Evaluating condylar distraction potential: a comparison between patients with unilateral disc displacement without reduction and asymptomatic volunteers

Abstract

Purpose

This study aimed to evaluate and compare the condylar distraction potential and joint space dimensions between patients with unilateral disc displacement without reduction (DDwoR) and asymptomatic individuals, using dynamic jaw tracking and MRI imaging.

Materials and methods

A total of 47 participants were included: 34 patients diagnosed with unilateral DDwoR and 11 asymptomatic volunteers. Dynamic mandibular movements were recorded using the Zebris JMA 3D ultrasonic jaw tracking system. The amount of condylar distraction was measured using the Electronic Position Analysis (EPA) module. Additionally, MRI scans were performed to assess joint space distances. Statistical comparisons were conducted using appropriate parametric and non-parametric tests, with significance set at p < 0.05. The findings provide insight into the differences in condylar distraction between patients with unilateral DDwoR and healthy volunteers.

Results

Patients with DDwoR showed significantly reduced condylar distraction potential an on the affected side compared to controls (p < 0.05). MRI analysis revealed a narrower anterior joint space and limited increase in superior joint space in the DDwoR group. No significant differences were found on the non-affected side compared to the control group.

Conclusion

Patients with unilateral DDwoR demonstrate limited condylar distraction capacity and altered joint space dynamics compared to asymptomatic individuals. These findings highlight the mechanical constraints associated with disc displacement and emphasize the importance of functional and morphological evaluation in temporomandibular joint disorders.

Distraction is defined as the intentional separation of a joint’s osseous components without compromising the continuity of the ligamentous and capsular structures. In medical terminology, it specifically refers to the application of controlled mechanical forces to increase joint space while preserving the integrity of the periarticular soft tissues [1]. More than 70 years ago, Dr. Sears was among the first to speculate about the possibility of achieving distraction in the temporomandibular joint (TMJ) [2]. Sears and Gerber [2, 3] reported that distraction effectively alters condylar position and reduces compressive loading on the TMJ soft tissues.

The TMJ has a unique capacity for regeneration, likely due to its fibrocartilaginous composition, which differs from the hyaline cartilage found in most other synovial joints [4, 5]. Thanks to this regenerative potential, many TMJ disorders can improve with conservative treatments—such as reducing mechanical load and promoting muscular relaxation—without the need for surgery or permanent occlusal changes [6].

Especially in cases of narrowed joint spaces, the primary goal of distraction is to expand the TMJ space, thereby creating a more favorable environment for healing. Radiologically, this space is defined as the area between the glenoid fossa and the condyle where the articular disc normally resides. Excessive mechanical loading can lead to internal derangement of the TMJ [2], often accompanied by a reduction in the superior joint space. Narrowing of the joint space has been consistently observed in cases of disc displacement without reduction, as reported in multiple studies [7–9].

Previous studies [10, 11] have suggested that condylar distraction may help reduce pressure on TMJ structures, but only if occlusal contacts are established between the posterior teeth on both sides. However, the biomechanical characteristics of the mandible limit the practicality of bilateral distraction. Specifically, the vector forces generated by the masticatory muscles are directed posteriorly relative to the tooth contact point, meaning that bilateral distraction would require an upward force applied to the anterior mandible—something difficult to achieve [10–12].

Most research to date has focused on increasing joint space in asymptomatic individuals with morphologically intact TMJs [12–14]. Yet, these attempts remain largely theoretical due to insufficient empirical evidence. Only one study [15] has proposed that unilateral joint space distraction may be feasible, but only in pathologically narrowed joints. However, this study’s lack of a control group and failure to quantify the distraction potential of the unaffected contralateral joints within the same subjects limits its internal validity and prevents firm conclusions. Therefore, the current study was conducted to address these limitations by evaluating and comparing the condylar distraction potential between patients with unilateral disc displacement without reduction and asymptomatic volunteers.

