The term temporomandibular disorders (TMD)

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1 PAIN MEDICINE Volume 2 Number The Association Between Research Diagnostic Criteria for Temporomandibular Disorder Findings and Biting Force and Endurance in Patients with Temporomandibular Disorders Thomas E. Rudy, PhD,* Carol M. Greco, PhD, Giselle A. Yap, DMD, MS, Hussein S. Zaki, DDS, MS, Joseph K. Leader, MS, and J. Robert Boston, PhD *Professor, Departments of Anesthesiology/CCM, Psychiatry, and Biostatistics; Assistant Professor, Department of Anesthesiology/CCM; Department of Prosthodontics; Professor, Departments of Prosthodontics and Otolaryngology; Department of Bioengineering; and Associate Professor, Department of Electrical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania ABSTRACT Objective. This study was designed to evaluate the association between examination findings based on Research Diagnostic Criteria for Temporomandibular Disorders and performances on bite force and endurance tasks. Methods. Subjects were 126 patients with temporomandibular disorder and 34 pain-free controls. A subset of patients with temporomandibular disorder (n 56) also were evaluated following a brief conservative 4-week treatment intervention. Results. A multivariate analysis of variance indicated that female patients (P.001) but not males (P.17) had lower bite forces than age- and gender-matched controls. Dental examination findings were significantly but modestly predictive of bite task performance (R ), with higher joint pain and smaller maximum unassisted jaw opening associated with lower bite force. The brief treatment interventions resulted in both self-reported and clinically determined improvements. Clinical and selfreported improvement significantly predicted pre-post treatment changes in biting force among female subjects (R ). Specifically, reduction in joint palpation pain and self-reported pain (McGill Pain Questionnaire short form) were the primary predictors of increases in bite force. The improvement in biting force was modest (mean 7 lb), and the force levels of female patients remained lower than those of control subjects. Treatment did not significantly increase endurance time. Discussion. The brief conservative treatments used resulted in improvements in pain and jaw opening, and 81.8% of patients reported moderate to major improvement. The modest association of the bite task with Research Diagnostic Criteria for Temporomandibular Disorders examination findings and treatment improvement in this heterogeneous sample suggests that the bite and endurance tasks have limited diagnostic utility and sensitivity to treatment effects. Key Words: Temporomandibular Disorders; Treatment Outcome; Bite Force and Endurance; RDC/TMD The term temporomandibular disorders (TMD) refers to a set of clinical conditions that can include temporomandibular joint (TMJ) problems Reprint requests to: Thomas E. Rudy, PhD, Pain Evaluation and Treatment Institute, University of Pittsburgh, 4601 Baum Boulevard, Pittsburgh, PA Tel: (412) ; Fax: (412) ; rudyte@anes.upmc.edu. such as pain and limitations in opening, and pain in the masticatory musculature. Considerable research has established that patients with TMD exhibit lower biting forces than do persons without TMD. Several studies have found lower biting force among patients with TMJ abnormalities or dysfunction compared with controls [1 3]. Larheim and Floystrand [4] found lower bite forces in a group of adults with Blackwell Science, Inc /01/$15.00/

2 36 Rudy et al. rheumatoid arthritis and TMJ abnormalities, in comparison to a group without joint disease. A similar discrepancy between biting forces was found among children with and without chronic juvenile arthritis [5]. However, a sample of dental students with TMD signs such as limitations in opening, clicking with pain, or painful joints did not differ from asymptomatic students in biting force [6]. TMD of neuromuscular origins also may be associated with decreased biting forces [7,8]. Additionally, several studies have shown decreased endurance time at maximal or submaximal levels of force among patients with TMJ articular disorders [5,9] or myofascial pain [7], compared with healthy controls. Several studies have examined the effects of treatment on patients biting forces or endurance. Helkimo et al. [10] evaluated biting force before, during, and after conservative treatments in 30 patients with muscle and joint pain. Initially lower than that of control subjects, biting force increased during treatment and was reported as similar to that of controls following treatment. In contrast, Ow et al. [8] studied bite force in 10 patients with craniomandibular disorders before and after 5 to 8 weeks of conservative treatments, and found no improvement in force, despite reduction in clinical dysfunction and patient reports of symptoms. Tzakis, Dahlström, and Haraldson [11] evaluated endurance and chewing efficiency of 12 patients with TMD of neuromuscular origins before splint treatment and 1 month later, and found that endurance at 50 N increased significantly. Sinn et al. [3] evaluated biting force and mandibular range