The morphological effects on the rat mandibular condyle of section of the lateral pterygoid muscle

European Journal of Orthodontics 5 (1983) 315-321© 1983 European Orthodontic Society

The morphological effects on the* ratmandibular condyle of section of thelateral pterygoid muscle

M. Goret-Nicaise, M. Awn and A. DhemLouvain, Belgium

Summary. The effect of section of the lateral pterygoid muscle on the morphology of themandible was studied in 32 ten-week-old Sprague-Dawley rats of both sexes. The rats weredivided into four groups: control, sham-, unilaterally- and bilaterally-operated. Some of therats received tetracycline after operation and alizarin before sacrifice.

All the operated rats with section of the lateral pterygoid muscle showed resorption of theanterior border of the mandibular condyle and growth at the posterior border, confirmed byfluorescent microscopy. Both resorption and bone formation led to deformation of mandibularcondyle.

Introduction

Durkin, Heeley and Irving (1979), in a recentreview of current literature on the temporo-mandibular joint, summarize existing know-ledge of the growth of the condyle "Thecontribution of the condylar cartilage to thegrowth of the mandible has been debatedfor a considerable length of time with verylittle progress being made towards a totalconcept, leaving many questions unansweredor debatable".

One factor which appears to influencethe activity of the condylar cartilage is thelateral pterygoid muscle. Petrovic andStutzmann (1972) conclude, from the reduc-tion in the number of cells undergoingmitosis, that cutting the lateral pterygoidmuscle reduces the growth of the condylarcartilage. In the present study this experimentwas repeated but the growth of the mandibularcondyle was examined by microscopy underultra-violet light after administration offluorescent markers to show calcification.The observations indicate that section of

the lateral pterygoid muscle does not leadto a diminished growth rate but producescomplex deformations of the condyle.

Material and methods

The investigation was carried out on 3210-week-old Sprague-Dawley rats of bothsexes (Table 1).

Table 1

Number of rats

unilaterallysham operated bilaterally

controls operated (right) operated

Study ofdecalcifiedmandible 4 J 4 /

Study ofundecalcifiedmandible 10 4 6 4

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316 LATERAL PTERYGOID SECTION

The condylar process was exposedusing a pre-auricular approach and thelateral pterygoid muscle was cut at itsmandibular insertion. The muscle, fascia andskin were individually closed with sutures.

To study undecalcified bone, 50 mg/Kgtetracycline was injected intraperitoneallyimmediately after the operation and 60 mg/Kgalizarin red S was injected 6 hours beforesacrifice. All the rats were sacrificed 15 daysafter operation.

The heads were prepared and embeddedin methyl methacrylate and then cut intosections 120 microns thick without decalcifica-tion.

After reduction to a uniform thicknessof 80 microns by manual grinding eachsection was placed on a fine grain spectro-scopic plate and exposed to long wave-length X-radiation at 13 KV and 18 mA for15 minutes with a film focus distance of61 mm, and 45 minutes at 106 mm.

The sections were examined under afluorescent-light microscope prior to stainingwith Alcian blue—periodic acid Schiff tech-nique (Herlant, 1960).

For the control and unilaterally operatedrats decalcified paraffin sections were alsoprepared.

The rats were given ground food on theday of the operation and their usual food onthe following days. Five of the 10 unilaterally-operated rats increased in weight, thebilaterally-operated rats decreased in weight;3 of the sham-operated rats increased inweight and 2 decreased.

Observations

Microradiographs of sagittal sections of theright temporo-mandibular joint in uni-laterally-operated and control rats are shownin Figures 1-7. The effects of cutting thelateral pterygoid muscle can be seen inFigure 4 where the condyle is surrounded bya calcified plate that is rTearly continuous butof variable thickness. The condyle is betterdelineated than that of the control animal(Fig. 3) which appears to be formed by

calcified pillars whose extremities are clearlyseparated from each other in the areaadjacent to the articular cavity.

The shape of the condyle is altered onthe operated side. The curve between thetwo white arrows in Figure 4 represents aregion of excessive growth compared withthe same region in the control rat. At theanterior side, however, the calcified pillarsare shorter than normal (Fig. 5) with manyHowship's lacunae, corresponding to activeor recent osteoclastic resorption.

In the bilaterally-operated animals,excess of growth was observed at the posteriorborder on one or both sides. Resorption ofthe anterior border of the mandibularcondyle was regularly observed.

Fluorescent microscopy was used tostudy the growth changes in the intermediatearea.

In the control animals the osteocartila-ginous pillars are vertically orientated andincrease in width with increasing distancefrom the articular surface (Fig. 6) whereasin the animals whose lateral pterygoid musclehad been sectioned the pillars had no specificorientation (Fig. 7).

