Comprehensive Three-Dimensional Evaluation of Temporomandibular Joint Changes Following Stabilization Splint Therapy

Abstract

Introduction and aims

While stabilization splints (SSs) have shown promising therapeutic effects for temporomandibular disorders (TMDs), comprehensive studies evaluating temporomandibular joint (TMJ) changes following SS therapy are necessary. This study aimed to assess TMJ structural, positional, and condylar remodelling changes in TMD patients by using advanced three-dimensional assessment and shape correspondence analysis.

Methods

This retrospective study included 80 adult TMD (arthralgia) patients treated with SS. Pre- and post-treatment cone beam computed tomography scans were analysed using three-dimensional Slicer software. The following measurements were evaluated: (1) volumetric condylar changes, (2) bone mineral density, (3) joint spaces, (4) condylar position, (5) condylar rotation, and (6) condylar remodelling (resorption or apposition). Statistical comparisons between time points and condylar sides were performed via paired t tests and Wilcoxon rank-sum tests.

Results

Treatment duration was 6 to 12 months (mean: 9.8 months). Study results indicated a slight increase in condylar volume and bone mineral density, but no statistically significant changes were observed. However, significant differences were noted in the anterior joint space on both sides. Condylar positional changes demonstrated inferior, lateral, and anterior translation, along with forward rotational movement on both sides. Localized condylar remodelling revealed bone formation predominantly in the posterior and superior regions, while slight bone resorption was mainly observed in the anteromedial and medial regions.

Conclusions

SS therapy promotes favourable condylar remodelling and TMJ realignment, as evidenced by reduced anterior joint space, anterior-inferior condylar displacement, and forward rotational changes, along with localized bone apposition. These findings highlight its role in facilitating adaptive changes in patients with TMD.

Clinical relevance

This study demonstrates that SS therapy improves TMJ function and condylar dynamics, offering a noninvasive treatment option that reduces mechanical stress and enhances patient outcomes. These insights provide clinicians with valuable evidence for incorporating SS therapy into TMD management strategies.

Introduction

The temporomandibular joint (TMJ) is a complex and delicate structure that connects the temporal bone to the mandible, enabling essential jaw movements.¹ It consists of bilateral mandibular condyles that work together, moving either in the same or opposite directions. The two main components of the TMJ are the mandibular condyles and the glenoid fossa, with a disc between them that acts as a cushion to absorb stress and facilitate smooth condyle movement during jaw actions.²

Temporomandibular disorder (TMD) is a broad term that encompasses various clinical conditions affecting the TMJ, the masticatory muscles, and their related anatomical structures.³ Evidence indicates that approximately 31% of adults experience TMD, highlighting its significant prevalence in the general population. By contrast, the prevalence among children and adolescents is notably lower, at around 11%.⁴ Furthermore, the prevalence of TMD varies across geographic locations.⁵ Common symptoms associated with TMD include the loss of cartilage integrity, disc displacement, mandibular deviation during opening or closing, persistent pain, joint noises (such as popping or clicking), restricted jaw mobility, muscle tenderness, earaches, and headaches,⁶ all of which can significantly and negatively affect an individual’s quality of life.⁷ TMD has a multifactorial origin, with potential contributing factors such as stress, genetic predisposition, psychological influences like personality type,⁸ bruxism, jaw clenching, and musculoskeletal issues affecting the jaw joint and injuries.⁹

Conventional two-dimensional imaging was once the main radiography technique used for evaluating the TMJ. However, this method has limitations, such as the overlapping of adjacent structures and low sensitivity in detecting changes in the condylar and temporal bone components, making it undependable.¹⁰ The introduction of three-dimensional (3D) imaging and magnetic resonance imaging has significantly improved the precision of TMJ analysis.¹¹ Cone beam computed tomography (CBCT), in particular, offers high-resolution imaging with less radiation exposure compared with conventional computed tomography (CT), making it highly accurate for TMJ examination.¹²

TMD treatment options depend on its severity and underlying cause. These options can be categorized as invasive, minimally invasive, or noninvasive. Invasive procedures include open TMJ surgery and arthroscopy,¹³ while minimally invasive techniques, such as injection of corticosteroids, platelet-rich plasma, or hyaluronic acid, help alleviate inflammation and pain.^14,15 Noninvasive approaches comprise manual therapy, physiotherapy, short-term pharmacotherapy, psychotherapy, and the use of occlusal splints.16, 17, 18, 19 Among noninvasive treatments, occlusal splints are widely utilized for TMD patients,²⁰ with stabilization splints (SSs) being, particularly effective.²¹ SSs are essential in managing many oral and extraoral disorders, including TMD, headaches, myofascial pain, migraines, and bruxism.^19,22, 23, 24 They also function as muscle relaxants, efficiently reducing muscle pain,^25,26 and help the condyles in achieving their most stable musculoskeletal position by replicating functional occlusion.²⁷

