Abstract
Correct tooth position in all planes of space while respecting the boundaries of the underlying bone is important for stability of teeth after orthodontic treatment as well as the health of the supporting periodontium. The aim of this study was to determine: 1) if mandibular posterior teeth are more centered over basal bone when they are more upright or close to WALA Ridge norms proposed by Andrews ; 2) if mandibular posterior teeth are more centered in alveolar bone when they are more upright or close to the WALA Ridge norms; 3) if the estimated center of resistance mandibular posterior teeth is most often centered in alveolar bone; and 4) if the WALA Ridge is located at or near the estimated center of resistance of mandibular posterior teeth. A sample of 34 pre-treatment CBCT scans and mandibular casts of patients ages 12–18 were included in the study. CBCT scans were digitized and analyzed using the Carestream 3D Imaging Software Version 3.5.7. Casts were digitally scanned using the Ortho Insight 3D scanner. The WALA Ridge horizontal measurements were made using the Six Elements™ software (MotionView, Chattanooga, TN). The WALA Ridge vertical measurements were obtained from the casts using a digital caliper. Coronal CBCT images were used to measure tooth positions of pre-treatment mandibular posterior teeth relative to surrounding bone. Centeredness of teeth within the bone was quantified and compared to their inclination and to the WALA Ridge location: D1, D2, D3 and D4. Data were analyzed using the JMP version 10 SAS Software. Descriptive statistics were used to calculate the mean, standard deviation, minimum, and maximum values for the distance between WALA Ridge vertical and CR, D2, D3 and D4 for each of the posterior teeth. Single linear regression analysis was performed to evaluate the relationship between both the long axis inclination and WALA Ridge variables compared to the D1, D2, D3, variables. No statistical significance was found for centeredness of mandibular posterior teeth over basal bone when they were more upright or approached WALA Ridge norms. No statistical significance was found for centeredness of mandibular posterior teeth in alveolar bone when they were more upright or approached the WALA Ridge norms. Significant differences were found for the mandibular posterior teeth center of resistance being centered in the alveolar bone regardless of the long axis inclination or WALA Ridge norms (p-value <0.05). Significant differences were also found for the Wala Ridge being located at or near the center of resistance of mandibular posterior teeth (p-value <0.05). 1) More upright mandibular posterior teeth based on long axis inclination or mandibular posterior teeth more closely related to the WALA Ridge landmark are not more centered over basal bone. 2) More upright mandibular posterior teeth based on long axis inclination or teeth more closely related to the WALA Ridge landmark are not more centered in alveolar bone. 3) The center of resistance of all mandibular posterior teeth can most often be found in the center of the alveolar bone regardless of inclination. 4) The WALA Ridge is located at or near the center of resistance for all mandibular posterior teeth. 5) The WALA Ridge may be a useful landmark for customizing mandibular arch form if teeth are tipped to an upright position.
Introduction
Correct tooth position in all planes of space while respecting the boundaries of the underlying bone is important for stability of teeth after orthodontic treatment as well as the health of the supporting periodontium. Despite all the research over many decades, clinicians are still divided, leading to debate on extraction versus non-extraction. It is generally believed that when tooth mass is too small relative to basal bone, interdental spacing or diastemas will likely occur. Conversely, if the basal bone in the body of the mandible is too small relative to tooth mass, teeth will be crowded. To this point, successful alignment of the teeth, among other factors, is dependent on the size of the basal bone in relation to the tooth mass. Does this mean that teeth should be centered over basal bone?
The term “basal bone” has been used loosely for decades to describe the bone over which teeth should be positioned to obtain stability for both function and health. According to several authors, basal bone is the bone that underlies, supports, and is continuous with the alveolar process. The term “apical base” was first introduced by Lundstrom in 1923 but failed to stimulate a sufficient response until Tweed presented the concept again in 1944 as “basal bone”. Tweed defined basal bone as the bony ridge over which the mandibular central incisors must be situated to produce permanence of orthodontic results. The focus of Tweed’s research was to find the most stable lower incisor position relative to the underlying basal bone to prevent post orthodontic relapse.
Lundstrom (1925) theorized that the apical base did not change to fit the normal occlusion but rather the establishment of normal occlusion was controlled by the apical base. In contrast Damon (2005) suggested that the use of light continuous orthodontic force could be used in crowded cases to expand the alveolar bone and maintain its integrity. Previous studies including Howes (1947) and Downs (1948) have attempted to locate basal bone with little consensus. Not surprisingly, confusion still exists among clinicians and researchers as to the location of basal bone and its true relevance to stable clinical orthodontic treatment.
The Six Elements of Orofacial Harmony™ developed by Andrews presents a set of parameters and guidelines to aid in obtaining optimal goals for the teeth, arches, and jaws. Element I states that an optimal arch exists when teeth are centered over basal bone and the clinical crowns are optimally inclined so that the occlusal surfaces can interface and function ideally with the teeth in the opposing arch. Andrews proposed using the WALA Ridge to serve as a landmark for assessing mandibular arch form (size and shape) which in turn can provide a template for the maxillary arch form. The WALA Ridge is a band of soft tissue immediately coronal to the mucogingival junction of the mandible and suggested to be at or near the level of the center of resistance of the teeth ( Fig.1 ). This landmark aids the clinician in establishing the correct arch form leading to the most ideal tooth position in the mandible relative to the basal bone.
The objectives of this study were to investigate 1) if mandibular posterior teeth are more centered over basal bone when they are more upright or close to the WALA Ridge norms; 2) if mandibular posterior teeth are more centered in alveolar bone when they are more upright or close to the WALA Ridge norms; 3) if the estimated center of resistance of mandibular posterior teeth is most often centered in alveolar bone; and 4) if the WALA Ridge is located at or near the estimated center of resistance of mandibular molar and premolar teeth.
