Effect of proximal wall height on all-ceramic crown core stress distribution: a finite element analysis study

Mechanical analyses of idealized crown-cement-tooth systems through finite element analysis (FEA) has provided valuable insight concerning design parameters and materials that favor lower stress patterns. However, little information regarding variation of basic preparation guidelines in stress distribution has been available. The primary objective of this study was to evaluate maximum principal stresses on a molar crown veneer plus core system natural tooth configuration preparation with variations in the ratio of proximal axial length (PAL) to buccal axial length (BAL) as well as loading condition and position.

Three-dimensional models comprising a crown veneer (porcelain), crown core (zirconia), cement layer, and tooth preparation (4.2 mm BAL with PAL reductions of 0.8 mm, 1.0 mm, and 1.2 mm) yielding BAL:PAL ratios of 1.23, 1.31, and 1.4 were designed by computer software (Pro/Engineering). The models were imported into an FEA software (Pro/Mechanica), with all degrees of freedom constrained at the root surface of the tooth preparation. Each tooth preparation crown configuration was evaluated under a vertical (axial) 200 N load, and under a combined vertical 200 N and horizontal (buccally) 100 N load applied at different positions from the central fossa to the cusp tip. Maximum principal stress (MPS) was determined for the crown core for each crown BAL:PAL ratio, loading condition, and position.

Under both vertical and combined loading conditions, the highest MPSs were located at the occlusal region and in the occlusogingival region of the ceramic core. MPS values increased in the proximal region as the BAL:PAL ratio increased. Combined loading resulted in a general increase in MPS compared to vertical loading.

Increasing the BAL:PAL ratio (reducing the proximal axial length of the preparation) acted as a stress concentrator at regions near the crown margins, suggesting this area may be vulnerable to damage from fit adjustment as well as during function. Such increases in stress concentration should be considered in clinical scenarios, especially when inherent flaws are present in the material, since extensive high-magnitude tensile stress fields have been noted under all loading conditions.