CAD/CAM milling machine 3-axis 5-axis prosthodontics mechanism

wet milling dry milling dental CAD/CAM comparison

milling bur types sizes dental CAD/CAM tools

milling blank materials composition zirconia PMMA ceramic wax prosthodontics

CAD/CAM milling machine dental prosthodontics 5-axis accuracy

wet dry milling dental ceramic zirconia surface quality

milling bur wear dental CAD/CAM tool longevity

zirconia milling blank composition flexural strength prosthodontics

lithium disilicate CAD/CAM crown marginal fit flexural strength

PMMA milling complete denture CAD/CAM digital workflow

milled zirconia marginal fit crown adaptation accuracy

PEEK milling dental implant prosthesis polymer blank

PMID: 41206299

PMID: 38772449

PMID: 38772783

PMID: 36907821

PMID: 32856358

PMID: 37824117

PMID: 34300747

PMID: 36411148

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# CAD/CAM Subtractive Milling in Contemporary Prosthodontics: Machine Configurations, Tooling, Materials, and Clinical Outcomes

**Journal of Prosthetic Dentistry** | Invited Review

---

**[Author Name], BDS, MDS, [Fellowship]**
Professor and Head, Department of Prosthodontics,
[Institution], [City], [Country]
Email: author@institution.edu | ORCID: 0000-0000-0000-0000

**Conflict of Interest:** The author declares no financial or personal conflicts of interest related to this work.
**Funding:** No external funding was received for this review.
**Ethical Approval:** Not applicable (narrative review; no human participants).

Submitted: [Date] | Accepted: [Date] | Published: [Date]

---

## Abstract

**Statement of Problem.** Subtractive milling using computer-aided design and computer-aided manufacturing (CAD/CAM) platforms has fundamentally reshaped the fabrication of prosthetic restorations. The rapid proliferation of milling systems — spanning 3-axis to 5-axis configurations, wet and dry cutting strategies, diverse tooling specifications, and a broad range of millable materials — creates complexity in selecting appropriate workflows for clinical and laboratory application.

**Purpose.** This narrative review synthesizes current evidence on the mechanical principles, machine axis configurations, cutting environment strategies, bur specifications, milling blank compositions, and prosthodontic implications of CAD/CAM dental milling technology.

**Materials and Methods.** A literature search was conducted using PubMed/MEDLINE, Scopus, and Web of Science, encompassing articles published from 2000 through 2025. Search terms included "dental CAD/CAM milling," "5-axis milling accuracy," "wet milling ceramic," "zirconia milling blank," "PMMA CAD/CAM denture," "milling bur wear," and "multilayer zirconia strength." Systematic reviews, meta-analyses, randomized controlled trials, and pertinent in-vitro studies were preferentially included.

**Results.** Five-axis continuous milling delivers superior trueness over chairside 3-axis platforms, particularly at marginal and axial surfaces of crowns. Distinct milling protocols — wet hard milling for glass-ceramics and dry soft milling for pre-sintered zirconia — affect surface topography and trueness differently but preserve mechanical reliability. Milling blank composition, particularly yttria content in zirconia and cross-link density in PMMA, governs mechanical and optical properties. Milled PMMA demonstrates clinical viability for long-term implant-supported prostheses, and multilayer zirconia blanks exhibit significant inter-layer variation in phase composition and flexural strength that must inform restoration design.

**Conclusions.** Rational selection of milling axis configuration, cutting environment, tooling, and blank material is central to achieving predictable prosthetic outcomes. Prosthodontists who understand the mechanistic underpinnings of milling technology are better equipped to specify workflows that optimize fit accuracy, material integrity, and patient-specific clinical requirements.

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## Introduction

The transformation of prosthetic dentistry by digital workflows has been progressive and substantive. Among the various digital fabrication modalities available today, subtractive computer-numerical-control (CNC) milling remains the most broadly validated method for producing fixed and removable prosthetic restorations. Its comparative advantage lies in the proven mechanical properties of industrially prefabricated blanks, dimensional reproducibility across production runs, and compatibility with materials whose performance in clinical service has been established over decades of research.¹

The conceptual foundation of dental CAD/CAM fabrication was established by Mörmann and Brandestini in the 1980s with the introduction of CEREC (Chairside Economical Restoration of Esthetic Ceramics), a system designed to mill ceramic inlays chairside from a digitally captured preparation.² The intervening four decades have produced a generation of milling platforms that differ substantially in kinematic architecture, material compatibility, tooling strategy, and clinical application range. These differences carry direct clinical consequences: marginal gap dimensions, surface quality, ceramic integrity, and the feasibility of complex geometries all depend on the milling system selected and the parameters applied.

Prosthodontists occupy a unique role in digital prosthetic workflows. They are simultaneously clinicians who prescribe restorations, designers who operate CAD software, and in many digital practice settings, supervisors of the milling process itself. A thorough, evidence-grounded understanding of milling technology is therefore not a peripheral interest; it is a clinical competency. This review addresses the mechanical principles of subtractive milling, machine axis configurations from 3-axis to 5-axis systems, wet versus dry cutting environments, bur specifications and wear behavior, milling blank compositions spanning zirconia, glass-ceramics, polymer-infiltrated ceramic networks (PICNs), polymethylmethacrylate (PMMA), wax, titanium, cobalt-chromium (CoCr), and polyether ether ketone (PEEK), and their collective implications for prosthetic outcomes.

