Restorative Mission: No Longer Impossible
How chairside CAD/CAM improves a complicated procedure
Mike Moroni, DDS
Replacing a crown that is an abutment for an existing removable partial can present a struggle using conventional techniques, often requiring several impressions of the preparation with and without the partial in place. With chairside digital dentistry (CAD/CAM), it does not have to be a daunting task. We don’t have to wonder if the crown will fit, if the clasps will engage the tooth correctly, if the partial will have to be remade, or even have to submit the partial with the case. This is all easily done with digital technology in dentistry today—chairside and in one appointment.
A healthy 72-year-old woman presented with an uncomplicated medical history, and she was on no medications. She presented to the office with the porcelain on her existing PFM fractured and her partial feeling like it rocked some. Upon clinical examination, the partial was well made and still in excellent shape, having successfully served the patient for 10 years. The patient was comfortable with the partial, liked the fit, and was on a limited income with social security. She wanted to spare the additional expense of a new partial. The porcelain had fractured off of the occlusal lingual. Upon further inspection, the rest seat was still in tact, and the clasps were engaged correctly, with the exception of the buccal being slightly non-engaged. The porcelain was not fractured on the buccal. The clasp was adjusted so that it properly engaged the porcelain on the buccal of the restoration.
The fractured crown was scanned prior to removal. This preoperative image is critical to get an exact fit of the partial and is used as a copy (preoperatively) for the design. If the crown is missing entirely, a clinician can place resin on the prepped tooth, place petroleum jelly on clasps and rest seat of the partial, and place on the resin. The resin will form around the contour of clasps, and the clinician spot cures to harden. The clinicians then removes the partial, spot cures again, and removes flash and excess resin so that a crown is formed. After this step, the clinician places the partial to check the fit again, and then scans the resin. This is then a preoperative image that the final crown will mimic. Once the prior crown is removed, the clinicians removes decay if present, and scans the preparation.
This patient’s preparation followed proper guidelines for a lithium disilicate milled restoration and soft tissue was managed so all margins were visible. The preparation and neighboring dentition was scanned with Planmeca’s PlanScan (part of the full Planmeca Fit System).
Once this scan was completed, we proceeded with designing the crown from the shape, size, and contours of the previous crown. If the preoperative scan of the tooth is not available, it is necessary to scan the teeth directly opposing the prepared tooth. The buccal bite is then imaged while having the patient bite down. The bite is critical when creating a crown so that the occlusal of the restoration is aligned and designed in accordance to the patient’s bite. With the preoperative image of the previous crown, no opposing or buccal bite is required.
The scan is then handed off to the dental assistant or dental auxiliary. This varies by state, but the dental auxiliary can scan the prepared tooth, the opposing arch, and the buccal bite. The margin is then identified and drawn, and the proposal is adjusted according to the patient’s bite or pre-designed image. The restoration was then milled with the PlanMill 40. After milling, the restoration should be tried in and adjusted if needed. Contacts and margins are checked at this stage. Occlusion is not checked at try-in, as the patient could fracture it upon checking, or cause micro-fractures in the materials, which could cause restoration breakage. Once tried in, the restoration is characterized (if desired), and fired in an oven to crystalize the material (IPS e.max, Ivoclar Vivadent, www.ivoclarvivadent.com).
The finished restoration for this patient was bonded onto the prepared tooth with an acceptable bonding agent and cement. The partial was then tried in to ensure fit and function with the existing partial denture.
This type of clinical case may present in the dental office a few times per year because this is not a very common case. However, it is more common to have to replace a crown on a patient that wears a retainer, occlusal guard, sleep apnea device, etc. It is most beneficial for the patient to be able to fabricate a replacement restoration that conforms precisely to the appliance that they have, saving the additional costs associated with replacing the appliance because it does not fit anymore with a new crown that is shaped differently.
The author has no relevant financial relationships to disclose.
About the Author
Mike Moroni, DDS
Castle Rock, Colorado
For more information, contact:
Planmeca CAD/CAM Division