Physiology and pathophysiology of skeletal muscle contractions. Part I. Dynamic activity

Recent advances in 3D bioprinting of musculoskeletal tissues

The musculoskeletal system is essential for maintaining posture, protecting organs, facilitating locomotion, and regulating various cellular and metabolic functions. Injury to this system due to trauma or wear is common, and severe damage may require surgery to restore function and prevent further harm. Autografts are the current gold standard for the replacement of lost or damaged tissues. However, these grafts are constrained by limited supply and donor site morbidity. Allografts, xenografts, and alloplastic materials represent viable alternatives, but each of these methods also has its own problems and limitations. Technological advances in three-dimensional (3D) printing and its biomedical adaptation, 3D bioprinting, have the potential to provide viable, autologous tissue-like constructs that can be used to repair musculoskeletal defects. Though bioprinting is currently unable to develop mature, implantable tissues, it can pattern cells in 3D constructs with features facilitating maturation and vascularization. Further advances in the field may enable the manufacture of constructs that can mimic native tissues in complexity, spatial heterogeneity, and ultimately, clinical utility. This review studies the use of 3D bioprinting for engineering bone, cartilage, muscle, tendon, ligament, and their interface tissues. Additionally, the current limitations and challenges in the field are discussed and the prospects for future progress are highlighted.

A Revised View of the Dynamics, Physiology, and Treatment of Occlusion: A New Paradigm

A new perspective is proposed regarding the functional dynamics of occlusion, the masticatory muscles, and mandibular placement. Each is analyzed on the basis of a new criterion: force. Stomatognathic function is represented as an ongoing equilibrium maintained between the three-dimensional set of individual occlusal forces and the three-dimensional set of bilateral muscle forces. Mandibular placement is of prime importance, mediating as it does between the two. The conclusion is drawn that occlusion and the muscles can correlate optimally with each other only when the mandible occupies its neuromuscular position (NMP), herein defined as that placement where muscle accommodation is at its absolute minimum. The conclusion is that only the muscles themselves are capable of establishing the NMP and that customary conventional technology is inadequate. A new technology, hydrostatics, is needed to create the special (not existing naturally) occlusal conditions essential to optimally integrating occlusion, mandibular placement, and muscles. Preliminary EMG data is offered in support of this conclusion.

Rotational and translational loading of the temporomandibular joint

Following an introduction to the functional properties of a three-dimensional instantaneous helical axis pertaining to circular (rotatory) and linear (translatory) motions of the mandible, this feasibility study applied the concept of a mandibular average finite helical axis to the maneuver of cyclic opening and closing of the mouth in three healthy subjects. Through the accelerations and decelerations of a mandibular incisor point (instead of a mandibular condylar point) as well as the laws of physics, the kinetic reaction forces and reaction pressures in the upper and lower cavities of the temporomandibular joint (TMJ) were estimated over opening-closing distances of five and ten mm from centric occlusion. The translatory reaction pressures in the upper TMJ cavity (17-29 mm Hg) exceeded the rotatory reaction pressures in the lower TMJ cavity (5-12 mm Hg). The estimated reaction pressures were in close agreement with synovial fluid pressures measured in vivo in the TMJ of humans and pigs, and the biologic significance of frequent and/or prolonged increased TMJ hydrostatic pressures is discussed.

A randomized double-blind clinical trial of the effect of amitriptyline on nocturnal masseteric motor activity (sleep bruxism)

Using a double-blind and randomized experimental design, ten adult subjects with sleep bruxism were administered amitriptyline (25 mg/night) and placebo (25 mg/night), each compound over a period of one week. Neither the intensities and locations of pains nor the nocturnal masseteric electromyographic activities were significantly affected by the tricyclic antidepressant. In fact, intake of amitriptyline was unpredictably associated with either an increase or a decrease in masseteric electromyographic activity (microV.s/min of sleep). On the basis of this study, small doses of amitriptyline cannot be recommended for the control of sleep bruxism and associated discomforts.