of motion in 25 females before and after surgical treatment for various articular disorders. Bite force assessments repeated 6 months after surgery indicated significant improvement compared with preoperative levels. Mandibular opening ability also improved following treatment. By 1 year posttreatment, all but 4 patients no longer had pain, and their bite force levels were not significantly different from those of control subjects. Stegenga, Broekhuijsen, De Bont, and van Willigen [9] evaluated pretreatment and posttreatment endurance times at a standard force level of 50 N in 51 patients with various articular TMD (reducing disc displacement, permanent disc displacement, or synovitis). Treatments were not standardized, but were based on diagnosis and patient preference. Pain was decreased or eliminated in all patients, and significant improvements in endurance time were found across all 3 diagnostic categories. The results of studies that evaluate the effects of treatment on bite force and/or endurance are mixed, which may be at least in part the result of differing diagnostic procedures and diagnostic groups, diverse methodologies, or sample size issues. Additionally, in none of the above studies were specific dental examination measures of improvement used to predict change in biting force levels. Several studies support the idea that pain is a major limiting factor in biting force and endurance among healthy subjects [7,12 14] as well as those with myofascial pain dysfunction [7] and articular TMD [9]. If pain and dysfunction limit biting force and endurance in patients with TMD, reductions in clinical symptoms following treatment should be directly associated with improvements in biting force and endurance task performance. The primary objectives of the present study were twofold: (1) to evaluate the sensitivity of a bite force and endurance task to severity of joint and muscle problems, and (2) to determine whether improvements in clinical and self-reported symptoms predict improvement in bite force and endurance. As an initial evaluation of sensitivity to jaw pain problems, patients performances were compared with those of control subjects. Previous literature supports the idea that both myofascial pain and articular disorders are associated with decreased biting force and endurance relative to the abilities of asymptomatic controls. We hypothesized that control subjects would exhibit greater bite force and endurance compared to patients with TMD. A second objective was to evaluate the associations between task performance and severity of Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) [15] examination findings of signs and symptoms prior to treatment. We hypothesized that increased severity of myofascial pain and/or articular disorders would be associated with poorer performance on bite and endurance tasks. An additional aim was to evaluate the sensitivity of the bite tasks to treatment effects. Specifically, we wanted to determine whether oral/dental examination measures of improvement following conservative treatments predict changes in the physical capabilities of patients with TMD, as measured by biting force and endurance levels. Pretreatment to posttreatment changes in patient reports of pain and activity disruption and RDC/TMD-based oraldental evaluation measures of muscle pain, joint pain, joint sounds, and jaw opening were used to predict changes in patients biting force and endurance. In addition, since biting force and endurance times are inherently voluntary efforts rather than pure measures of physical capacity, surface electromyography (EMG) and patient reports of effort,

3 Biting Force and Endurance in TMD Patients 37 pain, and fatigue were used to supplement the bite force and endurance information. In evaluating the sensitivity of biting force and endurance tests to clinical treatment effects, we hypothesized that improvement on dental examination variables and self-reported reduction in pain and interference with activities would be associated with increases in biting force and endurance. Methods Subjects Subjects were 126 patients with TMD from the University of Pittsburgh s Pain Evaluation and Treatment Institute. These patients met the following inclusion criteria: (1) between 18 and 60 years of age, (2) TMD pain symptoms of at least 3 months duration, and (3) natural posterior occlusion. Patients were excluded from the study if they presented with: (1) occlusal splint therapy that they were unwilling to discontinue for the study duration, (2) previous TMJ surgery, (3) moderate-tosevere periodontal disease, (4) removable dentures, Table 1 Subject demographics and pain-related descriptive statistics for TMD subjects (5) third molar problems such as pericoronitis or supereruption, (6) pregnancy, or (7) major psychiatric disorders. A comparison group of 34 individuals without TMD or other pain conditions were recruited from the Pittsburgh community through advertisements to serve as control subjects. All subjects completed Institutional Review Board approved written informed consent procedures prior to study entry. Demographics for all subjects and pain-related descriptive statistics