Introduction

The unilateral posterior crossbite with a functional shift (FUPXB) is one of the most common early adolescent malocclusions with a prevalence of 7%−23%. ^, Frequency of unilateral posterior crossbite (UPXB) occurs in 5.9% to 9.4% of the total population, while FUPXB is the most common form of posterior crossbite occurring in 80% to 97% of all posterior crossbite cases. ^, The prevalence of FUPXB is 8.4% in the primary dentition and 7.2% in the mixed dentition. The etiology of posterior crossbite is unclear but has been related to a combination of many factors that included dental, skeletal, soft tissue, respiratory, functional neuromuscular, or habitual abnormalities. Posterior crossbite can be caused by a deficient maxilla relative to the mandible, resulting in a convenient shift of the mandible to one side for better interdigitation. This functional shift may cause a right-to-left-side differences in the condyle fossa relationship, resulting in temporomandibular joint (TMJ) problems. Much attention has been given recently to the effects of this functional shift on the condyles.

In animal studies, altering mandibular position with bite planes or occlusal grinding results in skeletal growth pattern changes at the ramus and condyle, change in cartilage thickness, and gene expression differences with insulin-like growth factor-1 (IGF-1) and fibroblast growth factor-2 (FGF2) and mRNA expression on protruded and non-protruded condylar sides. Liu, found that rats exposed to a 2 mm shift of the condyle to the left side developed asymmetric mandibles. The length of the condylar head was greater on the protruded side, and the mandible on the protruded side grew in a more anterior superior direction.

Only a few studies have evaluated condylar position within the TMJ using CBCT, with varying and inconclusive results. In 2009, Ikeda and Kawamura published a study where limited CBCT was used to find the optimal position of the mandibular condyle within the glenoid fossa. In all subjects, the joints were completely symptom free, and the position of the articular disc was verified by MRI analysis. The joint spaces found were found to be significantly altered. The anterior joint space (AS) was 1.3 mm, superior joint space (SS) was 2.5 mm, and the posterior joint space (PS) was 2.1 mm. Hesse, performed a tomographic analysis of the condylar position in patients with a FUPXB in 1997 on patients before and after maxillary expansion. The non-crossbite condyle moved posteriorly and superiorly from before to after expansion treatment, and the superior joint space was the greatest on the non-crossbite side before treatment. Relative condylar position was more anterior on the non-crossbite side before treatment, but both sides were similar after treatment. In 2012, Leonardi performed a low dose CT study analyzing the crossbite and non-crossbite side condyles pretreatment and post treatment. No differences in position of the condyles pre-treatment were found, but significant increases in superior joint space on the non-crossbite side, and relative increases in anterior and posterior joint spaces on the non-crossbite sides occurred post treatment. In addition, the posterior joint space increased only on the crossbite side after treatment.

The objective of this study was to determine if the presence of a unilateral posterior crossbite with a functional shift results in altered condylar position within the TM joint. In specific, the condylar width and condylar angle to the midsagittal plane, positional differences between crossbite side and non- crossbite side condyles, and condylar osseous changes, such as progressive condylar resorption (PCR) will be determined in both the study and the control groups.

Methods and materials

The subjects were randomly selected from the office of one of the investigators (T.S.). The clinical findings together with CBCT scans were used to place subjects in either the control or the study group. The CBCT images were all taken on the same i-CAT machine with a setting of 14.7 acquisition time, 20.27 mA, 120 kVp, a field of view of 17 mm x 23 mm and voxel size of 0.3 mm x 0.3 mm. The inclusion criteria included adolescent patients seeking orthodontic treatment with a good quality DICOM file image. The study group had a maxillary transverse deficiency with posterior crossbite (involving greater than one tooth) on one side only with the teeth at maximum intercuspal position as indicated by the clinical exam. Subjects were excluded based on the presence of artifacts, developmental or acquired craniofacial deformity with or without mandibular/condylar involvement, systemic disease, history of orthodontic treatment, anterior crossbite, signs or symptoms of TMD according the AAO medical history/exam, missing teeth (excluding third molars), carious lesions, extensive restorations, or pathologic periodontal status. The final sample consisted of 31 subjects in the control group without a functional crossbite and 29 in the study group with a crossbite.

The sixty DICOM files were analyzed using the In Vivo Dental 4.1 imaging software. Each file was oriented according to criteria set forth by Cho. The sagittal plane was derived from a best fit of the landmarks Nasion, Crista galli, Sella, and Basion. The axial plane was set parallel to Frankfort Horizontal, and the axial pane was set parallel to the frontozygomatic points (FZ). A lateral cephalometric radiograph and posterior anterior cephalometric radiograph were formed from oriented CBCT images. Maxillary position (SNA), mandibular position (SNB), relative position of the maxilla to the mandible (ANB), mandibular plane (SN-MP and FMA), maxillary incisor inclination (Upper I to SN), and mandibular incisor inclination (IMPA) were measured from the lateral cephalometric radiograph, and the maxillary width (AG-GA) and mandibular width (JR-JL) were measured from the P-A cephalometric radiograph to calculate the maxillomandibular transverse differential index. Within each CBCT image the maxillomandibular difference according to Miner was completed ( Fig.1 ). The axial angle of the maxillary and mandibular first molars compared to the functional occlusal plane and the maxillary and mandibular mid-alveolar process widths were measured.

Image of the dental transverse measurements and the molar inclinations as described by Miner.

Each condyle had a sagittal section determined by a vertical plane bisecting the long axis. In the axial section, anterior joint space (AS), superior joint space (SS), and posterior joint space (PS) were measured at the bisected sagittal section and 5 mm medial and lateral to this section. The angle of the long axis of the condyle was measured from the midsagittal plane ( Fig.2 ). Each joint measurement was made twice over a 2-week period for reliability.

View of the imaging planes for joint space analysis.

Signs of active and reparative progressive condylar resorption and/or degenerative joint disease were recorded. Defects were classified according to the following criteria, flattening, osteophytes, cup shaped defects, cortical surfaces defined but not corticated, and beaking, but due to inadequate resolution of some of the images, an exact identification was not always possible.

Results

The experimental design was reviewed and considered Exempt by the Institutional Review Board at West Virginia University (protocol number 1,310,115,425).

The final sample consisted of 60 total subjects; 31 subjects in the control group and 29 in the study group. The mean age of the control group was 9.8 ± 1.3 yrs and the study sample was 9.4 ± 2.0 yrs. ( Table1 ). A total of 120 temporomandibular joints, including 62 from the control group and 58 (29 crossbite sides and 29 non-crossbite sides) from the study group were analyzed. Among the 29 study group subjects, 19 had crossbite to the left and 10 had crossbite to the right.