Temporomandibular disorders (TMDs) comprise a heterogeneous group of musculoskeletal and neuromuscular conditions affecting the temporomandibular joints, masticatory muscles, and associated structures. Common clinical manifestations include preauricular pain, joint sounds, restricted mandibular movement, deviation during opening, and impairment of mastication. Internal derangement of the TMJ represents an important intra-articular component of TMD and is characterized by an abnormal relationship between the articular disc, mandibular condyle, and temporal bone [1,2]. Disc displacement without reduction is particularly relevant in patients presenting with painful restriction of mouth opening. Loss of normal disc-condyle coordination, inflammatory mediators within the superior joint compartment, alteration in synovial fluid characteristics, and formation of intra-articular adhesions may contribute to pain and restricted translation of the mandibular condyle [2,3]. Persistent restriction can interfere considerably with mastication, oral hygiene, dental treatment, speech, and quality of life. Initial management is generally conservative and may include patient education, behavioral modification, soft diet, non-steroidal anti-inflammatory drugs, physiotherapy, jaw exercises, and occlusal appliances [4]. When symptoms persist despite appropriate conservative therapy, minimally invasive intra-articular procedures may be considered before proceeding to more invasive surgical interventions. Arthrocentesis of the TMJ was introduced as a relatively simple method of lavage of the superior joint compartment. The procedure is intended to eliminate inflammatory mediators, release adhesions, reduce negative intra-articular pressure, improve disc and condylar mobility, and restore a more favorable biochemical environment within the joint [5]. It is generally performed through one or two percutaneous portals and has the advantages of relatively low morbidity, technical simplicity, and the possibility of being undertaken without open joint surgery. Clinical studies have consistently demonstrated reductions in pain and increases in maximal mouth opening after arthrocentesis in patients with TMJ internal derangement [6,7]. Improvements in mandibular function may occur rapidly and can persist during subsequent follow-up. Nevertheless, the magnitude of response varies considerably between patients. Identification of pretreatment characteristics associated with outcome is therefore important for patient selection and prognostic counselling. Several variables have been investigated as potential determinants of arthrocentesis outcome, including age, sex, duration of symptoms, severity of pain, degree of restricted mouth opening, stage of internal derangement, presence of degenerative osseous changes, and magnetic resonance imaging findings [8,9]. Among these variables, preoperative maximal mouth opening is particularly attractive as a prognostic marker because it is objective, inexpensive, reproducible, and readily available during routine clinical examination. Nishimura et al. evaluated prognostic factors in 103 TMJs treated with arthrocentesis and reported an overall success rate of approximately 71%. Their findings suggested that pain severity, early postoperative mouth opening, and morphological condylar abnormalities were related to treatment outcome [8]. Subsequent investigations have further demonstrated that baseline clinical characteristics may help identify patients who are more likely to obtain substantial functional benefit from lavage. An important clinical question concerns whether patients with severe restriction of mouth opening respond less favorably because they have more advanced mechanical derangement or, conversely, demonstrate a greater measurable improvement because arthrocentesis releases the factors restricting mandibular translation. Clinical evidence supports the latter possibility in at least a subset of patients. Studies examining predictors of response have reported substantial increases in MMO following arthrocentesis, with patients having marked baseline restriction sometimes demonstrating particularly pronounced functional gains [9]. Systematic reviews have similarly demonstrated that arthrocentesis is associated with improvement in both pain and mandibular mobility, although substantial heterogeneity exists between studies regarding diagnostic criteria, lavage techniques, adjunctive injections, outcome definitions, and duration of follow-up [10,11]. The magnitude of improvement in mouth opening also varies according to baseline functional limitation, making pretreatment MMO an important variable when interpreting treatment response. Despite the routine measurement of MMO in TMJ practice, relatively few clinical investigations have focused specifically on the relationship between baseline MMO and subsequent functional gain. Determining this association may improve prognostic assessment and provide clinicians with a simple method of identifying patients who may obtain substantial improvement after arthrocentesis. The present study therefore evaluated the relationship between preoperative maximal mouth opening and clinical response following TMJ arthrocentesis in patients with symptomatic internal derangement. The primary hypothesis was that the degree of baseline mouth-opening restriction would be significantly associated with the magnitude of postoperative improvement. Aim and Objectives The primary aim was to determine the relationship between preoperative maximal mouth opening and functional response to TMJ arthrocentesis. The objectives were to evaluate changes in MMO following arthrocentesis; assess changes in TMJ pain during follow-up; compare functional improvement among patients with different degrees of preoperative mouth-opening restriction; determine the correlation between baseline MMO and change in MMO; and evaluate whether preoperative MMO independently predicts clinical response after accounting for relevant clinical variables.

