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Hip dysplasia and dog breedingJ. Bouw aa Department of Genetics and Breeding , ZoologicalInstitute of the Faculty Veterinary Medicine , Yalelaan17, Utrecht, the NetherlandsPublished online: 01 Nov 2011.

To cite this article: J. Bouw (1982) Hip dysplasia and dog breeding, Veterinary Quarterly,4:4, 173-181, DOI: 10.1080/01652176.1982.9693859

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Hip dysplasia and dog breeding

J. Bouwl

SUMMARY

Hip dysplasia is considered to be one of the most serious problems in dog breeding.In the past 20 years the studies of, and the attempts to control, this condition havebeen directed mainly to the overall picture of the abnormality of the hiPs. Thevarious efforts have resulted in a decrease in the incidence of HD in several breedsof dogs. For other breeds the results have not been very convincing, however. Insome instances they are even said to be conflicting with other aims of breeding.Based on these data some separate studies have recently been performded inrespect of the details of the disorders in thehip joints and of the heritability of theseelements.The present study concerns itself with the roots of the disorders as they areobserved in the various breeds. A great variety of data indicates that selectionwithin the canine species for specific morphological traits must be considered asone of the most important roots of the occurrence of HD. The collected datastrongly indicate that improvements in the HD status in the various breeds can bereached by selection against specific disorders of the hip joints in these breeds.

INTRODUCTION

Hip dysplasia is considered to be a syn-drome. According to Webster's diction-ary a syndrome is 'a group of signs andsymptoms that occur together and char-acterize an abnormality'.

So far the efforts to eliminate hip dyspla-sia have been concentrated mainly on thereduction of the signs and symptoms.Van der Velden (30) and Brooymans-Schallenberg (3) considered the methodsand the criteria used for the evaluation ofthe signs and symptoms to be insufficient.In their reports they proposed moreobjective and more exact classificationsof the signs which were observed on radi-ography. Discussions on this approach

have led to the question of whether theseanalyses and classifications can lead tosolutions for the ultimate elimination ofhip dysplasia.The present report will deal with consider-ations of the roots of the disease andwith the questions of whether and howthese roots can be analyzed an attacked.For that purpose we have made an analy-sis of the syndrome and of the variouselements of the body involved in the dis-order. An inventory of the factors knownto contribute to the phenomenon leads tothe conclusion that there are some princi-pal elements in this distortion of the hipjoints:

the hip joints are the hinges of the hindquarters of the animal body,

I Department of Genetics and Breeding of the Zoological Institute of the Faculty Veterinary Medicine,Yalelaan 17, Utrecht, the Netherlands.

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the divergence from the normal hipjoints involves bones, muscles, andligaments of the hip and the interac-tion between these elements,artificial selection within breeds ofdogs has in several cases led to drasticchanges in the construction and thecomposition of the hind quarters.

There is still considerable variation in therate of distortion within the breeds ofdogs which are affected by hip dysplasia.This variation is due partly to genetic,partly to environmental factors and of-fers possibilities for improvement. Theimprovements so far obtained have notalways been very convincing for the dogbreeders. In some breeds the improve-ment of the HD status is said to be con-flicting with other aims of selection andbreeding.Upon the basis of these considerations wehave developed a proposal to investigatepossibilities for improvements withinbreeds. These improvements should bebased on the existing aberrations of thenormal structure of the hips, with recog-nition of the other aims of selectionwithin the breeds.

THE DIAGNOSIS

Hip dysplasia in dogs cannot be character-ized satisfactorily on the basis of clinicalsymptoms. Dogs with serious aberra-tions in their hip joints may not show anysigns of hindrance, while animals withmerely slight bone proliferations can pre-sent severe symptoms of impeded move-ment. On the basis of these observationsit has been generally accepted that thediagnosis of hip dysplasia should be re-lated to the signs observed in the radio-graphs of the hip joints.Since there is a good deal of variation inthe elements of the hip joint which can beimpaired as well as in the degree of im-pairment of these elements, there is alsomuch variation in the interpretation ofthe radiographs. This variation in inter-pretation can be reduced considerablyif the bodies offiCially appointed for theassessment of radiographs are prepared tofollow uniform rules. At the request ofthe Scientific Committee of the Feder-