Reflex jaw motions and jaw stiffness pertaining to whiplash injury of the neck

Because a so-called mandibular whiplash injury requires the absence of short-latency jaw-closing reflexes in order to explain the postulated mechanism of injury (excessive jaw opening); the authors studied the presence and absence and more importantly, the kinematics (duration, displacement, velocity, acceleration) of monosynaptic and possibly, polysynaptic myotatic (stretch) reflexes in the jaw elevator muscles. In six healthy adults jaw jerk maneuvers were elicited through a brisk tap on the chin, and surface electromyography identified elevator reflexes while translational electrognathography identified the kinematics of the reflexes. The maneuvers were done while maintaining the rest position (3% MVC) and moderate clenching of the teeth (30% MVC). Electromyography was also used to identify phasic elevator excitations during a passive brisk neck extension maneuver. A sudden and unexpected elongation of the jaw elevators released autogenic reflex responses that, in conjunction with augmented tissue elasticity (stiffness), elevated the mandible into centric occlusion within approximately 150 milliseconds. In 86% of trials, the responses occurred regardless of the prevailing resting and clenching contractile activities. There was no evidence of a depressor force that consistently would and could anchor the mandible in a position of extreme or moderate depression, the theoretical linchpin of the mandibular whiplash injury. It was concluded that the mandibular locomotor system is very efficient in maintaining the rest and intercuspal positions of the mandible. This study found no evidence corroborating the mechanism claimed to release a so-called mandibular whiplash injury.

Surface electromyography in the identification of chronic low back pain patients: the development of the flexion relaxation ratio

OBJECTIVE: To develop a reliable and repeatable way to monitor changes in the flexion relaxation phenomenon of the lumbar paraspinal muscles during forward flexion by the development of a flexion relaxation ratio and observation of the sEMG activity in standing and during forward flexion in patients with chronic low back pain (CLBP) and healthy controls. DESIGN: Two experiments were conducted, the first to assess the test-retest reliability of the measure in a group of CLBP (n = 11) patients; the second compared the results between a group of normal healthy controls (n = 20) and a group of CLBP patients (n = 70). RESULTS: Repeated measurements over 4 weeks demonstrated between session reliability of between 0.81 and 0.98 for the dynamic activity. The levels of sEMG activity in the fully flexed position was significantly greater in the fully flexed position in the CLBP group than the controls. The flexion relaxation ratio (FRR), a comparison of the maximal sEMG activity during 1 s of forward flexion with activity in full flexion, demonstrated significantly lower values in the CLBP than the control group. The combined discriminant validity for the FRR for all four sites resulted in 93% sensitivity and 75% specificity. CONCLUSION: The FRR clearly discriminated the patients from the healthy controls. These results indicate that dynamic sEMG activity of the paraspinal muscles can be reliably measured and is useful in differentiating CLBP patients from normal controls. RELEVANCE: Analysis of the pattern of different levels of muscle activity during a forward flexion can be used in CLBP where normalization of the sEMG signal to the maximum voluntary contraction may be difficult. The FRR may be used in the assessment of change in the flexion relaxation phenomenon following treatment interventions.

Bilateral masseteric contractile activity in unilateral gum chewing: differential calculus

Twelve healthy subjects performed 10 s, 15 s, 20 s, and 25 s of right-sided and, subsequently, left-sided gum chewing. The contractile activities of the ipsilateral (chewing side) and contralateral (non-chewing side) masseter muscles, mainly the concentric contractions of the phase of jaw closing and the isometric contractions of the phase of dental occlusion, were recorded through cumulative surface electromyography (EMG). A linear function (y = ax + b) described the association between an increase in the duration (x) of unilateral gum chewing and the cumulative EMG (y) of both the ipsilateral and the contralateral masseter muscle, and because of different slopes (a) of the two straight lines a geometric function (y = aqx) described the progressively larger differences between the paired and straight lines. When differential calculus was applied to the exponential functions, it became evident that the chewing forces generated by the ipsilateral masseter muscle continually exceeded those generated by the contralateral masseter muscle, and that the positive work (force x distance) produced by the concentric contractions of the ipsilateral masseter muscle continually exceeded that produced by the concentric contractions of the contralateral masseter muscle. It was inferred that mechano-physical work plays a major role if clinical muscle fatigue develops during prolonged unilateral gum chewing.

Gum chewing and jaw muscle fatigue and pains

To study possible associations between gum chewing and fatigue and pains in the jaw muscles, eight healthy adults performed prolonged idling, prolonged unilateral chewing of gum, and brief vigorous clenching of the teeth (MVC). Through surface electromyography (EMG), the authors monitored the cumulative (microV.s) as well as the average rates (microV.s-1) of contractile activities in the right and left masseter muscles. During 10 min of idling there was an absence of muscle fatigue and muscle pains when the EMG rates of the right and left masseter muscles were 2% and 3%, respectively, of those required to elicit isometric muscle pains through MVC. During 10 min of right-sided gum chewing at a rate of 1.2 Hz, the majority of subjects (75%) experienced weak jaw muscle fatigue-not jaw muscle pains-when the EMG rates of the right and left masseter muscles were 38% and 19%, respectively, of those required to elicit isometric pains through MVC. In comparison with 10 min of idling, the weak muscle fatigue of 10 min of unilateral gum chewing appeared when the total contractile activities of the right and left masseter muscles were increased by 1664% and 519%, respectively. It seemed as if prolonged unilateral gum chewing and previous pain-releasing MVC caused some sensitization of muscle nociceptors which, in turn, aggravated subsequent isometric jaw muscle pains elicited through MVC. Even though the right masseter muscle was the most frequent site of clinical fatigue and pains, the authors found no evidence supporting the theoretical foundation of the myofascial pain/dysfunction syndrome.