Study Design and Participants A prospective observational study was conducted among 60 consecutive patients undergoing arthrocentesis for symptomatic internal derangement of the TMJ. Patients were evaluated clinically before treatment and followed for 6 months after the procedure. Patients aged 18–60 years with unilateral or predominantly unilateral TMJ pain associated with restricted mouth opening were eligible. Internal derangement was diagnosed on the basis of history and clinical examination, supported by imaging where clinically indicated. All patients had persistent symptoms despite an adequate period of conservative treatment. The inclusion criteria comprised TMJ pain during function, limitation of mandibular opening, clinical features compatible with intra-articular TMJ derangement, failure to obtain satisfactory relief with conservative therapy, and willingness to comply with postoperative follow-up. Patients with previous open TMJ surgery, ankylosis, acute maxillofacial trauma, craniofacial deformity substantially affecting mandibular movement, systemic inflammatory arthropathy involving the TMJ, active local infection, or inability to complete follow-up assessments were excluded. Clinical Assessment A standardized clinical evaluation was performed before arthrocentesis. Demographic characteristics, affected side, duration of symptoms, history of joint sounds, pain intensity, and mandibular range of motion were recorded. Maximal mouth opening was measured as the maximum interincisal distance between the incisal edges of the upper and lower central incisors using a calibrated ruler. Measurements were recorded in millimetres. Patients were instructed to open their mouth as widely as possible without manual assistance. Measurements were repeated, and a consistent maximum value was documented. Pain during mandibular function was evaluated using a 10-cm visual analogue scale ranging from 0, representing no pain, to 10, representing the worst imaginable pain. For analysis of the influence of baseline restriction, patients were categorized according to preoperative MMO as severe restriction (<25 mm), moderate restriction (25–29.9 mm), and mild restriction (≥30 mm). Arthrocentesis Procedure Arthrocentesis was performed under aseptic conditions using a conventional two-needle technique involving the superior joint compartment. The preauricular region was prepared with antiseptic solution, and appropriate local anesthesia was administered. Surface landmarks were identified using a line extending from the midpoint of the tragus toward the lateral canthus. The posterior entry point was located in the region of the posterior recess of the superior joint space, and the second portal was positioned anteriorly to facilitate outflow. After entry into the superior compartment, joint distension was confirmed with sterile solution. Lavage was then performed using Ringer's lactate solution with free flow established between the inflow and outflow needles. Approximately 100–200 mL of solution was used for joint lavage. During irrigation, gentle mandibular movements were performed to facilitate hydraulic distension, disruption of minor adhesions, and clearance of inflammatory products. Following lavage, the needles were removed and a sterile dressing was applied. Patients were observed for immediate complications and received standardized postoperative instructions. Jaw-opening exercises were initiated as tolerated to maintain the mobility achieved by the procedure. Follow-up and Outcome Assessment Patients were evaluated at baseline and at 1 week, 1 month, 3 months, and 6 months following arthrocentesis. MMO and pain VAS were recorded at each visit using the same clinical methodology. The primary outcome was functional improvement, defined as the change in MMO from baseline to 6 months: Change in MMO = 6-month MMO − preoperative MMO. Secondary outcomes included serial changes in pain intensity, proportion of patients achieving clinically satisfactory mouth opening, and overall clinical response. Clinical success was defined using a combined functional and symptomatic endpoint incorporating a final MMO of at least 35 mm together with substantial reduction in pain during function. The use of combined pain and functional criteria was selected because assessment based solely on change in mouth opening may overestimate treatment success in patients with persistent symptomatic disease. Statistical Analysis Continuous variables were summarized as mean ± standard deviation or median and interquartile range according to distribution. Categorical variables were expressed as frequencies and percentages. Changes in MMO and VAS scores between baseline and follow-up were evaluated using paired statistical tests. Repeated measurements across follow-up intervals were assessed using repeated-measures analysis where distributional assumptions were satisfied. The association between preoperative MMO and absolute improvement in MMO at 6 months was evaluated using Pearson correlation analysis. Differences in functional improvement among the three baseline MMO categories were assessed using one-way analysis of variance followed by appropriate post-hoc comparisons. Multivariable linear regression was used to determine whether preoperative MMO independently predicted the magnitude of improvement in mouth opening after adjustment for age, baseline pain score, and duration of symptoms. Logistic regression was additionally used to explore factors associated with the predefined clinical success endpoint. Regression coefficients, odds ratios, 95% confidence intervals, and p values were calculated as appropriate. A two-sided p value <0.05 was considered statistically significant.

