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Activators: An orthopedic puzzle
Activators: John
J.
Cunat,
D.D.S.,
An orthopedic puxxle M.S.*
Bufnlo, N. Y.
T
his article will discuss some of the pros and cons associated with the use of appliances commonly called “activators,” which are designed to hold the mandible forward in order to correct Class II malocclusions. It will also include a preliminary report on a study designed to assess changes in craniofacial morphology through modification of posture. The value of activators has frequently, but not exclusively, been supported or rejected on the basis of whether or not this repositioning of the mandible has a stimulatory effect on its growth. The crux of the problem lies in one’s concept of whether the areas usually named as growth sites of the craniofacial structures are primary or secondary in nature. That is, little disagreement exists when the head of the condyle, the posterior border of the ramus, and the alveolar process are listed as growth sites of the mandible. The controversy begins when one describes them as being inherently the regulators of the growth process or, conversely, if they are thought to be simply a bony manifestation of some other biologic or mechanical process. If one believes that the prime sites of growth lie within the bones themselves, the use of activators to modify the growth process makes little sense. The most obvious problem is encountered after the mandible is advanced onto the artieular eminence (Fig. 1). If the head of the condyle is a primary growth site, its increments would have to be in precisely the right direction in order to reestablish the original condyle-fossa relationship, Another factor that has to be considered before such a procedure is undertaken lies in the fact that after treatment the articular disc might be forced to occupy a perverted position, either temporarily or permanently. This is because of the anatomy of the structures of the joint. Certain anatomy texts, including that by CunninghamP after describing the insertion of the external pterygoid muscle on the neck of the condyle as well as the anterior surface of the disc, conclude that the disc is, therefore, pulled forward by the contraction of this muscle. Read before the Chicago Association of Orthodontists, March 22, *Chairman, Department of Orthodontics, School of Dentistry, New 16
York
at Buffalo.
1971. State
University
of
Volume Number
65 1
Fig. 1. A semidiagrammatic joint with (A) jaw at rest, B. G.: The Temporomandibular Thomas, Publisher.)
Activators
17
representation of the structures of the temporomandibular (B) mouth open, (C) extreme opening of mouth. (After Sarnat, Joint, second edition, Springfield, III., 1964, Charles C
Sicher,” however, casts doubt on this action because, if it were true, one would then expect to find an antagonistic muscle on the posterior aspect of the disc which would pull it back to its initial position when the condyle retreats to its centric relationship. It is his contention that everywhere else in the body when a muscle contracts to move a part, there is another muscle strategically placed to return the part to its original position. In his section on “Functional Anatomy” in the monograph entitled The Temporomandibular Joint, he states: “It has to be pointed out, however, that the discs passively follow the normal movements of the mandible because they are tightly fixed to the poles of the condyle. If muscular action would be necessary to protract the discs, the lack of a retractor of the discs could not be explained. It is furthermore known that also in the cadaver the discs follow the movements of the mandible though here only passive movements are possible.” Sicher states further that the attachment of the fibrous capsule of the temporomandibular joint is vastly different between the disc and mandibular fossa in comparison with its relationship to the disc and the condylar process of the mandible. The former attachment is loose and flexible, while the latter is snug, so that the spatial relationships of the disc are at the mercy of the condylar position. As the condyle is pulled forward by the external pterygoid muscle, the disc also advances because of its intimate attachment to the condyle. Those fibers of the muscle that attach to the disc are said to be more for the balancing fixation of the disc than as its protractor. Consequently, when the condyle is returned to the fossa, the disc again follows passively, thereby obviating the necessity of a muscle for its retraction. If all of this is true, then it follows that the anterior positioning of the condyle by orthopedic appliances also advances the disc to a relationship well onto the articular eminence. Consequently, even if the condyle grows back into its initial position in the fossa, this would have to be synchronized with a harmonious growth pattern of the disc, capsule, and related anatomic structures if future temporomandibular joint dysfunction is to be avoided. The vulnerability of the structures in this protracted position, even for a brief time interval, is
18
Fig. the 19:
Curd
2.
Internal
view
superficial masseter 55-61, 1961.)
of
mandible muscle
on had
(left)
been
the removed.
normal (After
side Avis,
and
(right) V.:
Am.
the
side
J. Phys.
on
which
Anthropoi.