for TMD subjects are presented in Table 1. Analysis of variance (ANOVA) and chi-square analyses indicated no significant differences existed between control and TMD subjects for gender composition, age, percentage graduating from high school, and marital status (Table 1). Procedures Oral/dental evaluation. All subjects underwent an oral/dental evaluation based on RDC/TMD [15] examination procedures conducted by 1 of 3 trained and calibrated dentists. The examination includes bilateral palpation (2 lb pressure for external, 1lb Demographic Variable TMD Subjects Control Subjects P-value Gender Male Female Age (yrs) Mean SD High school graduate (%) Marital status (%) Single Married Separated/Divorced 8 3 Duration of pain (yrs) Mean 6.9 SD 6.8 Constant pain (%) Yes 45 No 55 Pain onset (%) Gradual 57 Abrupt 43 Reported analgesic usage (%) Any 79 3 or more days/wk 38 Number of previous treatments for TMD Mean 1.9 SD 2.4 RDC/TMD diagnoses (%) Group I: Myofascial 95 Group II: Disc Displacement 43 Group III: Arthralgia, arthritis, arthrosis 45 Laterality of pain (%) Right 16 Left 17 Both 67

4 38 Rudy et al. for internal) of 10 internal and external muscle areas, palpation of the TMJ, measurements of maximal jaw opening, and joint sounds. All control subjects also received a comprehensive oral/dental evaluation, and none of the 34 controls met research diagnostic criteria for the diagnosis of TMD. RDC/TMD examination findings indicated that the vast majority of TMD subjects received a myofascial diagnosis (Group I), and a substantial number also received Group II and III diagnoses (Table 1). Additionally, examination findings indicated the majority of TMD subjects reported their pain was bilateral (Table 1). TMD subjects also received a clinical interview and standardized questionnaires related to symptoms and associated dysfunction and distress. Following the evaluation, TMD subjects were assigned randomly to 1 of 3 conservative 4-session treatment programs: flurbiprofen therapy, physical therapy exercises, or biofeedbackassisted relaxation training. Following the dental evaluation and prior to starting treatment, subjects completed the bite force and endurance task, described below. A random subset of 56 patients was recruited to repeat the bite force and endurance task following their 4-session treatment program. Bite force and endurance tasks. Bite force and endurance trials were based on equipment and procedures described by Clark and Carter [13] and Jow and Clark [12]. Subjects were seated in a radio frequency shielded, sound- and light-attenuated, temperature-controlled (20 C 24 C) room. Maximum Voluntary Bite Force (MVBF) was measured with a Sensotec (Columbus, OH) load cell with 500 lb capacity and infinite resolution held in place intraorally by two 1-mm thick 17-4 PH Stainless Steel plates. The sampling frequency of the load cell was 60 Hz. The lower or mandibular plate contained a centrally located metallic housing for the load cell. The metal plates were fitted to the first molar-second premolar areas of the maxillary and mandibular arches with Duralay acrylic resin (Reliance Mfg. Co., Worth, IL) sleeves approximately 1 mm in thickness. While the acrylic was in the dough stage, the bite plates with the acrylic sleeves on their outer ends were adjusted to be parallel to one another while in the mouth. The removable acrylic sleeves were labeled and retained for posttreatment testing. To assess muscle activity during the tasks, 4-mm Ag/AgCl bipolar electrodes were taped bilaterally over the surface of the masseter in the position recommended by Lippold [16]. Data were collected, stored, and analyzed by means of a Data Translation (Marlboro, MA) 12-bit resolution analog-todigital converter, a Pentium-based microcomputer, and the ASYST software package (Macmillian Software Co., New York, NY). A bandpass filter of Hz was used, and the EMG recordings were digitized at 2000 Hz. Experimental procedures. The bite force and endurance trials were conducted by a single experimenter who was kept masked to dental examination findings. Subjects were asked to rate their current pain on an 11-point scale (where 0 no pain and 10 severe pain) prior to placement of the bite apparatus. With the bite plates and load cell in place intraorally, the inter-incisal distance (using teeth 8 and 25) was measured with a millimeter ruler and recorded. A saliva ejector was provided to the subject for use between trials. Subjects were instructed to bite as hard as they could on the biteplates 3 times for a period of 3 seconds each. Each maximum clench was followed by a 20-second rest period. The subject then was asked to rate pain by pointing to the 11-point rating scale held by the examiner. Next, the subject was given a chance to practice holding 30% 10% of the mean of their 3 maximum voluntary bite force (MVBF) trials using visual feedback on the computer monitor. Following 5 to 15 seconds of practice, the bite plates were removed from the subject s mouth for a 5-minute rest period. The bite plates and load cell were re-inserted and inter-incisal distance was checked. Using visual feedback on a computer monitor, subjects were asked to keep