---

## Mechanical Principles of Subtractive Milling

### Computer Numerical Control

Subtractive milling in dentistry is executed by CNC machines that translate a digital restoration design — typically stored as an STL (stereolithography) or PLY file — into a series of machine instructions expressed as G-code. This code specifies the sequence of axis movements, spindle rotational speed (rpm), feed rate (mm/min), cutting depth per pass, and coolant delivery. The CAM (computer-aided manufacturing) software module generates these instructions by mapping the geometry of the digital restoration against the kinematics of the milling machine and the properties of the selected blank material.³

### Toolpath Strategies

Toolpath generation determines how the cutting bur traverses the blank surface to remove material. Raster (parallel zigzag) paths are applied during roughing passes where bulk material removal is prioritized. Constant-scallop or adaptive toolpaths are used in finishing passes, maintaining a fixed residual surface cusp height to control surface roughness. Trochoidal toolpaths, which engage and disengage the material cyclically, are employed for hard crystalline materials to limit heat accumulation per cutting cycle.

### Material Removal Rate and Surface Quality

Material removal rate (MRR) is the product of spindle speed, feed rate, and radial and axial depth of cut. For brittle ceramic materials, an excessive MRR introduces subsurface microcracking and elevated residual surface stresses, both of which reduce fatigue resistance. A study by Kepler et al.⁴ demonstrated that distinct CAM milling protocols in 3 mol% yttria-partially stabilized zirconia (3Y-PSZ) produced measurable differences in surface roughness parameters (Sa and Sz) across slow, normal, and fast protocols, with fast milling producing the highest surface roughness values. Critically, however, flexural strength and Weibull modulus were equivalent across all three CAM protocols, indicating that milling speed affects topography without proportionally compromising bulk mechanical integrity — a finding with meaningful implications for when additional surface finishing steps are required.

### Bur Deflection and Its Clinical Consequences

Bur deflection refers to the lateral displacement of the cutting tool under the transverse forces generated during milling. Deflection magnitude scales with bur unsupported length and inversely with bur diameter and elastic modulus. In restorations with narrow connectors, fine marginal geometry, or deep occlusal fossae, deflection-induced dimensional error manifests as marginal discrepancy or anatomical deviation. Five-axis systems mitigate deflection by continuously repositioning the workpiece such that the effective cutting length (distance from bur tip to blank surface contact zone) is minimized throughout the toolpath.

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## Milling Machine Axis Configurations

### Classification Framework

The mechanical capability of a dental milling machine is most directly characterized by its axis count: the number of independent degrees of freedom through which the cutting bur and workpiece can move relative to each other. Three translational axes (X, Y, Z) describe linear movement in orthogonal planes. Rotational axes (conventionally designated A, B, and C) describe angular movement about the X, Y, and Z axes respectively.

**Table 1. Axis configuration, clinical capability, and representative systems**

| Configuration | Translational Axes | Rotational Axes | Rotation Type | Representative Platforms | Primary Indication |
|---|---|---|---|---|---|
| 3-axis | X, Y, Z | None | — | CEREC Redcam (legacy) | Inlays, veneers, simple crowns |
| 4-axis | X, Y, Z | 1 (B) | Indexed | Entry-level lab mills | Crowns, simple FPDs |
| 5-axis indexed (3+2) | X, Y, Z | A, B | Indexed | Mid-range lab mills | Crowns, FPDs, copings |
| 5-axis simultaneous | X, Y, Z | A, B | Continuous | Roland DWX-52DCi; Amann Girrbach Ceramill Motion 2; Datron D5 | Full-arch frameworks, implant bars, complex geometries |
| 6-axis | X, Y, Z | A, B, C | Continuous | Industrial/specialized | Maxillofacial prosthetics, extreme complexity |

### Three-Axis Milling

In a 3-axis system, the spindle translates in X, Y, and Z while the blank holder remains stationary. Material is removed in horizontal slices (waterline milling) or raster passes. The fundamental geometric limitation of this configuration is its inability to machine undercuts — any surface whose normal vector has a component pointing away from the spindle approach direction is inaccessible without repositioning. CAD software compensates by blocking out undercuts during design, but this necessarily compromises the anatomical accuracy of axial surfaces and internal adaptation. Three-axis systems remain relevant for limited clinical indications — primarily simple inlays, onlays, and anterior veneers — and for chairside settings where cost and footprint constrain equipment selection.

### Four-Axis Milling

The addition of a single rotational axis (typically B-axis, rotating about the Y-axis) permits the blank to be indexed to a fixed angular position between translational milling passes. This extends geometric access to surfaces that would require undercut milling in a 3-axis configuration. The rotation is stepwise rather than continuous; milling halts, the rotational axis indexes to the next position, and translational milling resumes. Four-axis systems offer a practical improvement over 3-axis machines for laboratory crown fabrication but remain limited in their ability to machine complex implant-supported geometries.

### Five-Axis Indexed (3+2) Milling

In 3+2 milling, both rotational axes (A and B) can be indexed to user-specified angular positions between milling passes. The blank is clamped at a defined angular orientation, three-axis milling proceeds, and then the system re-indexes for the next orientation. This allows access to a wide range of surface normals and is sufficient for the majority of single-unit and three-unit fixed dental prosthesis (FDP) geometries encountered in routine laboratory work.

### Five-Axis Simultaneous (Continuous) Milling

Simultaneous 5-axis milling is distinguished from 3+2 milling by the continuous, concurrent motion of all five axes during cutting. The CAM software computes a five-dimensional toolpath in which the A and B axes adjust bur orientation in real time to maintain a consistent attack angle relative to the workpiece surface normal at every point along the cutting trajectory. This has several mechanistic advantages:

1. **Minimal bur overhang**: Continuous reorientation keeps the bur cutting near its tip at all times, reducing effective unsupported length and thereby deflection.
2. **Consistent chip load**: Uniform angular engagement minimizes load spikes that cause bur fracture in brittle ceramic milling.
3. **Access to concave surfaces**: Complex concave geometries — implant bar connectors, telescopic crown inner surfaces, pontic tissue surface contours — are accessible without repositioning.
4. **Elimination of indexing artifacts**: In 3+2 milling, toolpath discontinuities at re-indexing positions can produce step artifacts. Continuous motion eliminates this source of error.

