X-ray - Hip Dysplasia

The British Veterinary Association (BVA) and The Kennel Club in the UK (TKC) have joined together to run a hip scoring scheme for breeders, in order to test for canine hip dysplasia, the most common inherited joint problem of large breed dogs. This current system came into effect in 1984.
Dogs are scored from the age of 12 months, and can only be scored once in their lifetime.

The BVA/TKC publish a list of Breed Mean Scores (BMS) for each breed, together with the number of dogs tested to date and the range of scores achieved. It is then recommended that breeders ensure their breeding stock have scores which are well below the BMS.

From an x-ray taken at the owner’s vets, the BVA scores each of the hip joints — the norberg angle (0-6); subluxation (0-6); cranial acetabular edge (0-6); dorsal acetabular edge (0-6); cranial effective acetabular rim (0-6); acetabular fossa (0-6); caudal acetabular edge (0-5); femoral head and neck exostosis (0-6); femoral head recontouring (0-6), thus each hip can receive a maximum score of 53 and overall score of 106. The lower the score the better, so 0/0 would be perfect, while 53/53 would mean that the dog had no hips at all!

The Diagram shows scores of 0.
AF – Acetabular fossa
AN – Acetabular notch
FH – Femoral head
FV – Foveal defect
DAE – Dorsal Acetabular edge
CaAE – Caudal acetabular edge
CrAE – Cranial acetabular edge
CrEAR – Cranial effective acetabular rim

Norberg Angle
Measures two features — the degree of congruence between the FH and acetabulum and the length of the CrAE which gives a relative indication of acetabular depth.

Evaluation is based principally on the degree of congruence between the femoral had and acetabulum. The general ‘fit’ of these two components is assessed by the relationship between the FHC and the underlying image of the DAE; the configuration of the cranial joint space is also taken into account, particularly at the lower end of the scoring scale, when abnormalities are relatively minor. The cranial joint space is seen as a radiolucent shadow between the CrAE and adjacent cranial articular margin of the FH. For a zero score the FHC must be medial to the DAE and the cranial joint space uniformly narrow, with the curve of the CrAE eactly following that of the FH.

Cranial Acetabular Edge
Minor alterations in the shape, contour and possibly the length of the CrAE are generally believed to be indicators of poor articular congruence, while more severe changes are clearly consequences of chronic instability, abnormal marginal wear and remodelling of the joint. For a score of zero the CrAE should be convex, uniformly curved and match exactly the countour of the adjacent femoral head with no lateral or medial divergence of the cranial joint space.

Dorsal Acetabular Edge
On good quality radiographs, the DAE forms a well-defined density interface which traverses the fH almost vertically and extends a little beyong it cranially and caudally. The clarity whith which it can be seen is highly variable. A minor change in the contour of the DAE, which is normall slightly curved, merits a score of 1, but small localised irregularities, seen only on radiographs of exceptionally high quality, are considered to be within normal limits. Higher scores are related to definitive pathological changes associated with degenerative joint disease (osteoarthritis), the smallest being minor exostosis, which is usually seen cranially.

Cranial Effective Acetabular Rim
The earliest detectable abnormalities of the CrEAR are either minor exostosis, usually in the form of a small, well-modelled osteophyte, or slight ’rounding off’ of the junction between the CrAE and DAE, which may be seen in the presence of a non-biblabiated CrAE.

Acetabular Fossa
Detectable bone deposition in the AF is almost always associated with marked subluxation and the amount tends to increase in proportion with the increase in the other radiographic hallmarks of secondary degennerative joit disease (osteoarthritis). Objective numerical scoring of this criterion is difficult, but as changes are likely to be recorded only in dogs with a total score considerably higher han average, the importance of a high level of accuracy is diminished.

Caudal Acetabular Edge
The CaAE is the segment of the acetabulum which is subject to the widest range of normal radiographic variation. Radiographic abnormalities are scored between 0 and 5 and changes are due mainly to exostosis with signs of wear being apparent only in advanced cases.

Femoral Head and Neck Exostosis
The normal FH should have a smooth roundish profile, but the shape may vary considerably.

Recontouring of the Femoral Head
This criterion is used to record the extent to which the FH shape is altered as a consequence of instability. Thus scores are likely to be awarded only in cases in which secondary changes are well established, when remodelling occurs as a result of the combined effects of periarticular new bone formation and loss of subchondral bone following total destruction of articular cartilage.

References: BVA/KC scoring scheme for control of hip dysplasia: interpretation of criteria; Christine Gibbs, BVSc, PhD, DVR, MRCVS

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