New Advances in Power Tool Technology

Today, most manufacturers of surgical power tools adequately meet the performance requirements expected by surgeons in large bone, reconstructive surgery, trauma and sports medicine procedures. But as new industry players enter the power tool market, the focus is moving toward technological advances in areas such as ergonomics, motors and batteries, and the interchangeability of handpieces, power sources and attachments. In this context of change, new benefits that arise are not always widely known, hence this review.

In a previous issue of this journal, Dr. E. Marlowe Goble, M.D. spoke of the “rhythm” that a surgeon develops to help him move through a procedure. (See “Surgeon Interview: E. Marlowe Goble, M.D.,” U.S. Orthopaedic Product News, November/December 2006.) The characteristics of the power tools used by the surgeon are crucial to achieving that rhythm. The latest generation of power tools to be available offers greater precision and accuracy than ever before, and a wealth of other features which enhance their ease of use and

value to surgeon and hospital alike. This article provides a convenient review of these factors, to help practitioners to choose the power tool range that most closely fits their needs.

Less Weight, More Power
How power tools feel in the hand and how they perform are key aspects of their usefulness. Tool manufacturers therefore need to devote particular attention to the design and actual shape of their power tools, so that they function as a natural extension of the hand. But power is important too, among other considerations to ensure that operating speed is maintained under all conditions. The ideal tool design would combine weight reduction with increased power, a requirement that represents a considerable challenge to manufacturers' ingenuity.

Yet the latest generation of power tool technology to become available does indeed offer smaller, lighter and more compact tools that virtually eliminate fatigue in use, with additional power to enable the surgeon to operate with greater precision, particularly in restricted areas. This characteristic enhances their compatibility both with minimally invasive surgery and with more demanding procedures such as hip resurfacing.

Versatility
A significant factor in the overall usefulness of power tools is reduction of the number of tools needed to perform the usual procedures. Traditional tool manufacturers offer a range of separate drilling, reaming, sawing and wire driving tools, while other manufacturers have opted for a more modern, simplified approach, offering the ability to drill, saw or ream with the same handpiece - for all types of bone work, including large bones.

With this universal approach, only the attachment needs to be changed. Products of this kind typically feature a time-saving, quick-release, quick-fit mechanism. Moreover, since different surgeons may have their own power source preferences for the tools they use (AC power, battery or compressed air), a further technical plus is the ability to use any attachment with any handpiece, whatever the power source, although not many manufacturers offer this degree of flexibility.

Simpler Administration and Lower Costs
A further benefit of the universal handpiece with interchangeable attachments lies in inventory management, and will be of particular interest to the administrator. A single, universal handset with interchangeable attachments can replace several separate devices. Therefore, fewer items of equipment (including batteries) need to be purchased, thus reducing investment and inventory costs, battery charging routines and maintenance requirements. It may be prudent to invest in back-up machines, but here again, adopting a universal system brings the number of devices to be purchased and managed down to one single handpiece.

Sterilisation can be a threat to battery life. To combat this, the latest trend is to provide drop-in batteries which are inserted under sterile conditions into a sterilisable battery box. In aseptic transfer systems of this kind, only the battery box needs to be sterilised in the autoclave. However, not all manufacturers offer this valuable money- and time-saving feature.

Electric Motors
A key area of technological change concerns tool motors, in which manufacturers' research and development efforts have recently brought about some particularly interesting technological advances. Most tools are powered by 9.6 to 12-volt direct-current electric motors, but a new generation of much more powerful, 18-volt devices is now available.

The earliest models used motors with carbon brushes. This relatively primitive technology was not always reliable and did not perform efficiently. Its successor was the brushless motor, a technology that replaced the purely mechanical contact between carbon brushes and commutator with an electronic system for breaking up the direct current into tiny impulses. These brushless motors were equipped with Hall sensors to adjust speed, but these are delicate components with many wires and soldered joints which have proved fragile, especially when sterilised in an autoclave. The most advanced form of motor now beginning to be used in orthopaedic power tools is the brushless, sensorless, fully electronically-controlled motor. This new technology enables a significant increase of the power available, combined with a finer degree of user control and improved reliability.

