Pathogenesis of canine hip dysplasia

Question 1

The combination of what factors likely predisposes a patient to development of hip dysplasia?

Answer 1

Combination of genetically predisposed animals exposed to environmental factors that enhance the expression of genetic weakness.

Question 2

What are some changes that can be seen in dysplastic hips as early as 30 days of age?

Answer 2

Edematous ligament of the head of the femur with torn fibers and hemorrhage, increased volume of the ligament, and increased synovial fluid volume.

Question 3

How does muscular pull on the canine hip joint potentially impact the development of hip dysplasia in dogs with hip joint laxity?

Answer 3

When the hip is in the swing phase of the gait, subluxation of the hip joint is thought to occur secondary to parallel pull of the rectus femoris, cranial sartorius and iliopsoas muscles. During the stance phase of the gait the gluteal and adductor muscles exert large forces across the hip joint causing traumatic reduction and damage to the articular cartilage.

In a lax hip joint the adductor and gluteal muscles also have to increase their force of contraction to maintain reduction of the joint, and the forces are acting over a smaller area (the dorsal labrum of the acetabulum).

Question 4

When is hip joint laxity at its maximum?

Answer 4

When the joint is in a neutral weight bearing position (pulling the pelvic limbs into extension causes wind-up of the joint capsule and limits lateral movement of the femoral head).

Question 5

What are three radiographic measures of hip joint laxity?

Answer 5

1) Norberg angle.
2) Distraction index.
3) Femoral head coverage.

Question 6

What formed the basis of the PennHip testing?

Answer 6

The understanding that displacement of the femoral head from the acetabulum is maximized in the neutral position and is largely independent of the distraction force (above a minimum force). That is, above a certain force (30N), distraction of the hip joint should be fairly uniform based on degree of laxity.

Question 7

What is a normal Norberg angle?

Answer 7

> 105 degrees.

Question 8

What is the primary risk factor for development of hip dysplasia?

Answer 8

Hip joint laxity.

Question 9

What is the issue with the genetic tissue for hip dysplasia as developed for use by IDEXX in Labradors?

Answer 9

Testing was performed in animals with a mean age of 21-64 months, and doesn’t therefore account for animals that may develop hip dysplasia later in life.

Question 10

The risk for development of osteoarthritis increases beyond what distraction index score?

Answer 10

> 0.30

Question 11

What are some proposed causes of joint laxity in dogs with hip dysplasia?

Answer 11

1. Joint fluid: higher volumes of synovial fluid, thickened ligament of the head of the femur. Unclear whether these are primary or secondary changes.
2. Pelvic muscle mass: may be decreased in dogs with hip dysplasia.
3. Hormonal factors: high levels of relaxin might contribute to development of hip dysplasia. Gonadal hormones also appear to have an effect (non-neutered Golden retrievers half as likely to develop hip dysplasia as compared to dogs neutered before 12-months of age).
4. Weight and growth: early closure of the acetabular growth plates in rapidly growing dogs may cause dysplastic joints. Body weight is unlikely to cause dysplasia but may exacerbate phenotype through increasing pressure on the joints.
5. Nutrition: excess calcium and vitamin D might contribute to the development of disease through disturbances/delays of endochondral ossification.
6. Environmental: food restriction in juvenile patients may reduce the severity of dysplasia development, weekly intramuscular polysulfated glycosaminoglycans was shown to reduce incidence in one study.

None of these factors fully explains the development of hip dysplasia.

Question 12

How is increased joint fluid volume proposed to impact the pathophysiology of hip dysplasia?

Answer 12

With increased joint fluid volume there is loss of passive intracapsular pressure and delayed mechanoreceptor activation during subluxation of the hip joint during the swing phase of the gait. This results in delayed activation of the periarticular muscles and traumatic reduction of the hip joint during the stance phase of the gait.

Question 13

What are the two forms of hip dysplasia?

Answer 13

Juvenile or severe form: patients present between 5-12 months of age. Clinical signs are thought to be related to extreme joint laxity with stretching and tearing of the joint capsule, ligaments and muscles, along with microfracture of the dorsal acetabular rim. As dogs age the clinical signs may resolve due to periarticular fibrosis, before recurring later in life with severe DJD.

Chronic form: variable onset in older patients associated with progressive DJD of the joint.

Question 14

What clinical signs might be observed in dogs with hip dysplasia?

Answer 14

Initially a wide based stance to try and prevent hip luxation. Once this fails, a narrow stance may be adopted to try and prevent pain associated with hip reduction.

Swaying of the hips, bunny-hopping gait, shifting of the weight to the thoracic limbs with hyperextension of the tarsi and stifles is also common.

Question 15

What are some tests that can be used to assess hip joint laxity in the puppy?

Answer 15

1. Bardens: laterally directed pressure on the femur (lifting of the femur from the medial aspect). Any movement greater than 1/2 inch is suggestive of hip joint laxity.
2. Barlow: first part of the Ortolani procedure (prior to reduction).
3. Ortolani: subluxation and reduction of the hip. Some surgeons use the angle of subluxation and reduction to determine the minimum and maximum angles required for TPO.

Question 16

What percentage of Ortolani negative dogs have hip joint laxity as determined by distraction index?

Answer 16

50%. This is likely due to dorsal acetabular remodelling in these cases.