ation Cynologique Internationale, Brasset al. (3) published proposals for suchrules in 1978. In 1980 sixteen such bodiessigned a declaration of agreement withthese rules.Mutual agreement on the classificationof degrees of hip dysplasia can reduce agood deal of disagreement among themembers of interpreting panels and ofdissatisfaction among dog breeders.Such agreement may also offer a generalbasis for selection against the disease. Itshould be kept in mind, however, thatwithin the various degrees of dysplasiathere is still a good deal of variation insymptoms at similar rates of hip dysplasia.In addition there remain variations in theappreciation of the signs by different in-terpreting bodies.Although van der Velden (30) andBrooymans-Schallenberg (4) stronglysupported the efforts to reach mutualagreements on HD classifications theywere not satisfied with the degrees of theclassication as,tools for selection againsthip dysplasia. In a thorough study of apopulation of Rottweilerdogstheysearch-ed for exact and reliable tools of selec-tion. The results of these studies led tomore objective criteria within two maincategories: the fitting of the various el-ements of the hip joint and the prolifera-tion of the bones.Discussions about this approach led us tothe question of whether the strategy ofselection against signs should be contin-ued along the same lines. On the basis ofthese discussions it was decided to inves-tigate the possibilities to trace the roots ofthe signs and to consider whether theseroots could be attacked in the fightagainst hip dysplasia.

STRUCTURE AND FUNCTION OF THE HIPJOINT

The hip joint is composed of at least threedifferent types of tissues: bones,.muscles,and connective tissues. Each of these tis-sue types is composed of a variety ofdifferent elements: five different bonesand a large variety of muscles, tendons,ligaments and other connective tissues.All elements of the hip joint are present inthe newborn animal. The final composi-

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tion and structure of these elements andthe total construction of the joint is achiev-ed during the development of the animalbody. Olsson and Audell (18) consideredthe bones as the main elements of thejoint. They explained that these bones inthe newborn dogs were composed mainlyof flexible cartilage. The shape of thebones could be influenced during de-velopment by the qualities and the quanti-ties of the forces acting upon them. Basedon experiments in which parts of the hipjoint were replaced, Arnoczky (1) andPrieur (20) made detailed studies of mech-anical forces in the hip joint. They dem-onstrated that abnormalities in the for-mation of the hip could lead to enormousforces on specific parts of the bones.The ossification of the cartilage is knownto be influenced also by various enzymesand hormones. Influences on the biosyn-thesis of proteins specifically the regula-tion of the ratio of soluble to nonsolublecollagen in the tissues of the hip joint hasbeen studied by Lust et a/. (16), Gus-tafsson et al. (8), and Kasstrom et al. (14).If we compare the configuration of thehip of dogs of different breeds it be-comes quite clear that there are consider-able differences. These differences donot only concern anatomical differencesin the construction of the skeleton, alsothe muscles and connective tissues areinvolved in these differences. Striking ef-fects of differences in the conformationof the hips on the development of thejoints have been presented by Riser andShirer (22). For 20 greyhounds (all HD)and for 12 July hounds (all HD+) theycalculated a 'pelvic muscle index' by di-viding the total body weight by the totalweight of all pelvic muscles. The authorsfound a negative correlation (P < 0.001)of this index. The same authors madecomparable studies with a group of 60German shepherd dogs. They divided thedogs of this breed into groups with andwithout hip dysplasia. The results of thisstudy confirmed the negative correlationbetween HD and the pelvic muscle indexwithin a breed.Although the data are usually less exactand convincing, various other indica-tions of relations between the conforma-tion of the body and the occurrence of hip