their biting force within a specified distance (or window) from a line on the screen for as long as physically possible. The line represented a bite force of 30% of their previously produced mean MVBF, and the 2 lines representing the upper and lower limits of the window were 10% above and below the 30% mean MVBF line. In addition to the visual feedback, subjects were provided with verbal encouragement to continue the task. Once the subjects stopped their endurance trial, they were asked to rate pain and fatigue by pointing to the rating scale. Dependent Measures Oral/dental examination measures. Six scales were derived from the oral/dental evaluation: (1) a muscle palpation pain index, (2) a joint palpation pain index, (3) an index of TMJ clicks, (4) a crepitus index, (5) unassisted jaw opening without pain, and (6) maximum unassisted jaw opening. For the first 4 scales, item response theory (IRT) [17] was used to

5 Biting Force and Endurance in TMD Patients 39 create summated scale scores, based on the number of positive findings that comprised each of these RDC/TMD indices. This scoring approach corrects for the nonlinearities that exist in simple summated scales [18], and has been used effectively in other areas of TMD research [19]. More specifically, without linearizing corrections, raw scores are biased in favor of central scores and against extreme scores. IRT corrections to raw scores provide equal, interval level measurement throughout the range of a scale, which is not the case for raw scores. The resulting logit scores, which can take on negative values, were rescaled from 0 to 100, with 0 no positive findings, and 100 all possible findings positive. Patient reports of pain and disability. At the pretreatment and posttreatment evaluations, TMD patients filled out questionnaires concerning pain and functioning. Self-reported pain was assessed by total score on the McGill Pain Questionnaire (MPQ) short form [20]. The MPQ-short form is a 15-item instrument on which patients rate the intensity during the past week of various pain descriptive adjectives. Disability was assessed using the 11-item Interference scale of the Multidimensional Pain Inventory (MPI) [21], which measures activity disruption due to pain. Table 2 Laboratory test measures for control and TMD subjects pretreatment Bite force and endurance task measures. Bite force was measured in pounds, and endurance measured in seconds. A work index was computed by multiplying endurance time and 30% of the mean MVBF. Supplemental measures included self-reports of pain before, during, and after the tasks, as well as estimates of fatigue, effort exerted, and difficulty of the tasks. To estimate muscle fatigue, we calculated the mean of the EMG median frequency at 3 phases of the endurance task: the beginning, middle, and end. Median frequency for 2-second intervals were calculated from the power spectrum, and then 5 of these windows (10-second) were averaged for each of these 3 phases for each subject. EMG power spectrum shifts during sustained contraction have been frequently used to identify muscle fatigue [22]. Data Analysis To control for experiment-wise error rates, multivariate analysis of variance (MANOVA) was used to test for differences between control and TMD subjects at the time of the pretreatment assessment. Significant MANOVAs were interpreted further by analysis of variance (ANOVA). A mixed-model MANOVA was used to test for differences in the EMG data, and repeated-measures MANOVA was used to evaluate significant changes for TMD subjects completing both the pretreatment and posttreatment assessments. Multiple regression models were used to test the association between oral-dental and self-report findings with findings from the laboratory bite task. A P-value of.05 was considered statistically significant. Results Pretreatment and Control Mean MVBF and Biting Endurance Mean maximum voluntary bite force (MMVBF), endurance times, and work indices for the 126 TMD pretreatment and the 34 control subjects are presented in Table 2. These data are presented separately by gender, since a significant gender main effect occurred for MMVBF (F 1, , P.001) (because of the large sample size discrepancies between gender, which led to significantly reduced power for interaction analyses that involved the gender factor, separate analyses by gender were computed). A MANOVA for females indicated significant differences between TMD and control subjects for these measures (F 3, , P.001). As displayed in Table 2, follow-up ANOVAs indicated that female controls had significantly higher MMVBF and work indices, compared with female TMD subjects. Although male control subjects had higher MMVBF and work values compared with TMD Subjects Control Subjects P-value Female Subjects (TMD n 97, Control n 26) Mean Maximum Voluntary Bite Force (MMVBF) (lbs) (3.86) a (7.46).001 Endurance Time (ET) (sec) (20.40) (39.40).192 Work Index (30% of MMVBF ET) (282.65) (545.95).012 Male Subjects (TMD n 29, Control n 8) MMVBF (lbs) (10.20) (13.89).172 Endurance Time (sec) (39.45) (12.16).439 Work Index (524.70) (556.01).097 a Numbers in parentheses are standard errors of the mean.