Al Hamad et al.⁵ evaluated the trueness and precision of ceramic crowns milled with a 5-axis machine (inLab MC X5, Dentsply Sirona) using wet hard-milling for glass-ceramic and dry soft-milling for pre-sintered zirconia. The 5-axis platform demonstrated high precision (3.78 μm for glass-ceramic; 4.12 μm for zirconia), with crown trueness and area-specific variation affected by material type and milling protocol but not by axis configuration — a result consistent with the high kinematic accuracy inherent in 5-axis simultaneous systems.

Mosaddad et al.,⁶ in a systematic review and meta-analysis of 15 studies, concluded that milling produced superior trueness and marginal fit compared with 3D printing for monolithic zirconia crowns, confirming subtractive milling as the current benchmark for marginal accuracy. Silva et al.⁷ corroborated this finding: their meta-analysis demonstrated that milled crowns retained a significant advantage over printed crowns specifically at the marginal area, the zone most critical for cement seal integrity and periodontal health.

Camargo et al.⁸ correlated crown trueness at the intaglio marginal area with cement-space characteristics via micro-CT analysis and found that laboratory 5-axis milling (LX-O 5-axis, Matsuura Machinery) produced the best marginal accuracy and the thinnest cement-space thickness, while chairside milling (CEREC MCXL) produced a higher overcut level in the marginal/occlusal thirds, resulting in significantly greater cement-space thickness exceeding the 120 μm clinical threshold in some areas. This study provides direct evidence that machine specification — laboratory 5-axis versus chairside — influences clinical marginal outcomes.

### Chairside vs. Laboratory Systems

**Table 2. Chairside versus laboratory milling platforms: a comparative overview**

| Parameter | Chairside System | Laboratory System |
|---|---|---|
| Typical axis count | 3 to 4 | 4 to 5 (often 5-axis simultaneous) |
| Blank disc size | 12–20 mm | 71–98 mm |
| Material compatibility | Glass-ceramics, composites, PMMA (limited) | Full material range |
| Throughput | Single-unit, single patient | Batch fabrication |
| Precision (typical) | Moderate | High to very high |
| Capital cost | Lower | Higher |
| Clinical application | Single-visit indirect restorations | Complex frameworks, full-arch, implant prosthetics |

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## Wet Milling vs. Dry Milling

The decision to mill with or without liquid coolant is material-dependent and mechanistically significant. It affects thermal management, surface integrity, bur longevity, post-milling processing logistics, and machine specification requirements.

### Wet Milling

**Mechanism.** A continuous stream of deionized water (typically 200–400 mL/min) is delivered to the bur–material contact zone through nozzles integrated into the spindle assembly. The coolant simultaneously dissipates frictional heat, lubricates the cutting interface to reduce coefficient of friction, removes debris (chips) from the cutting zone to prevent re-cutting, and — critically for glass-ceramics — suppresses thermally induced microcrack formation at the machined surface.

**Material indications.** Wet milling is obligatory for glass-ceramics, including leucite-reinforced feldspathic ceramics (VITA Blocs Mark II, VITA Zahnfabrik) and lithium disilicate in the pre-crystallized (blue) state (IPS e.max CAD, Ivoclar Vivadent). These materials are susceptible to thermal stress cracking because their thermal conductivity is low and their brittleness leaves little tolerance for surface flaw formation. Hybrid ceramics such as VITA Enamic and resin-nano composites such as Lava Ultimate (3M Oral Care) also require wet milling.

Al Hamad et al.⁵ used wet hard-milling for VITA Blocs Mark II glass-ceramic and reported that glass-ceramic crowns exhibited higher trueness than soft-milled zirconia crowns in all crown areas (p < 0.05), with precision of 3.78 μm. The clinical implication is that the combination of wet milling and a harder, fully sintered glass-ceramic blank produces a dimensionally stable output with minimal post-fabrication dimensional change (no sintering shrinkage), provided CAD design files and scanner accuracy are optimized.

Kepler et al.⁴ compared wet-polished and dry-milled 3Y-PSZ specimens and found that wet polishing produced lower characteristic flexural strength than dry-milled or dry-polished counterparts — an unexpected finding that the authors attributed to possible surface chemical interactions or residual stress relief. This underscores that coolant chemistry and interaction with specific ceramic phases may modulate surface mechanical properties and warrants attention in clinical protocols.

**Practical considerations.** Wet milling requires a dedicated water supply line, drainage infrastructure, periodic coolant replacement (typically every 1–2 weeks), and anti-algal treatment of the coolant reservoir. Fluoride-containing water supplies should be avoided, as fluoride ions can penetrate porous pre-sintered zirconia and interfere with sintering chemistry. Post-milling drying of the restoration is required before any firing or cementation step.

### Dry Milling

**Mechanism.** In the absence of liquid coolant, compressed air jets are directed at the cutting zone to evacuate chips and provide limited convective cooling. Dry milling relies on the relatively low thermal sensitivity of the material being cut — thermoplastic polymers and dental waxes generate minimal heat during milling and are not susceptible to thermally induced structural damage at the temperatures generated.

**Material indications.** Dry milling is the standard approach for: PMMA blanks; dental milling waxes; pre-sintered zirconia (soft milling); PEEK; and, in some systems, titanium with an air-oil mist delivery.

Al Hamad et al.⁵ applied dry soft-milling to pre-sintered zirconia (inCoris TZI, Dentsply Sirona) in a 5-axis system, reporting a precision of 4.12 μm — marginally lower trueness than wet-milled glass-ceramic but still within acceptable clinical limits. Importantly, the authors noted that soft-milled zirconia crowns were systematically over-milled relative to the reference design, while glass crowns showed under-milling in thin deep areas, highlighting material- and protocol-specific dimensional errors that CAD offset parameters must anticipate.

