Feline Hip Dysplasia Study Guide

Overview and Clinical Importance

Feline hip dysplasia (FHD) is a developmental orthopedic condition characterized by abnormal formation of the coxofemoral joint, resulting in joint laxity, subluxation, and progressive osteoarthritis. While historically considered rare in cats compared to dogs, recent studies demonstrate FHD is significantly underdiagnosed due to cats' ability to mask clinical signs and the subtlety of their lameness patterns.

The reported prevalence varies dramatically between studies, ranging from 6.6% to 46.7% depending on the population studied. Purebred cats, particularly large-bodied breeds, have a significantly higher incidence than domestic shorthairs. Understanding this condition is essential for NAVLE success as it represents an important differential diagnosis in feline orthopedic lameness.

Hip Joint Anatomy in Cats

The feline coxofemoral joint is a ball-and-socket synovial joint formed by the articulation of the femoral head (ball) with the acetabulum (socket) of the pelvis. In cats, the acetabulum is naturally shallower than in dogs, which has important implications for joint stability and dysplasia assessment.

Key Anatomical Components

• Acetabulum: Cup-shaped depression in the pelvis formed by fusion of the ilium, ischium, and pubis. The acetabulum should cover at least 50% of the femoral head for normal function.
• Femoral Head: The spherical proximal end of the femur that articulates with the acetabulum. Covered by articular (hyaline) cartilage for smooth joint movement.
• Round Ligament (Ligamentum Teres): Intracapsular ligament connecting the fovea capitis of the femoral head to the acetabular fossa. Provides blood supply to the femoral head in young cats.
• Joint Capsule: Extends from acetabular rim to the femoral neck, enclosing the joint and containing synovial fluid.

Etiology and Pathophysiology

Genetic Factors

Feline hip dysplasia is a polygenic inherited condition, meaning multiple genes contribute to its development. Studies in Maine Coon cats have established a heritability of approximately 0.36 (95% CI: 0.30-0.43), indicating a moderate genetic contribution. Importantly, a genetic correlation with body mass (0.285) has been identified, suggesting that selection for larger body size in certain breeds may inadvertently select for hip dysplasia.

Environmental Factors

• Obesity: Excess weight increases mechanical stress on hip joints, accelerating cartilage degeneration and osteoarthritis development.
• Rapid Growth: Fast growth rates in kittens may contribute to abnormal joint development.
• Concurrent Orthopedic Conditions: Patellar luxation and other musculoskeletal disorders increase stress on hip joints.

Disease Progression

In FHD, the femoral head and acetabulum develop abnormally, leading to joint incongruity and laxity. Unlike dogs, joint subluxation is not consistently present in cats with FHD. The primary pathological finding is a shallow acetabulum with degenerative changes developing primarily on the craniodorsal acetabular margins rather than the femoral head and neck.

Progressive changes include: cartilage erosion, subchondral bone sclerosis, osteophyte formation (enthesophytes at acetabular margins), and eventual secondary osteoarthritis. Degenerative changes tend to develop later and are less marked than in dogs.

Breed Predispositions

Board Tip - Memory Aid: 'M-P-H' = Maine coon, Persian, Himalayan are the top 3 predisposed breeds. Remember: MPH = Miles Per Hour - these big, heavy cats put more 'miles' on their hips!

Clinical Presentation

Challenges in Recognition

Clinical signs of FHD are often subtle and gradual in onset, making recognition challenging. Cats are notorious for hiding pain and compensating for orthopedic dysfunction. Additionally, bilateral disease (present in approximately 56% of cases) makes lameness even harder to detect as there is no normal limb for comparison.

Clinical Signs

Physical Examination Findings

• Pain on hip manipulation: Cats with FHD typically resent hip abduction, sometimes more than flexion/extension
• Crepitus: Palpable grinding sensation during passive range of motion
• Decreased range of motion: Limited hip extension and abduction
• Ortolani sign: May be positive, indicating joint laxity (less reliable than in dogs)
• Muscle atrophy: Pelvic limb musculature may be reduced compared to thoracic limbs

Diagnosis

Radiographic Evaluation

Radiography is the primary diagnostic modality for FHD. The standard view is the ventrodorsal (VD) extended-leg projection, identical to the positioning used in dogs. Sedation or anesthesia is typically required for proper positioning and accurate assessment.

Radiographic Findings in Feline Hip Dysplasia

Norberg Angle in Cats

The Norberg angle (NA) is measured as the angle formed by a line connecting the centers of both femoral heads and a line drawn between the center of a femoral head and the craniodorsal rim of the acetabulum.

• Normal cats: Approximately 95 degrees (lower than dogs)
• Dysplastic cats: Typically less than 84 degrees
• Distraction Index: Greater than 0.6 associated with increased risk of osteoarthritis development

Differential Diagnoses

• Primary osteoarthritis (age-related degenerative joint disease)
• Patellar luxation
• Traumatic hip luxation or fracture
• Capital physeal fracture (slipped capital femoral epiphysis)
• Lumbosacral disease
• Neoplasia

Treatment

The majority of cats with FHD respond well to non-surgical (conservative) management. Surgical intervention is reserved for cases that fail to respond adequately to medical therapy.

Non-Surgical Management

Environmental Modification

• Provide ramps or steps to access favorite resting spots
• Use litter boxes with low entry sides
• Place food and water at floor level or easily accessible heights
• Provide soft, padded bedding
• Maintain warm environment (arthritis worsens in cold)

Weight Management

Critical component of management. Obesity significantly increases mechanical stress on joints and accelerates disease progression. A structured weight loss program with caloric restriction and controlled exercise is essential for overweight cats.

Pharmacological Management

Surgical Treatment

Surgery is indicated when conservative management fails to provide adequate pain relief and quality of life. Two primary salvage procedures are available:

1. Femoral Head and Neck Excision (FHNE)

• Procedure: Removal of the femoral head and neck, creating a pseudoarthrosis (false joint)
• Mechanism: Scar tissue forms between pelvis and femur; muscles support limb function
• Advantages: Less expensive; widely available; eliminates bone-on-bone pain
• Disadvantages: Inconsistent results; mechanical lameness may persist; affected limb slightly shorter
• Recovery: Most cats recover within 6 weeks; physical therapy recommended

2. Total Hip Replacement (THR)

• Procedure: Complete replacement of hip joint with prosthetic components (micro-THR in cats)
• Advantages: Restores normal biomechanics; excellent outcomes reported (91% success); near-normal function
• Disadvantages: Expensive; limited availability; requires specialist surgeon; potential complications
• Complications: Luxation, aseptic loosening, infection, periprosthetic fracture

Prognosis

The prognosis for cats with hip dysplasia is generally good with appropriate management. Most cats respond well to multimodal non-surgical therapy and can maintain good quality of life. FHD does not typically shorten life expectancy.

• Conservative management: Majority of cats respond well; 61% of owners report improvement with meloxicam therapy
• FHNE: Variable outcomes; most cats can walk, run, and jump but may have mild mechanical lameness
• THR: Excellent outcomes in 91% of cases; near-normal function expected

Prevention

• Breeding programs: Radiographic screening of breeding cats in at-risk breeds; do not breed affected individuals
• Weight management: Maintain healthy body weight from kittenhood; avoid rapid growth
• Breed selection: Consider FHD risk when selecting purebred cats, especially large breeds