## 1 Introduction  
Pycnodysostasis (PDS) results from mutations in the cathepsin K gene (CSTK), leading to osteogenic skeletal dysplasia [1]. The clinical features of PDS include short stature, craniofacial anomalies, retrognathia, a maxillary transverse deficit, and an inferiorly-posteriorly positioned hyoid bone [2]. This condition can lead to the development of obstructive sleep apnea (OSA), which is defined as the repetitive closure of the upper airway during sleep, resulting in sleep fragmentation and daytime hypersonnolence. The severity of OSA is typically quantified by the apnea-hypopnea index (AHI), measured as the number of apneas plus hypopnea per hour of sleep. OSA is categorized as mild (5-14.9 events/hour), moderate (15-29.9 events/hour), or severe (>30 events/hour) [3, 4].  
However, the limited number of case reports and case series of PDS in the literature suggests that obstructive respiratory diseases are highly prevalent; patients with craniofacial anomalies have a threefold greater risk of experiencing OSA compared to healthy controls [5].  
[PERSON] et al. retrospectively observed 10 PDS patients over 10 years; continuous positive air pressure treatment (CPAP) was required in six of these patients. Among the three patients who underwent uvulopalatopharyngoplasty (UPP), two still needed CPAP during their follow-up, while only one patient never developed OSA [6]. Management of this syndrome is multidisciplinary. OSA in PDS patients can be treated with either a conservative (non-surgical) or surgical approach. The use of CPAP represents the gold standard for non-surgical treatment in young PDS patients. CPAP devices deliver air through a mask worn over the nose or both the nose and mouth, applying constant pressure throughout the respiratory cycle to keep the airway open. This positive air pressure prevents airway collapse during inspiration, thereby increasing functional residual capacity.  
In pediatric patients, surgical approaches are indicated when CPAP is not well accepted due to facial morphology. Surgical treatment may involve UPP in childhood or bimaxillary orthognathic surgery (BOS), typically performed in adulthood. UPP entails the resection of the uvula and part of the soft palate. Despite its efficacy, UPP can have adverse effects, such as prolonged pain and postoperative bleeding. Conversely, BOS provides a sagittal increase in the upper airway, but this approach carries surgical risks related to the high susceptibility to osteomyelitisitis in PDS. Therefore, BOS is usually reserved for adult PDS patients with severe OSA who have undergone \"phase I\" surgeries (adenotonsillectomy and uvulopalatoplasty) and are aged 6 years or older.  
Given the craniofacial anomalies associated with PDS, orthodontic treatment aims to address the maxillary transverse deficit and retrognathia [7]. Currently, there is no evidence regarding the influence of orthodontic therapy on the level of OSA among PDS patients. The most recent study on early ENT surgery in a PDS patient had a short-term follow-up [8]. However, there are no studies with larger cohorts of PDS patients concerning the long-term effects of UPP on the recurrence of obstructive symptoms. The aim of this study is to assess the functional-(AHI) and morphological-dimensional changes in the upper airways following UPP and orthodontic camouflage in a PDS patient.