SS was described for the first time in its modern form by Jeffrey Okeson. He mentioned a number of factors that may clarify why occlusal splints alleviate the symptoms related to TMDs, including changes in occlusal conditions, condylar positions, increases in vertical dimensions, cognitive awareness, alterations in peripheral nervous system input, natural recovery of the musculoskeletal system, placebo effects, and regression to mean.²⁵ A systematic review by Al-Moraissi et al¹⁹ found that all occlusal splints likely provide better results for TMD patients than no treatment or nonoccluding splints.

Previous studies have explored the effects of SS therapy on TMJ changes in TMD patients. Derwich et al²⁸ investigated alterations in joint space and condylar position, concluding that SS therapy combined with physiotherapy had no significant effect on joint space dimensions or mandibular condyle repositioning into centric relation. Steinbaum et al²⁹ evaluated condylar remodelling changes following SS treatment in patients with degenerative joint disease of the TMJ. Their findings suggest that SS therapy may promote favourable adaptive changes in the condyles of these patients. Ramachandran et al³⁰ investigated the effects of deprogramming splint therapy on condylar position in patients with TMD. The study aimed to assess clinical symptoms and condylar positioning, and it found significant improvements in condylar positioning after treatment.

Although previous studies provided valuable insights, most of them primarily focused on specific aspects of TMJ changes following SS therapy. This specificity has resulted in a knowledge gap owing to the lack of a comprehensive approach that evaluates multiple parameters simultaneously, as is done in the present study. This comprehensive approach provides a more holistic understanding of the outcome of SS therapy on TMD patients. Another limitation in previous research is the relatively small sample sizes, which may restrict the reliability and generalizability of the findings. By contrast, this study benefits from a larger sample size, enhancing the robustness of its results. Furthermore, this study employed voxel-based superimposition of models reconstructed from CBCT scans, utilizing two suggested registration protocols and automatic shape correspondence calculations (SPHARM-PDM) with 3D Slicer software. This advanced approach for quantifying TMJ morphological changes reduces reliance on examiner expertise, minimizes errors associated with intra- and inter-rater variability, ensures standardized results, facilitates novel discoveries, and aids in identifying new imaging-based risk markers. As such, this methodological approach represents one of the key strengths of this research.

This study aimed to analyse overall volumetric changes of the condyle, bone mineral density (BMD), joint spaces, condylar position, condylar rotation, and localized bony changes on each condylar surface after SS therapy through shape correspondence analysis, which compares the same anatomical points across different time models and between different sides.

Methods

Study design

This retrospective clinical study was conducted at the First Affiliated Hospital of Xi’an Jiaotong University in Xi’an, Shaanxi Province, China, in compliance with the ethical principles outlined in the Declaration of Helsinki. The study protocol received ethical approval (No. XJTU1AF2022LSK-027) from the Institutional Review Board of the hospital before the commencement of data collection and analysis. Written informed consent was obtained from all participants prior to their involvement in the study.

Sample size calculation

The study data were obtained from the full medical records of patients diagnosed with TMD (intra-articular joint disorders/arthralgia) and treated with SS therapy between July 2017 and July 2024. The sample size was determined using G*Power software (Version 3.1.3; Franz Faul, Universität Kiel), with an alpha level of 0.05 and a power of 80%. The calculation was based on the findings of Ahmed et al,³¹ who analysed CBCT 3D images of the mandible and TMJ before and after the application of SSs in patients with TMD. Their study reported changes in the superior joint spaces (SJSs) on the deviated side of –0.48 ± 0.86 mm. The power analysis indicated that a minimum of 32 patients was required to achieve statistical significance; nevertheless, the sample size was subsequently increased to a minimum of 80 patients.

