Morphologic Variability of Bone and Soft Tissue in Microtia With Hemifacial Microsomia

Abstract

Objective: Microtia patients with hemifacial microsomia (HFM) have a host of distinct anatomical disorder of skeletal and soft tissue asymmetries. The purpose of this study was to assess soft tissue discrepancies in microtia patients with HFM and their correlation with skeletal discrepancies. Methods: A total of 42 patients were enrolled and studied prospectively using a 3-dimensional superimposition and color mapping of the soft and hard tissues. Mirroring techniques created perfectly symmetric models for comparison. Differences between affected and normal sides were evaluated in 5 areas: retroauricular mastoid, malar, maxillary frontal, mandibular frontal, and gonion areas. Pearson correlations were used to assess the relationship between skeletal and soft tissue asymmetry. Results: Hard tissue asymmetry ranged from 0.79 mm (mandibular frontal) to 1.29 mm (malar), while soft tissue asymmetry ranged from 1.34 mm (maxillary frontal) to 5.26 mm (retroauricular mastoid). Correlations between skeletal and soft tissue asymmetry varied, with the strongest correlation observed at the retroauricular mastoid area and the weakest at the maxillary frontal area. Conclusion: There was a high correlation between bone and soft tissue hypoplasia at the retroauricular mastoid area, while the other evaluated areas showed poor correlation between skeletal and soft tissue asymmetries. Clinicians should assess each component separately for optimal treatment planning in microtia patients with HFM.

Introduction

Microtia is a congenital malformation characterized by an underdevelopment or absence of the auricle altogether. ¹ Microtia can present in isolation or as part of abnormalities such as hemifacial microsomia (HFM), the underdeveloped bone and soft tissue of unilateral face with resulting asymmetric balance of the face, low hairline, depression of the temporoparietal joint, and low-set ear vestige. ² Microtia patients with HFM have a host of distinct anatomical disorders that pose surgical challenges and often hinder ideal aesthetic outcomes following microtia reconstruction. ³

The morphologic variability of bone and soft tissue in microtia with hemifacial microsomia has been the subject of considerable study. Understanding the anatomical differences between affected and unaffected side can aid in the development of effective treatment plans, including surgical interventions.

Previous studies commonly used 2-dimensional (2D) radiographs to evaluate skeletal or dental asymmetries. However, these studies are susceptible to errors of projection, errors of identification, and magnification errors. ⁴ While 3-dimensional (3D) reconstructions of cone beam computed tomography images have conquered the above limitations as the preferred modality for assessing asymmetries. ²

Utilizing 3D evaluations of facial and skeletal morphology in clinical practice can provide superior information and understanding of microtia with HFM. Although it is commonly accepted that unilateral skeletal and soft tissue discrepancies in microtia with HFM are prevalent, there has been little investigation of the correlation between the 2. The objective of this study was to assess the relationship between soft and hard tissue discrepancies in different facial regions, especially retroauricular mastoid regions by utilizing a 3D superimposition and color mapping.

Patients and Methods

Study Population

From July 2021 to January 2023, patients in Plastic Surgery Hospital of Chinese Academy of Medical Scien-ces were prospectively enrolled by inclusion criteria as follows: (1) diagnosed as unilateral congenital microtia with HFM refers to the diagnostic criteria of MacQuillan ⁵ and (2) were willing to participate in the study (CAMS 2021—I2M-C&T-B-076) and sign the informed consent before treatment. The study was approved by the Ethics Committee of our hospital.

