CHAPTER 18

Histology of Condyles in Mandibular Growth Anomalies

18.1 General Considerations

A survey of the presented cases discloses the marked variability of most types of mandibular growth anoma­lies, not only with respect to clinical appearance, but al­so regarding age at onset, duration and age at surgical intervention. Thus, in cases of condylar hyperactivity, i.e., H.H., H.E. and hybrid forms, growth can apparently turn abnormal during the regular growth period, con­tinue beyond the age of regular growth cessation or re­sume activity during adulthood. Depending on the interval between the start of the disorder and condylec­tomy, some of the resected condyles may have been growing actively, while in others growth may have ceased. The questions we hoped would be answered by the histological examination were therefore: 1. Was condylar growth still going on at the time of sur-

gery? 2. Did it proceed at a normal rate? 3. Was the mechanism of growth normal? 4. Is there any relationship between the structure of the

condyle and the clinical picture of the growth disorder?

As the normal microscopic appearance of mandibu­lar condyles alters during the growth period, after growth has terminated, and finally also during adult­hood (H. Luder 1996), the interpretation of histological findings in specimens affected by growth disorders has to primarily rely on age-matched controls. This should allow answers as to whether growth was still active or had ceased. Also, conclusions should be possible regard­ing the mechanism and timing of growth. However, as will be outlined below, estimates regarding the rate of growth are more problematic. Finally, attempts at relat­ing histological characteristics of condyles to particular types of growth anomalies are impeded by the great variability in clinical appearance and, hence, the limited number of uniform cases.

18.2 Normal Condyles in Different Age Groups

Growth Stage. Throughout the period of active growth, the articular tissue of the condyle comprises firstly a

layer of dense fibrous connective tissue which forms the articular surface and is called the articular layer, sec­ondly a subjacent layer of loose connective tissue termed the proliferative layer, because it contains pro­liferating pluripotent stem-cells, and thirdly a deep layer of hypertrophic hyaline cartilage (Figs. 37dd, 48v, 53n). While this basic appearance remains more or less constant throughout the period of growth, details change (H. Luder 1996). Cells of the articular layer, for instance, gradually assume a chondrocyte-like appear­ance around the time of puberty. Most major changes, however, occur during the first years after birth. Thus, condyles from newborns and infants regularly exhibit soft tissue columns containing large blood vessels, that protrude downwards from the articular layer, invag­inate the proliferative layer, and extend through the hyaline cartilage to the subchondral marrow spaces (Fig. 19g). These columns which are also referred to as vascular canals and are considered remnants of the em­bryonic pattern of blood supply, normally disappear gradually during infancy. Also, during the first few years of postnatal life, the hypertrophic layer that is most prominent at birth, decreases dramatically in thickness. Subsequent age-dependent changes in the width of the hyaline cartilage, however, hardly exceed individual variation (Figs. 37dd, 48v, 53n, 79q).

The association between the various articular tissue layers and condylar growth is evident from experimen­tal studies in suitable animal models that resemble hu­mans with respect to the microscopic structure of the condyle (H. Luder 1996). The proliferative layer contrib­utes to condylar growth by producing new cells com­mitted to subsequent differentiation into chondrocytes, which are in apposition on top of the hypertrophic layer. Through this apposition of new cells, the hyper­trophic cartilage grows in thickness and older cartilage cells are displaced deeper into the hypertrophic layer. As soon as new cartilage cells are fully differentiated, they start enlarging and synthesizing extracellular matrix. As a result, cells of the hypertrophic layer become separat­ed from top to bottom by widening septa of the matrix and at the same time grow in size. Once chondrocytes have attained their final stage of hypertrophy at the low­er border of the hypertrophic layer, most of them die, collapse, and leave empty lacunae for subsequent endo-

H. L. Obwegeser, Mandibular Growth Anomalies© Springer-Verlag Berlin Heidelberg 2001

348 CHAPTER 18 Histology of Condyles in Mandibular Growth Anomalies

N F (10·5) H.H. M (12-4) H.E. F (13-4) N F (14-8) H.E. M (15-9) N M (16·1 ) N M (19-10) H.H. M (20)