A total of 60 patients with symptomatic TMJ internal derangement completed the 6-month follow-up and were included in the analysis. The mean age was 32.8 ± 9.4 years, with an age range of 18–56 years. Forty-two patients (70.0%) were female and 18 (30.0%) were male. The right TMJ was predominantly affected in 32 patients (53.3%) and the left TMJ in 28 (46.7%). The median duration of symptoms was 7 months (interquartile range: 4–12 months). The mean preoperative maximal mouth opening (MMO) was 27.6 ± 5.1 mm. Twenty patients (33.3%) had severe restriction with an MMO <25 mm, 21 (35.0%) had moderate restriction between 25 and 29.9 mm, and 19 (31.7%) had an MMO ≥30 mm. Mean preoperative pain intensity was 7.1 ± 1.2 on the visual analogue scale (VAS). Table 1. Baseline demographic and clinical characteristics of the study population Variable Value Number of patients 60 Age (years), mean ± SD 32.8 ± 9.4 Female, n (%) 42 (70.0) Male, n (%) 18 (30.0) Right TMJ affected, n (%) 32 (53.3) Left TMJ affected, n (%) 28 (46.7) Symptom duration (months), median (IQR) 7 (4–12) Preoperative MMO (mm), mean ± SD 27.6 ± 5.1 Preoperative VAS pain score, mean ± SD 7.1 ± 1.2 MMO <25 mm, n (%) 20 (33.3) MMO 25–29.9 mm, n (%) 21 (35.0) MMO ≥30 mm, n (%) 19 (31.7) SD: standard deviation; IQR: interquartile range; MMO: maximal mouth opening; VAS: visual analogue scale; TMJ: temporomandibular joint. Changes in Maximal Mouth Opening A progressive improvement in mouth opening was observed following arthrocentesis. Mean MMO increased from 27.6 ± 5.1 mm preoperatively to 33.8 ± 5.0 mm at 1 week and 36.7 ± 4.8 mm at 1 month. Further improvement was observed at 3 months, when mean MMO reached 39.0 ± 4.6 mm, and this improvement was maintained at 6 months, with a mean MMO of 40.2 ± 4.7 mm. The mean absolute increase in MMO from baseline to 6 months was 12.6 ± 5.8 mm. The difference between baseline and 6-month measurements was statistically significant (p < 0.001). Most of the improvement occurred during the first 3 months, although a smaller additional functional gain was observed between 3 and 6 months. Table 2. Change in maximal mouth opening following TMJ arthrocentesis Assessment MMO (mm), mean ± SD Mean change from baseline (mm) p value* Preoperative 27.6 ± 5.1 — — 1 week 33.8 ± 5.0 +6.2 <0.001 1 month 36.7 ± 4.8 +9.1 <0.001 3 months 39.0 ± 4.6 +11.4 <0.001 6 months 40.2 ± 4.7 +12.6 <0.001 *Compared with preoperative value. The proportion of patients achieving an MMO ≥35 mm increased from 8.3% preoperatively to 91.7% at the 6-month assessment. Changes in Pain Intensity Pain decreased substantially after arthrocentesis. The mean VAS score declined from 7.1 ± 1.2 preoperatively to 4.5 ± 1.4 at 1 week and 3.1 ± 1.3 at 1 month. At 3 months, the mean score was 2.1 ± 1.2, decreasing further to 1.6 ± 1.1 at 6 months. The mean reduction in pain between baseline and the final follow-up was 5.5 ± 1.5 VAS units (p < 0.001), corresponding to a reduction of approximately 77% from the baseline level. Table 3. Change in pain intensity following TMJ arthrocentesis Assessment VAS pain score, mean ± SD Mean reduction from baseline p value* Preoperative 7.1 ± 1.2 — — 1 week 4.5 ± 1.4 2.6 <0.001 1 month 3.1 ± 1.3 4.0 <0.001 3 months 2.1 ± 1.2 5.0 <0.001 6 months 1.6 ± 1.1 5.5 <0.001 *Compared with preoperative value. Relationship Between Preoperative MMO and Functional Improvement A significant relationship was observed between the severity of preoperative restriction and subsequent improvement in mouth opening. Patients with the greatest restriction before treatment demonstrated the largest absolute increase in MMO. Among patients with baseline MMO <25 mm, mean opening increased from 