The Knee — Architecture & Function
The knee joint connects the femur (thigh bone) to the tibia (shin bone) and incorporates the patella (kneecap) as a third bone. It is classified as a modified hinge joint — primarily flexing and extending, but also allowing a degree of rotation that is essential to normal gait mechanics.
The knee is far more than a simple hinge — it is a bicondylar synovial joint capable of flexion, extension, and a degree of rotation that increases as the knee flexes. This complexity is both its greatest functional asset and the reason it is uniquely vulnerable to injury and degenerative disease.
Three articulations coexist within the knee: between the medial femoral condyle and the medial tibial plateau, between the lateral femoral condyle and the lateral tibial plateau, and between the posterior surface of the patella and the anterior femur (the patellofemoral joint). All three are enclosed within a single joint capsule and lined by a continuous synovial membrane.
The knee has evolved to meet two fundamentally competing demands: stability (it must remain controlled under loads that can exceed five times body weight) and mobility (it must allow the range of movement needed for walking, sitting, climbing and rising from a chair). It achieves this balance through an elaborate system of bony geometry, fibrocartilaginous menisci, cruciate and collateral ligaments, and surrounding musculature — components that work together seamlessly in a healthy knee, and whose failure contributes to the development of arthritis and dysfunction.
The Three Compartments of the Knee
The knee joint contains three distinct compartments, each of which can be affected independently by arthritis — an important fact that determines whether total or partial knee replacement is appropriate for a given patient.
Medial Compartment
Between the inner femoral condyle and the medial tibial plateau. The most commonly affected compartment in osteoarthritis — particularly in patients with varus (bow-legged) alignment, which concentrates load medially. Medial compartment OA often produces pain on the inner side of the knee, especially on stairs and walking.
Lateral Compartment
Between the outer femoral condyle and the lateral tibial plateau. Less commonly affected in isolation. More often involved in patients with valgus (knock-kneed) alignment. Lateral compartment OA is a feature of rheumatoid arthritis and post-traumatic arthritis as well as primary OA.
Patellofemoral Compartment
Between the posterior surface of the patella and the trochlear groove of the femur. Pain is characteristically felt at the front of the knee — worse on stairs, rising from a chair and kneeling. Isolated patellofemoral OA may be amenable to partial (patellofemoral) replacement.
The Bones of the Knee
The Femur (Thigh Bone)
The distal femur expands into two rounded prominences — the medial and lateral femoral condyles — which articulate with the tibia below. Between them posteriorly is the intercondylar notch, which houses the cruciate ligaments. Anteriorly, the condyles blend into the trochlear groove, the articulating surface for the patella. The shape of the condyles — rounder posteriorly and flatter anteriorly — determines the changing roll-glide mechanics of the knee through its arc of flexion.
The Tibia (Shin Bone)
The proximal tibia forms the tibial plateau — a flat bony surface divided by the tibial spine into medial and lateral portions. The medial plateau is slightly concave; the lateral plateau is convex, a geometry that increases joint mobility at the cost of intrinsic bony stability. The tibial tubercle on the anterior surface is the attachment point of the patellar tendon. The tibial plateau is covered in articular cartilage and supported by the menisci.
The Patella (Kneecap)
The patella is the largest sesamoid bone in the body — a bone that develops within the quadriceps tendon and articulates with the trochlear groove of the femur. Its primary mechanical role is to increase the lever arm of the quadriceps muscle, enhancing the efficiency of knee extension by approximately 30–50%. It acts as a bony shield protecting the knee joint anteriorly. The posterior surface of the patella is covered in articular cartilage — the thickest in the body at up to 7 mm.
The Fibula
The fibula is not part of the knee joint itself — it lies alongside the tibia and does not bear significant weight through the knee. However, the fibular head is an important anatomical landmark: the lateral collateral ligament attaches to it, and the common peroneal nerve winds around its neck — explaining why this nerve is at risk in lateral knee injuries, valgus deformity correction and certain surgical approaches.
The Menisci — The Knee's Shock Absorbers
The menisci are two C-shaped wedges of fibrocartilage — one on the inner (medial) side and one on the outer (lateral) side of the knee — that sit between the femoral condyles and the tibial plateau. They are among the most functionally important structures in the knee, and their loss or damage has significant consequences for joint health.
Functions of the Menisci
- Load distribution: The menisci increase the contact area between the femur and tibia by approximately 70%, reducing peak contact stresses on the articular cartilage. Without the menisci, contact stresses increase dramatically — accelerating cartilage wear.
