Context: Dental fragility post-radiotherapy

Approximately 30% of patients treated with radiotherapy (RT) for head and neck cancer develop radiation-related caries within 6 to 12 months following treatment. Beyond induced hyposalivation, RT directly alters the chemical structure and micromechanical properties of enamel, dentin, and cementum, drastically reducing their wear resistance. In this critical context where maintaining oral hygiene is imperative to prevent osteoradionecrosis, the impact of ultrasonic instrumentation remains poorly defined. Currently, no standardized protocol specifies the setting parameters or safe scaling techniques for these structurally weakened tissues.

Study objectives and hypotheses

The objective of this in vitro study was to quantify the substance loss induced by robotized ultrasonic scaling on 24 human molars previously irradiated at doses of 0, 30, or 70 Gy. Researchers tested the hypothesis that tissue damage depends on specific clinical variables: the type of tip (wide G6 vs. thin P20), the working angulation (15° vs. 45°), the tip orientation (back vs. lateral), and the targeted dental region (crown vs. root). By simulating these parameters in a standardized manner with a force of 1 N, the study aimed to determine whether the irradiation dose and the surgical technique interact to aggravate iatrogenic wear.

Study methodology

This in vitro study analyzed the impact of ultrasonic scaling on the integrity of human dental tissues previously subjected to ionizing radiation. The protocol involved a sample of 24 human molars, divided into three groups according to the irradiation dose received: 0 Gy (control group), 30 Gy or 70 Gy.

Ultrasonic instrumentation was performed using a standardized robot to eliminate human variability, applying the following clinical parameters:

• Support force: 1 N constant.
• Tested tips: G6 (wide tip) and P20 (fine periodontal tip).
• Technical configuration: 15° and 45° angulations, back and lateral insert orientations.
• Settings: Medium power with water spray.

Researchers evaluated the loss of substance in two distinct regions: the crown and the root. Damage was quantified on intra-oral scans by three blinded observers, using an ordinal scale from 0 to 5 (increasing according to the depth of tissue loss). Data analysis was based on cumulative logit ordinal mixed models to evaluate the median and maximum depth of the induced lesions.

Impact of irradiation and instrumentation parameters on tissue loss

The results of this standardized in vitro study reveal that ultrasonic instrumentation induces a loss of dental substance, the extent of which is closely linked to the prior irradiation dose and the technical configuration of the scaler.

Effect of the irradiation dose: a threshold effect identified

Prior irradiation of dental tissues modifies their resistance to mechanical abrasion. Notably, a dose of 30 Gy significantly increased the probability of deeper surface loss, particularly with a back orientation of the insert (back orientation):

• Median depth: β = 2.43 (p = 0.039)
• Maximum depth: β = 2.10 (p = 0.022)

Curiously, no significant contrast was observed for the 70 Gy group compared to the control group, suggesting the existence of a threshold effect rather than a linear dose-dependent response.

Influence of the insert and angulation

The choice of the tip appears to be the most decisive technical factor. The wide tip (G6) caused substantially greater damage than the thin periodontal tip (P20), with strong statistical significance (p < 0.001) for all evaluated depth criteria.

Regional vulnerability: Crown vs Root

The study confirms differential susceptibility depending on the anatomical region treated. The root proved significantly more vulnerable to instrumentation than the crown (β ≈ 2.95, p < 0.001). This radicular fragility is exacerbated by irradiation, making cementum and dentin particularly prone to measurable wear as soon as the insert angulation reaches 45°.

In summary, qualitative observations from intra-oral scans show that the use of thin inserts (P20) with a low angulation (15°) and a lateral orientation preserves tissue integrity, even on teeth that have received therapeutic doses of radiation.

Clinical and iatrogenic risk analysis

This study demonstrates that irradiation, starting at a dose of 30 Gy, induces significant structural vulnerability of dental tissues when subjected to ultrasonic scaling. The major finding is the identification of a threshold effect: a dose of 30 Gy considerably increases the risk of substance loss (β = 2.43; p = 0.039), particularly when the tip is used on its back. For the practitioner, this means that the mechanical resistance of the tooth is compromised well before reaching maximum cumulative doses (70 Gy). The aggressiveness of the instrumentation is directly correlated here to the geometry of the tip and its angulation, with the wide G6 tip causing substantially deeper damage than the thin P20 model.

Study limits and framework

Although this in vitro study on 24 molars uses rigorous robotic standardization (1 N force), it does not reproduce all clinical conditions, notably the influence of microbial flora and the absence of salivary remineralization, often compromised by post-radiation hyposalivation. However, the precision of measurements on intra-oral scans by three blinded observers reinforces the robustness of the data on the depth of iatrogenic wear.

Implications for clinical practice

These results corroborate the literature on the alteration of the micro-mechanical properties of enamel and dentine after radiotherapy. The critical point lies in the increased susceptibility of the root compared to the crown (β ≈ 2.95; p < 0.001). Given that radiation caries (RC) often begin on cervical surfaces, an unsuitable scaling technique risks creating surface defects that promote plaque retention and rapid progression towards pulpitis or osteoradionecrosis.

Summary of results

This in vitro study conducted on 24 molars demonstrates that irradiation starting at 30 Gy significantly increases the risk of substance loss during scaling (p=0.039). The wide G6 insert causes much deeper damage than the thin P20 model (p < 0.001), particularly at a 45° angulation on root surfaces, which prove to be more vulnerable than crowns (β ≈ 2.95).

In concrete terms, for the practitioner:

• Select your tips: Systematically use fine periodontal tips (P20 type) for the follow-up of irradiated patients in order to limit iatrogenic abrasion.
• Correct the angulation: Maintain a low angulation (15°) and prioritize a lateral orientation of the tip; a 45° angulation with a posterior orientation increases the risk of surface defects.
• Caution at the root level: Since post-RT dentin and cementum are less resistant, the use of ultrasounds on necks and roots must be lighter and better controlled to avoid lesions that promote plaque.

Technical lexicon of the study

Radiation-related Caries (RC): Creeping carious lesions developing early (6 to 12 months) after head and neck radiotherapy, preferentially affecting cervical areas and cusp tips.

Surface loss: Iatrogenic wear or abrasion of dental hard tissues caused by ultrasonic instrumentation, measured here via intra-oral scans according to an ordinal scale from 0 to 5.

G6 and P20 inserts: Specific ultrasonic tips used in the study; the G6 is a wide insert while the P20 is a slim periodontology insert (slim tip), the latter being associated with superior tissue preservation.

Orientation (Back vs Lateral): Technical parameter defining the face of the insert in contact with the tooth. Back orientation significantly increases the depth of damage compared to lateral orientation.

Irradiation dose (0, 30, 70 Gy): Levels of exposure to ionizing radiation tested in vitro, where a dose of 30 Gy was sufficient to significantly increase the susceptibility of tissues to mechanical wear.

Osteoradionecrosis: Serious bone complication following radiotherapy, which can be triggered by odontogenic infections resulting from untreated post-radiation caries.

Information intended for healthcare professionals. This content may contain errors or truncated summaries. We recommend always verifying with the original source article. Delynov disclaims all responsibility regarding the use of this information. This document is not intended for patients or the general public.