emphasized by SicheP when he states: “If the jaws are forcibly closed while a piece of food intervenes between the teeth, the position of the condyles and discs on the posterior slope of the articular eminences is in jeopardy.” It therefore follows that if one is to employ -this form of treatment, he must be fully prepared to accept the consequences of creating morphologic havoc if he is incorrect in his contention that these structures are capable of self-adjustment. At the other end of the growth philosophy spectrum lie the several schools of thought which contend that function, in various ways, is the predominant force in molding skeletal form. Representative of this group are such workers as WashburnI and Avis.= In their studies, various muscles which attach to the craniofacial complex are removed or paralyzed and the resultant anatomy is Washburn, 18 for example, removed the temporalis described and interpreted. muscle from rats and, after noting the subsequent absence of development of the mandibular coronoid process at the site of the operation, concluded that lack of muscle tension was responsible for deficient formation of that structure. Similarly, Avis,’ after diagrammatically dividing the mandible into its funetional components, discusses an experiment in which she severed the internal pterygoid and masseter muscles in the rat. When the angle of mandible did not develop subsequently, she concluded that it was because of the absence of functional stimulation (Fig. 2). Boyd2 and his co-workers refute this by reasoning that bone does not develop at the sites where muscles have been removed, not necessarily because of lack of tension but rather because the existence of bone is dependent upon adequate vascularity. They state, further, that since most of the blood vessels that supply these bony parts travel through the muscles, removal of the muscle also eliminates arterial supply to the bone. To substantiate their contention, they removed the attachment of the temporalis muscle in guinea pigs but left the blood supply intact. They concluded, after assessing the development of the coronoid process in operated animals, that the blood supply alone is enough to maintain this morphologic entity. MOS@ has written widelv in support of the concept of functional cranial development. He states that”the head consists of a number of relatively independent components which are divided into two parts. These are, first of all, the functional soft-tissue elements which he calls the “functional matrix” and, second, the bone and cartilage that exist to support a specific functional matrix.
Volume Number
65 1
Activators
19
C Fig. 3. Comparsion of the orientation cranial region (B and D) in the human Human Face, New York, 1968, Harper
of the foramen magnum (A and C) and the anterior (above) and rodent [below). [After Enlow, D. H.: The & Row.)
For example, this concept contends that the growth of the cranial sutures is not primary in nature but, rather, a secondary fill-in response to the growth of the brain. The bony mandible and maxilla are also composed of a number of functional elements but, according to Moss, they, like all classically described osteologic elements, have no biologic reality. Instead, he states that the mandible grows as the orofacial capsule in which it is embedded expands. Thus, the growth of the condylar cartilage is compensatory and, hence, secondary to the expansion of the capsule. The confusion that exists over the nature of growth of the craniofacial structures led me to pursue a method of modifying cranial growth without directly performing surgical procedures on any of its parts. It seemed that if one interferes with muscle function or growth sites in the skull, a variety of interpretations are possible, which could be confusing at best and contradictory at worst. Therefore, the method chosen was that of modifying the posture of an animal and then observing what, if any, impact this had on craniofacial morphology. The assumption of an upright posture in man has been given credit for dramatic morphologic changes on a phylogenetic basis. Enlow, for example, describes the difference in position of the foramen magnum and cribriform plates in the rodent and man with the resultant flexure of the cranial base (Fig. 3). In their monograph entitled The Adaptive Chin, Du Brul and Sicher5 describe, in detail, various phylogenetic modifications in mandibular morphology
20
Am. J. Orthod. January1974
Cunnt
nom0
Hyloboter
Ccbus
Ccrcopithecur
Tupoia
Fig. 4. Changes
in the relationship of border of the ramus in primates. (After Springfield, Ill., 1954, Charles C Thomas,
the symphysis, mandibular Du Brul, E. L., and Sicher, Publisher.)
H.:
plane, The
and posterior Adaptive Chin,
brought about in response to the physiologic implications of postural change (Fig. 4). They describe such structural changes in the mandible as the reduction of the gonial angle with a consequent change in its acuity, the repositioning of the condyle, the increase in angulation between the two halves, and the outward roll of the lower border. It is suggested that these evolutionary changes have been brought about to avoid mandibular impingement on the vital structures in the neck as animals became bipedal. It seemed of interest to explore these phylogenetic changes on an ontogenetic basis. For this purpose, bipedal rats are being produced through modification of a method described by Colton in 1929. By amputation of the forelimbs of these quadrupedal animals, Colton caused them to rise on their hind legs for a substantial part of their locomotion. His early interests, in fact, concerned the examination of the Lamarckian concept of the heritability of traits that he had imposed on his experimental animals. He chose the rat for his studies because it, like man, is a plantigrade animal. That is, it walks with its heels on the ground, in contrast to digitigrade animals which walk with the posterior part of the foot raised.
vo1ums Number
Activators
65
1
Fig. 5. Bipedal
21
rat.
Other worker+ 15,I69ID have used these bipedal rodents in investigations which centered largely around orthopedic research concerned primarily with the spinal column as well as the growth and development of the bone and muscles of the hindlimbs. Some craniofacial changes in this type of laboratory animal have been described by Fenart,’ while Moss,lZ working with rats whose forelimbs had been removed and also with animals whose hindlimbs had been amputated, described the rotation of the otic capsule as a function of posture. The method used in the present study was to force the self-amputation of the forelimbs and tails of Sprague-Dawley white rats at 3 to 10 days after birth. Because of their age, hibernation anesthesia was used. In this method, the rats were placed in glass tubes which were then immersed in a bath of ice water while the open end of the tube was kept above the water line so that the animals could breathe. In the first series of rodents, surgical procedures were performed to remove the tails and the forelimbs at the clavicle, with silk sutures used to close the wounds. The high rate of cannibalism following this procedure made it unfeasible to continue. Therefore, we reverted to a method of self-amputation which, although it does not remove the limb as completely as surgery, does do it without producing bleeding ; perhaps for this reason, cannibalism by the mother was greatly reduced. To accomplish this, several different kinds of cotton and gut sutures were employed. The type which proved to be most successful, however, was a conventional orthodontic elastic ligature. After the animal was anesthetized, this was tied tightly around the forelimbs as close to the shoulder girdle as possible and also around the tail close to the body. The tail was included in the procedure as well as the forelimbs because we wanted to avoid any tripodal effect which the tail might afford with a subsequent modification of posture. Edema was produced distal to the ties, and this added to the security of the ligature. Self-
22
Am. J. Orthod. 1974 January
Cunat
Fig. 6. magnum
Angular (F.M.)
relationship in (A) control
between rat and
the palatal plane (6) bipedal littermate.
