E$ects of mandibular hyperpropulsion on the prechondroblastic zone of young rat condyle

Jean-Paul Charlier, D.D.S., Alexandre Petrovic, M.D., D.Sc., and

Jeanne Herrmann-Stutzmann, LSc. Strasbourg, France

In a previous article1 we reported that, as evidenced in organ culture, the spheno-occipital synchondrosis and the cartilage of the nasal septum, where growth takes place by the division of differentiated chondroblasts, manifest an independent growth potential but that the calvaria and facial sutures, where growth occurs by the division of young connective cells, do not.

We also reported that only when growth takes place by proliferation of young connective cells are the mechanical forces able to stimulate or inhibit it.?

In the present article we will describe research conducted to study the effects of an orthopedic treatment type of hyperpropulsion in young rat con- dylar cartilage, in which growth occurs mainly by mitoses of the young cells in the prechondroblastic zone and only to a minor degree by mitoses of the differentiated cells of the chondroblastic zone.

Methods

Eight litters of 4-week-old rats (Sprague-Dawley strain), six in each, were ancst,hetizcd for 4 hours each day with Nembutal. One half of the animals in each litter wore a device which put the mandible in hyperpropulsion, whereas the other half served as controls. The mandible is put in hyperpropulsion by means of a wire-net intraoral dericc which keeps the lower incisors ahead of the upper incisors. The device is made fa.st by a strip set around the snout so that it keeps the mouth slightly open.

After 4 weeks, the mandibular condyles wcrc fixed in 10 per cent formalin, demineralized with EDTA, sectioned at 5 microns, and stained with hematoxy- lin and eosin, Van Giesons, Mallorys, P.A.S., toluidinc blue, and alcian blue stains.

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72 Charlier, Petl-ovic, and Berrnza?l?a-l\t,lLtgnzcrll,r

Results

When compared to the controls, the condyles of the rats subjected to hg-per- propulsion showed the following characteristics :

1. The articular disc was significantly thiclrcr. 2. The articular zone was distinctly higher and the cells were rounder. 3. The prechondrololastic zone (the so-eilllcd inkmediate zone) was

significantly elongated (Figs. 1 and 2). This certainly results from an increased cell proliferation, as evidcncctl by a statistically significant rise in the number of cell divisions as reported in a previous article. Individ- ually, cells generally ha\-e a slightly larger and clearer nucleus. (Mean surface increase is about 12 per cent.)

Fig. 1

Fig. 2

Figs. 1 and 2. Condyles of control and treated rats. These rats came from the same litter,

one (Fig. 1) as a control and the other [Fig. 2) being put for a J-week period under man-

dibular hyperpropulsion. The above figures show sagittal sections through the condyles. A comparison of the two figures shows that the prechondroblastic zone in the treated rat is

significantly increased in length.

4. The chondroblastic zone becomes also elongated, but the comparison of results obtained after 1 to 4 weeks seems to favor the interpretation that one is dealing here with a phenomenon secondary to the increased act,ivity of the prechondroblastic zone.

5. The angle drawn through the condyle to the inferior border of the mandible in the treated animal is 3 degrees larger. At the same time, as evidenced by tetracycline labeling, the periosteal bone apposition on the posterior border and the rear of the inferior border of the mandible was increased.

Discussion

The problem of whether the condylar cartilage actually responds to ortho- dontic therapy, although fundamental, remains unanswered. Some evidence has been reported in its favor, the most significant being that of Baume and L)erichsweiller3 and H~ffer.~ However, most of the authors think that more in- formation is needed before any firm conclusion can be drawn.

Experiences reported here show that an orthodontic treatment type of hyperpropulsion does hare an action on condylar cartilage growth and that this action consists primarily of stimulation of mitotic activity of the pre- chondroblasts. In other words, mechanical forces do not play a role in the growth of cpiphyseal cartilage of long bones, but, they definitely do in the growth of condylar cartilage.

Ontogenetically, the explanation for this difference might bc found in the modalit,& of growth, which is different in both cases-exclusively interstitial in long bones and mainly appositional and only slightly interstitial in condylar cartilage. In this connection, young cells of the prechondroblastic zone may be compared to the young cells of periosteal ossification, and it is generally agreed that mechanical factors can stimulate osteogenesis in periosteal boric.

Phylogenetically, the explanation for this peculiar responsiveness to me- chanical force might be found in the fact t,hat the temporomandibular joint in mammals appeared as the result of a secondary (in a way, adaptive) joining of the squamosal and the dentary bones, along with the subsequent formation of a eondylar cartilage.

Conclusion

Our short-term experiments in young rats show that hyperpropulsion brings about an additional growth of condylar cartilage by stimulating the prechon- droblastic zone cells.

They also establish the existence of a causal relationship between the intensity of condylar growth and the amount of periostcal bone apposition on the posterior border and the rear of the inferior border of the mandible.

REFERENCES

1. Petrovic, A., and Charlier, J. P.: La synchondrose sphbno-occipitale de jeune rat en culture dorganes-mise en evidence dun potentiel de croissancc indkpendant, Compt. rend. Acad. SC. 265D: 1511-1513, 1967.

2. Charlier, J. P.: Les facteurs mkaniques dans la croissance de larc basal mandibulaire 2

la lumierc dc lnnalyse des cnrnct&res struct.ur:lns et ilcs propri6t6s biologiqucs ilu cnrtilngo condylicn, Orthodont. franc. 38: 177-184, 1967.

3. Ihume, J. CL. and Ik~ri~lrswrillrr, II. : Is the c:o~~d~l~r c:krt,ilagc growth cwfer rcsponsivc 10 ortllodont ic tllrrapy? Oral Surg., Oral Med. & Orxl I:ttll. 14: 347-362, 1961.

4. TIoffvr, 0. : 1,~s modifications dc larticulation tomporo-l~:nllihulnirc: par lac+on rlcs moyone orthopCdiqucs, Orthodont. frang. 29: 9i-146, 1958.