Comparing occlusal contact quality after aligner and xed appliance treatment using computerized occlusal analysis during 6 months of retention

Objective Less than ideal contacts have been reported following aligner therapy, which is believed will resolve with settling, despite settling improving occlusal balance has not been scientically conrmed. The aim of this study was to compare the outcome quality of occlusal contacts in patients treated with xed appliances or clear aligners. 39 orthodontic patients (14 treated with aligners; 25 with xed appliances) were evaluated with a digital occlusal analysis system (T-scan10 ™), assessing Maximum Intercuspation contact simultaneity, symmetry, and relative force distribution. The Occlusion Time, the Right/Left force percentage (%R/L), the Anterior/Posterior contact ratio (RAP), and the anteroposterior Center of Force (COF) locations were recorded at treatment completion, and 3 and 6 months after. shim stock removal T-Scan limitations of and techniques accurately quanties many differing contact force and contact timing sequence for clinical occlusal statistically analyzed


Background
Obtaining a stable and functional occlusion is a main responsibility of the orthodontist. Occlusal aims for closure intercuspated contacts include achieving simultaneous, evenly distributed contacts, that are stronger posteriorly and very light anteriorly (1). Respecting these objectives has been suggested as a key factor to ensure long-term stability of the orthodontic result (2,3). In addition, many authors have reported that occlusal disharmonies could be correlated with potential problems such as temporomandibular disorders (4), periodontal disease (5), cervical abfraction or less e cient mastication (6, 7), though these relations remain controversial.
Whether xed bracket therapy or aligner therapy obtains better occlusal contact outcomes remains a modern-day and unanswered orthodontic question. In 2019, a systematic review comparing the results obtained with aligner therapy or conventional xed appliances, concluded that aligners were less effective in producing adequate occlusal contacts (8), when evaluated on plaster casts with the Objective Grading System (9). Fortunately, it is expected that the quality of contacts would improve during the retention period (10), as teeth once freed from the orthodontic appliances would have a natural tendency to come into contact through the physiologic eruption process (settling). However, little scienti c evidence supports that settling results in ideal occlusal contact force and timing relationships. Independent studies (11)(12)(13) reported that occlusal balance did not always result following orthodontic treatment, when evaluated using a digital occlusal analysis technology. Furthermore, the type of retention, consisting of classic Hawley type appliances, bonded lingual retainers or Essix-type with occlusal coverage could in uence the occlusal settling quality or duration (14)(15)(16).
The primary objective of this study was to determine whether a difference in contact distribution (surface, intensity and symmetry) could be observed in patients that were treated with xed appliances compared with those treated with aligners, at appliance removal. The secondary objective was to follow these two groups during retention, after three and six months, to describe the quality and timing of settling.
To objectively measure and determine occlusal contact quality in the xed appliance or aligner groups, the T-Scan 10 Novus System was chosen (Tekscan Inc. S. Boston, MA, USA). The T-scan 10 is an innovative technology, that records in real-time the contact force distribution as it changes functionally from 1st contact through until complete Maximum Intercuspation (MIP). This method has been proven precise and reproducible (17), while eliminating the clinician subjectivity of occlusal paper mark interpretation (18-22) or shim stock removal "feel". The T-Scan overrides the limitations of waxes and silicone impressions techniques that are static, as it accurately quanti es many differing relative occlusal contact force parameters and contact timing sequence durations for clinical occlusal diagnosis and treatment, that can be statistically analyzed (23).

Participants
All included patients had completed non-extraction, non-surgical, comprehensive orthodontic therapy into an ideal class I occlusion at either a university orthodontic clinic, or at a senior orthodontist's practice. The study population included 39 patients total, comprised of 25 patients (mean age 18.7 ± 5.2; 6 females, 19 males) that received self-ligating xed appliance therapy (Speed™ brackets, Canada or Empower™ brackets, American Ortho, USA), while 14 patients (mean age 20.6 ± 7.3; 10 females, 4 males) received clear aligner therapy (Invisalign™, Align Technology, San Jose, CA, USA).
Subjects were excluded if they presented with occlusal coverage retainers, tooth agenesis (excluding 3rd molars) microdontia (unless related to 3rd molars), severe periodontal disease or had had large restorations installed during the time of data ( Figure 1).