(P.P.)
and
plane
of
the
foramen
amputation of the part followed in 4 to 6 days (Fig. 5). Sacrifice began at 4 months and is being continued on the bipedal animals and their littermate controls at l-month intervals. The findings thus far are offered only as a preliminary report, since the study is still in progress and statistical analysis of the data has not begun. I realize that this is a dangerous method of presentation, for my observations may be construed as conclusions. I have purposely avoided any reference to the mandible, since the changes observed here have the most far-reaching implications and, until the study is completed, I think it would be very unfair to make inferences at this time. Of a general nature, however, certain cranial changes have occurred in the bipedal animals which are worthy of note.
Volume Number
65 1
Fig.
Activators
7. Basal
view
of
skull
of control
rat
(C5)
and
bipedal
littermate
23
(BS).
For example, it seems from the evidence available at this time that the foramen magnum has rotated in many of the experimental animals. Representative of these are a biped and a littermate control which show the angular relationship between the palate and foramen magnum to be 106 degrees in the control and 116 degrees in the biped (Fig. 6, A and B). This, of course, is the direction of migration of the foramen magnum phylogenetically, and if these data prove to be valid statistically, the work will support that of Fenart. In addition, posterior cranial base comparisons seem to demonstrate a shortening of this structure which would also be in agreeement with Fenart (Fig. 7). Many more gross and histologic observations are being made. Until the evidence can be weighed, however, further comment seems inappropriate. I have only mentioned the study since the possibilities of change in morphology in a single generation are of great concern in orthopedic procedures. Success, however, with the use of activators is not exclusively dependent on changes in size or form of the bony parts. One other reason offered in support of the method has been advanced by Harvold,s who indicates that if one can control the eruption of the maxillary and mandibular posterior teeth, Class II problems can, in some measure, be corrected by this mechanism alone. That is, Harvold’s studies have indicated that if the maxillary molars are prevented from erupting, and the mandibular molars are free to advance in an occlusal direction, a Class II case may “erupt” into Class I. To illustrate this, 1 have taken the lateral head film of a Class II patient and superimposed upon it another film of the same patient with the mandible dropped and the lower molar erupted in a purely vertical direction (Fig. 8). While it is granted that this amount of probable, the point is that it is possible to eruption may not be the most approach neutrocclusion in this way. As the skeletal pattern varies, so will the
24
Am. J. Orthod.
&mat
Fig. 8. Class II, Division Class I relationship.
January1974
1 case
Fig. 9. Diagrammatic representation grows at end to the right. (After J. & A. Churchill, Ltd.)
illustrating
how
molar
eruption
can
of direction in which periosteum La Croix, P.: The Organization of
assist
in establishing
a
slips (arrow) as bone Bones, London, 1951,
feasibility of correction by this method. However, if activators are constructed with the ability to allow these movements, it would seem that it could add a dimension to the treatment program. A second factor which may allow activators to work without directly modifying the amount of growth concerns the slippage of the periosteum across the surface of a bone. This is described very well in a work by La Croix.l” It is his contention that periosteum does not grow along conventionally with bone growth but, rather, that it is stretched over the surface of the bone as the bone grows at either or both ends. Through this mechanism, La Croix explains the change in spatial disposition of the nutrient canals of a bone (Fig. 9). That is, as growth pushes a bone in a certain direction, the periosteum is held back and slips in the opposite direction, thereby causing the opening of the nutrient canal to face more and more in the direction of the growing end. He uses this reasoning to explain the direction the nutrient vessels travel in long bones, and MosP uses it to explain the change in direction of opening of the mental foramen. That is, this opening in a child faces forward, but as the condylar process grows, whether
Volume Number
66
Activators
1
Fig. 10. Change in direction of opening of mental foramen hood. (After Moss, M. L.: J. Prosthet. Dent. 10: 1149-l 159,
from 1960.)
birth
to 5 years
25
to adult-
primarily or secondarily, the body of the mandible moves forward ; the periosteum is held back, however, so that the direction of opening of the mental foramen is changed to an upward and, finally, a posterior direction (Fig. 10). Whatever the mechanism, slippage of muscles across the surface of the mandible must occur if we consider the natu.re of growth of the lower jaw. Few will deny that the mandible of the infant grows to adulthood by bony apposition in three main locations-the head of the condyle, the posterior border of the ramus, and the alveolar process. Further, bone is resorbed from the anterior border of the ramus, thereby retaining proportionality. Given these factors, it follows that the muscle fibers which attach to the mandibular ramus must also slide across its surface during growth or some fibers would be left behind when its anterior border is resorbed. Further, the posterior additions of bone to the mandibular ramus would be devoid of muscle attachment unless the muscles slid posteriorly. These contentions then bring about the possibility of a mode of action of activators which, through tension, might cause the muscles of the mandible to slide more rapidly and therefore establish a new position of attachment. Much work has to be done to support or refute these considerations. They are simply offered as possibilities.
