A subluxation describes a partial or incomplete dislocation of a joint, where the bones briefly move out of their normal alignment before spontaneously returning to position. This temporary misalignment frequently occurs in the knee, particularly among active individuals and athletes. A knee subluxation is a significant injury because it compromises joint stability and can lead to recurrent episodes of instability. Understanding the mechanics, causes, and management of this condition is essential for effective treatment and prevention.
Defining Subluxation in the Knee Joint
A knee subluxation is a mechanical issue where one of the bones forming the joint slips partially out of its proper anatomical articulation. While the knee joint involves the femur (thigh bone) and the tibia (shin bone), the term “knee subluxation” most commonly refers to patellar subluxation, involving the kneecap. The patella normally glides within a groove on the femur called the trochlear groove as the knee bends and straightens. In a patellar subluxation, the kneecap temporarily slides partially out of this groove, most often toward the outside of the leg.
This partial displacement is distinct from a full dislocation, where the kneecap completely moves out of the groove and remains locked until a medical professional manually relocates it. Because a subluxation is temporary and self-reducing, it is generally considered less severe than a complete dislocation, but it still often causes damage to the surrounding soft tissues. The underlying instability that allows the patella to partially slip out is known as patellofemoral instability.
Common Causes and Risk Factors
Knee subluxation involves a combination of acute forces and underlying structural or functional weaknesses that predispose the joint to instability. Acute episodes frequently occur during athletic activities involving sudden twisting, pivoting, or rapid changes in direction while the foot is planted. A direct blow or impact to the front or side of the knee can also provide the force needed to push the kneecap out of its central track.
Predisposing anatomical factors are often a greater determinant of chronic subluxation than a single traumatic event. A significant risk factor is a shallow trochlear groove in the femur, which provides less of a bony restraint for the patella. Patella alta, a condition where the kneecap sits higher than normal, is another common structural issue that reduces stability within the groove.
Chronic subluxation is often linked to soft tissue imbalances, such as weakness in the vastus medialis obliquus (VMO) portion of the quadriceps muscle, which is responsible for pulling the patella inward. This muscle weakness allows the stronger lateral structures to pull the patella outward during movement, encouraging the partial slippage. Ligamentous laxity, or general looseness in the ligaments and tendons supporting the knee, also increases the risk by providing insufficient passive restraint. Individuals with a history of a previous subluxation have a significantly higher chance of experiencing a recurrence due to the initial stretching or tearing of stabilizing ligaments, such as the medial patellofemoral ligament (MPFL).
Recognizable Signs and Symptoms
The immediate experience of a knee subluxation is characterized by sudden, sharp pain and the distinct sensation of the knee “giving way.” Individuals frequently report feeling a “pop” or a “shift” as the kneecap temporarily moves out of alignment. Since the kneecap often returns to its position immediately, the initial severe pain may quickly subside, but a dull ache and instability remain.
Following the event, localized swelling develops rapidly around the kneecap due to trauma to the joint capsule and surrounding soft tissues. This swelling is often accompanied by tenderness directly over the area where the kneecap shifted. Patients may also experience a sensation that the joint is “catching” or “locking” when they attempt to bend or straighten the leg, which can be due to tissue damage or continued instability.
Diagnosis and Treatment Approaches
The medical management of a knee subluxation begins with a thorough physical examination to assess the alignment and stability of the kneecap. The doctor performs specific tests to check for tenderness, swelling, and the degree of passive movement of the patella. Imaging studies are routinely used to confirm the diagnosis and rule out related injuries.
X-rays are typically the first imaging step, assessing the bony anatomy, including the depth of the trochlear groove and the position of the kneecap, and ensuring no fractures occurred. To evaluate the integrity of the soft tissues, such as stabilizing ligaments and cartilage, a Magnetic Resonance Imaging (MRI) scan may be ordered. The MRI is useful for identifying tears in the medial patellofemoral ligament (MPFL), a common injury with patellar instability.
Initial treatment focuses on conservative measures to manage pain and inflammation, following the R.I.C.E protocol: Rest, Ice, Compression, and Elevation. Physical therapy is a central component of non-surgical management, aiming to restore stability and prevent future episodes. Therapy focuses on strengthening the muscles that control the patella’s tracking, particularly the VMO, and improving hip and core strength.
Bracing or specialized taping techniques are often employed temporarily to provide external support and guide the kneecap along its proper path. For patients who experience recurrent subluxations despite consistent physical therapy, or where significant ligament damage is present, surgical intervention may be considered. A common surgical procedure is a medial patellofemoral ligament (MPFL) reconstruction, which replaces the damaged ligament with a tendon graft to restore the primary passive restraint against the kneecap shifting outward.