Craniofacial CT and 3D image reconstruction

Craniofacial computed tomography (CT) was performed after admission with a 64-slice scanner (Brilliance CT 64-slice system, Philips Medical Systems; Cleveland, OH, USA). All CTs were taken with the following setting: tube voltage, 120 kV; tube current, 220 mAs; section thickness, 1 mm; pitch, 0.8; rotation time, 0.75 s; and matrix, 512 × 512 pixels. The original data were transferred to Mimics Research 21.0 (Materialise, Belgium) for skeletal and structural 3D reconstruction separately, each of which was exported as a 3D surface model in a stereolithography (.stl) file. Then, 3D surface models were imported in Meshmixer 3.5 (Autodesk, Inc. San Rafael, CA, USA). The unaffected side of each 3D skeletal and soft tissue facial surface model was mirrored within the midsagittal plane as a reference, resulting in a mirrored model composed of 2 unaffected sides, which was also exported as .stl files (Figure 1). Both original and mirrored 3D surface models were imported into 3D Slicer 4.11.2 (http://www.slicer.org) ⁶ with superimposition by using the unaffected side as reference. The differences between the surfaces of 2 models were quantified by the 3D Slicer extension “Model to Model Distance,” creating a new colormap model (Figure 2). Applying the extension “Pick and Paint,” the landmark was positioned on the interested area of this new colormap model. Centered on the landmark, the radius was determined as 5 mm, and the region of interest was created. Then the “Mesh Statistics” extension was used to measure the linear differences in the interested regions (in mm). The measurements were exported based upon X, Y, and Z planes of space. The 3 planes were combined so that an absolute difference between the 2 models on the selected area was exported.

Figure 1.

Original/mirrored skeletal and soft tissue.

Figure 2.

Colormap 3D model showing the extent of skeletal and soft tissue asymmetries.

The 5 areas that were selected to evaluate the differences between the affected and unaffected side of the models were the following: retroauricular mastoid area, malar area, maxillary frontal area, mandibular frontal area, and gonion area (Figure 3).

Figure 3.

Evaluated area in skeletal and soft tissue.

Statistical Analysis

The correlations between the amount of skeletal asymmetry and soft tissue asymmetry were evaluated by Pearson correlations. The mentioned analysis above was performed by IBM SPSS Statistic, version 26.0 (IBM Corporation; Armonk, NY, USA), with P < .05 was considered statistical significance.

Results

From July 2021 to January 2023, 42 patients were diagnosed with unilateral microtia with HFM at Plastic Surgery Hospital of Peking Union Medical College, and met the study criteria (G power analysis, power = 0.79). Of the 42 patients, 31 (75.6%) were male and 10 (24.4%) were female with a mean age of 13 ± 6 years.

Regarding the variations between the assessed skeletal and soft tissue regions on the affected and unaffected sides, the asymmetry in skeletal tissue ranged from 0.79 mm (mandibular frontal area) to 1.29 mm (Malar area), while the soft tissue asymmetry ranged from 1.34 mm (maxillary frontal area) to 5.26 mm (retroauricular mastoid area). The soft tissue areas exhibited higher levels of asymmetry compared to the skeletal areas (Table 1).

Table 1.

Skeletal and Soft Tissue Asymmetries Between Affected and Unaffected Sides on the Evaluated Areas and Their Correlation.

Area	Skeletal asymmetry (mm)				Soft tissue asymmetry (mm)				Pearson correlation coefficient (r)	P value
	Min	Max	Mean	SD	Min	Max	Mean	SD
Retroauricular mastoid area	−0.08	3.37	1.13	0.78	2.49	7.16	5.26	0.92	0.41	.007
	−1.59, 2.07	2.24, 5.67	0.46, 4.23	0.52, 1.46	0.33, 6.32	3.64, 11.65	2.53, 8.73	0.69,1.37
Malar area	−0.84	3.22	1.29	0.72	2.15	4.22	3.36	0.49	0.04	.80
	−1.75, 0.41	1.85, 4.66	0.36, 2.48	0.45, 1.21	0.93, 4.03	2.80, 7.24	1.91, 5.67	0.35,0.74
Maxillary frontal area	−0.51	2.04	0.95	0.60	−0.11	2.81	1.34	0.55	−0.12	.44
	−1.23, 0.15	1.16, 3.01	0.31, 1.15	0.41, 0.79	−0.84, 0.76	1.82, 4.81	0.36, 2.64	0.36,2.64
Mandibular frontal area	−0.17	1.89	0.79	0.63	0.47	3.43	2.27	0.92	0.01	.96
	−1.47, 0.15	1.34, 3.01	0.32, 1.25	0.40, 0.95	0.60, 1.93	1.37, 6.35	0.35, 4.12	0.49,1.31
Gonion area	−0.62	3.29	1.17	0.81	3.47	6.53	4.76	0.70	0.24	.14
	−1.28, 0.71	1.76, 6.20	0.51, 3.86	0.59, 1.57	1.24, 7.13	3.82, 10.66	2.18, 8.73	0.37,1.14

Abbreviation: SD, standard deviation.

