Clinical Anatomy of the Patellofemoral Joint

Transcription

1 Clinical Anatomy of the Patellofemoral Joint Moira O Brien Anterior knee pain is one of the common knee problems in athletes. The anatomy of the patellofemoral joint and the role the quadriceps plays in the movements of the joint are discussed. Key Words: patella, quadriceps, patellar dislocation Key Points: The patellofemoral articulation totally depends on the function of the quadriceps. The articular cartilage covering the patella is one of the thickest in the body. Wasting of the oblique portion of the vastus medialis results in tilting of the medial facet, affecting its nutrition. Squat exercises produce patellofemoral joint-reaction forces 7.6 times body weight. Introduction The patellofemoral joint is the portion of the knee joint between the patella and the femoral condyles. It has only 1 degree of freedom with a simple active interaction. The patellofemoral articulation totally depends on the function of the quadriceps. The patella forms a mobile yet firm site for the attachments of ligaments and tendons on the extensor side of the knee. It increases the angle of pull of the patellar tendon, improving the mechanical advantage of the quadriceps in knee extension. The patella, the quadriceps tendon, ligamentum patella, and the retinacular fibers form the capsule of the knee joint anteriorly and provide protection for the anterior portion of the knee joint. Articular Surface The articular surfaces consist of the patella (see Figures 1 and 2) and the trochlear surface of the femoral condyles. The patella is a sesamoid bone in the tendon of the quadriceps and is the largest sesamoid bone in the body. It is triangular, the base is superior and has rounded margins, and the apex points inferiorly. The patella has anterior and posterior surfaces, 3 borders, and an apex. Its thickness varies by a maximum of 2 3 cm, excluding the cartilage. The articular cartilage covering the patella is one of the thickest in the body. The articular cartilage on the medial facet is thicker than that 1

2 2 O Brien on the lateral facet, 2,3 but the thickest part of the articular cartilage (6.4 mm) is found on the median ridge of the patella, which lies in the hollow of the patellar surface of the femur when the knee is bent. Articular cartilage gets its nutrition from synovial fluid, but the articular surfaces must be in contact. Wasting of the oblique portion of the vastus medialis results in tilting of the medial facet, affecting its nutrition. The cartilage is subjected to high pressure, making it liable to nutritional disturbances. 2 The anterior surface of the patella is related to the quadriceps tendon, and the prepatellar bursa (which, if inflamed, is commonly known as housemaid s knee) separates it from the skin. The posterior surface is divided into a larger superior articular and lower extra-articular portion. The upper three quarters of the posterior surface is divided into 2 concave facets by a smooth vertical ridge, with the lateral facet larger than the medial, on average with a ratio of 1.4 to 1. Two horizontal lines separate the articular Figure 1 Anterior aspect of lower end of left femur and patella. Figure 2 Anterior and posterior surfaces of the right patella.

