Layered Approach to the Anterior Knee: Normal Anatomy and Disorders Associated with Anterior Knee Pain

Abstract

Anterior knee pain is a common complaint that can be caused by a wide spectrum of disorders affecting the many varied tissues at the anterior knee. The anatomy and pathologic conditions of the anterior knee can be approached systematically by organizing the region into four layers: (a) superficial layer of fat, fascia, and bursae; (b) functional layer composed of the extensor mechanism and patellar stabilizers; (c) intracapsular extrasynovial layer containing the fat pads; and (d) intra-articular layer. The superficial layer is composed of delicate tissues that are predisposed to blunt and penetrating trauma, irritation, and infection. The extensor mechanism forms the functional layer, is responsible for knee extension and patellar stabilization, and is engaged in repetitive movements; overuse disorders dominate in this layer. The fat pads of the anterior knee are discussed collectively as an extracapsular extrasynovial layer, functioning to improve congruence and protect the articular surfaces during motion. Diseases involving the fat pads can be primary or secondary to pathologic conditions in the rest of the joint. The synovial lining and cartilage surface are in the fourth and final intra-articular layer; pathologic conditions are centered around arthritides and internal derangement. Symptoms in the anterior knee may be due to conditions affecting one or more of these interrelated layers. © RSNA, 2018 • radiographics.rsna.org Dyan V. Flores, MD Catalina Mejía Gómez, MD Mini N. Pathria, MD Abbreviations: ACL = anterior cruciate ligament, FNS = focal nodular synovitis, MPFL = medial patellofemoral ligament, PD = proton density RadioGraphics 2018; 38:2069-2101 ). © RSNA, 2018After completing this journal-based SA-CME activity, participants will be able to:This journal-based SA-CME activity has been approved for AMA PRA Category 1 Credit TM . See rsna.org/learning-center-rg.

Magnetic resonance imaging of the knee

“…The extensor mechanism of the knee allows for dynamic knee extension and patellofemoral stabilisation [69]. It primarily consists of the quadriceps tendon, patella, patellar tendon, tibial tuberosity, medial and lateral patellar retinacula, and the vastus musculature [79].…”

“…Tears may also be secondary to systemic disease or medication use. Tears usually occur 1-2 cm proximal to the patella and at the patellar insertion, where the tendon is relatively avascular [79][80][81]. Partial tears most commonly affect the anterior rectus femoris portion [79][80][81] and appear as a partial thickness, fluid-filled gap on MRI, without complete disruption of the tendon.…”

“…Tears usually occur 1-2 cm proximal to the patella and at the patellar insertion, where the tendon is relatively avascular [79][80][81]. Partial tears most commonly affect the anterior rectus femoris portion [79][80][81] and appear as a partial thickness, fluid-filled gap on MRI, without complete disruption of the tendon. Complete quadriceps tendon tears appear on MRI as complete disruption of the tendon fibres, a full thickness, fluid-filled gap, a retracted tendon stump, an inferiorly positioned patella (patella baja), and a wavy patellar tendon (Figure 31).…”

“…Often, there is triangular intermediate signal involving the posterior half of its patellar attachment, and occasionally there may be intermediate signal its tibial insertion. This is probably secondary to a combination of magic angle phenomenon and histological changes within the tendon provided there are no other signs of tendinopathy [79,83]. When the knee is in full extension and the quadriceps muscles are relaxed, the tendon can buckle and be associated with multiple transversely oriented bands of intermediate signal intensity at the sites of buckling that are also related to magic angle phenomenon [84].…”

“…Occasionally, there may be an osseous MPFL avulsion from its medial patellar attachment, which may not be diagnosed at the time of initial injury and be associated with persistent instability. The MPFL may be injured at any location, but is most commonly injured at its femoral attachment [79,80]. MRI findings of MPFL injury include thickening, intra-ligamentous fluid, wavy fibres, and fibre discontinuity [86]; often, fluid extends along the vastus medialis oblique (VMO) muscle, which comprise the inferior vastus medialis muscle fibres that attach directly onto the MPFL.…”