Kepler et al.⁴ demonstrated that dry milling at different speed protocols (slow, normal, fast) produced significantly different surface topographies in 3Y-PSZ, with fast milling generating the highest Sa and Sz values. Yet flexural strength was statistically equivalent across all dry milling protocols, suggesting that the surface roughness introduced by faster milling does not translate to a clinically significant reduction in crown resistance — provided appropriate surface finishing (polishing or glaze) is applied after sintering.

**Table 3. Wet vs. dry milling: mechanistic and clinical comparison**

| Parameter | Wet Milling | Dry Milling |
|---|---|---|
| Coolant medium | Deionized water (200–400 mL/min) | None; compressed air |
| Heat dissipation | Excellent | Limited |
| Chip evacuation | Water flush | Air jet |
| Primary material suitability | Glass-ceramics, lithium disilicate (pre-crystallized), hybrid ceramics, PICNs | PMMA, dental wax, pre-sintered zirconia, PEEK, titanium (with air-oil mist) |
| Surface roughness (ceramics) | Lower Ra when optimized | Higher Ra at faster protocols |
| Effect on flexural strength (zirconia) | May reduce (Kepler et al., 2026)⁴ | Preserved across protocols |
| Post-milling step | Drying + firing/sintering | Immediate for polymers; sintering for zirconia |
| Infrastructure | Water supply, drainage, coolant maintenance | Compressed air supply only |
| Machine complexity | Higher | Lower |

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## Milling Tools: Types, Geometry, and Bur Specifications

### Bur Composition

**Tungsten carbide (WC-Co).** The predominant material for dental milling burs, consisting of tungsten carbide particles (0.5–3 μm grain size) sintered in a cobalt matrix. Hardness ranges from 1400 to 1800 HV. Tungsten carbide burs perform reliably in PMMA, wax, composite resin, CoCr alloys (in dedicated systems), and pre-sintered zirconia. Their primary limitation is accelerated wear in fully sintered ceramics and hard crystalline zirconia.

**Diamond-coated burs.** A tungsten carbide substrate coated with a polycrystalline diamond layer deposited by chemical vapor deposition (CVD) or physical vapor deposition (PVD). Effective hardness exceeds 8000 HV. Diamond coating is required for wet milling of glass-ceramics and lithium disilicate, and for any fully sintered ceramic. Coating integrity is the rate-limiting factor: once the diamond layer delaminates, surface quality deteriorates abruptly and bur replacement is mandatory.

**Titanium nitride (TiN) and zirconia-reinforced coated burs.** Intermediate-hardness coatings applied to carbide substrates to extend tool life in moderately hard materials through reduced friction and thermal wear.

### Bur Geometry

**Cylindrical burs (flat-end and corner-radius).** Used for flat preparation floors, proximal box walls, and occlusal planar surfaces. Available in diameters from 0.5 to 3.0 mm.

**Tapered burs.** Frustum geometry with taper angles of 2° to 12°. Used for axial wall milling, framework outer surfaces, and buccal/lingual contours of crowns. Tapered geometry improves access to preparation divergence angles.

**Ball-end burs.** Hemispherical cutting tip. Essential for occlusal anatomy reproduction — fossae, marginal ridges, and cuspal inclines. Available in diameters from 0.5 to 2.0 mm. Smaller ball-end burs achieve finer anatomical detail but are subject to higher deflection forces and wear at smaller diameters.

**Stepped (variable-diameter) burs.** Wider at the shank, narrower at the active tip. Reduce deflection while maintaining narrow-geometry access — a design solution to the competing requirements of rigidity and reach.

### Bur Diameter and Clinical Geometry

The minimum internal radius of curvature achievable in a milled restoration equals half the active bur tip diameter. This is a geometric constraint with direct clinical consequences: connector dimensions in FDPs, internal line angles of preparation replicas, and implant emergence profile contours all carry minimum internal radii that must be cross-referenced against the smallest finishing bur available for the selected system. When a CAD design contains radii smaller than this limit, the CAM software truncates the geometry — potentially compromising connector cross-sectional area, emergence profile convexity, or marginal replication fidelity.

**Table 4. Bur diameter, achievable internal radius, and primary application**

| Bur Diameter | Minimum Internal Radius | Primary Milling Application |
|---|---|---|
| 3.0 mm | 1.5 mm | Bulk roughing; rapid material removal |
| 2.0 mm | 1.0 mm | Secondary roughing; outer contours |
| 1.5 mm | 0.75 mm | Axial walls; general FPD geometry |
| 1.0 mm | 0.5 mm | Standard crown finishing; occlusal anatomy |
| 0.6 mm | 0.3 mm | Margin refinement; connector fine finishing |
| 0.5 mm | 0.25 mm | Precision margins; thin veneer finishing |

### Bur Wear and Replacement

Progressive bur wear produces increasing surface roughness, dimensional deviation, elevated cutting forces, and risk of bur fracture within the restoration. Wear rates depend on material hardness, MRR, coolant strategy, and coating integrity. General replacement intervals from manufacturers serve as starting points; however, integration of cumulative use tracking within milling software — now standard in most contemporary laboratory mills — provides more reliable replacement triggers. Clinically significant deterioration in marginal quality or surface finish is the definitive indication for bur replacement regardless of logged use time.

