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The functional significance of centric relation as demonstrated by electromyography of the lateral pterygoid muscles
Complete dentures
The functional significance of cen,ric relation as demonstrated by electromyography of the lateral pterygoid muscles Stephen E. Owens, Jr., D.D.S.,* Robert P. Lehr, Jr., Ph.D.,** and Norman L. Biggs, Ph.D.***
Baylor College o/Dentistry, Dallas, Texas
The location of centric relation, while a major controversy in dentistry, is now accepted by most dentists (1) as the most posterior, superior, and medial unstrained position of the mandible with the condyles in the glenoid fossae and (2) as a position from which lateral movement can be made at any given degree of jaw separation? The controversy involving centric relation relates to whether it is merely a reference point for treatment procedures or whether it exists as a functional position. Sicher 2 discussed the biologic significance of centric relation with regard to the occlusion of natural or restored teeth. He contended that centric relation is the optimal physiologic position with regard to the temporomandibular joints and the teeth and that centric relation is a functional position assumed during mastication, deglutition, and respiration. Woelfel and associates :~ recognized the anatomic-mechanical functions of the lateral pterygoid muscles and studied their relationship to centric relation electromyographically. The results of their study gave evidence that the lateral pterygoid muscles are inactive during a pure hinge movement of the mandible. Naylor, 4 Ramfjord, "~ and Dawson ~ all discussed the sig-nificance of the lateral pterygoid muscles as they relate to mandibular deviation. They contended that the lateral pterygoid muscles are involved in any propulsive displacement of the mandibular condyles out of the glenoid fossae. During the course of one of our electromyographic investigations, it became apparent that the role of the lateral pterygoid muscle in centric relation might be functionally significant. Previous electromyographic investigations of the lateral pterygoid muscles have been limited in number and success due to the relative dif*~Department of Orthodontics. **Associate Professor, Southern Illinois University, School of Medicine, Carbondale, Ill. *~*Professor, Department of Anatomy. 5
6
Owens, Lehr, and Biggs
j. ProsLhet. Dent. January, 1975
Table h Duration of electromyographic activity for elevating and depressing strokes of the chewing cycle
Activity Chewing cycle Elevating stroke Depressing stroke •
•
m
ill
,J
Duration in milliseconds (mean ± &E.) Chew right Chew left 901 ± 99 821 ± 72 456 ± 47 390 ± 34 360 ± 24 315 ± 20 i
i
i i
ficulty of electrode placement and resultant inability of the muscle to perform in a reasonably normal physiologic manner, r-'* Using improved electromyographic techniques now available, the lateral pterygoid muscle and its role in the chewing cycle were investigated. MATERIALS AND METHODS
Data were obtained by means of eight Hewlett-Packard model 350-2700-C highgain preamplifiers, ~ and permanent rccords were produced on a Honeywell Visocorder model 2106.t The preamplifiers were calibrated so that an input of 250 /xv produced a deflection of 1 cm. Bipolar fine-wire electrodes, prepared according to the technique of Basmajian,'" were inserted bilaterally through the oral mucosa into the medial and lateral pterygoid muscles of 14 subjects with Angle Class I occlusion and a full complement of teeth. Insertion of the electrodes into the lateral pterygoid muscles was facilitated by means of a hypodermic needle, 25 mm. in length, which was passed through the oral mucosa just lateral to the maxillary tuberosity in a posterior and superior direction. Then, the needle was removed, leaving the fine-wire electrodes midway between tile origin and insertion of the lateral pterygoid muscle, thereby avoiding the medial pterygoid muscle inferiorly and the temporal muscle superiorly. No attempt was made to differentiate between the two heads of the lateral pterygoid muscle. Insertions of the electrodes for the medial pterygoid muscle were through the apex of the pterygomandibular triangle in a posterior and inferior direction, entering the muscle in the lower one third. The intraoral electrodes were ligated bilaterally to the maxillary first permanent molars with 0.010 inch stainless steel ligature wire. Accuracy of placement of the bipolar fine-wire electrodes was determined on the basis of three criteria. These were: (1) laboratory examination of cadayers noting the anatomic relationships of the muscles in the infratemporal fossa; (2) measurement of the distance between the tuberosity of the maxilla and the neck of the condyle in dried prepared skulls; and (3) pilot electromyographic studies involving these muscles. Each subject was seated in a dental chair inside a copper-shielded cage, and a ~Hewlett-Packard, Palo Alto, Calif. tHoneywell Inc., Denver, Colo.
