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related talks: broken kneecap, tibial stress fractureshin splintsmetatarsal fracture

What is a Kneecap Dislocation (Patellar Instability)?

The kneecap (called the “patella”) is a bone located within your quadriceps tendon. The kneecap’s function is to maximize the strength of the quad muscle by increasing its mechanical advantage at all positions of knee flexion (remember Achimede's famous line "give me a place to stand and i will move the world", well its something like that).  To give your muscle this kind of extra power, its constantly adjusting its position by moving up and down the front of the knee.  Its always on the move.  This motion is called “tracking”, the kneecap “tracks” along the front of the knee, like a train on a track. 

The thigh bone (femur) has a deep groove (called the trochlea) which functions as the track to contain the kneecap while it moves.  Additionally there are ligaments attached to both sides of the kneecap to also keep it in line (called the retinaculum and the patello-femoral ligaments). 

If the structure of a person’s knee is abnormal, the kneecap may not “track” smoothly and this puts it at risk for dislocating. If the groove in the femur is too shallow, or the medial sided ligament (the medial patellofemoral ligament: MPFL) is too loose, or the lateral sided ligament is too tight, the kneecap wont track normally.  Also if a person is “knock-kneed” this will change the way our muscles pull on the kneecap and it will encourage dislocation (the kneecap pops out of the groove and gets stuck along the outside of the knee).

The kneecap is at risk to dislocate when the leg is fully straight.  It cannot dislocate when the knee is bent because the ligaments are too tight to allow any side to side motion. Just feel your own kneecap, you can move it a little with your leg straight.  

This condition is much more common in women than men (although it can happen to anyone).  Women are typically slightly more knock kneed, which naturally pulls the kneecap laterally, and they are more prone to ligament laxity. The instability usually begins during adolescence or in someones 20s.

How is a Kneecap Dislocation (Patellar Instability) Diagnosed?

Most commonly a young female will come to the office reporting that their kneecap feels unstable, and may report an incident where the kneecap became dislocated completely.  When a kneecap dislocates, it typically will relocate very quickly and so its uncommon for someone to come into the emergency room with their kneecap on the side of their leg (but anything can happen).  When examining the affected knee, a doctor will push the kneecap to the outside looking for excess mobility.  X-rays are usually ordered to look at the alignment of the knee, and the kneecap, and to look at the thickness of the groove (trochlea).  Sometimes a CAT scan is ordered (a cat scan gives lots of x-rays from three different views so the bones can be understood in 3D).

If someone reports a complete kneecap dislocation, an MRI is often ordered to look for injury to the MPFL and to see if any cartilage was chipped off (a cartilage injury usually occurs along the inside half of the kneecap when it pops back into position).

How is a Kneecap Dislocation Treated? (Patellar Instability) 

Treatment of kneecap instability depends on many factors.  It depends on the number of times (if any) the kneecap dislocated, and it depends on whether any pieces of cartilage chipped off and are floating within the knee.

In most cases of knee instability following a kneecap dislocation, the knee is immobilized for a short period (two weeksn) and then motion exercises are slowly started, with emphasis on quadraceps muscle strengthening (a strong thigh muscle can help with stability).  Physical therapy is crucial to providing improved stability.  Many times people will continue to wear a knee brace while playing sports to provide some extra stability and to act as a reminder that they need to be careful with their knee.  The majority of cases of kneecap instability are effectly treated with this nonsurgical approach.

Sometimes a piece of cartilage gets chipped off and is floating inside the joint.  In these cases, a small arthroscopic surgery (using a camera and a small incision) is required to remove that piece of cartilage.  Some surgeons will also recommend surgery if the MPFL is torn on MRI (however, current research suggests that non-surgical treatment has an equally good long term outcome as surgical repair of this ligament).

There are other surgical procedures with proven benefits if non-surgical treatment fails to help with stability.  A “Fulkerson” procedure moves the patellar ligament (kneecap ligament) so the kneecap tracks more to the inside.  Also, if the kneecap is tilted abnormally, a lateral release, can be used.  The lateral release was used at one time for all sorts of abnormal kneecap motion, however, studies have shown this procedure to be minimally effective, and may put the kneecap at risk for losing its blood supply if the procedure isn’t done correctly.  

What is the long term outcome?  

Nonsurgical treatment is usually effective in preventing recurrent dislocations.  Surgery, if necessary, will provide the stability necessary to prevent dislocations.  

Cartilage injury caused by the  trauma of a dislocation is another concern because it may cause increased risk for arthritis at the kneecap (which is a common site of knee arthritis). Studies show that some degree of cartilage injury occurs in almost all dislocations, but the effect of this on future arthritis requires further study.


1)  Wijdicks CA et al. Radiographic identification of the primary medial knee structures. JBJS 2009; 91:521-9. full article

1) Nomura E et al. Chondral and osteochondral injuries associated with acute patellar dislocation. Arthroscopy 2003; 19: 717-21. full article. medial patellar facet is site of injury.

2) Inoue M et al. Subluxation of the patella. Computed tomography analysis of patellofemoral congruence. JBJS 1988; 70: 1331-7. full article.

3) Laurin CA et al. The abnormal lateral patellofemoral angle: a diagnostic roentgenographic sign of recurrent patellar subluxation. JBJS 1978; 60: 55-60. full article

4) Redfern J et al. Anatomical confirmation of the use of radiographic landmarks in medial patellofemoral ligament reconstruction. Am J Sports Med 2010; 38: 293-7. full article.

5) Nomura E et al. Correlation of MR imaging findings and open exploration of medial patellofemoral ligament injuries in acute patellar dislcoations. Knee 2002; 9: 139-43. full article. mr is 80% sensitive, 95% mpfl tears if patella dislocates.

6) Fithian DC et al. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med 2004; 32: 1114-21. full article. females higher risk bc inc q-angle. ages 10-17, prior instability also risk factors.

8) Nomura E et al. Anatomical analysis of the medial patellofemoral ligament of the knee, especially the femoral attachment. Knee Surg Sports Trauma Arthro 2005; 13: 510-515. full article. MPFL is major restrained to displacement, contributes 50% of restraint.


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