The measurement of needle sharpness

Importance

Controlled sharpness of needles is important for four reasons:

Accuracy of puncture of structures such as vein walls.
Pain to the unanaesthetised patient from blunt or hooked needles.
Tissue damage from bad needles.
The need for resistance to penetration by short bevel needles to indicate traverse of fibrous anatomical layers.

Measurement versus descriptive adjectives

Rating by scalar adjectives is invalid beyond five steps, and more important cannot be reliably communicated from one observer to another without a large proven pool of common experience. A Japanese Industrial Standard (T1301-1965) attempts to quantify sharpness as the weight in grams (either 30 or 50) taken for a needle to penetrate aluminium foil .02 mm. thick.

An analogy is found in the clinically more valid force required to close artery forceps, measured easily with a cheap kitchen scale. The force should be between 3 and 4½ pounds weight. Outside these limits forceps become increasingly too stiff or too slack.

An unpublished personal study of five microsurgeons showed the ideal closing pressure for watchmakers' forceps to be in the range of 40 to 60 grams. Yet for 20 instruments from a reputable maker tested in March 1976, the average closing pressure was 160 grams, with one pair taking 360 grams to close, and completely unsuitable for fine manipulation.

Existing guidelines

Apart from the Japanese Standard, the appearance of needles of varying quality seen under the microscope has been studied by Noel Cass, the Melbourne anaesthetist¹, and a crude testing method using skin stretched over an aperture has been published by Patkin²Traditionally, chamois leather has been used to check the sharpness of cataract knives, both in theatres and in instrument factories. Chamois and the cylinder to stretch it over are in several current surgical instrument catalogues.

Studies for the present submission

Using a kitchen scale, a postal weighing scale, or a simple jig in which increasing weight was applied by steadily filling a measuring cylinder with water at 20 ml. per second, approximately 520 measurements were made of the force required for needles of gauge 19, 21, 23, and 25 to penetrate the following substances: Silastic, polythene from tubing, x-ray film, unprocessed 35 mm. film, synthetic chamois sold for cleaning motor cars, card paper, and two samples of recent refrigerated human skin, one a prepuce from an adult male and the other a large sheet of abdominal skin dissected from an apronectomy specimen.

The x-ray and negative film gave the most consistent results, with little variation evident with the crude apparatus used over ten measurements for each test needle. Deliberate blunting was carried out in several cases by pressing the needle point with a steady measured force of 1½ pounds weight against a piece of stainless steel. This gave the point a hook palpable on sliding against the finger, and a clear-cut consistent force increase needed for film penetration. Most needles, irrespective of their gauge (indicating the limitation of the test material) took a force of 12 to 14 ounces to penetrate the film. .

The apronectomy skin specimen was divided into 29 pieces each just over 2 centimetres square, supported on a wire mesh resting on the open end of a small jar, and ten measurements were made for each piece using a fresh needle. Prior to analysis of variation and deviations, no clear-cut pattern of readings emerged, except that the skin was consistently tougher the closer the site to the umbilicus, readings varying from 2 to 18 ounces. Toughness of prepuce skin varied much less, by a factor of two. In these tests there were no consistent blunting effects.

Tests on synthetic chamois ("Terumo 2162", bought from a local garage) varied by a factor of four from the lowest to the highest, with a clear-cut effect from deliberate blunting, and also affected by whether the chamois was dry or damp.

Plastic cannulae (" Dwellcath" and "Medicut" ) were also tested on x-ray film, with utter failure, as the outer plastic sheath wrinkled-up on the needle each time instead of following it through, reflecting the lack of elasticity of the material.

Provisional conclusions

Omitting discussion for the sake of brevity, the following personal opinions emerge:

Studies to establish a method of determining a method of measuring sharpness must be carried out jointly by a practising surgeon, an anaesthetist, a qualified scientist used to designed experiments, and an engineer familiar with needle manufacture.

Measuring equipment should take the form of a metric motorised universal test stand, made by Chatillon of New York, and incorporating their dial gauge. Such equipment is presently in transit to the writer thanks to the generosity of Messrs. Davis and Geck of New York. Alternative equipment would be electronic digital strain gauge test stands,- not available with a motorised drive to the knowledge of the writer.

Test material will comprise human skin from multiple sites and subjects of different ages and physical types, and a wide range of synthetic materials to select those with correlate with the real world situation in a numerically reproduceable test. For example using x-ray film to test plastic cannulae is obvious nonsense. Large numbers of measurements can be made in a short time once each method is set up, and can be processed quickly by standard EDP.

Fundamental components of the piercing process will need to be examined, comprising at least three: point entry, bevel cutting and stretching, and shaft friction. This will be the biggest difficulty in correlating synthetic materials with skin, with its special viscoelastic-plastic properties for which a mathematical model has yet to be established. Studies in. this: area . were possibly foreshadowed by Vorotyntseva⁴ of the U.S.S.R. in 1971and by others^5,6but further information from these sources has not been pursued.

Final statement: A laboratory test is quite feasible, but will need a careful choice of test material, and validation against test pieces of human skin.

References

1. Cass, Noel (1973) Disposable Hypodermic Needles, Pre-Printed Papers, 46th Annual GSM, Royal Australasian College of Surgeons.
2. Patkin, M. Surgical Instruments and Effort, Referring Especially to Ratchets and Needle Sharpness (1970), Med.J.Aust.,1,225-6.
3. Japanese Industrial Standard (T1301-1965).
4. Vorotyntseva, M.I. (1971) Classification and Terminology of Medical Piercing Instruments, Meditsinkskaya Tekchnika,1,39-42, January-February 1971.
5. Atraub, W. and Retzlaff, K. (1971) Quality Checks of Microsurgical Ophthalmologic Instruments (in German)
6. Ber.Dtsch.Ophthal.Gesellsch.1970 Band 70 (325-330), abstracted in Excerpta Medica Ophthalm.v.25,1971, p.242,Abstract No.i5i3. (Plastic foil, spring resistance measured, 50% of 593 used needles were damaged at the tip.)

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