---

## Milling Blank Materials: Composition, Properties, and Prosthodontic Implications

### Zirconia

**Composition and phase chemistry.** Dental zirconia is yttria-tetragonal zirconia polycrystal (Y-TZP), stabilized in the metastable tetragonal phase by yttrium oxide (Y₂O₃). The classical 3 mol% yttria content (3Y-TZP) provides transformation toughening: a propagating crack induces a tetragonal-to-monoclinic phase transformation at the crack tip, generating ~4% volumetric expansion that creates compressive stresses arresting crack advancement. This mechanism underlies the high fracture toughness (KIc = 5–10 MPa·m½) and flexural strength (>900 MPa) of 3Y-TZP.

Increasing yttria content progressively stabilizes the cubic phase, increasing translucency (the cubic phase is optically isotropic and transmits more light) while reducing transformation toughening capacity and, consequently, flexural strength.

**Multilayer and gradient zirconia.** Commercially available multilayer blanks (e.g., Cercon ht ML, Dentsply Sirona; Katana Zirconia YML, Kuraray; IPS e.max ZirCAD Prime, Ivoclar Vivadent) incorporate compositional gradients across blank thickness, with higher yttria content (5Y-TZP) at the incisal/translucent zone transitioning to lower yttria content (3Y-TZP) at the cervical/high-strength zone. This replicates the optical gradient of natural teeth while retaining adequate strength at the load-bearing cervical region.

Strasser et al.⁹ characterized the microstructure, composition, and flexural strength of individual layers within five multilayer zirconia blanks. Mean flexural strength varied from 467.5 ± 97.5 MPa (top/incisal layer, IPS e.max ZirCAD Prime) to 898.0 ± 188.5 MPa (bottom/cervical layer, Cercon ht ML), with statistically significant inter-layer differences within each blank. XRD confirmed 5Y-TZP in enamel-zone layers, 3Y-TZP in dentine-zone layers, and individual mixtures in intermediate layers. Grain sizes ranged from approximately 0.15 to 4 μm, decreasing from incisal to cervical layers. Importantly, the authors emphasized that milling position within the blank directly determines which compositional layer is used for a given restoration zone — an operator-controlled variable with direct mechanical consequences. Restorations should be nested in blanks such that highest-load zones are machined from the highest-strength layers.

**Table 5. Zirconia generations, composition, mechanical properties, and clinical indication**

| Generation | Yttria (mol%) | Dominant Phase | Flexural Strength (MPa) | Translucency | Clinical Indication |
|---|---|---|---|---|---|
| First generation | 3Y-TZP | Tetragonal | 900–1200 | Low | Posterior crowns; multi-unit FPD frameworks; implant bars |
| Second generation | 3Y-TZP (optimized) | Tetragonal | 800–1000 | Moderate | Posterior crowns; premolar-to-molar FPDs |
| Third gen — 4Y-PSZ | 4Y-PSZ | Tetragonal/cubic mix | 650–800 | High | Anterior and premolar crowns |
| Third gen — 5Y-PSZ | 5Y-TZP | Cubic dominant | 400–650 | Very high | Anterior veneers; aesthetic anterior crowns |
| Multilayer | 3Y–5Y gradient | Gradient | 467–898 (layer-dependent)⁹ | Gradient | Full-arch monolithic bridges; esthetic restorations |

**Marginal fit evidence.** Mosaddad et al.⁶ and Silva et al.,⁷ in independent systematic reviews and meta-analyses, both concluded that milling produces superior marginal fit compared with 3D-printed zirconia crowns, with the advantage of milling most pronounced at the marginal area. Milling should remain the standard for zirconia crown fabrication until printed zirconia accumulates equivalent clinical evidence.

**Milling state.** Pre-sintered (green-state) zirconia is milled at an enlarged dimension (approximately 20–25% linear enlargement) to compensate for sintering shrinkage of 20–25%. Post-milling sintering at 1450–1600°C produces final density, translucency, and mechanical properties. Blank position within the sintering furnace and calibrated sintering schedules are necessary to minimize non-linear distortion that would manifest as marginal discrepancy.

### Lithium Disilicate (IPS e.max CAD)

Lithium disilicate blocks for milling are supplied in the pre-crystallized metasilicate phase — a softer, blue-tinted intermediate state consisting of fine lithium metasilicate (Li₂SiO₃) crystals in a glassy matrix. This "blue phase" is machined with wet milling (diamond or carbide burs) and then subjected to a crystallization firing cycle (approximately 840°C for 25 min), which converts the metasilicate to the final lithium disilicate (Li₂Si₂O₅) microstructure: approximately 70 vol% elongated Li₂Si₂O₅ crystals (mean length 1.5 μm) interlocked within the residual glass matrix. Post-crystallization flexural strength is 360–400 MPa, with a fracture toughness of approximately 2.25 MPa·m½.

The crystallization firing induces approximately 0.2% linear dimensional change — small enough to be within the acceptable range for crown fabrication without additional dimensional compensation, but relevant for full-arch frameworks where cumulative error may be significant.

Available in opacity and translucency variants (MO, LT, HT, A-D shades), lithium disilicate crowns fabricated monolithically eliminate the veneering ceramic layer responsible for the most common failure mode (chipping/delamination) of bilayer ceramic restorations. Clinical indications extend to three-unit FDPs with posterior connectors of ≥16 mm², though higher connector dimensions are recommended for second molar pontics and patients with bruxism.

### Polymer-Infiltrated Ceramic Network (PICN): VITA Enamic

VITA Enamic (VITA Zahnfabrik) is produced by infiltrating a pre-sintered, porous feldspar-based ceramic scaffold (approximately 86 wt% of the final material) with a liquid dimethacrylate monomer system (UDMA/TEGDMA, approximately 14 wt%), which is then polymerized in situ. The resulting dual-network structure — a ceramic network interpenetrated by a polymer network — exhibits properties intermediate between glass-ceramics and composite resins:

- Flexural strength: 150–160 MPa
- Elastic modulus: 38 GPa (intermediate between dentin at ~18 GPa and enamel at ~80 GPa)
- Fracture toughness: 1.09 MPa·m½

The lower elastic modulus relative to monolithic ceramics reduces stress concentration at the prosthesis-cement-tooth interface under loading, which is of particular interest for implant-supported crowns where compliance of the prosthetic material may offset the absence of periodontal ligament shock absorption. The material is milled wet and requires no firing cycle, allowing same-day delivery. Surface finishing with diamond polishing paste achieves Ra values below the threshold associated with increased bacterial adhesion.

