Fractures of the patella disrupt the connection between the quadriceps and the tibia and thus disrupt knee extension. Even if that acute impairment is treated and extension is restored, painful patello-femoral arthritis may result.
Patellar fractures represent about 1% of all fractures. They are injuries typically sustained by young men but not exclusively so.
Structure and Function
The patella is a sesamoid bone within the quadriceps tendon just proximal to its insertion into the tibia. (The soft tissue distal to the patella itself is called the patellar tendon or infra-patellar ligament, but functionally this is a continuation of the quadriceps tendon).
The patella places the quadriceps in a more anterior position relative to the femur, which by leverage, augments the force of the quadriceps. Viewed in the sagittal plane, knee extension can be appreciated as a rotational movement of the tibia around the center of the knee. That rotation is powered by the quadriceps. The further the quadriceps are from the center of the knee, the more powerful its force will be, just as a long handled wrench can turn a bolt a short one may not. Hence the patella, which places the quadriceps in a more anterior position relative to the femur, is able to make the quadriceps functionally more powerful.
The anatomical arrangement of the patella as sesamoid bone within the quadriceps tendon has four important clinical consequences: first, and most obvious, a complete patellar fracture will be associated with an inability to extend the knee; second, such a fracture will be easily displaced by the pull of the quadriceps, as shown in this drawing (modified from http://en.wikipedia.org/wiki/File:Gray350.png):
Third, because the patella is, in a sense, a fulcrum between the pull of the quadriceps (extending the knee) and the pull of gravity on body weight (which resists the quadriceps), there are high reactive forces compressing the patella into the femoral trochlea, making arthritis there particularly painful; and last, without a patella and the leverage it brings, the quadriceps will is functionally weaker (about 30% weaker); so much so that many routine activities such as stair climbing are possible only with a normal patella.
The first two of these suggest that diagnosis and treatment are initially geared to detecting extensor failure and remedying that; the latter two suggest that the long term goal of treatment is to preserve the patello-femoral joint. Indeed, that is the case.
Figure 2. These axial plane drawings of the patello femoral joint, normal to the left, fractured on the right, illustrate a few important anatomic facts regarding patellar fracture. First, the patella is subcutaneous and thus a fracture is easily an open fracture. There is also a thick cartilage layer on the bone, shown here in olive green, that makes up the joint. In this drawing, the amount of bony displacement is less than the cartilage displacement (and therefore xrays will understate the injury). In the configuration shown on the right, the amount of cartilage contact that remains is only about 50% of normal, thereby doubling the focal stress on that area. The synovial pouch of the knee holds fluid normally; this fluid can have blood or fat in it when there is a fracture.
Fractures of the patella can result from direct blows to the knee, falls or a combination thereof. A direct blow can break the bone directly. A fall even without contact on the knee can cause a patellar fracture indirectly, by the eccentric pull of the quadriceps, decelerating the body. (This is the same mechanism that leads to tendon rupture. Whether a given load will break the bone or the soft tissue depends on the speed of the force: owing to their viscoelastic properties, the mechanical strength of tendon and bone depend on the rate of loading. Faster loads will tend to injure the soft tissue preferentially.)
These two mechanisms of injury will lead to distinct presentations. When an indirect forces breaks the patellar, there is a transverse fracture pattern, often with superior displacement of the proximal fracture fragment. The skin in typically undisturbed. A direct blow creates a comminuted stellate (shattered) fracture, with a greater risk for open, articular or anterior soft tissue injuries.
If there is any blood on the skin, take particular efforts to exclude an open fracture.
A patellar fracture must be suspected in all patients who have sustained a direct high-energy blow to the anterior knee or who cannot actively extend their knee after a flexion injury or fall.
An inability to extend actively, with passive motion retained, points to a patellar fracture or rupture the quadriceps or patellar tendon. Radiographs can exclude a bony injury.
With a direct blow, the skin must be examined to exclude an open fracture.
Consider a cartilage injury alone, that xrays won't see.
Consider patellar dislocation with spontaneous relocation. Tenderness at the sides of the patella with apprehension suggest this diagnosis.
