Overview of Patellar Instability
Patellar instability is a medical condition where the patella (kneecap) dislocates or moves out of its normal position in the joint it shares with the thigh bone (femur). This could be a subluxation where the patella moves but does not fully dislocate, or a complete dislocation. Different factors can contribute to this condition such as a traumatic injury, loose ligaments, bone misalignment, connective tissue disorders, or other anatomical issues. Over time, this can lead to severe pain, difficulty performing everyday tasks, and long-term arthritis.
Patella dislocations make up 3% of all knee injuries. It is predominantly seen amongst younger individuals, particularly females between the ages of 10 and 16. The likelihood of getting this condition is 5.8 in 100,000 for the general population and increases to 29 in 100,000 for those aged between 10 and 17. While most first-time dislocations that do not involve loose body parts or damage to the joint surface are treated without surgery, there’s a chance of it recurring. Patients who have experienced two or more dislocations are more likely to experience it again. Also, a previous dislocation puts the person at a high risk of future instability.
Patellar instability can be categorized as:
– Single dislocation due to trauma.
– Chronic instability – in cases such as Ehlers-Danlos syndrome.
– Misaligned bones – conditions like femoral anteversion, genu valgum, and external tibial torsion can lead to a condition known as “Miserable Malalignment Syndrome.”
– Anatomical discussion affecting the natural depression on the femur bone, such as trochlear dysplasia.
Patients with any of these can experience pain, functional decline, and long-term arthritis over time.
The condition is classified broadly as:
– Acute (first dislocation)
– Subluxation or dislocation
– Traumatic
– Patellar instability
– Recurrent
– Habitual dislocation – involuntary dislocation of the patella
– Passive dislocation – dislocation that can be provoked by certain maneuvers
– Syndromic – patellar dislocation associated with a neuromuscular disorder, connective tissue disorder, or syndrome
To initiate diagnosis, doctors consider the following:
– Patient’s age and gender
– A record of previous dislocations or subluxation events
– Complaints about instability
– History of ligament laxity
– Any previous surgeries
– Location of pain
– Physical examination
During a physical exam, doctors evaluate several areas including overall limb alignment, hip and knee rotation, the presence of a swollen joint in the knee, absence of signs of trauma, any pain on the inner side of the knee over medial patellofemoral ligament (MPFL), increased movement of patella on one side in comparison to the other, and they assess the Q-angle – formed by a line from the anterior superior iliac spine to the center of patella and from the center of the patella to the tibial tubercle.
Radiographic examination is carried out to rule out any loose bodies and evaluate overall lower extremity alignment and patellar height. Based on the images obtained, various ratios can be calculated to estimate the level of patella. This helps in deciding the treatment course.
Anatomy and Physiology of Patellar Instability
The patellofemoral joint is made of the bottom of the kneecap, or patella, and the surface of the thigh bone that it touches. It works properly when the groove in the thigh bone and the forces acting on the patella line up so that the kneecap slides smoothly across the groove. If anything disrupts this smooth movement, the kneecap can slip out of place.
The structure of the bones in the patellofemoral joint gives some stability to the kneecap. If anything goes wrong with these bones, it can make the kneecap unstable. Most of the time, the kneecap touches the side of the groove on the thigh bone. A normal groove is deep and steep, which helps to keep the kneecap stable. However, if the groove is shallow or flat, it can cause the kneecap to be unstable. The underside of the kneecap has a cartilage layer that is 6-7 mm thick, which is the thickest in the body. The kneecap itself is a special type of bone surrounded by a part of the quadriceps tendon and it helps to increase the efficiency of the knee extension movement. The kneecap has seven areas or facets which are divided into sections and any injury or defect in these can cause pain and instability and may need to be fixed.
In addition to the bones, the soft tissues also help to keep the kneecap stable. These include the quadriceps that attach on top of the kneecap and combine with the patellar tendon to make up the extensor mechanism of the knee, which helps to keep the kneecap in line. If anything happens to this mechanism, it can cause the kneecap to be unstable.
There are also several ligaments that are important for preventing the kneecap from moving too much to the side and keeping it in line with the groove in the thigh bone. The strongest of these ligaments is the medial patellofemoral ligament (MPFL) which connects the thigh bone to the top edge of the kneecap and primarily stops the kneecap from becoming unstable. When doctors repair the MPFL, they focus on where to place it on the thigh bone to restore a normal knee movement. A specific point named after Dr. Schottle can be used to find the accurate place to insert the ligament. The MPFL is responsible for 50-60% of preventing the kneecap from moving too far to the side. Any tear or injury to the MPFL can lead to the kneecap becoming unstable.
