DDS students should complete this experience during their second year working with IPT 2 faculty and designated clinical crown block faculty. IDS students should make every effort to initiate crown block during fourth quarter of their first year and complete the experience no later than the first quarter of their second year.
Crown block is a highly structured initial fixed prosthodontics experience. It is designed to familiarize each student with the clinical protocols in fixed prosthodontics. This experience is closely supervised to ensure an optimal outcome for both the student and patient. Each student must identify a patient for crown block. This patient must be willing to come in on the same day of the week until the prosthesis is delivered. Once the student has obtained a patient for crown block, he/she must select a faculty member to work with throughout the entire crown block. This faculty member will closely mentor each step of the procedure, demonstrate cord packing, and assist with the final impression and delivery of the definitive prosthesis. Each student must obtain and manage the “Crown Block Form.”
You must have mounted models and pressed form splints and putty matrix. Accurate pressed formed splints are required on all clinical fixed prosthodontic procedures. These should be fabricated from a duplicate cast of the diagnostic wax-up or pre-treatment cast. A clear splint will also help you determine the correct amount of reduction needed.
Prior to any crown or bridge preparation, the pupal vitality of proposed teeth should be determined. Ideally, this will be performed during the treatment planning phase. All custom trays must be perforated.
A checklist (paper form) must be completed and signed by an instructor prior to beginning a Fixed Prosthodontics or Implant case. A start check will not be given if the checklist is not completed and signed. This checklist should be attached to the lab form. Prior to preparation, instructors will verify that mounted study models are present with diagnostic wax-up, pressed form splint and putty matrix are present, tooth vitality is checked with recent PA available and the MI contacts are recorded using shim stock.
One of the main reason cases are returned to the laboratory for modification is incorrect shade selection. The shade selection must be performed at the beginning of the final impression appointment. The shade must be verified and signed by two instructors. Once the instructor has confirmed the shade, he/she will write their instructor number and initials next to the shade selection on the laboratory prescription form. For porcelain restorations located in the esthetic zone, a 1:2 ratio retracted photograph with selected shade tab(s) must be sent to the laboratory. For
E.max restorations, photographs of the stump shade must be taken as well. The image should be sent via e- mail to email@example.com. The image should be labeled with the patient number and name.
First Appointment – Crown Prep
- Anesthetize the patient
- Check occlusion if the tooth is intact, and visualize what the prep should look like after the preparation.
- Complete the prep, making sure that you have adequate occlusal reduction. Ideal amounts of reduction are:
- FVC – 1.5 mm occlusally with a chamfer around the tooth
- PFM – 2.0 mm (at least) occlusally for porcelain occlusal with a 1.2 mm shoulder facially and a chamfer lingually.
- You can verify the reduction using bite registration and your Boley or Crown gauge, warmed green wax, or red rope wax. With the wax technique you can hold it up to the light to see which areas get thin, or you can also carefully insert your perio probe to estimate the thickness. The pressed form clear splint can also be used to visualize tooth reduction.
- FVC – 1.5 mm occlusally with a chamfer around the tooth
- Make a temporary with your splint or putty and trim it.
- Cement with TEMPBOND NE. Measure out equal amounts from the packet. When you are ready to cement, mix them together, place it in the temporary and seat the temporary.
- Eugenol is a sedative agent that is great for calming an inflamed tooth, but does interfere with the bond strength of the resin cements and resins for temporary restorations. If your patient’s tooth isn’t uncomfortable, always use N.E. Temp-bond or Ultratemp (a carboxylate based temporary cement) so that you won’t restrict the cementing options in the future. If you still plan on refining your prep later, then you can use eugenol cement, as you will prep away the contaminated tooth structure. IRM cement, sometimes used for temporary cementation, contains eugenol.
- Have the patient bite down slowly on a cotton roll and hold it.
- Give your patient post-op instructions. Instruct the patient that it is normal to have some sensitivity for several days after the procedure. The patient should be informed that it is only a weak cement holding the temporary in place and so they should avoid eating anything sticky or hard on that side. To keep it clean, they should floss the tooth and pull the floss out the side. They should also brush as normal.
