vastus lateralis, and rectus femoris. The quadriceps muscle group exerts force on the patella through the quadriceps tendon onto the patella, and through the patellar tendon onto the tibia, which acts as a leverage pulley to increase the muscle power of the quadriceps (Hartigan et al., 2011). Other muscles that assist with knee extension near end range are the sartorius, tensor fascia lata and gluteus maximus (via their attachments to the iliotibial band) although these muscles can also assist with knee flexion when the knee is in a more flexed position. The primary knee flexors are the hamstring muscles (semitendinosus and semimembranosus medially, and biceps femoris laterally), with secondary assistance coming from the gastrocnemius muscle of the calf (Hartigan et al., 2011; Figure 2). Figure 2: Muscles of the Posterior Knee Semitendinosus
Table 1: Ligaments of the Knee Ligament Origin
Insertion
Action
ACL
Posteromedial aspect of lateral femoral condyle within intercondylar notch. Anterolateral aspect of medial femoral condyle.
Anterior aspect of tibial plateau.
Restricts anterior translation and anterolateral rotation of tibia on femur. translation of tibia on femur and tibial external rotation. Restricts valgus stress forces and anteromedial rotation. Restricts posterior Restricts varus stress forces and posterolateral rotation of tibia on femur.
PCL
Posterior slope of tibial plateau.
MCL
Medial femoral epicondyle of distal femur.
Periosteum of proximal tibia posterior to pes anserinus. Posterior to anterior point of fibular head.
Semimembranosus
Bicep Femoris
Gracilis
Plantaris
LCL
Lateral femoral epicondyle of distal femur.
Gastrocnemius
The muscles of the knee provide power and dynamic stability. The primary knee extensor is the quadriceps muscle group, which is composed of the vastus medialis, vastus intermedius, Biomechanics The three compartments of the knee contact each other in a complex way during physiological active range of motion (AROM), not only moving the knee joint into flexion and extension but also into rotation and translation (a shearing or sliding movement) in a three-dimensional manner (Hartigan et al., 2011). When ambulating with a normal gait pattern with a natural knee, during knee flexion there are approximately 2 to 4 mm of posterior translation of the medial femoral condyle as compared with 21 mm of translation of the lateral condyle, creating an effective lateral rotation of the femur on the tibia during flexion (Crockarell & Guyton, 2013; Massin, Boyer, Hajage, Kilian, & Tubach, 2010). This natural bias toward rotation of the knee is mirrored in the “screw home mechanism” seen during end-range knee extension during static standing, where there is relative medial femoral rotation and lateral tibial rotation (Crockarell & Guyton, 2013). A computerized study of tibiofemoral rotation and posterior displacement of healthy and osteoarthritic knees demonstrates these knee kinematics when the ACL is intact (Massin et al., 2010). However in patients with end-stage OA and increased anterior-posterior laxity, greater self-reported disability is noted when reporting functional ability (Kauppila et al., 2009). Normal ROM of the knee joint ranges from 0º to 5º of hyperextension to 130º to 140º of flexion (Hartigan et al., 2011). During the performance of ADLs, kinematic studies show that the natural knee joint goes through 67º of flexion during the swing phase of gait, 93º when moving from sit to
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stand, 83º with stair climbing, and 90º when descending stairs (Crockarell & Guyton, 2013). The long-term success of a total knee replacement is dependent on normal limb alignment and the restoration of adequate AROM to reduce abnormal stresses on the new joint and decrease the incidence of implant loosening, tibiofemoral or patellofemoral instability, and residual physiological stiffness (Crockarell & Guyton, 2013). As the largest joint in the human body and one in which weight bearing directly occurs through the joint, the knee is subject to forces that exceed the patient’s body weight many times over during ambulation due to muscular contraction and gravitational forces. This loading force can result in the typical “wear and tear” pattern of degenerative OA (Vincent, Conrad, Fregly, & Vincent, 2012). Initially, joint cartilage is thickest in the region where the joint experiences the most repetitive loading, but as small shifts in accessory joint movements occur over time, the points of contact between the femur, tibia, and patella change, creating abnormal translation (shearing) and rotational (torque) forces at the joint. As the articular cartilage begins to degenerate in response to increased friction, the areas of highest load in the knee joint degrade the most rapidly in response to normal loading forces; thus, OA is often the result of subtle changes in joint mechanics over time (Vincent et al., 2012). Other factors of a nonmechanical nature, such as joint inflammation found in RA or gout, can also hasten the progression of degenerative changes inside the joint.
CRITERIA FOR SURGERY
When knee pain and disability persist despite conservative measures, such as anti-inflammatory medications, activity modification, offloading the painful joint by using a cane or brace, and muscle strengthening, surgery may become necessary. TKA is considered the gold standard for pain relief and the restoration of functional mobility in patients with symptomatic end-stage OA (American Academy of Hip and Knee Surgeons [AAHKS], 2015; Billante & Diduch,
2009). Although no consensus has yet been reached between orthopedic surgeons, rheumatologists, family medicine physicians, and physical therapists regarding the most important indicators for TKA surgery, there are several variables that are usually taken into consideration. Most frequently, patients are selected on the basis of consistent knee pain that significantly interferes with functional activities in the setting of radiographic evidence of knee OA.
Book Code: PTNY3622B
Page 159
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