musiba no tiryou ni masui wo tukaimasu.

の connects two nouns.

Here, 虫歯の治療 means:

• treatment of cavities
• cavity treatment
• treatment for tooth decay

So の is not only for possession like John’s. It very often links nouns in a broad way, and the exact relationship depends on context.

Examples:

• 日本の文化 = Japanese culture / the culture of Japan
• 歯の痛み = tooth pain / pain in a tooth
• 虫歯の治療 = treatment for cavities

In this sentence, に marks the purpose or target use of something.

A very common pattern is:

A に B を使う
= use B for A

So:

• 虫歯の治療に = for cavity treatment
• 麻酔を使います = use anesthesia

Put together: 虫歯の治療に麻酔を使います
= We use anesthesia for cavity treatment.

This same pattern appears in sentences like:

• 料理に塩を使います = Use salt in/for cooking
• 勉強に辞書を使います = Use a dictionary for studying

Because 麻酔 is the thing being used, so it is the direct object of 使います.

• 麻酔 = anesthesia / anesthetic
• を = object marker
• 使います = use

So 麻酔を使います literally means use anesthesia.

麻酔を使います is a very natural way to say use anesthesia.

Japanese often expresses this idea with 使う when focusing on using something as a tool or method.

You may also hear 麻酔をします, especially in medical contexts. That usually means something like:

• administer anesthesia
• give anesthesia

So the nuance is a little different:

• 麻酔を使います = focus on using anesthesia
• 麻酔をします = focus on performing/administering anesthesia

In many real situations, both can appear, but this sentence is built around the pattern A に B を使う.

Japanese often leaves out the topic if it is already understood from context.

In a clinic, the speaker might mean:

• We use anesthesia for cavity treatment
• Our clinic uses anesthesia for cavity treatment
• The dentist will use anesthesia for cavity treatment

All of that can be left unstated.

If you wanted to add a topic, you could say something like:

• 虫歯の治療には麻酔を使います。

That は adds emphasis or contrast, something like:

• As for cavity treatment, we use anesthesia
• For cavity treatment, anesthesia is used

So the version without は is just a more neutral statement.

It is the polite non-past form, so it can mean either:

• use
• will use

Japanese non-past often covers both present/habitual and future meaning.

So depending on context, this sentence could mean:

• We use anesthesia for cavity treatment
  or
• We will use anesthesia for your cavity treatment

Context decides which one is intended.

That is normal in Japanese.

The subject is omitted because it is probably obvious from context. Possible implied subjects include:

• the dentist
• the doctor
• we (the clinic/staff)
• they

Japanese often leaves out subjects when they are not necessary.

So even though English usually wants a subject, Japanese does not always need one.

Japanese nouns usually do not show singular vs. plural the way English does.

So 虫歯 can mean:

• a cavity
• cavities
• tooth decay in a general sense

In this sentence, English might translate it as either:

• treatment for a cavity
• treatment for cavities
• treatment for tooth decay

The exact choice depends on the situation.

Because 虫歯の治療 is a noun phrase, and that fits very neatly into the pattern:

A に B を使う
= use B for A

So:

• 虫歯の治療に麻酔を使います
  = use anesthesia for cavity treatment

If you want to use the verb 治療する, you would need a different structure, such as:

• 虫歯を治療するのに麻酔を使います。
  = We use anesthesia to treat cavities.

That is also possible, but it is a different grammar pattern.

Usually に is the more natural choice with 使う when you mean for a purpose.

So:

• 虫歯の治療に麻酔を使います = natural, standard

If you use で, it can sound more like in the context of or during the treatment, rather than directly marking purpose. In some situations you may hear で, but for the clear pattern use X for Y, に is the better particle here.

A helpful way to remember it:

• 目的 / purpose → often に
• 手段 / means, location, or setting → often で

It is read:

むしば の ちりょう に ますい を つかいます。

Word by word:

• 虫歯 = むしば
• 治療 = ちりょう
• 麻酔 = ますい
• 使います = つかいます

One pronunciation note:

• The particle を is pronounced お, even though it is written を.

So aloud, 麻酔を使います sounds like: ますい お つかいます