Needle deformation in infiltrative anesthesia: what impact for the practitioner?

In daily practice, maxillary infiltration anesthesia is a routine procedure, yet far from trivial regarding equipment integrity. Bone contact, inherent to vestibular and palatal techniques, can cause deformation of the needle tip (barb formation). This phenomenon is clinically critical: a blunt or hooked needle increases tissue trauma during withdrawal and can generate post-operative pain or even neurological complications. Despite this prevalence, few data compare the influence of operator experience on this specific risk during dental extractions.

This cross-sectional study aims to evaluate the prevalence, magnitude, and direction of needle tip deformation following maxillary infiltration. Researchers analyzed 180 samples (90 experimental needles and 90 new controls) to determine whether the level of training — comparing 4th-year students to residents — and the injection sequence (buccal alone, palatal alone, or sequential) influence needle morphology. The hypothesis tested is based on the idea that clinical experience and the injection site significantly modulate the incidence of structural deformations observed by field emission scanning electron microscopy (FE-SEM).

How to track invisible needle deformation?

For this cross-sectional study conducted at the University Dental Hospital Sharjah, researchers developed a rigorous protocol to compare the clinical dexterity of 4th-year students with that of interns. How to measure the real impact of an injection on the geometry of a needle tip? The team collected 180 samples, including 90 control needles (new) and 90 experimental needles (27G, 25 mm, triple bevel) used during maxillary extractions.

The clinical protocol, supervised by instructors to ensure perfect standardisation, followed precise steps:

• Vestibular infiltration: 2 to 3 mm insertion at the mucovestibular fold, injection of 0.6 to 1 mL of lidocaine (2% with adrenaline 1:100,000) over 30 seconds.
• Palatal infiltration: insertion 5-10 mm from the gingival margin with light bone contact, injection of 0.2 to 0.3 mL.
• Distribution: 6 subgroups (n=15 each) were established according to the technique (vestibular only, palatal only or sequential vestibular-then-palatal) and the operator's level.

For the analysis, the team used field emission scanning electron microscopy (FE-SEM) at magnifications of x100 and x500. Using ImageJ software and a 5.5 µm grid, the researchers quantified the deviations and classified the direction of the deformation into two categories: inward (towards the needle lumen) or outward (towards the outside). The data were then subjected to Student's t-tests and ANOVA analyses via SPSS 28.0.

Sample distribution and demographic data

The study focused on the analysis of 180 needles (27G, 25 mm), divided into two equal groups: 90 experimental samples (50%) and 90 controls (50%). The needles in the experimental group were collected after maxillary extraction procedures in 90 patients, predominantly men (82.2%, n=74) compared to 17.8% women (n=16). The patients' ages ranged from 15 to 65 years, with an average of 37.22 years.

Validation of the control group and quality control

Field emission scanning electron microscopy (FE-SEM) analysis of the 90 control needles (new, from sealed packaging) established the baseline morphology of the bevel. These observations confirmed the absence of manufacturing irregularities or handling-related defects, ensuring that any deformation observed in the experimental group results exclusively from the clinical procedure.

Experimental group structure and measurement protocol

The 90 experimental needles were subdivided into six subgroups (n=15 each) to compare the impact of the operator's experience level and the injection technique:

• Operators: 4th-year students versus dental surgery residents.
• Techniques: Buccal infiltration alone, palatal alone, or sequential (buccal then palatal with the same needle).

Deformation quantification was performed using ImageJ software on 5.5 µm grids, with magnifications of ×100 and ×500. The study systematically evaluated the vertical and horizontal deviations of the bevel tip, as well as the direction of the deformation (internal towards the lumen or external).

Sample category	Quantity (n)	Morphological observations (FE-SEM)
Control Group (New)	90	Intact basic morphology, no manufacturing defects.
Experimental Group (Post-clinical)	90	Analyse of the deformation according to the approach (Vestibular/Palatal).

Although the literature data cited in the introduction report deformation rates ranging from 50% to 100% depending on the techniques (notably 97% for conventional infiltrations), this study specifically standardized the insertion (2 to 3 mm buccally, light bone contact palatally) to isolate the effect of the training level on the integrity of the tip.

Clinical analysis of the bevelled deformation

This study, based on the analysis of 180 needles (90 experimental and 90 controls) using field emission scanning electron microscopy (FE-SEM x100 and x500), highlights a technical risk inherent to maxillary infiltration. The challenge goes beyond simple material physics: the formation of micro-hooks ("barbed hooks") after bone contact — almost systematic during palatal injection at a depth of 5 to 10 mm — is directly linked to tissue trauma and the risk of paresthesia during withdrawal.

The methodological design, comparing 4th-year students and residents, seeks to validate whether clinical experience reduces the incidence of these deformations. In the practice, the injection sequence (buccal then palatal with the same 27G needle) is a common practice, but this study suggests it could multiply the risks of bevel distortion, particularly during penetration of the palatal attached gingiva where mechanical resistance is higher.

Although limited to a single type of needle (25 mm tri-beveled), this research highlights that deformation is not a manufacturing defect (confirmed by the pristine control group) but a direct consequence of the clinical act. The precision of the analysis protocol on a 5.5 µm grid allows for the quantification of deviations invisible to the naked eye, yet potentially deleterious to nerve fascicles.

In concrete terms, for the practitioner:

• Prioritize needle replacement: If a palatal injection follows a vestibular infiltration, using a new needle limits the risk of trauma related to a bevel already deformed by a first bone contact.
• Master bone contact: During palatal infiltration, contact with the bone must be "gentle"; excessive pressure mechanically increases the formation of hooks on the bevel.
• Post-operative vigilance: In case of trismus or unusual pain upon removal, suspect a deformation of the needle tip that may have damaged soft tissues or terminal nerve fibers.

This study, analyzing 180 needles (27G) by scanning electron microscopy (SEM), highlights a quasi-systematic deformation of the bevel during maxillary infiltrations. The data confirm that insertion, particularly during bone contact at the palatal level, transforms the needle tip into a true microscopic hook (barb), significantly increasing the potential for tissue laceration during withdrawal.

Technical lexicon of the study

27-gauge needle: Standard 25 mm gauge used in the study, whose internal diameter constitutes a critical vulnerability factor against mechanical stress and plastic deformation.

G*Power Software: Biometric calculation instrument used to set the statistical power at 85% with an effect size of 0.4, ensuring the robustness of the results across the 180 analyzed samples.

Mucobuccal fold: Strategic impact point above the dental apex where a 2 to 3 mm insertion is performed for effective vestibular infiltration.

Surgical bevel: Bilateral sharpening geometry and reduced length, designed to optimize tissue penetration while minimizing the risk of distortion during bone contact.

Sequential infiltration: Clinical approach consisting of performing the vestibular injection followed by the palatal injection with the same needle, a practice that challenges the integrity of the tip through repeated insertions.

Hook deformation: A true sharp hook, this distortion of the tip tears the tissues during removal, at the risk of causing paresthesia and trismus.

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