Effect of apical root resection, orthodontic extrusion, and surgical crown lengthening on load capability

This study aims to investigate the load-to-fracture of procedures changing crown-to-root ratio (R_CRR) aimed to restore severely damaged upper central incisors to avoid tooth extraction compared to implant placement. There is no evidence on load capability after apical root resection (AR), orthodontic extrusion (OE), and surgical crown lengthening (SCL) in respect to R_CRR, respectively.

Human maxillary central incisors were endodontically treated, decoronated, and divided into 4 groups (n = 48). The following specimen preparation was performed: (I) adhesive core-and-post build-up (control), (II) as (I) and 2 mm apical root resection (AR), (III) before adhesive core-and-post build-up teeth were shortened 2 mm coronally (OE) (IV) as (I), but specimens were embedded 4 mm instead of 2 mm below the CEJ (SCL), group (V) implant-borne restoration with individual all-ceramic abutments (n = 12; ∅4.1/l = 12 mm) (IBR). All specimens received all-ceramic crowns, thermo-mechanical (TML), and subsequent linear loading (LL) until failure. R_CRR were calculated and log-rank, Kruskal-Wallis, Mann-Whitney U, ANOVA, and chi-square tests applied (p = 0.05).

Fracture loads after subsequent LL differed significantly (p = 0.001) between groups, while implants showed the highest values. F_max median (min/max) were as follows: (I) 252 (204/542), (II) 293 (243/443), (III) 253 (183/371), (IV) 195 (140/274), and (V) 446 (370/539). Pair-wise comparison showed significant differences (p = 0.001) between group I/IV and group V, I, and IV (p = 0.045), II and IV (p = 0.001), and III compared to IV (p = 0.033), respectively. R_CRR below 1 significantly increased load capability compared to R_CRR = 1.

OE appears to preferably ensure biomechanical stability of teeth that are endodontically treated and receive core-and-post and crown placement compared to SCL. AR has no adverse biomechanical impact. R_CRR < 1 is biomechanically beneficial.

For endodontically treated and restored teeth, orthodontic extrusion should be preferred compared to surgical crown lengthening prior single-crown restoration. As orthodontic extrusion, apical root resection has no adverse effect on load capability. Single-crown implant-borne restorations are most load capable.