3 Anatomy of the Patellofemoral Joint 3 area into 3 areas on each side. A narrow strip, which is broader above, is found on the medial aspect of the medial facet, (the so-called third facet), which meets the medial condyle of the femur in extremes of flexion. 4 A rough, sloping area extends between the lower margin of the articular surface and the apex; the upper portion is related to the infrapatellar fat pad. The distal portion of the wedge-shaped apex has no articular surface; it is completely ensheathed by the patellar tendon. 5 The base of the patella is roughened for the attachment of the vastus intermedius and the rectus femoris. The medial and lateral borders are rounded and roughened for the attachments of the vastus medialis and lateralis. Ossification The patella is cartilaginous at birth. It ossifies from a single or several centers between the age of 3 years and puberty. Occasionally it is absent or bipartite. 4 In developmental patella alta, the patella is aligned to the shallow part of the proximal trochlear groove when the knee is in the functional position of 45 flexion. Hughston 6 showed that at 90 flexion, the patella is pointed toward the ceiling, whereas it is low and close to the tibia in patella baja. Blood Supply The blood vessels to the patella derive from a vascular ring formed by the anastamosis of the genicular arteries. Most of them enter the anterior surface, but the blood supply to the apex enters posteriorly below the articular surface, and, consequently, marginal fractures often do not unite after a transverse fracture. If a fragment is to be removed, it should be the upper one, because the lower fragment has its own blood supply. Femoral Articular Surface The femoral condyles only project slightly in front of the shaft of the femur but project quite a distance posteriorly. The anterior aspects of both condyles are included in the articular area for the patella. The patellar articular surface is larger on the lateral femoral condyle than on the medial. There is an anterior projection on the lateral femoral condyle lateral to the patellar groove. This is the bony factor, which prevents lateral dislocation of the patella. Dysplasia of the projection predisposes one to patellar dislocation. The trochlea is on the anterior, distal end of the femur. The anteroposterior trochlear groove is in the middle of the trochlea and divides it into 2 facets. The groove is continuous posteriorly with the intercondylar notch of the femur. The lateral facet is more prominent and has a greater radius. The

4 4 O Brien upper margin overhangs the supratrochlear fossa. There is a smooth transition between the supracondylar fossa and the lateral facet but a rougher transition to the medial facet. The lateral facet is more prominent and blocks lateral movement of the patella once it enters the groove as flexion begins. The terminal sulcus is the junction of the femoropatellar and femorotibial articular surfaces. The angle of the trochlea is 140. Quadriceps The quadriceps tendon is the fused portion of the quadriceps, proximal to the patella. The quadriceps does not pull vertically over the patella to the tibial tuberosity but exerts an oblique pull that forms a laterally obtuse angle called the quadriceps, or Q, angle. The Q angle is 10 to 15 in extension. In flexion, the femur rotates laterally on a fixed tibia and the Q angle is 0. The quadriceps, which is trilaminar consists of the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius. The rectus femoris is the most superficial portion of the quadriceps and acts on both the hip and the knee. It inserts into the base of the patella, but part of its fibers pass superficial to the patella, forming the superficial portion of the ligamentum patella. The tendons of the vastus medialis and lateralis form the middle layer, and both consist of vertical and oblique fibers. The oblique portion of the vastus medialis obliquus (VMO) arises from the adductor magnus tendon, which is attached medially to the adductor tubercle. The medial patellar retinaculum is an extension of the VMO and is a dynamic medial stabilizer that resists the lateral displacement of the patella. 1 The oblique portion of the lateralis has its origin from the iliotibial tract and forms part of the lateral retinaculum. VMO and Vastus Lateralis Both the VMO and the vastus lateralis have their own nerve supply and insert into the sides of the patella. They control the position of the patella in the trochlear groove. The VMO is active, not just in full extension but also at 30, 60, and 100 of knee flexion. The deepest layer is the vastus intermedius, which is the most powerful extensor of the knee. Ligamentum Patella The quadriceps inserts into the tibia via the ligamentum patella and the retinacular fibers. The ligamentum patella is the central portion of the quadriceps tendon and has a broad attachment to the lower pole of the patella and to the depression on the distal posterior aspect of the patella. 5 It is attached inferiorly to the tubercle of the tibia. There is no laminar arrangement in the ligamentum patella; the femoral nerve (L2,3,4) supplies it.