Selection criteria

Patients were selected on the basis of the following inclusion criteria: adult patients aged ≥18 years; those with complete oral and medical health record; those who underwent a comprehensive AXIS I DC/MD clinical evaluation confirming a diagnosis of one of the following TMJ disorders: intra-articular joint disorders such as disc displacement with reduction, disc displacement with reduction and intermittent locking, or arthralgia; patients who complied with the treatment plan and showed a cooperative attitude; those whose treatment plan involved maxillary SS with no apparent skeletal mandibular asymmetry; those showing full eruption of all permanent teeth (excluding third molars) with healthy periodontal condition, those who met the SS retention requirements; and patients who had completed their SS treatment and had clear CBCT scans taken before and after the treatment. The exclusion criteria for patient selection were as follows: a craniomaxillofacial trauma history, active-phase idiopathic condylar resorption, a history of orthognathic surgery, orthodontic treatment or TMJ surgery, and the presence of metabolic diseases or systemic immune disorders.

Intervention

Once the SSs were manufactured, they were inserted into the patients’ mouths, and adjustments to the occlusion were carried out. Patients were advised to wear the SS for a minimum of 20 hours daily³²; the device was intended to be taken out while eating and teeth brushing. The success of treatment largely relied on the diligence of patients to attend all scheduled appointments, given that multiple visits were necessary for adjustments to the SS. After a week of splint utilization, a dentist examined the patient’s bite that marked on its surface and made additional adjustments by grinding it. This process helped establish a stable jaw position. Follow-up appointments were scheduled after 15, 30, and 60 days, followed by monthly visits until treatment completion.³² The total treatment duration ranged from 6 to 12 months (mean: 9.8 months). CBCT scans were obtained at two-time points: before treatment initiation (T0) and after treatment completion (T1). The patients’ progress was evaluated at each appointment, during which the joint area was examined, muscle tenderness was noted, and the splint was adjusted when needed. Patients received treatment relying exclusively on SS; no other interventions, such as medication or physical therapy, were used. The clinical criteria used to assess the completion of SS treatment included: pain relief in the masticatory muscles, TMJ, neck, and shoulders; normalization of the mouth opening range; clinical improvement in disc displacement; and the absence of joint sounds (eg, clicking or popping).³¹ After therapy, the treatment was assessed not only by patient feedback but also by the Helkimo index with an evaluation of condyle displacement. These assessments determined whether the jaw joint remained stable if the condyles were positioned near centric relation and whether TMD symptoms improved over three follow-up appointments.³¹ Finally, the patient was advised to discontinue use of the SS appliance after the TMJ achieved a stable position and symptoms were relieved.

CBCT assessment

A CBCT machine (KaVo 3D eXam; KaVo Dental, Bismarckring) was utilized to capture 3D images. The settings for imaging were configured at 120 kV and 37.1 mA, featuring a field of view of 23 × 17 cm, a voxel size of 0.3 mm, an exposure duration of 17.8 seconds, and a slice thickness of 0.3 mm. Throughout the procedure, patients were instructed to sit upright with their teeth in maximum intercuspation. The Frankfort horizontal plane was aligned parallel to the floor, while the midsagittal plane was oriented perpendicular to it. Patients are also instructed to avoid swallowing during the scan. Following the CBCT scan, the data were collected and converted into the Digital Imaging and Communications in Medicine file format. These data were then processed using a validated measurement method with 3D Slicer software (version 5.2.2).

Image analysis

The linear assessments of radiographic TMJ joint spaces were measured in millimetres using the method described by Alhammadi.^33,34 These measurements included anterior joint space (AJS), SJS, posterior joint space (PJS), and medial joint space, as illustrated in Supplementary Figure 1. The locations of the chosen points were modified across the sagittal, axial, and coronal planes.

The pre- (T0) and post-treatment (T1) CBCT scans underwent cranial base voxel-based registration using 3D Slicer software.³⁵ After the cranial base registration, semiautomated segmentation was conducted to generate a 3D model of the mandible. After segmenting the condyle using the Segment Editor module, we used the Segment Statistics module to calculate the average BMD in Hounsfield units (HU) for the segmented condyle, as well as the volume of the condyle in cubic millimetres.

Regarding condylar displacement and rotation measurements, landmarks for position and angulation were identified on the condylar heads. A total of four points were identified on the condylar head – two on the right side and two on the left side, with one point on the lateral pole and one on the medial pole for T0 and T1, as shown in Figure 1. The 3D Slicer software’s Quantitative 3D Cephalometric module was utilized to measure translational and rotational changes in the condyles, providing a direct assessment of condylar displacement within 3D models. After marking the medial and lateral poles of the condylar head, the software automatically calculated the translational and rotational displacements. Translational displacement was measured in three planes: sagittal (AP: anterior [+] and posterior [–]), vertical (SI: superior [–] and inferior [+]), and transverse (ML: medial [+] and lateral [–]). Rotational changes were assessed in the coronal plane (roll: inferior [+] and superior [–] rotation), axial plane (yaw: inward [+] and outward [–] rotation), and sagittal plane (pitch: anterior [+] and posterior [–] rotation).