26
Am. J. Orthod. January 1974
Cunat
Fig. 11. Activator
in position.
Fig. 12. Activator
from
lingual.
Now, if we construct an activator to take advantage of these factors it seems to broaden our base of operation so that we do not have to adhere to a philosophy of growth stimulation in order to try them clinically. Therefore, we are using them on a limited number of patients in the Orthodontic Department of the State University of New York at Buffalo and attempting to evaluate what we are seeing. The appliances are constructed along the lines advocated by Harvolds (Fig. 11). They, therefore, include a small spring against the mesial aspect of the maxillary molar to give it a slight distal thrust and a plastic ledge in contact with the maxillary posterior teeth in order to keep them from moving occlusally. Conversely, the mandibular molars are not covered with plastic, so their eruption will not be prevented. However, a flange does extend vertically from the palate well beyond the levels of the lower teeth to keep the tongue from splaying laterally and possibly partially impeding the eruption of the lower posterior teeth (Fig. 12). The lower jaw is taken forward to stretch the retractor muscles of the mandible isometrically in order to place a force on the upper molars and thus prevent their eruption. Liberal relief is given to the plastic lingual to the upper and lower incisors to minimize the forward force on these teeth. A labial
vozrme
Number
65 1
Activators
27
wire is used for retentive purposes as well as for delivering a lingual force on the upper incisors if reduction of their procumbency is desirable. It is clear that any technique involving such diametrically opposed philosophical schools of thought is bound to create fairly emotional offenses and defenses. If activators have merit, we are being derelict in our duty to our patients if we deny them this opportunity for treatment. If, on the other hand, the long-range risks outweigh the potential for success, the use of activators is a great disservice. I would hope that in the future we can consider and weigh the evidence as it accumulates and base our ultimate acceptance or rejection of the method upon realistic grounds. REFERENCES
1. Avis, Virginia: The significance of the angle of the mandible: An experimental and comparative study, Am. J. Phys. Anthropol. 19: 55-61, 1961. of the temporalis muscle from 2. Boyd, T. G., Castelli, W. A., and Huelke, D. F.: Removal its origin: Effects on the size and shape of the coronoid process, J. Dent. Res. 46: 9971001, 1967. 3. Colton, H. 5.: How bipedal habit affects the bones of the hind legs of the albino rat, J. Exp. Zool. 53: 1-11, 1929. 4. Romanes, G. J. (editor) : Cunningham’s textbook of anatomy, ed. 10, London, 1964, Oxford University Press. 5. Du Brul, E. Lloyd, and Sicher, Harry: The adaptive chin, Springfield, Ill., 1954, Charles C Thomas, Publisher. 6. Enlow, Donald H.: The human face, New York, 1968, Harper & Row. 7. Fenart, R.: Changements morphologiques de l’encephale, chez le rat ampute des membres anterieurs, J. Hirnforsch. 8: 493-501, 1966. 8. Harvold, Egil, P.: Activators, Lectures presented a.t the State University of New York at Buffalo, Sept. 26-27, 1970. 9. Kimura, Fumio, and Suzuki, Ryohei: A study on the bipedal rats as laboratory animals for orthopaedic research, Fukushima J. Med. Sci. 12: 111-120; 1965. 10. La Croix, P.: The organization of bones, London, 1951, J. & A. Churchill, Ltd. 11. Moss, Melvin, L.: Functional analysis of human mandibular growth, J. Prosthet. Dent. 10: 1149-1159, 1960. 12. Moss, Melvin, L.: Rotation of the otic capsule in bipedal rats, Am, J. Phys. Anthropol. 19: 301-307, 1961. 13. Moss, Melvin, L.: The primacy of functional matrices in orofacial growth, Dent. Pratt. 19: 65-73, 1968. 14. Sarnat, B. G., and Laskin, D. M.: In Sarnat, Bernard G. (editor) : The temporomandibular joint, ed. 2, Springfield, Ill., 1964, Charles C Thomas, Publisher. 15. Sato, Yoshishige: Studies on deformation of the spinal column in bipedal mice, Shikoku Igaku Zasshi 15: 1888-1900, 1959. 16. Saville, Paul D., and Smith, R.: Bone density, breaking force and leg muscle mass as
functions of weight in bipedal rats, Am. J. Phys. Anthropol. 25: 35-39, 1966. 17. Sicher, Harry: In Sarnat, Bernard G. (editor) : The temporomandibular joint, ed. 2, Springfield, Ill., 1964, Charles C Thomas, Publisher. 18. Washburn, 8. L.: The relation of the temporal muscle to the form of the skull, Anat. Rec. 99: 239-248, 1947. 19. Yamada, K., Sakamoto, K., Ushikubo, S., and Sato, Y.: Study of intervertebral disc herniation in bipedal rats, Tokushima J. Exp. Med. 7: 93-103, 1960. $36
Capen
Hall
J.