Fluorescent markers permit the apposi-tion and cartilaginous calcification to bedated. The tetracycline administered im-mediately after surgery appears white in theillustrations showing the outline of thecalcified cartilage at the time of the firstlabelling. No trace of tetracycline wasobserved in the pillars above the labelledregion (Fig. 6B and 7B) but the tops of thepillars were marked by alizarin red S whichappears red under direct examination andlight grey in the photographs.

In Figure 7B the traces of alizarinlabelling are more numerous than in thecontrol animal indicating that, at thatmoment, the ossifying tissues were morenumerous in the operated side than in thecontrol animal. The cartilaginous pillars inthe operated and control animals were ofapproximately the same length in the areasconsidered.

The illuminated areas with blurred out-lines corresponded to primary fluorescence

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Figure 1 Microradiograph of a sagittal section fromthe right temporo-mandibular joint of a control rat.(x4).

T: temporal boneMC: mandibular condyleAP: angular processA: alveoli.

Figure 2 Microradiograph of a sagittal section fromthe right temporo-mandibular joint of a unilaterallyoperated rat (x4).

T: temporal boneMC: mandibular condyleAP: angular processA: alveoli.

Figure 3 Magnification of the area reproduced in figure 1 (xl5). The frame delineates the area photo-graphed in figure 6A.Figure 4 Magnification of the area reproduced in figure 2 (Xl5). The right frame delineates the areaphotographed in figure 5. The left frame delineates the area photographed in figure 7A.Figure 5 Magnification of the anterior border of the condylar process (x91).

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7c x105

IFigure 6A Magnification of the area delineated by a frame in figure 3 ( x 105). The white asterisk designatesthe most important area for analysis.Figure 6B Image, under ultra-violet light, of the area which is shown in figure 6A (xl05). The whiteasterisk designates the most important area for analysis.Figure 6C Photograph of the same area as figure 6B after staining by Alcian blue—P.A.S. ( x 105).Figure 7A Magnification of the area delineated by the left frame in figure 4 (X105). The white asteriskdesignates the most important area for analysis.Figure 7B Photograph, under ultra-violet light, of the area shown in figure 7A (xl05). The whiteasterisk designates the most important area for analysis.Figure 7C Photograph of the same area as figure 7B after staining by Alcian blue—P.A.S. ( x 105).

of the vessels and to a redistribution oftetracycline liberated by resorption.

The appearance of Figures 6B and 6cis characteristic of endochondral ossification(Coutelier, Dhem and Vincent, 1963).

Discussion

The microradiographs of the control ratshow that the condylar process grows byendochondral ossification. The micrographicappearance of the mandibular condyle iscomparable to the non-growing extremity ofa small long bone (Dhem, Goret Nicaise,

1979), illustrated by Lacroix, Dhem andVincent (1969), and not to a growth cartilage.

Unilateral section of the lateral pterygoidmuscle is followed by marked deformationof the homolateral mandibular condyle, withincreased growth at its posterior border,resorption of the anterior border and forma-tion of a calcified cartilage plate in theintermediate area.

No changes were observed in the sham-operated rats.

In the urulaterally-operated rat theintermediate part undergoes morphologicalchanges as well as the posterior and anteriorparts. In regions of comparable thickness

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there is an increase in osteogenesis andendochondral bone formation. This can becompared with the appearance (Dhem, 1961),after section of the Achilles tendon, of acuteosteoporosis of the calcaneus which wasshown by autoradiographic study to be theresult of an increase of both osteoclasticresorption and osteogenesis.

According to Geiser and Trueta (1958)this acute osteoporosis is due to vascularstagnation consequent upon immobilization.In our investigation vascular stagnation, butnot osteoporosis, was observed.

Petrovic et al. (1972, 1975) analysed theeffects of cutting the rat lateral pterygoidmuscle by determining the number ofmitoses, identified by tritiated thymidineadministered one houi before sacrifice, andconcluded from the diminished number ofmitoses that section of the muscle results in adecrease in the growth of the condyle.

The present study shows, however, thatoverall numerical data do not providesufficient information about all the morpho-logical changes in the mandibular condyleafter cutting the lateral pterygoid muscle.The mitotic index of the prechondroblasts atany given moment is a measure of instan-taneous multiplication of the cell populationand gives only indirect information about therate of growth.

Moreover, as shown by Koskinen (1977),after unilateral section of the temporal andmasseter muscles there is an important time-dependent variation in cell proliferation asstudied by tritiated thymidine labelling. Thenumber of labelled cells increased 2 daysafter the operation but dropped below thenumber of labelled cells in the control animalafter 5 days. In the same preparationKoskinen measured the bone formation bytritiated glycine and observed enhanced boneformation during the post-operative period.Thus, although evaluation of the labelledcells after 15 days by Petrovic et al. (1972)showed diminution of the mitoses in theoperated group this did not necessarilymean that growth had slowed.

