Patella dislocation and patella stabilisation
What is the patella?
The patella, or kneecap, is a bone that lies at the front of the knee. It sits within the quadriceps and patellar tendons, which form the main mechanism that straightens the knee.
The patella improves the mechanical efficiency of this mechanism, allowing the quadriceps muscle to straighten the knee more effectively. It also forms a joint with the front of the femur (thigh bone). This is called the patellofemoral joint.
The undersurface of the patella and the groove in the femur, called the trochlea, are covered with smooth articular cartilage. This allows the patella to glide smoothly within the trochlear groove as the knee bends and straightens.
What is a patella dislocation?
The patella normally moves up and down within the trochlear groove as the knee bends and straightens. Occasionally, it can move completely out of the groove. This is called a patellar dislocation.
The vast majority—approximately 99% or virtually all standard traumatic patellar dislocations—are lateral.
A patellar dislocation is different from a patellar subluxation. With a subluxation, the patella partially moves out of its normal position but does not completely dislocate.
How does the patella dislocate?
A first-time patellar dislocation most commonly occurs during a twisting movement on a slightly bent knee. It is often a non-contact injury, meaning that there is no collision or direct blow to the knee. It can occur when changing direction, landing from a jump or pivoting during sport. A direct blow to the patella can also cause it to dislocate.
At the time of the injury, patients often describe feeling the kneecap move out of position. The knee is usually immediately painful and commonly swells rapidly. Sometimes the patella moves back into position by itself. In other cases, the patella remains dislocated.
If the patella remains dislocated
A persistently dislocated patella should be assessed urgently.
If there is no concern about an associated fracture or other injury, gentle attempts to straighten the knee may allow the patella to return to its normal position. However, the quadriceps and hamstring muscles can go into spasm following a dislocation, making this difficult and painful.
If the patella does not reduce easily, it should be relocated by an appropriately trained medical professional. Pain relief and, occasionally, sedation may be required to allow the muscles to relax and the patella to be safely reduced.
What happens to the knee when the patella dislocates?
A patellar dislocation stretches and usually tears the soft tissues on the inside of the knee that normally help prevent the patella from moving towards the outside. One of the most important of these structures is the medial patellofemoral ligament (MPFL). The MPFL runs between the medial (inner) side of the patella and the medial femur and acts as an important restraint against lateral movement of the patella.
The MPFL is commonly injured during a first-time patellar dislocation.
The patella can also damage the articular cartilage as it moves out of and back into the trochlear groove. This can cause a cartilage injury on the patella or on the lateral femoral condyle. Occasionally, a piece of cartilage together with the underlying bone is knocked off. This is called an osteochondral fragment. A large osteochondral fragment may require surgical treatment.
What happens after a first time patella dislocation?
After the patella has been relocated, the knee is often swollen and painful. The knee may initially be placed in a brace or splint, usually for comfort and security and to help with walking.
The early focus of treatment is to control pain and swelling and to restore movement and muscle function.
The quadriceps muscle can become inhibited after a knee injury. Pain and swelling can make it difficult to voluntarily contract the muscle.
It is therefore important to start restoring quadriceps muscle activation as soon as pain allows. This usually begins with simple isometric quadriceps exercises, progressing to a straight-leg raise and then to more demanding strengthening exercises as recovery progresses.
Physiotherapy is an important part of recovery, helping to restore muscle strength, movement, balance and confidence in the knee.
What are the risk (predisposing) factors for patella dislocation?
The stability of the patellofemoral joint depends on a combination of static and dynamic stabilisers.
The static stabilisers are the bones and ligaments that provide structural stability. The dynamic stabilisers are the muscles that control the position and movement of the leg.
STATIC STABILISERS
Important anatomical factors include:
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Patellar height
The position of the patella relative to the trochlear groove is important. A high-riding patella is called patella alta. This means the patella sits higher than normal and does not enter the trochlear groove until the knee bends further.
As a result, the patella has less support from the groove when the knee is straight or only slightly bent, making it easier for the patella to dislocate.
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Trochlear dysplasia
The trochlea is the groove at the front of the femur in which the patella runs. In a normally shaped knee, the trochlea has a defined groove that helps contain the patella. The undersurface of the patella also has a corresponding shape similar to the keel of a boat that fits within this groove.
In some people, the trochlea is unusually shallow or has an abnormal shape. This is known as trochlear dysplasia. A shallow or dysplastic trochlea provides less bony restraint to the patella and is an important risk factor for recurrent patellar instability.
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Alignment of the leg
The overall alignment and rotational anatomy of the lower limb can also influence the position and movement of the patella. The angle between the quadriceps mechanism and the patellar tendon is described using the Q-angle. A greater lateral pull on the patella can contribute to instability.
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Femoral and tibial rotation
The rotational alignment of the femur and tibia can influence the tracking of the patella. For example, increased femoral anteversion can contribute to an inward rotation of the femur and alter the position of the patella during knee movement. In some patients with severe rotational abnormalities, this may contribute to recurrent instability.
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Joint hypermobility
Ligamentous laxity is a risk factor for patellar dislocation because the soft-tissue structures that help restrain the patella, including the medial patellofemoral ligament (MPFL), may be more lax. This can reduce the passive stability of the patella and make it more prone to dislocation.
DYNAMIC STABILISERS
The muscles around the hip, thigh and knee also play an important role in controlling the position of the patella.
These include:
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Quadriceps – controls knee extension and helps control movement of the patella.
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Hamstrings – control knee flexion and contribute to control of tibial rotation.