21.8 ± 2.1 mm to 39.0 ± 4.3 mm at 6 months, representing an improvement of 17.2 ± 4.5 mm. In patients with baseline MMO between 25 and 29.9 mm, the corresponding increase was from 27.5 ± 1.4 mm to 40.3 ± 4.2 mm, representing an improvement of 12.8 ± 4.3 mm. Patients with a baseline MMO ≥30 mm showed a smaller absolute increase, from 33.8 ± 2.7 mm to 41.3 ± 5.1 mm, with a mean improvement of 7.5 ± 4.1 mm. The difference in functional gain among the three baseline groups was statistically significant (p < 0.001). Table 4. Functional response according to preoperative maximal mouth opening Preoperative MMO n Baseline MMO (mm) 6-month MMO (mm) Improvement (mm) p value <25 mm 20 21.8 ± 2.1 39.0 ± 4.3 17.2 ± 4.5 25–29.9 mm 21 27.5 ± 1.4 40.3 ± 4.2 12.8 ± 4.3 ≥30 mm 19 33.8 ± 2.7 41.3 ± 5.1 7.5 ± 4.1 Overall comparison 60 — — — <0.001 Post-hoc comparisons demonstrated significantly greater improvement in the <25-mm group compared with the ≥30-mm group (p < 0.001). The difference between the <25-mm and 25–29.9-mm groups was also significant (p = 0.006). Despite the greater absolute improvement among patients with severe initial restriction, the final 6-month MMO values were relatively similar among the groups. This indicated substantial functional convergence following treatment. Correlation Between Baseline MMO and Treatment Response Correlation analysis demonstrated a moderate-to-strong inverse relationship between baseline MMO and absolute improvement at 6 months (r = −0.58, p < 0.001). Thus, lower preoperative mouth opening was associated with greater subsequent improvement. A weaker inverse association was observed between duration of symptoms and improvement in MMO (r = −0.29, p = 0.026), suggesting that patients with longer-standing symptoms tended to demonstrate somewhat smaller functional gains. Baseline pain showed a positive correlation with the magnitude of pain reduction at 6 months (r = 0.62, p < 0.001). Age was not significantly correlated with the final improvement in MMO (r = −0.18, p = 0.17). Predictors of Improvement in Mouth Opening Multivariable linear regression was performed with absolute change in MMO at 6 months as the dependent variable. Baseline MMO remained independently associated with functional improvement after adjustment for age, symptom duration, and baseline VAS score. Each 1-mm increase in preoperative MMO was associated with approximately 0.58 mm less absolute improvement in mouth opening at 6 months (B = −0.58; 95% CI: −0.78 to −0.38; p < 0.001). Longer duration of symptoms showed a smaller negative association with functional gain (B= −0.17; 95% CI: −0.32 to −0.02; p = 0.031). Age and baseline pain were not independently associated with change in MMO in the adjusted model. Table 5. Multivariable linear regression for predictors of improvement in maximal mouth opening at 6 months Predictor Regression coefficient (B) 95% CI p value Preoperative MMO (per 1 mm) −0.58 −0.78 to −0.38 <0.001 Age (per year) −0.07 −0.17 to 0.03 0.164 Symptom duration (per month) −0.17 −0.32 to −0.02 0.031 Baseline VAS pain score 0.24 −0.29 to 0.77 0.368 CI: confidence interval; MMO: maximal mouth opening; VAS: visual analogue scale. Clinical Success At the 6-month follow-up, 51 of 60 patients (85.0%) fulfilled the predefined combined criteria for clinical success. The success rate was 90.0% among patients with baseline MMO <25 mm, 85.7% among those with MMO of 25–29.9 mm, and 78.9% among patients with baseline MMO ≥30 mm. No major procedure-related complications were observed. Transient postoperative discomfort and mild preauricular swelling occurred in a small proportion of patients and resolved with routine postoperative management.

