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.
Optimum characteristics of cutting tools
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.
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.
What other characteristics should we look for in selecting power tools?
- Customized Performance
Core power tools in Orthopaedics are :
- Drill Systems
- Bone saws
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
- 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.
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
- 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 Dr S Roberts , Mr G Ashcroft 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 orthopaedic 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 legislated guidelines. This constitutes a noise hazard and carries a significant, but unquantified risk for noise-induced hearing loss.
3. Aerosol / Droplet Infection
Precautionary measures against contact with infectious aerosols and droplets have to be mandatory during use of high-speed orthopaedic power tools. Every surgical suite attendant must wear personal protective clothing including surgical gloves, a water impermeable resistant surgical gown with long sleeves, a surgical mask, and full-face protection with a face-shield. It is recommended that the surgery be performed by experienced surgeons and anesthetists and sufficiently trained personnel to reduce operative time and the duration of exposure to infectious aerosols. Use of diathermy and power tools should be kept to a minimum. It is essential to wash the hands prior to touching the face, eyes, or nose after removal of protective apparel.
Power Tools ergonomics
Power tools ideally should be used wherever feasible. Motor power is better than muscle power. Yet design considerations and surgeon comfort and awareness both, are equally important towards delivering a good result and avoiding operator and equipment breakdown.
What design characteristics need be considered in their selection?
- Span: Recommended maximum distance between 2 handles of a span type tool is 3.5 inches for men and 3.12 inches for women.
- Curve:The curve of the handle should not be greater than 0.5 inches over its length.
- Diameter: Optimum diameter is dependant on hand size, grip required and task repetitions with the tool..
- Length: Minimum handle length for any tool is 4 inches and for tools used with gloves, 0.5 inches is added.
- Shape:The shape of the handle should allow the forces to be distributed over as large an area as possible.
- Handle: Although in some cases, a larger handle decreases the forces to the small joints of the hand, they may require the hand to utilize muscle groups for which the motor cortex in our nervous system may not be ideally suited.
- Ability to grip the equipment: The pressure of the handle should be borne over fat pads of hands avoiding mechanical stresses. Also handles with grooves often do not fit the shape and size of the users hands and thus may place pressure on the delicate structures of the hands. Non- grooved handles may actually be better ergonomically.
- Phalyngeal ends: These should prevent hands from sliding onto dangerous parts of tools that may be either hot or sharp. They should minimize need for tight pinching or grasping.
- Texture: Adequate friction helps to avoid fatigue. Rubber handles provide more friction for a good grip and reduce pressure overall.
- Vibrations: No more than 30 minutes of direct exposure to vibrating tools is recommended at a stretch and only at particular frequencies deemed safe.
Safety requirements and compliance standards
The process of designing in safety is a must to ensure that electrical medical products succeed in meeting both regulatory requirements and market prospects. Projects are often delayed because the product safety requirements are underestimated in the initial design phase.
The European Economic Council (EEC) publishes the Medical Devices Directives (MDD; 93/42/EEC; http://europa.eu.int/comm/enterprise/medical_devices/index.htm), which declare EN 60601-1 (identical to IEC 60601-1) a harmonized standard. For products complying with EN 60601-1, this declaration gives a “presumption of conformity” to a majority of the essential requirements of the directives. Meeting the essential requirements is a major step toward receiving a CE mark for a device.
Guidelines for the surgeon
The National Institute for Occupational Safety and Health has researched and has developed many helpful guidelines for hand tool use to minimize occupational hazards.
- Keep the wrists straight. Avoid bending or rotating the wrists.
Solution: Bend the tool, not the wrist. Many types of bent-handle tools are available. The 'handshake' is a neutral wrist position.
- Avoid standing still in one place when using a heavy tool.
Solution: Reduce the size and weight of the tool. To help avoid neck strain, keep the elbows low and slightly bent when using heavy tools.
- Avoid stressing soft tissues such as the palms of the hands or fingers. Tools that create pressure on the base of the palm can interrupt circulation and nerve function.
Solution: Look for well-designed tools with finger grooves that fit the worker's hand. Short-handled tools may be less stressful to soft tissues.
- Avoid tools that require a lot of grip force to use or to hold.
Solution: Certain tools are made with a compressible gripping surface. This is usually better than hard plastic.
- Avoid tools requiring finger grip.
Solution: Select tools that use a full-hand power grip.
- Avoid tools with sharp edges and pinch points.
Solution: Select tools that will not cut or pinch the hands or fingers, even when gloves are worn.
- Avoid tools that require trigger-finger action to operate.
Solution: Select tools with large switches that enable the tool to be operated using all four fingers.
- Keeps hands free from heat and cold. Excessive temperatures can affect manual dexterity.
Solution: If possible, change the work location and/or wear gloves.
- Avoid excessive vibration. Excessive vibration can have an adverse affect on circulation.
Solution: Select tools with features that control or limit the transmission of vibration to the extremities and whole body.
- Wear gloves that fit. Tight-fitting gloves can put pressure on the hands. Loose-fitting gloves reduce grip strength. Gloves reduce friction, making it harder to hold the device. They reduce the sensory input so that the power exerted by the operator on the tissue is difficult to judge. In general, gloves affect dexterity and the ability to grip.
Solution: Provide a selection of gloves of different sizes.
- Transmission and Prevention of Occupational Infections in Orthopaedic Surgeons, Kwok Chuen Wong, Kwok Sui Leung. The Journal of Bone and Joint Surgery (American) 86:1065-1076 (2004)
- Noise exposure in the orthopaedic operating theatre: a significant health hazard, Hamish Love. ANZ Journal of Surgery Vol. 73 Issue 10,Pages 836 - 838. Published online: 9