- Make your next appointment for the impression as soon as possible
General Impression Protocols
Generally, stock trays are provided for all final impressions. In dental arches that do not fit stock trays, the instructor may advocate the use of a custom tray. Custom trays should be fabricated using Pacific protocol and be available at the preparation appointment. When using a custom tray, only light body impression material should be utilized. For multiple units, a full arch impression is mandated. After 8 final impressions have been successfully submitted to PSL, quadrant impression trays or Triple Trays impression trays may be used. Quadrant trays and Triple trays must be utilized only for single unit posterior restorations with adjacent teeth in proper occlusal contact.
For each first attempt at a final impression, the student doctor may attempt the procedure without instructor help or guidance (except for crown block). Dual cord technique is taught and practiced at Pacific. If the student is unsuccessful with the first attempt, the instructor is required to provide verbal guidance during the second attempt. The instructor may also serve as the “dental assistant” during the second attempt. If the student is unsuccessful in capturing the second impression, the instructor is required to take the third impression and demonstrate the proper techniques for the student doctor.
- Cover your diagnostic cast with a thin layer of Vaseline.
- Warm up a section of pink baseplate wax under hot water or with a flame and mold it to the cast (two layers).
- Once the wax is adapted, cover it with a little Vaseline so that the wax will not stick to your tray.
- Cut out three occlusal stops (tripod), trying to stay 2 teeth away from the tooth to be prepped.
- Adapt TRIAD to the wax-covered cast, saving some material for the handle in front.
- Cure the tray for 2 minutes (do not leave it for too long as the heat from the curing light will tend to melt your wax) then remove the tray from the cast and cure it for an additional 3-4 minutes.
- Smooth and finish the edges of the tray for your patient’s comfort with your Brasseler. Make 12-20 holes of adequate size in the tray for retention of impression material.
Clinic Tip: If your patient has an amalgam that is below the gumline on the tooth to be prepped, make sure that you have discussed the possibility of crown lengthening surgery. Crown lengthening surgery will require 4-6 weeks of healing before you can continue with the crown preparation.
Second Appointment - Taking an Impression
For the impression you will need an assistant. Ask a classmate or instructor. Spending 25 minutes on one good impression is much better than spending an hour on five bad impressions. Although it may not seem like it, making a custom tray is really worth it.
- Cotton Rolls – Start with good isolation, cotton rolls on the buccal and lingual of mandibular teeth, and buccal of maxillary teeth. You can ask your patient to place their tongue to the roof of their mouth when placing lingual cotton rolls.
- Cord #1– Packing a smaller cord circumferentially around the tooth, starting interproximal and ending with no overlap. Be sure that this cord is vertically below the margin of your prep: this will give you a lot of flash.
- Cord #2 – You may need to pack a larger cord in areas that still need retraction. If your prep is sub-gingival, be sure that the tissue is stretched out horizontally adequately. You will want to remove this cord right before you impress, but leave it in for about 5 minutes to allow the gingiva to expand so that the tissue will stay away from your margins while you impress.
- Hemostatic Agent – You may wish to scrub bleeding areas with hemostatic agent or apply directly to the margins for moisture control. (Make sure that there are no medical contraindications.) Ferric sulfate tastes terrible, so be sure to be sympathetic as well as warn your patient. After this dries, and when you remove your second cord, you can wash everything with water or a 2x2, and then dry again.
- Rope Wax – If your patient has a lot of bridgework, implants or heavy undercuts; you may need to use wax to block out some areas. This will make it a lot easier and less painful to remove the tray.
- Air-dry everything – While you keep everything dry and remove the second cord, then dry everything again. Your assistant should fill your tray after you have pulled the cord and inspected the prep for clear margins. Before you inject, be sure that everything is clean and dry and that you can see all of your margins. You may need to adjust your first cord again if it has come out of the sulcus. When the tray is almost filled, start injecting around your prep with the MOJO syringe.
- Syringe – Start injecting on the distal margin of the tooth (or the area that is most difficult to access) and inject into the gingival sulcus, and circumferentially move slowly around and up the tooth. Always keep the tip inside the impression material and if you hear any bubbles, repeat the area.
- Tray – Fully seat your impression tray. Have the dental mirror ready to remove any excess material from the back of the palate.
- Tray – Remove tray, your air water syringe may help to break the seal. You should also floss your patient and have some mouthwash ready.