The present study demonstrated substantial improvement in mandibular opening and pain following TMJ arthrocentesis. More importantly, preoperative maximal mouth opening (MMO) showed a significant inverse relationship with the magnitude of functional improvement. Patients presenting with greater restriction before treatment demonstrated larger absolute increases in MMO, whereas those with relatively preserved baseline opening showed smaller gains. Despite these differences in absolute improvement, final MMO values were relatively similar across the baseline categories, suggesting convergence toward a functional range of mandibular opening following treatment. The mean MMO increased from 27.6 ± 5.1 mm preoperatively to 40.2 ± 4.7 mm at 6 months, representing a mean absolute improvement of 12.6 ± 5.8 mm. This magnitude of functional recovery is consistent with previous reports demonstrating improvement in mandibular mobility following arthrocentesis in patients with internal derangement and restricted mouth opening [6,7]. Arthrocentesis is believed to improve joint mobility through lavage of the superior joint compartment, hydraulic distension, disruption of minor adhesions, reduction of negative intra-articular pressure, and restoration of more favorable conditions for mandibular translation [5,6]. The principal finding was the relationship between the degree of preoperative restriction and subsequent functional gain. Patients with baseline MMO <25 mm demonstrated a mean improvement of 17.2 ± 4.5 mm, compared with 12.8 ± 4.3 mm among those with an MMO of 25–29.9 mm and 7.5 ± 4.1 mm among patients with baseline MMO ≥30 mm. Correspondingly, baseline MMO demonstrated a significant inverse correlation with absolute improvement at 6 months (r = −0.58, p < 0.001). These findings indicate that marked restriction before arthrocentesis does not necessarily imply an unfavorable functional prognosis. Previous investigations have attempted to identify clinical characteristics capable of predicting arthrocentesis outcomes. Nishimura et al. evaluated 103 TMJs in 100 patients with internal derangement and reported successful treatment in approximately 71% of cases [8]. Successful cases demonstrated greater mouth opening during the early postoperative period and less severe preoperative pain, whereas morphological condylar abnormalities were more frequently observed among unsuccessful cases. These findings indicate that treatment response is multifactorial and that clinical mobility should be considered alongside pain severity and structural joint characteristics. Emshoff and Rudisch also investigated predictor variables associated with treatment outcomes following arthrocentesis and hydraulic distension [9]. Their findings further emphasized that baseline clinical characteristics can influence therapeutic response. Collectively, these studies support the use of readily measurable clinical variables as components of prognostic assessment while demonstrating that no single preoperative parameter is sufficient to determine treatment outcome. A plausible explanation for the greater absolute improvement among patients with marked baseline restriction relates to the pathophysiology of internal derangement. Restricted mandibular movement may result from a combination of pain-related muscular guarding, altered intra-articular pressure, impaired lubrication, inflammatory changes, and adhesions within the superior joint compartment. These factors may restrict translation of the mandibular condyle even in the absence of irreversible structural obstruction. Lavage and hydraulic distension can potentially reverse some of these components, allowing patients with severe functional restriction to achieve substantial improvement [5,10]. The biological effects of arthrocentesis extend beyond mechanical lavage. Inflammatory mediators within TMJ synovial fluid are considered important contributors to pain and intra-articular dysfunction. Kaneyama et al. investigated the removal of bradykinin, interleukin-6, and protein during arthrocentesis and demonstrated progressive clearance of these substances during joint lavage [15]. This provides a biological basis for the reduction in pain and improvement in function observed after the procedure. In a subsequent investigation, Kaneyama et al. compared inflammatory mediator profiles between successful and unsuccessful arthrocentesis cases [16]. Differences in the concentrations or detection of mediators such as bradykinin and leukotriene B4 were observed between outcome groups, suggesting that the biochemical inflammatory environment within the TMJ may contribute to variability in clinical response. Thus, the effects of arthrocentesis are likely to reflect an interaction between mechanical restoration of joint movement and modification of the inflammatory intra-articular environment. Pain reduction represented another major treatment response in the present study. Mean VAS pain score decreased from 7.1 ± 1.2 before treatment to 1.6 ± 1.1 at 6 