Materials and methods

The study sample consisted of patients who sought treatment at the Istanbul Medipol University Faculty of Dentistry between 2017 and 2019. The study protocol was approved by the Ethics Committee of the same university, and informed consent was obtained from all participants. Thirty-four patients diagnosed with unilateral disc displacement without reduction (UDDwoR) and thirteen asymptomatic volunteers were enrolled in the study. All participants provided informed consent and met the inclusion criteria. The diagnosis of UDDwoR was established using the Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) in conjunction with magnetic resonance imaging (MRI) findings. All clinical examinations were performed by an experienced prosthodontist.

The patient group included individuals with natural teeth numbered 5, 6, and 7 bilaterally, Angle Class I occlusion, and no signs of periodontal disease that could cause tooth mobility. None of the patients wore removable prostheses, had a history of orthodontic treatment or occlusal device use, and all demonstrated adequate cooperation. Participants were also free from temporomandibular disorders associated with masticatory muscles (e.g., myofascial pain, muscle spasm, or contracture), as well as facial asymmetry or developmental anomalies.

Placement of intraoral pivots

Maxillary alginate impressions were taken from all subjects (Cavex Impressional, Cavex, Wost Chester, USA), and dental stone models were subsequently fabricated. A 0.75 mm thick hard thermoplastic resin sheet (Essix A+^® Plastic, Dentsply Sirona, Netherlands) was vacuum-formed over each model. The fit of the splint was verified intraorally, and the material covering the maxillary second and third molars (teeth 7 and 8) was trimmed. Grooves were prepared in the regions corresponding to teeth 4, 5, and 6 to ensure accurate repositioning of the acrylic pivot after removal. Following groove preparation, baby oil (Johnson’s Baby Oil, Johnson & Johnson, USA) was applied to the designated areas using a fine brush to facilitate later separation of the resin. With the splint in place intraorally, a 0.5 mm thick tin foil was folded three times and placed bilaterally at the level of the second molars (tooth 7), where subjects were instructed to gently bite down. During this procedure, the mandible was guided into centric relation without the application of additional force. Since the tin foil deformed immediately under biting force, it did not restrict or alter mandibular movement. While the foil was positioned on the maxillary second molars (tooth 7), a temporary crown and bridge acrylic material (Dentalon Plus, Heraeus Kulzer, Germany) was applied in two small portions onto the previously prepared grooves, covering the regions corresponding to teeth 4, 5, and 6. The splint was then removed from the mouth, and polymerization of the acrylic material was completed extraorally (Fig. 1). Once the material had fully set, the acrylic pieces on both sides were easily separated from the splint due to the baby oil applied beforehand (Fig. 2). After the pivots were prepared, they were placed onto the intraorally positioned splint and subjects were instructed to bite with maximal force on each side separately. In three subjects, where occlusal contact occurred on the contralateral side during unilateral clenching, the height of the pivot was increased. This adjustment was necessary because unintended contralateral tooth contacts could interfere with or prematurely terminate the distraction movement.

Fig. 1.

Temporary crown and bridge acrylic is placed on the groove

Fig. 2.

After set of the material both pieces removed from the splint

Measurement of the amount of distraction

The distraction movement were measured with a 3-D ultrasonic jaw motion analyzer (JMA, Zebris Medical GmbH, Isny, Germany). The JMA system measures jaw motions in six degrees of freedom and is supported by the software program WinJaw (Zebris Medical GmbH, Isny, Germany) Version 10.6.50, which records mandibular movement. The JMA device consists of a facebow comprised of a plastic frame and four fixed receiver sensors mounted onto a paraocclusal solid metal bite fork. A temporary crown and bridge acrylic material (Dentalon Plus, Heraeus Kulzer, Germany )was used to attach the bite fork to the labial surface of the mandibular dentition between the right and left molars. Prior to the registration process, a reference plane was defined in the WinJaw software. In accordance with a previous study [16], the maximum intercuspation position was chosen as the condylar reference. The condylar position was then recorded using the Electronic Position Analysis (EPA) module through jaw tracking.

