SHARONLAY - A new onlay design for endodontically treated premolar.

Root-canal-treated teeth are structurally compromised as a result of loss of tooth structure due to caries, iatrogenic cavity preparation, and dehydration. Given that a direct relationship exists between the amount of remaining tooth structure and the ability to resist occlusal forces, it is vital to provide a restoration allowing cuspal coverage as soon as possible following completion of the root canal treatment. A decision to provide a full crown or an onlay depends on the remaining tooth structure; if the cuspal width to length ratio is 1:2 or more, an onlay can be placed. When the ratio is less than 1:2, a full crown has to be planned. In single-rooted teeth requiring post-endodontic restoration cast post and core or a prefabricated post can provide resistance to fracture with comparable results. However, in case of premolars contradictory to the practice of providing only cuspal coverage through Onlays, it would in addition require cervical reinforcement also to counter horizontal forces acting in cervical region. A new onlay design SHARONLAY patented design with I.P. no 1956475 dated 27/04/2010 with a post extending into the radicular portion of the premolar providing the required reinforcement in a conservative manner and protecting it against both vertical and horizontal forces is proposed herewith.

INTRODUCTION

Root-canal-treated teeth are structurally compromised as a result of loss of tooth structure due to caries, iatrogenic cavity preparation, and dentin dehydration. A direct relationship exists between the amount of remaining tooth structure and the ability to resist occlusal forces,[1] it is vital to provide a post endodontic restoration allowing cuspal coverage as soon as possible following completion of the root canal treatment.[2] A decision to provide a full crown or an onlay depends on the remaining tooth structure; if the cuspal width to length ratio is 1:2 or more, an onlay can be placed.[3]

When the ratio is less than 1:2, a full crown has to be planned. In single-rooted teeth, cast post and core or a prefabricated post have shown similar long-term results.[4] However in case of premolars, where mostly cuspal coverage is being practised, it becomes imperative to provide sufficient cervical reinforcement to counter the horizontal force at cervical region.[5678910]

A new onlay design (SHARONLAY) with a post extending into the radicular portion of the premolar providing the required reinforcement in a conservative manner and protecting it against both vertical and horizontal forces is proposed herewith. In this design, the onlay component protects the endodontically treated premolar from splitting under compressive loading and the radicular extension serves dual function of retention as well as protection from fracture at the neck due to tensile (horizontal) forces.

INDICATIONS

In single-rooted premolars, Sharonlay can be fabricated for mesio-occlusal, disto-occlusal, or mesio-occluso-distal lesions involving the pulp as a post-endodontic restoration. In case of teeth with short clinical crown requring additional retention. It is also indicated in premolars with two roots where at least one of the roots is parallel to the line of draw planned for the onlay. Also indicated in single-rooted molars where additional retention by means of extention into root canal is required. Also best suited for teeth with short clinical crown.

Step wise clinical procedure

On completion of root canal treatment, the chosen canal is enlarged to a minimum depth of 7 mm from the canal orifice. Additional retention may be achieved with increase in depth to more than 7 mm if the canal configuration and root length permits. The canal is minimally enlarged up to Peeso reamer # 3 (corresponding to file size 110) (Dentsply). The canal is further enlarged to peeso 4 (corresponding to file size 130) or 5 (corresponding to file size 130) depending on the initial canal diameter avoiding excessive tooth structure removal at the cervical region.

Since SHARONLAY is indicated for cases with adequate coronal tooth structure, post extension beyond 7 mm into the root canal may not be required because the post provides reinforcement at the neck of the tooth, but in cases with compromised coronal tooth structure (where one of the cusps is badly damaged) maximum apical extension of the post without jeopardizing the apical seal and radicular dentin has to be undertaken for better resistance and retention. Internal walls of the coronal cavity are finished with 5° taper on each wall and the buccal and lingual cusps are reduced from 1-2 mm depending on the material used for the restoration i.e. metal/zirconia respectively.

Acounter bevel of 0.5 mm on buccal cusps for esthetic reasons and 1 mm on lingual cusps is placed for the esthetic requirements and to obtain the hooding effect. Post space is reproduced in wax (GC Fuji Inlay wax) or with light body rubber base (Additional silicone, Zhermack) material and full arch impression is made with rubber base (Additional silicone, Zhermack). A removable die is prepared and an indirect wax pattern made (Type II wax, GC Fuji) and casting is done. Try in is done on the die before trying in the patients mouth. Once the occlusion, contour and contact have been checked, the casting is polished and cemented into the tooth with a luting cement (Type 1, GIC Fuji I).

SHARONLAY can be fabricated using base metal alloy, gold alloy, as well as ceramic (CAD-CAM).

Clinical cases

Case 1

A 25-year-old male patient reported for post endodontic restoration in relation to 35 (root canal treatment was done elsewhere). On removal of temporary access filling, the tooth was examined clinically and radiographically and was found suitable for SHARONLAY and the treatment was rendered [Figure 1a-d].

Figure 1

(a) Tooth prepared to receive SHARONLAY (b) Coronal tooth preparation with buccal and lingual reverse bevel (c) Cemented SHARONLAY on 35 (d) Post cementation radiograph of 35 with SHARONLAY

Case 2

Patient reported with pain in relation to 25, which was diagnosed as acute irreversible pulpitis and planned for routine endodontic treatment.

