**Core Concept**
Lingual fixed appliances, also known as lingual orthodontic appliances, are a type of orthodontic device that is attached to the back of teeth, making them invisible from the front. This is particularly useful for patients who are concerned about the aesthetics of traditional metal braces.

**Why the Correct Answer is Right**
The lingual fixed appliance is used in cases where the patient requires extensive orthodontic treatment, but also wants to maintain a discreet appearance. The appliance is custom-made to fit the individual's teeth and is attached to the lingual surface of the teeth using brackets and wires. This type of appliance is commonly used for correcting complex malocclusions, such as Class II or Class III bites, and for aligning teeth that are severely crowded or rotated.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because lingual fixed appliances are not typically used for treating simple orthodontic cases, such as mild crowding or spacing.
* **Option B:** This option is incorrect because lingual orthodontic appliances are not usually used for patients who are under the age of 12, as their teeth and jaw are still developing.
* **Option C:** This option is incorrect because lingual fixed appliances are not typically used in conjunction with removable appliances, such as retainers or aligners.

**Clinical Pearl / High-Yield Fact**
It's essential to note that lingual fixed appliances require a higher level of oral hygiene due to their location on the back of the teeth, making regular cleaning and maintenance crucial to prevent plaque buildup and gingivitis.

**Correct Answer:** C.