Question 17

What is the accuracy of hip-extended radiography in diagnosis of canine hip dysplasia?

Answer 17

Wind-up of the hip joint during extension can lead to false negative results.

Is typically considered inaccurate in juvenile patients, but accuracy may increase with age (92-95% by 2 years of age, compared to 69% at 1 year, and 32% at 6 months). This is because definitive radiographic signs of dyplasia become more evident.

Question 18

What are some additional radiographic signs that may be useful to help predict future development of hip dysplasia on hip-extended radiography?

Answer 18

1. Caudolateral curvilinear osteophyte (CCO). Needs to be distinguished from the ‘puppy line’ that occurs in the same area but that disappears at 18-months of age and has no correlation with development of dysplasia.
2. Circumferential femoral head osteophyte (CFHO).

Question 19

What are some described hip scoring systems?

Answer 19

1. OFA: seven point, ordinal grading system. Dogs must be 2 years of age. Voluntary submission (‘normal’ hips are 8.2 times more likely to be submitted). 54% of hips scored excellent on the OFA system have DI > 0.30.
2. FCI: letter grades A through E, with subgroups 1 and 2 (grade A1 being the best grade). Norberg angles are used to assess laxity (>100 degrees grade A or B). Interobserver agreement between grades is low.
3. BVA (AVA/NZVA): each hip joint is evaluated for nine radiographic features with a score between 0 to 6, for a maximum score of 53 per hip (0 being ideal). Only 2 parameters assess laxity. Film submission is voluntary. Predictive accuracy for hip dysplasia is low.
4. PennHip: most evidence based screening test to date. Can be performed from 16 weeks of age. All radiographs must be submitted.
5. Dorsolateral subluxation: femoral heads are forced to subluxate in a dorsolateral direction and the degree of subluxation is quantified. Doesn’t seem to be accurate in predicting dogs susceptible to OA.
6. Fluckkiger subluxation index: similar to the dorsolateral subluxation index, but the dog is in dorsal recumbency. No long term studies assessing predictive accuracy.

Question 20

Describe how supposed normal phenotype on canine hip dysplasia screening with bad genotype could influence the gene pool.

Question 21

What are the three radiographic views performed for PennHip evaluation?

Answer 21

1. Ventrodorsal hip extended.
2. Neutral compression radiograph.
3. Neutral distraction radiograph.

Question 22

Why is the dorsolateral subluxation index thought to be inaccurate in the prediction of dogs developing hip dysplasia?

Answer 22

1. The hips are tested with the legs slightly extended and adducted. This reduces passive laxity by 30%.
2. The degree of subluxation is sensitive to errors in positioning.
3. Magnitude of subluxation is likely influenced by chronic arthritic changes.

Question 23

What hip distraction measurement technique is depicted?

Answer 23

Question 24

Can ultrasound be used in the assessment of hip dysplasia?

Answer 24

Dynamic ultrasound be used to assess laxity prior to femoral head ossification (8-16 weeks). However, there is large imprecision in repeated measures precluding routine use.

Question 25

What two angles based on CT of the hip joint were shown to positively correlate with hip distraction angle?

Answer 25

1. Center edge angle.
2. Dorsal acetabular sector angle.

Question 26

Why is a well defined phenotype essential for development of genomic associations for hip dysplasia?

Answer 26

It will be possible to establish genomic associations between these well-characterized “screening phenotypes” and single nucleotide polymorphism markers, quantitative trait loci (QTLs), or genes themselves,.

Question 27

What are the characteristics of an ideal hip screening tool?

Answer 27

1) Should be accurate (i.e. closely associated with the unwanted metric such as OA).

2) Should be precise.

3) Should have a continuous ratio scale (not an ordinal scale).

4) Should be measurable as early as possible in life and should remain constant throughout life.

5) Must have a heritable component (heritability is a number between 0 and 1. Heritability values >0.5 allows for rapid genetic change within a breeding population).

6) Must be sufficient range within the population to apply selection pressure.

Question 28

Why are estimated breeding values useful?

Answer 28

They allow for more accurate determination of an individuals genotype for lower heritability traits.

They combine pedigree and phenotypic information.

Result in much more rapid genetic change as compared to a simpler approach of mass selection.

Question 29

What determines the rate of genetic change within a population?

Answer 29

The heritability and selection differential.

Question 30

Why is the PennHip distraction index an ideal measure for enacting genetic change within a population for hip dysplasia?

Answer 30

It has a high heritability for phenotype (hip joint laxity), and the phenotype has a high correlation with development of OA in short and long-term studies compared to other measures.

The Norberg angle and hip-extended radiographs have low correlation with development of OA. And other scoring systems have low heritability or predictive value for OA.

Question 31

What is one of the problems of mass selection in a small cohort of dogs?

Answer 31

Inbreeding may result.

A moderate strategy whereby the dogs in the top 40% of the breed for hip scoring is selected for breeding (rather than selection of the best hip scores each time), might balance the issues of selection pressure and inbreeding.

Question 32

In summary, why has relatively slow genetic progress been made in the improvement of canine hip dysplasia with use of the current hip screening tools (aside from PennHip)?

Answer 32

1) Low heritability of the hip-extended phenotype.

2) Poor relationship between the hip-extended phenotype and later development of OA.

3) A breed distribution that leaves little room for applying meaningful selection pressure (e.g. the average Labrador retriever hip score is OFA good).