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Accuracy and Reliability of a System for the Digital Capture of Infant Head Shapes in the Treatment of Cranial Deformities

Geil, Mark D. PhD; Smith, Aaron CO, LO

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JPO Journal of Prosthetics and Orthotics: April 2008 - Volume 20 - Issue 2 - p 35-38
doi: 10.1097/JPO.0b013e318169c439
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Increases in incidence of cranial deformities in infants have enhanced the importance of accurate means to capture head shape for the purposes of diagnosis, orthosis design, and outcome measurement. One such deformity is deformational plagiocephaly (DP), a multiplanar nonsynostotic cranial flattening occurring either pre- or postnatally in infants. DP can occur secondary to abnormal forces accentuated by postnatal posture. Posterior (occipital) forces have been applied to infants through a combination of the advice of pediatricians to place infants supine when sleeping to reduce sudden infant death syndrome and the use of child carriers and car seats that reduce alternative daytime positioning.1–3 Multiple births are also cited as a common cause for DP,4 with Peitsch et al.5 noting cranial flattening in 56% of twin births in one sample.

Because DP is a shape- and symmetry-related condition, cranial anthropometric measurement and documentation is essential. Several different means exist to measure three-dimensional head shape. Hand tools, including linear calipers and flexible tape measures, are commonly used to measure diameters and circumferences based on key landmarks, including the left and right tragion and the sellion. Published studies on head anthropometrics in DP6–13 have used differences between linear measurements to determine the need and/or success of treatment strategies, including repositioning and orthotic management. Plank et al.14 documented three-dimensional head shape in infants diagnosed with DP using a noninvasive laser shape digitizer. Three-dimensional shape was used to quantify outcomes after treatment with cranial remolding orthoses. The study identified four specific anthropometric measures that were determined to be of particular importance in head shape assessment. Of those four, only one measure can be taken using hand tools in the absence of a digitizer. The digitizer used for that study, the STARScanner (Vorum Research Corp., Vancouver, BC; Orthomerica, Orlando, FL), captures infant head shape using four eye-safe lasers that create circumferential light beams whose contours are recorded by eight cameras in less than 2 seconds. The scanner is used in over 35 clinics worldwide; its scans are incorporated into specialized software for measurement and shape modification in preparation for central fabrication of cranial remolding orthoses. Although the scanner has been bench-tested to confirm accuracy within 0.5 mm, the authors are not aware of any independent investigations into the accuracy and reliability of the scanner for clinically relevant shapes. Because research has noted significant growth effects in anthropometric values on the order of 2 to 3 mm,14 scanner accuracy and consistency should be verified.

METHODS

A STARScanner Laser Data Acquisition System (Orthomerica) was used for repeated scans of a model of an infant head. The scanner was calibrated to ambient light conditions in the treatment room. Scans were conducted by two certified practitioners at the Children’s Healthcare of Atlanta Orthotics and Prosthetics Department. The practitioners had specific training in the use of the scanner and software and at the time of the study at least 5 years of experience in the clinical use of the device. The model used in the study was a positive foam carving produced from a foam blank by a five-axis router. The model was that of a 10-month-old female child with a diagnosis of DP. Clinically, the model was significant for right posterior flatness, left anterior flatness, and right anterior ear shift. The foam model was covered with a cotton stockinette, leaving the face and ears exposed, to stay consistent with clinical practice. The tragion landmark was identified on the foam carving by a black dot. The foam model was attached to a steel pipe for ease in placement within the scanner.

To test various degrees of consistency, each practitioner scanned the model five times on each of the two testing sessions. The practitioners alternated scans, and the model was removed from the scanner after each scan. The two sessions were separated by a week during which the scanner was used for routine clinical measurement.

After the scans, landmarks were placed on the computerized model in the Yeti shapebuilder software. The right tragion, left tragion, and sellion were identified. Once landmarks were added, the file was saved and Yeti was closed. Next, the practitioner opened the scanned file in the Cranial Comparison Utility software (Version 2.2). The scan was aligned on a cross-sectional grid, and a report of anthropometric measurements was generated. The study focused on measurements of particular clinical relevance14: circumference in the plane created by the tragion and sellion landmarks at level 3 and the cranial vault asymmetry index (CVAI). To calculate the CVAI, diagonal measurements at 30° diameters from the midsagittal line were compared. This method was repeated by each practitioner for each of the scans they performed. Three dimensions were compared in the analysis: the diagonal distance from left anterior to right posterior, labeled LDIAG; the diagonal distance from right anterior to left posterior, labeled RDIAG; and circumference at level 3, labeled CIRC.

To develop “benchmark” comparison values, the practitioners, blinded of the study results, recorded the study measurements using accepted hand tools. The circumference was measured using a Gulick tape measure with a spring used to standardize tension.15 The diagonals were measured using a standard ML caliper.

Analyses focused on multiple potential sources of variability, including repeated measures by a given practitioner, day-to-day variation between measurement sessions, and the results of one practitioner compared with another. Accuracy was assessed by comparing practitioner results with the benchmark comparison values. Means were compared with hand tool results for each measurement. In addition, repeated measures analysis of variance was used to assess differences in measurement results within subjects across measurement sessions and between subjects. Because of sample size limitations, Greenhouse-Geisser’s epsilon was used to adjust the within-subjects effects.

RESULTS

Measurements of each dimension were consistent across practitioners and sessions. Within subjects, Greenhouse-Geisser’s epsilon revealed no significant differences for any measurements (Table 1). Similarly, between-subjects analysis of variance showed no significant difference in comparisons of the measurements of one practitioner with the other (Table 1).