Prior to doing the measurement, the program established the following definitions for points:

1. CN: The mouth was closed on the natural teeth without the acrylic pivot.

2. CMax: The mouth was closed on the natural teeth with maximal force, without the acrylic pivot.

3. RLxN: The mouth was closed on the acrylic pivot placed on the right side.

4. RLxMax: The mouth was closed with maximal force on the acrylic pivot placed on the right side.

5. LLxN: The mouth was closed on the acrylic pivot placed on the left side.

6. LLxMax: The mouth was closed with maximal force on the acrylic pivot placed on the left side.

One of the acrylic pivots was positioned in a posterior location on the occlusal splint and then inserted into the patient’s mouth. Distraction movement was observed while subjects were unilaterally biting on an acrylic pivot, which is compatible with the occlusal morphology of the first and second premolars and the first molar. An electronic condyle position program on an ultrasonic jaw motion analyzer helps to detect distraction movement (Fig. 3, Jaw Motion Analyzer, Zebris, Germany). In the EPA mode, the condylar position was determined among the six points defined above. In the event that there is a displacement of the facial arch, it is advisable to revert back to doing a functional analysis. The item was then returned, and the measurements were subsequently redone.

Fig. 3.

Distraction movement is detected by the condyle position with the aid of electronic condyle position programme

Examination of MRI images

All MRI examinations were performed with the patients in the supine position using a standard head coil. During image acquisition, head positioning was carefully adjusted to ensure that the Frankfort (orbitomeatal) plane was parallel to the scanning table. This standardized positioning allowed the Frankfort plane to correspond to a line parallel to the upper and lower margins of the sagittal MRI images, thereby enabling consistent and reproducible joint space measurements.

Joint space, articular disc, and condyle position measurements were performed on magnetic resonance images and OsiriX imaging software. For each joint, three slices were selected (the most lateral slice in which joint borders were visible; the central slice; the most medial slice in which joint borders were visible), and on each slice, a circle was drawn to approximate the border of the condyle, with the center of the circle coincident to the center of the condyle. Next, a reference line was drawn through the center of the condyle parallel to the Frankfort plane (which was parallel to the upper and lower image margins of the sagittal slices because patients’ heads were positioned parallel to the Frankfurt plane), another line was drawn through the center of the condyle perpendicular to the Frankfurt plane, and two additional lines were drawn at 60° angles from the vertical. The area between the condyle and fossa borders falling within the angulated lines was then automatically calculated (mm) by the imaging software [15] (Figs. 4 and 5). Joint space area measurements were performed on the lateral, central, and medial slices of parasagittal MR images of both joints. Due to insufficient image quality, some datasets had to be excluded.

Fig. 4.

The determination of the center of the condyle head and the limits of the joint spacing according to the template

Fig. 5.

Measurement of Anterior, Posterior, and Total Joint Space Areas

Statistical analyses were performed using IBM SPSS Statistics version 22 (IBM Corp., Armonk, NY, USA). The normality of data distribution was assessed using the Shapiro–Wilk test. As most variables did not meet the assumptions of normal distribution, non-parametric statistical tests were applied. Comparisons between joints with disc displacement without reduction (DDwoR) and their contralateral joints within the same patients were treated as paired data and analyzed using the Wilcoxon signed-rank test, in order to account for within-subject dependence. Comparisons involving the healthy control group, which consisted of joints obtained from different individuals and therefore represented independent samples, were analyzed using the Mann–Whitney U test. Associations between continuous variables were evaluated using Spearman’s rho correlation coefficient. Statistical significance was set at p < 0.05 for all analyses. Intraclass correlation coefficient (ICC) analysis was used to assess reliability of repeated measurements.

Results

The patient group consisted predominantly of females (85.3%), while males accounted for 14.7% of the group. The mean age of patients was 38.12 ± 14.97 years (range: 15–67 years). In the control group, females constituted 53.8% and males 46.2% of the participants, with a mean age of 22.36 ± 2.59 years.Information regarding disease duration could not be reliably obtained and was therefore not included in the analysis.

Mean anterior, posterior, and total areas were smallest in joints with disc displacement and largest in joints of healthy subjects. There was no statistically significant difference between the contralateral joints (DDwoR) and the joints healthy controls, only in terms of anterior areas. Statistically significant differences were found in all other groups (Tablo 4.1) (Table 1).

