What is a dislocating kneecap, or patellofemoral instability?
Patellofemoral instability refers to a condition in which the patella (kneecap) does not properly track or move within the femoral groove (thigh bone) during knee flexion and extension. This can result in a range of symptoms, including pain, swelling, stiffness, and a sensation of the knee "giving way" or feeling unstable.
If the kneecap moves completely out of the groove in the thighbone that it normally sits in, this is known as a dislocating kneecap, or patellofemoral dislocation. An example is shown below, with the pictures taken from the foot end.
Kneecap dislocation viewed from foot end with Xray
My kneecap dislocated very recently, what do I do?
The best treatment for a recent dislocation is:
Treatment for recent (acute) kneecap dislocation
Manage swelling with rest, ice, compresison, elevation (RICE)
seek a specialist opinion as to whether further tests eg X ray or MRI required
seek out a good physiotherapist who can help rehab
What causes patellofemoral instability (dislocating kneecap) ?
There are several potential causes of patellofemoral instability, including:
Anatomy: Certain anatomical factors, such as a shallow femoral groove or a high-riding patella or wrongly aligned patellar tendon, can increase the risk of patellar instability. Higher up the leg, a malrotated femur (thighbone) can also mean the kneecap sits in the wrong place at the knee.
Trauma: A traumatic injury to the knee, such as a dislocated patella or a medial patellofemoral ligament tear, can lead to patellar instability.
Hyperlaxity or hypermobility (elastic collagen).
Muscle imbalances: Weakness or imbalance in the muscles surrounding the knee can affect the patella's alignment and stability.
Overuse: Repeated stress on the knee joint with hyperlaxity, such as from running or jumping, can lead to patellar instability over time.
Causes of kneecap dislocation or maltracking
How do you diagnose patellofemoral instability/dislocating kneecap
Diagnosis is always based on taking a history of symptoms including:
what made the kneecap come out?
how often it dislocates or sublimes (half disloates”
how the person is affected by this: eg pain, can’t do daily activities, can’t play sport
if physio has been tried, if so what physio?
family history of dislocation
history of hypermobility
This is followed by a thorough examination as below.
How do you examine a dislocating kneecap?
Examination should look at:
signs of hypermobility (Beighton scale)
Hip or knee malalignment
How the kneecap tracks in both flexion and extension (bending and straightening)
Whether the MPFL (medial patellofemoral ligament) is loose
other structures one the knee (eg ligaments, meniscus cartilages)
flat feet that affect knee alignment
What does inverse J sign mean?
The inverse J is a sign of patellar maltracking where the kneecap deviates outwards (laterally), usually when the knee straightens. Here is an example in this patient’s left knee. This patient has undergone stabilisation of the other (right) kneecap, which does not display the inverse J sign.
What tests/investigations for a dislocating kneecap?
These include:
X ray to look for fractures (broken/chipped bone) and bony anatomy
MRI scan to look for fractures, soft tissue damage especially MPFL (medial patellofemoral ligament) tear, and bony anatomy.
Special angle Xrays (skyline views) showing normal and abnormal grooves in trochlea.
What does a torn MPFL look like?
Here are MRI scan and keyhole arthroscopy pictures of a torn MPFL.
Torn MPFL on MRI and on keyhole arthroscopy
How do you treat a dislocating kneecap or maltracking kneecap?
Treatment for patellofemoral instability depends on the underlying cause and the severity of symptoms.
This almost always involves physiotherapy, and in some case may involve operative stabilisation of the kneecap.
What physiotherapy for kneecap dislocation?
Physiotherapy often includes:
swelling management with ice massage and rest
strength and conditioning of the gluteal (buttock) hamstring and quadriceps muscles, especially the inner range quads.
massage, sometime to release tight lateral structures
balance and proprioception exercises
building up confidence in landing and jumping in later rehab
When is surgery required for kneecap dislocation/maltracking?
Surgery is considered when:
kneecap dislocation or instability continues despite good physiotherapy.
there are clear anatomical issues that physio is not solving (eg incompetent MPFL, tendon malalignment, shallow groove)
there is a fracture associated with the dislocation
the kneecap dislocation is significantly affecting the patient’s day to day or sporting life despite physio.
What are the different surgeries for kneecap dislocation?
There are many different types of surgery for kneecap dislocation. Which surgery or combination of surgeries is required depends on the underlying anatomical abnormality or abnormalities.