The physio-pathological conditions ofKoskinen's study are similar to those of the

present investigation of the condyle and theconclusions are in accord, but differ fromthose of Petrovic et al. (1972).

New bone formed from the time of theoperation must clearly be measured. Theuse of fluorescent markers is the most precisemethod of determining the rate of calcifica-tion or of osteogenesis (Harris, 1960).However, this method is open to criticismbecause of toxicity of tetracycline (Filippiand Mela, 1957). Since this toxicity dependson the type of tetracycline and on the lengthof treatment it presumably affects control,sham- unilaterally- and bilaterally-operatedrats equally, and does not influence thecomparability of the results. Alizarin red wasonly used at the end of the experiment andits toxicity (Paff and Eksterowicz, 1950)is not relevant in this study.

The deformation of the mandibularcondyle after section of the lateral pterygoidmuscle could be compared with a mech-anically induced osteoarthritic state. Thedeformations could be regarded as anadaptive stabilization by resorption of theanterior border and increased growth of theposterior border but if this is true the roleof the lateral pterygoid muscle on thetemporo-mandibular joint must be revised.

Acknowledgements

This work was supported by a grant of the"Fonds de la Recherche ScientifiqueMedicale" (F.R.S.M., Brussels, Belgium).

We wish to thank Mrs Deserranno, MrsDemirci, Mrs Deridder, Mrs Dzelzgalvis andMiss Coumans for their devoted assistance.

Address for correspondence

Dhem, A., Professor of Human Anatomy,Unite d'Anatomie humaine,Tour Vesale,Avenue Em. Mounier 52,1200 Bruxelles (Belgium).

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References

Coutelier, L., Dhem, A. and Vincent, A.(1963). La microscopie de fluorescencedans l'etude de l'ossification endochondrale—Bulletin de I'Academie Royal de Mede-cine Belgique. Vile serie, 3: 675-689.

Dhem, A. (1961). Recherches experimentalessur Posteoporose aigue. Annals of Anatomyand Pathology, 6: 497-501.

Dhem, A. amd Goret-Nicaise, M. (1979).Role du cartilage condylien dans lacroissance mandibulaire. Archives ofAnatomy, Histology and Embryology, 62:95-102.

Durkin, J., Heeley, J. and Irving, J. (1979).Cartilage of the mandibular condyle. In:Ed. Munksgaard. Temporomandibular jointFunction and Dysfunction. Copenhagen,Denmark, 457 pp.

Filippi, B. and Mela, V. (1957). Malfor-mazioni congenite facciale e degli artida tetraciclina. Minerva Chirurgica, 12:1106-1110.

Geiser, M. and Trueta, J. (1958). Muscleaction, bone rarefaction and bone forma-tion. An experimental study. Journal ofBone Joint Surg. 40 B/2: 282-311.

Harris, W. H. (1960). A microscopic methodof determining rates of bone growth.Nature, 188: 1038-1039.

Herlant, M. (1960). Etude critique de deuxtechniques nouvelles destinees a mettreen evidence les differentes categories cellu-laires presentes dans la glande pituitaire.Bulletin de Microscopie Applique 3:153-162.

Koskinen, L. (1977). Changes after unilateralMasticatory Muscle Resection in Rats. AMicroscopic Study. Proceedings of the Fin-nish Dental Society, 73, Suppl. 10-11: 1-80.

Lacroix, P., Dhem, A. and Vincent, A. (1969).Anatomie radiologique generate dusquelette. In: R. Trial (ed.). Traite deradiodiagnostic, Tome 10, Paris, Masson,1-36.

Paff, G. H. and Eksterowicz, F. C. (1950).The selective stoppage of bone growth intissue culture. Anatomical Record. N. Y.,108: 44-55.

Petrovic, A. and Stutzmann, J. (1972). Lemuscle pterygoidien externe et la croissancedu condyle mandibulaire. Recherchesexperimentales chez le jeune rat. Ortho-dontie Francaise. 43: 271-283.

Petrovic, A., Stutzmann, J. and Oudet, C.(1975). Control processes in the post-natalgrowth of the condylar cartilage of themandible. In: J. A. McNamara, Jr. (Ed.).Determinants of Mandibular Form andGrowth, Monograph no. 4, Granio-facialGrowth Series,, Center for Human Growthand Development, The University ofMichigan, Ann Arbor, U.S.A., 101 pp.

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The morphological effects on the rat mandibular condyle of section of the lateral pterygoid muscle