Marginal leakage is one of the biggest challenges in Class II composite restorations. It commonly results from poor adaptation of the composite or polymerization shrinkage, leading to microleakage, recurrent caries, and eventual restoration failure. To overcome this problem, several modified techniques have been developed, with the Snow Plow technique and the Injection Molding technique being two of the most discussed approaches.
This article compares these two methods, explains how they work, and highlights their advantages and limitations based on current evidence.
Why Marginal Leakage Matters in Class II Restorations
- Marginal leakage compromises the long-term success of restorations.
- It occurs mainly in the gingival margin of proximal boxes, especially when adaptation to tooth structure is poor.
- A proper lining strategy can improve adaptation, act as a stress absorber, and reduce the risk of failure.
read our guide about Stabilization of Matrix Bands in Class II Composite Restorations
The Snow Plow Technique
How It Works
- After etching, bonding, and curing, a thin layer of flowable composite (≈1 mm) is injected into the gingival area.
- Without curing, packable composite is placed and adapted over it.
- Both materials are cured together for 30 seconds.
Advantages
- Flowable composite acts as a stress-breaking layer, improving marginal sealing.
- Reduces polymerization shrinkage stress compared to bulk packable use.
Limitations
- Risk of uncured flowable in deep areas, especially when using thick layers (>1 mm).
- Requires proper curing from multiple directions to ensure complete polymerization.
- Limited wear resistance and fracture toughness of flowable resin.
The Injection Molding Technique
How It Works
- After etching and applying adhesive, do not cure the bonding agent.
- Inject a thin layer of flowable composite (≈1 mm) without curing.
- Immediately place a thin layer (≈1 mm) of packable composite on top.
- Cure the adhesive, flowable, and packable together as a single mass.
Advantages
- Creates a monolithic mass with excellent marginal adaptation.
- Flowable and adhesive act as wetting agents, enhancing adaptation of the packable composite.
- Significantly reduces microleakage compared to traditional methods.
- Thinner flowable layer (10–20%) decreases risk of marginal ridge fracture.
Limitations
- Requires correct matrix band and wedge placement for success.
- Technique sensitive—risk of overhang if flowable escapes in beveled margins.
- Preheating composites (sometimes recommended) may improve flow but can also increase polymerization shrinkage and add clinical complexity.
| Feature | Snow Plow | Injection Molding |
|---|---|---|
| Flowable use | Thin uncured layer under packable | Incorporated as part of uncured sequence |
| Curing step | Flowable + packable cured together | All materials cured at the same time |
| Adaptation | Flowable improves marginal seal | Entire restoration molded as one unit |
| Shrinkage stress | Reduced if minimal flowable used | Distributed across mass but technique-sensitive |
| Evidence | Well-documented in literature | Promising but less long-term data |
Evidence from Studies
- Injection molding generally shows lower microleakage than the Snow Plow technique.
- Some studies found no significant difference, while others confirmed injection molding provides superior results.
- Both techniques outperform traditional lining or bulk-fill approaches in marginal sealing.
read our guide about Deep Caries Management: Step-by-Step Guide
Clinical Takeaways
- Use flowable composites cautiously, keeping thickness ≤1 mm.
- Ensure proper curing protocols to avoid uncured resin.
- Injection molding is often preferred due to better marginal adaptation and reduced microleakage.
- Always use proper matrix systems and wedges for optimal results.
- Consider the trade-offs of preheating composites before incorporating it routinely.
Conclusion
Both the Snow Plow and Injection Molding techniques were designed to address the issue of marginal leakage in Class II composite restorations. While Snow Plow offers improvement over conventional methods, Injection Molding has shown more consistent results in minimizing microleakage, thanks to the formation of a well-adapted monolithic composite mass.
For clinicians, the choice of technique depends on case selection, operator skill, and proper execution. However, when done correctly, the Injection Molding technique appears to be the superior option for long-term clinical success.
Reference:
- Influence of Sandwich, Snow Plow, and Injection Molding Modified Composite Lining Strategies on the Microleakage of Class II Proximal Box Cavities (2020)