There are some tissues on the outer side of the knee that can also affect the stability of the kneecap, though they contribute less than the MPFL. These include the deep lateral retinaculum, epicondylopatellar ligament, and the patellotibial band. The tightness of the iliotibial band (ITB), which is a large tendon that runs down the side of the leg, can also affect the stability of the kneecap.
How stable the kneecap is, also depends on your muscles. Specifically, two muscles in the thigh, the vastus medialis and the vastus lateralis, help to control the movement of the kneecap.
Having the right knee alignment is also critical for kneecap stability. Usually, the knee joint is set at a 6-degree angle, with the line between the hip and ankle crossing through the center of the knee. If this angle increases, it can push the kneecap out of line with the groove in the thigh bone, causing instability.
If the thigh bone is rotated more than the usual 15-20 degrees, this can increase the chances of the kneecap slipping out to the side because it can cause a greater force being directed sideways across the kneecap. Similarly, a condition where the shin bone (tibia) is rotated too much externally can also contribute to kneecap instability.
A condition called “miserable malalignment syndrome” where there’s an increased rotation in the thigh bone, increased genu (knee) valgus (an inward angulation of the bones), and an increased outward rotation of the tibia increase the chances of the kneecap becoming unstable.
In some cases, the groove in the thigh bone might be shallow which is a condition known as trochlear dysplasia. This leads to the loss of the bone stability in the patellofemoral joint.
Finally, diseases that impact the body’s collagen like Ehlers-Danlos syndrome or Down Syndrome can lead to a gradual loosening of the ligaments in the knee. As these structures become lax, the kneecap can lose its stability.
How is Patellar Instability performed
If your kneecap (patella) pops out of place, this is known as a patellar dislocation, which often happens to the side (lateral). Interestingly, many of these types of kneecap dislocations will naturally pop back into place on their own. But, if not, a doctor can use a method called ‘reduction’ to put it back in place, and while it may seem a little nerve-wracking, it’s typically not necessary to have any kind of pain relief for this.
Before the procedure, your doctor will explain to you what’s believed to have happened to your knee, describe what they’re going to do to fix it, and assure you that when it’s finished, your knee should feel a lot better. For the procedure, you’ll be made to lie down or sit comfortably, with your hip slightly bent. This position helps to keep the tendon that’s connected to your kneecap relaxed.
Then, if your kneecap has moved to the side, your doctor will gently straighten your knee, pushing it a little bit past its usual straight position (hyperextension). At the same time, they will carefully press inward on the dislocated side of your kneecap to guide it back into place. Sometimes, one person can do this by themselves. They would hold the back of your ankle in one hand, which not only helps to control the position of your leg, but also allows them to feel if your thigh muscles begin to tighten—since this can make the reduction more difficult. In some cases, if a patient is particularly nervous or in a lot of pain, some mild pain relief might be given to help with the procedure.
After the reduction, the doctor will take X-rays from different angles of your knee to make sure that your kneecap is back in its correct location and to check for any other related injuries. These could include a patellar avulsion (when a tendon or ligament pulls a piece of bone away) or a problem with the cartilage or bone in the part of your knee where the thigh bone meets the kneecap.
Possible Complications of Patellar Instability
When treating conditions related to the knee, there are a variety of possible risks and complications that might occur. These include:
Arthritis, which is a condition where the joints become inflamed causing pain and stiffness.
Another complication that may occur is Arthrofibrosis, a stiff joint condition due to excessive formation of scar tissue during the healing process after a surgery or injury.
Osteonecrosis is another serious condition that could happen. It’s a disease caused by reduced blood flow to bones in the joints, which can lead to bone death.
When reconstructing MPFL (medial patellofemoral ligament, a knee ligament), “overconstraining” the patella is a risk. This means making the MPFL too tight at a point when your knee is bent (60-90 degrees of flexion), which can cause issues.
Conversely, if the doctor performs an “excessive lateral release”, it can cause the knee to become weak or dislocate. “Lateral release” means cutting ligaments on the outside of your kneecap to stop it from dislocating. If it’s done too much, though, it can lead to issues.
In addition, undergoing procedures on the tibial tubercle (a bony bump on the shin bone beneath the knee), can lead to developing a deformity known as Recurvatum, which means your knee bends backward more than it should.
Some individuals may also experience “Recurrent dislocation”, where your knee continues to dislocate!
Finally, any surgery runs the risk of infection, which can delay healing and cause additional complications.
