Biomechanical Impact of Crown-to-Root Ratio Altering Treatments of Damaged Incisors

Objectives:
This is the first ex-vivo study investigated the biomechanical impact of procedures altering the crown-to-root ratio R_CR in the aesthetic zone, aimed to restore severely damaged teeth. There is no evidence on the impact of apical surgery (AS), orthodontic extrusion (OE), and surgical crown lengthening (SCL) on load capability of upper central incisors. Procedures were compared to control and dental implant-born restoration (IBR) by 5-year dynamic (TCML) and subsequent linear loading (LL).
Methods:
Human maxillary central incisors were endodontically treated, decoronated and divided into 4 groups (n=48). Following specimen preparation was performed: (I) adhesive core-and-post build-up, (II) as (I) and 2mm apical root resection (AS), (III) before adhesive core-and-post build-up 2mm of the coronal part was removed (OE) (IV) as (I) (SCL), group (V) individual abutments on titanium implants (n=12; Æ4.1/l=12mm) (IBR). Only specimens of group IV were embedded 4mm instead of 2mm below the CEJ to simulate SCL. All specimens were restored with all-ceramic crowns, crown-to-root ratios calculated, exposed to TCML, and subsequent LL until failure occurs. Statistics: log-rank, Kruskal-Wallis, Mann-Whitney-U, ANOVA, and Chi-Square tests (p=0.05).
Results: Fracture loads differed significantly (p=0.001) between groups. F_max median; min/max: (I) 275 (204/542), (II) 308 (243/443), (III) 270 (183/371), (IV) 206 (140/274), (V) 447 (370/539). Pair-wise comparison showed significant differences between teeth (group I-IV) and implants (group V) (p=0.001), and between I and IV (p=0.045), II and IV (p=0.001) and III and IV (p=0.033). R_CR differed significantly between group I compared to group II, III and IV (p=0.001). A R_CR below 1 (group IV) significantly decreased capability.
Conclusions:
SCL has a pronounced adverse effect on R_CR and subsequently on biomechanical behaviour, i.e. load capability of restored severely damaged teeth. OE appears to be a better option to ensure a 2mm ferrule to restore severely damaged teeth.