Cunat,
D.D.S.,
An orthopedic puxxle M.S.*
Bufnlo, N. Y.
T
his article will discuss some of the pros and cons associated with the use of appliances commonly called “activators,” which are designed to hold the mandible forward in order to correct Class II malocclusions. It will also include a preliminary report on a study designed to assess changes in craniofacial morphology through modification of posture. The value of activators has frequently, but not exclusively, been supported or rejected on the basis of whether or not this repositioning of the mandible has a stimulatory effect on its growth. The crux of the problem lies in one’s concept of whether the areas usually named as growth sites of the craniofacial structures are primary or secondary in nature. That is, little disagreement exists when the head of the condyle, the posterior border of the ramus, and the alveolar process are listed as growth sites of the mandible. The controversy begins when one describes them as being inherently the regulators of the growth process or, conversely, if they are thought to be simply a bony manifestation of some other biologic or mechanical process. If one believes that the prime sites of growth lie within the bones themselves, the use of activators to modify the growth process makes little sense. The most obvious problem is encountered after the mandible is advanced onto the artieular eminence (Fig. 1). If the head of the condyle is a primary growth site, its increments would have to be in precisely the right direction in order to reestablish the original condyle-fossa relationship, Another factor that has to be considered before such a procedure is undertaken lies in the fact that after treatment the articular disc might be forced to occupy a perverted position, either temporarily or permanently. This is because of the anatomy of the structures of the joint. Certain anatomy texts, including that by CunninghamP after describing the insertion of the external pterygoid muscle on the neck of the condyle as well as the anterior surface of the disc, conclude that the disc is, therefore, pulled forward by the contraction of this muscle. Read before the Chicago Association of Orthodontists, March 22, *Chairman, Department of Orthodontics, School of Dentistry, New 16
York
at Buffalo.
1971. State
University
of
Volume Number
65 1
Fig. 1. A semidiagrammatic joint with (A) jaw at rest, B. G.: The Temporomandibular Thomas, Publisher.)
Activators
17
representation of the structures of the temporomandibular (B) mouth open, (C) extreme opening of mouth. (After Sarnat, Joint, second edition, Springfield, III., 1964, Charles C
Sicher,” however, casts doubt on this action because, if it were true, one would then expect to find an antagonistic muscle on the posterior aspect of the disc which would pull it back to its initial position when the condyle retreats to its centric relationship. It is his contention that everywhere else in the body when a muscle contracts to move a part, there is another muscle strategically placed to return the part to its original position. In his section on “Functional Anatomy” in the monograph entitled The Temporomandibular Joint, he states: “It has to be pointed out, however, that the discs passively follow the normal movements of the mandible because they are tightly fixed to the poles of the condyle. If muscular action would be necessary to protract the discs, the lack of a retractor of the discs could not be explained. It is furthermore known that also in the cadaver the discs follow the movements of the mandible though here only passive movements are possible.” Sicher states further that the attachment of the fibrous capsule of the temporomandibular joint is vastly different between the disc and mandibular fossa in comparison with its relationship to the disc and the condylar process of the mandible. The former attachment is loose and flexible, while the latter is snug, so that the spatial relationships of the disc are at the mercy of the condylar position. As the condyle is pulled forward by the external pterygoid muscle, the disc also advances because of its intimate attachment to the condyle. Those fibers of the muscle that attach to the disc are said to be more for the balancing fixation of the disc than as its protractor. Consequently, when the condyle is returned to the fossa, the disc again follows passively, thereby obviating the necessity of a muscle for its retraction. If all of this is true, then it follows that the anterior positioning of the condyle by orthopedic appliances also advances the disc to a relationship well onto the articular eminence. Consequently, even if the condyle grows back into its initial position in the fossa, this would have to be synchronized with a harmonious growth pattern of the disc, capsule, and related anatomic structures if future temporomandibular joint dysfunction is to be avoided. The vulnerability of the structures in this protracted position, even for a brief time interval, is
18
Fig. the 19:
Curd
2.
Internal
view
superficial masseter 55-61, 1961.)
of
mandible muscle
on had
(left)
been
the removed.
normal (After
side Avis,
and
(right) V.:
Am.
the
side
J. Phys.
on
which
Anthropoi.
