Material and methods

A grid system was created for identification of palatal sites used for bone analysis and stability testing ( Fig 1 ). The rationale was to include the most commonly reported palatal insertion sites, as well as those that could be used for miniscrew insertion, without encroaching on important anatomic structures such as nerves and blood vessels. The midpalatal suture was excluded from analysis due its variability and possible effect on growth in young patients. In the custom-designed grid system, the green region represents the currently recommended areas for safe placement ( Fig 1 ).

Grid system representing the predetermined insertion sites. Coordinate 0,0 represents the posterior border of the incisive foramen at the midpalatal suture ( blue circle ). M-L , Mediolateral, 4-mm increments starting ± 2 mm from the midpalatal suture. A-P , Anteroposterior, 6-mm increments starting 3 mm posterior to the incisive foramen. The green area represents the currently reported safe area for insertion of TADs.

The 0,0 coordinate of the grid system represents the posterior border of the incisive foramen at the midpalatal suture. X and y axes were constructed from this reference point. The y-axis ran through the midpalatal suture, and the x-axis was tangent to the posterior border of the incisive foramen, perpendicular to the y-axis. The x-axis was coincident with the distal aspect of the maxillary permanent canines as described previously.

Parallel to both the x and y axes, the anteroposterior and mediolateral directions were created. In the anteroposterior direction, the initial line was 3 mm posterior to the x-axis, with subsequent lines in increments of 6 mm from the initial line, to correspond with dental landmarks ( Fig 1 ). A numeric order (1-5) was given to each line according to its proximity to the incisive foramen. For the mediolateral direction, the initial lines were 2 mm lateral to the y-axis, with subsequent lines located in 4-mm increments from the initial lines. These lines were named according to their proximity to the midpalatal suture, with the closest termed the parasagittal line, the middle termed the sagittal line, and the farthest termed the lateral line.

To test all intersection points of the grid system and to preserve an acceptable distance between the miniscrews for testing, the insertion sites were staggered between the right and left sides of each specimen ( Fig 1 ). This technique was deemed acceptable, since Gracco et al demonstrated no significant difference in BT between contralateral sides of the palate for the same subject. A total of 13 sites were tested on each specimen.

All palatal specimens were obtained with permission from the body-bequeathal program at Western University, London, Ontario, Canada, in accordance with the Anatomy Act of Ontario and Western’s Committee for Cadaveric Use in Research. A total of 10 embalmed cadaveric heads were used ( Table I ). Sex of the specimens was not a specific requirement because Gracco et al showed no statistically significant difference in palatal BT between sexes. Age at death was also not a specific criterion due to the limited number of specimens available. For inclusion in this study, the specimens were required to have an intact and complete (right and left) maxillary palatal bone, full or partial dentition with at least 4 anterior teeth (to ensure comparable incisive canal outlines), and no palatal torus and bony pathology involving the maxilla.

The embalmed heads were cut so that the maxilla was separated from the rest of the skull. All soft tissues were dissected to eliminate specimen variability and standardize the LSE during miniscrew placement. The crowns of the teeth were subsequently removed, eliminating any metal substance from dental fillings that could interfere with x-ray imaging.

The specimens were stabilized by embedding in standardized acrylic blocks with the teeth facing superiorly for miniscrew insertion testing. The acrylic level was carefully controlled, leaving approximately 1 cm of clearance below the nasal floor to allow assessment of miniscrew penetration into the nasal cavity and to prevent interferences during insertion. A stencil with the predesigned grid pattern (described above) was custom fabricated and used for accurate reproduction of the grid system onto the specimens.

One hundred thirty VectorTAS orthodontic miniscrews (Ormco, Orange, Calif; length, 6 mm; diameter, 1.4 mm) were used for insertion at the predetermined sites (n = 10). All miniscrews were self-drilling; thus, predrilling of the cortical bone was not required.

To replicate the average palatal soft tissue thickness as determined by Vu et al, a 1-mm silicone spacer was installed over the soft tissue collar of each miniscrew before insertion ( Fig 2 ), leaving 5 mm of the body length available for insertion into bone. The miniscrews were manually inserted by the first author (C.B.), simulating the typical clinical technique. The corresponding peak torque value reached during miniscrew insertion was recorded in Newton-centimeters (Ncm).

Custom apparatus used for TAD insertion: A, specimen mounted on the apparatus; B, close-up of a VectorTAS miniscrew during insertion. The insertion apparatus incorporated a multiaxis clamp and stabilization bar with 6 degrees of freedom motion. These allowed for specimen and screwdriver adjustment before insertion, ensuring that TAD positioning was always perpendicular to the palatal bone surface.

The insertion device consisted of a screwdriver with a chuck, which secured the manufacturer’s specific driver adaptor and engaged the miniscrew head ( Fig 2 ). The screwdriver handle was held in place by a stabilizing bar, which was specifically designed to support the driver shaft and prevent oblique forces during manual screw placement. This allowed the miniscrews to be inserted vertically, without introducing off-axis loading along their length.

For insertion of the miniscrews, the embedded specimens were fixed at the base of the insertion device within a custom adaptor and secured onto the torque sensor (6 DOF load cell; Advanced Mechanical Technology, Watertown, Me). A multiaxis clamp was used to position the specimens at the center of a torque sensor and adjusted as required to ensure perpendicular insertion of the miniscrew to the bone surface. The torque sensor and associated software program (WaveMatrix Software; Instron, Norwood, Mass) measured and recorded torque values during miniscrew insertion. Calibration of the sensor was completed before insertion of each miniscrew to reduce any systematic errors in the setup.

