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Azerbaycan Saytlari

 »  Home  »  Endodontic Articles 7  »  The ferrule effect: a literature review
The ferrule effect: a literature review
Summary of studies with artificial crowns - References.



Summary of studies with artificial crowns.
These studies showed that coronal extension of dentine above the shoulder increases the resistance to failure (Fig.2).This ferrule effect may be significantly enhanced when the coronal extension of dentine is at least 1.5 mm. This effect has been demonstrated in teeth restored with cast post and cores. A ferrule in teeth restored with direct posts and resin cores may not offer any additional benefit.
In all the in vitro studies, both with and without crowns, only single rooted teeth have been investigated. The influence of the ferrule effect on multi-rooted teeth remains an area for further research.

Figure 2. Coronal extension of dentine above the shoulder provides an effective ferrule.

Coronal extension of dentine above the shoulder provides an effective ferrule

The ferrule effect in vivo.
There appear to be no reports of prospective clinical investigations of the ferrule effect. A retrospective study of the survival rate of two post designs was conducted by Torbjorner et al. (1995).Their study involved reviewing the records of 638 patients, with a total of 788 posts; the patients were not examined. Seventy-two of the posts failed. Most (46 cases) failed owing to the loss of retention. It was observed that all the post fractures (six cases) occurred in teeth with a ‘‘. . . lack of a ferrule effect of the metal collar at the crown margin area . . .’’. The remainder failed owing to root fracture. Their study did not state how many crowns surveyed had a ferrule. From the work of Sorensen & Engelman (1990) and Libman & Nicholls (1995) it has been shown that the position and length of the ferrule is of significance. Torbjorner et al. (1995) did not indicate whether these design features were recorded in the patients’ files. Furthermore, radiographs do not allow assessment of the amount of tissue remaining under a crown with a metal substructure. Therefore, it would be prudent for future studies to make some record of the ferrule, oreven consider keeping dies.
A classification of the single-rooted pulpless teeth based upon the amount of remaining supra gingival tooth structure has been recommended (Kurer 1991). Five classes of pulpless tooth were described; class I has sufficient coronal tissue for a crown preparation, II has coronal tissue but requires a core, and III has no coronal tissue. The classes IV and V refer to the complications of intra osseous fracture and periodontal disease. However, this classification does not account for a minimum effective ferrule length. It would perhaps be of value to add a further subclassification of ferrule length to the Class II type of pulpless tooth. A suitable distinction would be less than or at least2 mm-ferrule length.

Preoperative tooth assessment.
In assessing whether a tooth should be restored, the clinician must consider the amount of remaining supragingival tissue. Libman & Nicholls (1995) demonstrated in vitro that the minimum effective ferrule should be 1.5 mm of coronal dentine extending beyond the preparation margin. Below this length, there is a significant decrease in resistance to preliminary failure. Without supportive in vivo research the clinician is left to question whether satisfactory treatment can still be provided where a ferrule is absent or that is shorter than that advised in this in vitro study.
A minimum ferrule length of 2 mm to compensate for the difficulties of intraoral tooth preparation has been recommended (McLean1998). Citing a paper by Freeman et al. (1998), it was noted that even where a1-mmferrule was attempted extraorally, it was difficult to achieve. Overcompensation is thus advisable to achieve an adequate length of parallel dentine, to produce an effective ferrule.
The ferrule length that may be obtained will be influenced by the ‘biologic width’. This is defined as ‘‘. . . the dimension of the junctional epithelial and connective tissue attachment to the root above the alveolar crest’’ (Sivers & Johnson 1985). If unpredictable bone loss and inflammation is to be avoided, the crown margin should be at least 2 mm from the alveolar crest. It has been recommended that at least 3 mm should be left to avoid impingement on the coronal attachment of the periodontal connective tissue (Fugazzotto & Parma-Benfenait 1984).Therefore, at least 4.5 mm of supra-alveolar tooth structure may be required to provide an effective ferrule.
In those clinical situations where there is insufficient ferrule length, even where margins are placed subgingivally, the clinician may consider surgical crown lengthening or orthodontic extrusion. This allows the distance between the crown margin and alveolar crest to bewidened, and increases the potential ferrule length. Methods to increase ferrule length will reduce the root length and result in more tooth loss, possibly making the crown to root ratio unfavourable. Furthermore, both procedures will add to the cost of restoring the tooth, prolong treatment time and cause discomfort to the patient. The study by Al-Hazaimeh & Gutteridge (2001) suggests that the use of a resin-bonded direct post and resin core may be a preferable alternative where a ferrule can not easily be obtained.
The effect of crown lengthening to establish a ferrule on static load failure was investigated (Gegauff 2000). Using restorative composite resin to make root analogues, two groups of10 teeth were tested. One group simulated teeth that had received crown lengthening and a ferrule in the preparation; the other group were not crown lengthened and were without a ferrule and no remaining clinical crown. All the teeth had cast post retained cores and crowns cemented prior to testing. Both the alveolar bone and periodontal ligament were simulated in the study. The teeth were subjected to a load by a universal testing machine until failure (peak load). The group that had crown lengthening and a ferrule demonstrated a significantly lower failure load.
Gegauff (2000) noted that the apical relocation of the finish line following crown lengthening resulted in a decrease in the cross section of the preparation. Subsequently, this reduction in tissue combined with an altered crown to root ratio may result in weakening of the tooth. He also pointed out that orthodontic extrusion may be preferable to crown lengthening as it results in a smaller change in the crown to root ratio. The results of that study suggested that the overall importance of a ferrule at the expense of remaining tooth structure was unclear.
A balance between the ferrule length obtained and the remaining root is there for eneeded. These considerations are obviously best made prior to root canal treatment. If a suitable ferrule length cannot be obtained, the patient should be informed of the potential compromise.

References.

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  Freeman MA, Nicholls JL,Kydd WL,Harrington GW(1998) Leakage associated with   load fatigue-induced preliminary failure of full crowns placed over three different   post and core systems. Journal of Endodontics 24, 26-32.
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