6 40 Rudy et al. Table 3 MANOVA results for median EMG frequencies (in Hz) during the biting endurance task Patients (n 126) Controls (n 34) P-values for F-tests Early Middle Late Early Middle Late Group(G) Time(T) G T Left Masseter (1.6) a (1.8) (2.0) (2.9) (3.3) (3.8) Right Masseter (1.8) (1.9) (2.0) (3.4) (3.6) (3.7) a Numbers in parentheses are standard errors of the mean. male TMD subjects (Table 2), a MANOVA analysis indicated only marginal significant differences between male TMD and control subjects for these measures (F 3, , P.06), and separate ANOVAs indicated only the work index approached statistical significance (Table 2). The lack of differences in work indices for male subjects may be due in part to: (1) lower endurance times of control males compared to TMD males, perhaps because they were working under a higher load; and (2) lower statistical power because of the small number (8) of control males. Median EMG frequencies for the early, middle, and late portions of the biting endurance trial are presented in Table 3. As can be seen in Table 3, both TMD and control subjects displayed significant reductions in median EMG frequencies for both the left and right masseter muscles as the endurance trial progressed from early to late phases, indicating muscle fatigue. No group differences, however, were found for median EMG frequencies over the duration of the endurance trial. These EMG changes suggest that both groups of subjects, TMD and controls, were exerting comparable effort during the endurance task, which is confirmed by the self-report effort measures described below (Table 4). Table 4 presents the subjective ratings collected during the biting experiment. As would be expected, TMD subjects reported significantly higher pain levels after both the maximum clenching trials and the endurance trial. Additionally, TMD subjects rated the maximum clenching task significantly Table 4 Task Rating Subjective experimental task ratings for control and TMD subjects pretreatment TMD Subjects (n 126) more difficult for them to do than did control subjects. As displayed in Table 4, despite equivalent EMG changes over the duration of the endurance task, TMD subjects reported higher muscle fatigue than controls at the end of the task. Both TMD and control subjects, however, reported exerting a high degree of effort during the task (mean ratings and 93.71, respectively, out of a maximum rating of 100). Association between Dental Examination and Bite Findings in TMD Subjects Regression analyses indicated a significant association between TMD subject s MMVBF values and pretreatment dental examination findings (R , F 6, , P.001). As displayed in Table 5, two examination findings primarily accounted for this significant association, maximum unassisted opening and TMJ palpation pain. Larger opening were positively associated with larger MMVBFs, and more TMJ palpation pain was associated with lower MMVBF values (standardized coefficients, Table 5). Dental examination findings, however, were not significantly associated with the work index, which was used to characterize TMD subject s performances on the biting endurance trial (R , F 6, , P.56). Pre-Post Treatment Results for TMD Subjects MMVBF, endurance time, and work indices results for the 56 TMD subjects completing the bite experiment following treatment are presented in Table 6. These values for the 34 control subjects (Ta- Control Subjects (n 34) P-value Pain level before testing (0 10) 3.13 (0.16) a 0.00 (0.00).001 Pain level after max clench trials (0 10) 4.56 (0.18) 0.83 (0.35).001 Effort used on max clenching task (0 100) (0.79) (1.49).506 Difficulty of max clenching task (0 10) 4.78 (0.27) 1.86 (0.51).001 Pain level after endurance trial (0 10) 6.89 (0.19) 3.57 (0.37).001 Muscle tiredness after endurance trial (0 10) 7.98 (0.16) 5.80 (0.31).001 Effort used on endurance task (0 100) (0.64) (1.16).078 Difficulty of endurance task (0 10) 6.79 (0.24) 6.03 (0.45).132 a Numbers in parentheses are standard errors of the mean.

