Patella baja, also known as patella infera, is an orthopedic condition where the kneecap rests lower than its normal position within the groove of the thigh bone (femoral trochlear groove). This abnormal positioning compromises the biomechanics of the knee joint. The condition can impair the ability to bend and straighten the leg, leading to chronic pain and functional limitations. While some cases are congenital, patella baja is frequently acquired following trauma or surgical procedures affecting the knee.
The Anatomic Abnormality and Functional Impact
The patella acts as a fulcrum, increasing the mechanical advantage of the quadriceps muscle group during knee extension. In a healthy knee, the kneecap tracks smoothly within the femoral trochlear groove, allowing for efficient leverage.
When the patella is positioned too low, it engages the groove prematurely, remaining deep even when the knee is almost straight. This low position shortens the lever arm of the quadriceps tendon, demanding more muscle force for full extension. This inefficiency of the extensor mechanism is often described as weakness or “extensor lag.”
The altered mechanical alignment significantly increases contact pressure between the back of the kneecap and the femur. This excessive pressure causes chronic anterior knee pain, particularly during deep knee bending activities like squatting or climbing stairs. The abnormal loading patterns accelerate wear on the joint cartilage, potentially leading to early patellofemoral arthritis.
Patients commonly experience restricted range of motion, specifically an inability to fully bend the knee (restricted knee flexion). The low-lying patella can physically impinge against the tibia or soft tissues at the extremes of motion. This mechanical block, combined with pain and stiffness, severely limits daily activities.
The abnormal rubbing can cause a grinding sensation or noise within the joint, known as crepitus. Limited motion contributes to muscle weakness, as the quadriceps cannot be effectively exercised through a full range of movement. This cycle of weakness and stiffness reinforces the functional impairment.
Etiology: Why the Patella Shifts Low
Patella baja is most commonly acquired following injury or surgery to the knee. The underlying cause of the downward shift is the shortening or scarring of the patellar tendon and surrounding soft tissues, resulting from the body’s healing response.
A frequent cause is infrapatellar contracture syndrome, characterized by dense scar tissue (fibrosis) formation in the fat pad beneath the kneecap. This fibrosis pulls the patella distally, reducing the patellar tendon length. The risk is elevated following surgeries like anterior cruciate ligament (ACL) reconstruction, especially when a patellar tendon graft is used, or after complex knee fractures.
Total knee arthroplasty (TKA) frequently precedes patella baja, sometimes caused by true shortening of the patellar tendon due to scarring. However, “pseudo-patella baja” can occur when the surgical implant placement raises the joint line of the tibia relative to the patella. In this case, the tendon length is normal, but the kneecap’s position relative to the new joint surface is functionally too low.
Traumatic injuries, such as a direct blow or a tibial fracture, can also lead to the condition without surgery. The subsequent immobilization and inflammatory response can trigger the shortening of the quadriceps tendon or the formation of restrictive adhesions. In rare instances, the condition is congenital, resulting from a structurally abnormal knee or a shorter patellar tendon.
Clinical Identification and Imaging Measurements
The definitive diagnosis and quantification of patella baja rely on specialized imaging, primarily a lateral view X-ray of the knee taken at a specific angle of flexion. Radiographic measurements are necessary to confirm the diagnosis and severity, as physical examination alone is insufficient. These measurements involve calculating a ratio between the length of the kneecap and the length of the patellar tendon.
Two widely used methods for assessment are the Insall-Salvati ratio and the Caton-Deschamps index. The Insall-Salvati ratio compares the length of the patellar tendon (from the lower pole of the patella to the tibial tuberosity) to the greatest diagonal length of the patella. A normal ratio is approximately 1.0, and a ratio below 0.8 indicates patella baja.
The Caton-Deschamps index is calculated by dividing the distance from the inferior articular surface of the patella to the anterosuperior angle of the tibia by the length of the patellar articular surface. A normal range is 0.6 to 1.3, with a ratio below 0.6 indicating patella baja. These indices provide objective, numerical values for tracking the condition and planning corrective treatment.
Magnetic resonance imaging (MRI) is often utilized alongside X-rays to provide a detailed view of the soft tissues surrounding the knee. MRI assesses the extent of patellar tendon shortening, the presence of infrapatellar fat pad fibrosis, and potential damage to the articular cartilage. This approach helps differentiate between true patella baja (tendon shortening) and pseudo-patella baja (joint line changes).
Management Strategies for Restoration of Function
Initial management of patella baja involves conservative measures, though these are often limited in fixed, acquired cases. Physical therapy focuses on stretching exercises to gently lengthen the quadriceps mechanism and improve the flexibility of stiff periarticular tissues. Pain is managed using non-steroidal anti-inflammatory drugs and local modalities to reduce discomfort.
If conservative treatment fails and the low position is fixed, surgical intervention is required to restore normal knee function. The most common surgical solution is a proximalization tibial tubercle osteotomy (P-TTO). This procedure involves cutting the bony prominence on the tibia where the patellar tendon attaches and repositioning it higher on the shin bone.
Moving the tibial tubercle proximally raises the entire patellar complex, restoring the normal length of the extensor mechanism’s lever arm. This correction allows the patella to track properly within the femoral groove, reducing patellofemoral joint pressure and increasing range of motion. For severe tendon contracture, the surgeon may also perform a patellar tendon lengthening procedure, sometimes augmented with a graft.
The goal of surgical management is to correct the patellar height to within the normal range defined by radiographic indices. This restoration of proper biomechanics alleviates anterior knee pain and improves the patient’s ability to fully bend and straighten the knee. While P-TTO can significantly improve function, patient outcomes may remain diminished compared to healthy knees, highlighting the complexity of treating this condition.