3D evaluation of tooth-borne and bone-borne surgically assisted rapid maxillary expansion
A scientific essay in Medical Sciences
DOCTORAL THESIS defended in public on 19th of February 2013
Chapter 1 is a brief introduction about distraction osteogenesis (DO) and the Eurocran Distraction Study in general and Surgically Assisted Rapid Maxillary expansion (SARME) in particular. The limitations of the application of tooth-borne appliances and the rationale behind the introduction of bone-borne appliances are then presented. SARME has been the topic of numerous investigations, however, the presence of a wide variety of expansion devices and treatment regimes makes it difficult to draw definite conclusions from the literature. Moreover, most studies were performed using dental plaster models despite the fact that SARME does not only influence the position of the teeth but also the alveolar bone, the hard and soft tissues of the mid-face, the nasal cavity and the soft tissues of the nose. Finally, the potentials of three dimensional (3D) imaging modalities like cone beam computed tomography (CBCT) and stereophotogrammetry in enhancing our evaluation of treatment outcomes are explored.
In chapter 2 the results of a web-based survey, set out to investigate the current practice of DO in Europe, are described. The aim of the survey was to get more insight into the opinion of European surgeons and orthodontists on the use of DO for patients with different diagnoses and treatment protocols. A web-based survey was set up, showing records of four patients with different conditions: hemifacial microsomia, bilateral mandibular deficiency, cleft lip and palate and Crouzon syndrome. Surgeons and orthodontists of 181 Eurocleft centres were asked to fill out a questionnaire for each patient. There was lack of consensus among the respondents about many aspects of DO. Out of six different treatment parameters, an acceptable degree of agreement was only seen in two: a latency period of 3-7days and a distraction rate of 1 mm per day. Furthermore, there was noticeable disagreement on the ideal age for treatment, surgical technique, distraction device, and retention period. The results showed that there is a wide variety in treatment approaches for craniofacial anomalies in Europe. There is disagreement on essential steps in the distraction procedures.
Chapter 3 describes a study to test the accuracy and reproducibility of CBCT superimposition on the anterior cranial base or the zygomatic arches using voxel based image registration. 16 pairs of 3D CBCT models were constructed from pre- and post- treatment CBCT scans of 16 adult dysgnathic patients. Each pair was registered on the anterior cranial base three times and on the left zygomatic arch twice. Following each superimposition, the mean absolute distances between the 2 models were calculated at 4 regions: anterior cranial base, forehead, left and right zygomatic arches. The results showed that voxel based image registration on both zones could be considered as an accurate and a reproducible method for CBCT superimposition. The left zygomatic arch could be used as a stable structure for the superimposition of smaller field of view CBCT scans where the anterior cranial base is not visible.
In chapter 4 a prospective cohort study to three-dimensionally assess the long-term effects of tooth-borne and bone-borne SARME is presented. The study comprised 45 consecutive skeletally mature nonsyndromic patients with transverse maxillary hypoplasia. In 28 patients, a tooth-borne distractor (Hyrax) was used for expansion, whereas in the remaining 17 a bone-borne distractor (transpalatal distractor, TPD) was used. CBCT scans were performed before treatment and 22 months later, after fixed appliance treatment. 3D models were constructed from CBCT data and superimposed using voxel-based matching. Distance maps between the superimposed models were computed to evaluate the amount of skeletal changes. The distance maps of the superimposed models showed positive distances on the right and left posterior alveolar segments of the maxilla indicating lateral expansion. The anterior maxillary region showed negative distances or posterior displacement and remodelling of the anterior alveolar region. There was no statistically significant difference between TPD and Hyrax for the three alveolar segments (p values ranged from 0.63-0.81). Bone-borne and tooth-borne SARME were found to produce comparable results at the end of fixed appliance treatment regarding skeletal changes.
In chapter 5 the soft tissue changes in the orofacial region following tooth-borne and bone-borne surgically-assisted rapid maxillary expansion (SARME) were three-dimensionally assessed. The prospective cohort study included 40 skeletally mature patients with transverse maxillary hypoplasia. A tooth-borne distractor (Hyrax) was used for expansion in 25 patients. In the remaining 15, a bone-borne distractor (Transpalatal Distractor, TPD) was used. CBCT scans were acquired before treatment and 22 months later. 3D models were constructed from CBCT data and superimposed using voxel-based matching. Distance maps between the superimposed 3D models were computed to evaluate the degree of skeletal and soft tissue changes in the maxillary region. Distance maps showed negative distances (mean -1.25, SD 1.5 mm) in the middle of the upper lip, indicating posterior repositioning of this area. The cheek region showed positive changes (mean 1.66, SD ± 1.1 mm), reflecting the underlying increase in maxillary width. There was no significant difference between the two groups in all measured distances (p > 0.05). Retro-positioning of the upper lip accompanied skeletal remodeling in the anterior alveolar region at a mean ratio of 88%, while the cheek region followed 32% of the alveolar expansion. Soft tissue changes following SARME include posterior repositioning of the upper lip and increased projection of the cheek area. These changes were comparable between bone-borne and tooth-borne appliances.
Chapter 6 describes a study to assess the effects of bone-borne and tooth-borne SARME on the volume of the nose and nasal airway two years post-surgically. Thirty two patients with transverse maxillary hypoplasia were included in this study. In 19 patients, a tooth-borne distractor (Hyrax) was used for expansion, in the remaining 13 a boneborne distractor (Transpalatal Distractor, TPD) was used. CBCT scans and 3D photographs of the face, acquired prior to treatment and 22 ±7 months later, were used to evaluate the volume of the nose and nasal airway. Nasal volume increased by 1.01 ± 1.6 % in the Hyrax group and by 2.39 ± 2.4 % in the TPD group. Nasal airway volume increased by 9.7 ± 5.6 % in the Hyrax group and 12.9 ± 12.7 % in the TPD group. The changes in nasal volume as well as nasal airway were statistically significant between T0 and T1 (p < 0.5), but were not significant between the groups (p > 0.5). Twenty-two months after SARME alar width, volume of the nose, and nasal airway have increased. These changes where comparable between tooth borne and bone borne devices.
Chapter 7 is a general discussion of the methodological problems encountered during this investigation as well as the clinical significance of the results of the different studies. Finally, the chapter ends with suggestions for future research.