The patellofemoral joint, located at the front of the knee, is where the kneecap, or patella, meets the thigh bone, known as the femur. This articulation is a fundamental component of the knee, enabling a wide range of movements necessary for daily activities. Understanding its structure and mechanics helps explain knee function and common issues. This article explores the anatomy, function, and common problems associated with the patellofemoral joint.
Anatomy of the Patellofemoral Joint
The patellofemoral joint comprises two primary bones: the patella and the distal end of the femur. The patella, commonly called the kneecap, is the largest sesamoid bone in the human body, meaning it is embedded within a tendon rather than articulating directly with other bones. It is triangular, with its wider base located superiorly and a pointed apex inferiorly.
The femur’s role in this joint involves a specialized area on its anterior, distal surface called the trochlear groove. This groove acts as a track, guiding the patella as the knee bends and straightens. The trochlear groove features medial and lateral facets, with the lateral facet being more prominent, which assists in stabilizing the patella.
Covering the posterior surface of the patella and the femoral trochlear groove is a smooth, slippery tissue called articular cartilage. This cartilage is designed to reduce friction and absorb shock during movement.
Soft tissues provide additional stability and facilitate movement within the patellofemoral joint. The quadriceps tendon connects the large thigh muscles to the superior aspect of the patella. Extending from the patella’s inferior pole to the shin bone (tibia) is the patellar tendon. These tendons, along with the retinaculum and the medial patellofemoral ligament (MPFL), help maintain the patella’s position and influence its tracking.
Function of the Patellofemoral Joint
The patellofemoral joint plays a significant role in the biomechanics of the knee, primarily by enhancing the mechanical advantage of the quadriceps muscles. The patella acts as a pulley, increasing the lever arm of the quadriceps tendon. This leverage allows the quadriceps to generate more force, making it more efficient to extend and straighten the leg.
Beyond its role in leverage, the patella also serves as a protective shield for the deeper structures of the knee joint. Positioned at the front of the knee, it safeguards the underlying femur and its articular cartilage from direct impact and external forces. This protective function is relevant during activities that involve kneeling or falls.
Another important function is patellar tracking, which describes the smooth gliding motion of the kneecap within the trochlear groove during knee flexion and extension. Proper tracking is dependent on the coordinated action of the quadriceps muscles, the integrity of surrounding soft tissues, and the specific geometry of the patella and trochlear groove. As the knee bends, the patella engages with different parts of the trochlear groove, and the contact area increases with greater knee flexion to distribute joint forces effectively. This intricate movement is observed during common actions such as walking, running, jumping, and squatting.
Common Issues Affecting the Patellofemoral Joint
The patellofemoral joint is susceptible to several common issues that can cause discomfort and limit movement. One frequently encountered condition is Patellofemoral Pain Syndrome (PFPS), often referred to as “runner’s knee.” This syndrome is characterized by pain around or behind the kneecap, which can worsen with activities like squatting, climbing stairs, or prolonged sitting with bent knees. PFPS can result from factors such as overuse, sudden increases in activity, muscle imbalances, or abnormal patellar tracking.
Patellar instability and dislocation represent another set of problems. Patellar instability occurs when the kneecap does not track correctly within the femoral groove, potentially causing it to slip partially (subluxation) or completely out of place (dislocation). Dislocations are often traumatic injuries, where the patella shifts to the outside of the knee. Factors contributing to instability include anatomical variations like a shallow trochlear groove or a high-riding patella.
Osteoarthritis can also affect the patellofemoral joint, similar to other joints in the body. This condition involves the breakdown and wear of the articular cartilage covering the patella and the trochlear groove. As the cartilage deteriorates, it can lead to pain, inflammation, stiffness, and a grinding sensation during knee movement. Patellofemoral osteoarthritis can develop due to aging, prior injuries, or long-term abnormal patellar mechanics.