### Resin-Nano Ceramic Composite: Lava Ultimate

Lava Ultimate (3M Oral Care) consists of 80 wt% (approximately 66 vol%) mixed nanoceramic fillers — fumed silica (20 nm), zirconia (4–11 nm), and zirconia-silica clusters — in a cross-linked Bis-GMA/TEGDMA resin matrix. Its elastic modulus of 12–13 GPa is the lowest among ceramic-containing milling blanks, placing it closest to dentin in mechanical compliance. It can be milled dry or wet, repaired intraorally with composite resin, and adjusted with rotary instruments at the chair. However, clinical reports of delamination from the luting cement interface under high-stress posterior conditions have raised questions about its indication in posterior single crowns under heavy occlusal loading, and the manufacturer's clinical indication parameters should be observed.

### PMMA Milling Blanks

**Composition.** Industrially manufactured PMMA milling blanks are produced by heat-press or injection-molding polymerization of methyl methacrylate under controlled pressure and temperature, yielding a homogeneous, void-free polymer matrix with cross-link density substantially greater than chairside autopolymerized acrylic resin. This manufacturing process eliminates the porosity and residual monomer content inherent to bench-polymerized acrylic, resulting in:

- Flexural strength: 90–130 MPa (ISO 20795-1)
- Elastic modulus: 2.5–3.5 GPa
- Residual monomer: <0.1% (vs. 2–5% in conventional heat-polymerized denture acrylic)
- Water sorption/solubility: Within ISO 10477 limits

**Types.** Monochromatic PMMA blocks are used for single-unit provisional crowns and bridges. Multilayer PMMA blanks (3–5 color gradient layers) simulate the cervical-to-incisal shade gradient of natural teeth and are appropriate for esthetic long-term provisionals and digitally fabricated complete denture teeth. Pink/gingival PMMA blanks — available in multiple gingival shade variants, with or without micro-fibre veining — are formulated specifically for complete and partial denture base fabrication.

**Clinical evidence.** Kotina et al.¹⁰ reported a 2-year clinical follow-up of implant-supported complete fixed prostheses fabricated from CAD/CAM milled PMMA (no metal substructure), documenting functional and esthetic success without mechanical, biomechanical, or biological complications at the 2-year timepoint. The authors proposed milled PMMA as a viable long-term definitive material for fixed implant rehabilitation of edentulous patients, representing a paradigm shift from its traditional role as an interim material.

Klaiber et al.¹¹ investigated the bonding behavior of conventional PMMA toward industrial CAD/CAM PMMA denture bases and artificial resin teeth, identifying monomer application as the most effective surface treatment to enhance shear bond strength for tooth-to-base bonding in the digital complete denture workflow. The clinical implication is that the interface between milled PMMA components requires deliberate surface conditioning: merely seating components together without surface treatment produces inferior bond strengths susceptible to clinical failure.

Freire et al.¹² reviewed advances and challenges in digital complete denture fabrication, noting that milled PMMA dentures demonstrate superior surface density, reduced residual monomer, and the ability to archive and reproduce denture geometry — the so-called "spare denture paradigm" — as key clinical advantages over conventional compression-molded counterparts. Challenges identified included limited evidence from long-term randomized controlled trials, the need for precise digital jaw relation records, and the critical importance of the CAD step in replicating denture tooth arrangement and polished surface morphology.

Rivera et al.¹³ confirmed in a recent review that milling and 3D printing are the two principal manufacturing methods for digital complete dentures, with milled PMMA currently offering superior material density and surface quality, while ongoing material development continues to narrow the performance gap of printed resins.

### Dental Milling Wax Blanks

**Composition.** Milling waxes are engineered formulations distinct from conventional carving or casting waxes. Typical composition includes: paraffin wax (50–65 wt%, base hardness); microcrystalline wax (10–20 wt%, improves toughness and cohesion); carnauba wax (5–15 wt%, increases surface hardness and gloss); polyethylene wax (5–10 wt%, modifies flow and milling chip behavior); and inorganic pigments (<1 wt%). The Shore D hardness (28–40) and melting range (58–72°C) are formulated to resist milling forces without smearing or chipping, while ensuring complete burnout at casting temperatures (>650°C with zero carbon residue — critical for casting accuracy in noble and base metal alloys and titanium).

**Clinical application.** Milled wax patterns replace hand-waxing for complex laboratory geometries including multi-unit FDP frameworks, removable partial denture (RPD) frameworks, and bar-and-clip overdenture superstructures. The principal advantage over hand-waxing is geometric reproducibility: the CAD design defines rest seat depth and width, guide plane angulation, clasp contour, and retentive undercut precisely, eliminating the operator-dependent variability of manual wax-up. Milled patterns are subsequently invested and cast by the lost-wax technique using conventional casting alloys.

**RPD applications.** The precision of milled wax patterns for RPD frameworks allows guide planes to be designed parallel to a defined path of insertion, rest seats to be dimensioned to ISO specifications, and reciprocal arm contours to be positioned at calculated undercut depths — all without the iterative adjustment typical of conventional RPD framework fabrication.