months, corresponding to a reduction of approximately 77.5%. Improvement occurred progressively throughout follow-up, with the greatest reduction occurring during the earlier postoperative period. This pattern is consistent with previous literature demonstrating clinically meaningful pain relief following arthrocentesis [6,10]. The therapeutic rationale for arthrocentesis was established by the early work of Nitzan et al., who described lavage of the superior joint compartment as a simplified treatment for severe limitation of mouth opening [5]. This represented an important development in the management of TMJ internal derangement because restoration of mandibular function could be achieved without direct open surgical repositioning of the articular disc. Subsequent investigations have demonstrated that lavage may improve symptoms even when the anatomical disc-condyle relationship itself is not restored. Murakami et al. compared arthrocentesis with nonsurgical therapy and arthroscopic lysis and lavage in patients with TMJ closed lock [12]. Arthrocentesis produced favorable short-term outcomes, although arthroscopic treatment showed a higher success rate in that investigation. These findings supported arthrocentesis as an intermediate therapeutic option between conservative management and more invasive joint surgery. Longer-term observations have also indicated persistence of clinical benefit. Hosaka et al. evaluated patients treated with arthrocentesis for closed lock and reported favorable outcomes during extended follow-up [13]. Similarly, Alpaslan et al., in a five-year retrospective evaluation, demonstrated sustained clinical benefit following TMJ arthrocentesis while emphasizing that long-term outcome may be influenced by patient-, symptom-, and treatment-related variables [14]. These studies are important because immediate increases in mouth opening following hydraulic distension do not necessarily establish durable recovery. The clinical success rate in the present study was 85.0% at 6 months. Although this value falls within the broad range reported in the literature, direct comparison of success rates requires caution. Studies have used different diagnostic criteria, patient populations, definitions of successful treatment, lavage techniques, postoperative rehabilitation protocols, and durations of follow-up [10,18]. Some studies define success primarily according to pain reduction, whereas others require improvement in mouth opening or employ composite functional and symptomatic criteria. Kim et al. investigated factors influencing the prognosis of arthrocentesis and reported successful outcomes in the majority of their patients [17]. Clinical and behavioral variables were examined as potential predictors, with oral parafunctional habits showing an association with poorer outcomes. Such observations demonstrate that treatment response cannot be explained entirely by the anatomical severity of restriction. Behavioral loading, bruxism, muscular activity, chronic pain mechanisms, and adherence to postoperative management may influence recovery. In the present study, longer duration of symptoms demonstrated a modest negative association with functional gain. This finding is clinically plausible because persistent internal derangement may be accompanied by progressive adhesions, fibrosis, degenerative changes, and adaptive alterations in mandibular movement. Limitation caused predominantly by pain, inflammatory changes, or minor adhesions may be more readily reversible than long-standing restriction associated with advanced structural changes. Nevertheless, symptom duration has not been consistently demonstrated to be an independent determinant of arthrocentesis outcome. Differences in patient selection and definitions of chronicity complicate comparisons between studies. Duration of symptoms should therefore be considered as one component of the overall prognostic assessment rather than as an isolated indication or contraindication for arthrocentesis. Age was not significantly associated with functional improvement in the present study. Chronological age alone may be an inadequate surrogate for the pathological condition of the TMJ. Structural abnormalities, inflammatory activity, parafunctional loading, chronicity of symptoms, and the degree of functional adaptation may be more relevant to treatment response than age itself. Accordingly, treatment decisions should be based primarily on the clinical phenotype and severity of joint dysfunction rather than demographic characteristics alone. An important feature of the present results was the convergence of final mouth-opening measurements. Although baseline MMO differed considerably among the three groups, mean 6-month MMO ranged from approximately 39 to 41 mm. Patients with severe initial restriction therefore achieved a final functional range comparable to that of patients who began treatment with less severe limitation. This observation suggests that the larger improvement among patients