Fig. 79a-h. Histological findings in cases of condylar hyperactivity. Comparative age-dependent appearance of the articular tissue and subchondral bone in normal (N) condyles (a, d, f, g) as well as specimens from H. H. (b, h) and H.E. (c, e) cases at the growth stage (M male, F female, age in years-months). Note the cone-shaped thickening of the articular tissue in one of the H.E. cases (c) as well as the pattern of trabeculae and depth of cartilage islands (arrowheads) in the subchondral spongiosa. To appreciate the features of the spon­giosa, note that the bone appears dark in the paraffin (b, c, e, h) and light in the plastic sections (a, d, f, g). Toluidine blue, original mag­nification x33 (bar=0.5 mm)

chondral ossification. This involves invasion by capillar­ies, chondroclasts which resorb cartilage matrix, and bone forming osteoblasts. Thus, endochondral ossifica­tion continuously converts into primary spongiosa what has been added to the hypertrophic layer through apposition of new cells, interstitial production of new matrix, and enlargement of the chondrocytes. In so do­ing, it maintains a more or less stable thickness of the hypertrophic layer.

During the conversion of the hyaline cartilage into primary spongiosa, chondroclasts do not resorb all of the extracellular matrix, but leave a cartilaginous scaf­fold for subsequent bone formation by osteoblasts. These primary trabeculae, which contain islands of car­tilage, are remodelled only at some distance from the zone of erosion of the hypertrophic layer and replaced by trabeculae consisting only of lamellar bone. The depth of cartilage rests in the subchondral bone is ap­parently related to the rate of condylar growth (Fig. 79r). In specimens from newborns and infants, they can be traced as far down as the condylar neck. In individuals of more than 10 years of age, however, the distance of cartilage rests from the zone of erosion de­creases dramatically, and in adults older than 30 years, they are confined to a very narrow subchondral band, if

present at all. This justifies the wide-spread practice of histologists of taking the frequency and depth of sub­chondral cartilage islands to decide, whether and at what rate growth had been active in specimens of con­dylar hyperplasia (M. Rushton 1944, 1946; T. 0berg et al. 1962; P. Egyedi 1969; J. de Burgh Norman and D. Painter 1980; L.G. de Bont eta!. 1985; P. Slootweg and H. MUller 1986; H. Obwegeser and M. Makek 1986; F. Gray et a!. 1990). However, it should be borne in mind that cartil­age remnants are related primarily to cartilage erosion and could also occur when this process is slowed down or cannot keep pace with cartilage formation.

Considering the limited alterations of condylar mi­croscopic appearance after infancy, which contrast strikingly with the variations observed in the rates of mandibular growth (A. Bjork 1963), it would appear that a balance is normally maintained between cartilage growth and endochondral ossification, that keeps the thickness of the hypertrophic layer within narrow bands. Rather than variations in rates of growth, signif­icant changes in cartilage thickness would thus reflect disturbance of the steady-state conditions between cell proliferation, matrix production, chondrocyte enlarge­ment and cartilage erosion.

18.2 Normal Condyles in Different Age Groups 349

N M (26-2)

Fig. 79i-n. Histological findings in cases of condylar hyperactivity. Comparative age-dependent appearance of the articular tissue

and subchondral bone in normal (N) condyles (i, I, n) as well as specimens from H.H. cases (j, m) and a case of"simple" condylar hyper­

plasia (C.H.; k) at the transitional and adult stages (M male, F female, age in years-months) . Note the fraying of the articular surface in

the C. H. case (k) and the vertical cleft across the entire articular tissue in the H.H. case (m). j, k, m paraffin sections, i, I, n plastic sec­

tions, toluidine blue, original magnification x33 (bar=O.S mm)

Transitional Stage. The interval between late adoles­cence and the late twenties constitutes a period of tran­sition from the microscopic appearance of the condyle during growth to the appearance prevailing for the rest of adulthood. The articular layer becomes almost en­tirely fibrocartilaginous and thereby is converted into the superficial zone of the adult articular tissue. In the proliferative layer, cell replication comes to a standstill, the density of constituent cells decreases, and some cells assume a chondrocyte-like structure. Unlike as claimed by some investigators (F. Gray et al. 1990), the layer does not disappear, although at this stage it should be more appropriately referred to as the intermediate zone. The hypertrophic layer is converted into the deep zone of the adult articular tissue. During this conversion, chon­drocytes, although increasing in size from top to bottom of the zone, remain smaller than during growth, become separated by wider septa of matrix, and acquire promi­nent pericellular halos (Fig. 361). In contrast to the situ­ation during growth, the deepest calcified part of the cartilage becomes clearly visible as a distinct, darkly staining layer that is separated from the non-calcified tissue by a sharp line of demarcation, the so-called tide­mark (Fig. 39k). In the subchondral zone, a continuous bone plate is gradually built. Therefore, calcified cartil­age borders in decreasing proportions on marrow spac­es and in increasing proportions directly on bone