Functional significance o/ centrw relation
voa,,~,: 3:~ Nttmher 1
7
Table I!. Duration of electromyographic activity for lateral pterygoid muscles for
the chewing cycle (measured in milliseconds)
Ac.tivity i
i
i
ii
i
.
.
.
.
.
Chew right Right Le[t ptery.~oid pterygoid
Chew lelt Right Le[t pterygoid pterygoid
383 ± 28
324 ± 36
351 ± 26
309 ± 30
420 ± 47
462 ± 49
362 ± 28
402 ± 33
157 ± 37
--
--
155 ± 36
129 ± 35
193 ± 45
.
Delay between lateral pterygoid activations at initiation of cycle
Duration of activation of lateral pterygoid muscle Duration of delay between activations of right and left lateral pterygoid muscles in chewing cycle Duration of overlap of lateral pterygoid muscle in elevation
86 ± 15
137 ± 34
.grounding electrode was placed on the subject's wrist. Then, the subject was asked to chew a peanut first on one side of the mouth and then on the other as electromyographic records were made. Data were analyzed in the following manner. The amplitude of activity present in the records was not measured but only the presence or absence of activity in the lateral pterygoid muscles, and the sequential relationship of this activity (in milliseconds) to the elevating musculature was evaluated. The end of the first elevating stroke in the masticatory cycle was used as a reference point to initiate the sequential measurements. Elevation of the mandible was considered to have occurred between the first and last discernible action potentials of 100 /xv or more, measured from peak t o peak, in any of the three pairs of elevating muscles (temporal, masseter, or medial pterygoid muscle). Measurements were made during four chewing strokes on both the right and left sides of each of the 14 subjects. These data were then averaged for the 14 subjects, and the average values for the test group were obtained. RESULTS
The averaged values in milliseconds for each portion of the chewing cycle for both the right- and left-side chewing strokes are shown in Tables I and II. However, a more complete appreciation for the sequential relationship is demonstrated by the composite drawing of a chewing stroke on the right side (Fig. 1). DISCUSSION
Perusal of the composite illustration (Fig. 1) obtained from the data in this investigation reveals several interesting facts with regard to lateral pterygoid muscle activity during the chewing, cycle. The most significant part of the composite illustration is that period during elevation of the mandible in which bilateral activity of the lateral pterygoid muscles is absent. This period of inactivity occurs as the mandible is elevated to effect the crushing of food. This "lateral pterygoid muscle lull" which occurs during the elevating portion of the chewing cycle may have much clinical significance. During this period of inactivity of the lateral pterygoid nmscles in the elevating portion of the chewing cycle,
8
Owens, Lehr, and Biggs
j. Prosthet. Dent. January, 1975
Chewing cycle
Chewing cycle
Left
Lateral pterygoid Right
Lateral pterygoid Elevation
Oepression
Elevation
Depression
SCALE H=20 ms :=100 ms
Fig. I. Bars indicate the composite of the activity of the lateral pterygold muscles during chewing of a peanut on the right side of the dental arch. The box labeled A is the time of inactivity of the lateral pterygoid muscles (lateral pterygoid muscle lull).
the mandibular condyles are under the influence of the elevating musculature. The masseter muscle and its synergist, the medial pterygoid muscle, exert a combined bilateral elevating force on the condyles. The temporal muscle exerts a combined bilateral elevating-retruding force on the condyles. These elevating and retruding vectors of force on the condyles coupled with the inactivity of the lateral pterygoid muscles tend to indicate that, at this point during the chewing cycle, the mandibular condyles are seated in their most posterior, superior, and medial unstrained position or ira centric relation. Given the anatomic characteristics of the glenoid fossae, discs, and the mandibular condyles and the direction of force applied by the elevating musculature, it is puzzling as to how.the mandibular condyles could be situated in any position in the fossae other than a posterior, superior, and medial one. If this position does exist when functional chewing occurs, then it is coincidental with centric relation. This would indicate that centric relation is a functional position, which has been questioned by clinical and scientific investigators for many years. On the basis and interpretation of these results, it appears that the principle of centric relation shoukl be incorporated into treatment procedures involving the gross rearrangement of occlusal surfaces of teeth, either orthodontically or prosthodontically. The incorporation of this principle into treatment is advised, not merely because centric relation is a repeatable reference point but because it exists as a functional entity. Centric relation is the optimal physiologic position from which the functional movements of the stomatognathic system occur. SUMMARY
A study was made using electromyography to determine the activity of the lateral pterygoid muscles during mastication. Resuhs indicated that the lateral gterl,'goid muscles were inactive for a period of time as the mandible was elevated during mastication, l)uring this period of inactivity, tile elevating and retruding muscular forces tended to position the mandible in centric relation. Thus, centric relation is a functional as well as a reference position of mandibular movement.