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dysplasia have been presented in the lite-rature. A striking example of these indi-cations is presented by Wegner (31). Ac-cording to this author the high incidenceof hip dysplasia in German shepherddogs is mainly due to selection for over-angulated hind legs. The incidence ofHD is considerably less in comparableshepherd dog breeds with more naturallyangulated hind legs.Not only factors of the animal body itselfcan influence abnormal developments ofthe hip joints. Also environmental fac-tors, such as feeding and excercise of thegrowing animals, can have appreciable ef-fects. Hedhammar et al. (9) and Kas-strom (13) made comparative studies ofgroups of puppies fed on normal and onexcessive diets. In both studies markedincreases in disorders of the hips wereobserved in the groups of dogs whichwere greatly overdosed with foodstuffsand with specific nutrients. Although wedo not know of any reliable studies inthat respect it seems reasonable to as-sume that not only extravagant feedingbut also inadequate excercise can have aconsiderable influence on the normal de-velopment of the structure and thestrength of the hip joint. It stands toreason that unnatural and extravagantforces on a joint which is still partly com-posed of cartilage and of flexible connect-ive tissues can modify the normal andnatural information of the hip.

BREED DIFFERENCES

The above studies of Riser and Shirerdemonstrated that the occurrence of hipdysplasia was not confined to any specialbreeds of dogs. Within the population ofGerman shepherd dogs some animals dopresent signs of hip dysplasia and_ otherdo not. This does not imply, however,that the characteristics of body confor-mation of the breeds have no relation toHD. Van der Velden (29) presented acomparison of data on the occurrence ofhip dysplasia in various breeds of dogs.In his report he stated that the data hadonly a limited value since they were basedon subjective interpretations of radiogra-phic data. Nevertheless a comparison ofthe 1974 data of the Dutch Committee

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for HD with those of the OrthopedicFoundation for Animals (OFA) in theU.S.A. does reveal a great deal of agree-ment as far as the overall figures regard-ing the incidence of HD in the variousbreeds of dogs are concerned.

Besides the previously presented data wemade calculations of the incidence of hipdysplasia in dog breeds in the Nether-lands as observed in the years 1976-1979.Results of these calculations are present-ed in Table 1. In this table only thosebreeds are reported of which the radio-graphs of at least eighty animals wereexamined in the past four years.

The data of this table demonstrate thatthere is considerable variation in the per-centages of dogs which are examined perbreed. The reliability of the data in thistable increases with the percentage ofanimals examined. In some breeds with

low percentages of examined animals theactual incidence of HD may be consider-ably higher. Some owners of dogs ofthese breeds are interested mainly or evenexclusively in official certificates of HD-free animals. Radiographs with obvioussigns of hip dysplasia are not sent in bythese owners for assessment.Several other breeds of dogs with relativ-ely high numbers of official registrationsare lacking in this table. This concernsfirst of all the small sized breeds. Severalof the small sized breeds were never seri-ously considered for hip dysplasia. Inpractice, only the small sized animals ofthe hypochondroplastic breeds came tobe considered for HD examinations. TheDutch HD Committee has establishedthat the hips of these hypochondroplasticbreeds cannot be judged on the basis ofthe criteria used for the large sizedbreeds.

Table I. Breeds of dogs examined in the Netherlands in the period 1976-19799

Breeds Number ofregistration

number of % examinations % of HDexamined of registrations positive dogsdogs

St. Bernards 5048 81 2 94

Newfoundlands 1324 226 17 82

Kuvasz 220 91 41 79

Bernese Mountain Dogs 473 82 17 76

Irish Setters 4949 256 5 67

German Shepherd Dogs 39267 1109 3 66

Appenzeller Mountain 169 93 55 66DogsBriards 556 100 18 61

Bouviers de Flandres 32002 645 2 60

Boxers 13784 160 59

German Short Haired 9100 243 3 56

PointersGreat Danes 10097 305 3 55

Labrador Retrievers 3313 410 12 50

Leonbergers 421 223 53 50

Rottweilers 2790 819 29 50

Golden Retrievers 4103 716 18 48

Hovawarts 130 92 71 47

Drentse Patrijshond 2843 273 10 41

Flat Coated Retrievers 398 119 30 40Dutch Shepherd Dogs 1198 141 12 30

Collies 2305 90 4 27

Dobermans 5901 283 5 23

Siberian Huskies 317 138 44 18

Belgian Shepherd Dogs2 5621 67 15

Long Haired Smooth Haired Rough, or: Collies, coat not specified.

I The numbers presented are derived from the working data of the Dutch Committee for hip dysplasiain dogs.