[Present status of cortisone myopathy]

Steroid myopathy is one of the various side effects of prolonged treatments, particularly with the 9 alpha fluorinated steroids. The receptor protein probably plays a major role with a decrease in protein synthesis. There is no mean to detect this myopathy before clinical signs appear. Neither muscular testing nor enzymes nor electromyography give arguments and muscular biopsy is aggressive. Low density scanner and MNR could help diagnosis. Frequency of disease is not known. A total cumulative maintenance steroid dosage of 400 mg can be sufficient. Proximal muscles are usually involved, quadriceps and other pelvic girdle muscles being more severely affected. Fast twitch glycolytic type IIB fibers are particularly susceptible. Physical exercise is effective in preventing myopathy. Nevertheless, no special rehabilitation program has been tested for the moment in human beings.

The effect of litigation status on adjustment to whiplash injury

STUDY DESIGN

This retrospective study examined the effect of civil litigation on reports of pain and disability in chronic pain patients who sustained whiplash injuries after a motor vehicle accident.

OBJECTIVES

To examine the effect of litigation on adjustment to chronic pain.

SUMMARY OF BACKGROUND DATA

A common methodologic weakness with many studies in this area is the composition of the nonlitigant group, which often includes individuals who have completed litigation as well as those who opted not to litigate. This introduces a confound in that litigant and nonlitigant groups differ not only with respect to litigation status but with respect to any factors that predispose one to litigate.

METHODS

Questionnaire data were obtained from 41 patients (current litigants) in the process of litigation and 21 patients (postlitigants) who had completed litigation. Subjects completed self-report measures assessing demographic characteristics, psychological distress, sleep disturbance, employment status, and various pain indices.

RESULTS

There were no significant group differences in demographic characteristics, employment status, or psychological distress. Litigants, however, reported more pain than did postlitigants. Group differences in pain reports remained statistically significant even after controlling for length of time since accident and initial severity of the injuries.

CONCLUSIONS

That litigation status did not predict employment status suggests that secondary gain does not figure prominently in influencing the functionality of these patients. The rather robust effect of litigation status on pain reports is discussed with respect to the potential mediational role of the stress of litigation.

Experimental occlusal interferences. Part V. Mandibular rotations versus hemimandibular translations

Frontal plane mandibular rotations and corresponding hemimandibular translations were studied in vitro by using direct observations of a human cadaver mandible and in vivo by using the indirect observations of rotational electrognathography. A comparison between the two methods showed that rotational electrognathography erred in measuring the clinically relevant hemimandibular translations resulting from mandibular rotations having a unilateral molar point (simulated occlusal interference) as the pivot of frontal plane torque. In vitro frontal plane rotations about a unilateral mandibular molar tooth (simulated occlusal interference) suggested that the resulting hemimandibular upward translations of the lateral portion of the mandibular condyle, contralateral to the molar tooth, would cause considerable compressive loading of the temporomandibular joint disc.

Experimental occlusal interferences. Part IV. Mandibular rotations induced by a pliable interference

In 12 subjects, a pliable, yet unbreakable, intercuspal interference (aluminum shim onlay splint; uniform height of 0.25 mm) was placed between either the right or left maxillary and mandibular second premolars and first molars. During brief and forceful biting (dynamic chewing stroke of about 20 kg force) the interference emulated a semisoft food bolus, and at the end of biting (subsequent static clenching stroke of about 20 kg force) it emulated a rigid metal interference. During dynamic/static biting, rotational electrognathography measured maximum frontal and horizontal plane torque of the right and left mandibular condyles. Eleven subjects (92%) showed frontal plane upward rotation (mean of 1.0 degree) of the condyle contralateral to the interference, and one subject (8%) showed frontal plane upward rotation (0.4 degree) of the condyle ipsilateral to the interference. Two subjects (17%) showed no horizontal plane rotation; seven subjects (58%) showed backward rotation (mean of 0.4 degree) of the condyle contralateral to the interference; and three subjects (25%) showed backward rotation (mean of 0.3 degree) of the condyle ipsilateral to the interference. It is suggested that, in the presence of an occlusal interference, mastication may have both short- and long-term detrimental effects.

