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Centric occlusion and the free-way space
CENTRIC
OCCLUSION GEORGE Emory
H.
University
AND MOULTON, School
THE B.S., of Dentistry,
FREE-\VAY
SPACE
II.D.S.* Atlmtn,
Ga.
HERE HAS BEEN MUCH in our literature concerning the free-way space and its relationship to all branches of dentistry. The works of iXiswonger,* However, there seems to be Thompson,2 and others would seem to be conclusive. a vacuum existing between this knowledge of the stomatognathic system and its application to successful diagnosis and treatment planning. I will discuss this viewpoint as related to the dentition and crolvn and bridge prosthodontics. We certainly accept the fact of physiologic rest position.” We also know that when the mandible is in physiologic rest position, a space described as the freeway space (interocclusal distance) exists between the mandibular and maxillary teeth. This free-way space, or interocclusal distance, is an all-important criterion upon which any restorative procedure that increases the length of the clinical crowns of a dentition must be based. This same principle applies to the construction of a splint, or to any type of construction within the interocclusal distance. A practical procedure for determining the free-way space is to take two measurements : first, the distance between the nasion and gnathion (or two similar dots or lines placed on the face) while the mandible is in physiologic rest position; and second, the distance between these points when the teeth are in Physiologic rest position is relatively easy to determine. It centric occlusion. is a position of complete relaxation of all structures almut the mandible. A patient can be told to relax the mandible as he does his arm in dropping the arm to his side. Physiologic rest position is assumed also after swallowing, and after the pronunciation of certain words and letters. This measurement should be checked a number of times, but normally it can be done in a matter of a few minutes. Centric occlusion is the initial point of contact of the mandibular with the maxillary teeth when the mandible closes on the hinge axis from physiologic rest position. The distance between the two reference points is measured when the teeth are in contact. The free-way space is the difference between the measurements at physiologic rest position and at centric occlusion. If, for example, the nasion-gnathion measurement in physiologic rest position is 50 mm., and in centric occlusion it is 47 mm., the free-way space is 3 mm. or a normal free-way space. This might be referred to as the approximate minimum free-way space allowance which must be maintained throughout all restorative procedures.
T
Read before the American Academy of Crown and Bridge Prosthodontics, Feb. 4, 1956. Received for publication Feb. 21, 1956, revised by the author Sept. 27. 1956. *Professor and Chairman, Department of Crown and Bridge Prosthodontics.
Chicago,
Ill.,
210
3IOLiI,TON
J Pros. Marcl~.
Den. 1957
To place further emphasis on the accuracy of centric occlusion, we sometimes This emphasizes the importance refer to this position as “true centric occlusion.” of any premature contact on closure from physiologic rest position that results in displacement of the mandible to obtain increased tooth contact. The mandible is still in centric position when this first contact of teeth is made, whether only one cusp of each of two opposing teeth is in occlusion, or whether all teeth make contact in a position of so-called normal occlusion. This first point of contact must not only be noted but it should also be registered, and the study casts should This is the occlusion that occurs when the be mounted from this registration. mandible is in centric relation, and is centric occlusion. The mounted study casts will then allow the operator to duplicate on the articulator the movements of the mandible required to obtain a maximum occlusion. If such a movement be necessary, it is strictly a functional relationship which results from displacement of the mandible. Such a case is described in Fig. 1,A. The functional occlusion (Fig. 1,B) in this case is a Class III relationship, and it points out the obvious error if diagnosis and treatment planning were based on the functional relationship. This also points to the frequent error in diagnosis resulting from articulating study casts instead of determining centric occlusion, and accurately mounting the casts in centric occlusion (Fig. 1,C). I have knowledge of more than one such case in which all the maxillary teeth were extracted and then, upon the recording of centric relation for complete denture construction, it was noted the ridge relationship It is was normal instead of the Class III relationship which had been expected. from such cases that we should be able to appreciate the importance of centric occlusion, and to differentiate from that of functional occlusion and its