# The Nallan–Nickel Effect: A Mechanistic Perspective on Burning Sensations and Lichenoid Reactions in Long-Serving Porcelain-Fused-to-Metal Restorations

## TL;DR

This paper proposes a new theory explaining why some long-lasting dental crowns cause burning sensations and lichenoid reactions in the mouth.

## Contribution

The Nallan–Nickel Effect is introduced as a novel theoretical model linking nickel ion release from dental crowns to neuro-immune reactions.

## Key findings

- Nickel ions from corroded crowns may trigger inflammation and burning through immune and nerve pathways.
- Factors like low saliva and acidic pH can lower the threshold for these reactions.
- Replacing nickel-containing crowns may alleviate symptoms by restoring tolerance.

## Abstract

Porcelain-fused-to-metal (PFM) crowns continue to serve as a cornerstone of restorative dentistry owing to their strength, affordability, and esthetics. However, late-onset complications such as oral burning and lichenoid reactions have been observed in long-serving PFMs, suggesting complex host–material interactions that extend beyond simple mechanical wear. This Perspective introduces the Nallan–Nickel Effect, a theoretical model proposing that a host- and environment-dependent threshold of bioavailable nickel ions (Ni2+), once exceeded, may trigger a neuro-immune cascade culminating in a burning phenotype. Within this framework, slow corrosion at exposed PFM interfaces releases Ni2+ into saliva and crevicular fluid, facilitating epithelial uptake and activation of innate immune sensors such as TLR4 and NLRP3. The resulting cytokine milieu (IL-1β, IL-6, TNF-α) drives NF-κB, mediated inflammation and T-cell activation, while neurogenic mediators—including nerve growth factor (NGF), substance P, and CGRP—sensitize TRPV1/TRPA1 nociceptors, establishing feedback loops of persistent burning and neurogenic inflammation. Modifying factors such as low salivary flow, acidic oral pH, mixed-metal galvanic coupling, and parafunctional stress can lower this threshold, whereas replacement with high-noble or all-ceramic materials may restore tolerance. The model generates testable predictions: elevated local free Ni2+ levels and increased expression of TLR4 and TRPV1 in symptomatic mucosa, along with clinical improvement following substitution of nickel-containing restorations. Conceptually, the Nallan–Nickel Effect reframes PFM-associated burning and lichenoid lesions as threshold-governed, neuro-immune phenomena rather than nonspecific irritations. By integrating corrosion chemistry, mucosal immunology, and sensory neurobiology, this hypothesis offers a coherent, testable framework for future translational research and patient-centered management of PFM-related complications.