Orthopedics
Patellar instability: causes, diagnosis and treatment options
Patellar instability is a common orthopedic challenge that can affect both young and elderly patients. It is caused by inadequate guidance of the kneecap (patella) in the gliding surface of the femur, the so-called trochlea. This can cause the patella to slip out of position, often accompanied by pain, swelling, and limited mobility. Repeated dislocations, in which the kneecap usually pops outward (laterally), are particularly stressful for those affected. These dislocations can lead to permanent cartilage damage and osteoarthritis over the long term.
The patella plays a central role in knee biomechanics. As the largest sesamoid bone in the human body, it acts like a lever, transferring the power of the quadriceps muscle to the extension movement of the leg. Its stability is the result of a complex interplay of bony structures, soft tissues such as ligaments and muscles, and the dynamics of the entire leg axis. A dysfunction in any of these areas can compromise the stability of the patella.
Causes and mechanisms of instability
The causes of patellar instability are diverse and often multifactorial. The anatomy of the knee joint plays an important role. A shallow or poorly developed trochlea, which normally functions as a guide channel for the kneecap, can cause the patella to slide out of position more easily. Likewise, a patella alta, or kneecap that is too high, can increase the risk of instability. Both changes contribute to the kneecap no longer being optimally held in the trochlea, especially during sharp bending of the knee.
The soft tissues surrounding the knee also influence the stability of the patella. The medial patellofemoral ligament (MPFL), which stabilizes the kneecap internally, is particularly often affected. During a dislocation, this ligament is often stretched or torn, further reducing stability and increasing the risk of recurrence. In addition, muscular imbalances, such as weakness of the quadriceps muscle or a dominant pull of the outer portion of the muscle, can contribute to an external displacement of the patella.
Symptoms and long-term consequences
The first patella dislocation often occurs in connection with a sudden load or trauma, such as a fall or an abrupt change in direction during a sport. A sudden protrusion of the kneecap is typical, accompanied by pain and a visible misalignment. The knee is often severely swollen, as bleeding into the joint can occur.
Repeated dislocations or subluxations, in which the patella partially slips out of the trochlea, lead to long-term damage to the articular cartilage. The constant microtrauma can cause osteoarthritis in the patellofemoral joint, which is accompanied by pain, stiffness, and limited mobility. These chronic changes not only impair daily life but also the ability to participate in sporting activities.
Diagnostic procedures
The diagnosis of patellar instability begins with a thorough medical history, focusing on previous dislocations, pain, and limited mobility. A clinical examination is essential to assess factors such as mobility, leg axis, and ligament stability. Tests such as the apprehension test, in which the examiner gently pushes the kneecap outward, can reveal the patient's feelings of insecurity and fear of recurrence.
Imaging techniques play a central role in diagnosis. X-rays of the knee in various planes allow the assessment of the bony structures and can reveal deformities such as a patella alta or a flat trochlea. Magnetic resonance imaging (MRI) is particularly useful for diagnosing soft tissue damage such as MPFL tears or cartilage damage. In complex cases, computed tomography (CT) with 3D reconstruction can help better analyze the exact anatomy and any bone defects.
Treatment options
Treatment for patellar instability depends on the severity of the instability and the underlying causes. After the initial dislocation, conservative treatment is often possible. This includes physical therapy to strengthen the stabilizing muscles, particularly the quadriceps, as well as the use of orthoses or taping to stabilize the patella. Anti-inflammatory medications and cooling can also reduce pain and swelling.
If repeated dislocations occur despite conservative measures, surgical treatment is often necessary. The goal of the operation is to permanently restore the stability of the patella and prevent further damage to the knee. A frequently performed method is reconstruction of the MPFL, in which a tendon is used as a graft to replace the function of the damaged ligament. In cases of severe bony deformities, trochleoplasty can also be performed, in which the trochlea is deepened to provide better support for the patella. Another option is the transfer of the tibial tuberosity to change the direction of traction of the patella.
Follow-up treatment and long-term prospects
After surgery, structured follow-up care is crucial for treatment success. During the first few weeks, the knee is usually protected by a brace that allows controlled mobilization. Physiotherapy begins in parallel, gradually restoring mobility and muscle strength. A full return to sporting activities is usually possible after about six to twelve months.
In the long term, many patients can achieve stable and pain-free knee function through consistent treatment. The prognosis is favorable, especially with early treatment and individually tailored therapy. However, preventing recurrent dislocations remains a key goal, as repeated injuries can promote the development of osteoarthritis.
Patellar instability requires careful diagnosis and individually tailored treatment to prevent long-term damage. Modern conservative and surgical treatment methods now offer effective options for restoring patella stability and significantly improving the quality of life of those affected.