Steven Haas, MD
Davis V. Reyes, PT, DPT, OCS
Benjamin F. Ricciardi, MD
Dr. Haas or an immediate family member has received royalties from Innovative Medical Products and Smith & Nephew; is a member of a speakers’ bureau or has made paid presentations on behalf of Smith & Nephew; serves as a paid consultant to Smith & Nephew; has stock or stock options held in Ortho Secure; has received research or institutional support from Smith & Nephew; and has received nonincome support (such as equipment or services), commercially derived honoraria, or other non-research–related funding (such as paid travel) from APOS Medical & Sports Technologies. Neither Dr. Reyes nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this article.
The knee is a modified hinge-type synovial joint. It consists of four bones: the tibia, femur, fibula, and patella. There are three articulations: the tibiofemoral joint, patellofemoral joint, and the tibiofibular joint.
The distal femur forms the proximal aspect of the tibiofemoral joint. It consists of the medial and lateral condyles, which serve as attachment sites for many soft-tissue stabilizing structures around the knee. The medial condyle is larger, more symmetrical in the sagittal plane, and extends farther distal relative to the lateral condyle. Anteriorly, the femoral trochlea separates the medial and lateral condyles, and contains the patellofemoral articulation. Distally, the intercondylar notch separates the two condyles and contains the origins of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). The lateral epicondyle and medial epicondyle are bony prominences of the lateral and medial condyles, respectively, that serve as attachment sites for the lateral collateral ligament (LCL) and medial collateral ligament (MCL). The epicondyles are important anatomic structures in total knee arthroplasty (TKA) because a line drawn connecting these two structures in the axial plane helps identify the appropriate plane for rotation of the femoral component. Malrotation of the femoral component can contribute to poor patellofemoral joint tracking and soft-tissue imbalance after TKA.
The proximal tibia forms the distal aspect of the tibiofemoral joint. It contains soft-tissue insertions for structures such as the MCL and patellar tendon, which inserts anteriorly onto the tibial tuberosity. The medial tibial plateau has a relatively concave articular surface, while the lateral tibial plateau is not concave and has a posterior slope. This bony structure allows for a rotational moment to occur between the tibia and femur through the flexion-extension axis, which is centered on the medial side of the knee. The intercondylar eminence separates the medial and lateral tibial plateau and serves as an attachment site for the menisci and cruciate ligaments. The fibula articulates with the tibia at the tibiofibular joint. It serves as an attachment site for soft-tissue structures such as the LCL, biceps femoris muscle, and ligaments of the posterolateral corner. There is a synovial aspect to the tibiofibular joint proximally; however, the distal part of the joint is a syndesmosis with little motion, secured together by a strong interosseous ligament.
The patella is the largest sesamoid bone (a bone embedded within a tendon or muscle) in the body, and it has the thickest articular cartilage of any joint. Its posterior aspect contains hyaline cartilage and consists of medial, lateral, and odd facets, which articulate with the femoral trochlea forming the patellofemoral joint. It serves as the insertion point for the quadriceps tendon, and increases its functional lever arm during active knee extension.
Intra-articular and extra-articular soft tissue structures contribute a significant portion of the inherent stability of the knee joint. Two important intra-articular structures in the native knee are the medial and lateral menisci. The menisci are fibrocartilaginous structures that help deepen and improve the conformity of the articular surfaces of the medial and lateral tibiofemoral joint, distributing forces across the articular surface. The lateral meniscus covers a greater proportion of its tibial articular surface (75%–93%) relative to the medial meniscus (51%–74%). Loss of the meniscus from injury or meniscectomy results in increased peak loads across the articular cartilage. The lateral meniscus has weaker attachments to the tibia and is more mobile than the medial meniscus, making it less likely to tear than the medial meniscus, which has more firm attachments to the tibia and MCL. The outer periphery of the meniscus has good vascularity, while the inner aspect of the meniscus is avascular, which has important implications in the decision to repair the meniscus or excise the torn aspect of the meniscus (meniscectomy) during surgery.
Two intra-articular, extrasynovial cruciate ligaments (ACL and PCL) connect the distal femur and proximal tibia centrally. The ACL inserts anterior to the intercondylar eminence and originates on the posteromedial aspect of the lateral femoral condyle. It has two bundles: the anteromedial bundle, which tightens as the knee flexes, and the posterolateral bundle, which tightens as the knee extends. The primary role of the ACL is resistance to anterior tibial translation, while also providing resistance to tibial rotation. Reconstruction of the ACL to its proper anatomic location is critical to restoring these functions; improper tunnel placement during ACL reconstruction is a major cause of failure. The ACL has a variety of nerve endings, suggesting that it also plays a significant proprioceptive role in the knee. The PCL inserts posterior to the intercondylar eminence, approximately 1 cm distal to the posterior tibial joint line, and originates from the posterolateral aspect of the medial femoral condyle. It displays maximal tension at 90° of knee flexion and serves as the primary resistance to posterior translation of the tibia.