Abstract
Occluso-cervical preparation height is an important determinant of retention and resistance in zirconia crown preparation, especially when treating short clinical crowns. As axial walls become shorter, the preparation becomes increasingly sensitive to total occlusal convergence, bur angulation and cementation strategy. This article reviews the evidence behind preparation height and translates it into a practical MR.Bur workflow designed to preserve useful axial-wall geometry without unnecessary tooth reduction.
What Is Occluso-Cervical Preparation Height?
Occluso-cervical preparation height refers to the vertical height of the prepared axial wall from the cervical finish line toward the occlusal surface.
It is different from the overall height of the natural tooth. What matters clinically is the amount of prepared axial wall that remains after occlusal and axial reduction.
Greater preparation height generally provides more surface area for retention and more resistance to rotational or lateral displacement. Shorter preparations provide less geometric resistance and therefore become more dependent on other variables such as taper, auxiliary resistance features and luting protocol.
Is There a Minimum Occluso-Cervical Height?
There is no universal minimum that applies to every tooth and every restoration.
However, Goodacre, Campagni and Aquilino's influential review proposed a minimum occluso-cervical dimension of approximately 4 mm for molars and 3 mm for other teeth when total occlusal convergence is maintained within approximately 10° to 20°. They also recommended an occluso-cervical-to-faciolingual ratio of at least 0.4.
These values should be understood as biomechanical guidelines, not targets for additional tooth reduction.
A preparation that already has sufficient height should not be shortened simply to reach a textbook measurement. In posterior crown preparation, the more relevant question is how much useful axial-wall height can be preserved after creating the restorative space required by the selected zirconia.
Why Does a Short Preparation Lose Retention?
Preparation height does not act alone. It interacts strongly with total occlusal convergence, or TOC.
Ersu and colleagues compared zirconia crown preparations with 4 mm and 7 mm axial heights and with 6° and 20° taper. Retention was greater in taller preparations and in preparations with less taper. Taller preparations also demonstrated greater fracture strength.
This produces an important clinical relationship:
Short axial walls combined with excessive taper create a less favorable resistance form.
When a preparation is already short, repeatedly tilting the bur inward to eliminate an undercut may further increase convergence and reduce the remaining effective wall height.
What Does Recent Zirconia Research Show?
A 2020 CAD/CAM zirconia study evaluated preparation heights between 1 and 3 mm at a controlled 2° convergence angle. Preparation height significantly affected crown retention, while the authors recommended preparation heights greater than 3 mm for the specific system tested.
More recently, Soleimani and colleagues evaluated monolithic zirconia crowns cemented onto molars prepared to 2 mm or 4 mm abutment heights. Increasing abutment height increased pull-out force with both tested cement types. The study also showed that cement selection significantly affected retention, demonstrating that preparation geometry and cementation should be considered together.
These studies should not be used to create one universal millimetre rule because the experimental geometries, zirconia systems and cements differ.
The more consistent evidence-based conclusion is:
Preserving axial-wall height generally improves retention, particularly when the preparation is also kept from becoming excessively tapered.
Why Is Total Occlusal Convergence Critical in Short Molars?
A short crown preparation does not automatically need more tooth removal or an auxiliary groove.
The first priority is to preserve the resistance form that is already available.
Roudsari and Satterthwaite studied short molar preparations with 3 mm occluso-cervical height and 22° TOC. Adding proximal grooves improved resistance, but reducing cervical TOC from 22° to 4° produced an even greater improvement in resistance form.
This suggests a useful clinical principle:
Before adding auxiliary retention features, control the convergence of the existing axial walls.
MR.Bur Workflow for Preserving Preparation Height
Direct evidence comparing individual commercial bur sequences is limited. Bur selection should therefore follow the geometry required at each stage rather than implying that one bur independently improves crown retention. MR.Bur's own zirconia preparation guidance follows the same principle.
Step 1: Assess the Available Crown Height
Before beginning zirconia crown preparation, evaluate:
- Existing axial-wall height
- Occlusal clearance
- Caries or previous restorations
- Opposing dentition
- Planned zirconia thickness
- Path of insertion
- Presence of short clinical crown anatomy
A tooth with limited crown height should immediately be considered a height-sensitive preparation.
Step 2: Perform Anatomical Occlusal Reduction
MR.Bur 39C Occlusal Surface Reduction Peach Coarse Diamond Bur FG
Use the MR.Bur 39C Occlusal Surface Reduction Peach Coarse Diamond Bur FG to reduce the posterior occlusal surface while following cusp inclines, fossae and existing anatomy.
MR.Bur includes 39C within its posterior occlusal preparation sequence specifically to support anatomical rather than completely flat reduction.
This matters because unnecessary occlusal reduction directly shortens the remaining axial walls.
The objective is sufficient zirconia clearance while preserving as much useful preparation height as possible.
Step 3: Open the Proximal Areas Carefully
MR.Bur 52 Coarse FG Pointed Cone Bur
Use the MR.Bur 52 Coarse FG Pointed Cone Bur for controlled interproximal opening and refinement.
The instrument should remain aligned with the intended path of insertion. Excessive inward angulation can create unnecessary proximal convergence, unsupported enamel or an excessively deep groove.
MR.Bur specifically lists the 52 Pointed Cone for controlled interproximal access in crown preparation.
Step 4: Establish Axial Walls and the Finish Line
MR.Bur 34A Taper Round End Coarse Diamond Bur FG
The MR.Bur 34A Taper Round End Coarse Diamond Bur FG is particularly relevant when managing a short preparation because the long tapered profile provides a visual reference for axial-wall direction.
Keep the bur aligned with the planned path of insertion while performing axial reduction and forming the chamfer.
MR.Bur identifies 34A for chamfer formation and advises maintaining the instrument along the intended insertion path rather than indiscriminately engaging the full bur diameter.
For a short crown, excessive inward tilting can convert limited wall height into a short, over-tapered preparation.
Step 5: Refine Without Changing the Geometry
MR.Bur 36F Fine Grit FG Taper Round Bur
Use the MR.Bur 36F Fine Grit FG Taper Round Bur for final surface and margin refinement.
At this stage, the objective is not additional reduction. Smooth axial surfaces, refine the finish line and round minor internal irregularities while preserving the height and convergence already established.
MR.Bur positions 36F for axial-wall smoothing, finish-line refinement and rounding of internal transitions.
What If the Crown Preparation Is Already Too Short?
A short preparation should trigger reassessment rather than automatic additional cutting.
Depending on the clinical situation, management may include:
- Improving excessive convergence where possible
- Adding proximal grooves or boxes
- Selecting an appropriate adhesive cementation strategy
- Reconsidering restorative design
- Considering crown-lengthening procedures where clinically indicated
Goodacre and colleagues recommend auxiliary grooves or boxes when ideal resistance features are absent. Research on short molars also shows that grooves can improve resistance, although controlling TOC may produce an even greater effect.
Conclusion
Occluso-cervical height is more than a measurement in zirconia crown preparation. It is part of the preparation's overall resistance geometry.
The evidence consistently shows that taller axial walls generally improve retention, while short preparations become increasingly sensitive to excessive convergence and cementation strategy.
A practical MR.Bur sequence is:
MR.Bur 39C Occlusal Surface Reduction Peach Coarse Diamond Bur FG → MR.Bur 52 Coarse FG Pointed Cone Bur → MR.Bur 34A Taper Round End Coarse Diamond Bur FG → MR.Bur 36F Fine Grit FG Taper Round Bur
The key principle is simple: create the restorative space zirconia requires while preserving the axial-wall height and resistance form the tooth already provides.