Battery Technologies
The largest share of the tool market is now represented by battery-powered instruments. The rechargeable battery technologies traditionally used for surgical power tools are the same as those employed in many other non-clinical applications. First, the nickel-cadmium or NiCd battery, invented as long ago as 1899, followed in 1983 by the nickel-metal hydride (NiMH) battery, and around 1990 by the lithium-ion battery.

One of the prime criteria for judging the value of a battery technology is its energy-to-weight ratio, or energy density. NiCd batteries achieve the lowest energy density while lithium-ion the highest. (See Exhibit 1.)

Exhibit 1. Graph of energy densities (Wh/kg) for different batteries. Lithium-ion battery technologies have over three times the energy density of NiCd batteries, without the drawbacks of the NiCd's memory effect.

A further consideration is memory effect, or a battery's tendency to lose capacity if it has not been fully discharged before it is recharged, a problem which is completely eliminated by lithium-ion technology.

Surgical power tools are therefore best powered by lithium-ion batteries, which over the years have proved beyond a doubt their superior energy density, low maintenance needs and the freedom they give to recharge them at any time. Lithium-ion technology is now used in many medical applications, and is recognised as being completely safe. It has also enabled an increase of the battery voltage to 18 volts, and thus improvements in motor power and efficiency, and elimination of the risk of stalling even in the most demanding conditions.

In cases where no charged battery is available during a procedure, it is of course desirable to be able to quickly connect an AC power cord with a step-down transformer and rectifier to the battery-powered handpiece. Some manufacturers offer this facility.

Pneumatic Motors
Around 25% of the power tools used are pneumatically powered. These have proved popular, as they are easily connected to the hospital's compressed air network or, in many surgical centers, to a tank. But traditional designs of pneumatically-powered tools suffered from various weaknesses. The technology has evolved little in recent years, and until now, pneumatically-powered tools have tended to be noisy and heavy on maintenance. Users of these tools should look out for the innovations in the pneumatic motor mechanism now coming to the market, offering reduced noise and maintenance needs, enhanced reliability and lower incidence of breakdowns.

Pneumatic motors consist of a cylindrical core which rotates within a chamber by the action of the compressed air on vanes protruding from the core. In the most modern pneumatically-powered tools, noise and wear are reduced in two ways. First, by siting the motor immediately behind the chuck, in the upper part of the handpiece. The position adopted in earlier models was in the handle, approximately at right angles to the direction of thrust, a design which required the inclusion of a system of gears. This increased both the noise level and opportunities for wear, and consequently represented a greater risk of frequent maintenance being required. Placing the motor in the upper part of the handpiece, in line with the chuck, obviates the need for gears, reducing noise levels and increasing tool reliability. Further noise reduction can be achieved by placing phonic insulation materials in the space thus freed up in the handle of the tool. Exhibit 2 shows an illustration of this type of design.

Exhibit 2. Section through a pneumatic handpiece with in-line motor. In-line positioning of the pneumatic motor reduces noise and wear, yet still enables cannulated drilling.

Additionally, the most advanced manufacturers of modern pneumatically-powered tools have implemented ways of reducing motor wear and noise by choosing a ceramic material for the chamber in which the rotor moves, and a fiber-reinforced, self-lubricating compound for the mobile components. The effects of this are to eliminate the need for lubrication, and to offer pneumatic tools that are lighter in weight, easier to handle, virtually silent and maintenance-free.

Experience, Vision and Innovation
Although the technologies employed at the “cutting edge” of orthopaedic power tools are largely stabilised, considerable advances have been made in the areas of ergonomics, motors and batteries. This article recommends readers to evaluate manufacturers' offerings against each of these technological advances. By incorporating these advances into the design of their products, manufacturers of the most advanced tools continue to show how their experience, vision and flair for innovation can make a significant contribution to the orthopaedic surgeon's professional excellence.

Jacques R. Essinger, Ph.D. has over 20 years of experience as entrepreneur and manager in orthopaedic companies. He founded Symbios, a Swiss implant company, and later joined Modex shortly after inception, which is now IsoTis, a public company focusing on bone substitutes. Dr. Essinger has also performed consulting for several orthopaedic companies in the power instrument area. He can be reached at jacques@essinger.ch.