New trends in Ultrasonic cleaning – responses to the threat of prion transmissible illnesses

The need to thoroughly clean re-useable instruments as an essential part of the sterilisation cycle has long been apparent to those working in dentistry and allied fields. Pockets of debris and bio burden, if left on the surface of an instrument, can act as a protective shield to infectious agents during the sterilisation process, increasing the risk of cross contamination. Traditional hand scrubbing of instruments has been shown to be ineffective for removing debris, especially from equipment where hinges, bind holes, threads and fine fissures are common, such as dental instruments. Ultrasonic cleaning baths are generally considered the preferred alternative to hand scrubbing in the cleaning cycle. The Glennie report[1], during its review of sterile services provisions for the National Health Service (NHS) in Scotland found that over 90% of surgeries surveyed contained an ultrasonic cleaner.

Ultrasonic baths comprise of an ultrasonic generator connected to a number of piezoceramic transducers bonded to the base of a tank which operate by pulsing high frequency mechanical waves through the fluid within the tank. During the rarefaction period of the wave, the molecular bonds of the fluid are stretched beyond their tensile strength, causing a microscopic void, or bubble, to form. This void can then collapse violently during a compression phase of a wave; a process known as cavitation. The size of the collapsing bubble is dictated by the driving frequency of the sound wave input, and it is the formation and collapse of millions of these micron sized bubbles throughout the liquid that provides the pervasive cleaning action of ultrasonics. The implosion of each bubble forms a high speed, high pressure shock wave that removes contaminants without damaging the substrate of the instrument being cleaned.

Ultrasonic cleaning has been shown to be far more effective that manual scrubbing [2], whilst lowering the risks of sharps injuries[3] and can penetrate delicate features, removing debris and exposing regions of the instrument to fresh cleaning chemistries. Yet ultrasonic cleaners are only truly effective if used correctly; variables in the cleaning process such as fluid level, temperature, surfactant dosing, input power, frequency and above all cleaning cycle time can make a large difference to the cleaning efficacy. Traditional ultrasonic cleaners also make it possible to remove cleaning loads before a cycle has been completed and add items mid cycle, running the risk of a partially completed clean. Recent issues in health care have highlighted the need to remove as much variance as possible from the cleaning cycle to ensure each workload placed in an ultrasonic cleaner receives the same consistent, thorough cleansing.

The vCJD threat and industry response

During the 1990�s, the UK suffered a mass outbreak of bovine spongiform encephalopathy (BSE, or �mad cow� disease) in its cattle herds. This outbreak in turn was strongly linked to a marked increase in a fatal human brain disease, variant Creutzfeldt - Jakob disease (vCJD). Evidence mounted that the agent responsible for the illness in cows, a prion disease, was the same agent causing human mortality[4]. The unknown prevalence of the asymptomatic vCJD infection gave rise to concerns over cross infection in a wide range of surgical procedures[5]. Although not on the scale of the UK outbreak, cases of BSE have been reported in the European Union, the USA, Canada and Japan with cases of vCJD reported in Europe and the USA[6]. Out of growing concern over cross infection from HIV and hepatitis strains, and now with the added impetus of vCJD and the removal of glutaraldehyde-based disinfectants from the UK market, the government published a document giving health practitioners a set of guidelines for the correct cleaning and disinfecting of re-useable medical and dental equipment to help prevent cross infection, Health Technical Memorandum 2030 (HTM2030)[7]. Compliance to the standards was not only seen as a matter of preventing cross contamination, but also a litigation and disciplinary issue with the General Dental Council (GDC) noting that �Failure to employ adequate methods of cross infection control would almost certainly render a dentist liable to a charge of serious professional misconduct.�[8]

HTM2030 laid down the principles not only for sterilization, but also for the cleaning process prior to sterilization, identifying areas of the cleaning process that were not addressed by the majority of ultrasonic cleaning units in place at the time, namely validation and traceability. The Glennie[9] report found that 96% of surgeries using ultrasonic devices failed to check the efficiency of the ultrasonic baths or monitor the cleaning efficacy, ultrasonic baths can drift in performance over time leading to a marked shift in its ability to remove stubborn debris. Potentially more alarming is that a single transducer may not function properly or fail, leading to areas within the ultrasonic bath with little or no cleaning action whilst the noise from the other transducers leads the user to believe the bath is operating as normal. HTM2030 recommends that validation of the cleaning efficacy and the operation of the transducers be carried out on a daily, weekly, quarterly and annual basis. Cleaning efficacy tests are performed by measuring the ultrasonic unit�s ongoing ability to clean a protein test soil within a specified cleaning cycle.

