NPG Post and Core: Is the NPG Alloy Toxic, and Does the Post Come Loose Early?
These days Instagram is full of posts about NPG post and cores. One says the NPG alloy is toxic, another says the post comes loose early, and another asks how you can use it when there is no RCT behind it. Many of these posts are mostly after saying something different and getting noticed. But we need to check whether what they say is true, so I reviewed the scientific evidence available on NPG.
What is the NPG alloy?
NPG is a copper-based casting alloy. More than 80 percent of it is copper [1], and the rest is aluminum plus small amounts of other metals such as nickel and iron. The word Gold in its name (Non-Precious Gold) refers only to its golden color. The NPG+2 version contains a small amount of gold [2]. In dentistry it is used mostly to fabricate cast post and cores.
What do the ADA and the FDA say about NPG?
The ADA classifies dental alloys by their noble metal content [3]. Like Ni-Cr, NPG falls into the base metal category. The manufacturer states in its catalogue that the ADA Council on Dental Materials has accepted this alloy [4], but I found no confirmation of this from the ADA itself. Nor did I find any other ADA opinion or recommendation on NPG.
The FDA has cleared NPG for sale on the US market [5]. But this clearance came through a pathway called 510(k), which is the simpler route. On this pathway, the manufacturer only has to show that its product is similar to products already on the market [2]. So FDA clearance means that selling NPG is legal, but it tells us nothing about its safety or about how durable the post is.
Is an NPG post and core toxic?
This claim has a true part. Laboratory studies have shown that NPG corrodes more readily than Ni-Cr [6]. In a study on NPG+2, the alloy released large amounts of copper, aluminum and nickel ions in an acidic environment, and its corrosion products were toxic to cells [7]. An intraoral study has also shown that these alloys corrode more in areas of the mouth that are not self-cleaning [8].
A post that has been cemented under a well-sealed crown has no contact with saliva. But if microleakage occurs, in my view its situation comes close to this one.
Even so, there is a long way from "it corrodes" to "it is toxic for the patient". The toxicity findings are all from the laboratory [6, 7], and the intraoral study showed only corrosion, not tissue damage [8]. In a study comparing three alloys, NPG was more biocompatible than another copper alloy but did not reach Ni-Cr, and the authors considered Ni-Cr the first choice [6]. In a PubMed search I also found no report of harm to a patient.
Does an NPG post come loose early?
I found no direct evidence for this claim. Only one study has directly compared the retention of NPG posts with Ni-Cr, and it found no significant difference [9]. Corrosion might weaken the post–cement bond over time, but I found no study that has measured this.
NPG has no clinical trial (RCT), so should it not be used?
It is a fair question, but we have to see how other materials fare by the same standard. In a PubMed search I found no dedicated RCT for Ni-Cr posts either, and the results were laboratory studies. Clinical trials have usually compared post types with each other, for example cast versus fiber, not alloy types. So if an RCT is the condition for use, Ni-Cr does not pass this filter either.
This does not mean the two alloys are equal, though. Every casting alloy must meet the requirements of ISO 22674, including on the amount of ions it releases [10]. I found no independent evidence that NPG meets this requirement, and the laboratory corrosion data do not favor it either [6, 7]. So NPG's problem is not the lack of an RCT. Its problem is that the very laboratory evidence we have shows its weakness against corrosion.
Does the elastic modulus of NPG protect the root?
This claim by NPG's supporters has not been proven either. The elastic modulus of these alloys is lower than that of Ni-Cr and closer to dentin [1], but still several times that of dentin [11]. The fracture resistance results do not agree with each other either. Several studies showed NPG to be stronger than Ni-Cr [1, 12, 13], one showed Ni-Cr to be stronger [9], and one found no difference between them [14].
NPG post and core: what does the evidence say?
| Claim | What the evidence says | Status |
|---|---|---|
| NPG is toxic | Corrosion and ion release have been seen in the laboratory [6, 7], but no report of patient harm was found | Not proven |
| The NPG post comes loose early | The only comparative study found no difference in retention [9] | No evidence |
| It has no RCT, so it should not be used | No dedicated RCT was found for Ni-Cr posts either | Applies to Ni-Cr too |
| Its elastic modulus protects the root | Fracture resistance results are contradictory [1, 9, 12, 13, 14] | Not proven |
| NPG corrodes more readily than Ni-Cr | A direct laboratory study showed exactly this [6] | Laboratory evidence |
Neither NPG's toxicity for the patient, nor early loosening of its post, nor its mechanical superiority has been proven. What the laboratory evidence shows is that NPG corrodes more readily than Ni-Cr [6]. That is not enough for a scary headline, but it is a good reason to prefer Ni-Cr when we have the choice.
∆ References
PubMed studies were reviewed on the basis of their abstracts.
- Fracture strength of central incisors with NPG and Ni-Cr posts, and the composition of NPG (abstract): PubMed 29807967
- FDA clearance for NPG+2: K972971
- ADA classification: Materials for Indirect Restorations
- The manufacturer's claim of ADA acceptance: Aalba Dent catalogue
- FDA clearance for NPG: K863782
- Corrosion and cytotoxicity, NPG versus Ni-Cr (abstract): PubMed 41210404
- Ion release and toxicity of NPG+2 (abstract): PubMed 18546197
- Intraoral corrosion of copper-aluminum alloys (abstract): PubMed 11005911
- Retention, and fracture resistance in favor of Ni-Cr (abstract): PubMed 26046108
- The ISO 22674 requirement on ion release (as cited in an article, not the text of the standard itself): Ion release from three different dental alloys
- Elastic modulus of experimental copper-aluminum-nickel alloys, not NPG itself (abstract): PubMed 25006386
- Premolar fracture resistance in favor of NPG (abstract): PubMed 28808455, DOI
- Fracture resistance in favor of NPG (abstract): Journal of Dental Biomaterial 2014
- No difference in fracture resistance: Nodehi et al., Journal of Mashhad Dental School, 2023