2 Included in this table for comparison with other shepherd breeds.

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Of the medium and large sized breeds,several with relatively high numbers ofregistered animals are also absent in thetable. Afghan hounds with 4220, Airdaleterriers - 2968, Schnauzers - 2888, Span-iels - 2612, and Dalmatians with 2224 reg-istrations in the past four years were notincluded, since fewer than eighty dogswere examined in that period. Of 1419registered Greyhounds, only 3 radio-graphs were sent in for examination. Thelow percentage of examinations in thesebreeds is mainly due to the fact that hipdysplasia does not form a serious prob-lem for those dogs.Previous discussions on this subject inthe literature and also our own data dem-onstrate quite clearly that calculationsof the incidence of hip dysplasia in breedsof dogs are to be considered with reserva-tions. The presented data are nonethelesssufficient to conclude that hip dysplasiain dogs is a specific disease of large sizedbreeds with a low index for the total bodyweight in relation to the weight of thepelvic muscles. Specific breed character-istics, such as overangulated hind legs,hypochondroplasia, and other disturb-ances in the natural development of thebones can offer serious contributions tothe syndrome.

INFLUENCES OF BREEDING

A review of the factors which play a rolein the development of the hip joint re-veals that we are dealing with a widevariety of genetic factors. The qualities ofthe enzymes and hormones are primarilya product of the determining genes. Alsothe structure and the constitution of thebones, muscles, and connective tissues areprimarily genetically determined. Qual-ities and quantities of foodstuffs. excer-cise, etc. determine whether and how thegenes can find full expression.In his review of the literature Van derVelden (29) demonstrated that each ofthe elements contributing to the total hipjoint was controlled by a variety of differ-ent genes. Each of these controllinggenes offered a more or less importantcontribution to the ultimate structureand or function of the respective el-ement(s). This meant that the significanceof the loss or change of a gene dependedTHE VETERINARY QUARTERLY, VOL. 4, No. 4, OCTOBER 1982

upon its influence on the respective el-ements and upon the influence if that el-ement on the total structure and functionof the hip joint.On the basis of this reasoning, the devel-opment of the hip joint may be con-sidered as a polygenically determinedevent. In practically all cases the ultimateeffects of the individual genes should beconsidered as fractions of the effect ofgroups of interacting genes. Disturb-ances in the normal development ofthe hipjoint can be caused by a wide variety ofdifferent genes and by disturbances in theinteraction of these genes. Genes interactnot only with other genes but also withthe environment. The ultimate effects ofthe genes are thus also dependent on en-vironmental factors. Enzymes and hor-mones regulating the biosynthesis of thehip joint cannot act properly if the requir-ed trace elements are not available insufficient quantities and in appropriateproportions. Fibres of muscles and con-nective tissues will not sufficiently de-velop without environmental stimuli.If we now look at the existing differencesin incidence of HD between breeds ofdog, we can conclude that there must beconsiderable differences in the numbersand in the respective effects of the genescontributing to the development of thehip joints. These differences may existbetween but also within breeds. In con-sidering these genetically determined dif-ferences more closely we may first of allexpect that these differences within thespecies Canis lupus familiaris are the re-sult of long term selection - natural aswell as artificial, or genetic drift.Mech (17) described an enormous varia-tion in characteristics between as well aswithin the wolf populations which wereobserved in the various parts of theworld. Senglaub (24) has by now present-ed sufficient evidence to allows us to un-derstand that the genetic composition ofthe original wolf populations has enabledthis species to adapt itself to an extremelywide variation of environmental condi-tions. Our domestic dogs have been select-ed from the natural sources of the wolfpopulations and have afterwards beenbred for a variety of different purposes,to be carried out in a variety of differentconditions.

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We previously demonstrated (2) thatthe establishment of dog breeds forstrongly diverging purposes has led togreat differences in morphological traits.This does not only concern the bodyweight and the height at the withers. Thevarious purposes for which dogs havebeen selected specially for the variouskinds of hunting or fighting have alsoled to enormous differences in the con-formation of the body. A close compari-son of the differences in body conforma-tion between Asiatic hounds and Englishbulldogs will reveal at least as many differ-ences as can be found in a comparisonbetween St. Bernards and Chihuahuas.