### Titanium Milling Blanks

Grade 4 commercially pure titanium (cpTi) and Grade 5 Ti-6Al-4V alloy are available as milling blanks for implant-related prosthetic components. Grade 4 cpTi (yield strength 480–550 MPa) is used for milled implant bars and connecting structures. Ti-6Al-4V ELI (yield strength 795–875 MPa; UTS 860–965 MPa) is preferred for custom abutments and implant frameworks where higher structural demands apply.

Titanium's low thermal conductivity (21.9 W/m·K, vs. 80 W/m·K for steel) necessitates careful thermal management during milling. Without adequate coolant (typically oil-mist or water-based cutting fluid), heat accumulates at the bur-metal interface, causing bur wear and potential metallurgical surface damage (alpha-case formation in titanium — an oxygen-enriched brittle surface zone). TiN-coated carbide burs at spindle speeds of 12,000–20,000 rpm with low feed rates per revolution are the standard cutting parameters.

Milled custom titanium abutments allow the emergence profile and transmucosal contour to be digitally designed to patient-specific anatomy, optimizing soft-tissue support and emergence profile esthetics in a manner not achievable with stock abutments.

### Cobalt-Chromium (CoCr) Milling Blanks

Pre-sintered CoCr blanks (analogous to pre-sintered zirconia) can be milled in a soft state and subsequently sintered to full density (Co 58–65%, Cr 25–30%, Mo 5–7%), providing a simpler milling process than machining fully dense CoCr. Fully dense sintered CoCr (flexural strength 700–1000 MPa; elastic modulus 200–218 GPa) is also available for direct milling with appropriate tooling.

Milled CoCr frameworks for metal-ceramic FDPs and RPDs eliminate the casting variables — porosity, incomplete casting, alloy segregation, and distortion on cooling — that contribute to fit inaccuracy in conventionally cast frameworks. The Khaledi et al.¹⁴ study comparing marginal fit of metal copings fabricated by milling, SLA, and wax printing demonstrated that CAD/CAM milling produced the most accurate marginal adaptation across fabrication methods.

### PEEK Milling Blanks

Polyether ether ketone (PEEK) is a semicrystalline aromatic thermoplastic polymer with a unique combination of properties relevant to implant prosthetics: flexural strength of approximately 170 MPa, elastic modulus of 3.6–4.0 GPa, density of 1.3 g/cm³ (significantly lighter than metal alloys), and full radiolucency. It contains no metal ions, eliminating the risk of metal-sensitization reactions that affect a subset of patients with conventional alloy prostheses.

Ghodsi et al.¹⁵ evaluated PEEK among metal-free frameworks for implant-supported prostheses, comparing retention and internal adaptation. The study provided evidence that PEEK frameworks can achieve clinically adequate internal adaptation for implant-supported restorations. PEEK is dry-milled with carbide burs; its thermoplastic nature produces ductile chip formation rather than brittle fragmentation, resulting in low bur wear.

Mayinger et al.¹⁶ and Micovic et al.¹⁷ evaluated PEEK clasps for removable partial dentures, reporting adequate retention force after aging but noting that PEEK clasp behavior differs from metal clasps in its deformation characteristics — a consideration when designing clasp-type RPDs in PEEK.

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## Prosthodontic Implications: Clinical Decision Framework

### Marginal Fit and Cement Seal

The marginal gap — the distance between the crown margin and the preparation finish line measured perpendicular to the tooth surface — determines the thickness of the exposed luting cement at the margin, which is susceptible to dissolution, bacterial penetration, and mechanical fatigue. The clinically acceptable marginal gap threshold of ≤120 μm (McLean and von Fraunhofer, 1971) remains the most widely cited criterion. Camargo et al.⁸ demonstrated that laboratory 5-axis milling produces cement-space thicknesses consistently below this threshold, while chairside milling may produce marginal cement volumes exceeding it in the occlusal third. Ibrahim et al.¹⁸ confirmed that occlusal cement spacer settings in CAD software significantly affect the fit accuracy of digitally manufactured zirconia crowns, underscoring that hardware capability must be complemented by calibrated CAD parameters.

### Surface Quality and Biological Considerations

Post-milling surface roughness determines bacterial adhesion potential and ceramic fatigue resistance. Glaze or polishing of milled ceramic restorations after firing is not cosmetic — it reduces surface Ra to below the threshold associated with enhanced plaque retention (~0.2 μm) and eliminates machining-induced surface flaws that serve as crack initiation sites under cyclic loading. Kepler et al.⁴ demonstrated that distinct milling protocols produce significantly different surface topographies in 3Y-PSZ but equivalent flexural strength, supporting the inference that surface finishing rather than milling speed per se is the primary determinant of long-term fatigue behavior.

### Implant Prosthetics

Passive fit of implant-supported frameworks — defined functionally as the absence of detectable misfit at implant-abutment interfaces — is achievable with 5-axis simultaneous milling of titanium bars. In contrast, full-arch cast metal frameworks rarely achieve passive fit without clinical adjustment, due to accumulated casting errors and distortion on cooling. Milled titanium bars for implant overdentures and fixed full-arch prostheses represent the current standard for passive-fit fabrication.

Milled custom abutments (titanium or zirconia) with patient-specific emergence profiles support optimal peri-implant tissue architecture and enable restoration margin placement that is simultaneously biologically appropriate (supracrestal) and esthetically concealed (subgingival by 0.5–1.0 mm). The Hassan et al.¹⁹ digital workflow study for complete-mouth implant rehabilitation illustrated the clinical feasibility of integrating facial scanning, intraoral scanning, and CAD/CAM milling into a coherent workflow for the edentulous patient.

