Introduction

Elbow dysplasias (EDs) have in common that they all cause degenerative joint disease (DJD) eventually.

There are at least 4 groups of entities known:

1.  Medial coronoid disease (MCD)¹, formerly known as fragmented coronoid process (FCP)

2.  Osteochondrosis (OC) - osteochondritis dissecans (OCD) - OCD-like disease referring to a radiological indentation at the medial side of the humeral

3.  Ununited anconeal process (UAP) previously known as ED

4.  Incongruity (INC) of the elbow joint, being the radiohumeral, the ulnar-humeral or the radioulnar joint.

At clinical investigation, differentiation is not very discriminating, especially since the entities are also seen in combinations (Table 1). Plain radiology has always been the first-line diagnostic tool, but also the first choice as screening tool, since it is easy to perform, widely available in veterinary practices and not too expensive. A variety of preferred views allow to visualize the entities listed above. Osteochondrosis lesions at anterioposterior (AP) views and/or AP medial oblique views, UAP on mediolateral (ML) flexed views, INC in the non-twisted ML (extended) view, and also the AP view can be supportive. Incongruity of the radioulnar joint cannot be visualized on plain radiographs. The sensitivity of radiographs to diagnose MCD ranges from 10–60%, and can even be false negative.²

Knowledge about the etiologies of the entities listed above will help to understand the experience of breeders and veterinarians alike, that screening will not always predict the elbow status of the dog at an older age nor the elbow status of the offspring.

Table 1. Distribution of primary diseases (%) encompassing ED in five dog breeds

n.k. = not known; - = not present
*Adapted from: D. Remy, P. Maitre, C. Carozzo, E. Viguier, D. Fau, J.P. Genevois. Elbow dysplasia in German shepherd dogs (GSD). In: Proceedings from 23rd annual meeting of the International Elbow Working Group (IEWG), Dublin, Ireland; 2008: 29–30.
+ Lavrijsen et al. Prevalence and co-occurrence of hip dysplasia and elbow dysplasia in Dutch pure-bred dogs. Prev Vet Med. 2014;114(2):114–122.

Etiology of fragmented Coronoid Process (FCP)

In Labradors, Golden retrievers, and German shepherd dogs (GSD), but not in Bernese Mountain dogs, there is a preference for male dogs.³

Labrador puppies born out of two MCD-positive parent dogs, radiographs did not detect abnormal medial coronoid development, whereas on CT in 50% of the dogs abnormal development, being 100% of the siblings with pathological medial coronoid process development, starting at the age of 15 weeks.⁴ Disturbed endochondral ossification with delayed cartilage mineralization and in some cases, a fracture line in the subchondral area could be noticed. The delay in endochondral ossification with retained cartilage makes the medial coronoid vulnerable for mechanical stress. When a crack occurs in the subchondral area, eventually the fissure line can extend into the overlaying articular cartilage. When the latter occurs, the fragment can freely move and irritate the bordering radius and ulna, and thus leads to DJD; this can occur at young (4–8 months), but also not earlier than at 2–8 years.^2,5 When this fracturing occurs at young age, the process of endochondral ossification can continue, leading to a separated ossicle of bone covered with joint cartilage. Mechanical stress can be caused by overactivity, overweight, and/or joint incongruity including a relatively too long ulna and/or an incongruity of the radioulnar joint. This mechanical stress can cause an avulsion of the vulnerable, cartilaginous fragment at the medial coronoid process. Radioulnar incongruence in canine elbow joints reduces the contact area between the humerus and the radioulnar surfaces. The contact becomes concentrated at the lateral aspect of the medial coronoid process in a location similar to that reported for fragmentation in clinical cases of elbow dysplasia. Although on CT the proximity of the bone contours in the radioulnar joints is tight, no objective methods could be applied in vivo to control the width of the joint cartilage forming the joint space.⁴ When the fragment moves freely, eventually blood supply can be hampered, causing a separated, sometimes dead and necrotic ossicle, surrounded by weakened subchondral bone and joint cartilage (chondro-osteomalacia) as reported by Theyse (Different Presentations of Medial Coronoid Disease at Different Ages: A Clinical, Radiological, CT, and Arthroscopic Study). Contact lesions between the ossicle and medial aspect of the humeral condyle can cause cartilage erosion, making the syndrome of 'medial coronoid disease' complete.