Methods and materials
IRB exemption was obtained from the West Virginia University Institutional Review Board prior to the start of this study (#1506708605). Pre-treatment orthodontic records including CBCT scans and mandibular casts were obtained from the orthodontic practice of Dr. CR. The inclusion criteria include patients 12–18years of age in the permanent dentition with no previous orthodontic treatment, who had a pretreatment cone beam computed tomography scan, and a mandibular study cast taken prior to orthodontic treatment. The exclusion criteria included presence of any craniofacial anomalies; absence of mandibular first and second molars; absence of mandibular first and second premolars; abnormal root morphology; and any previous orthodontic treatment.
The CBCT scans were digitized and analyzed using the Carestream 3D Imaging Software Version 3.5.7. Mandibular casts were digitally scanned using the Ortho Insight 3D scanner (MotionView Software, Chattanooga, TN). The WALA Ridge horizontal was measured using the Six Elements of Orofacial Harmony™ software (MotionView Software, Chattanooga, TN). Coronal CBCT images were used to measure tooth positions of pre-treatment molars and premolars.
WALA ridge vertical measurements
The WALA Ridge landmark according to Andrews was identified on each cast and marked with red pencil ( Fig.2 ). A stainless steel endodontic ruler (Miltex by Integra, Patterson Company, Saint Paul, MN) was then laid across the occlusal surface of each second molar (M2), first molar (M1), second premolar (P2), and first premolar (P1) and its contralateral counterpart ( Fig.3 ). A digital caliper was used to measure the distance in millimeters from the top surface of the ruler to the WALA Ridge on each tooth ( Fig.4 ). To account for the ruler thickness, 0.5 mm was subtracted from each measurement.
WALA ridge horizontal measurements
The WALA Ridge horizontal was measured using the Six Elements™ software ( Fig.5 ). The digital models were calibrated, and landmarks were identified according to the software specifications. WALA Ridge horizontal measurements (WH) of the second molars (M2), first molars (M1), second premolars (P2), and first premolars (P1) were obtained. Once the measurements were made, each of the values were subtracted from the norms proposed by Andrews ( Table1 ). This new value, DWALA, represents the difference between the actual values and the norm (DWALA = WALA Ridge horizontal measurement – norm value).
| Tooth Type | WALA Horizontal Norms (mm) |
|---|---|
| First Premolar | 0.8 |
| Second Premolar | 1.3 |
| First Molar | 2.0 |
| Second Molar | 2.2 |
Measurements from CBCT scans
All CBCT DICOM files were de-identified and downloaded onto the Carestream 3D Imaging Software Version 3.5.7 for data collection. Table2 shows the data points and reference line descriptions. Table3 shows the CBCT variables included in the study. The following measurements were made for each of the posterior teeth including the second molar (M2), first molar (M1), second premolar (P2) and first premolars (P1).
| Points | Description |
|---|---|
| ROP | Reference Occlusal Plane |
| CR | Center of Resistance |
| ABC2 | Alveolar Bone Center at level of Center of Resistance Location |
| ABC1 | Alveolar Bone Center at level of Apex Location |
| APA | Apex Point Alveolar Bone |
| APB | Apex Point Basal Bone |
| BBC | Basal Bone Center |
| IBB | Most Inferior Basal Bone Border |
| LAI | Tooth Long Axis Inclination (degrees) |
| WV | WALA Vertical |
| WH | WALA Horizontal |
| Variable | Definition |
|---|---|
| D1 | Distance from ABC2 to CR |
| D2 | Distance from ABC1 to APA |
| D3 | Distance from BBC to APB |
| D4 | Distance from WALA Vertical to CR |
| LAI | Angle measurement of tooth long axis at ROP |
Long axis inclination (LAI) of posterior teeth
Relative to the Reference Occlusal Plane, the long axis inclination (LAI) of each of the mandibular posterior teeth was measured from the long axis of the clinical crown., ( Fig.6 ).
Measurement of WALA ridge vertical (WV) to center of resistance (CR)
The center of resistance (CR) of each of the posterior teeth was first measured first from the sagittal view. The CR of molars were measured from the top of the clinical crown to the furcation area ( Fig.7 ). The CR of premolars were measured from the top of the clinical crown to 1/3 of the distance from the alveolar crest to the apex. The CR and WALA vertical measurements were then transferred to the coronal view ( Fig.8 ). The distance between CR and WALA vertical was then measured and designated as D4 .
Measurement of center of alveolar bone to center of resistance
Alveolar bone measurements were measured at the center of resistance point (CR) and the apex point alveolar (APA) for each of the posterior teeth. The buccal lingual distance from the alveolar bone internal cortex was measured at CR and APA. These values were divided in half to approximate the center of the alveolar bone at these two locations represented by ABC1 and ABC2, respectively. The distance of CR and APA to the alveolar center point was designated as D1 and D2 respectively ( Fig.9 ).
Measurement of center of basal bone (BBC) to apex of basal bone (APB)
Basal bone measurements were measured at the basal bone center point (BBC) which was located vertically by taking half the distance from the tooth apex to the most inferior basal bone border (IBB). Once this vertical position was identified, the buccal lingual distance from the basal bone internal cortex was measured. This value was divided in half to approximate the center of the basal bone (BBC). The apex point basal bone (APB) was then constructed with a line from the tooth apex perpendicular to the ROP to identify the apex location relative to the basal bone. The distance between BBC to APB was measured and designated as D3 ( Fig.10 ).
