Esler, & Harper, 2007). Actual implant survival on a per patient basis is hard to predict because it is dependent on surgical considerations, activity levels, and patient comorbidities. Large- scale studies of implant survival report 80% to 92% of implants are still intact 15 years after surgery. In patients who are over 70 years of age, with an expected lower level of activity, implant survivability typically is not an issue. However with the increase in knee arthroplasty patients under the age of 65, who have expectations of a higher level of activity, implant survivability is a greater concern (Aggarwal et al., 2007 Kim et al., 2012; Ravi et
al., 2012; Roberts et al., 2007; Santaguida et al., 2008). Recent data suggest that the underlying reason for revision may differ based on the age of the patient, with infection being the most common reason in patients over age 70 years and a higher rate of aseptic loosening in younger patients (Aggarwal et al., 2007). Aseptic loosening is a loss of fixation at the bone-implant interface that occurs most commonly on the tibial side of the joint. Polyethylene wear, which may be accelerated by higher intensity of physical activity, results in particulate debris–induced bone resorption and eventual loosening of the implant.
FUNCTIONAL KNEE ANATOMY
The knee is the largest joint in the body, and it is responsible for directly supporting the majority of body weight of the individual while working to control and advance lower extremity movement via the large muscles of the lower extremity. The major articulating bones of the knee joint are the femur and tibia, which enable knee flexion and extension. The femur, which is the longest bone in the human body, is divided into two condyles at its distal end where it articulates with the tibia, between which a smooth depression (intercondylar notch) forms a third surface that articulates anteriorly with the patella (Figure 1). The patella, which is the largest sesamoid bone in the body, acts as a pulley for the quadriceps muscle. It enters the intercondylar notch when knee flexion is around 20º and is then centered during motion by its soft-tissue attachments and the intercondylar boundaries (Hartigan, Lewek, & Synder-Mackler, 2011). In the natural knee, the articular surfaces of the femur and tibia are cushioned by cartilage, both on the ends of the bones and also between them by the menisci. The medial and lateral menisci, are semicircular fibrocartilaginous pads that help match the shapes of the femur and tibia to improve joint stability, disperse loading forces over a broadened surface to assist with shock absorption, and protect the articular cartilage of the femur and tibia from impact and friction (Hartigan et al., 2011). The three functional compartments In the past, the knee was viewed as a simple hinge joint that could only open and close into the gross movements of extension and flexion, but knee joint mechanics are much more complex than that of a hinge. Technically, the knee joint is now viewed as a “modified-hinge diarthrodial synovial joint” that comprises three main compartments, or zones of contact, that share a common joint capsule: the medial tibiofemoral, the lateral tibiofemoral, and the anterior patellofemoral (PF) compartments (Hartigan et al., 2011). When OA is present, it can affect any or all of the three compartments of the knee. Medial compartment The medial tibiofemoral compartment contains the bony convex medial femoral condyle and the concave medial tibial plateau. It also includes the adductor tubercle, medial meniscus, medial collateral ligament, and pes anserine (Hartigan et al., 2011). The medial compartment is the one most frequently involved with knee OA, and when the articular cartilage collapses and the medial meniscus disintegrates, medial joint space is lost, resulting in a varus deformity of the knee (Iverson & Westby, 2014). Lateral compartment The lateral tibiofemoral compartment contains the bony convex lateral femoral condyle and the concave lateral tibial plateau. It also includes the lateral meniscus, lateral collateral ligament, and popliteus tendon (Hartigan et al., 2011). Although the fibular head is also located at the lateral distal aspect of the knee, because it is inferior to the joint line it is not considered Musculotendinous and ligamentous attachments The knee joint is held firmly stable during range of motion (ROM) and functional movement against concurrent medial, lateral, rotational, and translational forces by its ligaments and muscles. The primary ligaments of the knee joint include the anterior
Figure 1: Knee Anatomy
Note. Reproduced with permission from OrthoInfo. ©American Academy of Orthopaedic Surgeons. http://orthoinfo.aaos.org .
a component of the lateral compartment. When the lateral compartment is affected by OA, the lateral joint space collapses and can result in a valgus deformity. Anterior compartment The anterior patellofemoral compartment contains the patella; the articulating surface of the intercondylar fossa (femoral trochlear groove) between the two femoral condyles; and the tendon that envelops the patella, known as the quadriceps tendon proximally and the patellar tendon distally (Hartigan et al., 2011). Alongside the patella lie fibers of the medial and lateral retinaculum, which are the endpoints of the vastus medialis and vastus lateralis muscles. The Q-angle of the knee, which is the angle between the line of the long axis of the femur and the line extending from the tibial tuberosity through the center of the patella, is important for normal patellofemoral mechanics because limbs with greater Q-angles are more prone to lateral subluxation of the patellae (Hartigan et al., 2011). During TKA, the surgeon strives to correct any varus/valgus malalignment that may contribute to poor patellar tracking (Williams, Garbuz, & Masri, 2010). Knee OA can also affect the anterior compartment. Previously this was considered a rare occurrence, but recent studies report isolated anterior knee compartment OA in 18% to 34% of females with knee pain (Gross et al., 2012; Hinman, Lentzos, Vicenzino, & Crossley, 2014; Kim & Joo, 2012).
cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL; Table 1).
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Book Code: PTNY3622B
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