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 Instability
There are several factors that can contribute to patellar instability:
- 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
Symptoms of Patellar Instability
Symptoms of chronic patellar instability are:
- A kneecap that dislocates frequently, sometimes with only minor injury or activity (in extreme cases, the kneecap can be voluntarily dislocated)
- A knee that buckles and cannot support weight with certain activities as the kneecap begins to dislocate (subluxation)
- Chronic swelling or pain in the front of the knee
The diagnosis of patellar instability can usually be made based on clinical history and physical exam. Your orthopedist will examine your knee and check for tracking of the patella during range of motion, check for laxity of the kneecap and generalized ligamentous laxity, and assess the status of the supporting ligaments of the kneecap. You will also be assessed for other alignment problems of the limb such as excessive rotation of the hip joint or distal femur, rotation of the tibia, valgus alignment of the knee, and even the alignment of your foot and ankle, as all of these can contribute to patellar instability. X-rays are useful to diagnose any bony loose fragments that may have broken off from the kneecap or the femur, and will show the depth of the trochlear groove, how well the patella sits in the groove, and if there is elevation of the patella (alta). A special tracking CT or MRI may also be used to measure the amount of bony malalignment or malrotation, which will help in surgical planning if necessary.
Treatment of Patellar Instability
If a recent dislocation has occurred, an initial period of rest and immobilization is followed by a course of physical therapy to restore the motion and strength of the knee. A patellar stabilizing brace may be used during activities or sports to help support the kneecap and prevent further dislocations.
If multiple dislocations have occurred, often conservative measures such as taping and bracing are not enough to keep the patella from dislocating during activities. In those cases, surgery may be necessary to address the anatomic causes of the instability.
In many cases of instability, the medial patellofemoral ligament (MPFL) is torn and/or lax and needs to be reconstructed to help keep the patella in place. This can be accomplished with a graft from your knee such as a hamstring tendon (autograft), or a donor (allograft) tendon.
If the maltracking is more severe, an MPFL reconstruction alone may not be enough to correct the problem. In these cases a bony procedure such as a tibial tubercle osteotomy may also be necessary to correct the patellar tracking and prevent any recurrent dislocations. In a tubercle osteotomy, the attachment site of the patellar tendon on the tibia is cut and moved to improve alignment. The attachment site, or tubercle, may be moved medially, anteriorly, or distally to improve alignment. In most cases, two or more of those movements are performed simultaneously. Other bony procedures may be needed in some cases to address certain issues such as an extremely shallow trochlear groove or an extremely rotated femur or tibia.
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. When an osteotomy is performed, there is a risk that the bone will not heal back to itself, creating a non-union that may need a separate procedure to correct. There is also a risk of a complete fracture of the tibia, especially if post-op restrictions are not followed.
The rehabilitation follow 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. It may take 4-6 months to return to sporting activity after a bony realignment procedure.
- 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.
- Is being double-jointed a risk factor for patellar instability?
The term “double-jointed” is generally used to encompass all types of ligamentous laxity. There are many causes and degrees of ligamentous laxity, but one of the most symptomatic is group of genetic conditions called Ehlers-Danlos Syndrome. In the hypermobility types of Ehlers-Danlos, there is generalized ligamentous laxity that can lead to instability of the joints, particularly the shoulders and the kneecaps. These cases are more difficult to correct as they do not respond as well to the ligament reconstruction techniques relied on to correct some of the instability problems.
- 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.
If you have more questions, please call my office at 502-394-6341.