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Hip abductors – including gluteus medius and gluteus minimus, help control the position of the pelvis and femur.
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Hip extensors – including gluteus maximus, contribute to control of the hip and lower limb.
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Hip external rotators – help control rotational movement of the femur.
Good neuromuscular control allows the leg to remain in positions that reduce the forces tending to push the patella towards the outside of the knee.
PATIENT AGE
First-time patellar dislocations most commonly occur in adolescents and teenagers, particularly between the ages of 10 and 18.
During adolescence, changes in skeletal development, muscle strength and neuromuscular control can influence the stability of the kneecap. Ligamentous laxity and the anatomy of the patellofemoral joint can also contribute.
The more risk factors that are present, the younger a patient may be when they experience their first patellar dislocation.
Diagnosis
The diagnosis of a patellar dislocation can often be made from the history and examination.
It is important to establish what happened at the time of the injury, whether the patella completely dislocated or partially slipped out of position, and whether it relocated spontaneously.
X-RAYS
X-rays are usually obtained following a patellar dislocation. They can confirm the position of the patella and identify associated fractures or other bony abnormalities. If the patella remains dislocated, X-rays may be obtained before reduction. Further X-rays are normally obtained after the patella has been relocated to confirm its position and assess for associated injury.
MRI Scan
An MRI scan may be recommended following a patellar dislocation, particularly if there is uncertainty about the diagnosis or concern about an associated injury.
An MRI can identify damage to the structures around the knee, including the MPFL, cartilage and bone. It is particularly useful for detecting a significant cartilage injury or an osteochondral fragment (a piece of bone and cartilage that may have broken off during the dislocation).
MRI can also provide information about the shape and anatomy of the patellofemoral joint and identify features that may have contributed to patellar instability.
In patients with recurrent instability, additional imaging may sometimes be needed to assess the alignment and rotational anatomy of the lower limb.
Treatment
Most people who sustain a first-time patellar dislocation do not require surgery.
The overall risk of another dislocation following a first-time patellar dislocation is approximately 30–40%, although the risk varies considerably between individuals. Younger patients and those with particular anatomical risk factors have a higher risk of recurrent instability.
REHABILITATION
The initial treatment usually involves:
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controlling pain and swelling;
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restoring knee movement;
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restoring quadriceps activation;
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strengthening the quadriceps and muscles around the hip;
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improving balance and neuromuscular control; and
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gradually returning to normal activities and sport.
Physiotherapy is therefore an essential part of treatment following a first-time dislocation.
Surgery is generally considered when the patella repeatedly dislocates despite appropriate rehabilitation, although there are some circumstances in which surgery may be considered after a first dislocation, such as a significant osteochondral injury requiring treatment.
Patella stabilisation surgery
The aim of surgery is to restore stability while preserving the normal movement and function of the patellofemoral joint. The operation required depends on the underlying factors contributing to patellar instability.
This is particularly important because recurrent instability is often caused by a combination of anatomical factors, rather than a problem with a single ligament. Surgical decision-making involves identifying which risk factors can be corrected with surgery, while considering the potential risks, benefits and recovery associated with each procedure.
Medial patellofemoral ligament (MPFL) reconstruction
The medial patellofemoral ligament is the main soft-tissue restraint preventing the patella from moving laterally, particularly during the early part of knee flexion. An MPFL reconstruction creates a new ligament to reproduce the function of the damaged MPFL, most commonly using one of the hamstring tendons.
For many patients with recurrent instability and otherwise suitable anatomy, an MPFL reconstruction is sufficient to restore stability.
Tibial tuberosity osteotomy
The tibial tubercle is the bony prominence at the top of the shin bone where the patellar tendon attaches.
A tibial tubercle osteotomy involves carefully cutting and moving this area of bone to change the position and/or tension of the extensor mechanism.
The tibial tubercle is most commonly moved downwards to correct a high-riding kneecap (patella alta). In selected patients, it may also be moved medially to improve the alignment and stability of the kneecap.
Trochleoplasty
In patients with severe trochlear dysplasia, the shape of the trochlea may be a major contributor to instability. A trochleoplasty is a procedure that reshapes the trochlea to create a more effective groove for the patella. This is a specialist procedure and is generally reserved for selected patients with significant trochlear dysplasia.
What does MPFL reconstruction involve?
Steps of MPFL reconstruction
1. Anaesthetic
The surgery is usually performed under a general anaesthetic.
2. Graft harvest
One of the hamstring tendons is harvested through a small vertical incision is made over the upper inner part of the tibia.
3. Graft fixation to the patella
A small incision is made along the inner border of the kneecap (patella). The tendon graft is then secured to the patella. This may be done by passing the graft through a small bone tunnel created in the patella or by using small anchors inserted into the patella.
4. Graft passage
The graft is passed underneath the soft tissues along the inner side of the knee, following the position of the native MPFL.
5. Graft fixation to the femur
The other end of the graft is attached to the femur (thigh bone) at the anatomical attachment of the MPFL. The position of the MPFL attachment is confirmed with the aid of an image intensifier (X-ray). Accurate positioning of this attachment is important to allow the reconstructed ligament to provide appropriate restraint while allowing the patella to move normally as the knee bends and straightens. Once appropriate graft tension has been determined the graft is secured in a bone tunnel created in the femur using an interference screw.
6. Checking the reconstruction
The knee is moved through its range of motion to check that the patella tracks normally and that the graft provides appropriate stability without over-tightening the patellofemoral joint.
7. Wound closure
The incisions are then closed using buried, dissolvable sutures, and a dressing is applied. A knee brace is applied to the knee.