This retrospective longitudinal cohort study was approved by the institutional review board at the University of the Pacific in San Francisco, California (#2022-183). A total of 119 patients were identified and categorized into 3 groups: IFS, hyrax, and untreated controls. The control group was obtained from the American Association of Orthodontists Foundation Craniofacial Growth Legacy Collection (Michigan and Oregon growth collections). Treatment groups were derived from 2 private practices (S.T. and M.B.), with both practitioners holding American Board of Orthodontics certifications.

To minimize selection bias, the practitioners were not involved in the sampling process. Consecutive eligible patients treated between September 2018 and June 2022 were screened. The inclusion criteria consisted of complete digital study casts at all 3 time points: T1 (pretreatment in mixed dentition), T2 (posttreatment in mixed dentition), and T3 (at least 1 year after T2, in permanent dentition) and ≥4.0 mm planned maxillary expansion. Exclusion criteria included the use of other appliances, such as mandibular advancement devices. Patients with bilateral posterior crossbites were also excluded, as both clinicians used hyrax expanders in these patients to achieve skeletal maxillary expansion. After applying the inclusion and exclusion criteria, 44 and 36 patients met the requirements for the IFS group and the hyrax group, respectively.

The control group consisted of 39 untreated subjects from the American Association of Orthodontists Foundation Growth Legacy Collection, with records available at comparable time intervals. The control group was matched to the treatment groups by age, sex, and Angle classification.

Sample size calculations were performed to ensure study power with a 2-tailed test α of 0.05 and 80% power. To detect a change of ≥1 mm in intermolar width, corresponding to an effect size of 0.7 based on a previous study, the required sample size was estimated to be 18 patients per group.

IFS treatment used removable clear aligners in both maxillary and mandibular arches, with sequential staging expansion, 0.25 mm expansion per aligner, and 7-day aligner changes. The patients were instructed to wear aligners primarily after school and at night. During retention (T2-T3), passive aligners were used as retainers until they no longer fit the erupting dentition, after which no retainers were used. Patients were monitored approximately every 4 months during the retention period.

In the hyrax group, tooth-borne hyrax expanders were banded to the permanent maxillary first molars and activated one-quarter turn per day (0.25 mm) for approximately 4 weeks, then 3 turns per week as needed until the lingual cusps of the maxillary molar approximated the buccal cusps of the mandibular molar. Expanders were retained passively for an additional 3 months, after which maxillary 2 × 4 fixed appliances were placed. In the mandibular arch, 40% patients received a lower lingual holding arch, 20% had lower 2 × 4 or 2 × 6 fixed orthodontic appliances, and 40% received no treatment. Retention consisted of clear maxillary retainers with palatal and molar occlusal coverage, incorporating cutouts for erupting teeth. For patients with mandibular fixed appliances, the lower lingual holding arch served as retention; otherwise, no mandibular retention was used.

Digital study models were measured with Ortho Analyzer software (3Shape, Copenhagen, Denmark). Three calibrated examiners identified dental landmarks: the cusp tips of canines, the central fossae of primary molars and permanent premolars, and the centroid of the permanent molars. The centroid measurement was determined by connecting the 4 main cusps of each first molar and identifying the geometric intersection point. Four transverse measurements were recorded at each time point (T1, T2, and T3) per arch for all 3 groups ( Fig 1 , A and B ). Interexaminer reliability was excellent (intraclass correlation coefficient = 0.97-0.98), and the average of the 3 examiners’ measurements was used for analysis. Lateral cephalometric radiographs were available for 94 patients (IFS group: 31, hyrax group: 30, and control group: 33). Cephalometric tracing was performed using Dolphin Imaging & Management Solutions (version 12; Chatsworth, Calif) by a single examiner (A.T.) and verified by a faculty member. A standard set of cephalometric measurements was recorded at T1 to confirm baseline comparability among the 3 groups. Two vertical skeletal measurements, SN-MP and FMA, were analyzed longitudinally to evaluate potential vertical changes associated with expansion.

Arch width measurements on the study casts: A, Mixed dentition; B, Permanent dentition. a, Maxillary C-C or 3-3; b, Maxillary D-D or 4-4; c, Maxillary E-E or 5-5; d, Maxillary 6-6; e, Mandibular C-C or 3-3; f, Mandibular D-D or 4-4; g, Mandibular E-E or 5-5; and h, Mandibular 6-6.

The study comprised 3 groups: control (n = 39; 17 males and 22 females), hyrax (n = 36; 12 males and 24 females), and IFS (n = 44; 21 males and 23 females). No statistically significant differences were observed in the age and Angle classification distribution among the 3 groups ( Table I ). The mean age at T1 was approximately 9.1 years across groups. The mean time interval from T1 to T3 was 3.8 years for control, 3.9 years for hyrax, and 3.3 years for IFS groups, with a minimum of 1 year of supervision between T2 and T3 for all groups. At the baseline (T1), no statistically significant differences were observed in the skeletal or dental cephalometric measurements ( Table II ). The IFS group showed a slightly greater maxillary intermolar width (46.8 ± 2.3 mm) than the control group (44.4 ± 2.5 mm) ( Table III ). Both treatment groups achieved significant expansion during phase 1 treatment compared with the control group ( P <0.0001 for all measurements; Table IV ). The hyrax group demonstrated greater maxillary expansion, with increases of 3.6 ± 1.9 mm in intercanine width and 4.4 ± 2.1 mm in intermolar width. The IFS group showed a smaller but still statistically significant increase of 2.9 ± 2.0 mm (intercanine) and 1.9 ± 1.3 mm (intermolar). In contrast, the control group showed minimal changes, ranging from 0.2 ± 0.7 mm to 0.7 ± 0.8 mm. In the mandibular arch, the IFS group showed significant increases in widths of 2.0 ± 1.8 mm (intercanine), 3.7 ± 2.6 mm (inter-first primary molar), 3.2 ± 2.2 mm (inter-second primary molar), and 2.4 ± 1.4 mm (intermolar).

Table IV

Arch width changes from T1 to T2

	Control (n = 39)		Hyrax (n = 36)		IFS (n = 44)		P value	Group comparison ∗
	Mean	SD	Mean	SD	Mean	SD
Maxilla
3-3	0.16 a	1.59	3.55 b	1.87	2.87 b	1.96	<0.0001	Control and hyrax; control and IFS
4-4	0.15 a	0.70	4.10 b	1.88	2.75 c	1.93	<0.0001	Control; hyrax; IFS
5-5	0.23 a	0.67	4.75 b	2.31	2.67 c	1.69	<0.0001	Control; hyrax; IFS
6-6	0.66 a	0.80	4.36 b	2.06	1.88 c	1.32	<0.0001	Control; hyrax; IFS
Mandible
3-3	–0.19 a	1.02	0.91 b	1.48	2.01 c	1.78	<0.0001	Control; hyrax; IFS
4-4	0.40 a	1.16	1.46 a	1.95	3.65 b	2.58	<0.0001	Control and IFS; hyrax and IFS
5-5	0.07 a	0.67	1.59 b	1.77	3.24 c	2.16	<0.0001	Control; hyrax; IFS
6-6	0.12 a	0.67	2.18 b	1.27	2.44 b	1.44	<0.0001	Control and hyrax; control and IFS

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