Fig. 1.

Condylar rotational changes measurements: (A) roll; (B) pitch; (C) yaw.

Rigid-regional registration was used to register the rami of T0 and T1, followed by clipping of the condyle. The rigid-regional registration was conducted by identifying and selecting stable areas that do not undergo significant remodelling (ie, region of interest registration). These areas include the coronoid process, sigmoid notch, and posterior ramus,^36,37 as illustrated in Figure 2.

Fig. 2.

Illustration of the segmented ramus model registration. (A) T0 and T1 rami before registration; (B) selecting ROIs at the coronoid process, sigmoid notch, and posterior ramus, followed by the selection of an identical mesh radius for each ROI on both models (red); (C) registered rami of T0 and T1.

Clipping of the registered condyles (T0 and T1) was conducted once for every patient at specific planes, which consisted of a horizontal plane parallel to the Frankfort plane, intersecting with a vertical plane at the lowest point of the sigmoid notch. The condylar remodelling was assessed by evaluating the localized surface remodelling changes, either resorption or apposition. Condylar surface remodelling was accurately examined by corresponding point calculations based on shape correspondence analysis of the same anatomical points using the SPHARM-PDM toolkit.^36,38,39 Localized condylar surface remodelling was assessed by identifying region of interest on the T1 condylar surface, and these localized condylar subregions included anterior (A), anteromedial (AM), anterolateral (AL), anterosuperior (AS), posterior (P), posteromedial (PM), posterolateral (PL), posterosuperior (PS), superior (S), superomedial (SM), superolateral (SL), medial (M), and lateral (L). They were transferred to the corresponding surface of the T0 condyles by utilizing the PickNPaint tool of the 3D Slicer software. Subsequently, the amount of resorption and apposition on each surface was calculated using the mesh statistic feature of the software with the mesh statistic extension as shown in Figure 3.³⁵

Fig. 3.

Demonstrates the points of the region of interest (ROI) on the condylar surface, used to measure the extent of localized surface remodelling in millimetres: (A) superior surface, (B) lateral surface, (C) anterior surface, (D) medial surface, and (E) posterior surface.

Statistical analysis

Statistical analysis was conducted using SPSS version 26.0 software (IBM Corp.). The Shapiro–Wilk test was used to evaluate the normality of the data distribution. Descriptive statistics, including the mean value (M) and standard deviation, were calculated and reported for T0 and T1 for the right and left sides of all 80 patients. Statistical significance is defined as a P value less than .05. Paired t tests and the Wilcoxon rank-sum test were employed to assess the statistical significance of the mean differences for intragroup and intergroup comparisons. The reliability of CBCT measurements, assessed using the intraclass correlation coefficient, was evaluated by two observers who reanalysed 20 randomly selected cases within a 2-week interval to determine intra- and interexaminer agreement.

Results

A total of 80 TMD adult patients with 160 condyles (average age: 23.88 ± 5.8 years; 28 males and 52 females) were included in this study. The patients underwent SS therapy for a treatment period that varied between 6 and 12 months, with a mean of 9.8 months. The measurements of condylar volume, BMD, and joint spaces are shown in Table 1. Condylar position and angulation measurements are presented in Table 2. Changes in condylar remodelling measurements are shown in Table 3 and Supplementary Figure 2. Excellent intra- and interobserver reliability was achieved for all the measurement outcomes, with intraobserver reliability ranging from 0.903 to 0.993 and interobserver reliability ranging from 0.901 to 0.997.

Table 1.

Pre- and post-treatment changes in condylar volume, BMD, and joint spaces with intercomparison.