with low baseline MMO was not merely an increase from an extremely restricted value while remaining functionally impaired. However, the inverse relationship between baseline MMO and absolute improvement requires cautious statistical interpretation. Change scores are mathematically dependent on baseline measurements. Patients beginning with an MMO of approximately 20 mm have substantially greater opportunity to demonstrate a large absolute increase than patients beginning at 35 mm. A ceiling effect and regression toward the mean may therefore contribute to the observed inverse correlation. For this reason, baseline MMO should not be interpreted solely on the basis of its correlation with the change score. The final functional status and adjusted analyses should also be considered. The multivariable analysis partly addressed this issue by examining baseline MMO together with age, symptom duration, and baseline pain. Preoperative MMO remained independently associated with the magnitude of functional gain. Nevertheless, future studies should supplement change-score analysis with models evaluating final MMO while adjusting for baseline MMO and other potential confounders. Such approaches may provide a more robust estimate of the independent prognostic contribution of baseline restriction. Systematic reviews have generally supported improvements in pain and mandibular function following arthrocentesis, while also highlighting considerable heterogeneity in the available evidence. Monje-Gil et al. reviewed the literature on TMJ arthrocentesis and reported generally favorable clinical outcomes, but identified substantial differences in study design, patient selection, diagnostic criteria, treatment protocols, and outcome assessment [10]. Such heterogeneity limits direct comparison of absolute changes in MMO across studies. Guarda-Nardini et al. similarly reviewed clinical evidence concerning TMJ arthrocentesis and concluded that the procedure generally results in improvement in jaw function and reduction in pain, while emphasizing methodological variability among published investigations [18]. Differences in lavage volume, number and position of needles, adjunctive intra-articular medications, postoperative exercises, and diagnostic subgroups remain important considerations when interpreting treatment outcomes. The effectiveness of arthrocentesis must also be considered relative to conservative treatment rather than exclusively according to within-patient improvement. Thorpe et al. performed a systematic review and meta-analysis of randomized controlled trials comparing arthrocentesis with conservative management for painful TMD [19]. Arthrocentesis was associated with a statistically greater improvement in MMO at 6 months; however, the pooled between-group difference was small. This distinction is clinically important because a large improvement from baseline following arthrocentesis does not necessarily indicate an equally large advantage over appropriately conducted conservative therapy. Consequently, restricted mouth opening alone should not be considered an indication for arthrocentesis. Conservative management remains an important first-line approach for many patients with TMD [4]. Arthrocentesis is more appropriately considered in selected patients with persistent intra-articular pain and functional limitation that have not responded adequately to conservative measures. Technical variations in arthrocentesis may also influence procedural characteristics and potentially treatment outcomes. Both single-puncture and conventional double-puncture approaches have been described. Nagori et al. systematically compared outcomes of single- and double-puncture arthrocentesis techniques and found no consistent major advantage in postoperative pain or MMO for either approach, although certain single-puncture techniques may offer procedural advantages [20]. This suggests that successful lavage and appropriate patient selection may be more important than the specific puncture configuration in determining clinical response. Lavage volume is another technical consideration. Removal of inflammatory substances increases progressively during irrigation, providing a biological rationale for adequate joint lavage [15]. However, an optimal universal lavage volume has not been established, and differences in joint anatomy, needle placement, flow characteristics, and procedural technique may influence the amount required to achieve satisfactory irrigation. From a clinical perspective, preoperative MMO represents an attractive prognostic variable because it is objective, inexpensive, non-invasive, and readily repeatable. The present findings indicate that pronounced restriction should not automatically be interpreted as evidence that arthrocentesis is unlikely to succeed. On the contrary, patients with severe baseline limitation may demonstrate substantial absolute functional improvement when restriction is driven by reversible intra-articular factors. Nevertheless, MMO should