Consideration of soft tissue status is important, as the patella is nearly subcutaneous. Foremost, an open fracture must be considered when there is a break of the skin. A full thickness skin laceration in the absence of a fracture should prompt an injection test to exclude a traumatic arthrotomy.
Here, a large volume of saline is injected into the knee joint far from the traumatic wound and the wound is then examined for leakage. Adding some lidocaine may help provide some pain relief and allow for a more detailed examination. Aspiration of a tense hemearthrosis (blood in the joint) may also be helpful.
AP, lateral plain radiographs must be obtained but often underestimate the degree of comminution and articular step off, especially in the axial plain. An axial view is helpful for evaluation, but may be difficult to obtain due to discomfort. The geometry of the fracture seen on radiographs might not match the anatomy seen at surgery, as proximal cartilage may be attached to distal bone (or vice versa); or broken free altogether.
The patella may appear to be "broken" on xray because it did not fuse during growth and development: a so called bipartite patella. This patellar fragment, in this case, is often in the superior lateral quadrant and is often seen bilaterally.
(The contribution of a reader who has a an xray of a bipartite patella similar to that shown on "www.docs4tri.com/2010/05/24/knee-pain-patellar-dislocation" to lend here would be appreciated !)
In the setting of extensor mechanisms failure but no fracture, radiographs obtained in flexion may offer evidence of quadriceps tendon or patellar ligament injury: with those soft tissue injuries, the patella can be found in an excessively distal or proximal position, respectively.
Fractures should be described by orientation and position (e.g. transverse, inferior pole), degree of comminution, and displacement (especially effecting the articular surface). Vertical fractures are possible but rare. Avulsion fractures are best considered as soft tissue injury---especially because the bony fragment may simply be calcifications of prior tendinopathy.
Fat droplets in the aspiration fluid suggests an occult fracture.
Risk factors and Prevention
Beyond the generic advice to "avoid falling " there is not much one can do avoid indirect patellar fractures. Direct patellar fractures typically occur from random trauma and likewise cannot be specifically prevented; except to note that protective equipment for high risk activities (eg, playing catcher in a baseball game) should be required. Of course, this equipment can also help prevent (the more frequently encountered) painful contusions too and therefore participants often do not need to be reminded to do their protection.
The goals of treatment are a functional extensor mechanism, articular congruity, and full, painless motion of the knee joint.
Non-operative treatment may be reasonable in patients with an intact extensor mechanism and minimal intra-articular step-off, especially if the fracture line is not transverse. (When the fracture line is not transverse, the pull of the quadriceps may displace the fracture.)
Surgery should not be chosen for patients whose pre-operative risk assessment is bleak, or in those who are not apt to regain function in any case (for instance, those patients with a prior failed extensor mechanism, joint anklyosis, a non-ambulatory status).
Small fragments can be excised or repaired with suture. Removal of the entire patella may be contemplated for massively comminuted fragments, but the typical result is a painless knee but unhappy patient. As noted, without a patella, 30% of effective quadriceps strength is lost; enough to preclude climbing stairs or getting out of a chair.
Transverse fractures can be treated surgically with so-called tension band technique: two wires woven around metal pins placed longitudinally. This configuration converts the tensile force of the quadriceps into a compressive forces at the fracture line.
Most patellar fractures fully unite with treatment. The functional outcome is inversely proportionate to the severity of the initial injury. Two caveats are worth bearing in mind: first, the severity of the initial injury may be understated on radiographs, which may not show the full extend of cartilage damage; second, because the patella is loaded with every step and because the average person takes thousands of steps per day, even mild post-traumatic arthrosis can be impairing. Also, it must be noted that it may take the better part of a year for a patient to recover from a patellar fracture, as most patients need some form of rehabilitation to regain strength and motion.
Because the patella is very sensitive, the malicious production of patellar injury, a process known as "kneecapping", can be used by criminals to reliably inflict enduring pain on the victim.
patella; quadriceps; open fracture; post-traumatic arthrosis;
recognize on examination failure of the extensor mechanism (inability to actively extend, with passive extension normal); correctly interpret radiographs and describe fracture pattern; recognize bipartite patella; aspirate the knee joint; apply splint or knee immobilizer until definitive treatment is obtained