Measurements	Right side		Difference	P value	Left side		Difference	P value	Intercomparison		ICC
	T0	T1			T0	T1
	M ± SD	M ± SD			M ± SD	M ± SD			R-T0 vs LT0	R-T1 vs LT1	Intra-	Inter-
Condylar volume (mm3)	1356.02 ± 315.68	1390.60 ± 335.08	34.58	.193	1388.24 ± 377.46	1420.03 ± 362.14	31.79	.294	0.409	0.468	0.974	0.950
BMD (HU)	305.76 ± 91.48	320.37 ± 75.48	14.61	.117	290.40 ± 73.78	306.70 ± 85.58	16.30	.064	0.142	0.167	0.926	0.901
Joint spaces
AJS (mm)	2.79 ± 0.97	2.54 ± 0.92	–0.25	.003**	2.49 ± 1.10	2.28 ± 0.90	–0.21	.029*	0.036*	0.056	0.973	0.958
PJS (mm)	2.88 ± 2.88	3.04 ± 3.04	0.17	.096	2.75 ± 0.83	2.91 ± 0.87	0.15	.088	0.170	0.252	0.966	0.970
SJS (mm)	3.16 ± 0.95	3.32 ± 1.02	0.16	.091	3.15 ± 0.85	3.30 ± 0.91	0.16	.082	0.885	0.855	0.956	0.980
MJS (mm)	2.83 ± 0.93	2.79 ± 1.04	–0.04	.147	2.90 ± 0.95	2.92 ± 0.91	0.02	.768	0.563	0.313	0.993	0.996

HU, Hounsfield unit; ICC, intraclass correlation coefficient; Inter-, interobserver; Intra-, intraobserver; L, left side; M, mean value; mm, millimetres; mm³, cubic millimetres; R, right side; SD, standard deviation; T0, before treatment; T1, after treatment.

*P < .05.

^⁎⁎P < .01.

Table 2.

Descriptive statistics of condylar positional and angular changes (T1-T0) with comparisons between right and left sides.

Measurements	Right side	Left side	P value R vs L	ICC
	(T1-T0) M ± SD	(T1-T0) M ± SD		Intra-	Inter-
Condylar position
AP (mm)	0.27 ± 0.43	0.28 ± 0.38	.924	0.928	0.931
SI (mm)	0.19 ± 0.32	0.17 ± 0.34	.696	0.971	0.969
ML (mm)	–0.15 ± 0.30	–0.16 ± 0.37	.772	0.946	0.934
Condylar angulation
YAW (°)	0.17 ± 0.27	0.15 ± 0.30	.645	0.903	0.946
PITCH (°)	4.66 ± 3.40	3.86 ± 4.12	.165	0.926	0.950
ROLL (°)	0.18 ± 0.31	0.20 ± 0.21	.714	0.948	0.935

°, degree; ICC, intraclass correlation coefficient; Inter-, interobserver; Intra-, intraobserver; L, left side; M, mean value; mm, millimetres; R, right side; SD, standard deviation; T0, before treatment; T1, after treatment.

Table 3.

Descriptive statistics of condylar remodelling changes (T1-T0) with comparisons between right and left sides.

Condylar remodelling changes (mm)
	A	AM	AL	AS	P	PM	PL	PS	S	SM	SL	M	L
Right side
Mean	–0.05	–0.11	0.15	0.23	0.41	0.04	0.21	0.23	0.32	–0.09	0.11	–0.12	0.25
SD	0.14	0.22	0.20	0.26	0.29	0.12	0.19	0.22	0.26	0.29	0.16	0.25	0.27
Min	–0.39	–0.54	–0.29	–0.28	–0.37	–0.24	–0.29	–0.33	–0.27	–0.54	–0.34	–0.66	–0.45
Max	0.27	0.37	0.55	0.66	0.78	0.41	0.61	0.64	0.68	0.53	0.52	0.30	0.65
Percentile
25th	–0.13	–0.30	0.03	0.05	0.41	–0.02	0.10	0.08	0.24	–0.32	0.01	–0.34	0.04
75th	0.03	0.07	0.33	0.42	0.61	0.12	0.30	0.38	0.51	0.15	0.23	0.11	0.45
Left side
Mean	–0.08	0.07	–0.09	0.19	0.31	0.03	0.24	0.15	0.38	0.07	0.19	–0.10	0.05
SD	0.17	0.18	0.13	0.25	0.25	0.12	0.21	0.22	0.25	0.15	0.22	0.27	0.13
Min	–0.45	–0.38	–0.43	–0.45	–0.27	–0.28	–0.29	–0.31	–0.25	–0.28	–0.20	–0.63	–0.33
Max	0.28	0.58	0.23	0.59	0.74	0.49	0.58	0.58	0.65	0.44	0.65	0.54	0.52
Percentile
25th	–0.21	–0.05	–0.19	0.05	0.23	0.01	0.09	0.00	0.38	0.01	0.01	–0.33	0.01
75th	0.03	0.17	–0.02	0.38	0.49	0.09	0.40	0.33	0.54	0.17	0.36	0.11	0.13
ICC
Intra-	0.988	0.993	0.980	0.965	0.983	0.957	0.970	0.983	0.964	0.975	0.978	0.969	0.957
Inter-	0.956	0.950	0.971	0.994	0.997	0.946	0.977	0.973	0.950	0.968	0.955	0.952	0.993
R vs L side significance
P value	.256	.000⁎⁎⁎	.000⁎⁎⁎	.174	.019⁎	.590	.399	.019⁎	.141	.000⁎⁎⁎	.022⁎	.641	.000⁎⁎⁎

Positive value demonstrates bone apposition; a negative value demonstrates bone resorption.