References 1. Academy of Denture Prosthetics: Glossary of Prosthodontic Terms, J. PROSTliET. DENT. 20: 443-480, 1968.
Volume 33 Number 1
Functional significance o/ centric relation
9
2. Sicher, H.: Biologic Significance of Hinge Axis Determination, J. PROSTtIET. DENT. 6: 616-620, 1956. 3. Woelfel, J., Hickey, J., and Rinear, L.: Electromyographic Evidence Supporting the Mandibular Hinge Axis Theory, J. PROSTHET. DEN]'. 7: 361-367, 1957. 4. Naylor, J. G.: Role of the External Pterygoid Muscles in Temporomandibular Articulation, J. PROST~IET. DENT. 10: 1037-1042, 1960. 5. Ramfjord, S. P.: Dysfunctional Temporomandibular Joint and Muscle Pain, J. PROSTHET. DENT. 11: 353-374, 1961. 6. Dawson, P.: Temporomandibular Joint Pain-Dysfunction Problems Can be Solved, J. PROSTHET. DENT. 29: 100-112, 1973. 7. Ahlgren, J.: Mechanism of Mastication, Acta Odontol. Stand. 24: 50-86, 1966. 8. Carlsoo, S.: An Electromyographic Study of the Activity and an Anatomical Analysis of the Mechanics of the Lateral Pterygoid Muscle, Acta Anat. 26: 339-351, 1956. 9. Moller, E.: The Chewing Apparatus, Acta Physiol. Scand. 69: 1-150, 1966. 10. Moyers, R. E.: An Electromyographic Analysis of Certain Muscles Involved in Temporomandibular Movements, Am. J. Orthod. 36: 481-515, 1950. 11. Zenker, W., and Zenker, A.: Die Tatigkeit des Kiefermuskeln und lhre Elektromyographische Analyse, Ergeb. Anat. EntwickIungsgesch. 119: 174-200, 1955. 12. Basmajian, J. V.: Electromyography Comes of Age, Science 176: 603-609, 1972. DRS. OWENS AND BIGGS BAYLOR COLLEGE OF I~ENTISTRY DALLAS, TEXAS 75226
DR. LEHR SOUTHERN ILLINOIS UNIVERSITY SCHOOL OF MEDICINE CARBONDALE, ILL. 62901
The functional significance of cen,ric relation as demonstrated by electromyography of the lateral pterygoid muscles Stephen E. Owens, Jr., D.D.S.,* Robert P. Lehr, Jr., Ph.D.,** and Norman L. Biggs, Ph.D.***
Baylor College o/Dentistry, Dallas, Texas
The location of centric relation, while a major controversy in dentistry, is now accepted by most dentists (1) as the most posterior, superior, and medial unstrained position of the mandible with the condyles in the glenoid fossae and (2) as a position from which lateral movement can be made at any given degree of jaw separation? The controversy involving centric relation relates to whether it is merely a reference point for treatment procedures or whether it exists as a functional position. Sicher 2 discussed the biologic significance of centric relation with regard to the occlusion of natural or restored teeth. He contended that centric relation is the optimal physiologic position with regard to the temporomandibular joints and the teeth and that centric relation is a functional position assumed during mastication, deglutition, and respiration. Woelfel and associates :~ recognized the anatomic-mechanical functions of the lateral pterygoid muscles and studied their relationship to centric relation electromyographically. The results of their study gave evidence that the lateral pterygoid muscles are inactive during a pure hinge movement of the mandible. Naylor, 4 Ramfjord, "~ and Dawson ~ all discussed the sig-nificance of the lateral pterygoid muscles as they relate to mandibular deviation. They contended that the lateral pterygoid muscles are involved in any propulsive displacement of the mandibular condyles out of the glenoid fossae. During the course of one of our electromyographic investigations, it became apparent that the role of the lateral pterygoid muscle in centric relation might be functionally significant. Previous electromyographic investigations of the lateral pterygoid muscles have been limited in number and success due to the relative dif*~Department of Orthodontics. **Associate Professor, Southern Illinois University, School of Medicine, Carbondale, Ill. *~*Professor, Department of Anatomy. 5
6
Owens, Lehr, and Biggs
j. ProsLhet. Dent. January, 1975
Table h Duration of electromyographic activity for elevating and depressing strokes of the chewing cycle
Activity Chewing cycle Elevating stroke Depressing stroke •
•
m
ill
,J
Duration in milliseconds (mean ± &E.) Chew right Chew left 901 ± 99 821 ± 72 456 ± 47 390 ± 34 360 ± 24 315 ± 20 i
i
i i
ficulty of electrode placement and resultant inability of the muscle to perform in a reasonably normal physiologic manner, r-'* Using improved electromyographic techniques now available, the lateral pterygoid muscle and its role in the chewing cycle were investigated. MATERIALS AND METHODS