Table 1.

Assessment of Joint Space Areas in MRI Images

	Joints with DDwoR1	Contralateral joints (DDwoR)2	Joints healthy controls3
Anterior Area	10,45 ± 4,34 (n = 34)	11,65 ± 3,96 (n = 34)	13,15 ± 4,06 (n = 22)	1–2 p = 0,027* 1–3 p = 0,027* 2–3 p = 0,227
Posterior Area	7,81 ± 3,94 (n = 34)	8,87 ± 3,11 (n = 34)	10,98 ± 3,41 (n = 22)	1–2 p = 0,007** 1–3 p = 0,001** 2–3 p = 0,015*
Total	19,13 ± 7,39 (n = 34)	21,79 ± 6,48 (n = 34)	25,45 ± 7,07 (n = 22)	1–2 p = 0,018* 1–3 p = 0,003** 2–3 p = 0,046*

1 Joints with DDwoR

2 Contralateral joints (DDwoR)

3 Joints healthy controls

When the movement of the ipsilateral condyle was examined during unilateral biting, a distraction of 0.12 ± 0.34 mm was observed in the joints with disc displacement. Compression was − 0.14 ± 0.24 mm in the joints of joints healthy controls and − 0.03 ± 0.31 mm in the contralateral joints (DDwoR). A statistically significant difference was found between the joints with disc displacement and the other groups (Table 2).

Table 2.

Condylar Movement in the X, Y, and Z Planes Measured Using the EPA Module

	Joints with DDwoR	Contralateral joints (DDwoR)	Joints healthy controls
Ipsilateral X	-0,17 ± 0,28	-0,08 ± 0,24	-0,11 ± 0,34	1–2 p = 0,144 1–3 p = 0,794 2–3 p = 0,477
Contralateral X	0,008 ± 0,38	0,02 ± 0,23	0,02 ± 0,24	1–2 p = 0,798 1–3 p = 0,866 2–3 p = 0,858
Ipsilateral Y	0,12 ± 0,34	-0,03 ± 0,31	-0,14 ± 0,24	1–2 p = 0,007** 1–3 p = 0,003** 2–3 p = 0,217
Contralateral Y	-0,16 ± 0,37	-0,30 ± 0,29	-0,30 ± 0,29	1–2 p = 0,019** 1–3 p = 0,26 2–3 p = 0,697
Ipsilateral Z	0,06 ± 0,29	0,04 ± 0,24	0,009 ± 0,28	1–2 p = 0,646 1–3 p = 0,499 2–3 p = 0,540
Contralateral Z	0,05 ± 0,24	0,06 ± 0,30	0,009 ± 0,28	1–2 p = 0,983 1–3 p = 0,464 2–3 p = 0,516

1 Joints with DDwoR, 2 Contralateral joints (DDwoR), 3 Joints healthy controls

[X-axis (sagittal plane), positive (+) value = anterior, negative (−) value = posterior movement, Y-axis (vertical): + = superior, − = inferior movement, Z-axis (transverse) + = lateral, − = medial movement]

During the unilateral compression, movement of the contralateral condyle was examined, compression was observed in all groups. The compression amount was found to be the lowest in the joints with disc displacement (-0.16 ± 0.37 mm), and a statistically significant difference was found between the contralateral joints (DDwoR)and the joints healthy controls (Table 2).

When the relationship between the total area in the sections and the amount of movement in the Y plane during maximum force biting was examined, no correlation was found between the total area and the amount of distraction in the lateral, central, and medial sections of the disc displacement group (r = 0.054, p = 0.788; r = -0.062, p = 0.740; r = -0.070, p = 0.735, respectively). A negative correlation was found between the total area in the lateral section and the contralateral joint and the amount of movement in the y-plane only in the joints with disc displacement (Table 3).

Table 3.