importance in treatment planning. The case shown in Fig. 1,A was first treated by an orthodontist (Fig. 1,D). The main objective was to move the lower anterior segment distally to allow for a degree of vertical and horizontal overlap, thereby allowing further closure in centric relation. The free-way space as measured from rest position to the first point of confact (centric occlusio~z) prior to orthodontic treatment was inadequate. Orthodontic treatment increased this free-way space to 2.5 mm. At the same time, space was obtained for the replacement of the upper left lateral incisor, and other rotations were accomplished to improve the possible restorative result. The patient was 35 years of age at the beginning of orthodontic treatment, and was treated for a period of 10 months. The rehabilitation by crown and bridge prosthodontics brought the case to a successful conclusion shortly thereafter (Fig. 1,E). It is not the technical procedures accomplished in this case that are important here, but rather the diagnosis of centric occlusion and the relationship of centric occlusion to the free-way space. The treatment plan allowed for adequate free-way space after the restorative procedures were completed. Fig. 2,A shows a case of a lesser degree but of no less importance in treatment planning. Fig. 2,B shows the result after a relatively simple equilibration prior to the construction of a partial denture. Physiologic occlusion has thus been restored. There are many more such cases, but the point is not to miss this important diagnostic approach. The correction of all such malfunctioning occlusions must be a part of treatment planning.
CENTRIC
OCCL’CSIOK
AiVD
FREE-KAY
SI’i\CII
Fig. 1.---A, Premature contact in the anterior region upon closure on the hinge axis into centric occlusion. B, The same patient closed into a functional relationship. Note the anterior C, The study casts mounted in centric occlusion. D, Orthodontic displacement of the mandible. (Courtesy of W. H. Day, Colonel, Dental Corps, United States Army.) E, The treatment. completed case.
212 ERRORS
IN
DImAGNOSIS
We can learn from our errors. One of the errors that has come to our attention is illustrated in Fig. 3,A. This patient had sought dental care because of pain in the temporomandibular joint. The erroneous diagnosis had been made that “the bite was closed.” As a result of the diagnosis, a splint which did not allow nine months, for normal free-way space was placed on the mandibular teeth. Within the posterior teeth had intruded into the bone (Fig. 3,B) so that, when the splint was removed, considerable space existed between the posterior maxillary and
A.
B.
Fig. 2.--A, Premature axis into centric occlusion.
contact on the B, Correction
upper right central is accomplished by
A.
Fig.
3.-A,
A removable splint eliminating the interocclusal posterior teeth caused by the splint built within
incisor simple
upon closure equilibration.
on hinge
B.
distance. B, The intrusion the interocclusal distance.
of the
mandibular teeth when the anterior teeth were in occlusion. The temporomandibular joint pain had decreased for a few weeks after the insertion of the splint, but then it returned with increasing severity. The problems of treatment are now considerably more complex than before as a result of the error in diagnosis and treatment planning. This patient did not have a free-way space in excess of the normal, and any treatment which increases her vertical dimension in centric occlusion is and was contraindicated. The splint was contraindicated.
A similar case is shown in Fig. 4,rl. The metabolic processes could not keep pace with the destructive forces,’ and au acute periodontal condition existed within nine months after the insertion of splints. The splints were removed, and the upper Fig. 4,R shows the space existing in left first and second molars were estractetl. In my opinion, these the posterior region \vhile the anterior teeth are in occlusion. posterior teeth have been intruded by the forces resultin, u from interference with the normal free-\vay space. ‘\\‘hen the metabolic processes cannot keep pace with
B. Fig. 4.--A, Splints cemented teeth do not occlude as the result normal interocclusal distance.
to place of the
within intrusion
the of
interocciusal the teeth
due
distance. to the
R, splint
The posterior being within
the destructive forces, there is a complete periodontal breakdown. The splints were used in this case to simplify the construction of a more esthetic anterior bridge. Kestorations were to have been constructed for all posterior mandibular teeth to decrease the vertical overlap and increase the horizontal overlap. Again, this points to the limitations imposed by the free-way space, and the ultimate failure resulting from making restorations beyond the physiologic limits of the interocclusal distance. A statement is sometimes made to the effect that the restorative pro-
214
MOULTON
J. Pros.
Den.