5 Anatomy of the Patellofemoral Joint 5 Capsule of the Knee Joint The capsule of the knee joint is divided into meniscofemoral and meniscotibial or coronary ligaments, the meniscotibial being the tighter portion. The patellofemoral ligaments are thickenings of the capsule that pass from the epicondyles to the patella. They are from 3 to 12 mm wide. The patellomeniscal ligaments play a role in the control of the patella, but they do not occur with equal prominence in everybody. 3 Retinacular Fibers The patellar retinacula consist of aponeurotic expansions from the vastus medialis and lateralis, with contributions from the deep fascia (iliotibial tract). They insert obliquely into the anterior aspect of the tibia, lateral to the patellar tendon. The medial retinacular fibers are fascial investments, which arise from the vastus medialis and interdigitate with the superficial medial ligament and the medial patellar tendon. The medial patellotibial ligament is an oblique condensation of the medial retinaculum, which originates inferiorly and medially from the patella to attach to the tibia 1.5 cm below the joint line on the anteromedial border of the tibia. 7 The lateral retinaculum is a richly innervated, fibrous connective-tissue structure on the lateral side of the knee. It consists of a superficial and a deep layer. The superficial layer consists of fibers from the iliotibial tract and the vastus lateralis. The oblique fibers from the iliotibial tract insert mainly into the lateral border of the patella and interdigitate with the longitudinal fibers of the vastus lateralis and the patellar tendon. These fibers give very little support to the patella. The deep layer is composed of dense transverse fibers that connect the deeper portion of the fascia lata directly to the lateral patella. Two distinct ligaments, the epicondylopatellar and patellotibial are located at the superior and inferior borders, respectively, of the deep layer, tethering the patella to the lateral epicondyle and anterolateral tibia. 7 The proximal fibers of the deep lateral retinaculum are the epicondylopatellar band, and this portion gives considerable support to the patella. 8 The lowest part of the deep lateral retinaculum is the lateral patellotibial band. It is the anterior margin of the fascia lata, parallel to the patellar tendon, that runs obliquely from the distal lateral patella to the tibia. 8 Infrapatellar Fat Pad and Bursae The infrapatellar fat pad lies posterior to the patellar tendon. It is covered on its posterior surface by the synovial membrane of the knee joint and gives rise to the alar folds. The deep infrapatellar bursa lies between the lower portion of the patellar tendon and the tibial tuberosity, below the

6 6 O Brien infrapatellar fat pad. The superficial infrapatellar bursa lies anterior to the tibial tuberosity and is known as Parson s knee when it is inflamed. The suprapatellar bursa is the largest in the body, extending 4 finger breadths above the base of the patella and lying between the quadriceps tendon and the distal anterior surface of the shaft of the femur. It is a complete bursa in infants and does not communicate with the knee joint, but the synovial sac usually breaks down so that the synovial membrane of the bursa communicates with the synovial membrane of the knee joint. The fibers of the articularis genu insert into its proximal aspect. Patellar Tracking Tracking of the patella depends on the quadriceps. The entire articular surface of the patella is never in full contact with the femur at any time. 3 As the knee flexes, the articular contact area migrates toward the proximal pole of the patella. In extension, the patella articulates purely with the patellofemoral surface and is in contact with the femur at a point just proximal to its apex. During flexion, the patella descends vertically over a distance twice its length, as much as 8 cm, to the intercondylar notch. 1 It also turns about a transverse axis, with the deep surface of the patella going from facing posteriorly to facing superiorly if the tibia moves. Normally this does not happen, because it is held against the femur by the quadriceps. This pressure increases during flexion and is loose in extension, which is why the patella dislocates in extension. As the knee flexes, the patella has to recenter to take up a more medial position. The lower third of its articular surface is in contact at 30, the middle third at 60, the upper third at 90, and the lateral borders and articular facets of the third facet beyond 90. At 135, full flexion, the patella lies in front of the intercondylar fossa of the femur, and only the lateral facet and the medial odd facet articulate. 3 The action of the quadriceps enables the patella to leave the trochlea and reach its highest position in the supratrochlear groove. The patella acts as a mobile pulley to longitudinally redirect the transverse tensile forces from the quadriceps tendon, patellar ligament, transverse retinacula, meniscopatellar ligaments, longitudinal retinacula, and tendons of the vastus lateralis and medialis, transmitting them to the femur in the form of patellofemoral compression. By spreading its force out horizontally, the quadriceps subjects the patella to lateral movement and anteroposterior pressure on the femur. Passive mobility at the joint occurs at the trochlea as a result of sliding. While the patella remains equidistant from the tibia, the trochlea slides progressively upward and backward; the patella is held in position by the patellar ligament. The patellar retinaculae are first called into play, particularly the medial retinacula. In the frontal plane, the force of lateral translation of the patella is highest in extension. 3