ICC, intraclass correlation coefficient; Inter-, interobserver; Intra-, intraobserver; L, left side; R, right side; SD, standard deviation.

*P < .05.

^⁎⁎⁎P < .001.

The condylar volume exhibited an increase, but no statistically significant differences were observed across the time points or between the sides. For the right side, the comparison between T0 and T1 resulted in a P value of .193. For the left side, the comparison between T0 and T1 resulted in a P value of .294.

Regarding BMD, the result indicated a slight increase, but no statistically significant differences existed between the time points or between the sides. The average differences and P values were 14.61 HU (P = .117) for the right side and 16.30 HU (P = .064) for the left side. Concerning TMJ spaces, significant differences were identified in the AJS on the right and left sides between T0 and T1. Additionally, the intragroup comparison of AJS scores demonstrated statistical significance (P = .003, P = .029, and P = .036, respectively). Meanwhile, the PJS and SJS increased but did not reach statistical significance.

The assessment of condylar positional changes from T0 to T1 indicated that the condyles were primarily translated inferiorly, laterally, and anteriorly on the right and left sides. The mean changes were 0.19, –0.15, and 0.27 mm, respectively, for the right side and 0.17, –0.16, and 0.28 mm, respectively, for the left side. In terms of condylar angular changes, the results showed a rotational forward movement of the condyle (pitch movement) on the right and left sides, with changes from T0 to T1 measuring 4.66° ± 3.40° and 3.86° ± 4.12°, respectively.

Concerning the localized condylar remodelling, bone formation on the right side of the condyle occurred in subregions AL, AS, P, PM, PL, PS, S, SL, and L, whereas bone resorption was observed in subregions A, AM, SM, and M. On the left side, bone formation occurred in subregions AM, AS, P, PM, PL, PS, S, SM, SL, and L, whereas bone resorption was observed in subregions A, AL, and M. The predominant areas for bone formation were the P and S regions, whereas the predominant areas for bone resorption were the AM and M regions. Figure 4 presents the condylar remodelling changes between T0 and T1.

Fig. 4.

(A) Semitransparent overlay illustrating the magnitude and distribution of condylar remodelling changes between T0 (baseline) and T1. (B) Colour map illustrating the magnitude of condylar resorption (– negative values) and apposition (+ positive values) across the condylar surface.

Discussion

TMD is one of the most prevalent conditions affecting the oral and maxillofacial regions.⁴⁰ Treatment options for TMD patients include occlusal splints, physiotherapy, manual therapy, counselling, arthroscopy or arthrocentesis, and oral and injectable medications. Occlusal splints are widely utilized as a noninvasive treatment option for patients with TMD. Among the various types of occlusal splints, SSs have been proven to be one of the most effective, ²¹demonstrating an acceptable therapeutic effect on the symptoms and signs of individuals with TMD.¹⁹

The 3D analysis conducted in this study was based on a suggested protocol for evaluating condylar remodelling through rigid-regional registration.³⁹ In addition to analysing overall volumetric changes of the condyle, BMD, joint spaces, condylar position, condylar rotation, and localized remodelling changes of the condylar surfaces were also examined by shape correspondence analysis, which compared the same anatomical points across different time models.

This study found that the condylar volume increased, but no statistically significant differences between the time points or between the sides were identified. These results are consistent with the findings of Kim et al,³⁸ who evaluated condylar volume and condylar remodelling changes and reported no statistically significant differences in condylar volume after SS treatment. Various techniques have been employed for assessing BMD, such as dual-energy X-ray absorptiometry,⁴¹ quantitative CT,⁴² and CBCT, with BMD values often reported in HU.^43,44 In the current study, the density of the condylar bone was evaluated using CBCT. The results indicated a slight increase; however, this increase was not statistically significant. This finding is contrary to the findings of some studies, such as that by Musa et al,³² which reported statistically significant changes in BMD in specific regions of the condyle. The difference in findings may be attributed to the fact that we did not measure BMD in small, localized areas, as was done in the study by Musa et al, where 2 mm² regions on the superior slope, anterior slope, and posterior slope of the condyle were selected. That study found statistical significance in all three slopes for the arthralgia group, as well as in the posterior slope for the myalgia TMD group, before and after treatment.

