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An after dinner speaker?
  1. C E Payne
  1. Department of Plastic and Reconstructive Surgery, Pinderfields and Pontefract NHS Trust, Pinderfields Hospital, Wakefield, UK
  1. Correspondence to:
 Miss C Payne, 98 Tamarin Gardens, Cherry Hinton, Cambridge CB1 9GJ, UK; 
 carolinep{at}doctors.org.uk

Abstract

Mutilation of the hand and digits are commonly seen after attempts at home maintenance. Adults, adolescents, and children present to accident departments with sometimes dramatic injuries requiring immediate attention and referral to specialised units. In this case, finger entrapment in the electromagnet of a “club” sized speaker required specific management from the accident and plastic surgery departments and utilisation of the local fire brigade for quick digital release.

  • injury
  • electromagnet

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A 16 year old adolescent was referred from the accident and emergency department after sustaining attachment of a powerful electromagnet to his right dominant hand. He had attempted, after dinner that evening, to mend a high performance speaker. His fingers became trapped after the material cover of the speaker was removed to gain access to the voice coil while the power was still active. On examination of the right hand, the index, middle, and ring fingers were completely compressed and fixed between the two speaker magnets almost to the distal interphalangeal joints (fig 1). The magnetic gap was only 0.6 cm and the complex weighed an estimated 8.5 kg. Power could not be returned as the father had removed the magnet complex from the speaker before calling the emergency services. Marcaine digital blocks and morphine were administered while waiting for the fire brigade as more powerful equipment was required to remove the magnets. Several attempts were made with power saws to cut through the outer electromagnet with no result. The device was ultimately removed by shattering the brittle outer magnet with two pneumatic “jaws of life” so as to break it way from the inner magnet (fig 2A, B). The total ischaemic time was three hours. On radiological examination he sustained tuft fractures to the distal phalanges of the index and middle fingers and required a local anaesthetic procedure to repair the nail beds of all trapped fingers (fig 3). He suffered no soft tissue loss and was discharged the following day.

Figure 2

(A) “Jaws of life”. (B) Fractured outer magnet.

Figure 3

Fingers after release.

DISCUSSION

Mutilation of the hand and digits are commonly seen after attempts at home maintenance. In adults, severe injury is either by direct use or while mending an electrical item. Careless use of home power tools,1 garden tools,2 and kitchen appliances are well known hazards. In children, it is the inquisitive fingers that are most at risk of major mutilation in the home.3,4 Another temptation by many unqualified home owners is to apply basic knowledge of power circuits to fix domestic electrical installations. Electrocution because of the low voltage, alternating current of the domestic supply can cause minor injury to electrical fatalities.5,6 Fixing a speaker highlights two possible dangers; electrocution (which may have been expected here) and the observed crushing effect of the magnetic piston mechanism.

Speakers are air pistons that move back (on the negative cycle of the signal) and forth (on the positive cycle), creating different degrees of air pressure at different frequencies. Alternate positive and negative voltages reach the voice coil rigidly mounted to the cone inside the speaker, creating an electromagnet that will either be repelled, or attracted by the fixed magnet at the bottom of the speaker. The magnets are unyielding and attracted when the power source is discontinued. The voice coil itself is mounted on a rigid cylinder, to which it is firmly glued. At its natural resting point, the voice coil is centred within a narrow magnetic field gap (0.6 cm). The patients fingers were pulled into this gap by the piston action of the two magnets and were solidly trapped for three hours (fig 4).

Figure 4

Schematic details of the functional units of a speaker.

Managing this injury in the accident department required the essential primary care, referral to the hand surgeons, and the necessity of the local fire brigade. The outer magnet is composed of various materials including neodymium, strontium, and barium, among other technologically advanced metal composites, with older units using alnico-5 and other less dense alloys. These are impossible materials to cut even with powerful electric saws, but can shatter if a large enough force is applied across the magnetic ring. The use of the fire brigade equipment was essential and the only way to release the digits from the magnetic field gap.

The piston-like crushing power of these speaker magnets are enormous, they have a substantial attractive force and are impossible to separate once removed from the power source. The immediate objective, in this case, to remove the magnets was attributable to the extreme patient, and family, distress rather than the digital ischaemic time, which can be for several hours. Fortunately the damage sustained to the digits only required a local debridment and distal tip repair, with full function and sensation returning to all finger tips at 10 day follow up.

Acknowledgments

Photographs supplied by Mr R Warner.

REFERENCES

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