- Shock absorption: Their viscoelastic properties allow them to deform under load and absorb energy, protecting the cartilage and underlying bone from impact.
- Joint stability: The menisci act as "wedges" that deepen the tibial surface, improving congruence between the femoral condyles and tibial plateau. The medial meniscus is particularly important as a secondary stabiliser against anterior tibial translation in ACL-deficient knees.
- Lubrication & nutrition: Their movement during flexion and extension helps distribute synovial fluid across the articular surface, contributing to cartilage lubrication and nutrition.
- Proprioception: The menisci contain mechanoreceptors that contribute to proprioceptive feedback — the sense of joint position — which is important for neuromuscular knee control.
Meniscal Tears & Osteoarthritis
Meniscal tears are one of the most common knee injuries. In younger patients, tears typically result from acute twisting injuries. In older patients, meniscal tears are often degenerative — occurring gradually as the tissue weakens with age, and frequently co-existing with early osteoarthritis. The loss of meniscal tissue — whether from a tear or surgical removal (meniscectomy) — significantly increases the load transmitted through the articular cartilage and accelerates the development of osteoarthritis. This is why preserving meniscal tissue wherever possible is now a central principle in modern knee surgery.
The Ligaments of the Knee
The knee relies heavily on its ligaments for stability — far more so than the hip, which benefits from deep bony congruence. Four main ligaments control and stabilise knee movement, each providing resistance to different directional forces.
Anterior Cruciate Ligament (ACL)
The ACL runs from the anterior tibial plateau to the posterior lateral wall of the intercondylar notch. It is the primary restraint against anterior tibial translation (the tibia sliding forward relative to the femur) and also resists rotational forces. It is one of the most commonly injured ligaments in sport. ACL deficiency leads to instability and, over time, accelerated cartilage wear. The ACL is removed during total knee replacement.
Posterior Cruciate Ligament (PCL)
The PCL runs from the posterior tibial plateau to the anterior medial wall of the intercondylar notch. It resists posterior tibial translation and is the primary stabiliser against the tibia being pushed backwards. The PCL is stronger than the ACL and less commonly torn in isolation. Whether to preserve or sacrifice the PCL during knee replacement is an important surgical decision that influences implant design selection.
Medial Collateral Ligament (MCL)
The MCL runs along the inner aspect of the knee from the medial femoral condyle to the medial tibia. It is the primary stabiliser against valgus stress (force pushing the knee inward). The MCL is preserved during knee replacement and carefully balanced to ensure equal tension on both sides of the joint throughout the arc of motion — "ligament balancing" — which is one of the most technically important steps in the operation.
Lateral Collateral Ligament (LCL)
The LCL runs along the outer aspect of the knee from the lateral femoral condyle to the fibular head. It provides the primary resistance to varus stress (force pushing the knee outward). It is a cord-like structure, distinct from the MCL which is a broad, flat band. The LCL is also preserved and balanced during total knee replacement, and its integrity is important for lateral stability after surgery.
Why ligament balancing matters so much in knee replacement: Knee replacement relies on the patient's own collateral ligaments for stability and controlled motion. If the medial and lateral structures are not in equal tension throughout the arc of flexion, the knee will feel loose in some positions, tight in others, or will track asymmetrically — all of which compromise the outcome. Achieving a well-balanced knee is the single most important technical determinant of a good knee replacement result.
Articular Cartilage & Synovium
The weight-bearing surfaces of the knee — the femoral condyles, tibial plateau and posterior patella — are covered in articular cartilage, a specialised tissue that provides a smooth, near-frictionless bearing surface capable of withstanding the extreme loads of daily activity.
Articular cartilage in the knee is composed predominantly of water, type II collagen and proteoglycans — the same composition as in the hip. It is avascular and has very limited capacity for self-repair. Damage to the articular cartilage — whether from a single injury, from cumulative wear, or from the chronic inflammation of arthritis — is progressive and does not heal reliably without intervention.
The entire joint is enclosed within a synovial membrane, which produces the synovial fluid that lubricates the articular surfaces and provides nutrition to the avascular cartilage. In inflammatory arthritis (such as rheumatoid arthritis), the synovial membrane becomes chronically inflamed — producing excessive fluid (an effusion) and releasing enzymes that actively destroy cartilage and bone. In osteoarthritis, the synovium responds to cartilage debris with low-grade inflammation that contributes to pain and swelling.