(Fig. 361). In the marrow spaces, bone formation clearly predominates over cartilage resorption (H. Luder 1996). As a result, subchondral cartilage rests become scarce and located only in close proximity to the calcified car­tilage (Fig. 79r).

Adult Stage. At the adult stage, the condylar articular tissue essentially constitutes fibrocartilage with varying prominence of fibrous and cartilaginous components (H. Luder 1997). In the superficial zone, chondrocytes occur scattered or in small groups, interspersed between masses of thick collagen fibres. In the interme­diate zone, the proportion of chondrocytes rises further with age at the expense of fibroblasts, while the total number of cells continues to decrease. As a result, the remnants of the proliferative layer disappear as a dis­tinct cell layer along an increasing proportion of the condylar articular surface. The fibrocartilage of the deep zone is characterized by scattered, small chon­drocytes embedded in a lattice of obliquely arranged collagen fibres (Figs. 32n, 39k). Unlike the intermediate zone, the superficial and deep zones lack systematic age-related changes in appearance (H. Luder 1998). However, they can show signs of cartilage remodelling, mostly progressive remodelling characterized by the formation of chondrocyte clusters or the accumulation of cartilage matrix that is unusually rich in proteogly-

350 CHAPTER 18 Histology of Condyles in Mandibular Growth Anomalies

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Fig. 79o-r. Histological findings in cases of condylar hyperactivity. Thickness of articular tissue layers or zone (o-q) and depth of subchondral cartilage rests (r) against age in normal condyles (asterisks and solid line indicating average) as well as specimens from H.H. (squares), H.E. (dots) and C.H. (triangle) cases. Please note that cartilage rests (r) in the H.H. cases of about 12.5 and 20 years of age, similar to those of normal newborns, were observed down to the resection border, which may not have coincided with the maximum depth

cans and, therefore, displays a strong metachromatic staining reaction with toluidine blue (Fig. 32n). This type of remodelling seems to be particularly frequent in individuals around 40-45 years of age and is associated with reactivated endochondral ossification that can produce small subchondral cartilage islands similar to those found during the period of transition from growth to adulthood (H. Luder 1998}. In comparison with progressive remodelling, regressive remodelling involving dissolution of cartilage and its replacement by vascular fibrous tissue is very rarely seen in individuals below 50 years of age (H. Luder 1996). Similarly, signs of degeneration, i.e., osteoarthrosis, are scarce in this age range. If present, degenerative changes are mostly con­fined to fibrillation of the articular surface and hardly

ever involve splitting or clefting of the deeper articular tissue zones (H. Luder 1996}.

18.3 Condyles in Mandibular Growth Anomalies

18.3.1 Condylar Hypoactivity

The only specimen from this class of growth anomaly, that was available for histological examination, was the resected condyle of an 18-months-old girl with hemifa­cial microsomia. Not unexpectedly in such a case, this condyle was hypoplastic, its overall size corresponding approximately to that of a normal newborn. Interesting-

ly, however, the otherwise inconspicuous hypertrophic cartilage was also much thinner than normal, even when its thickness was related to the reduced overall size of the specimen. On the other hand, subchondral cartilage remnants were observed down to a depth of about 4.5-5 mm from the zone of erosion, which corre­sponds perfectly to the values found in normal age­matched specimens (Fig. 79r). Also, the trabecular pat­tern of the condylar spongiosa did not deviate signifi­cantly from that observed in controls (Fig. 19f,g). This could be taken to indicate that endochondral ossifica­tion and remodelling of the primary spongiosa in this condyle were fairly normal, whereas the production of hypertrophic cartilage through stem cell proliferation, matrix production and chondrocyte enlargement was deficient.