Relation of Total Area in Sections and Movement in the Y Plane (IPS Y)

		Joints with DDwoR IpsY	Contralateral joints (DDwoR) IpsY	Joints healthy controls IpsY	Joints with DDwoR ContY	Contralateral joints (DDwoR) ContY	Joints healthy controls ContY
Lateral Section Total Area	Correlation Coefficient (r)	0,054	-0,364	-0,094	-0,402*	0,128	-0,396
	Significance (p) Sig.(2-tailed)	0,788	0,114	0,738	0,037	0,590	0,144
	N	27	20	15	27	20	15
Central Section Total Area	Correlation Coefficient (r)	-0,062	-0,313	-0,142	-0,260	-0,013	-0,378
	Significance Sig.(2-tailed)	0,740	0,071	0,529	0,159	0,943	0,083
	N	31	34	22	31	34	22
Medial Section Total Area	Correlation Coefficient (r)	-0,070	-0,261	0,027	-0,202	-0,050	-0,057
	Significance (p) Sig.(2-tailed)	0,735	0,149	0,917	0,322	0,785	0,827
	N	26	32	17	26	32	17

Speraman’s rho correlation

Furthermore, intra-rater reliability analysis demonstrated excellent measurement consistency, with intraclass correlation coefficient (ICC) values ranging between 0.95 and 0.97.

Discussion

The mechanical rationale of pivot splint therapy can be explained by the hypomochlion mechanism. When a pivot is placed in the region of the second molar, clenching forces are thought to induce mandibular rotation around this fulcrum. As a result, the condyles undergo slight distraction within the glenoid fossa, which may contribute to unloading of the temporomandibular joints. This biomechanical effect has been proposed as a therapeutic principle of pivot splints, facilitating joint decompression and potentially reducing intra-articular stress. Such a mechanism has been highlighted in the literature [17], supporting the concept that splint therapy exerts its benefits not only through neuromuscular modulation but also through mechanical lever effects. Moreover, Gerber’s joint resilience theory posits that in pathological joints, compromised resilience impairs the capacity for functional compression [3]. In the present study, according to the EPA module results, joints with DDwoR demonstrated lower compression values during contralateral mouth closure compared to the other groups. This outcome may be attributed, in accordance with Gerber’s hypothesis, to the already diminished joint space in such cases, which restricts additional compressive deformation.

Another notable finding of the present study is that only the joints with DDwoR exhibited distraction when the mouth was closed on the acrylic pivot placed on the ipsilateral side under maximal force. In contrast, healthy joints exhibited compression under maximal force during mouth closure on the acrylic pivot, both on the ipsilateral and contralateral sides. Yıldız et al. showed that condylar distraction could occur in joints with unilateral DDwoR, where the joint space was already narrowed [15]. However, the degree of distraction observed in their study was greater than that reported here. This discrepancy may be explained by methodological differences, particularly the use of a distraction splint by Yıldız et al., as opposed to the unilateral pivot employed in the present study. When patients clench their teeth on a distraction splint, premature contralateral contact may limit the extent of distraction. In the current study, acrylic pivots were specifically designed to permit distraction to proceed until joint movement reached its physiological limit.

In the present study, distraction was observed exclusively in joints with a narrowed joint space under maximal force. While it might be intuitively assumed that a reduction in joint space would facilitate greater distraction, the findings demonstrated that the extent of distraction was not directly correlated with joint space width. This observation aligns with previous reports suggesting that distraction capacity may be influenced by inter-individual variability rather than solely by anatomical constraints [18, 19]. It may be postulated that increased ligamentous laxity and compromised structural integrity of the collateral ligaments contribute more significantly to distraction potential than the mere presence of disc displacement and associated joint space narrowing. These results underscore the need for further investigation to elucidate the complex biomechanical factors governing condylar distraction.

In the present study, based on MRI images, the mean total joint space area was 19.13 ± 7.39 mm² in joints with DDwoR, 21.79 ± 6.48 mm² in the contralateral (clinically healthy) joints of patients with disc displacement, and 25.45 ± 7.07 mm² in the joints of healthy control subjects. These findings are consistent with previous studies reporting joint space narrowing in patients with DDwoR and temporomandibular joint osteoarthritis [7, 20–25]. Notably, a statistically significant difference was observed between the control group and the contralateral joints of patients with disc displacement, indicating that joint space was narrower even in the seemingly unaffected joints of symptomatic individuals. This observation supports earlier evidence suggesting that the non-displaced side in unilateral disc displacement cases may exhibit subclinical or early-stage pathological changes and should not be regarded as entirely healthy [26–28].

