The phenomenon of decreased mandibular arch

width in opening movements

Carl P. Regli, D.D.S.,* and Ellsworth K. Kelly, D.D.S.** University of California, School of Dentistry, San Francisco, Calif.

A lthough from the clinical standpoint we think of bone as rigid and unyielding, McDowell and Reglil measured significant changes in the width of the mandibular arch in extreme opening positions. DuBrul and Siche? and Weinmann and Sicher3 had previously suggested that the obliquely arranged external pterygoid muscles exerted a bending force on the mandible. They stated that the forceful contraction of these two muscles pulled the condyles medially and deformed the mandible. More recently, Osborne and Tomlin4 have corroborated the work of McDowell and Regli using an accurate intraoral electronic measuring device. They obtained somewhat smaller measurements than McDowell and Regli, but in principle sup- ported the original work.

EXPERIMENTAL PROCEDURE

The results from sixty-two patients in the series were determined with the help of three students working independently. Impressions were made of each patient, one with the jaw in the wide open position, and one with the mouth closed as far as the impression tray would permit. Casts were poured in each of the im- pressions.

Occlusal templates of cold-curing acrylic resin were constructed on one cast. Four of the templates were made, one in the first bicuspid, and one in the second molar region on each side. Fine measuring pits were made in the resin in each template before the resin cured. These pits received the measuring stylus of a specially constructed micrometer.

The cross-arch distance was recorded in the bicuspid region on one cast, and the templates were transferred to the second cast, and the difference was recorded. This procedure was repeated in the molar region (Fig. 1) .

Precautions were taken in impression-making and the pouring of the casts to eliminate variables. The water temperature was controlled, the water-powder

*Professor and Chairman of the Division of Denture Prosthesis.

**Associate Professor of Denture Prosthesis.

49

50 Regli and Kelly

Table I Kecordrd dirnmsional changes

Subject

1 2 3 4 5

6

7 H 9

1 (I 11 12 13 14 15 16 17 10 19 20 1 22 23 24 25 26 27 28 29 30 31 32 3 3 34 3 .i 36 3 7 38 39 40 41 42 43 44 45 46 47

/ Contraction in the wide op~?r / Contraction in the wide open I

position in the second j position in the bicuspid re,gion molar region

(mm.) I (mm .)

0.025 0.178 0.05 1 0.076 0.038 0.102 0.023 0.076 o.oou 0.1',2

0.0 I 9 O.lL7 O.I3:!

0.02:3 0.09 0.04:i o.ou 1 0.035 0.14; 0.042 0.1

0.2 0.043 0.18 0.02.i 0.127 0.0 1 ?I 0.048 0.025 0.06-l 0.048 0.165 0.036 0.1 73 0.02n 0.1 1-b 0.036 0. I 49 o.oi-I 0.1 78 0.079 0.178 0.064 0.155 0.048 0.089

0.05 0.025 0.076 0.025 0.1 0.025 0.05 0.025 0.076 0.025 0.05 0.025 0.076 0.015 0.05 0.038 0.05 0.025 0.076 0.025 0.089 0.05 0.076 0.0.51 0.102 0.025 0.05 1 0.05 0.014 0.025 0.051 0.025 0.076 0.013 0.025 0.03 0.076 0.025 0.025 0.051

0.089 0.01 3 0.076

Volume 17 Number I Decreased mandibular arch width in opening movements 51

Table I-Contd

Subject

48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

Contraction in the wide open Contraction in the wide open position in the second position in the bicuspid region molar region

(mm.) (mm.1 -

0.015 0.051 0.025 0.051

- - 0.025 0.051 0.025 0.076 0.051 0.076 0.051 0.165 0.025 0.076 0.025 0.102 0.051 0.127 0.025 0.076 0.051 0.076 0.025 0.102 0.05 0.076

ratio accurately measured, and the same batches of materials were used throughout the tests. The methods used followed clinical and prosthetic laboratory procedures and were not those of the physics laboratory.

RESULTS

Measurable differences demonstrated by our methods lead us to conclude that the width of the mandible changes during opening movements. The changes in dimension are shown in Table I and may have clinical significance.

The average distortion across the mandibular dental arch from first bicuspid to first bicuspid was 0.03 mm. (O.OOl+ inch) and that from second molar to second molar was 0.09 mm. (0.0038 inch). This average includes two subjects that showed no distortion and five that showed distortion in only one of the two regions mea- sured.

DISCUSSION

The amount of mandibular distortion demonstrated is sufficient to affect the fit of a removable partial denture and to put undue stresses on abutment teeth of a bilateral fixed partial denture. Perhaps this phenomenon has not been noticed clinically, since a continuous fixed restoration in the lower dental arch is somewhat rare. Also, the patient is usually relaxed with the mouth almost closed while the impression material is setting. Recently, however, periodontal prosthetics, wherein restoration of the lost teeth is secondary to the splinting of all remaining teeth into one solid unit, has become widespread.

Often proponents of cross-arch bracing and splinting of teeth exhibit photo- graphs and roentgenographs of old-wrought crown-fixed partial dentures that re- store the entire arch on very few remaining abutment teeth, often only the cuspids

52 Regli and Kell)

Fig. 1 7he measuring instrument is adjusted over the cast so that pins on the legs of the instrument fit into the pits in the templates. The micrometer dial on the instrument is then set at zerc~. Then, after this set of templates is transferred to the second cast, the instrument is agairl adjusted in the measuring- pits, and the dial records, in thousandths of inches, the difference in the width of the dental arch. This procedure is repeated for the second stt of templates.

and second molars on each side. These restorations have lasted a long time with

little bone destruction around the roots of the abutments. However, we have not seen one of these restorations for the lower arch. Perhaps the phenomenon of de- creased mandibular arch width in opening movements creates enough stress on abutment teeth with a fixed partial denture to bring about its early failure. This is an avenue for further investigation.

The measurements of our tests, those by Osborne and Tomlin, and those of McJ3owell and Keglis earlier study, record only the linear narrowing of the mandible. However, the mandible is not only constricted but is also torqued because the muscles causing the action are inserted near the condyles. Suc.h action would be more destructive to the tissue around the abutment teeth of a fixed restoration than a simple linear force.

SUMMARY

Recent measurements indicate that deformation of the mandible occurs during opening movements. This important phenomenon is of considerable clinical sig- nificance.

Further investigation, with a measuring apparatus rigidly attached to the teeth and possibly using strain gauges attached to fixed restorations, is indicated. Meanwhile, it is advisable not to make mandibular impressions with the mouth wide open and to question the advisability of rigidly joining the lower teeth to- gether with a cross-arch splint.

We are indebted to Richard C. Jenninss, D.D,S., Reynold Dean Robinson, D.D.S., and Arthur Rabitz, D.D.S., who worked on this project while they were student research assistants.

Volume 17 Number I Decreased mandibular arch width in opening movements 53

References

1. McDowell, J. A., and Regli, C. P.: A Quantitative Analysis of the Decrease in Width of the Mandibular Arch During Forced Movements of the Mandible, J. D. Res. 40: 118% 1185, 1961.

2. DuBrul, E. L., and Sicher, H.: The Adaptive Chin, Springfield, Ill., 1954, Charles C Thomas.

3. Weinmann, J. P., and Sicher, H.: Bone and Bones, Ed. 2, St. Louis, 1955, The C. V. Mosby Company.

4. Osborne, J., and Tomlin, H. R.: Medial Convergence of the Mandible, Brit. D. J. 117: 112-114, 1964.

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