Canine Hip Dysplasia Study Guide

Overview and Clinical Importance

Canine hip dysplasia (CHD) is the most common developmental orthopedic disease in dogs, characterized by coxofemoral joint laxity, progressive degeneration, and secondary osteoarthritis (OA). First described in the 1930s, CHD continues to affect millions of dogs worldwide, with prevalence ranging from 1% to over 70% depending on breed. Understanding CHD is essential for the NAVLE as questions frequently test pathophysiology, diagnosis, breed predisposition, radiographic interpretation, and treatment selection.

CHD is a polygenic, multifactorial disease where both genetic predisposition and environmental factors (nutrition, exercise, growth rate) influence expression. Dogs are born with normal hips, but joint laxity develops within the first few weeks of life, leading to abnormal joint mechanics, cartilage damage, and eventual OA.

Etiology and Pathophysiology

Genetic Factors

CHD is a polygenic trait with complex inheritance patterns. The heritability ranges from 0.2 to 0.6 depending on breed. Key genetic factors include abnormal collagen composition in the joint capsule leading to reduced stiffness, delayed endochondral ossification of hip structures, and variations in genes associated with cartilage and bone development.

Environmental Factors

Environmental factors modify genetic expression and disease severity. These include excess caloric intake causing rapid growth (particularly in large breed puppies), excessive calcium and vitamin D supplementation, high-impact exercise during growth, and overweight body condition. Notably, lifelong caloric restriction (25% reduction) has been shown to delay OA onset from 6 years to 12 years in Labrador Retrievers.

Pathophysiology Sequence

The pathophysiology follows a predictable cascade. Joint laxity develops in the first weeks of life, often detectable by 8 weeks of age. This laxity causes the femoral head to subluxate during weight-bearing, creating abnormal mechanical stresses. The dorsal acetabular rim experiences microfractures and remodeling. Joint capsule stretching and synovitis develop by 3-5 months. Progressive cartilage damage leads to secondary osteoarthritis, with osteophyte formation, subchondral sclerosis, and joint remodeling.

Breed Predisposition

CHD predominantly affects large and giant breed dogs, though it can occur in any breed including small dogs and cats. The following table summarizes breed-specific information that is commonly tested on the NAVLE.

Clinical Signs and Physical Examination

Bimodal Age Distribution

CHD presents with a characteristic bimodal age distribution. Juvenile dogs (5-12 months) present acutely, while mature dogs (typically over 3-5 years) present with chronic signs of OA.

Physical Examination Findings

Key physical examination findings include pain on hip extension (most consistent finding), decreased range of motion particularly in extension and abduction, muscle atrophy of hindlimb muscles (especially gluteal and quadriceps), positive Ortolani sign indicating joint laxity, and crepitus palpable during hip manipulation in chronic cases.

The Ortolani Test

The Ortolani test is a critical examination technique for detecting hip joint laxity. The test is performed with the dog in lateral or dorsal recumbency under sedation. With the stifle flexed to 90 degrees, axial pressure is applied along the femur toward the pelvis to subluxate the femoral head. The limb is then slowly abducted. A positive Ortolani sign is an audible or palpable "clunk" as the femoral head reduces back into the acetabulum.

Diagnostic Imaging

While clinical signs and physical examination can suggest CHD, radiography is the gold standard for definitive diagnosis. Several standardized evaluation systems exist worldwide.

OFA (Orthopedic Foundation for Animals)

The most widely used system in North America. Dogs must be at least 24 months old for official certification. A standard ventrodorsal hip-extended radiograph is evaluated by three independent radiologists. Positioning requires the dog in dorsal recumbency, femurs extended and parallel, internally rotated so patellas are centered over femoral condyles.

PennHIP (Pennsylvania Hip Improvement Program)

PennHIP provides a quantitative assessment of hip laxity and can be performed as early as 16 weeks of age. Three radiographic views are obtained: compression view (assesses joint congruity), distraction view (measures maximum passive laxity), and hip-extended view (assesses OA changes).

The Distraction Index (DI) is calculated as the distance between femoral head and acetabular centers divided by the femoral head radius. DI ranges from 0 to 1, with lower values indicating tighter hips. A DI ≤0.3 indicates minimal risk for OA development, while DI greater than 0.7 indicates high susceptibility to CHD and OA.

Norberg Angle

The Norberg angle is measured on ventrodorsal hip-extended radiographs. It is formed by a horizontal line connecting the centers of both femoral heads and a line from each femoral head center to the cranial acetabular rim. A Norberg angle ≥105 degrees is traditionally considered normal, though breed-specific variations exist (German Shepherds may have normal angles as low as 92-102 degrees).

Radiographic Signs of CHD

Treatment Options

Treatment selection depends on patient age, disease severity, presence of OA, and owner goals. Options are divided into conservative (medical) management and surgical intervention.

Conservative Management

Weight management is the single most important aspect of conservative therapy. Studies demonstrate that maintaining lean body weight is as effective as NSAID therapy for pain relief. Dogs should have easily palpable ribs without excess fat covering.

Surgical Options

Surgical options are categorized as preventive (performed before OA develops) or salvage procedures (performed after significant joint degeneration).

Juvenile Pubic Symphysiodesis (JPS)

Age window: 12-20 weeks (earlier is better; ideally 12-16 weeks). This minimally invasive procedure uses electrocautery to cause premature fusion of the pubic symphysis. As the pelvis continues to grow, the acetabula rotate ventrolaterally, improving femoral head coverage by 40-46%. JPS is indicated for puppies with documented hip laxity but NO radiographic evidence of OA. The procedure is contraindicated if OA changes are already present.

Double/Triple Pelvic Osteotomy (DPO/TPO)

Age window: 5-10 months (before skeletal maturity). Osteotomies of the ilium, ischium, and pubis allow manual rotation of the acetabulum to improve femoral head coverage. The ilium is stabilized with specialized bone plates. Success rates of 86-92% for return to normal function. Candidates must have NO or minimal OA changes. The procedure is contraindicated if significant OA is present.

Total Hip Replacement (THR)

Age: 12+ months (skeletal maturity). THR is considered the gold standard salvage procedure for dogs with severe CHD and OA. Both cemented and cementless systems are available. Success rates of 90-95% for return to normal function. THR is ideal for large breed dogs where optimal function is desired. Complications include implant loosening, luxation, infection (less than 10% overall).

Femoral Head Ostectomy (FHO)

Age: Any age after growth plates close. FHO involves surgical removal of the femoral head and neck. A fibrous pseudoarthrosis forms. FHO is best suited for smaller dogs (less than 20-25 kg) where soft tissue support can adequately maintain limb function. Post-operative physical therapy is essential. Expect some degree of limb shortening and gait abnormality. FHO is less expensive than THR but provides inferior functional outcome in large breed dogs.

Prevention and Breeding Recommendations

Prevention strategies focus on selective breeding and environmental modification. For breeding, only dogs with normal hip evaluations AND normal littermates/parents should be bred. PennHIP screening at 16 weeks can identify at-risk puppies for early intervention. Dogs with CHD should never be used for breeding.

Environmental prevention includes feeding large-breed puppy food to slow growth rate, avoiding overnutrition (maintain lean body condition), avoiding excessive calcium/vitamin D supplementation, and limiting high-impact exercise in growing puppies while encouraging controlled, moderate exercise.