3D craniometry: cranial analysis from a three-dimensional scan

3D craniometry uses a head scan to reconstruct the cranial surface and obtain objective measurements of cranial shape. When acquisition and landmark placement are standardised, the method can increase the reproducibility of measurements and provide a three-dimensional record of cranial morphology.

Beyond traditional indices such as CVAI and the cephalic index, three-dimensional analysis makes it possible to obtain measurements related to the spatial distribution of the asymmetry and to document how cranial shape changes over time.

How it works

The process starts with capturing the surface of the head through a three-dimensional scan. The result is a mesh that represents the external geometry of the skull.

From that reconstruction, landmark placement establishes a coordinate system to orient the head. In Mamini's protocol, four anatomical landmarks serve as references for defining this orientation, which removes the need to level the head manually and makes orientation more standardised across assessments.

Once orientation is defined, the system can establish planes and take the measurements used to calculate the cranial indices.

What 3D measures beyond CVAI and the cephalic index

Conventional craniometry relies mainly on linear measurements taken between specific anatomical landmarks. Three-dimensional analysis extends that assessment by using the geometry of the cranial surface to obtain further parameters.

The measurements that can be obtained include:

• CVAI: quantifies the asymmetry between oblique diagonals of the cranial vault.

• Cephalic index: relates the width of the skull to its anteroposterior length.

• ACAI (anterior cranial asymmetry index): quantifies volumetric asymmetry in the anterior region of the skull.

• PCAI (posterior cranial asymmetry index): quantifies volumetric asymmetry in the posterior region of the skull.

• 3D asymmetry index: assesses asymmetry taking the three-dimensional surface of the head into account.

• Head circumference: can be derived from the three-dimensional mesh.

• Other geometric measurements: can be calculated from the reconstruction of the cranial surface.

These measurements complement the clinical assessment and the traditional indices, allowing a broader analysis of cranial morphology.

Anterior and posterior asymmetry

ACAI and PCAI make it possible to quantify volumetric asymmetry in different regions of the head.

ACAI assesses asymmetry in the anterior region, while PCAI assesses asymmetry in the posterior region. Together, the indices help identify how the volume difference is distributed between those regions.

This information can complement CVAI, which focuses on the difference between diagonals of the cranial vault.

3D asymmetry index

The 3D Asymmetry Index, proposed by Linz et al. (2022), quantifies cranial asymmetry from the volumetric distribution of the four quadrants of the head.

After the head is divided into four quadrants, identified as Q1, Q2, Q3 and Q4, the index is calculated with the following equation:

3D Asymmetry Index = |ln((Q4 × Q2) / (Q3 × Q1))|

Where Q1, Q2, Q3 and Q4 are the volumes of the four cranial quadrants, obtained from the three-dimensional reconstruction of the head surface.

The calculation compares, through a logarithmic ratio, the product of the volumes of diagonally related quadrants. When the volumes are distributed symmetrically, the ratio approaches 1 and the index approaches 0. The greater the difference in volumetric distribution between the sides, the higher the index tends to be.

Unlike CVAI, which uses linear measurements between cranial diagonals, the 3D Asymmetry Index considers the distribution of volume across different regions of the head, allowing a three-dimensional assessment of the asymmetry.

Linz et al. showed that this parameter performed well in identifying and classifying plagiocephaly in their sample, and that it represents the three-dimensional asymmetry of the head in a way that complements traditional measurements.

Head circumference and z-score

Head circumference can be estimated directly from the three-dimensional mesh.

Because the patient's age and sex are mandatory fields in the system, the value obtained can be compared against the World Health Organization reference curves to derive the corresponding z-score.

The z-score expresses the measurement relative to the expected distribution for children of the same age and sex, which makes cranial growth easier to interpret over time.

Head circumference and its z-score are complementary to the analysis of cranial shape. They do not replace the assessment of the asymmetry indices or of the cephalic index.

How the measurement planes are defined

To take the measurements, reference planes must be established on the three-dimensional reconstruction.

In Mamini's standard protocol, the system identifies the plane of greatest head circumference. That plane serves as a reference for certain measurements and calculations.

The operator can also adjust the height and tilt of the plane, or use a reference plane defined by the protocol in use, when necessary.

Defining the planes mathematically reduces variation related to manual positioning and makes it easier to compare different assessments.

Advantages over manual measurement

Three-dimensional assessment

The 3D model represents the cranial surface in three dimensions, allowing you to visualise and analyse features that isolated linear measurements do not directly represent.