emphasized by SicheP when he states: “If the jaws are forcibly closed while a piece of food intervenes between the teeth, the position of the condyles and discs on the posterior slope of the articular eminences is in jeopardy.” It therefore follows that if one is to employ -this form of treatment, he must be fully prepared to accept the consequences of creating morphologic havoc if he is incorrect in his contention that these structures are capable of self-adjustment. At the other end of the growth philosophy spectrum lie the several schools of thought which contend that function, in various ways, is the predominant force in molding skeletal form. Representative of this group are such workers as WashburnI and Avis.= In their studies, various muscles which attach to the craniofacial complex are removed or paralyzed and the resultant anatomy is Washburn, 18 for example, removed the temporalis described and interpreted. muscle from rats and, after noting the subsequent absence of development of the mandibular coronoid process at the site of the operation, concluded that lack of muscle tension was responsible for deficient formation of that structure. Similarly, Avis,’ after diagrammatically dividing the mandible into its funetional components, discusses an experiment in which she severed the internal pterygoid and masseter muscles in the rat. When the angle of mandible did not develop subsequently, she concluded that it was because of the absence of functional stimulation (Fig. 2). Boyd2 and his co-workers refute this by reasoning that bone does not develop at the sites where muscles have been removed, not necessarily because of lack of tension but rather because the existence of bone is dependent upon adequate vascularity. They state, further, that since most of the blood vessels that supply these bony parts travel through the muscles, removal of the muscle also eliminates arterial supply to the bone. To substantiate their contention, they removed the attachment of the temporalis muscle in guinea pigs but left the blood supply intact. They concluded, after assessing the development of the coronoid process in operated animals, that the blood supply alone is enough to maintain this morphologic entity. MOS@ has written widelv in support of the concept of functional cranial development. He states that”the head consists of a number of relatively independent components which are divided into two parts. These are, first of all, the functional soft-tissue elements which he calls the “functional matrix” and, second, the bone and cartilage that exist to support a specific functional matrix.
Volume Number
65 1
Activators
19
C Fig. 3. Comparsion of the orientation cranial region (B and D) in the human Human Face, New York, 1968, Harper
of the foramen magnum (A and C) and the anterior (above) and rodent [below). [After Enlow, D. H.: The & Row.)
For example, this concept contends that the growth of the cranial sutures is not primary in nature but, rather, a secondary fill-in response to the growth of the brain. The bony mandible and maxilla are also composed of a number of functional elements but, according to Moss, they, like all classically described osteologic elements, have no biologic reality. Instead, he states that the mandible grows as the orofacial capsule in which it is embedded expands. Thus, the growth of the condylar cartilage is compensatory and, hence, secondary to the expansion of the capsule. The confusion that exists over the nature of growth of the craniofacial structures led me to pursue a method of modifying cranial growth without directly performing surgical procedures on any of its parts. It seemed that if one interferes with muscle function or growth sites in the skull, a variety of interpretations are possible, which could be confusing at best and contradictory at worst. Therefore, the method chosen was that of modifying the posture of an animal and then observing what, if any, impact this had on craniofacial morphology. The assumption of an upright posture in man has been given credit for dramatic morphologic changes on a phylogenetic basis. Enlow, for example, describes the difference in position of the foramen magnum and cribriform plates in the rodent and man with the resultant flexure of the cranial base (Fig. 3). In their monograph entitled The Adaptive Chin, Du Brul and Sicher5 describe, in detail, various phylogenetic modifications in mandibular morphology
20
Am. J. Orthod. January1974
Cunnt
nom0
Hyloboter
Ccbus
Ccrcopithecur
Tupoia
Fig. 4. Changes
in the relationship of border of the ramus in primates. (After Springfield, Ill., 1954, Charles C Thomas,
the symphysis, mandibular Du Brul, E. L., and Sicher, Publisher.)
H.:
plane, The
and posterior Adaptive Chin,
brought about in response to the physiologic implications of postural change (Fig. 4). They describe such structural changes in the mandible as the reduction of the gonial angle with a consequent change in its acuity, the repositioning of the condyle, the increase in angulation between the two halves, and the outward roll of the lower border. It is suggested that these evolutionary changes have been brought about to avoid mandibular impingement on the vital structures in the neck as animals became bipedal. It seemed of interest to explore these phylogenetic changes on an ontogenetic basis. For this purpose, bipedal rats are being produced through modification of a method described by Colton in 1929. By amputation of the forelimbs of these quadrupedal animals, Colton caused them to rise on their hind legs for a substantial part of their locomotion. His early interests, in fact, concerned the examination of the Lamarckian concept of the heritability of traits that he had imposed on his experimental animals. He chose the rat for his studies because it, like man, is a plantigrade animal. That is, it walks with its heels on the ground, in contrast to digitigrade animals which walk with the posterior part of the foot raised.
vo1ums Number
Activators
65
1
Fig. 5. Bipedal
21
rat.
Other worker+ 15,I69ID have used these bipedal rodents in investigations which centered largely around orthopedic research concerned primarily with the spinal column as well as the growth and development of the bone and muscles of the hindlimbs. Some craniofacial changes in this type of laboratory animal have been described by Fenart,’ while Moss,lZ working with rats whose forelimbs had been removed and also with animals whose hindlimbs had been amputated, described the rotation of the otic capsule as a function of posture. The method used in the present study was to force the self-amputation of the forelimbs and tails of Sprague-Dawley white rats at 3 to 10 days after birth. Because of their age, hibernation anesthesia was used. In this method, the rats were placed in glass tubes which were then immersed in a bath of ice water while the open end of the tube was kept above the water line so that the animals could breathe. In the first series of rodents, surgical procedures were performed to remove the tails and the forelimbs at the clavicle, with silk sutures used to close the wounds. The high rate of cannibalism following this procedure made it unfeasible to continue. Therefore, we reverted to a method of self-amputation which, although it does not remove the limb as completely as surgery, does do it without producing bleeding ; perhaps for this reason, cannibalism by the mother was greatly reduced. To accomplish this, several different kinds of cotton and gut sutures were employed. The type which proved to be most successful, however, was a conventional orthodontic elastic ligature. After the animal was anesthetized, this was tied tightly around the forelimbs as close to the shoulder girdle as possible and also around the tail close to the body. The tail was included in the procedure as well as the forelimbs because we wanted to avoid any tripodal effect which the tail might afford with a subsequent modification of posture. Edema was produced distal to the ties, and this added to the security of the ligature. Self-
22
Am. J. Orthod. 1974 January
Cunat
Fig. 6. magnum
Angular (F.M.)
relationship in (A) control
between rat and
the palatal plane (6) bipedal littermate.
