How does an onlay repair a tooth with a broken cusp?

What the drawing is measured from

1. Among members of a dental care plan followed for 105 days, the tongue-side cusps of lower back teeth broke twice as often as the cheek-side ones; 95 per cent of all the breaks exposed dentine, 3 per cent reached the pulp and 24 per cent extended below the gum. Bader et al. 2001, Community Dentistry and Oral Epidemiology. DOI
2. In 28 general dental practices in the Netherlands over three months, 238 complete cusp fractures were recorded: in lower molars 75 per cent were tongue-side cusps, and almost 77 per cent of the broken teeth had been restored on three or more surfaces. Fennis et al. 2002, The International Journal of Prosthodontics. PubMed
3. A review of tooth fracture found that large restorations and extensive decay tend to be associated with most fractures, that fractures are commoner in first permanent molars than in other teeth, especially in the lower jaw, and that covering the cusps may reduce the risk in restored teeth. Burke 1992, Journal of Dentistry, review. DOI
4. A clinical protocol for bonded inlays and onlays, drawing on the long-term clinical evidence, places an adhesive base (dentine bonding and cavity design optimisation) before the impression, so that the preparation stays more conservative and the steps are easier to follow. Dietschi and Spreafico 2015, The International Journal of Esthetic Dentistry. PubMed
5. Immediate dentine sealing means applying and curing a dentine bonding agent on the freshly cut dentine straight after the preparation, before the impression or scan; a review of more than 30 articles found that it appears to give better bond strength, fewer gaps, less bacterial leakage and less sensitivity. Magne 2005, Journal of Esthetic and Restorative Dentistry, review. DOI
6. In 30 upper premolars restored with milled lithium disilicate overlays, a butt-joint preparation fitted as closely as a hollow chamfer, with marginal gaps of about 17 and 16 micrometres after cementation, and significantly more closely than a conventional box design, about 34 micrometres. Athab Hasan and Mohammed-Hussain Abdul-Ameer 2023, The Saudi Dental Journal, laboratory study. DOI
7. In 160 men followed for 26 years, filling margins placed below the gum line were detrimental to gum and periodontal health; the extra loss of attachment began slowly and could be detected 1 to 3 years after the restorations were placed. Schätzle et al. 2001, Journal of Clinical Periodontology. DOI
8. In one private practice, 556 pressed lithium disilicate inlays and onlays in 304 patients were followed for up to 10.9 years: the estimated survival of the 305 onlays was 98.3 per cent at 9.8 years, the overall failure rate was 0.3 per cent a year, and all six failures were in molars. Malament et al. 2021, The Journal of Prosthetic Dentistry. DOI
9. A systematic review and meta-analysis of six randomised trials of onlays, occlusal veneers and partial crowns on back teeth estimated three-year survival at 93.7 per cent for lithium disilicate restorations; the authors note that medium and long-term trials are still needed. Prott et al. 2024, Journal of Esthetic and Restorative Dentistry. DOI
10. For glass-ceramics such as lithium disilicate, a systematic review of 83 laboratory studies found that etching with hydrofluoric acid followed by a silane is the most widely accepted surface treatment before a resin cement. Tian et al. 2014, Dental Materials. DOI
11. The case and the steps are drawn as Dr. Khalid works: a living lower first molar whose tongue-side cusp has broken off beside a large old filling, the break staying above the gum; the old filling removed and the broken surface smoothed flat, ending in a butt joint with no bevel; immediate dentine sealing on the freshly cut dentine and a composite base that fills the old filling's undercut; and a lithium disilicate onlay bonded with resin cement under rubber dam, keeping the strong cheek-side cusp. The old cavity and the line of the break are illustrative.
12. The enamel follows one measured dataset for this tooth, from cone-beam CT of 45 lower first molars: 1.58 mm at the front cheek-side cusp tip and 0.98 mm at the central fossa, its thinnest point. Zhang et al. 2026, BMC Oral Health. DOI
13. Two canals in the front root, as drawn, are found in 94.4 per cent of lower first molars, and three in 2.3 per cent, from a systematic review of 41 studies covering 18,781 teeth. de Pablo et al. 2010, Journal of Endodontics. DOI
14. Bone level: on cone-beam CT of 100 lower molar sites in 25 adults, the crest of the bone sat 2.03 to 2.11 mm below the enamel edge of first molars, with 1.14 to 1.19 mm of bone over the cheek side of the root near its neck. Othman 2026, The Saudi Dental Journal. DOI
15. The cheek-side cortical plate near the crest is drawn from cone-beam CT of lower first and second molars, where it measured 1.1 to 1.4 mm. Sayed et al. 2021, The Saudi Dental Journal. DOI
16. Further from the crest, on cone-beam CT of 100 people, between the lower second premolar and first molar: the cheek-side plate measured 1.93 mm at 4 mm below the enamel edge and 2.36 mm at 6 mm, and the tongue-side plate 2.18 and 2.09 mm. Gupta et al. 2025, BMC Oral Health. DOI
17. The gum is drawn 1.2 mm thick, within the 0.8 to 1.5 mm measured by ultrasound across lower teeth in 200 periodontally healthy men. Eger, Müller and Heinecke 1996, Journal of Clinical Periodontology. DOI
18. The tooth follows the textbook descriptions of a lower first molar rather than a symmetrical outline: the crown leans toward the tongue on its root, the front root's tip lies under the front cheek-side cusp, the roof of the pulp chamber sits at the level of the enamel edge with the floor in the root trunk, and the ligament around the root is drawn 0.2 mm wide, under the quarter of a millimetre the textbooks give, inside a thin lining of compact bone. Wheeler's Dental Anatomy, Physiology and Occlusion, 11th edition, and Woelfel's Dental Anatomy.
19. The roof of the pulp chamber sits at the level of the enamel edge, where it lay in 97 per cent of 100 lower molars measured on radiographs, and the drawing's distance from the cheek-side cusp tip to that roof, 6.36 mm, is the study's mean. Deutsch and Musikant 2004, Journal of Endodontics. DOI