Data were obtained by means of eight Hewlett-Packard model 350-2700-C highgain preamplifiers, ~ and permanent rccords were produced on a Honeywell Visocorder model 2106.t The preamplifiers were calibrated so that an input of 250 /xv produced a deflection of 1 cm. Bipolar fine-wire electrodes, prepared according to the technique of Basmajian,'" were inserted bilaterally through the oral mucosa into the medial and lateral pterygoid muscles of 14 subjects with Angle Class I occlusion and a full complement of teeth. Insertion of the electrodes into the lateral pterygoid muscles was facilitated by means of a hypodermic needle, 25 mm. in length, which was passed through the oral mucosa just lateral to the maxillary tuberosity in a posterior and superior direction. Then, the needle was removed, leaving the fine-wire electrodes midway between tile origin and insertion of the lateral pterygoid muscle, thereby avoiding the medial pterygoid muscle inferiorly and the temporal muscle superiorly. No attempt was made to differentiate between the two heads of the lateral pterygoid muscle. Insertions of the electrodes for the medial pterygoid muscle were through the apex of the pterygomandibular triangle in a posterior and inferior direction, entering the muscle in the lower one third. The intraoral electrodes were ligated bilaterally to the maxillary first permanent molars with 0.010 inch stainless steel ligature wire. Accuracy of placement of the bipolar fine-wire electrodes was determined on the basis of three criteria. These were: (1) laboratory examination of cadayers noting the anatomic relationships of the muscles in the infratemporal fossa; (2) measurement of the distance between the tuberosity of the maxilla and the neck of the condyle in dried prepared skulls; and (3) pilot electromyographic studies involving these muscles. Each subject was seated in a dental chair inside a copper-shielded cage, and a ~Hewlett-Packard, Palo Alto, Calif. tHoneywell Inc., Denver, Colo.
Functional significance o/ centrw relation
voa,,~,: 3:~ Nttmher 1
7
Table I!. Duration of electromyographic activity for lateral pterygoid muscles for
the chewing cycle (measured in milliseconds)
Ac.tivity i
i
i
ii
i
.
.
.
.
.
Chew right Right Le[t ptery.~oid pterygoid
Chew lelt Right Le[t pterygoid pterygoid
383 ± 28
324 ± 36
351 ± 26
309 ± 30
420 ± 47
462 ± 49
362 ± 28
402 ± 33
157 ± 37
--
--
155 ± 36
129 ± 35
193 ± 45
.
Delay between lateral pterygoid activations at initiation of cycle
Duration of activation of lateral pterygoid muscle Duration of delay between activations of right and left lateral pterygoid muscles in chewing cycle Duration of overlap of lateral pterygoid muscle in elevation
86 ± 15
137 ± 34
.grounding electrode was placed on the subject's wrist. Then, the subject was asked to chew a peanut first on one side of the mouth and then on the other as electromyographic records were made. Data were analyzed in the following manner. The amplitude of activity present in the records was not measured but only the presence or absence of activity in the lateral pterygoid muscles, and the sequential relationship of this activity (in milliseconds) to the elevating musculature was evaluated. The end of the first elevating stroke in the masticatory cycle was used as a reference point to initiate the sequential measurements. Elevation of the mandible was considered to have occurred between the first and last discernible action potentials of 100 /xv or more, measured from peak t o peak, in any of the three pairs of elevating muscles (temporal, masseter, or medial pterygoid muscle). Measurements were made during four chewing strokes on both the right and left sides of each of the 14 subjects. These data were then averaged for the 14 subjects, and the average values for the test group were obtained. RESULTS
The averaged values in milliseconds for each portion of the chewing cycle for both the right- and left-side chewing strokes are shown in Tables I and II. However, a more complete appreciation for the sequential relationship is demonstrated by the composite drawing of a chewing stroke on the right side (Fig. 1). DISCUSSION
Perusal of the composite illustration (Fig. 1) obtained from the data in this investigation reveals several interesting facts with regard to lateral pterygoid muscle activity during the chewing, cycle. The most significant part of the composite illustration is that period during elevation of the mandible in which bilateral activity of the lateral pterygoid muscles is absent. This period of inactivity occurs as the mandible is elevated to effect the crushing of food. This "lateral pterygoid muscle lull" which occurs during the elevating portion of the chewing cycle may have much clinical significance. During this period of inactivity of the lateral pterygoid nmscles in the elevating portion of the chewing cycle,
8
Owens, Lehr, and Biggs
j. Prosthet. Dent. January, 1975
Chewing cycle
Chewing cycle
Left
Lateral pterygoid Right
Lateral pterygoid Elevation
Oepression
Elevation
Depression
SCALE H=20 ms :=100 ms
Fig. I. Bars indicate the composite of the activity of the lateral pterygold muscles during chewing of a peanut on the right side of the dental arch. The box labeled A is the time of inactivity of the lateral pterygoid muscles (lateral pterygoid muscle lull).
