The inside of the knee is called the medial side, and doctors refer to the entire inner section as the medial compartment. “Medial” simply means closer to the midline of your body. This compartment contains its own set of bones, cartilage, ligaments, tendons, and fluid-filled cushions that work together to keep the knee stable and moving smoothly.
The Medial Compartment
Your knee is divided into three compartments. The medial compartment is the inner portion, the lateral compartment is the outer portion, and the patellofemoral compartment sits behind the kneecap. When a doctor says something is wrong with the “medial” side of your knee, they’re talking about any structure along the inside.
The medial compartment bears a disproportionate share of your body weight. Because of the way force travels through the leg during walking, the inner knee handles more load than the outer knee. This matters: osteoarthritis develops in the medial compartment 5 to 10 times more often than in the lateral compartment in Western populations.
Bones on the Inner Side
Two bones meet at the inside of the knee. The bottom end of the thighbone (femur) flares out into a rounded surface called the medial femoral condyle. It sits on top of a relatively flat shelf of the shinbone (tibia) called the medial tibial plateau. These two surfaces glide against each other every time you bend or straighten your leg. The tibial plateau isn’t perfectly flat. It has a slight slope and a secondary contact surface called the flexion facet, which engages the thighbone when the knee bends past about 20 degrees.
Just above the joint line on the inner side, there’s a small bony bump called the adductor tubercle, which serves as an attachment point for muscles and is a key landmark surgeons use to locate other structures.
The Medial Meniscus
Sitting between the femoral condyle and the tibial plateau is the medial meniscus, a crescent-shaped wedge of thick, rubbery cartilage. It acts as a shock absorber, spreading the force of each step across a wider area of bone so no single spot takes all the pressure. It also helps stabilize the joint by deepening the shallow surface of the tibial plateau, giving the rounded end of the thighbone a more secure seat.
The medial meniscus is less mobile than the lateral meniscus on the outer side. That relative stiffness makes it more vulnerable to tears, especially from sudden twisting motions during sports or even everyday slips.
The Medial Collateral Ligament (MCL)
The main ligament on the inner knee is the medial collateral ligament, or MCL. It has two layers. The superficial MCL is a broad band that runs from a point just above the inner knob of the thighbone down to the shinbone, spanning roughly 9 to 10 centimeters. The deep MCL sits underneath and connects directly to the medial meniscus through short, thick fibers.
The MCL’s primary job is to resist forces that push the knee inward, a stress called valgus force. If you’ve ever been hit on the outside of the knee during a sport, it’s the MCL that takes the brunt of that impact. It also acts as a secondary restraint against the shinbone rotating or sliding backward. The superficial layer provides stability from full extension all the way through deep flexion, making it active through virtually every knee position.
Tendons and the Pes Anserinus
Three tendons from different thigh and hip muscles converge on the inner side of the shinbone just below the knee joint. They’re collectively called the pes anserinus, Latin for “goose’s foot,” because the way they fan out at their attachment point resembles a goose’s footprint. These three tendons come from muscles that help bend the knee and rotate the lower leg inward.
Tucked between these tendons and the bone is the pes anserine bursa, a small fluid-filled sac that prevents friction. When this bursa becomes inflamed, a condition called pes anserine bursitis, it causes pain and swelling on the inner knee a few centimeters below the joint line. It’s common in runners, people with osteoarthritis, and those who are overweight.
The Medial Plica
One structure on the inner knee that most people haven’t heard of is the medial plica. It’s a thin fold of the synovial membrane, the tissue that lines the inside of the joint capsule. Plicae are leftover from fetal development, when the knee forms as separate cavities that eventually merge. Most people have a medial plica and never know it, because it causes no problems.
In some cases, though, repeated bending, a direct blow, or overuse can irritate the plica and trigger inflammation. This is called medial plica syndrome, and it produces a dull ache along the front-inner part of the knee, sometimes with clicking or a catching sensation during movement. It’s often mistaken for other knee problems because the symptoms overlap with meniscus tears and cartilage issues.
Common Problems in the Medial Compartment
Because the inside of the knee carries more mechanical stress than the outside, it’s where many knee problems originate. The most common conditions affecting the medial compartment include:
- Osteoarthritis: Gradual breakdown of cartilage on the inner joint surfaces. The medial compartment is the single most common location for knee arthritis.
- MCL sprains: Stretching or tearing of the medial collateral ligament, typically from a blow to the outer knee or a sudden change in direction.
- Medial meniscus tears: Damage to the inner cartilage pad from twisting, squatting, or age-related wear.
- Pes anserine bursitis: Inflammation of the bursa beneath the three converging tendons on the inner shinbone.
- Medial plica syndrome: Irritation of the synovial fold along the inner joint lining.
Pain that localizes specifically to the inner knee is a useful clue for narrowing down which of these structures is involved. Tenderness right along the joint line often points to the meniscus, while pain a few centimeters below the joint line suggests the pes anserine bursa. Pain that worsens when the knee is pushed inward against resistance is more characteristic of an MCL issue.