Contributors: Ruth Dennis
Species: Canine | Classification: Miscellaneous
Features assessed during scoring
- See BVA/Kennel Club hip dysplasia scoring scheme for protocol BVA/Kennel Club hip dysplasia scheme.
- The scrutineers work from a table detailing the scores to be allocated for various radiographic features.
- The Norberg angle is calculated using a medical orthopedic measurement device called an ischiometer .
- It represents an assessment both of the depth of the acetabulum and the presence of subluxation; a good Norberg angle exceeds 105° and penalty points are given for each block of 5° less than this, eg a Norberg angle of 92° scores 3.
- The center point of each femoral head is marked on the radiograph by finding one of the concentric circles drawn on the ischiometer which most nearly fits the femoral head.
- The centers are joined using a baseline, also drawn on the ischiometer, and a second line taken from the center of the each femoral head to the most craniolateral point of each acetabular surface.
- The angle between the two lines is measured for each hip .
- If the acetabulum is shallow or the femoral head is subluxated then the Norberg angle is reduced .
- Subluxation as a sole feature is described by assessing how much of the femoral head lies medial to the dorsal acetabular edge (DAE).
- For a score of zero the center lies well medial to the DAE and the cranial joint space is regular with no medial or lateral widening; if any congruency is present then a score of 1 is given.
- If the femoral head center lies on the DAE the score is 2, and if it lies lateral to the DAE, the scores are 3-6 for progressively worsening subluxation.
Cranial acetabular edge
- The cranial acetabular edge (CrAE) should be smoothly curved with a regular band of subchondral sclerosis.
- Slight flattening resulting in medial or lateral joint space divergence scores 1, while flattening of the whole edge scores 2 .
- Once erosion changes are recognized on the cranial effective rim as well, the score rises to 3-6 depending on their severity .
Dorsal acetabular edge
- On a radiograph of adequate quality the DAE is clearly defined and gently S-shaped.
- Irregularities arise from bone erosion and new bone production and give rise to scores of 1 to 6, the latter for massive exostoses along its whole length .
- Scoring of the DAE is very subjective, and is difficult on sub-standard images.
Cranial effective acetabular rim
- The cranial effective acetabular rim (CrEAR) is normally a well-defined, pointed projection .
- If the hip joint is loose then the femoral head will erode this region of the acetabular margin resulting in formation of a new articular surface, known as bilabiation (double lipping) or facet formation .
- In more advanced cases, spurs of arthritic new bone also form on the rim, where the joint capsule attaches .
- Scores are given for different degrees of pathology, for example, a score of 4 is given for an obvious facet or bilabiation or for an obvious exostosis.
- The acetabular fossa (AF) is especially difficult to score as its appearance is extremely sensitive to technical factors relating to positioning and image quality.
- The normal clarity of the AF is lost as hazy new bone is produced around the origin of the teres ligament, leading eventually to narrowing of the caudal groove of the fossa .
- In severe cases the AF becomes completely filled with new bone (a score of 6).
- Changes in the AF will not occur until degenerative changes are present elsewhere in the joint, where they are more readily recognized.
Caudal acetabular edge
- Assessment of the caudal acetabular edge (CauAE) is also rather subjective since it does not parallel the femoral head as does the cranial acetabular edge, so remodelling is harder to detect.
- The scrutineers look for evidence of irregularity and sclerosis indicating new bone production.
- This is best seen medially (where it encroaches onto the groove of the acetabular fossa) and laterally (where it protrudes laterally to the joint).
- The maximum score for this feature is only 5, and is given for gross distortion due to a mass of new bone in the acetabulum with loss of the groove.
Femoral head and neck exostoses
- The femoral neck should be smoothly concave and the head and neck should show a regular trabecular pattern .
- New bone will form around the neck at the attachment of the joint capsule.
- In the early stages all that is seen is a sclerotic 'Morgan line' running vertically across the neck (score 1); new bone visible on the skyline of the neck scores 2 and definite rings of new bone score 3-6 , the higher scores reflecting a mushroom or ice-cream cornet shape to the femoral head .
Femoral head re-contouring
- As a result of the new bone and of bone erosion, the femoral head loses its rounded shape and becomes conical, so that it no longer fits into the concentric circles drawn on the ischiometer, for example a score of 3 is for 'obvious bone loss and distinct exostosis giving slight conical appearance' .