The differences between the amount of skeletal and soft tissue asymmetry ranged from 0.39 mm (maxillary frontal area) to 4.13 mm (retroauricular mastoid area), and the correlation between the skeletal and soft tissue asymmetries varied across the areas evaluated. A statistically significant correlation was found between the skeletal and soft tissue asymmetries in the retroauricular mastoid area (r = 0.41, 0.87, P = .005). There was no statistical significance in the other evaluated areas, and the maxillary frontal area had the lowest correlation (r = −0.12, P = .44; Table 1).

Discussion

In our study, we conducted a comprehensive 3D morphometric measurement and assessed facial skeletal and soft tissue asymmetry of unilateral microtia with HFM by utilizing 3D colormap quantification, aiming to contribute to the understanding and management of this rare congenital malformation. This methodology has been employed in previous studies to accurately measure facial differences in patients with HFM. ⁷ Previous research has demonstrated that quantifying asymmetries through 3D analysis is highly accurate. ⁸ Unlike 2D measurements on 3D models or X-rays with anatomical variations and the error of method in positioning landmarks, 3D measurements using superimposition methods like colormap can accurately identify asymmetries by using thousands of points to quantify surface differences. ⁹

Additionally, our study assessed the correlation between skeletal and soft tissue asymmetry, which is important for determining appropriate surgical interventions for patients with different types of severity in the skeletal and soft tissues of the face. Surgical treatment is chosen based on the individual needs of microtia patients with HFM. Children with severe mandibular hypoplasia, for example, may require surgical interventions for both skeletal and soft tissue structures along with expansion microtia reconstruction. ¹⁰

To contextualize our study, we reviewed previous studies related to microtia and other facial abnormalities. We found a scarcity of prior research specifically focusing on 3D morphometric measurement of unilateral facial microtia with HFM. Most 3D measurement and 3D surgical plan for patients with HFM were focused on maxillofacial region, especially on distraction osteogenesis technique to mandibular lengthening^11–14 instead of microtia reconstruction. This observation highlights the novelty and significance of our work in a relatively unexplored retroauricular mastoid area of HFM malformation assessment.

Specifically, our results show that the soft tissue asymmetry was bigger in retroauricular mastoid area than the skeletal asymmetry, and the results of retroauricular mastoid area highlighting the fact that the magnitude of soft tissue asymmetry may vary considerably from the magnitude of the skeletal asymmetry.

The retroauricular mastoid area plays a crucial role in microtia reconstruction surgery, since the success of the surgery depends on achieving a symmetric location achieved between the reconstructed ear and the unaffected side. ¹⁵ Additionally, providing adequate soft tissue coverage for the reconstructed auricle is a significant concern. The higher level of differences between soft tissue and skeletal tissue asymmetry in retroauricular mastoid area may indicate the tauter retroauricular skin, the more underdeveloped temporoparietal fascia and underlying temporal muscle, and the more unreliable arterial anatomy. ³

This information is crucial for the surgeon as it highlights the importance of accurately assessing and correcting the soft tissue coverage to achieve optimal results. The majority of articles recommend a 3-stage approach for auricular reconstruction in patients with HFM, in which the initial stage involves the expansion of retroauricular skin. Ideal tissue expansion eliminates the requirement for a fascial flap and skin grafts, which can lead to irregular, patchy skin, more noticeable scars, and hair loss, according to reports.^3,16 Yamada et al. ¹⁷ preferred employing the temporoparietal fascia flap along with mastoid flap and skin graft, and Nuri et al. ¹⁸ suggested using a pericranial flap. Surgeons should exercise caution when considering the vascular supply quality to the fascial flap when applying the above techniques.

Conclusion

This study’s findings indicate considerable variation in the extent of skeletal and soft tissue asymmetries among microtia patients with HFM, especially elucidating the distinctive morphological features and correlation of bone and soft tissue hypoplasia at the retroauricular mastoid area. By providing a detailed and systematic 3D morphometric analysis of unilateral facial microtia with HFM, we suggest clinicians evaluate each component separately to achieve the optimum treatment plan for microtia patients with HFM.