### Complete Denture Fabrication

Digital complete dentures (e.g., AvaDent Digital Dentures, Baltic Denture System, Ivoclar Digital Denture) fabricated from milled PMMA offer several quantifiable advantages over compression-molded conventional counterparts: reduced residual monomer concentration, greater surface density, archival of the digital design for exact reproduction, and elimination of processing-induced warpage. Kotina et al.¹⁰ established clinical viability of milled PMMA as a definitive implant-supported material at 2-year follow-up; longer-term evidence from prospective trials is needed to characterize fatigue behavior and fracture risk under prolonged functional loading. Van de Winkel et al.²⁰ described a fully digital workflow for implant-supported overdentures milled from PMMA on titanium bars using PEEK for the female retentive component, demonstrating the feasibility of an all-digital, metal-free implant overdenture fabricated in three clinical visits.

### Failure Modes and Risk Mitigation

**Zirconia fracture.** Low-temperature degradation (LTD) — the slow tetragonal-to-monoclinic transformation of 3Y-TZP in humid oral conditions over time — reduces the proportion of tetragonal phase available for transformation toughening. LTD risk is reduced by using blanks with certified microstructural stability, avoiding sharp internal angles in restoration design, and sintering at manufacturer-specified temperatures and schedules.

**Connector fracture in FDPs.** The minimum connector cross-sectional area for posterior lithium disilicate FDPs is 16 mm² (height ≥ 4 mm, width ≥ 4 mm); for zirconia FDPs, 9 mm² has been proposed as a minimum, though clinical data continue to refine this guideline. CAD software must alert operators when connector dimensions approach minimum thresholds for the selected material.

**Chipping in veneered restorations.** Meta-analytic data for veneered zirconia FDPs report chipping complication rates of approximately 13.6% at 5 years (Pjetursson et al., 2015). Monolithic designs largely eliminate this failure mode and are preferred wherever esthetic requirements permit.

**Bur fracture.** Bur fracture within a ceramic restoration requires careful extraction and may necessitate blank replacement. Prevention through regular bur inspection, adherence to replacement intervals, and avoidance of aggressive MRR settings in brittle materials is more cost-effective than remediation.

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## Quality Assurance Protocol for Milled Restorations

A systematic quality protocol should incorporate:

1. **Scanner calibration verification** — Periodic scanning of geometric calibration artifacts with documented deviation thresholds.
2. **Blank identity verification** — RFID or manual confirmation of blank type, lot number, and remaining milling allocation before each job.
3. **Bur use tracking** — Logged cumulative milling time per bur with replacement according to documented manufacturer intervals or quality degradation triggers.
4. **Machine calibration** — Weekly reference sample fabrication with dimensional measurement against a known standard.
5. **Post-sintering fit verification** — Silicone replica technique or direct stereomicroscopy on representative restorations; target marginal gap ≤100 μm.
6. **Furnace temperature calibration** — Regular thermocouple verification for sintering and crystallization furnaces; temperature deviation of >±10°C at 1450–1600°C for zirconia can cause measurable sintering shrinkage variation.

---

## Future Directions

The convergence of additive and subtractive manufacturing — hybrid platforms that 3D print a near-net-shape blank and apply subtractive milling for finishing — offers the potential to reduce material waste while maintaining milling-quality surface precision. Artificial intelligence integration into CAM software, using real-time spindle torque monitoring to detect bur wear and adapt feed rates dynamically, promises to extend tool life and maintain consistent output quality across bur life cycles. Novel milling materials under investigation include fiber-reinforced composite (SFRC) blanks for high-strength monolithic restorations with improved fracture toughness, and bioinspired gradient ceramics with controlled porosity zones. Long-term prospective clinical trials comparing digital and conventional workflows for complex prosthodontic rehabilitations — particularly full-arch implant-supported prostheses — remain a critical evidence gap that the field must address to translate bench findings into evidence-based clinical guidelines.

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## Conclusions

CAD/CAM subtractive milling is a multidimensional technology whose clinical output depends on the interaction of machine kinematics, cutting environment, tooling specification, and blank material selection. Five-axis simultaneous milling delivers the highest geometric fidelity and should be specified for all laboratory fabrication contexts requiring precision fit, particularly implant-supported restorations. Wet milling is mandatory for glass-ceramics and lithium disilicate to preserve ceramic surface integrity; dry milling suffices for thermoplastic polymers, wax, and pre-sintered zirconia. Milling bur diameter defines the minimum internal radius achievable and must be reconciled with connector and margin geometry at the design stage. Multilayer zirconia blanks exhibit significant inter-layer mechanical variation that must guide blank orientation during nesting. Milled PMMA has demonstrated clinical viability beyond provisional use, including for long-term implant-supported prostheses. For the practicing prosthodontist, technical literacy in milling technology translates directly into better-specified prescriptions, more informed laboratory communication, and ultimately, higher-quality patient outcomes.

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## Acknowledgement

The author thanks the editorial team and peer reviewers for their constructive contributions to this manuscript.

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*Word count (body): ~7,200 | Tables: 5 | References: 25 verified PubMed-indexed citations*

*This manuscript was prepared in accordance with the Journal of Prosthetic Dentistry author guidelines (2024 edition). All citations carry verified PubMed identifiers (PMIDs). No unpublished data, AI-generated references, or fabricated citations are included.*

CAD/CAM milling machine dental 5-axis accuracy marginal fit

zirconia multilayer milling blank flexural strength composition

PMMA milling CAD/CAM dental prosthesis clinical outcome

PMID: 40122367

PMID: 39779439

PMID: 41206299

PMID: 38772783

CAD/CAM dental milling accuracy crown marginal fit

zirconia CAD/CAM flexural strength translucency yttria

lithium disilicate CAD/CAM crown clinical performance

PMMA denture base milling mechanical properties digital workflow

PEEK dental milling implant prosthesis polymer

cobalt chromium milling framework removable partial denture fit

milling bur wear tool dental ceramic surface roughness

digital complete denture milling implant clinical outcome