Reproducibility

Orientation and measurement planes can be defined mathematically, reducing sources of variation related to positioning and to manually defining the sections.

When acquisition, landmark placement and processing are standardised, the method can make serial assessments more consistent.

Fast acquisition without prolonged direct contact

Depending on the equipment used, the scan can be performed quickly and without prolonged contact with the infant's head, which can make acquisition easier in small patients.

A permanent record of morphology

Beyond the numbers, the scan produces a three-dimensional representation of the cranial surface.

That record makes it possible to document the shape of the head visually at a given moment and compare it with later assessments.

Comparing manual and 3D craniometry

Manual craniometry and 3D craniometry can both be used to quantify cranial shape, but they have different methodological characteristics.

Manual craniometry uses instruments and landmarks defined by the examiner. It is accessible, can be performed quickly and has good clinical utility when the technique is standardised.

3D craniometry uses a digital reconstruction of the cranial surface. Beyond linear measurements, it allows geometric analysis and a record of three-dimensional morphology.

A CVAI calculated from a 3D section and a CVAI obtained by manual craniometry can show similar trends, but they should not be treated as identical measurements. Differences in acquisition, positioning, landmark definition and calculation method can influence results.

For serial follow-up, it is therefore important to keep the same method and measurement protocol whenever possible.

Precautions and limitations

The quality of the 3D analysis depends directly on the quality of the acquisition.

Movement by the infant, missing regions of the surface, hair, accessories, poor positioning or reconstruction failures can all interfere with the measurements.

Landmark placement can also introduce variation. Automating certain steps does not remove the need for an adequate acquisition and review protocol.

Another important point is that indices obtained by different methods should not be compared as though they were necessarily equivalent.

A CVAI calculated from a 3D section and a CVAI obtained by manual craniometry can show similar trends, but they are not necessarily the same measurement.

Likewise, values from different devices or software may not be directly comparable without method validation and standardised protocols.

Quantitative analysis should therefore be interpreted together with the clinical examination and the individual course of the child.

3D craniometry in follow-up

The main advantage of 3D scanning lies not only in the possibility of a more detailed assessment at a given moment, but also in the ability to document progress.

By repeating assessments throughout treatment, the clinician can compare:

• CVAI over time;

• cephalic index over time;

• anterior and posterior asymmetry;

• head circumference;

• head circumference z-score;

• changes in the three-dimensional surface;

• response to the interventions carried out.

Seeing how the shape of the head evolves, both in the image and in the numbers, helps the clinician follow the response to treatment and communicate progress to the family.

Using 3D in cranial remolding orthosis assessment

3D scanning is widely used in the assessment and planning of cranial remolding orthoses, making it possible to document the shape of the head and provide data for the planning and custom manufacture of the device.

The same technology can also be used long before that stage, in routine follow-up, to measure, classify and document the progress of infants under conservative management.

Using 3D does not, on its own, determine whether an orthosis is needed. The indication should take into account the clinical assessment, the severity of the deformity, the child's age, the course of the condition and the response to conservative measures.

How Mamini does it

In Mamini, 3D scanning is integrated into the process of assessing and following cranial asymmetry.

The clinician scans the head and the system processes the three-dimensional reconstruction to identify the landmarks, establish orientation and calculate the indices.

Results are presented in an organised way, so you can view the measurements and follow how they change across assessments.

One important advantage is that scanning can be built into routine clinical follow-up, which means 3D data can be used not just for a one-off assessment but to document the patient's progress. That is possible because of Mamini's pricing model, which does not charge per measurement, unlike the common practice in this market.

What the clinician gets

From the scan, Mamini provides the main parameters used in the assessment of cranial shape, including CVAI, cephalic index, ACAI, PCAI, 3D asymmetry index, head circumference and z-score.

Results can be followed over time, allowing you to compare different assessments and observe how the measurements change.

Quantitative analysis complements the clinical assessment and offers a standardised way to document the progression of cranial shape.

References

• Congress of Neurological Surgeons. Evidence-Based Guidelines: The Role of Imaging. Guidelines for the Management of Patients with Positional Plagiocephaly.

• Linz C, et al. Three-dimensional analysis of positional cranial deformities and cranial asymmetry. Scientific Reports. 2022;12:20831.

• World Health Organization. WHO Child Growth Standards. Head circumference-for-age.

• Graham JM Jr, et al. Studies on positional cranial deformities and cranial morphology parameters.

• Likus W, et al. Studies on cranial parameters and the cephalic index. ScientificWorldJournal. 2014.