the mandibular condyles are under the influence of the elevating musculature. The masseter muscle and its synergist, the medial pterygoid muscle, exert a combined bilateral elevating force on the condyles. The temporal muscle exerts a combined bilateral elevating-retruding force on the condyles. These elevating and retruding vectors of force on the condyles coupled with the inactivity of the lateral pterygoid muscles tend to indicate that, at this point during the chewing cycle, the mandibular condyles are seated in their most posterior, superior, and medial unstrained position or ira centric relation. Given the anatomic characteristics of the glenoid fossae, discs, and the mandibular condyles and the direction of force applied by the elevating musculature, it is puzzling as to how.the mandibular condyles could be situated in any position in the fossae other than a posterior, superior, and medial one. If this position does exist when functional chewing occurs, then it is coincidental with centric relation. This would indicate that centric relation is a functional position, which has been questioned by clinical and scientific investigators for many years. On the basis and interpretation of these results, it appears that the principle of centric relation shoukl be incorporated into treatment procedures involving the gross rearrangement of occlusal surfaces of teeth, either orthodontically or prosthodontically. The incorporation of this principle into treatment is advised, not merely because centric relation is a repeatable reference point but because it exists as a functional entity. Centric relation is the optimal physiologic position from which the functional movements of the stomatognathic system occur. SUMMARY
A study was made using electromyography to determine the activity of the lateral pterygoid muscles during mastication. Resuhs indicated that the lateral gterl,'goid muscles were inactive for a period of time as the mandible was elevated during mastication, l)uring this period of inactivity, tile elevating and retruding muscular forces tended to position the mandible in centric relation. Thus, centric relation is a functional as well as a reference position of mandibular movement.
References 1. Academy of Denture Prosthetics: Glossary of Prosthodontic Terms, J. PROSTliET. DENT. 20: 443-480, 1968.
Volume 33 Number 1
Functional significance o/ centric relation
9
2. Sicher, H.: Biologic Significance of Hinge Axis Determination, J. PROSTtIET. DENT. 6: 616-620, 1956. 3. Woelfel, J., Hickey, J., and Rinear, L.: Electromyographic Evidence Supporting the Mandibular Hinge Axis Theory, J. PROSTHET. DEN]'. 7: 361-367, 1957. 4. Naylor, J. G.: Role of the External Pterygoid Muscles in Temporomandibular Articulation, J. PROST~IET. DENT. 10: 1037-1042, 1960. 5. Ramfjord, S. P.: Dysfunctional Temporomandibular Joint and Muscle Pain, J. PROSTHET. DENT. 11: 353-374, 1961. 6. Dawson, P.: Temporomandibular Joint Pain-Dysfunction Problems Can be Solved, J. PROSTHET. DENT. 29: 100-112, 1973. 7. Ahlgren, J.: Mechanism of Mastication, Acta Odontol. Stand. 24: 50-86, 1966. 8. Carlsoo, S.: An Electromyographic Study of the Activity and an Anatomical Analysis of the Mechanics of the Lateral Pterygoid Muscle, Acta Anat. 26: 339-351, 1956. 9. Moller, E.: The Chewing Apparatus, Acta Physiol. Scand. 69: 1-150, 1966. 10. Moyers, R. E.: An Electromyographic Analysis of Certain Muscles Involved in Temporomandibular Movements, Am. J. Orthod. 36: 481-515, 1950. 11. Zenker, W., and Zenker, A.: Die Tatigkeit des Kiefermuskeln und lhre Elektromyographische Analyse, Ergeb. Anat. EntwickIungsgesch. 119: 174-200, 1955. 12. Basmajian, J. V.: Electromyography Comes of Age, Science 176: 603-609, 1972. DRS. OWENS AND BIGGS BAYLOR COLLEGE OF I~ENTISTRY DALLAS, TEXAS 75226
DR. LEHR SOUTHERN ILLINOIS UNIVERSITY SCHOOL OF MEDICINE CARBONDALE, ILL. 62901