T1-2
Table 1:
P values for measurement results within- and between-subjects

When results for each clinician were compared with one another (Figure 1), differences were extremely small. The mean for all measurements of clinician 1 across sessions for diagonal 1 showed a 0.33-mm difference versus that for clinician 2. The same comparison showed a difference of 0.36 mm for diagonal 2 and 0.10 mm for circumference.

F1-2
Figure 1.:
Mean results for repeated measures of each type of measurement for each clinician. LDIAG, diagonal distance from left anterior to right posterior at 30° from the midsagittal line; RDIAG, diagonal distance from right anterior to left posterior at 30° from the midsagittal line; CIRC, circumference at the plane formed by the sellion and left and right tragion landmarks. Scanner results are compared with “standard” results using calipers for diagonals and a spring tape measure for circumference. Standard deviation bars are present above each bar but generally too small to observe.

Differences between digital and hand tool measurements were larger than those noted between subjects or sessions for scanner data. Figures 2 to 4 show data averages of each practitioner’s five repeated measurements for each session alongside the “reference” hand tool result. In each case, the means of each clinician’s measurements on either data collection session were consistently greater than or smaller than the reference standard, depending on the dimension. For LDIAG (Figure 2), scanner results were always greater than caliper results. For RDIAG (Figure 3), scanner results were always less than caliper results. For CIRC, scanner results were always less than tape measure results. Absolute differences versus hand tools were largest for CIRC, on the order of 4 mm, but this was also the largest dimension measured. Normalizing for differences in scale, the DIAG differences represented approximately 1.0% of the reference dimension, whereas CIRC differences represented approximately 0.90% of the reference dimension.

F2-2
Figure 2.:
Average results for diagonal distance from left anterior to right posterior at 30° from the midsagittal line measured by each clinician (C1 or C2) on each testing session (S1 or S2), along with the “standard” measurement made using calipers. Indicators show standard deviation for each repeated measure.
F3-2
Figure 3.:
Average results for diagonal distance from right anterior to left posterior at 30° from the midsagittal line measured by each clinician (C1 or C2) on each testing session (S1 or S2), along with the “standard” measurement made using calipers. Indicators show standard deviation for each repeated measure.
F4-2
Figure 4.:
Average results for circumference at the plane formed by the sellion and left and right tragion landmarks by each clinician (C1 or C2) on each testing session (S1 or S2), along with the “standard” measurement made using a spring tape measure. Indicators show standard deviation for each repeated measure.

DISCUSSION

Digital shape capture of infant head shapes is clinically useful for a multitude of reasons. As part of a computer-aided design/computer-aided manufacturing (CAD/CAM) system, digital shape capture and modification is a prerequisite to central fabrication of cranial remolding orthoses. Apart from CAD/CAM, digital capture provides an efficient and lasting means to quantify an infinite number of dimensions and compare those dimensions to monitor improvement or progression of asymmetry. Like all tools, systems for digital shape capture should be independently assessed for accuracy and reliability, and assessments should model actual clinical use as closely as possible. To our knowledge, this investigation represents the first such assessment of a widely used infant head shape laser digitizer.

The investigation attempted to closely match actual clinical practice. To enable multiple comparisons of the same dimensions, a model was used. This approach does produce some study limitations. In practice, repeated scans of the same infant may produce additional variability because of motion of the infant in the scanner. Because it would have been impossible to ascertain sources of variability associated with the scanner and infant motion, we chose to accept this limitation to investigate the scanner itself. An additional limitation is the absence of a true gold standard measure with which these results could be compared. In the study, commonly used hand tools were used to provide comparator measures, and though these were referred to as standard measures, measurement with hand tools in prosthetics and orthotics practice has been shown to include its own sources of error and variability.16

As expected, the scanner produced consistent measurements by different practitioners on different days. Although some differences were present, none were statistically significant, and most differences in means were less than 1 mm.

The largest differences in the analysis were those between the scanner results and the hand tool results. These differences, though generally less than 1.5 mm for measurements of diagonals and 4 mm for circumference, may be considered clinically significant. The difference in circumference measures is most likely due to measurement techniques. With the tape measure, circumference is measured in the transverse plane. With the scanner, circumference is measured on a plane defined by the sellion and tragion landmarks. In addition, scanner circumference is a direct trace of the digitized shape of the head on that plane, whereas measurement with a tape measure may tend to level out contours in head shape. In that sense, scanner measurements may be considered the more accurate of the two techniques.

It is interesting to note that the differences in caliper versus scanner measurement of the two diagonals were consistent but bidirectional, depending on site. The diagonal measuring the flattening of the skull was consistently smaller for the calipers versus the scanner, whereas the “high side” diagonal consistently measured larger by the calipers versus the scanner. Although we do not have a clear explanation for this phenomenon, the clinicians participating in the study noted that the scanner consistently measures diagonals at a 30° angle from the neutral fore-aft axis, whereas the location for measurement with calipers may be less consistently and accurately realized.

Given that all three measurements showed sources of inconsistency with hand tools, it may be incorrect to label the hand tool measurement as a gold standard in this case. Although it is certainly the more common source of information, it is likely that, based on the consistency revealed in this investigation, the scanner results may be the more useful standard for comparison.

ACKNOWLEDGMENTS

The investigators acknowledge the assistance of Brian Giavedoni, CP, LP, MBA and the statistical consultation of Teresa Snow, PhD.

REFERENCES

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Keywords:

orthotics; deformational plagiocephaly; cranial remolding; clinimetrics; CAD

© 2008 American Academy of Orthotists & Prosthetists