Quantification of Bony Depression in Mastoid Region After Expander Implantation for Microtia Reconstruction in Hemifacial Microsomia

Abstract

Object: Three-stage expansion method represents the most common form of microtia reconstruction in hemifacial microsomia (HFM). Although the complication related expander has lowered owing to the current advances, bony depression in mastoid region in microtia patients with hemifacial microsomia was observed in clinical work. The aim of this study was to quantify bony depression after retroauricular expander implantation and identify associated factors. Methods: 42 patients were enrolled and studied prospectively utilizing 3-dimensional (3D) evaluation. Craniofacial computed tomography (CT) was performed before the first (pre-expansion) and the second stage (post-expansion) and 3D quantification was done to quantify bony depression in mastoid region by using CT data. Univariate analysis was performed to identify factors associated with bony depression in mastoid region. Results: The mean level of mastoid depression was 0.83 mm (range: 0.07-4.08 mm), and the max level of mastoid depression was 1.40 mm (range: 0.20-6.65 mm). In univariate analysis, capsular duration of expansion and expansion volume were associated factors with mastoid depression. Conclusion: This study showed the possibility of mastoid depression following expander implantation for microtia reconstruction in hemifacial microsomia. Plastic surgeons should be aware of the possibility and associated factors of bony depression in mastoid region following expander implantation to optimize microtia reconstruction for patients with HFM.

Introduction

Hemifacial microsomia (HFM), which is also known as craniofacial microsomia, is one of the most prevalent congenital craniofacial anomalies after cleft lip and palate. ¹ HFM is now recognized as a spectrum that predominantly affects the unilateral derivatives of the first and second branchial arches, mainly involving the orbit, mandible, auricle, facial nerve, and/or soft tissue. ² Microtia itself has been assumed to be an isotype of HFM. ³ Despite significant technical development in recent years, reconstructing microtia continues to be a difficult task for patients with HFM.

Considering soft tissue coverage of the reconstructed auricle framework with taut retroauricular skin and underdeveloped fascia in HFM, most studies advocate for a three-stage expansion method, whereby the initial stage consists of the retroauricular expander implantation.^3,4 With the rising frequency of surgical procedures involving expander implantation, significant attempts have been undertaken to minimize expander-related complication, such as hematoma, exposure and inferior displacement.^4-6 We observed some cases of bony depression in mastoid region especially in microtia patients with HFM, after removing implanted expander during the second stage of microtia reconstruction. There is a scarcity of research discussing mastoid depression following expander implantation which may affect the aesthetic outcome due to unpredictable height of auricle framework.

The aim of this study was to quantify bony depression by utilizing 3D evaluation after expander implantation for microtia reconstruction in HFM and identify possible correlative factors.

Methods

Study Population

From July 2021 to January 2023, patients at the Plastic Surgery Hospital of the Chinese Academy of Medical Sciences were prospectively enrolled based on the following inclusion criteria: (1) diagnosed as unilateral congenital microtia with HFM according to the diagnostic criteria of MacQuillan, ⁷ and (2) willing to participate in the study (CAMS 2021-I2M-C&T-B-076) and provide informed consent before treatment. Additionally, the patients were required to follow the study protocol, which included undergoing craniofacial computed tomography (CT) before the first stage (pre-expansion) and the second stage (post-expansion) of microtia reconstruction. Patients who had experienced expander-related infection, exposure, or required emergent surgery were excluded from the study.

Patient demographics (age and sex), duration of expansion were assessed. And the study was approved by the Ethics Committee of our hospital.

Craniofacial CT and 3D Image Reconstruction

Craniofacial CT was performed 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 two-time data were transferred to software Mimics Research 21.0 (Materialize, Belgium) for pre-expansion and post-expansion skeletal 3D reconstruction separately along with expanded sac 3D reconstruction (Figures 1 and 2), and each of which was exported as a 3D surface model in stereolithography (.stl) file. Then, pre-expansion skeletal, post-expansion skeletal and expander sac 3D surface models were imported in software 3D Slicer 4.11.2 (http://www.slicer.org). ⁸ The volume of expanded sac 3D surface model (expansion volume) was measured and exported by using the module “models.” The pre-expansion skeletal and post-expansion skeletal 3D surface models were superimposed by the 3D Slicer extension “Surface Registration” (Figure 3). Then the differences between the surfaces of 2 models were quantified by the extension “Model to Model Distance,” creating a new model. Applying the extension “Pick and Paint,” the landmark was positioned on the expander-implant mastoid area of this new model. Centered on the landmark, the radius was determined as 10 mm, and the region of evaluation was created, which is approximately the center of depressed area (Figure 4). 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 evaluated area was exported.

