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Three Part Question

In [patients requiring anaesthesia to the finger] IS [metacarpal block better than ring block] AT [minimising pain during infiltration, achieving adequate anaesthesia and achieving rapid onset].

Clinical Scenario

A 25-year-old man presents to the A+E department following a crush injury to the middle finger. There is a laceration over the distal phalanx involving the nail bed. The nail bed is disrupted. The injury requires removal of the nail and repair of the underlying nail-bed laceration. You wish to perform this procedure under local anaesthesia and suggest using a digital nerve block for anaesthesia. Your colleague suggests that a metacarpal block would be a better method as it is less painful to perform.

Search Strategy

MEDLINE using OVID interface on the world wide web 1966-June 1999
[exp nerve block/ or "nerve block".mp OR exp anesthesia, local/ or exp anesthetics, local/] AND [exp metacarpus/ or "metacarpal".mp.]

Search Outcome

MEDLINE – 27 papers found of which one paper directly addressed the three part question.

Relevant Paper(s)

Author, date and country	Patient group	Study type (level of evidence)	Outcomes	Key results	Study Weaknesses
Knoop et al, 1994, USA	Convenience sample of 30 adult patients, with third or fourth finger injuries including and distal to the proximal interphalangeal joint that required digital anaesthesia.Patients received a dorsal metacarpal block on one side of the finger and digital nerve block on the other. 2% buffered lidocaine was used for the blocks.30 patients received 60 blocks	PRCTUnblinded. Patients acted as their own controls.Patients were randomised as to which injection was given first.	Time to onset of anaesthesia (as measured by pinprick testing)	Better for digital block (2.82 mins vs 6.35 mins, p<0.001)	It is possible that a metacarpal block may reduce pain on the opposing side if given first.Pain is analysed using parametric analysis (students t test). However, the pain scores were not normally distributed. Non-parametric analyses should have been used (though the raw data still suggests a benefit to digital block).No power study was performed, the study has low power.Digital nerve injury was not an exclusion criteria.
			Adequacy of anaesthesia (i.e. was additional anaesthesia required)	3% of digital blocks failed vs 23% of metacarpal blocks (p=0.227)
			Pain during administration of block (using 10 point VAS applied immediately after injection)	Better for digital block (2.53 vs 3.38, p=NS)

Comment(s)

The clinical scenario is based upon a real discussion between clinicians who both believed themselves to be correct. A search for the evidence has found a paper that directly addresses the clinical equipoise. The success rate and time to onset of anaesthesia appear to be better for digital blocks. Digital blocks show a trend towards less pain although this finding is not statistically significant and the study may be too small to detect a true difference. Metacarpal blocks may still have a role when analgesia in more than one finger is required or when a proximal finger injury has occurred. They may also offer safety benefits in patients in whom there is concern regarding the quality of the peripheral circulation. The results of a single study such as this must be interpreted with caution, practical procedures are operator dependent and the high failure rate of metacarpal block may be due to operator error. The use of within patient controls is interesting and may lead to some bias if it were possible that there is some interaction between the two injections. A better study would randomise between rather than within patients. This study examined the use of a dorsal metacarpal block, a volar method has also been described, the findings of this study cannot be extrapolated to other methods of performing the block. Despite it's limitations, the evidence suggests that digital nerve blocks are preferable for routine use in the emergency department.

Clinical Bottom Line

This evidence-based summary concludes that when digital anaesthesia is required in the emergency department a digital nerve block should be performed in preference to a metacarpal nerve block.

References

1. Knoop K, Trott A, Syverud S. Comparison of digital versus metacarpal blocks for repair of finger injuries. Ann Emerg Med 1994;23:1296-1300.