7 Biting Force and Endurance in TMD Patients 41 Table 5 Regression analyses between dental examination findings and bite task results MMVBF Task Endurance Task Dental Examination Finding Std Coefficient t-value Std Coefficient t-value Unassisted jaw opening without pain Maximum unassisted jaw opening * Click index Crepitus index Muscle palpation pain index TMJ palpation pain index * Multiple R-square 0.175** *P.01. **P.001. ble 2) also are presented in Table 6 for comparative purposes. As with pretreatment findings, analyses were computed separately by gender. As displayed in Table 6, only female TMD subjects displayed significant pre-post changes, and only for MMVBF values. These posttreatment MMVBF values for TMD female subjects remained significantly less than those found for control females (Table 6), but their work indices were only marginally lower than female controls (P.072). Male TMD subjects did not display significant pre-post changes, and as would be expected based on the results presented in Table 2, their posttreatment values for the bite experiment were not significantly different than those found for male control subjects. The treatment program received by TMD subjects (flurbiprofen, physical therapy, or biofeedback) was not significantly associated with the magnitude of MMVBF changes (F 2, , P.15), or pre-post changes in the work indices (F 2, , P.34). Table 7 presents the pre-post treatment subjective task ratings for the TMD subjects. As displayed in Table 7, TMD subjects displayed statistically significant reductions in their pain level before the task, their pain level following the maximum clenching trials, and their pain level following the endurance task. Additionally, they rated the clenching Table 6 Pre-post treatment laboratory test measures for TMD subjects Pre- Treatment task significantly less difficult for them to perform following treatment (Table 7). In order to evaluate TMD subject s magnitude of change following treatment, pre-post treatment analyses were computed for dental examination findings and selected standardized self-report instruments. As can be seen in Table 8, following treatment TMD subjects displayed major and significant reductions in self-report pain (McGill short form) and examination pain (muscle and joint palpation pain). Additionally, subjects reported significantly less interference due to pain (MPI Interference scale), and maximum unassisted jaw opening, with and without pain, significantly increased following treatment (Table 8). Additionally, 81.8% of TMD subjects following treatment rated that they felt they had experienced moderate (41.8%) or major (40.0%) improvement in their pain and TMD symptoms as a result of treatment. Taken together, these findings demonstrate that treatment was largely successful for these 56 subjects in the short term. Association between Laboratory Bite Changes and Examination and Self-Report Changes To evaluate the degree to which changes in the bite task (Table 6) were associated with examination Posttreatment Control P-value a Subjects P-value b Female Subjects (TMD n 45, Control n 26) MMVBF (lbs) (3.97) c (4.32) (7.46) Endurance Time (ET) (sec) (21.47) (26.59) (39.40) Work Index (30% of MMVBF ET) (284.62) (277.32) (545.95) Male Subjects (TMD n 11, Control n 8) MMVBF (lbs) (15.53) (20.01) (13.89) Endurance Time (sec) (64.65) (42.32) (12.16) Work Index (793.41) (864.92) (556.01) a P values for changes pretreatment to posttreatment for TMD subjects. b P values for comparing patients posttreatment scores with control subject scores. c Numbers in parentheses are standard errors of the mean.

8 42 Rudy et al. Table 7 Pre-post treatment subjective experimental task ratings for TMD subjects Task Rating Pretreatment Posttreatment P-value Pain level before testing (0 10) 3.26 (0.27) a 1.70 (0.23).001 Pain level after max clench trials (0 10) 4.58 (0.31) 2.72 (0.30).001 Effort used on max clenching task (0 100) (1.04) (1.28).242 Difficulty of max clenching task (0 10) 5.19 (0.42) 4.11 (0.41).016 Pain level after endurance trial (0 10) 6.87 (0.27) 5.44 (0.33).001 Muscle tiredness after endurance trial (0 10) 8.15 (0.19) 8.00 (0.25).480 Effort used on endurance task (0 100) (1.01) (0.79).224 Difficulty of endurance task (0 10) 6.72 (0.33) 6.91 (0.37).681 a Numbers in parentheses are standard errors of the mean. and self-report changes (Table 8), difference scores for these measures were computed and 2 regression analyses were computed. These analyses are presented in Table 9. Changes in examination and self-report findings from pretreatment to posttreatment significantly predicted pre-post changes in MMVBF (R , F 6, , P.036). As displayed in Table 9, changes in self-reported pain (McGill) and changes in joint palpation pain were the primary positive predictors of increases in MMVBF. However, changes in examination and self-report findings were not predictive of changes in the amount of work performed during the endurance task (R , F 6, , P.614). Discussion In this study, female TMD patients performed less well than female controls on maximum biting force, as well as work (force x endurance time) on a submaximal clench endurance task. Male controls had higher bite force and work indices compared with male patients, but the differences were not statistically significant. This may be due to the small number of male controls used in this study and/or the high variability in performance among the male TMD patients. Females biting force performances improved significantly following treatment, but remained lower than that of female controls. The posttreatment work indices of the female patients were only marginally lower than those of controls. Although biting force and work index gains were modest and did not