Sonics and ultrasonics in endodontics
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Sonics and Ultrasonics in Endodontics
1. Physical Principles
Ultrasonics
Ultrasonic devices operate at frequencies of 25-40 kHz, well above the audible range. Energy is generated by either:
- Piezoelectric transducers - produce a linear (back-and-forth) vibration mode; more precise and widely used in modern systems
- Magnetostrictive transducers - produce elliptical vibration; generate more heat and require water cooling
When an ultrasonically energized tip is placed in irrigant, two primary physical phenomena occur:
a) Acoustic streaming (microstreaming)
Rapid oscillation of the file/tip creates localized, high-velocity fluid flow patterns around it. This shear stress disrupts and removes biofilm, debris, and necrotic tissue. Streaming velocity is greatest near the tip and diminishes with distance.
b) Cavitation
Cyclic pressure changes in the fluid produce microbubbles that grow and then implode violently. This releases localized shock waves and hydroxyl radicals, directly damaging bacterial cell membranes. Two subtypes exist:
- Stable (non-inertial) cavitation - bubbles oscillate without collapsing; gentler, more sustained
- Transient (inertial) cavitation - bubbles collapse abruptly; more destructive to biofilm
Sonics
Sonic devices operate at 1-6 kHz (within the audible range). Key differences from ultrasonics:
| Feature | Sonics | Ultrasonics |
|---|---|---|
| Frequency | 1-6 kHz | 25-40 kHz |
| Tip motion | High-amplitude oscillation | Low-amplitude oscillation |
| Vibration mode | 3D movement | Linear or elliptical |
| Mechanism | Fluid turbulence, acoustic transmission | Acoustic streaming + cavitation |
| Tip material | Flexible polymer (e.g., polyamide) | Metal insert |
| Risk to canal walls | Very low | Moderate (metal tip can cut) |
2. Methods of Ultrasonic Use in Endodontics
2.1 Ultrasonic Cavity Preparation and Access
Ultrasonic diamond-coated tips can be used to refine access cavities, remove overhanging restorations, and achieve minimally invasive trephination of calcified canals - areas where rotary burs lack precision.
2.2 Passive Ultrasonic Irrigation (PUI)
The most widely applied ultrasonic technique. The term "passive" is a misnomer - it refers to the noncutting nature of the tip, but the process is physically very active.
Technique:
- Complete canal shaping first (PUI is a post-preparation step)
- Place a small-diameter, non-cutting metal insert (e.g., Irrisonic tip) at 1-2 mm short of working length
- Activate at 25-40 kHz - tip must vibrate freely without contacting canal walls
- Irrigant (typically NaOCl) is agitated for 3 cycles of 20-30 seconds each, with fresh irrigant replenished between cycles
- Two flush methods: continuous (irrigant constantly replenished via separate needle) or intermittent (tip placed in a static column)
Continuous ultrasonic irrigation (CUI) is considered superior as it ensures constant renewal of the active irrigant.
2.3 Ultrasonic-Assisted Procedures
Broken instrument retrieval: Ultrasonic tips are the standard method for bypassing and trephining around separated files. A thin tip is placed alongside the fragment and activated, gradually loosening it through vibration. Success is highest in the coronal and middle thirds.
Removal of pulp stones and calcified canals: Ultrasonic tips dissolve and dislodge calcified bridges to locate orifices without removing excess dentin.
Post removal: Low-power ultrasonic vibration transmits energy down a cemented post to break the cement bond, allowing post removal with minimal root damage.
Root-end preparation (retrograde): In periapical surgery, ultrasonic retrotips (angled, diamond-coated) allow preparation of a class I cavity along the long axis of the root in a restricted surgical field - far superior to the angled burs previously used.
Calcium hydroxide removal: PUI significantly improves removal of calcium hydroxide inter-appointment dressings from canal irregularities compared to syringe irrigation alone.
Smear layer removal: Combined with EDTA, ultrasonics enhances the chemical chelation of the smear layer in the critical apical third.
