What is Lateral Patellar Instability?
Lateral patellar instability is defined as a lateral shift or displacement of the patella (kneecap) as a result of disruptive changes in the medial patellofemoral ligament (MPFL) and medial patellar retinaculum.
Any damage to the supporting ligaments of the knee may cause the patella to slip out of the groove either partially (subluxation) or completely (dislocation). This misalignment can damage the underlying soft structures such as muscles and ligaments that hold the kneecap in place. Once damaged, these soft structures are unable to keep the patella (kneecap) in position. Repeated subluxation or dislocation makes the knee unstable. This condition is called knee instability. Patellar instability results from one or more complete or partial dislocations (subluxations).
Anatomy of the Patella
The patella is a small piece of bone in front of the knee that slides up and down the groove in the femur bone during bending and stretching movements. The ligaments on the inner and outer sides of the patella hold it in the femoral or trochlear groove and avoid dislocation of the patella from the groove.
Causes of Lateral Patellar Instability
Lateral patellar instability can be caused by variations in the shape of the patella or its femoral groove as the knee bends and straightens. Normally, the patella moves up and down within the femoral groove when the knee is bent or straightened. Lateral patellar instability occurs when the patella moves either partially (subluxation) or completely (dislocation) out of the trochlear groove. Common causes of lateral patellar instability include:
- Poor alignment of the kneecap
- Complete or partial dislocation
- Tight or weak thigh muscles
- Direct trauma to the knee
Signs and Symptoms of Lateral Patellar Instability
The signs and symptoms of lateral patellar instability include:
- Pain, especially when standing up from a sitting position
- Feeling of unsteadiness or tendency of the knee to give way or buckle
- Recurrent subluxation
- Recurrent dislocation
- Severe pain, swelling, and bruising of the knee immediately following subluxation or dislocation
- Visible deformity and loss of function of the knee, which often occurs after subluxation or dislocation
- Change in sensation such as numbness or even partial paralysis, which can occur below the dislocation because of pressure on the nerves and blood vessels
Diagnosis of Lateral Patellar Instability
To diagnose lateral patellar instability, your doctor will ask about your symptoms, medical history, any sports participation, and activities that aggravate your knee pain. Your doctor will perform a physical examination of your knee. Diagnostic imaging tests such as X-rays, MRIs, and CT scans may be ordered to check if your instability is due to damage to the structure of the knee or because of the tissues that attach to it.
Treatment for Lateral Patellar Instability
The treatment for instability depends on the severity of the condition and is based on diagnostic reports. Initially, your surgeon may recommend conservative treatments such as physical therapy, use of bracesand orthotics. Pain-relieving medications may be prescribed for symptomatic relief. However, when these conservative treatments yield an unsatisfactory response, surgical correction may be recommended.
Surgical corrective options include tibial tubercle osteotomy, medial patellofemoral ligament reconstruction, and sulcus-deepening trochleoplasty. Medial patellofemoral ligament repair or reconstruction is the most commonly employed procedure, whereas tibial tubercle osteotomy is employed to rectify a lateralized tibial tubercle and the related elevated lateral pull of the extensor mechanism of the knee. For a select group of patients with acute trochlear dysplasia, a sulcus-deepening trochleoplasty procedure can be an effective alternative to avert future patellar instability cases.