The effect of surface coatings on dental anesthesia needles

Abstract

Frequent complaints are made by dental patients and practitioners about penetration problems and residual soreness from the use of anesthesia needles. This laboratory investigation used surface inspection methods and analytical tools to evaluate common anesthesia needle types as-purchased and after scrupulous cleaning and re-coating with a low-surface-energy silane layer known to reduce friction. Force of penetration and withdrawal testing utilized standard pericardium tissue patches for mucosa-simulating punctures, with and without contacting a bone surface, as is often practiced in clinical dentistry. Residual internal tissue damage was assessed by confocal infrared imaging. It was discovered that all as-purchased dental anesthesia needles are coated by the manufacturers with a silicone layer to reduce the known friction of the stainless steel needles themselves, but with uncertain binding strengths and thicknesses. This substance was totally cleaned from some needles by the room-temperature process of Radio Frequency Glow Discharge Treatment, and a subset of those clean needles was re-coated with a covalently bound layer of octadecylsilane (ODS). Critical Surface Tension determinations, by contact angle methods, showed the initial manufacturers' coatings to be thinned, but retained in part, during repetitive tissue punctures, and that the surface-energy-equivalent applied silane was also retained. Surprisingly, it was seen that there was a needle-form-dependent, subsequent penetration force lessening when bone contact-induced tip bending was noted after the first encounter, suggesting that slightly duller needles may actually have lower tissue penetration forces in general. Withdrawal forces increased, however, and left increased internal tissue damage in the needle paths. ODS-coated needles were significantly less damaging, with lower pull out force, than as-purchased needles. Possible transfer of the initial manufacturers' coatings to the tissue path was indicated. It is speculated that this left-behind material, described as a polydimethylsiloxane (PDMS), could interfere with subsequent tissue healing. Thus, the investigation concluded that the uniformity, binding and thinness of the tissue-friction-reducing coating is important, that using bone contact as a position marker damages the needles' tips and increases internal tissue damage as a result, and that this can be mitigated by coating of octadecylsilane.