Data Collection
Patients were assessed at three points in time: on the day of treatment completion (T0), at 3 months' post treatment (T3), and at 6 months' post treatment (T6), by the same examiner (CB) for consistent data acquisition. Each assessment follow-up visit included: a T-Scan 10 digital occlusal analysis recording into MIP, patient self-report of their retainer compliance and self-assessment of occlusal comfort using a visual analog scale (VAS) ranging from 0 -10 (0 = very uncomfortable; 10 = maximum comfort). The examiner con rmed that each patient had not undergone dental restorative or occlusal adjustment treatments since the previous visit.

Digital Occlusal Analysis
The digital occlusal analysis employed the T-Scan 10 system, that records occlusal data with a 100µ thin, exible horseshoe-shaped Mylar sensor (Novus HD sensor, Tekscan, Inc. S. Boston, MA, USA), that contains 1370 active pressure sensing cells, known as sensels, arranged in a compact grid, shaped as a dental arch. The Mylar coating encases sophisticated printed electronic components, allowing the HD sensor to withstand occlusal stresses from between occluding teeth, and repeatedly record 256 relative occlusal force levels and occlusal contact timing sequences. This system has been validated by several laboratory and clinical studies from 2006 to 2020 (11,17,(23)(24)(25), and has shown consistency to report relative occlusal force mapping and timing data, without sustaining signi cant sensor damage breakdown from intercuspating teeth.
Prior to recording actual study data, the sensitivity of the sensor was adapted the patient's muscular strength to avoid oversaturating the sensor. Also, patients were given recording instructions and performed practice registrations that familiarized them with the Multi-bite recording procedure. Then, 3 consecutive self-intercuspated closure-into-MIP registrations were recorded, without removing the sensor from the patient's mouth. Each patient then rested for one minute before replicating the procedure a second time, if needed. When several recordings were obtained during a single visit, the one presenting the most consistency between the three consecutive intercuspations was used for analysis. Importantly, the same sensor was used during all 3 visits, unless the sensor demonstrated excessive wear.
An estimate of the occlusal contact surface area can be determined from the number of activated sensels on the sensor at maximum force intercuspation (MIP, maximum loading of the sensor). The total surface of contacts, and the area ratio between anterior (canine to canine) to posterior (premolar to second molar) contact surfaces were extracted from the underlying ASCII data. The overall relative force distribution was assessed by the position of the Center of Force (COF) Icon (the force summation location that averages all occlusal contact forces, similar to a barycenter). The COF position was calculated using ASCII data, to give a numerical value of the anteroposterior position of occlusal forces at MIP (ranging from 0 to 1). A greater value signi ed a more posterior concentration of forces. The symmetry of contact distribution was expressed as the percentage of contacts on the Right side to the Left side (%R/L). Lastly, the time-simultaneity of the closure into MIP contacts was calculated by the Occlusion Time (26) measurement, which is the duration in milliseconds between rst contact and the time MIP was reached. When the OT is prolonged (> 0.2 seconds), it indicates the presence of premature or prolonged closure occlusal contacts within the contact sequence (27).

Statistical Analysis
A two-sided two-sample t-test estimated that a sample size of 15 participants per group would provide a power of 80% to reject the null hypothesis that there were no signi cant between-group differences at an alpha level of 5%, assuming a difference in the anteroposterior position of COF = 5.5±1.4 mm, based on a pilot study of COF absolute position.
Statistical analyses were carried out using IBM-SPSS Statistics for Windows (Version 28, IBM, USA). All results were presented as mean ± standard deviation for continuous variables, as percentages (%) for nominal variables, with all values tested for normal distribution using the Shapiro-Wilk test. For descriptive statistics, both parametric and non-parametric tests (ANOVA-type statistic for repeated measures; Mann-Whitney U tests; Chi-square) were used, including mixed models for repeated measures. The null hypothesis was rejected at p < 0.05.

