up failing at some point.

The Autoclavable Motor Solution

The best design option is to use an autoclavable DC motor that can survive autoclave on its own, without the need of a redundant sealing system, thus reducing the tool size and keeping the sterilization procedure as simple as possible.

Only a very limited number of motor manufacturers are able to design autoclavable motors. The brushless slotted technology has been a key reference in the medical market for more than 20 years. By design, the slotted motor winding is already protected when inserted into the slots of the lamination stack. Additional coating or molding material can easily be added without impacting motor performances. (Fig. 3)

On the flip side however, brushless slotless motors are not a very good fit for autoclavable applications. The winding construction is such that it is exposed to the outside environment. Efforts to protect the winding with coating or molding will result in increased magnetic air gap, drastically reducing the motor performance and tool efficiency. (Fig. 4)

Portescap (West Chester, PA) is a supplier of autoclavable motors for powered surgical hand tools. Typical Portescap motor applications in the medical industry include arthroscopic shavers, sagittal saws, oscillating saws, orthopedic drills, medium and high speed drills, wire drivers, and staplers.

Recent test results show that Portescap autoclavable motors are able to survive in excess of 2,000 sterilization cycles, which far exceeds the useful life of a surgical hand tool. In addition to autoclavable features, manufacturers like Portescap offer complete motor customizations tailored around surgical hand tool manufacturer needs, including: shaft cannulation, cross holes, custom gear ratio, custom winding, pin connections option, and motor temperature optimization.

Conclusion

Design engineers should be very careful when selecting a motor for a surgical hand tool. While it might be tempting to select an inexpensive nonautoclavable motor, it may result in a more costly end product when additional sealing costs are taken into consideration. The motor selection will also have a direct impact on the tool reliability and servicing cost.

Most of the leading manufacturers of surgical hand tools now use autoclavable brushless slotted technology for their superior performances in autoclave and best-in-class power density.

This article was written by Simon Pata, Director of Engineering for Portescap, West Chester, PA. For more information, Click Here .