Hip dysplasia

What it is

Hip dysplasia is a developmental disorder of the coxofemoral joint. The puppy is born with normal hips, but during growth the femoral head and the acetabulum fail to fit properly: the joint becomes lax, the head subluxates with every step and the cartilage wears down. The long-term result is secondary osteoarthritis.

Who it affects

Large and giant fast-growing breeds: German shepherd, Labrador, golden retriever, rottweiler, Saint Bernard. It has a polygenic genetic basis modulated by environmental factors, especially excess weight and exercise during growth.

Anatomy involved

• Femoral head and neck: the head loses its spherical shape and flattens; the neck thickens with osteophytes.
• Acetabulum: becomes shallow; the dorsal rim rounds off and no longer contains the head.
• Ligament of the femoral head and joint capsule: stretch, which perpetuates the laxity.

Clinical signs

Pelvic limb lameness, "bunny hopping" gait when running, difficulty rising, pain on hip extension, atrophy of the thigh muscles. In puppies of 4 to 12 months painful laxity predominates; in adults, osteoarthritis.

Diagnosis

• Ortolani sign: the subluxation is reduced under sedation and a "clunk" is felt.
• Ventrodorsal radiograph with extended hips: the standard projection of screening schemes (OFA, FCI). Acetabular coverage of the head, the Norberg angle and the presence of osteophytes are assessed.
• PennHIP: measures the distraction index, a quantitative indicator of laxity, from 4 months of age.

Treatment

Conservative (weight control, controlled exercise, anti-inflammatories, physiotherapy) or surgical depending on age and grade: juvenile pubic symphysiodesis in very young puppies, triple pelvic osteotomy before osteoarthritis appears, and in adults with osteoarthritis total hip replacement or femoral head and neck excision arthroplasty.

In the cat

Hip dysplasia also occurs in cats and is more common than once thought. It is strongly represented in the Maine Coon, and is also described in the Persian, Himalayan and domestic cats; it is often bilateral. Unlike in the dog, radiographic changes concentrate mainly on the acetabulum, which is shallow and develops osteophytes on its craniodorsal margin, while the femoral head and neck remodel less.

Many cats show no obvious lameness: the signs of osteoarthritis are subtle, such as jumping less or hesitantly, reduced activity, poorer grooming or behavioural changes. Diagnosis relies on the ventrodorsal hip-extended radiograph, and screening registries exist for breeding cats. Most cases are managed conservatively: weight control, environmental adaptation, non-steroidal anti-inflammatories with renal monitoring and anti-nerve growth factor monoclonal antibodies licensed for feline osteoarthritis in some countries. In severe cases femoral head and neck excision or total hip replacement give good results. It must be distinguished from femoral capital physeal dysplasia of young cats.

Key studies in the cat

• Langenbach et al. (1998), a prospective study of 78 cats with standard and distraction radiographs. 25 (32%) had hip dysplasia, mostly mild. Cats with degenerative joint disease had more laxity (mean distraction index 0.6 versus 0.49; Norberg angle 84° versus 95°), and no cat with an index below 0.4 had degenerative joint disease. The cats were not randomly selected.
• Keller et al. (1999), 684 cats of 12 breeds, with radiographs read by three radiologists. Frequency was 6.6% (45 of 684) and depended on breed. The main sign was a shallow acetabulum with remodelling of its craniodorsal margin, with minimal remodelling of the femoral neck.
• Loder and Todhunter (2018), the OFA registry, with 2548 Maine Coons. Prevalence was 24.9%, slightly higher in males (27.3%) than in females (23.3%); 56% of cases were bilateral and severity increased with age. The data are submitted by breeders and may not represent the whole breed.
• Low et al. (2019), 5038 Maine Coons in a radiographic screening programme over 20 years. Prevalence was 37.4%, with no sex predilection, and severity increased with age and body mass. Heritability was 0.36, and selection against dysplasia reduced its severity in descendants, which were also smaller.
• Ball et al. (2022), repeated readings by three radiologists of 196 hip radiographs from 98 cats in a screening programme. Only 75% of majority-consensus scores agreed between readings, and the repeatability of a unanimous consensus could be as low as 83%. Screening grades carry considerable uncertainty.