Power Tools In Orthopaedic Surgery – An Update

Power tools form the backbone of modern orthopaedic surgical procedures. Power tools have undergone many improvisations and modifications in the past two decades and have improved tremendously in their functionality and versatility. Though a natural extension of ordinary orthopaedic armamentarium they have progressed to form the third arm and many a surgeon would be lost without these in their quiver.

Orthopaedic cutting tools often receive little medical or even engineering attention beyond the considerations of size and shape. Medical power tools are unfortunately a replica of those used in the metal cutting industry and have evolved as “smaller powered version” of the same. The machining techniques and manufacturing technology deployed for these medical instrumentation are frequently modeled after the these industrial designs.

This 'off-the-shelf ' information is then applied to the requirements of a particular orthopaedic application, saving time and allowing more effort to be spent on improving implant performance.

1. Exhaust pathway for bone shavings and metal debris
Shavings produced by a fluted face-cutting tool have a very difficult flow path. Following the action of the instrument they must make a right angle turn, then follow a long and narrow channel before clearing the tool. Additional torque and thrust that must be applied to maintain cutting action and to compensate for the power required to push the shavings outside the cavity. The heat produced as a by-product of this inefficiency can also cause thermal necrosis and delay bone healing.

In contrast, a rotary plane has a short and direct flow path that doesn't create the problems described above. Torque, thrust and heat are reduced as a result with lesser damage to biology.

2. Proper cutting edge geometry
Cutting edges intended for metal cutting must be very strong. The high tensile strength of metal requires a blunt angle at the cutting edge in order to resist the high forces generated during cutting. Since bone has a much lower tensile strength than typical metals, cutting edges intended for bone can have a much narrower angle and, as a result, will cut with less force and heat.

3. Sharp, burr-free cutting edges.

4. Metallurgy
The tools ability to retain a sharp cutting edge hinges on various factors. Hardness is only one parameter. The other is corrosion-resistance.

A tool made from type 420 martensitic stainless steel hardened to Rc 52 holds an edge longer than a tool made from custom 455 precipitation hardening stainless steel at Rc 52. This is because the hardening mechanism for martensitic alloys produces very hard carbides that are dispersed throughout the material.

Regardless of the alloy selected, cleaning and passivation should immediately follow finishing in order to maximize corrosion resistance.

• Power
• Simplicity
• Versatility
• Customized Performance
• Standardization

Core power tools in Orthopaedics are :

• Drill Systems
• Bone saws
• Burrs

Common power accessories used are :

• Power Pulse Lavage systems
• Cast cutters

Drill systems are classified and selected on the basis of use in small/large bone surgery and based on specific applications like orthopaedics, neurosurgery or maxillo-facial applications.

Various parameters considered are :

• Speed : available from 10,000 - 95,000 rpm
• Torque: Ideal is around 2.5 inches/oz.
• Debris channeling troughs: rotary plane type
• Size
• Weight
• Versatility and attachment combinations possible
• Reversibility of directions for drilling
• Heat profile
• Duty cycle
• Flexibility - Flexible or rigid
• Design of the instrument - ergonomics
• Throttled or throttle-less
• Teeth configuration - diamond tooth, serrated etc.
• Autoclavable options
• Low or high profile

Various characteristics determine saw selection. They are:

• Power Source: Pneumatic, Battery operated or Electric
• Positions of blades possible
• Compatibility with different selections of blades
• Compatibility with other attachments
• Throttled or throttle-less
• Gear ratio - 5:1 is optimum

Various specifications available are:
Micro – sagittal saw: 0-25,000 cycles per minute (cpm)

• Throttle controlled
• 90-110 pounds per square inch (psi)

Micro- oscillating saw: 0-35,000 cpm

• Throttle controlled
• 90-110 psi

Micro- reciprocating saw: 0-22,000 cpm

• Throttle controlled
• 90-110 psi

Pneumatic extended oscillating saw: 0-35,000 cpm

Burr functionality is determined by ball-tip design and configuration. Several designs are available depending on the procedure for which the burr may be required. Some configurations are:

• Barrel Tip Carbide
• Cross cut fissures
• End/Side cut carbide
• Lindemann Carbide
• Round cutting carbide
• Round diamonds
• Round end carbide
• Shannon 44, stainless
• Burr selection depends on application whether orthopaedic, neurosurgical or maxillofacial.

What to look for?

• A non-clogging design
• Fracture resistant spray stem
• Non-brittle suction tip
• PVC-free construction
• Ergonomically designed hand press
• Non-kinking hose preferably polyurethane
• Variable speed trigger
• Suction pressure: not exceeding 100 psi.

Requirements:

• Balance between weight and power to reduce user fatigue