Results
Thirty-nine subjects were enrolled in the study (25 in the xed therapy group and 14 in the aligner group), and 8 subjects were excluded during the course of the project (4 in each group), because of a change in the retention protocol or a missed visit (Figure 1).
There were no signi cant differences between the xed or aligner treated groups for age, Angle's classi cation, asymmetry nor facial type (Table  1) except more girls being in the aligner group (p = 0.007). The retention protocol was similar in both groups, without any difference in the total number of bonded lingual wires (p=1 Fisher's exact test) but subjects having received aligner therapy had signi cantly more bonded lingual retainers in the upper arch (10/12 subjects in the aligner group vs. 7/15 in the xed appliance group; p = 0.10 Fisher's exact test). Self-reported compliance with Hawley retainer was not different between groups (p = 0.83 Fisher's exact test). Table 1 Baseline sample description of xed appliance and aligner groups. *p < 0.01 At treatment completion, the total surface of occlusal contacts and the number of teeth out of contact were not signi cantly different between the aligner and the xed orthodontics groups (Table 2). Qualitatively, the type of teeth out of occlusion did not differ between groups, and mostly involved central or lateral incisors (incisors involved in 5/12 in aligner group, n=9/15 in xed appliance group). The relative proportion of anterior to posterior contact areas (RAP), the anteroposterior position of the COF and %R (estimating symmetry) were not signi cantly different in either group. Although the %R/L per group were not signi cantly different, neither treatment resulted in ideal occlusal balance. In natural occlusions, ideal %R/L balance measured with the T-scan has been described to be = 50±2% (28-30). Taking a wider normal range = 50±5%, only some patients reached this equilibrium at treatment completion: 3/25(12%) for the xed appliance group and 7/14 (50%) for the aligner group (Figure 3a). Right/Left 50±5% balance improved at T3 for both groups (11/24, 45.8% for xed appliance group vs 8/13, 61.5% for aligners in Figure 3b The Occlusion Time (an estimate of contact simultaneity) appeared slightly increased in the aligner group at treatment completion (0.17±0.62 vs 0.14±0.58), without reaching statistical signi cance. Occlusal comfort was similar in both groups at treatment completion: the Median score = 8 for both groups (range 6-10 for xed appliance group; 5-10 for aligners). During the rst 6 months of settling, there was a net increase with regards to patients' comfort level, with a Median of 10 (p < 0.0001), with both treatment modalities (range 7-10 for xed appliance; 8-10 for aligners). Comfort scores could not be correlated to any occlusal feature (OT, %R/L, RAP, COF) nor associated with sex, age or pretreatment malocclusion type.
The COF moved posteriorly in both groups from T0 to T6, in parallel with a decreased ratio of anterior to posterior surface area, while the %R/L and OT remained statistically unchanged. Figure 5 shows the anteroposterior position of the COF, which moved posteriorly during the rst three months of retention (p = 0.003), after which it did not vary signi cantly regardless of treatment modality (p = 1.000). A statistically signi cant difference was observed in the anteroposterior position of COF between sexes, being more anterior in females at all times (p < 0.002) (Figure 6).
Examples of Tscan-records can be seen in Figures 7 and 8, illustrating contrasting treatment outcomes: a balanced outcome vs. and an asymmetrical outcome with the COF anteriorly positioned.