Both in the current study and in previous research [15], condylar distraction was observed exclusively in joints exhibiting a narrowed joint space. This finding raises the hypothesis that greater narrowing of the joint space might facilitate increased distraction. However, the present study found no significant correlation between joint space area and the amount of distraction achieved. These results suggest that distraction capacity is not directly determined by anatomical narrowing, but may instead be influenced by individual biomechanical differences. This interpretation is in line with prior studies [18, 19] reporting that factors such as ligamentous laxity, joint compliance, or variability in neuromuscular function may play a more critical role in determining distraction potential than joint space dimensions alone.

Seedorf et al. [11] reported downward and forward movement of the condyle with pivot splints. This movement was explained as a cause of gliding movement of the mandible over pivots as a cause of fossa morphology followed by the condyles. On the other hand, when they used a protrusion- preventing pivot, cranial movement of the condyle was seen. However, in the present study, the authors prevented protrusive movement by using pivots designed in accordance with the occlusal morphology of the opposing teeth (numbers 4, 5, and 6). Rather than using protrusion-preventing pivots placed on a single tooth, it was considered that pivots involving three teeth would be more effective in restricting lateral and anteroposterior movements. Furthermore, while a chinstrap could potentially restrict protrusive movement, its use is limited in practice because of reduced patient compliance [10].

One of the limitations of the present study is the absence of electromyographic (EMG) data to assess masticatory muscle activity. As demonstrated by Ito et al. [12], asymmetrical activation of the masseter muscle—specifically, increased force generation on the working side compared to the balancing side—results in greater condylar loading on the working-side condyle. These findings suggest that the magnitude and direction of masticatory muscle forces may influence condylar displacement patterns. Accordingly, the lack of direct muscle activity measurements should be considered when interpreting the biomechanical outcomes of this study.

Another limitation of the present study is the relatively small size of the asymptomatic control group, which was constrained by ethical considerations related to MRI imaging and strict inclusion criteria. The control group was included to provide physiological reference values rather than to constitute a statistically matched cohort. Future studies integrating jaw tracking, imaging, and EMG recordings in larger control populations may further clarify the biomechanical mechanisms underlying condylar behavior in temporomandibular disorders.

Conclusion

The findings of the present study demonstrate that joint space is significantly narrowed in patients with unilateral disc displacement without reduction (DDwoR). Moreover, in contrast to healthy joints, DDwoR-affected joints exhibited a measurable capacity for condylar distraction. Notably, no significant correlation was identified between joint space width and the extent of distraction, suggesting that distraction potential is not solely dependent on anatomical joint space dimensions. Additionally, joints with DDwoR displayed significantly reduced compressive response during contralateral loading when compared to healthy joints. These results underscore the complex biomechanical alterations associated with disc displacement and highlight the need for comprehensive assessment strategies that account for both structural and functional joint parameters.

Abbreviations

TMJ

Temporomandibular joint

DDwoR

Disc displacement without reduction

EPA

Electronic Position Analysis

RDC/TMD

Research Diagnostic Criteria for Temporomandibular Disorders

MRI

Magnetic resonance imaging

Authors’ contributions

Ceren Funda Tamer: Data curation, Methodology, Formal analysis, Writing– original draft. Sina Saygılı: Data curation, Methodology. Çağatay Dayan: Writing– review & editing, Supervision. Tonguç Sülün: Methodology, Conceptualization, Project administration, Supervision.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the Ethics Committee of the Faculty of Dentistry, Istanbul Medipol University (Approval No: 93, Date: 08/03/2017). All procedures performed in this study involving human participants were conducted in accordance with the ethical standards of the institutional research committee and with the 1964 Declaration of Helsinki and its later amendments. Written informed consent was obtained from all participants prior to their inclusion in the study.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.