- Clean – Disinfect your impression, carefully remove any cord and cotton rolls that have been incorporated and look at your impression under the scope. You may want to soak your impression in water to loosen your cord. Spray it with disinfectant and place it in a bag with your patients name and number and your name and number. Fill out your paperwork, have it signed off, and take everything to the lab. All master casts for fixed prosthodontics must be mounted by the student doctor.
- Fast Track: After 4 full arch impressions of fixed prosthodontics have been submitted to the laboratory, all posterior unit restorations are eligible for the fast track process. No anterior teeth (including cuspids) are eligible for fast track process.
- Additionally, the student doctor must produce a diagnostic wax-up and custom incisal guide table when indicated (see Custom Incisal Tables below). In order to be eligible for the fast track process, final impressions must be submitted with a completed laboratory prescription, bite registration, mounted opposing cast and a diagnostic wax-up. Once these items have passed the quality assurance process, the case will be fabricated without further student involvement.
In Between Appointments Two and Three
You should receive your master cast back from the lab in about 4 working days. Along with the cast, you will receive your prescription form with a green checklist stapled to it, which details what you are expected to turn in. Your instructor must sign this checklist when you are done. These steps include the following:
- The cast must be mounted in MI to the opposing cast. During mounting, make sure to cover the die holes with a wet paper towel or Play-Doh.
- Create ideal occlusion by adjusting ONLY the master cast. Use articulating paper to create ideal occlusal contacts and eliminate interferences in excursive movements. Time spent perfecting the cast occlusion will save a lot of time at the CIMOE appointment.
- Once occlusion is established, make a custom incisal guide table. Be certain that the pin moves smoothly along the table and the teeth. Remove all excess material in front of the pin.
- If the tooth was whole prior to the prep, include the diagnostic cast when you turn it in to the lab. If the tooth was broken down, you must provide a wax-up that functions with the correct occlusion.
- Obtain signatures from your instructor.
- Turn in diagnostic casts, master cast, incisal guide table, and articulator, making sure to put both your name and number on your patient’s casts and the table. You must also include the condylar inclinations for both the right and left sides (usually between 30-50 degrees) as well as the prescription signed by your instructor. Your work must pass “quality control,” so be sure to check on it after you have turned it in.
- You may track it again online. In general it takes 7-10 working days for a single unit crown.
Custom Incisal Tables
Custom incisal tables are required for your first four cases. After, incisal tables are required for all cases involving any canine preparation or 2 or more posterior teeth involved in group function or 2 or more anterior preparations.
Third Appointment – Seating Crown
- Anesthetize the patient if necessary. Remember, when dentin is exposed, it will be sensitive to air and cold water. The patient can decide to not use anesthetic so that they can feel whether their bite is normal, but they must be informed that they might have sensitivity during the procedure.
- Use the CIMOE (Contacts, Internal, Margins, Occlusion, Esthetics) technique to seat your crown. Don't adjust contacts with high speed coarse diamonds because you may over adjust. Also check the restorations on proximal teeth for bur marks in contact areas from previous preparation. Polish these areas prior to contact adjustment of the new crown. Make sure to place gauze in the patient’s throat to prevent the patient from swallowing the crown as you take it in and out of the mouth. You will need “FIT-CHECKER” and a red pen for the internal aspect of the crown (FVC and PFM). Make sure also to have your porcelain or gold polishing kit available along with your slow-speed with the straight nose cone attachment.
- Once the crown seats fully, repeatedly and the occlusion is not heavy and has been adjusted to IDEAL (it is very hard to adjust later in the mouth) you are ready to cement.
- Discuss with an instructor which cement should be used. Please see section on cements.
- Place the cement into your crown in a fairly thin layer (do not fill completely) so that all surfaces are covered. Seat the crown on the tooth and while maintaining pressure on it, check the margins and occlusion to make sure they are the same as when you seated it before.
- Do not have patient bite on a cotton roll. Too often they will bite unevenly and unseat your crown.
- Hold crown in place for cement set up.
- Let the cement set and then clean it off making sure to clear interproximal areas.
- Check the contacts and the occlusion again.
- Give your patient post-op instructions. Let them know that the cement will not reach maximum strength for 48 hours, so the patient should not chew anything sticky or hard for the next few days. Let them know that their bite may change in the next few days since they are no longer anesthetized, and that if it continues, they need to come back for an adjustment.