7 Anatomy of the Patellofemoral Joint 7 The lateral facet of the trochlea is larger and more prominent and blocks lateral movement of the patella once it enters the groove as flexion begins. In strong internal rotation, the patella is passively compressed against the medial femoral condyle; the active tension of the vastus lateralis increases and counteracts this compression. The lateral part is generally under greater contact pressure than the medial facet. During strong external rotation of the tibia, the patella is compressed against the lateral femoral condyle with an increase in the action of the vastus medialis. 3 Patellar Dislocation The patella dislocates spontaneously laterally during flexion, only in the area of the terminal sulcus, the junction of the femoropatellar and femorotibial articular surfaces. Reduction occurs in extension, and it crosses the high lateral lip of the lateral condyle, causing an osteochondral flake fracture. If the tibia is internally rotated and the knee is extended against resistance, the forced contraction of the vastus lateralis results in dislocation. 3 Synovial Plicae A synovial plica is a redundant fold that is the remnant of a vestigial septum from the embryonic period. A free space might exist on the medial side between the femur and the patella and might contain a synovial fold or plica. The plica is usually crescent shaped, extending from the infrapatellar fat pad medially and looping around the femoral condyle, passing beneath the quadriceps tendon in the suprapatellar pouch and laterally over the lateral femoral condyle to the lateral retinaculum. 1 In patella alta, the patella is aligned in the shallow part of the proximal trochlear groove when the tibia and femur are flexed to a functional position. Extreme valgus of the long bones, a low lateral femoral condyle, or patella baja can also alter patellofemoral contact in a functional flexed-knee position. During level walking, the force across the femorotibial joint can reach 5 times body weight, while the force at the patellofemoral joint is half of body weight. Compressive forces at the patellofemoral joint have been found to be half body weight during normal walking, increasing to 3 times body weight during stair climbing. Squat exercises produce patellofemoral joint-reaction forces 7.6 times body weight. 3 Ascending or descending stairs does not affect the femorotibial joint, but there is an increase of force at the patellofemoral joint of one and a half to twice body weight when ascending stairs, and the force might be times body weight when descending stairs. 1 The greatest forces occur at the patellofemoral joint when one is using one s arms to get out of a chair unaided. The force exerted is 3.5 times body

8 8 O Brien weight at the patellofemoral joint and 4 times body weight at the femorotibial joint. Forces exerted are also higher during running and jumping. In jumping, forces can reach 24 times body weight at the femorotibial joint and 20 times body weight at the patellofemoral joint. 1 References 1. Anderson MK, Hall SJ. Sports Injury Management: Lower Limb Injuries. Baltimore, Md: Williams & Wilkins; Bandi W. Chondromalacia patellae und femoro-patellare arthrose. Helv Chri Acta (Suppl). 1972;11: Muller W. The Knee: Form, Function and Ligament Reconstruction. Berlin: Springer- Verlag; Plastanga N, Field D, Soames R. Anatomy and Human Movement. Oxford: Butterworth-Heinemann; Williams PL, Dyson M. Gray s Anatomy. 37th ed. London: Churchill Livingstone; Hughston JC, Walsh WM, Puddu G. Patella subluxation and dislocation. Philadelphia, Pa: WB Saunders; Terry GC. The anatomy of the extensor mechanism of the knee. Clin Sports Med. 1989;8(2): Fulkerson JP. Evaluation of the peripatellar soft tissue and retinaculum in patients with patellofemoral pain. Clin Sports Med. 1989;8(2):