Loss of Cusps vs. Marginal Ridge: Impact on Fracture Resistance
Loss of mesiolingual and mesiobuccal cusps significantly compromises fracture resistance and strength more than loss of the mesial marginal ridge alone, and cusp coverage is more critical than preserving marginal ridges for maintaining tooth strength.
Evidence-Based Rationale
The biomechanical hierarchy is clear: cusp coverage is more important than preserving tooth structure, including marginal ridges 1. This finding fundamentally challenges the traditional emphasis on marginal ridge preservation.
Marginal Ridge Preservation Has Limited Protective Value
- Preserving a marginal ridge in molars does not fully conserve the strength of adjacent cusps 1
- When marginal ridges remain intact but cusps are lost, the tooth still experiences significant structural compromise
- The protective effect of an intact marginal ridge is insufficient to prevent the biomechanical consequences of cusp loss 1
Cusp Loss Creates Critical Structural Weakness
Cuspal deflection increases progressively with cavity size and reaches maximum levels following endodontic access 2. The specific findings include:
- Cuspal deflections exceeding 10 microns occur with extensive preparations 2
- This degree of flexure creates substantial risk for marginal leakage and cuspal fracture 2
- Loss of cusps without coverage results in fracture resistance dropping to approximately 40% of intact tooth strength (60.54 kgf vs. 151.40 kgf) 3
Cusp Coverage Restores Structural Integrity
The evidence demonstrates that cusp coverage reinforces the compromised tooth structure effectively 1:
- Full occlusal coverage strengthens all cusps, with gold performing most consistently 1
- Selective cusp coverage reinforces only the capped cusps, not adjacent structures 1
- Cusp coverage with composite resin restores fracture resistance to near-intact levels (141.90 kgf vs. 151.40 kgf for intact teeth) 3
Biomechanical Mechanism
Finite element analysis confirms that cuspal coverage leads to higher fracture resistance by redistributing occlusal stresses 4:
- The highest safety factors are observed with cuspal coverage 4
- Without coverage, stress concentrates at the tooth-restoration interface 4
- Failure of the tooth-restoration interface is more likely than composite material failure when cusps are not covered 4
Clinical Algorithm for Decision-Making
When both mesiolingual and mesiobuccal cusps are lost:
- Cusp coverage is mandatory regardless of marginal ridge status 1, 3
- Direct or indirect composite restorations perform equivalently when providing full cuspal coverage 5
- The restoration must include 2.0 mm of cuspal reduction and coverage 3
When only the mesial marginal ridge is lost:
- Cusp coverage is still recommended if the preparation is extensive 2
- Marginal ridge preservation alone provides inadequate protection 1
Critical Pitfall to Avoid
Do not prioritize marginal ridge preservation over cusp coverage 1. The traditional teaching that marginal ridges are the primary structural element is contradicted by biomechanical evidence showing cusp coverage is the dominant factor in fracture resistance.