not be used as a stand-alone prognostic tool. Nishimura et al. demonstrated the importance of pain severity, early postoperative mouth opening, and morphological joint abnormalities [8], while other investigations have identified behavioral and clinical factors that may influence outcome [17]. A comprehensive prognostic assessment should therefore incorporate functional limitation, pain characteristics, symptom duration, clinical diagnosis, imaging findings where appropriate, parafunctional habits, and previous response to conservative treatment. Serial postoperative assessment may provide additional prognostic information. Early improvement in mandibular opening has previously been associated with successful outcome [8]. Failure to demonstrate expected early functional recovery should prompt reassessment of persistent mechanical restriction, structural joint disease, adherence to jaw exercises, ongoing parafunctional loading, and alternative sources of orofacial pain. Clinical Implications The findings support routine measurement and documentation of MMO before and after TMJ arthrocentesis. Baseline MMO may provide useful information regarding the magnitude of functional improvement that can reasonably be expected after treatment. Patients with severe restriction may experience greater absolute improvement and should not be excluded from minimally invasive treatment solely because of a low preoperative MMO. Prognostic counselling should nevertheless distinguish between absolute improvement and final functional outcome. A large increase in MMO is clinically meaningful only when accompanied by satisfactory final mandibular function and symptomatic improvement. Assessment of treatment response should therefore incorporate final MMO, pain reduction, functional ability, and patient-reported improvement rather than relying exclusively on change in mouth opening. Strengths and Limitations A principal strength of the study was its focused evaluation of the relationship between preoperative MMO and subsequent functional response rather than merely documenting improvement following arthrocentesis. Serial follow-up allowed evaluation of the trajectory of recovery, and simultaneous assessment of pain provided a broader measure of clinical response. Stratification according to the severity of baseline restriction permitted direct comparison of functional gains among clinically relevant groups. Several limitations should be considered. The sample size was relatively modest, and the study did not include a parallel conservative-treatment control group. Therefore, the observed improvement cannot be attributed exclusively to arthrocentesis, as natural adaptation, postoperative exercises, behavioral modification, and other aspects of management may have contributed. The follow-up period was limited to 6 months and therefore does not establish long-term durability. Structural characteristics on magnetic resonance imaging were not incorporated systematically into the predictive model. Psychological factors, parafunctional habits, and adherence to postoperative exercises were also not quantified, although these factors may influence pain and functional recovery. Finally, the use of change in MMO as an outcome introduces potential mathematical coupling with the baseline measurement and may be influenced by regression toward the mean and ceiling effects. Future prospective studies should incorporate larger samples, standardized imaging criteria, patient-reported outcome measures, and multivariable models evaluating final MMO adjusted for baseline function. Longer follow-up would also help determine whether the relationship between baseline restriction and functional response persists over time.

TMJ arthrocentesis was associated with substantial improvement in maximal mouth opening and reduction in pain during 6 months of follow-up. Preoperative MMO showed a significant inverse relationship with absolute functional gain, with patients presenting with greater initial restriction demonstrating larger improvements in mandibular opening. Severe preoperative restriction should therefore not necessarily be interpreted as an unfavorable prognostic indicator. Baseline MMO may serve as a simple and clinically useful component of prognostic assessment in appropriately selected patients undergoing arthrocentesis. However, it should be interpreted together with final functional status, pain severity, symptom duration, structural joint characteristics, and other clinical factors. Larger controlled prospective studies using baseline-adjusted outcome models are warranted to establish clinically useful prognostic thresholds and improve prediction of individual treatment response. Declarations Ethical considerations: The study was conducted in accordance with the ethical principles governing research involving human participants. Written informed consent was obtained from all participants before treatment and study-related assessment.

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