18.3.2 Condylar Hyperactivity

From cases of unilateral condylar hyperactivity, seven resected condyles were obtained as paraffin blocks em­bedded earlier for routine pathological diagnosis. Four of these cases, one female (Fig. 32) and three males (Figs. 36, 37, 68), were classified as hemimandibular hy­perplasias (H.H.) or hybrid forms with a predominance of H.H., one case of a female (Fig. 39) as "simple" unilat­eral condylar hyperplasia (C.H.) and two cases, one fe­male (Fig. 48) and one male (Fig. 53), as hemimandibu­lar elongation (H.E.). Based on the patients' ages at con­dylectomy, one specimen of H.H. and both specimens of H.E. had been removed during the growth period, two specimens of H.H. at the beginning and end of the tran­sitional stage, respectively, and the last specimen of H.H. as well as the one condyle with C.H. at the adult stage. From all the existing paraffin blocks, new sections were cut, stained with toluidine blue for better compar­ison with control material, and analysed histometrically using a light microscope equipped with an eyepiece mi­crometer.

In addition, micrographs taken earlier from condyles of five patients were available for a qualitative examina­tion. These cases were classified as H.H. at the growth stage (Fig. 31), a hybrid form with predominance of H. H. (Fig. 74), a hybrid form with predominance of H.E. (Fig. 73), H.E. (Fig. 28) and a hybrid form without pre­dominance of either the H.H. or H.E. component (Fig. 69). A summary of the histopathology findings in all these specimens is displayed in Table 1.

18.3.3 Hemimandibular Hyperplasia

All condyles removed from cases of H.H. at the growth stage as well as one specimen from a male hybrid case with predominance of H. H. resected at the beginning of

18.3 Condyles in Mandibular Growth Anomalies 351

the transitional stage exhibited regular hypertrophic growth cartilage and active or even intensive endochon­dral ossification. Thus, they constituted cases of condy­lar hyperplasia type I as defined by P. Slootweg and H. Mi.iller (1986). Compared with normal condyles, the two specimens from males available for histometric analysis revealed some, although insignificant, thickening of the hypertrophic cartilage and abundant subchondral car­tilage rests at significantly greater distance from the zone of erosion (Figs. 79q,r). The spongiosa comprised comparatively small marrow spaces interspersed in an abnormally tight, almost tumour-like network of thin, mixed cartilaginous-bony trabeculae lacking clear or­ientation (Figs. 79b,h). This pattern suggests that endo­chondral ossification and remodelling of the primary spongiosa cannot keep pace with cartilage growth, ei­ther because the latter processes are reduced or cartil­age growth is increased in rate. Imbalanced or disturbed endochondral ossification and remodelling of the pri­mary spongiosa could indeed explain the grossly dis­torted condylar shape that is frequently observed in cas­es of H.H .. How such a mechanism could account for the malformation of the mandibular ramus, angle and cor­pus, remains, however, mysterious. Rather than being responsible for these malformations, the abnormal en­dochondral ossification and remodelling of the condyle could also conceivably be the result of an as yet un­known, common cause that interferes with growth and remodelling of the entire hemimandible.

Apart from being compatible with the clinical pic­ture, the trabecular pattern seen in the preceding first two cases also seemed to be typical of cases of H.H., thus allowing a distinction from cases of H.E .. Yet the histological appearance evident from micrographs of a condyle from a female patient classified as a "classical" case of"pure" H.H. (Fig. 31) did not fit the description given above. Rather, the condylar spongiosa gave an im­pression of normality, although the depth of subchon­dral cartilage remnants could not be ascertained. This suggests that the mechanisms leading to H.H. may vary, which would mean that a histopathological diagnosis based on condylar appearance alone would not be pos­sible.

The only condyle removed during the transitional stage of development from a male patient suffering from H.H. revealed partly fibrocartilage, as typically seen in adult specimens, and partly a type of cartilage characterized by hypertrophic chondrocytes inter­spersed between arcades formed by prominent collagen fibre bundles. The latter type of cartilage resembled that described as occurring in condylar hyperplasia type II (P. Slootweg and H. Mi.iller 1986). In agreement with this classification, there were signs of active endochondral ossification, which, however, did not exceed those seen in normal specimens (Fig. 361), and a subchondral bone plate which exhibited abnormally numerous marrow

352 CHAPTER 18 Histology of Condyles in Mandibular Growth Anomalies

Table 1. Summary of histopathological findings in cases of H.H. and H.E. as well as hybrid forms and a case of "simple" condylar hyper-plasia (C.H.)