The total variability of more than 300recognized dog breeds has been devel-oped in some 10000 years. The specificcharacteristics of the respective breedshave been achieved by means of artificialselection for specific purposes. The aimsof selection were directed to the breedingof animals which could serve their spe-cific purpose as best possible. The resultsof this selection were primarily depend-ent on the available material, the herit-ability of the characteristics, and the skillof the breeders. The wide genetic varia-tion possessed by the original dogs enabl-ed the breeders to produce individualswhich could serve highly specialized re-quirements.Selection for specific genes and for geno-types however, inevitably leads to the re-duction of other genes and to the inter-acting effects of these genes. Thenumber of different genes in the individ-uals diminishes with the number of breed-ing generations and with the rate of effi-ciency of the breeding methods. As longas these specially bred animals are usedfor their specific purposes and as long asthe same vigorous methods of selectionare maintained, these methods of breed-ing will not lead to serious problems.Problems may arise, however, if the rea-sons for keeping the dogs and the selectioncriteria and methods change. If dogswhich were originally bred for hard la-bour and/ or specialized field work aresubjected to environments and require-ments common for pet animals and if themethods and criteria of selection are re-

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laxed, several unexpected traits may turnup in the phenotypes of these animals.It has been explained already that thedevelopment of the normal hip joint iscontrolled by a large variety of differentgenes and by the interaction of thesegenes with each other and with the en-vironment. The aggregate of all the con-trolling genes was in specifically breddogs directed to specific aims which wereto be reached under special condition. Ifthe criteria and the methods of breedingas well as the environmental conditionsare changed, it is only plausible to expectthat the interactions of the controllinggenes are subjected to disturbances. Thesevere and complex demands for opti-mum functioning of the proximal joint ofthe hindlegs of the dog as these are de-fined by Prieur (20) may very well explainthat disturbances like these can find ex-pression in the construction of the hipjoint.Several authors have already reported onthe existing variation in the degrees ofdistortion of the hip joints. Brass el al. (3)presented a series of pictures of radio-graphs with increasing degrees of aberra-tion of the joints. Van der Velden (30)and Brooymans-Schallenberg (4) re-cently presented a good deal of data onthe various elements in which abnormal-ities were observed.It is remarkable to find how little atten-tion has so far been paid to comparisonsof hip distortions between breeds of dogs.Among some three hundred reports onhip dysplasia in the veterinary literature,only a ..few presented any data in thatrespect. Henricson et al. (10), Freudigeret al. (6), and Van der Velden (25) pub-lished comparative data on the incidenceof hip dysplasia in various breeds butnone on specific deformations in particu-lar breeds. Riser and Shirer (22) com-pared the pelvic muscle mass indices ofgreyhounds, July hounds, and Germanshepherd dogs. Scartazzini (23) com-pared radiographs of St. Bernards, Ber-nese mountain dogs, German shepherds,Rottweilers, Boxers, and Labrador Re-trievers and observed marked differencesin the relative acetabular depths and in thetightness of the hip joints. Further studiesin this respect should be carried out.

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SELECTION AGAINST HIP DYSPLASIA

The above discussion offers a sufficientbasis for the assumption that hip dyspla-sia in dogs is a polygenically determinedphenomenon. The genes contributing tothis characteristic interact with eachother and with various environmentalfactors. The data have furthermore dis-closed that there is a great deal of varia-tion in both the signs and the roots of thesigns of hip dysplasia.Variation offers possibilities for improve-ment. Improvements in environmentalconditions depend upon the willingnessand the skill of the owners of the dogs.Improvement in the genetic compositionof the breeds of dogs can be supported byadvice to the breeders and their organiza-tions in respect of selection.It has been mentioned already that theheritability of a characteristic is one ofthe primary tools for effective selection.Pirchner (19) defines the h2-value, therate of genetic variance, as a fraction ofthe total variance for a characteristic. Ahigh h2-value a relatively high rate ofgenetic variance offers good possibil-ities of selection.