(P.P.)
and
plane
of
the
foramen
amputation of the part followed in 4 to 6 days (Fig. 5). Sacrifice began at 4 months and is being continued on the bipedal animals and their littermate controls at l-month intervals. The findings thus far are offered only as a preliminary report, since the study is still in progress and statistical analysis of the data has not begun. I realize that this is a dangerous method of presentation, for my observations may be construed as conclusions. I have purposely avoided any reference to the mandible, since the changes observed here have the most far-reaching implications and, until the study is completed, I think it would be very unfair to make inferences at this time. Of a general nature, however, certain cranial changes have occurred in the bipedal animals which are worthy of note.
Volume Number
65 1
Fig.
Activators
7. Basal
view
of
skull
of control
rat
(C5)
and
bipedal
littermate
23
(BS).
For example, it seems from the evidence available at this time that the foramen magnum has rotated in many of the experimental animals. Representative of these are a biped and a littermate control which show the angular relationship between the palate and foramen magnum to be 106 degrees in the control and 116 degrees in the biped (Fig. 6, A and B). This, of course, is the direction of migration of the foramen magnum phylogenetically, and if these data prove to be valid statistically, the work will support that of Fenart. In addition, posterior cranial base comparisons seem to demonstrate a shortening of this structure which would also be in agreeement with Fenart (Fig. 7). Many more gross and histologic observations are being made. Until the evidence can be weighed, however, further comment seems inappropriate. I have only mentioned the study since the possibilities of change in morphology in a single generation are of great concern in orthopedic procedures. Success, however, with the use of activators is not exclusively dependent on changes in size or form of the bony parts. One other reason offered in support of the method has been advanced by Harvold,s who indicates that if one can control the eruption of the maxillary and mandibular posterior teeth, Class II problems can, in some measure, be corrected by this mechanism alone. That is, Harvold’s studies have indicated that if the maxillary molars are prevented from erupting, and the mandibular molars are free to advance in an occlusal direction, a Class II case may “erupt” into Class I. To illustrate this, 1 have taken the lateral head film of a Class II patient and superimposed upon it another film of the same patient with the mandible dropped and the lower molar erupted in a purely vertical direction (Fig. 8). While it is granted that this amount of probable, the point is that it is possible to eruption may not be the most approach neutrocclusion in this way. As the skeletal pattern varies, so will the
24
Am. J. Orthod.
&mat
Fig. 8. Class II, Division Class I relationship.
January1974
1 case
Fig. 9. Diagrammatic representation grows at end to the right. (After J. & A. Churchill, Ltd.)
illustrating
how
molar
eruption
can
of direction in which periosteum La Croix, P.: The Organization of
assist
in establishing
a
slips (arrow) as bone Bones, London, 1951,
feasibility of correction by this method. However, if activators are constructed with the ability to allow these movements, it would seem that it could add a dimension to the treatment program. A second factor which may allow activators to work without directly modifying the amount of growth concerns the slippage of the periosteum across the surface of a bone. This is described very well in a work by La Croix.l” It is his contention that periosteum does not grow along conventionally with bone growth but, rather, that it is stretched over the surface of the bone as the bone grows at either or both ends. Through this mechanism, La Croix explains the change in spatial disposition of the nutrient canals of a bone (Fig. 9). That is, as growth pushes a bone in a certain direction, the periosteum is held back and slips in the opposite direction, thereby causing the opening of the nutrient canal to face more and more in the direction of the growing end. He uses this reasoning to explain the direction the nutrient vessels travel in long bones, and MosP uses it to explain the change in direction of opening of the mental foramen. That is, this opening in a child faces forward, but as the condylar process grows, whether
Volume Number
66
Activators
1
Fig. 10. Change in direction of opening of mental foramen hood. (After Moss, M. L.: J. Prosthet. Dent. 10: 1149-l 159,
from 1960.)
birth
to 5 years
25
to adult-
primarily or secondarily, the body of the mandible moves forward ; the periosteum is held back, however, so that the direction of opening of the mental foramen is changed to an upward and, finally, a posterior direction (Fig. 10). Whatever the mechanism, slippage of muscles across the surface of the mandible must occur if we consider the natu.re of growth of the lower jaw. Few will deny that the mandible of the infant grows to adulthood by bony apposition in three main locations-the head of the condyle, the posterior border of the ramus, and the alveolar process. Further, bone is resorbed from the anterior border of the ramus, thereby retaining proportionality. Given these factors, it follows that the muscle fibers which attach to the mandibular ramus must also slide across its surface during growth or some fibers would be left behind when its anterior border is resorbed. Further, the posterior additions of bone to the mandibular ramus would be devoid of muscle attachment unless the muscles slid posteriorly. These contentions then bring about the possibility of a mode of action of activators which, through tension, might cause the muscles of the mandible to slide more rapidly and therefore establish a new position of attachment. Much work has to be done to support or refute these considerations. They are simply offered as possibilities.