Testing the efficacy of the ultrasonic transducers is performed using a comparative foil sample; ultrasonic baths produce cavitational energy of a sufficient force to pit aluminum foil samples. The placement of transducers, coupled with the geometry of the bath will lead to a distinctive erosion pattern on a foil sample suspended in the cleaning liquid for a length of time. By examining the number and positions of the foil erosions, a comparison can be made to earlier samples to determine the continuity of the transducers performance. A more quantitative method of examining the power output of the ultrasonic bath is to use an ultrasonic power probe capable of giving a numerical readout of the cavitational energy. A further part of the reform of the decontamination cycle was the traceability of the process. Keeping a record of the details of the batch provides assurance the cycle was completed satisfactorily and within all the correct operational parameters.

One response to the guidelines has been the design of ultrasonic cleaners compliant with all the recommendations of HTM2030 and to reduce the amount of variability possible in a cleaning cycle. Ultrawave, manufacturers of ultrasonic cleaning equipment, currently manufacture the only HTM2030 compliant ultrasonic bath available on the market with a locking lid and integrated validation printer. The Hygea 1250 incorporates a user display panel that allows the details of the operator and cycle variables to be programmed in to the machine prior to a cleaning cycle. A separate circuit ensures the transducers are functioning and an automatic locking lid prevents tampering with the cleaning load during operation and guards against airborne pollutants whilst lowering harmonic noise pollution from the machine. Sophisticated controls are also included that alert the operator of field variables such as over/under temperature conditions to prevent coagulation of proteins, low fluid levels to ensure all instruments are properly submerged and an automatic cycle abort should the lid be lifted during a cleaning cycle. All this is coupled with a paper print out of details of the cycle parameters to assist with the recording and traceability of the process as recommended in HTM2030.

Cross contamination of patients has long been at the forefront of the healthcare professionals agenda and the vCJD epidemic served to highlight some of the short comings in the cleaning and decontamination systems being used at the time. In response to this threat, the ultrasonic cleaning industry has adapted to meet the needs of the health care industry by developed technologies that can validate, monitor essential environmental variables and provide a permanent, traceable copy for records. The opportunity for human error and subsequent risk has been minimized as much as possible. It is by continuing in collaboration and discussing needs that health care professionals and equipment manufacturers will continue to improve equipment that not only safeguards patients but also the operators themselves.

This article was an invited editorial for US Dentistry 2006, published in September 06.

References:
[1] NHS Scotland: Sterile Services Provision Review Group: 1st Report (The Glennie
Framework).  (2001) Available online at: http://www.show.scot.nhs.uk/sehd/publications/sspr/sspr-00.htm
[2] Detwiler MS, �Ultrasonic cleaning in the hospital�, J. Healthc. Mater. Manage. 1989 Apr; 7(3): 46-50
[3] Andrews N. �Infection control � optimum instrument management� J. Practical Hygiene 2003(May/June): 34-35
[4] Hill, A.F., Desbruslais, M., Joiner, S., Sidle, K.C.L., Gowand, I.,
Collinge, J., Doey, L.J. and Lantos, P.L. �The Same Prion Strain Causes vCJD and BSE�, Nature, 1997 389: 448�50
[5] Taylor DM, Fraser JR. �The potential risk of transmitting vCJD through surgery�. The Journal of Hospital Infection. 2004 (April);44(Pt 4):318-319
[6] Jackson GS, McKintosh E, Flechsig E, Prodromidou K, Hirsch P, Linehan J, Brandner S, Clarke AR, Weissmann C, Collinge J. �An enzyme-detergent method for effective prion decontamination of surgical steel.� J. Gen. Virol. 2005 Mar;86(Pt 3):869-78.
[7] �Health Technical Memorandum HTM2030, Washer disinfectors�. The
Stationery Office; (1997)
[8] �Maintaining standards � Guidance to Dentists on professional and personal conduct� General Dental Council  2001 May.
[9] NHS Scotland: Sterile Services Provision Review Group: 1st Report (The Glennie
Framework).  (2001) Available online at: http://www.show.scot.nhs.uk/sehd/publications/sspr/sspr-00.htm

Author bio
Jamie Lewis (BEng) Hon�s is a Research associate and PhD candidate with the University of Glamorgan. He is currently working in conjunction with Ultrawave Ltd and the Universities Center for Electronic Product Engineering (CEPE) under finance from the EPSRC on the finite element modeling of ultrasonic baths for improved design and performance. He has published manuscripts in scientific publications on Ultrasonics and has presented work to the Institute of Physics (IoP) and the Institute of Electrical and Electronic engineers (IEEE) on the subject.