The different surgeries include:
Medial patellofemoral ligament reconstruction: to reconstruct the MPFL
Tibial tubercle transfer to realign the patellar tendon
Trochleoplasty to deepen a shallow or dome shaped groove.
Different surgical treatments for patellar dislocation
What is Medial Patellofemoral reconstruction/MPFL reconstruction?
Medial patellofemoral ligament (MPFL) reconstruction is a surgical procedure used to treat recurrent patellar instability, a condition where the kneecap (patella) dislocates or subluxes frequently. The MPFL is a ligament that helps to stabilize the kneecap as it moves along the femur bone in the thigh. When the MPFL is damaged or torn, the kneecap can become unstable and move out of its normal position.
During the MPFL reconstruction procedure, the damaged or torn ligament is replaced with a new one made from a tendon or ligament from another part of the body, or from a donor.
Small incisions are made around the knee and the kneecap is anchored (either with implants or with a tunnel through its substance) to the thigh bone to prevent dislocation, rather like the reigns of a horse holding it back from bolting. This stabilizes the kneecap and prevents it from dislocating or subluxing.
Recovery from MPFL reconstruction surgery usually takes several months, and patients will need to undergo physiotherapy to regain strength and mobility in the knee. They may also need to wear a brace or use crutches for 4-6 weeks after the surgery to protect the knee and allow it to heal properly.
What are the scars like after MPFL reconstruction?
You can see the scars in the the video below.
Like any surgery, MPFL reconstruction carries risks, including infection, bleeding, nerve or vessel damage, a 2-5% risk of recurrent instability (higher in hyperlaxity) and complications from anaesthesia. However, the procedure is generally considered safe and effective for treating recurrent patellar instability.
Return to work depends on the patient's occupation; people with sedentary or office based work could return to work after one week but may need to adjust their seating positions, and may be distracted by postop pain medications.
People who work in heavy manual professions may require through 2-3 months off work depending on pain stiffness and strength of muscles.
Knee moving after MPFL reconstruction.
What is tibial tubercle transfer osteotomy (bony realignment)?
Tibial tubercle transfer is a surgical procedure that involves moving the bony prominence located on the front of the tibia (the tibial tubercle) to a new location on the bone. This procedure is typically performed in individuals who have a condition called patellar malalignment with a “raised tibial tubercle-trochlear groove distance TTTG”, or a high kneecap, which causes the kneecap to move out of place.
During the procedure, the surgeon makes an incision in the skin over the tibial tubercle and carefully detaches it from the underlying bone. The tubercle is then moved to a new location on the tibia and secured in place using screws or other fixation devices. This new position is intended to realign the patella and improve its stability within the knee joint.
Rehabilitation following tibial tubercle transfer typically involves a period of restricted movement in a brace for 4-6 weeks, followed by physiotherapy to help restore strength and range of motion in the knee.
Patients may need to wear a brace or use crutches for several weeks after the surgery to protect the healing bone and promote proper alignment of the patella.
Return to work depends on the patient's occupation; people with sedentary or office based work could return to work after one week but may need to adjust their seating positions, and may be distracted by postop pain medications.
People who work in heavy manual professions may require through 2-3 months off work depending on pain stiffness and strength of muscles.
As with any surgical procedure, there are risks associated with tibial tubercle transfer, including infection, bleeding, and nerve damage. Patients should discuss the potential benefits and risks of the procedure with their surgeon to determine if it is the right choice for them
Can you have MPFL reconstruction and tibial tubercle transfer together?
Yes most surgeons do this if:
There is clear MPFL insufficiency
AND there is an high kneecap and/or lateral displacement of the tibial tubercle. Typically if there is a “patellatrochlear index of <10% or a TTTG of more than 18-20mm.
This combination of surgeries can be very powerful way of stopping patella dislocation.
What is trochleoplasty?
Trochleoplasty is reshaping of the thigh bone bed (trochlea) that the kneecap sits on.
The normal trochlea has a fairly deep groove that serves to accommodate the kneecap and also holds it in place as well as the MPFL.
In some knees with dislocating patellae, the groove is either shallow or in extreme cases dome shaped (convex). This can contribute to the kneecap being unstable to tracking badly.
In trocheloplasty, the bony groove is created either by deepening the groove or by raise the side walls.
