Introduction
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.
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.
Optimum characteristics of cutting tools
1. Exhaust pathway for bone shavings and metal debris
Shavings produced by a fluted face-cutting tool follow a tortuous path for escape. 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 of bone and delay healing.
In contrast, a rotary planar tool 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 and improved chance of healing.
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 tool's ability to retain a sharp cutting edge hinges on various factors. Hardness is only one parameter. The other is corrosion-resistance.
What characteristics should we look for in selecting power tools?
- 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
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 surgical 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
Saws
Various characteristics determine saw selection.
They are:
- Power Source: Pneumatic, Battery operated or Electric
- Positions of blades possible
- Compatibility with different selections of blades and makes
- 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
Burrs
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 especially depends on application whether orthopaedic, neurosurgical or maxillofacial.
Power Pulse Lavage System
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.
Orthopaedic Cast Cutter
Requirements:
- Balance between weight and power to reduce user fatigue
- Ergonomically designed for the plaster room technicians for prolonged use
- Low noise
- Light weight
- Multi-speed control
- What are the risks to the orthopaedic surgeon from using power tools or vibrating machinery?
1. Risk from vibrating power tools:
According to studies by researchers Roberts S, Ashcroft G et al, orthopaedic surgeons are a unique group within the medical profession using vibrating hand-held air and battery powered tools in the regular course of their work. The vibration transmitted to the hand and upper limb by prolonged operation of vibrating tools such as orthopaedic saws can result in the disease known as Hand-Arm Vibration Syndrome (HAVS) in which individuals may complain of white finger, numbness or muscle and joint problems.
The results of their study shown are an eye-opener. Orthopaedic surgeons showed a significant excess of upper limb neurological symptoms related to exposure to the use of battery and air powered tools and peripheral nerve abnormalities were identified in the test group. This study confirms the possibility that orthopaedic surgeons may be a group at risk of developing the neurological and musculoskeletal symptoms of HAVS.
2. Risks from noise exposure in the orthopaedic operating theatre
Surgeons working in orthopedic operating theatres are exposed to significant noise pollution due to the use of powered instruments. This may carry a risk of noise-induced hearing loss. A study by Hamish Love attempted to quantify this noise exposure and establish whether this breaches occupational health guidelines for workplace noise exposure. He concluded that the overall total noise dose during orthopaedic surgery was acceptable but orthopaedic surgeons experience brief periods of noise exposure in excess of stipulated guidelines. This carries a significant, but unquantified risk for noise-induced hearing loss.
3. Risks from surgical smoke and aerosols generated by power tools
Surgical smoke is typically produced during procedures utilizing power equipment, lasers and other electrosurgical instrumentation.