Discussion
Despite published reports suggest that non-ideal contacts result following orthodontic treatments with aligners compared to xed appliances, the results of this study found no difference in occlusal parameter quality between patients treated with aligners or xed orthodontic appliances, with respect to contact surface area, overall force distribution and symmetry of contacts in MIP. Settling quality indicators, such as patient comfort and Occlusion Time (simultaneity), were also not statistically different between the two groups, suggesting that both therapies obtained similar closure contact occlusal outcomes.
It was a common nding at the end of treatment to see light contacts in the incisor area, and frequently, a lack of contacts on the lateral incisors, both of which should not be considered problematic. In a mutually protective occlusion, it has been advocated that the contacts should be stronger posteriorly, with molars anatomically shaped to sustain larger proportions of the total occlusal load, with light contact existing on the incisors in MIP.
The antero-posterior position of the COF six months after completion of orthodontic treatment stabilized in both groups in an identical position (0.55±0.11 vs 0.55±0.06), which corresponds with a position located qualitatively at the level of the mesial of the rst molar, which has been shown to be its ideal position in patients with normal occlusions (27). The COF moved posteriorly in both groups from T0 to T3 (p=0.003), after which time, no statistically signi cant difference was noted. This suggests that most of the settling occurred within the rst three months' posttreatment, which corresponds with conclusions drawn in earlier studies (31). Interestingly, a statistically signi cant difference was observed in the anteroposterior position of COF between sexes, with it being positioned more anteriorly in female patients at all times.
A scienti c basis for using the COF (equivalent to a barycenter of occlusal force) as an indicator of functional occlusal balance has been described by different research teams using different methods, that obtained similar overall results. In the early 1980's, two studies determined the physiologic equilibrium point of the mandible, using electronic means (32,33); when Class I patients clenched their teeth in centric relation with an applied force of 24 pounds the equilibrium point was estimated to lie within the mesial third of the mandibular 1st molar, while being close to the mid-sagittal plane. Using the Dental Prescale System™, another group determined that the Center of Force was not in uenced by ethnicity, gender or age in a non-orthodontic sample (26).
Another unexpected nding of this study, was that an asymmetrical force distribution between the left and right sides remained in 1/3 of the treated patients after 6 months of settling. Right/Left balance appeared worse in xed appliance group immediately after treatment completion and improved at T3. After 6 months of retention, less than 50% of all patients ended up with ideal symmetrical loading of the sensor, in the %L/R 50±5 range, and one third depicted a signi cant functional asymmetry. This study's occlusal force asymmetry ndings coincide with those reported by Qadeer et. al, where signi cant occlusal force imbalances were observed in 2 separate post orthodontic T-Scan studies (12,13).
Although not related to occlusal comfort or the appearance of any symptoms, this observed asymmetry, of unknown etiology, illustrated postorthodontic uneven contact force distribution. This asymmetry could re ect the patient's preference for chewing on a particular side, as a type of hemispheric laterality (34). One study showed signi cant and positive correlations existed between masticatory laterality, side differences in bite force, and side differences in occlusal contact area at MIP (35).
It is important to point out that the occlusal force asymmetry following the visual aligning of teeth to a predetermined ideal, appears to not translate into occlusal surface interactional force control. Although the teeth may t together visually well, the visual ideal does not guarantee the functional interocclusal forces will be balanced, well-distributed, and of only low force intensity.
It is plausible that orthodontic appliances signi cantly interfere with the normal functioning of the masticatory muscles and that total neuromuscular recovery might vary between individuals. The reestablishment of normal function after orthodontic treatment was studied by Winocur et al. (36), who found that neuromuscular adaptability started immediately at bracket removal, with a maximal bite force increase of about 15%, followed by another 15.5% increase in the rst 3 months, whereas only an additional 2% increase was noted after 6 months. Varga et al. using electromyography, reported weaker forces of mastication in females at the time of debonding, and their settling took longer than for male patients (31). The authors hypothesized that women, being cautious, avoided biting forcefully during their orthodontic treatment.

Study limitations
The ndings of no difference in the quality of occlusal parameters in MIP after orthodontic treatment with aligners or conventional bonded brackets, still need to be interpreted with caution. These results may be speci c to a university setting, where much care is conferred to obtaining ideal results, and may possibly not correspond to what can be seen following aligner therapy in a broader context. For the aligner group, on average, 3.9 modi cations were made to each patient's Clincheck™ (the virtual treatment planning with simulated movements software) prior to Funding This study was not funded but public or private funds, but the recording sensors were graciously provided by Tekscan, Inc, S. Boston, MA USA Authors' contributions Dr JCL, AM and RBK designed the study. Dr CB collected data and wrote the rst version of the manuscript. PR performed statistical analysis, critically reviewed conclusions and several manuscripts. Dr JCL and RBK reviewed and modi ed data presentation, wrote the discussion and updated references. All authors read and approved the nal manuscript.

Figure 1
Patient's owchart Histograms in the 3D view and the pixel areas on the 2D occlusal view, represent contact force intensity and surface area respectively. b -An occlusal view photograph of the same patient in Figure 2a, of the maxillary MIP contacts marked with horseshoe articulating paper. Note the false positive contacts due to staining of the paper ink on the incisal borders of the lateral and central incisors. Distribution of patients according to %R/L force in MIP, for xed appliance groups (in blue) and aligners (in red). Figure 3a at T0, 3b at T3, Figure  3c at T6.

Figure 4
Page 12/13 Estimation of occlusal contact symmetry and its evolution during the retention period between groups. Note the divergence of many of the colored lines away from the hyphenated midline (=50%) in both aligner and xed treatment patients. This indicates that settling for 6 months did not improve patient occlusal balance   A balanced symmetric outcome after tooth movement with the COF centered and the right to left force imbalance = 47.3% right -52.3% left. Despite the overall balance, multiple differing individual contact force levels resulted from treatment, stronger in the anterior teeth. COF is anterior, posterior contacts are located in the palatal cusps of molars, and 27 has very little contact.

Figure 8
An imbalanced and highly asymmetric aligner outcome with the COF located in the posterior left. The large occlusal force imbalance (29.7% right -70.3% left) did not change with settling.

Figure 9
The development of a "posterior open bite" in the course of aligner treatment has been linked to the intrusion of posterior teeth due to the interocclusal thickness of aligners, premature anterior contacts (upright maxillary incisors in this case) and the di culty of extruding posterior teeth with aligners.