Figure 1.

3D reconstruction of skeletal and expander sac surface model.

Figure 2.

3D pre-expansion skeletal, post-expansion skeletal and expander sac 3D surface models.

Figure 3.

Surface registration of pre-expansion and post-expansion skeletal models.

Figure 4.

Evaluated mastoid area in skeletal model.

Statistical Analysis

Comparisons of categorical variables were performed using Mann-Whitney test due to the abnormally distributed data. Continuous variables were analyzed using the Spearman rank correlation test. Univariate analysis was used to examine the correlation of each variable. The mentioned analysis above were performed by software IBM SPSS Statistic, version 26.0 (IBM Corporation, Armonk, NY, USA) with P < .05 were considered statistically significance.

Results

From July 2021 to January 2023, a total of 42 patients diagnosed with unilateral microtia with HFM were included in the study at Plastic Surgery Hospital of Peking Union Medical College, and met the study criteria.

The demographic data of the patient population were analyzed (Table 1). Out of the 42 patients, 31 (75.6%) were male and 10 (24.4%) were female with a mean age of 13 ± 6 years (range, 7-33 years). The average duration of expansion was 94 ± 28 days (range, 61–145 days).

Table 1.

Demographic Characteristics of the Total Patients.

Characteristic	Value
No. of patients (%)	42 (100.0)
Sex, n (%)
Male	31 (75.6)
Female	10 (24.4)
Age, years
Mean	13 ± 6
Range	7-33
Duration of expansion, days
Mean	94 ± 28
Range	61-145

In terms of the expansion volume and variations in the mastoid regions before and after expansion, the median of expansion volume was 82.1 ml (range, 56.0–132.8 ml). The mean level of mastoid depression was 0.83 mm (range, 0.07–4.08 mm), and the maximum level of mastoid depression was 1.40 mm (range, 0.20–6.65 mm) (Table 2).

Table 2.

3D Evaluation of Expansion Volume and Bony Depression in Mastoid Region.

Evaluated variables	Value
Expansion volume (ml)
Median	82.1
Range	56.0-132.8
Mastoid depression (mm)
Min [median (range)]	0.25 (−0.51, 1.86)
Max [median (range)]	1.40 (0.20, 6.65)
Mean	0.83
SD	0.29

“Min 0.25 (−0.51, 1.86)” means median for min is 0.25 with range mentioned in brackets.

Following univariate analysis, the duration of expansion (P = .048) and expansion volume (P = .034) were identified as significant factors associated with the maximum level of bony depression in mastoid region, and expansion volume (P = .010) was found to be a significant factor related to the mean level of bony depression (Table 3).

Table 3.

Univariate Analysis and Correlations Between Variables and Bony Depression in Mastoid Region.

Variables	Max. of bony depression			Mean of bony depression
	U	r	P	U	r	P
Sex	162.000		0.808	162.000		0.808
Age		−0.189	0.231		-0.186	0.237
Duration of expansion		0.307	0.048		0.249	0.112
Expansion volume		0.328	0.034		0.394	0.010

Abbreviations: r, Spearman rank correlation coefficient; U, Mann-Whitney U value.

The bold values means that P < 0.05 with statistic significance.

Discussion

The reconstruction of microtia in patients with HFM is a challenging procedure that requires meticulous planning and execution. One commonly employed technique for microtia reconstruction involves the use of an expander implant. This implant gradually stretches the soft tissues over it, creating sufficient coverage for the cartilage framework. ³ However, after decades of applying this method in our center, we have observed some bony depression in mastoid region, which has made the carving of the cartilage framework to be more intricate.