Regarding TMJ spaces, the current study found that the average AJS decreased significantly after treatment, whereas the PJS and SJS increased but not statistically significant. These changes in the TMJ spaces suggest a forward and downward movement of the condyle. Similarly, Ahmed et al³¹ and Hasegawa et al⁴⁵ reported anterior and inferior displacement of the condyle following SS therapy in their studies. The significant difference that was detected in the AJS pretreatment between the right and left sides may be due to asymmetrical disc positions. On the contrary, no significant difference was detected in the bilateral joint spaces in the intergroup comparison at T1, indicating that SS can effectively adjust and balance the variations in bilateral joint space. The superior bilateral joint space showed an increase after treatment, which is attributed to the downward movement of the condyle. This downward movement of the condyle, which led to an increase in the SJS, may contribute to reducing pressure on the tissues within the joint region. Conversely, Tăut et al⁴⁶ and Derwich et al²⁸ reported no significant changes in TMJ spaces following treatment, observing no notable alterations in the AJS, PJS, SJS, or medial joint space. These contrasting findings may be ascribed to several factors, including the complexity of condylar displacement in TMD patients, variations in the adjustment of the SS, and differences in sample size.

Some studies^12,47 indicate that the TMJ demonstrates a coordinated relationship between the condyle and the fossa under normal physiological conditions. Disruption of this relationship can lead to TMDs. The displacement of the condyle is strongly correlated with TMD, as it may result in painful joint sounds, restricted movement, and other degenerative changes. This correlation highlights the importance of maintaining the integrity of the condyle–fossa relationship to prevent the development of TMD. Furthermore, it emphasizes the significance of evaluating treatment outcomes related to changes in condylar position. The current study assessed condylar positional changes from T0 to T1, revealing that the condyles were primarily translated inferiorly, laterally, and anteriorly on the right and left sides. This finding is consistent with other research findings^31,32,45,48 which also reported that the condyles demonstrated forward and downward displacement following SS therapy. An investigation demonstrated that, prior to treatment, the condyle in patients with TMD was positioned superiorly, posteriorly, and laterally within the glenoid fossa.³³ The SS aims to reposition the mandibular condyle towards the centric relation. Although it may not achieve precise centric repositioning, it effectively alleviates symptoms associated with TMDs. The optimal condylar repositioning with SS treatment reduces articulation with the highly innervated posterior retrodiscal area,⁴⁹ particularly in cases of anterior disk displacement. This alleviates pain, arthralgia, and joint noise while improving mouth opening.⁵⁰ The downward movement of the condyle decreases intra-articular pressure, preserves joint space, and enhances synovial fluid circulation to promote tissue repair.⁴⁵ At the same time, muscle-driven biomechanical forces stimulate condylar remodelling, redistributing occlusal loads and reducing inflammation, contributing to musculoskeletal recovery.³²

With respect to rotational changes, Ackerman et al⁵¹ emphasized the importance of analysing the yaw, roll, and pitch movements of craniofacial structures in three dimensions to enhance diagnostic precision and treatment outcome. Our study assessed condylar rotational changes from T0 to T1 and found that the condyles rotated forward (pitch) on both sides. These changes could be due to the clockwise rotation of the mandible after SS, as demonstrated in Al-Hadad et al’s⁵² study. They reported significant increases in mandibular plane inclination and occlusal plane angles, suggesting that mandibular rotation during SS therapy contributes to adaptive skeletal and occlusal changes.

In relation to the localized condylar surface remodelling, the current study demonstrated bone surface remodelling following SS therapy. The majority of the measured condylar subregions showed bone formation, including AL, AS, P, PM, PL, PS, S, SL, and L on the right side, and AM, AS, P, PM, PL, PS, S, SM, SL, and L on the left side, with these changes predominantly observed in the posterior and superior regions. By contrast, a few other subregions exhibited condylar resorption, including A, AM, SM, and M on the right side and A, AL, and M on the left side, with these changes predominantly observed in the anterior and medial areas. Although disc changes can promote bone formation, alterations in joint space stress and mechanical loading on osseous structures resulting from splint-generated condylar repositioning – specifically, anterior and downward condylar displacement – are considered the primary contributing factors to bone formation. SSs relieve pressure in the articular fossa and modify the stress stimuli in the TMJ area, potentially affecting the osteo-immune microenvironment.²⁹ This condition, in turn, may lead to adaptive remodelling of the condylar head that favours bone deposition rather than degeneration.

