4.3 Mechanical Cleaning: Washer-Disinfectors & Ultrasonics

Why Mechanical Cleaning

Mechanical (automated) cleaning uses equipment — primarily ultrasonic cleaners, washer-disinfectors, and cart washers — to clean instruments with far more consistency and reproducibility than manual cleaning alone. Machines apply the same time, temperature, water action, and detergent dose every cycle, removing the human variability that makes manual cleaning hard to validate. Mechanical cleaning also reduces staff exposure to bioburden and sharps. For most rigid surgical instruments, point-of-use treatment and any required manual pre-cleaning are followed by ultrasonic and/or washer-disinfector processing.

A device is only sent to mechanical cleaning if its IFU permits it. Powered hand pieces, certain electronics, and some delicate or single-use items are excluded from immersion or from specific cycles, so the IFU is always checked first. Sending a non-immersible device through a washer is a guaranteed way to destroy it.

Order of Operations

The two machines do different jobs and usually run in sequence. The ultrasonic targets fine soil trapped in crevices and lumens; the washer-disinfector delivers broad cleaning plus a validated thermal disinfection step. A common flow is: point-of-use treatment, then manual pre-clean of grossly soiled or complex items, then ultrasonic, then washer-disinfector, then inspection. Each stage assumes the device arrived already free of gross, visible soil — the machines refine cleaning, they do not rescue a neglected instrument.

Ultrasonic Cleaning and Cavitation

The ultrasonic cleaner is built to reach the places brushes cannot — box locks, serrations, hinges, ratchets, and lumens. It works by cavitation.

Cavitation Theory

Transducers in the tank generate high-frequency sound waves (commonly around 40 kHz) that pass through the cleaning solution. These waves create alternating high- and low-pressure zones. In the low-pressure phase, microscopic vacuum bubbles form in the liquid; in the high-pressure phase, those bubbles implode (collapse) violently. Each implosion releases a tiny burst of energy that dislodges soil from the instrument surface — including from tight crevices the solution can reach but a brush never could. This implosion-and-cleaning effect is cavitation.

Operating an Ultrasonic Correctly

• Instruments enter the ultrasonic already grossly clean (after point-of-use and/or manual pre-cleaning); the ultrasonic removes fine residual soil, it is not a first-stage gross-soil remover.
• Open all hinges, box locks, and ratchets, and disassemble multi-part instruments so cavitation reaches every surface.
• Fully submerge instruments; any surface above the liquid line is not cleaned.
• Do not mix dissimilar metals in one cycle — pairing stainless steel with aluminum, chrome-plated, or copper items can cause electrolytic/galvanic damage and electroplating of one metal onto another, ruining finishes.
• Degas fresh solution (run it briefly with no load) so dissolved air does not cushion and blunt the cavitation, and change the solution per facility policy and the unit IFU (soil-loaded solution cleans poorly).
• Most ultrasonics do not disinfect; instruments are rinsed afterward and move to a washer-disinfector or to sterilization.

Why Dissimilar Metals Fail Together

When two different metals sit in the same conductive bath, the more reactive metal (aluminum) corrodes preferentially while ions migrate and plate onto the other. The result is pitting on one instrument and a discolored, plated film on another — both are permanent. Sort loads by metal type before the cycle starts.

Washer-Disinfectors and Cart Washers

A washer-disinfector is an automated chamber that cleans and then thermally disinfects instruments through a sequence of timed phases, each at a controlled temperature with spray-arm water action. A typical cycle:

A0 and Thermal Disinfection

Thermal-disinfection lethality is quantified by the A0 value, which combines exposure time and temperature into a single measure of microbial kill. The widely used surgical-instrument benchmark is A0 600, which equals 90 C (194 F) for 1 minute — or, equivalently, 80 C for 10 minutes or 70 C for about 100 minutes. Higher A0 values are specified where greater lethality is needed. The pre-rinse is intentionally cold/cool because hot water early in the cycle would coagulate protein and bake blood onto instruments before the detergent could lift it.

Loading and Orientation

Correct loading is what determines whether the cycle actually cleans:

• Open all hinges, box locks, and ratchets so water reaches mating surfaces.
• Disassemble multi-part instruments per IFU.
• Place items so water drains freely — concave/cupped surfaces and lumens are angled or connected to flush ports so they fill and drain rather than trap water.
• Do not overload, stack, or nest items, and do not place solid pans that shadow (block spray from reaching) instruments beneath them.
• Use the correct manifold/connections for cannulated and channeled devices so the washer flushes the lumens.
• Place heavy items on the bottom rack so they do not crush delicate instruments and so spray reaches lighter items above.

Cart Washers

A cart washer is a large washer used to clean case carts, transport carts, rigid container systems, and large equipment — not delicate instruments. It cleans bulky items with high-volume sprays and heat, supporting the same separation-of-clean-and-soiled workflow at a larger scale.