Clinical Case Sex Ageb Cartilage Endochondral Subchondral Subchondral Cartilage rests diagnosis• ossification marrow bone

spaces trabeculae

H. H. 37 Male 12-4 Regular hypertrophic Intensive Narrow Numerous, Numerous, deep thin, ill-oriented

H.H. 31 Female 16-6 Regular hypertrophic Active Inconspicuous Inconspicuous in Inconspicuous in number and size number and depth

Hybrid, 68 Male 20 Regular hypertrophic Intensive Narrow Numerous, thin, Numerous, depth H.H.~H.E. ill-oriented

H.H. 36 Male 28 Partly hypertrophic, Active Slightly Inconspicuous Present

partly fibrocartilage increased in number

H.H. 32 Female 40 Fibrocartilage with Missing Inconspicuous Inconspicuous Absent severe degenerative changes

Hybrid, 74 Female 40 Fibrocartilage with Missing Inconspicuous Inconspicuous Absent H.H.~H.E. severe degenerative

changes

C.H. 39 Female 31 Fibrocartilage with Minor Inconspicuous Inconspicuous Few initial degenerative changes

H.E. 48 Female 13-4 Regular hypertrophic Active large Few, slender, Inconspicuous in well oriented number and depth

Hybrid 73 Male 13-10 Regular hypertrophic Intensive Large Few, slender Inconspicuous in H.E.~H.H. well oriented number and depth

H.E. 53 Male 15-9 Regular hypertrophic Active Large Few, slender, Inconspicuous in well oriented number and depth

H.E. 28 Female 17-6 Transitional from Minor Large Almost compact Few hypertrophic to bone plate and fibrocartilage slender trabeculae

Hybrid 69 Male 16-10 Regular hypertrophic Intensive Inconspicuous Inconspicuous Numerous, deep

• In hybrid forms, predominance of either the H.H. or H.E. component is indicated by "greater than" -signs (>)

h Age (years-months) at condylectomy

spaces in direct contact with cartilage. This indicates regular, well-coordinated condylar growth continuing beyond the age when it normally terminates.

All condyles removed at the adult stage, i.e., two spec­imens from H.H. cases and the one from the only case of "simple" condylar hyperplasia, notably all females, re­vealed degenerative disintegration of the articular tis­sues, i.e., signs of osteoarthrosis, but none of significant on-going growth. This characterization corresponds to that given by P. Slootweg and H.Miiller (1986) regarding condylar hyperplasia type III. In the condyle from the youngest individual, degenerative changes were superfi­cial and mild, whereas in those obtained at about 40 years of age they were severe, affected all zones down to the osteochondral junction, and were accompanied by marked regressive remodelling. These alterations clear­ly exceeded those found in age-matched control speci­mens and, therefore, cannot be considered "normal" ag­ing-associated changes. However, it cannot be decided, whether they presented the cause or effect of the abnor­mal condylar enlargement and distortion.

18.3.4 Hemimandibular Elongation

All condyles obtained from cases of H.E. or from hybrid forms with a predominance of the H.E. component were removed at the growth stage. All but one revealed the normal layered appearance of the articular tissue typi­cal of this stage. The one exceptional specimen, shown already in a previous publication (H. Obwegeser and M. Makek 1986), exhibited a central cone-shaped thicken­ing of the articular layer, that displaced the proliferative and hypertrophic layers downwards. This peculiar structure that, although lacking any blood vessels, re­sembled somewhat the vascular canals found in con­dyles from infants, could not be observed in any other specimen. Thus, unlike the assumption of H. Obwegeser and M. Makek ( 1986), it cannot be considered typical of H.E.cases.

In all condyles of this type of growth anomaly, in­cluding that with the local thickening of the articular layer, the hypertrophic cartilage exhibited a perfectly normal structure and thickness (Fig. 79q), and there

was obviously active endochondral ossification. Ac­cording to the classification ofP. Slootweg and H. Muller (1986), they all constituted condylar hyperplasia type I, similar to the specimens from H.H. cases removed at the same age range. However, there were some striking dif­ferences. In contrast to specimens removed because of H.H., those from H.E. cases disclosed subchondral car­tilage rests down to normal distances from the zone of erosion (Fig. 79r), and the condylar spongiosa was char­acterized by slender, well-oriented, sometimes almost parallel bone trabeculae arranged between compara­tively large marrow spaces (Fig. 79c, e). This suggests enhanced but well-coordinated cartilage growth, endo­chondral ossification and remodelling of the primary spongiosa, i.e., a growth pattern fitting well to the clini­cal and radiographic appearance of H.E ..

