The femur, your thigh bone, articulates with three bones: the pelvis at the hip, the tibia at the knee, and the patella (kneecap) at the front of the knee. These three connections form two major joints that carry your body weight and allow you to walk, run, sit, and climb. Notably, the fibula, the thin bone running alongside the tibia, does not articulate with the femur at all.
The Pelvis: Hip Joint
At its top end, the femur has a smooth, rounded ball called the femoral head. This ball fits into a deep cup-shaped socket in the pelvis called the acetabulum, forming the hip joint (formally, the acetabulofemoral joint). It’s a classic ball-and-socket design, which gives the hip a wide range of movement: roughly 150 degrees of forward flexion, 25 degrees of backward extension, 45 degrees of outward movement, and 30 degrees of inward movement. That combination lets you swing your leg forward, backward, and side to side, and also rotate it inward and outward.
The articular cartilage covering the femoral head averages between 1.2 and 1.8 millimeters thick. That thin layer of cartilage absorbs shock and allows the ball to glide smoothly inside the socket with minimal friction. When this cartilage wears down, the result is hip osteoarthritis, one of the most common reasons for joint replacement surgery.
The Tibia: Main Knee Joint
At its bottom end, the femur widens into two rounded knobs called the medial and lateral condyles. These sit on top of the tibia’s relatively flat upper surface, forming the tibiofemoral joint. This is the primary weight-bearing connection in the knee.
The two sides of this joint are not symmetrical. On the inner (medial) side, the femoral condyle is roughly spherical and sits in a shallow, conforming socket on the tibia. On the outer (lateral) side, the femoral surface is more rounded only in one plane, and the tibial surface is flat. This asymmetry is what allows the knee to do more than simply hinge open and closed. As you bend your knee past about 20 degrees, the lateral side of the femur slides forward and backward on the tibia while the medial side mostly pivots in place. That’s why your lower leg rotates slightly as you bend and straighten your knee.
A healthy knee flexes to about 100 degrees from a fully straight position. Several major ligaments hold the tibiofemoral joint together. The medial collateral ligament (MCL) runs along the inner side and prevents the knee from bowing inward. The lateral collateral ligament (LCL) does the same on the outer side. Inside the joint, the anterior cruciate ligament (ACL) prevents the tibia from sliding too far forward, while the posterior cruciate ligament (PCL) prevents it from sliding backward. The PCL is the stronger of the two and plays a critical role in supporting body weight when the knee is bent.
The Patella: Front of the Knee
The patella is a small, triangular bone embedded in the tendon of your quadriceps muscle. It glides along a groove on the front of the distal femur called the trochlear groove, forming the patellofemoral joint. This joint doesn’t bear weight in the same way as the hip or tibiofemoral joint, but it handles enormous forces during activities like squatting, climbing stairs, and jumping.
When your knee is fully straight, the patella sits just above the trochlear groove with only light contact on its outer edge. As you begin to bend, the quadriceps tendon pulls the patella down into the groove. At around 30 degrees of flexion, the inner surface of the patella also engages. The groove acts as a track that keeps the kneecap centered. If the patella tracks poorly, whether from muscle imbalance, a shallow groove, or alignment issues, the result is anterior knee pain, often called “runner’s knee.”
Why the Fibula Doesn’t Count
The fibula sits on the outer side of the lower leg, right next to the tibia, so it’s natural to assume it connects to the femur. It doesn’t. The top of the fibula articulates only with the lateral condyle of the tibia, forming a small, nearly immobile joint. The fibula is connected to the tibia along most of its length by a tough membrane, but it never makes contact with the femur. Its primary roles are providing an attachment point for muscles and forming part of the ankle joint below.
How These Joints Work Together
The hip and knee joints share the femur, the longest and strongest bone in the body, but they serve different mechanical purposes. The hip’s ball-and-socket design prioritizes multi-directional movement and stability under load. The knee’s combination of the tibiofemoral hinge and the patellofemoral glide prioritizes powerful forward motion and the ability to lock into a stable, straight position for standing. The patella increases the mechanical advantage of the quadriceps by acting as a lever, letting the muscle straighten the knee with less effort than it would need without the kneecap in place.
All three articulations depend on smooth cartilage surfaces, strong ligaments, and balanced muscle forces. Problems at one joint tend to affect the other. A stiff hip changes how forces travel through the femur to the knee, and a painful knee alters your gait in ways that stress the hip. That interconnection is why rehabilitation for either joint almost always includes exercises targeting both.