Specimens were imaged using a cone-beam μCT scanner (eXplore Locus Ultra; GE Healthcare, Milwaukee, Wis) before and after miniscrew insertion. The scanner acquires 1000 projection images (1024 × 680 matrix size), over a single 16-second rotation at 120 kV(p) and 20 mA. These projection images were back-projected to reconstruct a 1024 × 1024 × 680 voxel 3-dimensional volume, with 150-μm isotropic voxel spacing, and scaled in Hounsfield units (HU) using known water and air regions. Du et al previously described the scanner characteristics. Included with each scan were 2 known density calibrators (water [0 mgHA/cm ⁻³ ] and a bone-simulating material SB3 [1073mgHA/cm ⁻³ ]).

Image analysis was performed using 3-dimensional analysis software (MicroView 2.2; GE Healthcare, London, Ontario, Canada). BMD and BT were calculated using coregistered scans and the miniscrew trajectory into bone, as described by Sabo et al. Custom software was used to create 2-mm diameter cylindrical regions of interest within the bone and around the miniscrew trajectory, with length specified by the screw length. The cylindrical regions of interest were superimposed onto the original (preinsertion) scans ( Fig 3 , A ) and used for BMD analysis. Voxels with values of −200 HU or less were excluded from BMD calculations and assumed to represent air. Voxels with values between –200 and –50 HU were forced to have a value of 0 mgHAcm ⁻³ , since these values would correspond to water, fat, or preservation medium.

Image analysis method used for quantifying BMD and BT. A, Computer software depicting the cylindrical region of interest and the longitudinal TAD trajectory used for BMD and BT analysis ( left image ), initial, preinsertion scan. The cylindrical regions of interest are truncated at the air boundaries and represent the actual regions used in the BMD analysis ( right image ), registered scan used for identification of the longitudinal TAD trajectory. B, Line profile along the longitudinal TAD trajectory. Line profile of an insertion site for a given specimen taken from the prescrew insertion scan. The graph shows grayscale values (HU) sampled along a line that represents the screw trajectory, where highlighted regions differentiate between air ( blue ) and bone ( orange ).

For BT measurements, grayscale line profiles were created following each longitudinal miniscrew trajectory (described above) and ensured to go through the entire bone length. These profiles generated plots of grayscale values (HU) as a function of line position (mm) and allowed for thickness measurements. Using a full width at half maximum technique, grayscale transitions from air to bone and bone to air were used to identify the entrance and exit of each miniscrew ( Fig 3 , B ).

Miniscrew perforations into the nasal cavity were identified and confirmed by visual inspection of the specimens ( Fig 4 ). Perforations were only observed in the nasal cavity and not in the maxillary sinus, since the miniscrews were inserted into the hard palate proper and not the alveolar bone where the sinus cavities are located ( Fig 3 , A ). Using the μCT data, subtraction of the miniscrew threaded body length (5 mm) from the BT showed the magnitude of the perforation (negative value). Perforations of more than 1 mm were judged to be clinically significant and recorded.

Visual inspection of perforation of TADs into nasal cavity.

Results

Descriptive statistics representing the IT, BMD, BT, LSE, and perforation values for each insertion site are given in Table II . Statistical analysis showed that IT, BT, and LSE data were not normally distributed. BMD was normally distributed, but outliers were detected. The median values were therefore interpreted ( Fig 5 ).

Table II

Descriptive statistics for bone mineral density (BMD), bone thickness (BT), length of screw engagement (LSE), insertion torque (IT), and perforation (Perf) at each insertion site

Insertion site	n	BMD (mg/cc)			BT (mm)			LSE (mm)			IT (Ncm)			Perf (n/10)
		Median	Mean	SD	Median	Mean	SD	Median	Mean	SD	Median	Mean	SD
PS1	10	318.77	338.03	104.1	7.58	7.88	3.17	5.00	4.63	1.17	13.05	16.48	8.15	1
S1	10	303.12	283.88	84.10	9.35	8.82	2.74	5.00	4.68	1.02	15.45	15.53	4.41	1
PS2	10	381.14	373.67	109.94	4.42	4.68	1.27	4.42	4.33	0.69	9.75	9.39	3.94	4
S2	10	408.68	410.50	103.99	3.73	3.65	1.69	3.73	3.45	1.37	15.15	14.68	7.25	6
L2	10	300.43	321.39	91.29	6.16	5.58	2.07	5.00	4.41	1.20	14.50	14.44	6.42	1
PS3	10	416.41	426.59	123.32	3.15	3.42	0.98	3.15	3.42	0.98	13.10	13.83	3.91	7
S3	10	422.77	441.99	134.13	1.96	1.78	0.81	1.96	1.78	0.81	9.00	12.27	6.99	10
L3	10	384.29	393.18	150.65	2.46	2.64	1.54	2.46	2.59	1.43	8.55	11.16	7.43	8
PS4	10	519.49	482.88	135.96	3.50	3.71	1.18	3.50	3.64	1.07	14.55	13.86	5.57	6
S4	10	337.69	319.02	144.33	1.31	1.31	0.52	1.31	1.31	0.52	8.10	8.63	4.86	10
L4	10	256.67	271.01	134.14	1.04	1.27	0.67	1.04	1.27	0.67	7.05	7.05	3.60	10
PS5	10	435.01	421.42	103.70	2.54	2.47	0.94	2.54	2.47	0.94	6.75	8.19	5.13	10
S5	10	314.04	311.93	95.48	0.96	1.12	0.44	0.96	1.12	0.44	4.65	5.38	2.84	10
P value		0.004			<0.001 ∗			<0.001 ∗			<0.001 ∗			<0.001 ∗

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