Disorders of the Extensor Mechanism of the Knee
The extensor mechanism of the knee comprises the quadriceps muscle and tendon
; the patella ; and the patellar tendon (also known as the infra-patellar ligament). Disruption of any of these components impedes a person’s ability to actively extend the knee or resist passive flexion. Such injuries are therefore incompatible with normal walking and standing. Failure of the quadriceps or patellar tendon is often, but not always, preceded by painful tendinopathy.
Structure and Function
The quadriceps muscle, as its name implies, is composed of four muscles: the vastus lateralis, vastus intermedius, vastus medialis and rectus femoris (see Figure
figure 1). All but part of the rectus femoris originate on the femur itself. These muscles converge at a point approximately 5 cm proximal to the patella to form the quadriceps tendon. The quadriceps tendon has multiple layers, with the rectus femoris as the most superficial layer, the vastus medialis and lateralis as the middle layer, and the vastus intermedius as the deepest layer. The superficial layer is well vascularized; however, there is a hypovascular zone in the middle and deep layers approximately 1-2 cm proximal to the patella. This is the most common site of rupture.
The patellar tendon originates at the distal end of the anterior aspect of the patella and inserts on the anterior aspect of the tibia at the tibial tuberosity. The tendon most commonly tears at its origination on the patella.
Figure 1: Anatomy
anatomy of the extensor mechanism. The quadriceps muscles are are outlined in red, and their common tendon and the infrapatellar ligament are in yellow. Both attach to the patella, outlined in gray. (Original x-rays from https://radiopaedia.org/cases/normal-knees-x-rays
The function of the patella is to increase the distance between the extensor mechanism and the center of rotation of the knee as seen in the sagittal plane. This distance creates a so-called moment arm; the greater the distance, the greater the moment arm and intern the greater the leverage (see Figure
figure 2). Without a patella, the effective strength of knee extension is at least 30% decreased.
Figure Fig 2: As shown by this
this drawing, extension of the knee is essentially a rotation around the center of the knee in the sagittal plane, powered by the pull of the quads. The greater the distance between the quads and the center of the knee, the longer the lever arm [shown in blue] and thus the greater the torque (rotation force). Without a patella, the pull of the quad is closer to the center of the knee and thus the lever arm is shorter and the force of extension weaker.
Both patellar tendinitis and infrapatellar bursitis are painful to the touch. Patellar tendinitis can be differentiated from infrapatellar bursitis in that the latter condition is painful with side to side pinching of the skin as well (Figure 3A and 3B
figure 3 a and 3 b).
(FIGURE 3 Need clinical photos: A) direct
Figure 3A: Direct palpation of inferior pole of the patella. This is painful if either patellar tendinitis or infrapatellar bursitis is present.
Figure 3B: Pinching the
bursa, as shown, will be painful if bursitis is
present and not painful if the underlying pathology is deep to that, e.g., in the tendon. If palpation is painful but pinching is not, the diagnosis of tendinitis
When either tendon is completely torn patients are unable to extend the knee, perform a straight leg raise, and often cannot bear weight on the injured leg. (That is because in stance phase, the center of gravity of the body is behind the center of rotation of the knee joint and tends therefore to flex it.
Plain radiographs should be obtained to rule out fractures if the mechanism suggests that one may be present. Radiography can also help identify a soft tissue injury, as a lateral view x-ray will confirm displacement of the patella. A distally displaced patella (patella baja) and a proximally displaced patella (patella alta
[see Figure figure 4) are signs of quadriceps and patellar tendon rupture, respectively.
Figure 4: “Patella alta”, that is, a proximally displaced patella, indicates disruption of the infrapatellar ligament. The normal position of the patella is outlined in red (from https://en.wikipedia.org/wiki/Patellar_tendon_rupture#/media/File:Patellar_tendon_rupture.JPG)
If there is any question whether a disruption is present, an MRI may be helpful. MRI cannot only differentiate between a partial and complete tear, it can localize the injury and provide information about the quality of the surrounding tissue (Figure
figure 5). This This latter information may be useful for surgical planning.
Figure 5: MRI showing a quadriceps disruption Case courtesy of Dr Ahmed Abdrabou, Radiopaedia.org, rID: 22644
The differential diagnosis for focal anterior knee pain includes not only tendinopathy but patellofemoral pain syndrome (with or without patellofemoral arthrosis) and bursitis. Of course, these can coexist. A detailed physical examination with imaging
usually can differentiate between these diagnoses.
Once it is determined that the extensor mechanism is disrupted, the differential narrows to injuries of the quadriceps tendon, the patella itself and the patellar tendon. Bony injuries in the absence of a direct blow can be assumed absent, though at times patients fall after they tear their tendon making the history a little less clear. X-rays are therefore helpful. Films obtained with the knee in flexion can also indirectly document a soft tissue injury as the patella will be in an abnormal position. Noting the patient’s age (again, age over 40 is likely to implicate the quadriceps, and age under 40, the patellar tendon) though of course a physical examination may pinpoint the problem more exactly.
In general, tendon injuries do not herald other problems, though the presence of one tendon injury may foreshadow another, and of course tendinopathy itself may suggest an impending rupture.
Treatment Options and Outcomes
Because tendinopathy is often an
over use injury, a good first line treatment is relative underuse.
Extensor mechanism, quadriceps muscle, rectus femoris, vastus medialis, vastus lateralis,
and vastus intermedius, quadriceps tendon, patella and patellar tendon
Recognize the cause of extensor mechanism disruption plain radiographs. Differentiate between active and passive loss of extension on physical examination, and localize disorders of the extensor mechanism.