For hip dysplasia, h2-values were calcu-lated by Henricson al. (11) as 0.30, byLeighton et al. (15) as 0.27, and by Jessen(12) also as 0.30. These values were calcu-lated in different breeds of dogs for theoverall picture of hip dysplasia. The valuescan be considered relatively high. Onthe basis of these values, hip dysplasia canbe considered as susceptible to selection.Van der Velden (30) was not satisfiedwith h2-values for the overall picture ofhip dysplasia. Since he was of the opinionthat selection against HD should bebased on more exact and objective cri-teria, he started investigating h2-values forspecific elements in the total picture.In the past decades several attempts ofselection have already been made in vari-ous countries. Marked effects of selec-tion were observed by Van der Velden(27) in a Rottweiler population in theNetherlands. In the period of 1966-1969he observed an increase in the percentageof nondysplastic dogs from 30 to 63 percent. Riser (21) reported an increase inthe percentage of nondysplastic German

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shepherd dogs of the US army from 60 to80 per cent. For the period 1965-1972Freudiger (7) reported an increase in HDfree Germans shepherd dogs from 55 to73 per cent in Switzerland, Felix Mailerand Saar (5) in the period 1965-1975 forHovawarts in Switzerland from 40 to 50per cent, and finally Hedhammer et al. (9)from 50 per cent in 1970 to 78 per cent in1975 in the German shepherds of theArmed Forces Dogs Centre in Sweden.After these positive results, mentionshould be made also of several other pro-grammes of selection against hip dyspla-sia in which no or even negative resultswere obtained. Most striking in this re-spect are Van der Velden's (26, 29) obser-vations in the Rottweiler population al-ready mentioned. After the first positiveselection results he observed a new in-crease in the frequency of HD-positiveanimals. Several reports special byHedhammer et al. (9) emphasize thepositive results in programmes againsthip dysplasia may be obtained and main-tained only as long as close cooperationwith the breeders' organizations is guar-anteed.The studies of Freudiger et al. (6), Hed-hammer et al. (9) and Van der Velden (29)demonstrate that 10-15 years of expens-ive examinations and of exclusions of dogsfor breeding, have led several breedersand also breeders' organizations toshowing signs of disappointment and ofloss of interest in programmes against hipdysplasia. The disappointment is mainlydue to the relative slow progress whichhas been achieved in several breeds. Re-markable in this respect are the criticismswhich complain that the specific type of abreed is becoming lost as a result of theselection against hip dysplasia.

DISCUSSION AND CONCLEAIONS

The current status of hip dysplasia indogs requires a serious consideration ofthe question of whether and how at-tempts to reduce hip dysplasia in the sev-eral breeds of dogs shall be continued.In considering this question we can con-clude that:

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hip dysplasia is a condition whichcauses a lifelong and in severalcases painful disability of the affect-ed animals;it has a high frequency rate of inci-dence in a large number of caninebreeds. In the Netherlands the breedsof which more than 50 per cent of theanimals show HD positive radio-graphs (Table 1) represent more than65 per cent of all officially registereddogs,man is mainly responsible for the es-tablishment of the breeds which havenow turned out to be sensitive for thiscondition.

On the basis of this reasoning the ques-tion of whether the fight against hip dys-plasia should be continued can be ans-wered only affirmatively.The data presented demonstrate furthermore that there is large variation in:

the degrees of distortion of the hipjoints,the signs corresponding to the syn-drome of hip dysplasia,the conformation of the various affect-ed breeds,

the factors influencing the abnormal-ities of the hip joints.

Further we may conclude that:the influences on the distortion of thehip joints are partly genetically deter-mined;comparative studies on influencingfactors in the various breeds of dogsare still to be worked out;the effects of selection depend uponthe degree of a genetic variation for acharacteristic - the h2-value;useful h2-values have been found forthe overall picture of hip dysplasia indogs;studies on h2-values for special el-ements in this overall picture are inprogress;selection against specific signs of hipdysplasia may conflict with otheraims of selection in certain breeds.

The summing up of these various itemsleads to the final conclusion that furtherstudies on specific signs of hip dysplasiawithin the various breeds of dogs shouldbe initiated. Selection based on moreexact and more objective signs shouldtake place in close cooperation withbreeders' organizations.