In this study, CBCT images were acquired before treatment and approximately 6 to 12 months after treatment. This interval is appropriate for comparing osseous changes in the condyle, as a previous study observed that the average time for condylar remodelling following SS treatment is within 7 months.⁵³ Another study found that adaptive bone remodelling, characterized by a ‘double contour’ appearance, occurred in 80% of patients over a 6-month period.⁵⁴

Several studies have investigated condylar remodelling following SS treatment in patients with TMD. Kim et al³⁸ revealed remodelling of the bone surface in patients with TMJ-osteoarthritis, identifying areas of bone resorption and bone formation following SS treatment. Ok et al⁵⁵ found bone formation and cortical thickening in patients with TMJ-osteoarthritis who received SS therapy. Liu et al⁵⁴ reported that approximately 80% of patients treated with splints exhibited a ‘double contour’ in condylar CBCT images, suggesting bone modelling. Zhou et al⁵⁶ found that the SS group displayed resorption in the anterior and medial regions compared with that in other areas. Steinbaum et al²⁹ indicated that SS therapy may lead to favourable adaptive changes in the condyles of patients with degenerative joint disease of the TMJ.

Conversely, Tăut et al⁴⁶ did not observe osseous structural changes in condylar dimensions. They suggested that occlusal splint therapy neither induced condylar remodelling nor contributed to further degenerative changes in the bone. This variation in results may be attributed to methodological differences. Relying solely on condylar morphology measurements, such as height and width, may not accurately capture whether remodelling or resorption has occurred. A more precise approach, such as the method employed in our study to determine the areas of remodelling and resorption, is necessary for a more reliable assessment.

The findings from our study on the presence of limited areas of bone resorption in the condyle indicate that, while splint therapy may stabilize the condition of the condyles undergoing repair, a few regions within the condyle may still experience relapse or progression of resorption. A previous finite element analysis study demonstrated that the anterior surface of the condyle experiences the highest principal stress during mandibular movements (such as protrusion and opening), while stresses are concentrated on the medial surface during clenching. This finding may explain why a few subregions of the anterior and medial surfaces of the condyle are still susceptible to bone resorption even after treatment. However, a longer follow-up study may be needed to assess complete remodelling and ensure the persistence of the treatment results. Additionally, the areas of bone resorption and apposition varied significantly between the right and left sides. These findings highlight the variability in condylar remodelling, which may have important implications for understanding the biomechanical and functional adaptations of the TMJ. The observed differences could be associated with an imbalance in bone metabolism, particularly in the processes of bone resorption and formation.

This study employed voxel-based superimposition of models reconstructed from CBCT scans. Two suggested registration protocols were utilized alongside automatic shape correspondence calculations using 3D Slicer software, which constitutes one of the strengths of this research.³⁹ However, the authors acknowledge that their study has limitations. These limitations include the lack of a control group due to its retrospective design and ethical concerns regarding TMD patients. Withholding treatment is ethically unjustifiable, given that these patients seek immediate pain relief and their treatment cannot be delayed for the completion of an experiment or the need for multiple CBCT scans. A control group of 80 participants would be necessary for accurate results and comparison, which raises additional ethical considerations. A long-term follow-up study should also be performed to evaluate the effects of treatment over an extended period. Another limitation is that the disc position was not evaluated with magnetic resonance imaging, which could influence the accuracy of measuring condylar movement in relation to the TMJ.

Conclusion

This study utilized advanced 3D CBCT analysis to comprehensively evaluate the structural and positional adaptations of the TMJ in patients with TMD undergoing SS therapy. The results revealed significant AJS reduction, condylar positional changes (inferior, lateral, and anterior translation), and forward rotational movement (pitch) post-treatment. Localized condylar remodelling indicated bone apposition in the posterior and superior regions, likely due to reduced mechanical stress on the articular fossa, while resorption in the anteromedial and medial areas may reflect persistent biomechanical strain. Although increases in volumetric and BMD changes were observed, these changes were not statistically significant. Nevertheless, the adaptive osseous changes, along with condylar joint space and positional adjustments, suggest that SS therapy facilitates functional TMJ realignment and promotes favourable remodelling. Clinically, these findings support SS therapy as a viable strategy for TMD management, highlighting its potential to modulate condylar dynamics and improve patient outcomes. While the study underscores the need for further long-term research, it provides valuable insights into the effects of noninvasive TMD treatments on condylar remodelling.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.