March,
1957
This would indicate a cedure :s should not open “the bite” beyond rest position. lack o f appreciation of the necessity for a free-way space and the ph ysiology of the m: lsticating apparatus. THE
USE
OF
SPLINTS
T ‘he routine use of splints prior to restoring occlusion by means of crown and bridge : prosthesis can well be questioned. With the knowledge we have concerning A.
B.
Fig in the cedures
5.--A, An upper Imandibular dentition. were accomplished
cast
showing B, The on mandibular
loss of completed dentition.
coronal upper
anatomy. restorations.
A
similar Similar
condi Ition existed rest orative pro-
CENTRIC
OCCLUSION
AKD
FREE-WAY
215
SPACE
the vertical dimension and the free-way space, we can arrive at a treatment plan which is scientifically correct, and need not resort to a “trial and error” method to ‘
We can conclude from the evidence we have, that free-way space is a physiologic necessity. We cannot successfully insert splints, he they removable or fixed, or increase vertical dimension by crown and hridge prosthesis, unless the normal free-way space is maintained after the treatment. Therefore, many cases of mutilated occlusion must be rehabilitated without increasing the vertical dimension of centric occlusion. Establishing a more normal plane of occlusion and restoring a more normal occlusal anatomy will result in bringing the case to a successful conclusion. REFERENCES
1. Niswonger,
M. E. : The Rest Position of the Mandible and Centric Relation, 21: 1572-1582, 1934. 2. Thompson! J. R.: The Rest Position of the Mandible and Its Significance Science, J.A.D.A. 33:151-180, 1946. 3. Thompson, J. R.: Concepts Regarding Function of the Stomatognathic System, 48 :626-637, 1954. 4. Grange>4Fry;;; R. : Functional Relations of the Stomatognathic System, J.A.D.A. , . 106 FORREST AVE., ATLANTA 3, GA.
N.E.
J.A.D.A. to
Dental
J.A.D.A. 48:638-
OCCLUSION GEORGE Emory
H.
University
AND MOULTON, School
THE B.S., of Dentistry,
FREE-\VAY
SPACE
II.D.S.* Atlmtn,
Ga.
HERE HAS BEEN MUCH in our literature concerning the free-way space and its relationship to all branches of dentistry. The works of iXiswonger,* However, there seems to be Thompson,2 and others would seem to be conclusive. a vacuum existing between this knowledge of the stomatognathic system and its application to successful diagnosis and treatment planning. I will discuss this viewpoint as related to the dentition and crolvn and bridge prosthodontics. We certainly accept the fact of physiologic rest position.” We also know that when the mandible is in physiologic rest position, a space described as the freeway space (interocclusal distance) exists between the mandibular and maxillary teeth. This free-way space, or interocclusal distance, is an all-important criterion upon which any restorative procedure that increases the length of the clinical crowns of a dentition must be based. This same principle applies to the construction of a splint, or to any type of construction within the interocclusal distance. A practical procedure for determining the free-way space is to take two measurements : first, the distance between the nasion and gnathion (or two similar dots or lines placed on the face) while the mandible is in physiologic rest position; and second, the distance between these points when the teeth are in Physiologic rest position is relatively easy to determine. It centric occlusion. is a position of complete relaxation of all structures almut the mandible. A patient can be told to relax the mandible as he does his arm in dropping the arm to his side. Physiologic rest position is assumed also after swallowing, and after the pronunciation of certain words and letters. This measurement should be checked a number of times, but normally it can be done in a matter of a few minutes. Centric occlusion is the initial point of contact of the mandibular with the maxillary teeth when the mandible closes on the hinge axis from physiologic rest position. The distance between the two reference points is measured when the teeth are in contact. The free-way space is the difference between the measurements at physiologic rest position and at centric occlusion. If, for example, the nasion-gnathion measurement in physiologic rest position is 50 mm., and in centric occlusion it is 47 mm., the free-way space is 3 mm. or a normal free-way space. This might be referred to as the approximate minimum free-way space allowance which must be maintained throughout all restorative procedures.
