Knee Deformity: Causes, Types, and Contributing Factors

What Causes Knee Deformity?

Knee deformities, characterized by an abnormal alignment of the bones forming the knee joint, arise from a complex interplay of genetic, developmental, traumatic, degenerative, inflammatory, and biomechanical factors that disrupt the joint's intricate structure and function.

Understanding Knee Deformity

A knee deformity refers to an abnormal angulation or rotation of the lower limb at the knee joint, deviating from its natural anatomical alignment. This can manifest as genu varum (bow-legged), genu valgum (knock-knees), genu recurvatum (hyperextension), or rotational deformities. These alterations can significantly impact gait, stability, pain levels, and the long-term health of the joint. The causes are diverse, ranging from congenital issues present at birth to acquired conditions developing over a lifetime.

Developmental and Congenital Factors

Some knee deformities originate during fetal development or early childhood, often due to genetic predispositions or growth disturbances.

• Congenital Anomalies: Rare conditions present at birth, such as congenital dislocation of the patella or tibia, or deficiencies in bone formation.
• Growth Plate Disorders: The epiphyseal growth plates (physes) are crucial for bone lengthening. Damage or abnormal growth in these areas can lead to angular deformities.
  • Blount's Disease (Tibia Vara): A progressive condition affecting the growth plate of the shin bone (tibia) near the knee, leading to severe genu varum. More common in early walkers and adolescents.
  • Rickets: A condition caused by prolonged vitamin D deficiency, leading to soft and weakened bones, which can result in bow legs or knock-knees as the bones bend under body weight.
  • Slipped Capital Femoral Epiphysis (SCFE): Although primarily a hip condition, severe SCFE can alter lower limb mechanics and contribute to compensatory knee deformities.

Traumatic Injuries

Significant trauma to the knee or surrounding bones can directly cause or contribute to the development of deformities, especially if not managed appropriately.

• Fractures: Fractures involving the growth plates in children (physeal fractures) or complex fractures of the femur (thigh bone) or tibia (shin bone) in adults can heal with malunion, leading to angular or rotational deformities.
• Ligamentous Injuries: Severe or chronic instability from torn ligaments (e.g., ACL, PCL, MCL, LCL) can lead to abnormal forces across the joint, causing cartilage degradation and eventual bone remodeling that results in deformity.
• Meniscal Tears: While less directly a cause of deformity, chronic meniscal pathology can alter joint mechanics, contributing to uneven load distribution and degenerative changes that may precede or exacerbate deformity.

Degenerative Conditions

Long-term wear and tear, coupled with the body's response, are primary drivers of acquired knee deformities, particularly in older adults.

• Osteoarthritis (OA): The most common cause of acquired knee deformity. As articular cartilage breaks down, bone-on-bone friction increases, leading to bone remodeling, osteophyte (bone spur) formation, and a collapse of the joint space, often unevenly.
  • Medial Compartment OA: Often leads to genu varum (bow-legged) due to greater cartilage loss and bone collapse on the inner side of the knee.
  • Lateral Compartment OA: Less common, but can lead to genu valgum (knock-knees) due to wear on the outer side.
• Post-Traumatic Arthritis: Arthritis that develops years after a significant knee injury, often progressing to deformity similar to primary OA.

Inflammatory Conditions

Chronic inflammation within the joint can lead to cartilage and bone destruction, ultimately resulting in deformity.

• Rheumatoid Arthritis (RA): An autoimmune disease causing chronic inflammation of the synovium (joint lining). This leads to cartilage and bone erosion, ligament laxity, and often results in significant genu valgum (knock-knees) or other complex deformities.
• Psoriatic Arthritis, Gout, Lupus: Other systemic inflammatory conditions can also affect the knee joint, leading to destructive changes and deformity over time.

Neuromuscular Disorders

Conditions affecting the nerves and muscles can lead to muscle imbalances, abnormal joint loading, and subsequent deformity.

• Cerebral Palsy: Can cause muscle spasticity and weakness, leading to contractures and deformities like genu flexum (flexion contracture) or genu recurvatum.
• Polio: Historically, muscle paralysis from polio could lead to severe knee deformities due to muscle imbalance and gravitational forces.
• Muscular Dystrophy: Progressive muscle weakness can lead to joint instability and contractures, including knee deformities.
• Charcot Arthropathy (Neuropathic Joint): A progressive degenerative joint disease that occurs in people who have nerve damage (neuropathy), often from diabetes. The loss of sensation leads to repeated trauma, microfractures, and rapid joint destruction, resulting in severe deformity.

Metabolic and Nutritional Deficiencies

Systemic conditions affecting bone metabolism can weaken the skeletal structure, making it prone to deformity.

• Paget's Disease of Bone: A chronic disorder characterized by abnormal bone remodeling, leading to enlarged and weakened bones that can bend and deform, including in the femur and tibia near the knee.
• Osteomalacia: The adult form of rickets, where bones fail to mineralize properly, leading to soft bones susceptible to bowing and deformity.

Tumors

While rare, benign or malignant tumors originating in or near the knee joint can cause bone destruction or abnormal growth, leading to deformity.

• Osteochondroma: A benign bone tumor that often develops near growth plates and can cause angular deformities if it interferes with normal growth.
• Other Bone Tumors: Less common, but cancerous tumors (e.g., osteosarcoma, Ewing's sarcoma) can destroy bone tissue, leading to structural collapse and deformity.

Iatrogenic Causes

Deformities can sometimes be an unintended consequence of medical interventions.

• Surgical Complications: Malalignment following fracture fixation, joint replacement (e.g., total knee arthroplasty), or ligament reconstruction can lead to persistent or new angular deformities.
• Radiation Therapy: High-dose radiation to the knee area, especially in children, can damage growth plates and lead to subsequent growth disturbances and deformity.

Biomechanical Factors and Chronic Stress

Even without a specific disease, chronic stress and improper mechanics can contribute to the development or progression of knee deformity.

• Malalignment: Pre-existing subtle malalignment (e.g., mild genu varum) can accelerate uneven wear in the knee, leading to progressive deformity.
• Muscle Imbalances: Weakness or tightness in surrounding muscles (quadriceps, hamstrings, hip abductors/adductors) can alter patellar tracking and distribute forces unevenly across the joint, contributing to degenerative changes and deformity over time.
• Obesity: Increased body weight places greater stress on the knee joints, accelerating cartilage degradation and potentially exacerbating or initiating deformities, particularly genu varum.
• Occupational or Athletic Stress: Repetitive high-impact activities or prolonged standing with improper mechanics can contribute to chronic stress and degenerative changes.

Understanding the underlying cause of a knee deformity is crucial for effective diagnosis and management, which often involves a combination of medical imaging, physical examination, and a detailed patient history to guide treatment strategies.