REFERENCES

I. Arnoczky, S. P.: Forces acting on the canine hip. Proc. Woorjaarsclagen 1980', Utrecht, p. 56, 1980.2. Bouw, J.: Fokkerij en gezondheidsbewaking bij honden. Tijdschr. Diergeneesk., 107, (1), 12-20. (1982).3. Brass, W., Freudiger, U., Muller, L. F., Paatsama, S., Velden, N. A. van der, and Watering, C. C. van

de: Bericht der Hilftgelenkdysplasiekommission. Kleintier- Praxis, 23, 169-180, (1978).4. Brooymans-Schallenberg, J. H. C.: Determination of canine hip dysplasia and selection against this

trait. (in preparation, 1983).5. Felix Willer, L. and Saar, Chr.: Erste Ergebnisse der tierarztlichzUchterischen Massnahmen zur

Bekampfung der Huftgelenksdysplasie beim Hovawart. 77erdrztl. Umsch., 27, (4), 176-177, (1977).

6. Freudiger, U., Scharer, V., Buser, J. C., and MUhlebach, R.: Die HUftgelenksdysplasie: Bekampfungs-verfahren unde Frequenz bei den verschiedenen Rassen. Schweiz. Arch. Tierheilk., 115,69-73, (1973 a).a).

7. Freudiger, U., Scharer, V., Buser, J. C., and MUhlebach, R.: Die Resultate der HUftgelenksdysplasie.Bekampfung beim D. Schafer in der Zeit von 1965 bis 1972. Schweiz. Arch. Tierheilk., 115, 169-173,

(1973 b).8. Gustafsson, P. O., KasstrOm, H., Olsson, S. 0., and Wennmann, B.: Skeletal development and sexual

maturation in German shepherds, greyhounds and their crossbreed offspring. 2nd Int. Conf. Vet.Radiol., 1970. Acta Radiologica Suppl. 319, Stockholm, 187-190, (1972 h).

9. Hedhammer, A.0 Olsson, S. E., Andersson, S. A., Person, L., Petterson, L., Olausson, A., andSundgren, P. E.: Canine Hip Dysplasia. J. Am. Vet. Med. Assoc., 174, 1012-1016, (1979).

10. Henricson, B., Norberg, 1., and Olsson, S. E.: On the etiology and pathogenesis of hip dysplasia, acomparative review. J. Small Anim. Pract., 7, 673-688, (1966).

II. Henricson, B and Ljunggren G.: Canine hip dysplasia in Sweden. Statistics and Genetics. 2nd Int.Conf. Vet. Radiol., 1970.

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Osteoarthrosis and Canine Hip Dysplasia, Helsinki, 1979.19. Pirchner, F. Ph. D.: Populationsgenetik in der Tierzucht, Parey, Hamburg, 1979.20. Prieur, W. D.: Biomechanics of the hipjoint. Proc. 'Voorjaarsdagen - 1980', Utrecht, p. 55, 1980.21. Riser, W. H.: Hip dysplasia in military dogs. Proc. Canine Hip Dysplasia Symp., St. Louis, p. 131,

1972.22. Riser, W. H. and Shirer, J. F.: Correlation between canine hip dysplasia and pelvic muscle mass: a

study of 95 dogs. Amer. J. Vet. Res., 28, 769-777, (1967).23. Scartazzini, R.: A radiological study of normal and dysplastic hip joints in six breeds of large dogs. 2nd

Int. Conf. Vet. Radiol., 1970. Acta Radiologica Suppl. 319, Stockholm, 183-185, (1970).24. Senglaub, K.: Wildhunde-Hauskunde, Urania, Leipzig, 1978,25. Velden, N. A. van der: De bestrijding van heupdysplasia bij de hond door middel van selectie. Een

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September, 1976.29. Velden, N. A. van der: Hip dysplasia in dogs (in preparation 1983 a).30. Velden, N. A. van der: Studies on hip dysplasia in a Rottweiler population 1983 b (in press).31. Wegner, W. (1976) Kleine Kynologie. Terra Verlag, Konstanz, 1981 b.

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