3. Sonic Irrigation Systems
3.1 EndoActivator
- Air-driven, disposable polymer tips
- Frequencies: ~100 Hz (low), ~166 Hz (medium), ~190 Hz (high)
- Placed 2-3 mm short of working length; 30-second activation cycles
- No cutting of dentin; works by creating turbulence and fluid dynamics
3.2 EDDY (VDW, Munich)
- Flexible polyamide tip activated at 5,000-6,000 Hz by an air-driven air scaler
- Three-dimensional high-amplitude oscillatory movement
- Generates both acoustic transmission and cavitation-like effects
- Non-cutting, disposable, and sterilizable
- Meta-analysis (Chu et al., 2023, PMID: 36932445) found EDDY comparable to ultrasonically-activated irrigation (UAI) in smear layer and debris removal across all canal thirds, with the advantage of not damaging canal walls
3.3 SAF (Self-Adjusting File)
- A mesh-like hollow NiTi file that simultaneously instruments and irrigates
- Connected to irrigation pump delivering continuous NaOCl through its lumen
- Combines sonic-like vibration with simultaneous irrigant delivery
4. Comparative Clinical Evidence
Irrigant Penetration
A 2023 systematic review and meta-analysis (PMID: 36748449) comparing activation techniques found:
- All irrigant activation techniques (IATs) achieved significantly better irrigant delivery to working length than conventional needle irrigation (CNI) - 51.94% improvement in straight canals
- Efficacy ranking: Apical negative pressure (ANP) > PUI > Sonic irrigation > Manual dynamic activation
- ANP achieved 91.70% improvement over CNI in straight canals
Sonic vs Ultrasonic - Smear Layer and Sealer Bond
A 2022 meta-analysis (PMID: 35430959) comparing sonic and ultrasonic activation found:
- Sonic activation achieved better smear layer removal at the apical level (MD -0.48, 95% CI: -0.92 to -0.04)
- Ultrasonic activation produced significantly higher push-out bond strength of sealer at both middle and apical thirds, suggesting better sealer-dentine tubule penetration and reduced apical leakage risk
EDDY vs Ultrasonics
As noted above, EDDY is equivalent to UAI for smear layer and debris removal, making it a practical alternative given its lower risk of canal transportation.
Irrigant Extrusion Risk
A 2023 systematic review (PMID: 35988128) on irrigant extrusion found that activation methods vary in their risk of pushing irrigant beyond the apex - an important safety consideration, especially with NaOCl (which causes severe tissue injury if extruded).
5. Specific Applications Summary
| Application | Preferred Technique |
|---|---|
| Canal debridement / biofilm removal | PUI or sonic activation (EDDY/EndoActivator) |
| Smear layer removal (with EDTA) | PUI - enhances chelation in apical third |
| Broken instrument removal | Ultrasonic trephination |
| Root-end preparation | Ultrasonic retrotips (surgical endodontics) |
| Post removal | Low-power ultrasonic vibration |
| Calcium hydroxide removal | PUI (superior to syringe irrigation) |
| Irrigant delivery to working length | ANP > PUI > Sonic |
| Retreatment (gutta-percha removal) | Ultrasonics as supplementary tool |
6. Safety Considerations
- Canal transportation: Ultrasonic metal tips that contact canal walls can strip dentin, particularly in curved canals. Tip must vibrate freely.
- File fracture: Over-torquing or wedging an ultrasonic tip can cause fracture
- Irrigant extrusion: Both sonic and ultrasonic activation can push irrigant beyond the apex; tip placement 1-2 mm short of WL and patent foramen increase risk
- Sound level: Devices at sonic frequencies are audible and may be uncomfortable for the patient without ear protection consideration
- Heat generation: Magnetostrictive ultrasonic units generate more heat than piezoelectric units
7. Emerging Technologies
GentleWave System uses multisonic (broadband acoustic energy) with a closed-system fluid delivery platform operating across a wide frequency range. It combines acoustic microstreaming with advanced fluid dynamics and hydrodynamic cavitation to clean the entire root canal system with minimal instrumentation. Particularly useful for necrotic cases and complex anatomies.
Key References:
- Paixão et al. (2022). Efficacy of sonic and ultrasonic activation during endodontic treatment: a meta-analysis. Acta Odontol Scand. PMID: 35430959
- Chu et al. (2023). Cleaning efficacy of EDDY versus ultrasonically-activated irrigation: a systematic review and meta-analysis. BMC Oral Health. PMID: 36932445
- Kumar et al. (2023). Comparative efficacy of different irrigant activation techniques. Eur Endod J. PMID: 36748449
- Azizi Mazreah et al. (2023). Irrigant extrusion following different irrigation techniques. Aust Endod J. PMID: 35988128
- Irrigants and irrigation activation systems in endodontics - PMC review (2023)
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