Feline Congenital Musculoskeletal Disorders Study Guide

Overview and Clinical Importance

Congenital and developmental musculoskeletal disorders in cats represent an important category of orthopedic disease that veterinarians must recognize. While less common than in dogs, conditions such as aseptic necrosis of the femoral head (Legg-Calve-Perthes disease) and osteochondrosis (OC)/osteochondritis dissecans (OCD) can cause significant pain and lameness in affected cats. Additionally, spontaneous capital physeal fractures and femoral neck metaphyseal osteopathy are unique feline conditions that affect young, neutered male cats and are increasingly recognized in clinical practice.

Understanding the pathophysiology, signalment, clinical presentation, and treatment options for these conditions is essential for the NAVLE examination and clinical practice.

Section 1: Aseptic Necrosis of the Femoral Head (Legg-Calve-Perthes Disease)

Definition and Pathophysiology

Legg-Calve-Perthes disease (LCPD), also known as aseptic or avascular necrosis of the femoral head, is a condition affecting the blood supply to the femoral head, which is the "ball" portion of the hip joint. When blood flow to the femoral head is disrupted, the bone undergoes ischemic necrosis, leading to collapse, deformity, and secondary osteoarthritis.

Pathophysiology: The exact cause of blood supply disruption remains unknown. The femoral head receives its blood supply primarily through the medial femoral circumflex artery and the ligament of the femoral head. Interruption of this vascular supply leads to osteocyte death, weakening of the subchondral bone, and eventual collapse of the articular surface. The necrotic bone is gradually replaced by fibrous tissue and new bone formation, but the femoral head becomes misshapen and no longer articulates properly with the acetabulum.

Signalment and Epidemiology

While LCPD is relatively uncommon in cats compared to dogs, it does occur. Key signalment features include young cats typically under 1 year of age, with both male and female cats affected. Unlike dogs where toy breeds are predisposed, there is no clear breed predisposition in cats. The condition may be unilateral or bilateral, though it typically begins in one limb and may later affect the contralateral hip.

Clinical Signs and Physical Examination Findings

Radiographic Findings

Radiography is the primary diagnostic modality for LCPD. Key radiographic findings include widening of the joint space between the femoral head and acetabulum, irregular opacity and moth-eaten appearance of the femoral head, femoral head fragmentation and collapse in advanced cases, flattening or deformity of the femoral head, and secondary osteoarthritic changes including osteophyte formation.

Treatment Options

Section 2: Spontaneous Capital Physeal Fractures and Metaphyseal Osteopathy

Definition and Pathophysiology

Spontaneous capital physeal fractures (SCPF) and femoral neck metaphyseal osteopathy are unique feline conditions that result in non-traumatic fractures of the proximal femur. These conditions affect young, male neutered, overweight cats and are increasingly recognized in veterinary practice.

Key Pathophysiology: Early gonadectomy delays physeal closure in cats. The capital physis (growth plate at the femoral head) normally closes by 9-12 months of age. However, in neutered male cats, this closure is delayed due to the absence of testosterone, which normally promotes cartilage maturation and physeal closure. This results in a weakened physis that is susceptible to fracture without significant trauma.

Additionally, increased body weight in neutered cats places greater mechanical stress on the already-weakened physis, predisposing to spontaneous slippage or complete fracture of the femoral epiphysis from the metaphysis.

Risk Factors (The Four Key Factors)

Clinical Signs

Clinical presentation may be acute or chronic. Signs include hindlimb lameness (may be unilateral or bilateral), reluctance to jump (often one of the earliest owner-noticed signs), pain on manipulation of the coxofemoral joint, crepitus palpable on hip extension, and decreased range of motion. In some cases, cats may have bilateral involvement either at presentation or develop contralateral fractures within months.

Radiographic Findings

Key Radiographic Features: Displacement of the femoral epiphysis (best visualized on frog-leg VD view), radiolucent line across the femoral neck in metaphyseal osteopathy, osteolysis and remodeling of the femoral neck, contralateral open physes (capital, distal femoral, proximal tibial), and bilateral involvement in up to 35% of cases.

Comparison: SCPF vs. Metaphyseal Osteopathy

Section 3: Feline Osteochondrosis and Osteochondritis Dissecans

Definition and Pathophysiology

Osteochondrosis (OC) is a focal disturbance of endochondral ossification in which articular cartilage fails to form subchondral bone normally. This creates a zone of necrotic, weakened cartilage susceptible to injury. When fissures extend to the articular surface and form a cartilage flap, the condition is termed osteochondritis dissecans (OCD). OCD leads to synovitis, joint effusion, lameness, and degenerative joint disease.

Important Note: OC/OCD is extremely rare in cats compared to dogs. Current literature consists primarily of case reports. However, it should be considered a differential diagnosis in young cats presenting with joint lameness.

Reported Locations in Cats

Signalment

Based on case series data, the mean age at presentation is 7-11 months (range 4-14 months). Male cats are more commonly affected than females (7:3 ratio in one case series). Domestic shorthair cats predominate in reports, likely reflecting the general cat population. Bilateral involvement occurs in approximately 30% of stifle OCD cases.

Clinical Signs and Diagnosis

Clinical Presentation: Unilateral or bilateral hindlimb lameness (stifle OCD), reduced ability to jump, stifle joint swelling and effusion, pain on stifle manipulation (flexion and extension), and acute onset may be associated with minor trauma or fall.

Radiographic Findings: Subchondral bone defect on the femoral condyle, flattening of the articular surface, subchondral sclerosis, possible mineralized fragments in the joint, and joint effusion.

Advanced Imaging: CT is particularly useful for confirming diagnosis when radiographic changes are subtle. CT can better delineate the size and location of the lesion and identify loose fragments.

Treatment Options

Section 4: Femoral Head Ostectomy (FHO) - The Definitive Treatment

Femoral Head Ostectomy (FHO) is the surgical procedure of choice for most conditions affecting the feline hip, including LCPD, spontaneous capital physeal fractures, and metaphyseal osteopathy. The procedure involves removal of the femoral head and neck, eliminating painful bone-on-bone contact and allowing formation of a fibrous pseudoarthrosis.

FHO Procedure Overview

FHO Recovery in Cats

Phase 1 (Days 1-7): Focus on pain management with NSAIDs and opioids as needed. Strict activity restriction (crate rest or small room confinement). Incision monitoring for infection. Passive range of motion exercises if tolerated.

Phase 2 (Weeks 2-6): Gradual increase in activity to strengthen the joint. Prevention of excessive scar tissue stiffening. Physical rehabilitation exercises. Most cats achieve full recovery by 6 weeks.

Key Success Factors: Active, fit cats recover faster than sedentary cats. Muscle mass around the hip helps stabilize the pseudoarthrosis. Early physical therapy improves long-term outcomes. The prognosis for cats undergoing FHO is excellent.

Summary: Differential Diagnosis Comparison