attain the levels of control subjects, patients reported less discomfort during the posttreatment tasks, despite exerting strong effort as assessed by the self-report measure of effort and EMG changes. Male patients posttreatment task performances were not significantly different from their pretreatment results. Pretreatment to posttreatment improvements in patients self-reports of pain severity and dental examination determination of joint pain were associated with improvement on posttreatment bite force but not work. In addition, oral-dental evaluation parameters significantly predicted task performance; specifically, greater maximum unassisted jaw opening was associated with higher bite force, and more joint palpation pain was associated with lower bite force. Our study revealed that submaximal clenching was maintained by TMD patients and controls for a comparable length of time, and the amount of work performed did not differ among the 2 groups, either at pretreatment or at posttreatment. This is in contrast with several studies that found shorter endurance times in TMD patients compared to controls at maximal bite [5] or submaximal force levels [9]. On the other hand, our results are consistent with several studies that showed improvements in biting force following treatment. A stronger treatment effect was noted by Helkimo et al. [23], who found bite force to rise steadily during a variety of treatments until patients had reached biting force consistent with controls at the end of treatment. Table 8 Pretreatment and posttreatment examination and self-report outcome measures Examination Measure Pretreatment Posttreatment P-value Muscle palpation pain (0 100) (2.43)* (2.58).001 TMJ palpation pain (0 100) (3.03) (2.92).001 Unassisted jaw opening without pain (mm) (1.29) (1.04).001 Maximum unassisted jaw opening (mm) (1.14) (0.92).035 McGill short form (0 100) (1.04) (1.16).001 MPI Pain Interference Scale (0 100) (2.70) (2.32).001 *Numbers in parentheses are standard errors of the mean.

9 Biting Force and Endurance in TMD Patients 43 Table 9 Regression analyses between pre-post treatment changes in dental examination and self-report findings and changes in bite task results MMVBF Task Endurance Task Examination and Self-Report Finding Std Coefficient t-value Std Coefficient t-value Unassisted jaw opening without pain Maximum unassisted jaw opening Muscle palpation pain index TMJ palpation pain index * MPI Pain Interference Scale McGill short sorm ** Multiple R-square 0.237* *P.05. **P.01. Similarly, Sinn et al. [3] found that patient levels of bite force increased to nonpatient levels following surgical treatment, particularly at 6- and 12-month follow-up assessments. Bite force improvements in the present study were modest and only occurred among females. Potential reasons for the comparatively modest improvement in bite force following treatment in our study include the relatively brief and conservative nature of our treatments and the assessment of bite force immediately after the posttreatment dental evaluation rather than several months later. In the current study, pretreatment dental evaluation parameters of maximum unassisted jaw opening and joint pain severity predicted performance on pretreatment maximum bite force, but not work. In addition, improvements on joint palpation pain as well as self-reported pain following treatment were associated with increases in posttreatment bite forces. Although muscle pain indices changed markedly as a result of treatment, they were not predictive of biting performance. This suggests that the bite force task may be more sensitive to changes in joint pain compared with muscle pain, which is consistent with several previous studies [3 5], but in contrast with the findings of Clark et al [7], who included only patients with myofascial pain and not TMJ problems in their assessments of biting force and endurance. Our results also differ from those of Widmark et al. [24], who did not find a significant association between maximum unassisted opening and bite force. It is generally accepted that pain is a limiting factor in the performance of bite and endurance tasks [7,12]. Our findings lend further support to this idea, in that treatment was associated with reduction in muscle and joint pain, improvements in jaw opening, self-reported pain (MPQ), and less discomfort during the posttreatment biting task. At posttreatment testing, the female subjects (the majority of our sample) were able to bite harder, and they produced work that approached the levels of female controls. Although changes in clinical joint pain and self-reported pain levels on the MPQ predicted changes in bite force, change following treatment did not predict work on the endurance task. The diversity of methodologies, experimental procedures, and equipment used limit researcher s ability to compare biting force and endurance across studies. Researchers have used a biting fork to measure unilateral force [4,8,10,24,25] and various types of transducers [9,13]. Our apparatus has the advantage of placing the load over a broader area, bilaterally. Our methodology, including apparatus, placement in the mouth, use of protective acrylic, and distance between opposing dentition at bite was similar to that of others [3,7,9,13]. More importantly, the same vertical dimension was used for a subject between the time of their pretreatment and posttreatment testing. There seems to be no gold standard regarding the best method for measuring endurance. Endurance at maximal effort [24], endurance at a standard force level for all subjects [9,11], and endurance at a proportion of the individual s maximum voluntary force [7] have been reported in the literature. The present study included a large sample size of both patients and controls, which enabled us to disentangle the wide performance variability between genders by analyzing males and females separately when necessary. By including a comprehensive set of predictors of bite task performance, comprised of both clinically determined and selfreported symptoms, a more complete view of the pertinent areas associated with the bite task was provided. Our experimental findings are enhanced by the inclusion of EMG and patient reports of effort, pain, and fatigue during the tasks. Random assignment to standardized treatments as opposed to clinically determined treatments of various lengths