Experimental occlusal interferences. Part III. Mandibular rotations induced by a rigid interference

A rigid intercuspal interference (minimum mean height of 0.24 mm) was placed on either the right or left mandibular second premolar and first molar of 12 subjects. During brisk and forceful biting on the interference, rotational electrognathography measured maximum torque of the right and left mandibular condyles in the frontal and horizontal planes of orientation. All subjects showed frontal plan upward rotation (mean of 0.7 degrees) of the mandibular condyle contralateral to the interference. In 33% of the subjects there was no horizontal plane backward rotation. In 58% of the subjects there was horizontal plane backward rotation (mean of 0.5 degrees) of the mandibular condyle ipsilateral to the interference, and in one subject (8%) there was backward horizontal plane rotation (0.1 degree) of the mandibular condyle contralateral to the interference. It was inferred that the masseter muscle, ipsilateral to the interference, generated negative work in order to decelerate frontal plane 'unseating' of the mandibular condyle ipsilateral to the interference. It was inferred that the masseter muscle, contralateral to the interference, produced positive work in order to accelerate frontal plane 'seating' of the mandibular condyle contralateral to the interference. Finally, it was speculated that the impact forces of frontal plane 'seating' of the mandibular condyle, contralateral to the interference, might lead to 'vacuum sticking' of the temporomandibular joint disc because of the formation of negative hydrostatic pressures.

A comparison of the effects of group function and canine guidance interocclusal device on masseter muscle electromyographic activity in normal subjects

This study investigated the role of occlusal balance and canine guidance or group function guidance in masseter muscle function of normal subjects. Two types of interocclusal devices were constructed for each of the 10 subjects. Two ME 1020 EMG analyzers and bipolar Ag/AgCI electrodes were used to record the electromyographic activity of masseter muscles during clenching in centric occlusion, during left and right laterotrusive movements, and during clenching at the extremities of the jaw both with the natural dentition only and with adjusted and intentionally unadjusted interocclusal devices in place. This study demonstrated that there was no difference in masseter muscle electromyographic activity between the use of interocclusal devices designed for canine guidance or for group function guidance in normal subjects. It showed also that altering the occlusal balance significantly reduced muscle activity.

Experimental occlusal interferences. Part II. Masseteric EMG responses to an intercuspal interference

In 12 subjects, a rigid unilateral intercuspal interference (minimum mean height of 0.24 mm) was placed on either the right or left mandibular second premolar and first molar (sagittal physiological equilibrium point of the hemimandibular dental arch). During brisk and forceful clenching on the interference, bipolar surface electromyograms were obtained from the right and left masseter muscles. On the side opposite the interference, myoelectric clenching activity was significantly reduced. Correlation analyses showed that the interference elicited a non-linear (complex) co-ordination of the amplitude, but not the duration, of bilateral masseteric clenching activity, i.e. frequently there was significant motor facilitation on the side of the interference, and significant motor inhibition on the side opposite the interference. Theoretical considerations predicted that brief clenching on the interference would easily lead to frontal plane rotatory motions of the mandible which, indeed, occurred clinically.

The role of psychological factors in chronic pain. I. A half century of study

This review examines a half century of thought about the role of psychological factors in chronic pain. Changing views are discussed, and representative examples of pain research based on psychoanalytic, behavioural, cognitive, and psychophysiological theories are presented and evaluated. The evolution of thought from linear causal models of pain to multicausal explanations provides a conceptual framework for discussion. Studies reviewed show that an earlier concept, based on simple formulations of psychological causation, has been replaced by more comprehensive explanations comprising both physical and psychological influences. Further methodological and conceptual problems are discussed in the second paper of this 2-part review.

Electromyographic changes resulting from orthodontic correction of Class III malocclusion

This study measured the electromyographic (EMG) amplitudes of the interference pattern (IP) and the integrated EMG (IEMG) of the masseter and temporal muscles in 10 children with Class III malocclusion before, during, and after treatment with a myofunctional appliance (the monobloc), and compared the EMG values with those obtained from 10 children with Class I occlusion. The results showed that before treatment the mean amplitude values of IP were significantly higher in the Class III than in the Class I group. The mean amplitude values for IP activity in both muscles decreased during and after treatment; however, mean values of the IEMG activity did not show significant change for either muscle. These results are attributed to changes in the length of the muscle fibres as well as to neural modifications. This study demonstrates a non-invasive method that can be used to study the functional effects of orthodontic treatment.