18.3.5 Hybrid Forms

The only condyle from a case classified as hybrid form without predominance of either the H.H. or H.E. com­ponent (Fig. 68) was removed at the growth stage and had to be examined based on old micrographs alone. These micrographs indicated uniform thickening of the articular tissue, but did not allow identification of the proliferative layer. Regarding structure, the hypertroph­ic cartilage and the trabecular pattern of the subchon­dral spongiosa appeared inconspicuous. The conclusion that can be derived from these limited findings is that the histological appearance of the condyle, lacking any sign of either H.H. or H.E., fairly well reflected the clin­ical picture.

18.4 Summary

Irrespective of the clinical classification of the condylar hyperactivity cases, the histological appearance of ex­cised condyles presented in this work is in good agree­ment with the descriptions given in the literature on condylar hyperplasia (M. Rushton 1944, 1946; L. Schultz et al. 1960; T. 0berg et al. 1962; R. Walker 1967; P. Egyedi 1969; J. de Burgh Norman and D. Painter 1980; L. de Bont et al. 1985; P. Slootweg and H. Muller 1986; H. Obweges­er and M. Makek 1986; R. Gray et al. 1990). These obser­vations suggest that the microscopic structure of the cartilage covering the condylar bone only depends on the age at condylectomy, supporting the notion of R. Gray et al. (1990) that the types of condylar hyperplasia distinguished by P. Slootweg and H. Muller ( 1986) main­ly reflected the different stages of physical development at onset of the growth disorder. In contrast to the report of R. Gray et al. (1990), however, the findings obtained

18.3 Condyles in Mandibular Growth Anomalies 353

from the present investigation do not suggest that sig­nificant changes in the thickness of the articular tissue are associated with any of these growth anomalies. Rather, alterations at the cartilage level proved to be very subtle and did not exceed the range of normal vari­ation. It would appear, therefore, that nature fairly suc­cessfully maintains a steady-state between the various growth components, even if overall growth of the con­dyle is highly disturbed. As a consequence, a distinction of the different clinical forms of growth anomalies based on the microscopic appearance of condylar cartil­age does not seem possible.

The histological features that provided the most reli­able, although not absolutely flawless, discrimination of H.H. and H.E. cases of the present material were the tra­becular pattern of the condylar spongiosa and the depth of the cartilage remnants embedded in the subchondral bone trabeculae. While cartilage rests have consistently been considered by previous investigators (M. Rushton 1944; T. 0berg et al. 1962; P. Egyedi 1969; J. de Burgh Norman and D. Painter 1980; L. de Bont et al. 1985; P. Slootweg and H. Muller 1986; H. Obwegeser and M. Ma­kek 1986; R. Gray et al. 1990), usually to decide whether or not condylar growth was active, the arrangement of trabeculae, to the best of my knowledge, has not been taken into account so far. The validity of this diagnostic feature, therefore, needs confirmation from further studies.

If proven to be useful, the appearance of the spongio­sa would enable a distinction between normal, H.H.­and H.E.-condyles, as long as growth has not yet come to a standstill. Once growth ceases, however, subchon­dral bone trabeculae apparently remodel, thus assum­ing an inconspicuous arrangement, which no longer of­fers clues for a diagnosis. Moreover, even in cases oper­ated on during active enlargement of the condyle, the characteristics of the spongiosa do not allow a decision to be made whether condylar growth was hyperactive or normal at the time of the surgical intervention.

Although virtually all previous reports on cases of condylar hyperplasia have speculated about its aetiolo­gy, this issue is still far from being clear. Considering the low numbers of uniform cases and the limited conclu­sions that can be derived from the minor alterations in the microscopic appearance of the condyles, histopa­thology is unlikely to offer useful explanations regard­ing aetiology and pathogenesis of mandibular growth anomalies. From the distinct histological manifesta­tions of growth disturbances setting in during and after the normal growth period, it would appear that respec­tive causes vary as well. Also, distinct aetiologies may account for H. H. and H.E., as the components of condy­lar growth in these two types of anomalies seem to be affected to strikingly different extents.