In Labradors, but not in Bernese Mountain dogs, family clusters with MCD were identified with a higher incidence of MCD (45–60%) than other family groups.⁶ However, the h² is quite low, therefore also other non-genomic factors influence the occurrence of MCD. Environmental factors can play a role in disturbed endochondral ossification.

Etiology of the Other Elbow Dysplasias

There is a variety of risk factors presented in literature influencing the normal development of the elbow joint of dogs at risk for EDs. Food quality, especially calcium and vitamin D excess, has been discovered to be responsible for disturbances in endochondral ossifications of growth plates and joint surfaces, confirming the original findings of Hedhammar et al.⁷ in young growing Great Danes raised on excess of a balanced food. In Labradors ad libitum feeding caused overweight in comparison with Labradors raised on 70% of that amount of food, but in all Labradors of 11–13 years of age of that study, primary osteoarthritis was diagnosed of both elbow joints, since no signs of primary cause could be found at necropsy.⁸ Due to physiological or pathological joint incongruity (e.g., Bernese Mountain dogs with a relatively too short radius) or an FCP due to an incongruent radioulnar joint can cause fragmentation of the overloaded medial coronoid process. OC(D) was also seen more frequently in some familial clusters, not connected to the presence of MCD.⁶ UAP can only occur in those breeds where the anconeal process is a separate ossification center connected with a cartilaginous plate to the olecranon, like the German Shepherd dogs, and Bloodhound, but not in the Labradors who ossify the anconeal process gradually from the base to the apex.² Sharing forces, as due to elbow incongruity with a too long radius (as in radius curvus syndrome and chondrodystrophy as in Basset hounds) can cause UAP in the breeds with the separated ossification center.

Prevalence of Elbow Dysplasias (EDs)

There are different population reviews available, which give an indication of the incidence of the different entities grouped under EDs. However, it should be realized that these figures can be biased by the difference of age at screening (OFFA this is 2 years of age vs. IEWG 1 year of age), the amount of views and thus the sensitivity of the screening method.

The sensitivity of radiography to detect FCP ranges from 10–62%. Multiple views are advocated (Lappalainen:Scientific Basis for More Views and More Care for Overinterpretation), thus increasing the chance to detect one or more indications of ED (Tellhelm: Grading Primary Elbow Dysplasia Lesions and Elbow Osteoarthrosis According to the IEWG Protocol). Prescreening by owners and their local veterinarian may influence the prevalence for ED of the breed and certain breeders and thus evaluation of progress of breeding measurements.

Genetic Aspects

The heritability estimates differ per investigated cohort and are published to be for Bernese Mountain dogs between 0.24 and 0.43 (combination of MCD and joint incongruity), for Labradors 0.77 and Golden Retrievers 0.45 (both breeds a combination of OCD and MCD), for Rottweilers 0.25–0.28.

Recent studies revealed much lower heritability (Labradors 0.17; Golden Retrievers 0.24; Bernese Mountain dogs 0.06).⁹ The lower the heritability estimates, the higher is the influence of the non-genetic factors on the expression of the entity. These factors include the above-listed etiological factors including quality of the diet, and overloading of the joint, but also the sensitivity of the screening procedure.

From a variety of studies, all focused on MCD in Labradors, it is considered as a polygenetic disorder. Genetic research is hampered by the fact that dogs positive for MCD are not always detectable on radiographs, since some dogs only develop signs at an older age⁵ [Theyse: Different Presentations of Medial Coronoid Disease at Different Ages: A Clinical, Radiological, CT, and Arthroscopic Study] or may even not bring the genetic defect to expression due to a variety of environmental and unknown influences. Further progress in detecting ED during screening, and thus by abandoning the causes of elbow DJD, is to make the next step. This can be performed by improving the radiological techniques (e.g., more views, better quality films, addition of CT in suspected cases), the quality of the readers (independent, experienced radiologists and veterinary surgeons, uniform screening techniques), the evaluation of the breed based on the screening results (analysis of estimated breeding value to include results of relatives), the awareness of the consequences not coping with the screening procedure. Also a radical change in detection method (e.g., development of DNA-screening technique) should be considered by the breeders' world. This allows for detecting carriers of the responsible gene(s) irrespective of the occurrence of the actual disease state (i.e., ED). It will thus be irrespective of radiological findings, will facilitate the availability of sensitive screening for the breeders and owners by shipping only a blood sample. It warrants however a serious investment to develop these molecular genetic techniques by well-equipped laboratories on request of the breeders clubs.

