Anatomy of the Knee
The bones of the knee, the femur and the tibia, meet to form a hinge joint. The joint is protected in front by the patella (kneecap). There is a V-shaped notch in the femur called the trochlear groove that the patella fits into and moves along as the knee bends. Several things can happen that cause the kneecap to slide out of the groove during knee flexion, resulting in a partial or complete dislocation of the kneecap.
It generally takes a large amount of force to dislocate the kneecap (especially the first time), and significant damage to the underside of the patella and/or the end of the femur can occur. Often the surrounding ligaments will incur damage, especially on the medial side (inside). Sometimes small fragments of bone and cartilage chip off the patella or femur during dislocation to form loose bodies in the knee. If a kneecap dislocates multiple times, that condition is called chronic patellar instability.
Causes of Patellar Dislocation
There are several factors that can contribute to patellar dislocation:
- pre-existing laxity of the ligaments holding the kneecap in place, either from a previous injury or genetic laxity of the connective tissue
- an anatomic variation leading to a shallow or non-existent trochlear groove
- malalignment of the patella in relation to the rest of the knee joint
- a direct blow to the patella while the knee is flexing which forces it out of the trochlear groove
Symptoms of Patellar Dislocation
Symptoms of acute patellar dislocation are:
- Kneecap slides off to the side
- Extreme initial pain until relocation
- Rapid, acute swelling
- Pain and discoloration along the site of ligament injury
Immediate Treatment of a Patellar Dislocation
If a dislocation occurs, try to straighten the knee to see if the kneecap will go back into place. This simple maneuver is often all that is required to reduce a patellar dislocation. If the kneecap does not realign, go to an immediate care center or emergency room to have it manipulated back into the trochlear groove. The RICE protocol, a combination of rest, ice, compression, and elevation can be used to reduce swelling.
Most of the time the kneecap will be back into place before you see your physician. The doctor will discuss the event during which the dislocation occurred and perform a physical exam. You may be asked to perform motions such as walking around and bending the knee. X-rays can help determine whether the patella has properly realigned into the groove and to look for bone fragments. If a loose body or cartilage damage is suspected, an MRI may be ordered.
Treatment of Patellar Dislocation
If no loose bodies are found and no large cartilage injury is suspected, initial treatment for most first time patellar dislocations is non-surgical. An initial period of rest and immobilization is followed by a course of physical therapy to restore the motion and strength of the knee. Special taping techniques or a patellar stabilizing brace may be used during activities or sports to help support the kneecap and prevent further dislocations.
If a loose body or cartilage injury is seen on MRI after a patellar dislocation, then surgical intervention may be recommended to remove the loose body and to address the cartilage injury. The technique of addressing the cartilage injury may vary depending on the size and location of the lesion as well as the patient’s age. This can usually be done arthroscopically, but in some cases with a large cartilage injury an incision may need to be made to open the knee joint.
In addition to addressing the loose bodies and/or cartilage lesions, the stability of the patella can also be addressed. If there is only mild malalignment of the patella, arthroscopic soft tissue procedures such as a lateral release in addition to a medial reefing may be performed to help loosen the structures on the tight lateral side of the knee while tightening (or reefing/placating) the structures on the medial side. If the maltracking is more severe or there is chronic patellar instability (LINK), an open medial patellofemoral ligament reconstruction may be performed and in some cases a bony procedure such as a tibial tubercle osteotomy may also be necessary to correct the patellar tracking and prevent any recurrent dislocations.
As with any surgery on the knee, infection and blood clots are always a concern. These risks are slightly higher with an open procedure compared to an arthroscopic procedure. There is also a risk that the patella may remain unstable even after the procedure and future dislocation may be a concern. If this is the case, a more extensive procedure may be needed to stabilize the patella.
The rehabilitation following patellar stabilization surgery depends on the extent of the procedure. If an all-arthroscopic procedure is performed, then the knee is typically protected in a brace but knee motion is not restricted for more than a few weeks if at all. Weight bearing on the knee will be allowed unless there was significant cartilage damage requiring microfracture, and even then weight bearing may be allowed with the knee in full extension.
If a MPFL reconstruction is performed, the rehabilitation is similar to the all-arthroscopic procedures except the knee may be protected a few weeks longer in a brace. Generally, weight bearing and motion will not be restricted for more than two weeks.
If a bony procedure such as a tibial tubercle osteotomy is performed, then there will be more restrictions following surgery. Weight bearing is protected for approximately six weeks, and knee motion will slowly be advanced over the first six weeks. If X-rays show early healing of the osteotomy at six weeks, then the weight bearing and motion restrictions will be lifted, and strengthening exercises will begin.
- Do all patellar dislocations require surgery?
While this is a somewhat unsettled topic in orthopedics, the general consensus is that first-time dislocations with no loose bodies or large cartilage injuries can be treated non-operatively. If multiple dislocations occur, then surgery may become necessary to correct the instability.
- Can a brace help prevent further patellar dislocations?
Yes, braces can help reduce the chances of repeat patellar dislocation if the bony anatomy is normal. However, if the initial dislocation was caused by significant patellar malalignment or bony deficiencies, then a brace may not be enough to prevent repeat episodes.
- What is the difference between patellar dislocation and patellar instability?
An otherwise normal knee can sustain a traumatic patellar dislocation, but if the bony anatomy is normal then repeat dislocations will be more unlikely. However, if there is an anatomic deficiency such as lateral patellar alignment, dysplasia of the trochlea (a very shallow groove), a high patella (patella alta), or excessive laxity of the ligaments, then after the first dislocation there may be very little anatomic restraint to further dislocations and the patella may dislocate with minimal trauma or even normal activities. This case would be described as patellar instability and may need surgery to prevent further dislocations and cartilage damage to the knee.
If you have more questions, please call my office at 502-394-6341.