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Extensor mechanism (Patella and Quad tendon) rupture

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There are many muscle-tendon units that flex the knee, but there is essentially only one muscle group that extends it: the quadriceps. As such, disruption of the quadriceps leads to profound impairment. A patient with a disrupted extensor mechanism is limited not only from actively extending the knee, but from resisting passive flexion (the latter being necessary for standing with the knee slightly bent). The extensor mechanism can be disrupted either by a fracture of the patella or by failure of the quadriceps or patellar tendons.

Structure and function

The quadriceps comprised of four muscles: the rectus femoris, the vastus intermedius, the vastus lateralis and the vastus medialis). The quadriceps originate from the femur, with the exception of the rectus femoris, which originates from the pelvis, proximal to the hip. These four muscles ultimately insert on the tibial tubercle, but pass first through the patella (a sesamoid bone within the extensor mechanism). Nevertheless, it is reasonable to consider the quadriceps inserting into the patella, and in turn considering that the patellar tendon originates from the patella. (This anatomy gives rise to a question of nomenclature: is the soft tissue attachment of the patella to the tibia best called a ligament or a tendon? As you see, there is a logical argument for both.)

The quadriceps actively extends the knee, but also resists passive flexion. Indeed, ruptures of the extensor mechanism are more likely to occur with the latter, for example, when eccentrically contracting to decelerate the forced flexion of landing from a jump. (Think about it: if the muscle were not firing, a rupture would not occur, as clearly, the knee could not be forced into a position of more than maximal flexion (by definition!) and if the patient is completely relaxed, the knee can maximally flex with disrupting the quadriceps)

Beyond the muscle tendon unit itself, one must also consider the retinaculum, that is the soft tissue on the medial and lateral side of the patella. When the retinaculum is intact, a patient with a focally torn quadriceps tendon may be able to hold the leg in full extension, via this indirect attachment of muscle to bone.


Quadriceps tendon disruptions are much more common in males. Diagnosis can be difficult and failure rates have been reported between 10%-50. The age of the patient provides a clue as to the location of an extensor mechanism disruption: under the age of 40, the disruption is "under", that is distal to the patella, in the patellar tendon; above the age of 40, the disruption is above the patella, in the quadriceps tendon.

Clinical presentation

Patients usually present with either one of two different mechanisms of injury either a direct force to the anterior aspect of their knee or excessive eccentric loading of the knee in a partially flexed position.  The common scenario for quadriceps ruptures is an elderly patient who misses a step while descending stairs and attempts to prevent oneself from falling.

Extensor tendon ruptures are usually characterized by immediately popping or tearing sensation upon injury. It is followed by swelling, anterior pain and inability to extend the injured leg.  Extreme difficulty ambulating is noted with a feeling of the knee giving out when attempting to bear weight in complete tears. However some patients are able to walk with a knee immobilizer. Patients with partial tears may walk with a forward leaning gait to help support the knee in extension.

Upon examination clinician should note soft tissue defects superior or inferior to the patella depending on the type of rupture.  The patella should be freely mobile and flexion should still be intact.  In partial tears extension may be intact but reduced. It is useful to note that patients with complete tears may be able to keep knee extended against gravity if the medial and lateral retinaculms are intact. It can be beneficial to aspirate and inject the knee with lidocaine when performing the exam to truly assess whether there is a mechanical deficit that limits extension versus limitations due to pain. 

Red flags

Note that significant hemarthrosis can make palpation for soft tissue defects very difficult. Therefore lack of a soft tissue defect can be misleading in diagnosis injury. 

Differential diagnosis

In patients with large joint effusions and inability to extend knee extensor mechanism injuries should be strongly considered. Evaluation of cruciate ligaments for injury should still be performed. Other causes of anterior knee pain similar to extensor mechanism injuries include Sindig-Larsen-Johanssen disease, Osgood Schlatter Lesion, Chondromalacia of the patella as well as Extensor tendon tendonitis.

Objective evidence

Plain radiograph can be  helpful in the diagnosis of an extensor tendon rupture but they can also appear normal. Anteroposterior and lateral views are standard in evaluating for extensor tendon injuries and should be examined for patellar displacement and osteochondral defects. Avulsion fractures of the patella and calcification densities of the tendon can indicate injury to the extensor tendons. Vertical displacement of the patella can performed using Insall-Salvati ratio or Blackburne-Peel method.

Assessment of the radiographs for an obliterated quadriceps tendon shadow or suprapatella mass caused by retraction of the quadriceps muscle is useful in diagnosing a quadriceps tendon rupture. The patella may also be displaced inferiorly known as patella baja.

In patella tendon ruptures displacement of the tendon superiorly or avulsion fractures of the apical pole of the patella can be seen.

Other imaging modalities include ultrasound, which is very good in assessing the integrity of the both extensor tendons. MRI is the most sensitive study that can be performed and is very useful determining location of tears and when large effusions make other modalities less diagnostic. 

Risk factors and prevention

There are many factors that can contribute to the weakening of the patellar and quadriceps tendon resulting rupture. Systemic diseases expecially those inflammatory in nature such as rheumatoid arthritis and systemic lupus erythematosus can increase risk of tendon rupture. Other risk factors include diabetes, infections chronic, and chronic renal failure. Drugs can also contribute to tendon rupture including corticosteroids and fluoroquinolones. 

Treatment options

Immediate surgical repair is necessary for complete tears of the patella or quadriceps tendon. After the surgery is performed patients are immobilized in a cast and non-weight bearing for a minimum of 3 weeks. Physical therapy is also important in restoring patients’ function.  Incomplete tears can usually be treated non-surgically by immobilization for 3 to 6 weeks and followed by gradual flexion and extension of knee.


Outcomes are usually good with early detection and repair of the ruptured tendon. Studies performed have shown that time to between injury and repair is correlated with outcomes leading some to suggest repair within 1 week.  With early repair and proper rehabilitation patients can usually regain most of the range of motion, strength and return to pre-injury activities including sports.

Holistic medicine

Instructions for authors 
Nutritional factors, psychosocial impact of disease and economic effects.


Instructions for authors 
In this section include everything a professor can mumble, without necessarily having evidence to support the assertions:

  • Random factoids to help students remember important stuff. E.g.: Why are sailors called “limey”? Sailors at sea where prone to scurvy from Vit C deficiency (imagine the toothless deck hand). Once that was known, they were issued limes to eat - to help the collagen cross link
  • Clinical pearls
  • Favorite facts for exam writers. it would be great if you can compose a question or two for students to ponder
  • Frontier of science - what is coming down the pike from our basic science friends
  • What we don’t know

Key terms

Insall-Salvati ratio; Blackburne-Peel method


Insall-Salvati ratio

Used to determine the height of the patella by comparing the patella tendon length to the length of the patella. This method of determining patella height is very quick but lacks in reproducibility of results.

Step 1:

Step 2:

Insall-Salvati ratio is A:B. A value of >1.5 is patella alta and <.74 is patella baja.

Blackburne-Peel method

Alternative method to asses the height of the patella. This method has a higher rate of reproducibility and ratios are consistent between 30 and 50 degrees of flexion.

Step 1:

Step 2:

Step 3:

Blackburne-Peel ratio A:B Normal value for males .805 and female .806. A:B values >1 are considered patella alta and <0.5 patella baja 


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