T
Read before the American Academy of Crown and Bridge Prosthodontics, Feb. 4, 1956. Received for publication Feb. 21, 1956, revised by the author Sept. 27. 1956. *Professor and Chairman, Department of Crown and Bridge Prosthodontics.
Chicago,
Ill.,
210
3IOLiI,TON
J Pros. Marcl~.
Den. 1957
To place further emphasis on the accuracy of centric occlusion, we sometimes This emphasizes the importance refer to this position as “true centric occlusion.” of any premature contact on closure from physiologic rest position that results in displacement of the mandible to obtain increased tooth contact. The mandible is still in centric position when this first contact of teeth is made, whether only one cusp of each of two opposing teeth is in occlusion, or whether all teeth make contact in a position of so-called normal occlusion. This first point of contact must not only be noted but it should also be registered, and the study casts should This is the occlusion that occurs when the be mounted from this registration. mandible is in centric relation, and is centric occlusion. The mounted study casts will then allow the operator to duplicate on the articulator the movements of the mandible required to obtain a maximum occlusion. If such a movement be necessary, it is strictly a functional relationship which results from displacement of the mandible. Such a case is described in Fig. 1,A. The functional occlusion (Fig. 1,B) in this case is a Class III relationship, and it points out the obvious error if diagnosis and treatment planning were based on the functional relationship. This also points to the frequent error in diagnosis resulting from articulating study casts instead of determining centric occlusion, and accurately mounting the casts in centric occlusion (Fig. 1,C). I have knowledge of more than one such case in which all the maxillary teeth were extracted and then, upon the recording of centric relation for complete denture construction, it was noted the ridge relationship It is was normal instead of the Class III relationship which had been expected. from such cases that we should be able to appreciate the importance of centric occlusion, and to differentiate from that of functional occlusion and its importance in treatment planning. The case shown in Fig. 1,A was first treated by an orthodontist (Fig. 1,D). The main objective was to move the lower anterior segment distally to allow for a degree of vertical and horizontal overlap, thereby allowing further closure in centric relation. The free-way space as measured from rest position to the first point of confact (centric occlusio~z) prior to orthodontic treatment was inadequate. Orthodontic treatment increased this free-way space to 2.5 mm. At the same time, space was obtained for the replacement of the upper left lateral incisor, and other rotations were accomplished to improve the possible restorative result. The patient was 35 years of age at the beginning of orthodontic treatment, and was treated for a period of 10 months. The rehabilitation by crown and bridge prosthodontics brought the case to a successful conclusion shortly thereafter (Fig. 1,E). It is not the technical procedures accomplished in this case that are important here, but rather the diagnosis of centric occlusion and the relationship of centric occlusion to the free-way space. The treatment plan allowed for adequate free-way space after the restorative procedures were completed. Fig. 2,A shows a case of a lesser degree but of no less importance in treatment planning. Fig. 2,B shows the result after a relatively simple equilibration prior to the construction of a partial denture. Physiologic occlusion has thus been restored. There are many more such cases, but the point is not to miss this important diagnostic approach. The correction of all such malfunctioning occlusions must be a part of treatment planning.
CENTRIC
OCCL’CSIOK
AiVD
FREE-KAY
SI’i\CII
Fig. 1.---A, Premature contact in the anterior region upon closure on the hinge axis into centric occlusion. B, The same patient closed into a functional relationship. Note the anterior C, The study casts mounted in centric occlusion. D, Orthodontic displacement of the mandible. (Courtesy of W. H. Day, Colonel, Dental Corps, United States Army.) E, The treatment. completed case.
212 ERRORS
IN
DImAGNOSIS
We can learn from our errors. One of the errors that has come to our attention is illustrated in Fig. 3,A. This patient had sought dental care because of pain in the temporomandibular joint. The erroneous diagnosis had been made that “the bite was closed.” As a result of the diagnosis, a splint which did not allow nine months, for normal free-way space was placed on the mandibular teeth. Within the posterior teeth had intruded into the bone (Fig. 3,B) so that, when the splint was removed, considerable space existed between the posterior maxillary and
A.