The Patellofemoral Joint
The patellofemoral joint — where the patella tracks in the trochlear groove of the femur as the knee flexes and extends — generates some of the highest pressures of any joint surface in the body. During stair climbing, the patellofemoral joint reaction force can reach 3–4 times body weight; during deep squatting it may exceed 7 times body weight. This explains why patellofemoral arthritis produces pain that is characteristically worse on stairs, rising from a chair and kneeling — movements that dramatically increase the load on this compartment.
The patella is guided in the trochlear groove by the balance of forces from the quadriceps muscles, the patellar tendon and the medial and lateral patellofemoral ligaments. Abnormal patellar tracking — whether due to muscle imbalance, anatomical variation or malalignment — increases contact stresses on the articular cartilage and is an important contributing factor to patellofemoral arthritis and anterior knee pain.
The Muscles Acting on the Knee
The muscles that cross the knee joint provide dynamic stability, generate the forces needed for movement, and protect the joint from adverse loading. Their strength — particularly that of the quadriceps — is one of the most important determinants of knee joint health and of recovery after knee replacement surgery.
The Quadriceps — The Most Important Muscle Group
The quadriceps femoris — comprising rectus femoris, vastus medialis, vastus lateralis and vastus intermedius — is the primary extensor of the knee. It is the most important muscle group for everyday function: rising from a chair, climbing stairs and walking on level ground all require adequate quadriceps strength. The vastus medialis oblique (VMO) fibres, inserting at an oblique angle into the patella, are particularly important for patellar tracking and for the final degrees of full knee extension.
Quadriceps weakness — whether from disuse, pain inhibition or the temporary effects of surgery — is the single most common cause of functional limitation after knee replacement. Strengthening the quadriceps is the central focus of pre-operative physiotherapy (prehabilitation) and post-operative rehabilitation alike. The stronger the quadriceps at the time of surgery, the faster and more complete the recovery.
- Hamstrings (biceps femoris, semitendinosus, semimembranosus) — the primary knee flexors and secondary stabilisers. They also play an important role in controlling tibial rotation during the gait cycle.
- Gastrocnemius — though primarily an ankle plantarflexor, the two heads of the gastrocnemius cross the posterior knee and contribute to knee flexion. Tightness in the gastrocnemius can limit full knee extension.
- Popliteus — a small but important muscle at the back of the knee that "unlocks" the fully extended knee at the start of flexion by internally rotating the tibia relative to the femur (or externally rotating the femur relative to the tibia in a closed-chain position).
- Iliotibial band (ITB) — the thick fascial band running down the lateral thigh. It contributes to lateral knee stability and, when tight, can cause lateral knee pain (iliotibial band syndrome) in active patients.
How the Knee Works
The knee is not a simple hinge — its motion involves a complex combination of rolling, sliding and rotation that changes continuously through the arc of movement. Understanding this complexity helps explain why knee replacement, while highly effective, requires careful surgical technique to restore natural movement patterns.
The Screw-Home Mechanism
As the knee approaches full extension, the tibia automatically externally rotates relative to the femur — or equivalently, the femur internally rotates relative to the tibia in a weight-bearing position. This "screw-home" mechanism tightens the cruciate ligaments and collateral ligaments, effectively "locking" the knee into a stable extended position with minimal muscular effort. It is the action of the popliteus muscle that unlocks this mechanism at the start of knee flexion.
Loading Through the Knee
The forces transmitted through the knee during everyday activity substantially exceed body weight. During level walking, the knee joint reaction force is approximately 2–3 times body weight. On stairs this rises to 3–4 times body weight, and during activities such as running or squatting the load can exceed 6–7 times body weight. The distribution of this load between the medial and lateral compartments is influenced critically by the alignment of the leg — in a varus (bow-legged) alignment, 70–80% of the total load passes through the medial compartment alone, explaining why medial compartment OA is so much more common than lateral.
Alignment & Its Importance
The mechanical axis of the lower limb — the straight line from the centre of the femoral head to the centre of the ankle — should ideally pass through or near the centre of the knee joint. When this axis is deviated medially (varus) or laterally (valgus), load is concentrated in one compartment, accelerating cartilage wear. Restoring a neutral mechanical axis is one of the primary surgical goals of total knee replacement, and is a key reason why precise component positioning and limb alignment are so important in achieving a durable result.
The "joint line" in knee replacement: The knee joint line — the level at which the femur and tibia meet — must be accurately restored during knee replacement surgery. Raising or lowering the joint line by more than a few millimetres alters the biomechanics of the patellofemoral joint, changes the tension in the collateral ligaments through the arc of motion, and can produce an unsatisfactory functional result even when the implant appears radiographically well-positioned. Meticulous surgical planning and intraoperative attention to bony landmarks is essential to achieve this.