Methodological observations on positive and negative work (teeth grinding) by human jaw muscles

Through electrognathography and surface electromyography of the right and left masseter muscle (MM), this methodologic study examined the excentric (lengthening) and concentric (shortening) contractions of brief (2000 ms) voluntary teeth grinding performed through canine as well as group function of the opposing teeth. We inferred that the most anterior and superior portions of the right and left MM were lengthened and shortened by at least 4.23 mm (right MM) and 4.82 mm (left MM). For the negative work efforts (excentric contractions) of canine function, the tensions of the ipsilateral MM ranged from 10 to 20% MVC and those of the contralateral MM from 4 to 14% MVC. For the positive work efforts (concentric contractions) of canine function, the tensions of the ipsilateral MM ranged from 11 to 44% MVC and those of the contralateral MM from 20 to 41% MVC. A comparison between canine and group function showed that the negative as well as positive work efforts of group function exceeded those of canine function.

Stimulus-response latencies of two instruments delivering transcutaneous electrical neuromuscular stimulation (TENS)

Stimulus-response latencies of low-frequency transcutaneous electrical neuromuscular stimulation (TENS) were studied in 15 healthy subjects, applying the two different pulse configurations delivered by the Biotens and the Myomonitor instruments. Latencies, in milliseconds, were determined on bipolar raw surface electromyograms (EMG) of the suprahyoid muscles, using the skin surface over the sigmoid notches of the mandible as the site of stimulation. Stimulus-response times were measured from the onset of the stimulus artefact to the first response peak on EMG, and their mean values showed ranges of 3.79-4.49 ms for Biotens and 5.10-5.34 ms for Myomonitor. It was concluded that low-frequency TENS caused direct stimulation of motor nerves, and that the timing of the contraction response was not affected by altered electrode placement, lead-wire reversal or unbalanced (right/left) stimulation.

Observations in the time and frequency domains of surface electromyograms of experimental brief teeth clenching in man

Maximum voluntary teeth clenching was performed for about 1 s to study the interactions between subjective sensory-motor events and changes in the time and frequency domains of surface electromyograms of the masseter and anterior temporalis muscle. Isometric jaw muscle contractions were examined for their speeds of contraction, the total energy content and median frequency of their power density spectra, and for the specific rate of change and the efficiency of their voluntary and involuntary activation efforts. The observations suggested that, in general, brief maximum isometric contractions were not preprogrammed, but rather were regulated by a subconscious proprioceptive feedback mechanism; the mechanism, possibly a transcortical loop, appeared to have a low gain and to be based on recruitment/decruitment of motor units.

Quantitative observations on maximum static work efforts and associated pain of the human masseter muscle

The single case experimental design was used to study day-to-day variations in the onset (PL), tolerance (PT), and intensity (VAS) of masseter muscle pain. Pain was induced by maximum voluntary teeth clenching, with no artificial feedback-control of the level of isometric activity, and static work efforts were quantified by cumulative electromyography. A continual effort to produce maximum static work, about 40% increase from baseline work, elicited in 30-40 s an initial sensation of muscular pain that had an intensity of about 25% of maximum possible score. A further increase in maximum static work effort, about 60% increase from baseline work, caused in about 2 min an intolerable increase in the intensity of muscular pain, about 50% of maximum possible score. Whereas PL showed day-to-day variation, PT and the ratio PL:PT did not. Intensity (VAS) scores and maximum static work efforts showed no day-to-day variations. Onset (PL) and tolerance (PT) showed no linear associations with VAS scores, and VAS scores showed no linear associations with maximum static work efforts; if anything, the latter associations tended to resemble a cubic parabola. Two different central neural processes, not associated with maximum static work efforts in a simple linear manner, might have been instrumental in: (i) the establishment of criteria for onset and tolerance of pain, and (ii) the discrimination of variable levels of pain intensity.

Physiology and pathophysiology of skeletal muscle contractions. Part II. Static activity

An overview is presented of the physiology and so-called pathophysiology of static muscle contractions in the intact living organism. Static muscle activity produces no external mechanophysical work, but is used for fixation. Contingent upon the levels of the generated forces, and the duration of the isometric contractions, static activities give rise to fatigue and pains. The discomforts are viewed as somatic responses that are normally within physiological limits. That is, upon cessation of isometric contractions there is usually rapid recovery from the discomforts, and they do not normally induce chronic lesions.