B.
Fig. 2.--A, Premature axis into centric occlusion.
contact on the B, Correction
upper right central is accomplished by
A.
Fig.
3.-A,
A removable splint eliminating the interocclusal posterior teeth caused by the splint built within
incisor simple
upon closure equilibration.
on hinge
B.
distance. B, The intrusion the interocclusal distance.
of the
mandibular teeth when the anterior teeth were in occlusion. The temporomandibular joint pain had decreased for a few weeks after the insertion of the splint, but then it returned with increasing severity. The problems of treatment are now considerably more complex than before as a result of the error in diagnosis and treatment planning. This patient did not have a free-way space in excess of the normal, and any treatment which increases her vertical dimension in centric occlusion is and was contraindicated. The splint was contraindicated.
A similar case is shown in Fig. 4,rl. The metabolic processes could not keep pace with the destructive forces,’ and au acute periodontal condition existed within nine months after the insertion of splints. The splints were removed, and the upper Fig. 4,R shows the space existing in left first and second molars were estractetl. In my opinion, these the posterior region \vhile the anterior teeth are in occlusion. posterior teeth have been intruded by the forces resultin, u from interference with the normal free-\vay space. ‘\\‘hen the metabolic processes cannot keep pace with
B. Fig. 4.--A, Splints cemented teeth do not occlude as the result normal interocclusal distance.
to place of the
within intrusion
the of
interocciusal the teeth
due
distance. to the
R, splint
The posterior being within
the destructive forces, there is a complete periodontal breakdown. The splints were used in this case to simplify the construction of a more esthetic anterior bridge. Kestorations were to have been constructed for all posterior mandibular teeth to decrease the vertical overlap and increase the horizontal overlap. Again, this points to the limitations imposed by the free-way space, and the ultimate failure resulting from making restorations beyond the physiologic limits of the interocclusal distance. A statement is sometimes made to the effect that the restorative pro-
214
MOULTON
J. Pros.
Den.
March,
1957
This would indicate a cedure :s should not open “the bite” beyond rest position. lack o f appreciation of the necessity for a free-way space and the ph ysiology of the m: lsticating apparatus. THE
USE
OF
SPLINTS
T ‘he routine use of splints prior to restoring occlusion by means of crown and bridge : prosthesis can well be questioned. With the knowledge we have concerning A.
B.
Fig in the cedures
5.--A, An upper Imandibular dentition. were accomplished
cast
showing B, The on mandibular
loss of completed dentition.
coronal upper
anatomy. restorations.
A
similar Similar
condi Ition existed rest orative pro-
CENTRIC
OCCLUSION
AKD
FREE-WAY
215
SPACE
the vertical dimension and the free-way space, we can arrive at a treatment plan which is scientifically correct, and need not resort to a “trial and error” method to ‘
We can conclude from the evidence we have, that free-way space is a physiologic necessity. We cannot successfully insert splints, he they removable or fixed, or increase vertical dimension by crown and hridge prosthesis, unless the normal free-way space is maintained after the treatment. Therefore, many cases of mutilated occlusion must be rehabilitated without increasing the vertical dimension of centric occlusion. Establishing a more normal plane of occlusion and restoring a more normal occlusal anatomy will result in bringing the case to a successful conclusion. REFERENCES
1. Niswonger,
M. E. : The Rest Position of the Mandible and Centric Relation, 21: 1572-1582, 1934. 2. Thompson! J. R.: The Rest Position of the Mandible and Its Significance Science, J.A.D.A. 33:151-180, 1946. 3. Thompson, J. R.: Concepts Regarding Function of the Stomatognathic System, 48 :626-637, 1954. 4. Grange>4Fry;;; R. : Functional Relations of the Stomatognathic System, J.A.D.A. , . 106 FORREST AVE., ATLANTA 3, GA.
N.E.
J.A.D.A. to
Dental
J.A.D.A. 48:638-