Assuming that you have been examined and treated appropriately for gum disease, there are basically two reasons for sensitivity of the gum tissues around the necks of crowns.
1. The type of metal that the crown was made of may be problematic.
More often, where the edge of the crown is placed may be infringing on what is called the biologic width, the necessary thickness of healthy skin. Sometimes, the biologic width can be compromised by the actual contour of the crown and not necessarily the position of the edge of the crown.
About 10% of the population is allergic to base metals, like nickel. Sometimes dentists opt to use base metal alloys for the fabrication of lower-cost crowns. Sometimes these may be appropriate. However, the risks of allergy must be accepted by the patient and thoroughly explained by the dentist. If an allergy is discovered once the crown is placed, then the patient should be willing to accept the financial responsibility to replace the crown.
While it may be impossible for patients to understand all of the nuances of the type of materials used for a filling or crown because they have not likely taken countless hours and courses of education about the science of dental materials, the dentist should be having a basic discussion about the general types of materials that are reasonable for a given situation as well as the common risks and benefits of each so that the patient can make an informed decision. For example, why would all ceramic crowns be chosen over crowns that have porcelain laminated over high quality or low quality metal alloys or over full metal crowns? Patients should be able to answer this before consenting to crown therapy.
2. More commonly, chronic irritation of the gums around a crown is due to infringement of the biologic width.
The minimal thickness necessary between the outer layer of the skin and the surface of the bone is normally about 2-3 millimeters. Within this critical area reside blood vessels, connective tissues, fat, nerves, and multiple layers of maturing skin. If there is not enough room for normal skin growth, then redness, pain, and chronic inflammation occur. Eventually, bone loss may occur to allow healthy gum tissue to form because the body likes to be healthy.
Oftentimes tooth decay extends below the gum line. When a dentist repairs the tooth with a filling or a crown, he/she must get below the decay to sound tooth structure. Therefore, violation of biologic width is quite common, and sometimes it is even with intention out of necessity. Fortunately, the body can sometimes adapt if the skin is thicker than 3 millimeters in the area of the restoration.
When the biologic width is obviously violated, a decision must be made to either endure the chronic discomfort created by the restoration or to create adequate space for mature skin to develop. Creating this space is usually accomplished by a surgical procedure called crown lengthening. Crown lengthening involves a minor gum line incision and some bone sculpting to allow ideal biologic width to develop below the neck of a restoration, which can be done by conventional surgical techniques (using a scalpel, sutures, etc.) or even by laser.
Ideally, the need for this procedure is determined before the restoration is completed and performed 6-8 weeks in advance of the restorative appointment. However, due to various reasons, the need to recreate ideal biologic width may not be recognized until several months after the restoration is completed. The treatment is the same, but recession and root exposure below the neck of the crown may occur, which usually does not compromise the function of the crown.
As always, communication with your dentist about any discomfort that you are experiencing is essential. Your dentist may choose to refer you to a periodontist, gum specialist, for evaluation. Depending on the reason for gingival sensitivity, localized cleaning of the root, crown replacement, minor surgery, or a combination of all three may be necessary.