Analyzing the remaining tooth structure and the economic condition of the patient it was planned for SHARONLAY as the post endodontic restoration. The tooth has one palatal and one buccal canal. After completion of endodontic treatment, it was decided to prepare the palatal canal to receive the post and the buccal was blocked with polycarboxylate cement and the coronal tooth structure prepared to receive the onlay. In this design, since the palatal canal is in line with the line of draw of the onlay, a single component restoration was possible [Figure 2a]. The restoration cast in chrome-cobalt alloy was made [Figure 2b] and cemented in the endodontically treated 25 [Figure 2c]. The post cementation radiograph shows that the restoration is seated and contoured and also the radicular extension of the post is adequate for this case [Figure 2d].

Figure 2

(a) Prepared tooth to receive SHARONLAY on left maxillary second premolar (b) Finished and contoured SHARONLAY (c) SHARONLAY after cementation on left maxillary second premolar (d) Post cementation radiograph on left maxillary second premolars

Case 3

This male patient aged 35 years reported with signs and symptoms suggestive of periapical abscess in relation to 35, after completion of endodontic treatment it was planned for SHARONLAY to give the patient the advantage of radicular reinforcement and a functional post endodontic restoration which was lacking only in the esthetic parameters which did not matter as it was a lower second premolar and patient was a middle aged male patient. The post space and the coronal tooth structure was prepared [Figure 3a] the SHARONLAY fabricated [Figure 3b] and the restoration was cemented with luting glass ionomer cement (GC Fuji1) [Figure 3c]. The immediate radiograph shows the SHARONLAY in place with good adaptation [Figure 3d].

Figure 3

(a) Prepared tooth to receive SHARONLAY on left mandibular second premolar (b) Finished and contoured SHARONLAY (c) SHARONLAY after cementation on left mandibular second premolar (d) Post cementation intraoral periapical (IOPA) on left mandibular second premolar

Case 4

A 30-year-old male patient reported to my clinic with complain of fractured lingual cusp in relation to 25 (maxillary left second premolar), patient gave history of root canal treatment 5 years back. Taking into consideration the esthetic requirement of patient, SHARONLAY in zirconia using CAD-CAM (Cerec) was planned [Figures 4a-c]. The only disadvantage with SHARONLAY with ceramic is that it requires extensive tooth preparation; there is possibility of fracture at the post onlay junction in case the post is narrow. Hence, the post space preparation also has to be greater to avoid the fracture of the post and also post length of >7 mm is difficult to fabricate through CAD-CAM procedure. The follow-up in this case is not long, as it has been delivered in June 2012.

Figure 4

(a) Buccal view of SHARONLAY on left maxillary second premolar (b) Oclussal view of SHARONLAY on left maxillary second premolar (c) Post cementation radiograph on left maxillary second premolar

DISCUSSION

An onlay is the most conservative posterior post-endodontic restoration indicated when adequate tooth structure is available on buccal as well as lingual sides and for posterior teeth that are subjected to compressive loading. Onlay is also indicated when the vertical crown is inadequate for a full crown.

The premolars, which are generally single rooted and located anterior to the molars, are subjected to both compressive and tensile forces.[11] SHARONLAY is a design consisting of an onlay with post extending into the radicular portion casted into a single component giving the advantages of the onlay and radicular post extension. Single component restorations have a greater surface area for dissipation of stresses, thereby taking more load before fracturing compared to two unit components. In conventional post and core restorations, the post is extended 3-5 mm short of the apex, whereas in SHARONLAY, the radicular extension can be kept as minimal as possible (minimum 7 mm) so as to enhance resistance at the cervical region. Since, the post is used to provide resistance at the neck retention is not a major concern; however, in cases where the coronal tooth structure is weakened, the length of the post can be proportionately increased. The diameter of the post would depend upon the final preparation of the canal, with minimal enlargement with size # 3 peeso reamer in order to orient the post to the overlying onlay and provide adequqte strength to the post.

The onlay is designed conserving the tooth structure and preserving any healthy marginal ridge. The design is planned keeping in mind the aesthetic requirement and the hooding effect required to prevent splitting of the crown.

A preliminary in vitro study was carried out which compared the fracture resistance of endodontically treated tooth restored using this novel design. (Group I) with a two component restoration, i. e., post with separate onlay (separated by 2 mm) (Group II). SHARONLAY showed maximum resistance to fracture with a mean fracture resistance of 514.67N, which is higher than the maximum voluntary bite force recorded in the premolar region (422 ± 22N in males and 349 ± 24N in females). The fracture lines for both control group (Group III — only onlay without radicular extension) and Group II were seen at the cervical level splitting the cusps while in the SHARONLAY group, it was seen at the apical extent of the post. These findings indicate cervical reinforcement of premolars with this novel design.[12]

CONCLUSION

Premolars are subjected to both tensile and compressive stresses at the cervical region. Clinicians pay minimal attention to cervical reinforcement especially when there is adequate coronal tooth structure. In our in vitro study, the findings indicate that the post gives cervical reinforcement contrary to some of the studies.[13]

SHARONLAY design which is a single component (onlay with post) has been tested clinically for more than a decade. Evaluation of these restorations after 10 years shows promising long-term success; however, selection of appropriate case is of prime consideration for the success of the restoration.

One of the limitations is the visibility of metal on the premolars. Alternatively, with the advancement and availability of CAD-CAM technology, the same design may be fabricated using zirconia but this may require extensive tooth preparation which is unlikely in an endodontically treated tooth.