26
Am. J. Orthod. January 1974
Cunat
Fig. 11. Activator
in position.
Fig. 12. Activator
from
lingual.
Now, if we construct an activator to take advantage of these factors it seems to broaden our base of operation so that we do not have to adhere to a philosophy of growth stimulation in order to try them clinically. Therefore, we are using them on a limited number of patients in the Orthodontic Department of the State University of New York at Buffalo and attempting to evaluate what we are seeing. The appliances are constructed along the lines advocated by Harvolds (Fig. 11). They, therefore, include a small spring against the mesial aspect of the maxillary molar to give it a slight distal thrust and a plastic ledge in contact with the maxillary posterior teeth in order to keep them from moving occlusally. Conversely, the mandibular molars are not covered with plastic, so their eruption will not be prevented. However, a flange does extend vertically from the palate well beyond the levels of the lower teeth to keep the tongue from splaying laterally and possibly partially impeding the eruption of the lower posterior teeth (Fig. 12). The lower jaw is taken forward to stretch the retractor muscles of the mandible isometrically in order to place a force on the upper molars and thus prevent their eruption. Liberal relief is given to the plastic lingual to the upper and lower incisors to minimize the forward force on these teeth. A labial
vozrme
Number
65 1
Activators
27
wire is used for retentive purposes as well as for delivering a lingual force on the upper incisors if reduction of their procumbency is desirable. It is clear that any technique involving such diametrically opposed philosophical schools of thought is bound to create fairly emotional offenses and defenses. If activators have merit, we are being derelict in our duty to our patients if we deny them this opportunity for treatment. If, on the other hand, the long-range risks outweigh the potential for success, the use of activators is a great disservice. I would hope that in the future we can consider and weigh the evidence as it accumulates and base our ultimate acceptance or rejection of the method upon realistic grounds. REFERENCES
1. Avis, Virginia: The significance of the angle of the mandible: An experimental and comparative study, Am. J. Phys. Anthropol. 19: 55-61, 1961. of the temporalis muscle from 2. Boyd, T. G., Castelli, W. A., and Huelke, D. F.: Removal its origin: Effects on the size and shape of the coronoid process, J. Dent. Res. 46: 9971001, 1967. 3. Colton, H. 5.: How bipedal habit affects the bones of the hind legs of the albino rat, J. Exp. Zool. 53: 1-11, 1929. 4. Romanes, G. J. (editor) : Cunningham’s textbook of anatomy, ed. 10, London, 1964, Oxford University Press. 5. Du Brul, E. Lloyd, and Sicher, Harry: The adaptive chin, Springfield, Ill., 1954, Charles C Thomas, Publisher. 6. Enlow, Donald H.: The human face, New York, 1968, Harper & Row. 7. Fenart, R.: Changements morphologiques de l’encephale, chez le rat ampute des membres anterieurs, J. Hirnforsch. 8: 493-501, 1966. 8. Harvold, Egil, P.: Activators, Lectures presented a.t the State University of New York at Buffalo, Sept. 26-27, 1970. 9. Kimura, Fumio, and Suzuki, Ryohei: A study on the bipedal rats as laboratory animals for orthopaedic research, Fukushima J. Med. Sci. 12: 111-120; 1965. 10. La Croix, P.: The organization of bones, London, 1951, J. & A. Churchill, Ltd. 11. Moss, Melvin, L.: Functional analysis of human mandibular growth, J. Prosthet. Dent. 10: 1149-1159, 1960. 12. Moss, Melvin, L.: Rotation of the otic capsule in bipedal rats, Am, J. Phys. Anthropol. 19: 301-307, 1961. 13. Moss, Melvin, L.: The primacy of functional matrices in orofacial growth, Dent. Pratt. 19: 65-73, 1968. 14. Sarnat, B. G., and Laskin, D. M.: In Sarnat, Bernard G. (editor) : The temporomandibular joint, ed. 2, Springfield, Ill., 1964, Charles C Thomas, Publisher. 15. Sato, Yoshishige: Studies on deformation of the spinal column in bipedal mice, Shikoku Igaku Zasshi 15: 1888-1900, 1959. 16. Saville, Paul D., and Smith, R.: Bone density, breaking force and leg muscle mass as
functions of weight in bipedal rats, Am. J. Phys. Anthropol. 25: 35-39, 1966. 17. Sicher, Harry: In Sarnat, Bernard G. (editor) : The temporomandibular joint, ed. 2, Springfield, Ill., 1964, Charles C Thomas, Publisher. 18. Washburn, 8. L.: The relation of the temporal muscle to the form of the skull, Anat. Rec. 99: 239-248, 1947. 19. Yamada, K., Sakamoto, K., Ushikubo, S., and Sato, Y.: Study of intervertebral disc herniation in bipedal rats, Tokushima J. Exp. Med. 7: 93-103, 1960. $36
Capen
Hall