Considerations for Future Screening

Since each of the entities of ED has a polygenic inheritance, each genetic abnormality explains a relatively small part of the total genetic variation occurring in case of ED. Therefore a simple gene-test cannot be developed for these diseases. It might even be impossible to identify all chromosomal regions affecting a single trait. However, it does not imply that DNA data cannot be used for genetic improvement of poly-genetically inherited traits such as ED.⁹ The knowledge of chromosomal regions affecting a trait (so-called Quantitative Trait Loci = QTLs), is not required as long as there are a sufficient number of markers (e.g., SNPs),¹⁰ genotyped in the candidates. Estimated breeding values (EBVs) are traditionally calculated using health/disease recordings combined with information of relatives. EBVs based on DNA data are called genomic EBVs (gEBVs): chromosomal regions that are identified to have an effect on a selection of ED should receive more weight when DNA information is used to calculate EBVs.^9,10 However, first molecular genetic investigations should be performed, initiated by the breeders' world. Till that time radiological screening with improved techniques and, when indicated, expanded with CT scanning will be the best way to overcome this deficit.

See for complete text and references www.vet-iewg.org/joomla/index.php/archive/

References

1.  Fitzpatrick N, Smith TJ, Evans RB, Yeadon R. Radiographic and arthroscopic findings in the elbow joints of 263 dogs with medial coronoid disease. Veterinary Surgery. 2009;38:213–223.

2.  Lau Seng Fong. Development of medial coronoid disease in Labrador retrievers: diagnostic and pathogenic studies, Thesis Utrecht University October 2013 ISBN 978-90-393-5962-5964.

3.  Lavrijsen IC, Heuven HC, Voorhout G, Meij BP, Theyse LF, Leegwater PA, Hazewinkel HA. Phenotypic and genetic evaluation of elbow dysplasia in Dutch Labrador retrievers, Golden retrievers, and Bernese Mountain Dogs. The Veterinary Journal. 2012;193:486–492.

4.  Lau SF, Wolschrijn CF, Hazewinkel HA, Siebelt M, Voorhout G. The early development of medial coronoid disease in growing Labrador retrievers: radiographic, computed tomographic, necropsy and micro-computed tomographic findings. The Veterinary Journal. 2013;197:724–730.

5.  van Bruggen LW, Hazewinkel HA, Wolschrijn CF, Voorhout G, Pollak YW, Barthez PY. Bone scintigraphy for the diagnosis of an abnormal medial coronoid process in dogs. Veterinary Radiology & Ultrasound. 2010;51:344–348.

6.  Ubbink GJ, Hazewinkel HA, van de Broek J, Rothuizen J. Familial clustering and risk analysis for fragmented coronoid process and elbow joint incongruity in Bernese Mountain Dogs in The Netherlands. American Journal of Veterinary Research. 1999;60:1082–1087.

7.  Hedhammar A, Wu FM, Krook L, Schryver HF, De LaHunta A, Wahlen JP, Kallfelz FA, Nunez EA, Hintz HF, Sheffy BE, Ryan GD. Overnutrition and skeletal disease. An experimental study in growing Great Dane dogs. The Cornell Veterinarian. 1974;64(Suppl. 5):1–160.

8.  Huck JL, Biery DN, Lawler DF, Gregor TP, Runge JJ, Evans RH, Kealy RD, Smith GK. 2009. A longitudinal study of the influence of lifetime food restriction on development of osteoarthritis in the canine elbow. Veterinary Surgery. 2009;38:192–198.

9.  Lavrijsen IC, Heuven HC, Meij BP, Theyse LF, Nap RC, Leegwater PA, Hazewinkel HA. Prevalence and co-occurrence of hip dysplasia and elbow dysplasia in Dutch pure-bred dogs. Preventive Veterinary Medicine. 2014;114(2):114–122.

10. Meuwissen TH, Goddard ME. Prediction of identity by descent probabilities from marker-haplotypes. Genetics, Selection, Evolution. 2001;33(6):605–634.