To the best of our knowledge, this study is the first to evaluate the bony depression in mastoid region in patients with HFM, who undergo microtia reconstruction using the expansion method. In this study, duration of expansion and expansion volume were associated with development of mastoid depression. By using 3D quantification to assess the expansion volume and bony depression in mastoid region, we were able to obtain more accurate and objective measurements by using superimposition methods that involved setting up numerous points to measure the surface differences. This approach allows for improved outcomes compared to traditional 2D measurements on 3D models or X-rays with anatomical variations and potential errors in collecting expansion volume from medical histories.

In this study, we aimed to evaluate the severity of mastoid depression after expander implantation for microtia reconstruction in HFM. Our results showed that the mean and max level of bony depression in mastoid region was 0.83 and 1.40 mm respectively. The average depression level, whether measured in terms of mean or maximum level, was found to be within 1.5 mm. However, it is important to note that the severity of depression varied across a wide range, ranging from mild to severe. The maximum level of bony depression in the most severe case in our study was 6.65 mm, which is quite distinct from the reference range of 19.6 to 21.3 mm, as reported in Bo’s study ⁹ for the mean ear protrusion at the tragal level. In mild cases, the depression is usually not noticeable and does not cause cosmetic issues with the reconstructed auricle. However, in severe cases, the bony depression significantly increases the amount of costal cartilage required and poses challenges in carving the auricular framework to achieve sufficient height compensation for the bony depression. Besides, the severe depression further aggravate the mastoid or even entire facial asymmetry on the basis of the original HFM.

The complications associated with tissue expansion have been extensively discussed in the past few decades. While most of these studies focused on the adverse effects on the skin and soft tissues, there is also well-documented evidence of the side effects on the bones. Hemmer et al. ¹⁰ initially reported a case of calvarial erosion after scalp expansion. Additionally, Fudem and Orgel. ¹¹ documented a complete erosion of the skull bone caused by scalp expansion. Wang et al. ¹² described a cranial bone deformity after forehead tissue expansion. Apart from the cranial bone, tissue expansion could also affect other bones, as McKinney et al described a chest-wall deformity resulting from tissue expansion for breast reconstruction. Although the expansion volume for microtia reconstruction is relatively small, the congenital skeletal maldevelopment may cause reduced bone mineral density on the affected side in HFM, ¹³ which keep the bone in mastoid area to be more susceptible to remodeling under expansion stress. Previous studies^14,15 have suggested that osteoporosis can be considered a risk factor for chest-wall deformity after tissue expansion. Therefore, it is likely that this issue is observed primarily in patients with HFM in clinical practice.

According to our study, we found a positive correlation between expansion volume, duration of expansion, and the severity of bony depression in mastoid region. This means that a higher expansion volume and longer expansion time lead to a greater degree of bony depression. As Wolff's law describes, bones remodel and adapt to the stresses placed upon them by altering their structure and mass. ¹⁶ Compression loaded on the bone in mastoid area is the definitive factor determining the changes of the bony structure. As an implant, tissue expander is also associated with capsule formation due to chronic inflammation, ¹⁷ especially with longer expansion time. Capsular contracture may induce the effect of constant pressure on one point by holding the expander in place as hypothesized by Kim. ¹⁸ Additionally, with the continuous increase in pressure during expansion, it can be assumed that the bone in mastoid area will undergo more noticeable remodeling.

One limitation of this study is that the sample size is relatively small, which may limit the generalizability of the findings. Future research should aim to replicate these findings using a larger sample size.

Despite these limitations, the current study provides valuable insights into the of bony depression in mastoid region and factors that can influence the occurrence of bony depression in mastoid region after expander implantation for microtia reconstruction in HFM. These findings can be used to inform clinical practice and optimize the outcomes of this procedure.

Conclusion

In conclusion, this is the first study showing the severity and possibility of bony depression in mastoid region following expander implantation for microtia reconstruction in HFM. Plastic surgeons should be aware of the possibility and associated factors of bony depression in mastoid region following expander implantation to optimize microtia reconstruction for patients with HFM.