10 44 Rudy et al. is an additional strength in terms of study design. However, treatment effects, and possibly bite and work effects, may be stronger when lengthier, multicomponent treatments are provided. One limitation of the present study, which is common to all studies of bite force and endurance, is its potential lack of ecological validity. The artificial nature of the laboratory tasks and the fact that they did not closely simulate masticatory functions such as chewing and swallowing may have adversely affected performance, and limits our ability to make inferences about masticatory function. The discrepancy between sensations produced by biting at the vertical dimension used with the apparatus and the vertical dimension of normal clenching are presumably great, and may have produced an inhibition of performance in general. However, this interocclusal distance is not dissimilar to that reported in other studies [11,14]. An additional limitation is that of the smaller number of males, which reduced the power of the statistical analyses compared with those for female subjects. Although the direction of our findings is consistent with that of most other investigators who assessed changes in bite performance following treatment, it is interesting that pre-post bite force and work differences were modest. There are several possible explanations for this in addition to the questionable ecological validity of the tasks. First, the male subjects in our TMD sample were not significantly different from control males on bite force or work initially, indicating little room for improvement. Either the males had less severe problems, or they viewed the bite tasks more competitively than did female subjects (numerous male, but not female, subjects asked after the task if they were the best yet). However, all subjects rated their efforts during the task as high. Second, posttreatment performance could have been influenced by patients prior experience. Patients motivation may have been affected by factors such as memory of the pretreatment discomfort caused by the tasks. And finally, TMD subject s motivation may also have been adversely affected by factors common to all 3 of the treatments. During treatment, all patients received education on the benefits of jaw rest and avoidance of behaviors that can exacerbate pain, such as clenching and eating hard foods. In addition, patients were encouraged to use discomfort as a cue to discontinue clenching or other parafunctional oral behaviors. This training, although beneficial to patients in terms of self-management and prevention of pain, may have competed with their motivation to perform the biting tasks, particularly the endurance task, which had been rated as most painful pretreatment. Conclusions The objectives of the study were to determine whether RDC/TMD-based examination findings could predict patients performances on bite force and endurance tasks, and whether improvement on specific symptoms led to improvement on bite tasks. Before treatment, bite force and work in female TMD subjects was significantly lower than that of female controls; however, male patients and controls did not differ to a statistically significant degree. Pretreatment dental examination findings predicted patients performance on bite force. Specifically, higher joint pain and smaller maximum unassisted jaw opening were associated with poorer performance on bite force. The brief conservative treatments used in the present study resulted in both self-reported and clinically determined improvements in pain and jaw opening, with 81.8% of patients reporting moderate to major improvement immediately following treatment. Among female TMD subjects, bite force after treatment showed a modest but significant improvement compared to pretreatment levels. However, both male and female TMD subjects were able to complete the posttreatment bite force and endurance assessments with significantly less pain than they had prior to treatment, even though self reported effort and EMG-determined muscle fatigue remained high. These results suggest several possibilities: Pain improvements may not immediately be associated with functional improvements; rather, increases in ability may take months to develop. Alternatively, patients performance may be affected by prior learning. Despite rating their effort as high, patients may have been modifying their behavior due to their knowledge that the tasks had previously caused discomfort, and they had been trained during treatment to avoid oral behaviors that can increase pain. The results of the present study support the use of brief conservative treatments for TMD, such as 4 weeks of flubiprofen, biofeedback, or physical therapy exercises. Clinical and self-reported outcomes were positive. However, the associations between outcomes and bite and endurance task improvements were modest at best. Although perhaps interesting from a research perspective, our modest findings suggest that these laboratory tasks have limited diagnostic utility and may not be as sensi-

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