The patella, a sesmoid bone hanging in the quadriceps femoris tendon, should normally reside between the femoral trochlea and the tibia (in the trochlear groove). If the patella leaves this space it is known as patellar dislocation and if it moves around in that space it is known as patellar instability or patellar sublaxation.
Normal forces on the patella
The quadriceps femoris muscle is actually made of four parts. The most important part concerning the patella is the vastus medialis oblique muscle. It and the medial retinaculum (medial patello-femoral ligament) a ligament that pulls the patella inward, keep the patella in place. A lateral movement of the patella might cause it to pop out – or dislocate.
Causes of patellar instability
Patellar dislocation can be traumatic or non-traumatic. The more common type is the multi-factorial non-traumatic patellar sublaxation.
Some common anatomical causes for non-traumatic patellar instability are:
- “Knock knees” – Genu Valgum – The knees turn inward and the tibia is turned outward
- Outward turning tibia
- Hip anteversion – outward turning hip, causing inward turned toes. Another, related cause, is a weak gluteus medius. The gluteus medius muscle is responsible for external hip rotation. A weak gluteus medius results in mal-alignment of the hip and therefore the knees, and knee pain on that side.
- “Flat feet” – Pes Planus – with inward turning knees
All these reasons may start off differently, but they result in the same way – The Q angle is abnormal (more than 15 degrees).
Other causes for patellar dislocation are not anatomical per say:
- Trochlear dysplasia – Normally, the medial trochlear facet is lower than the lateral facet. This prevent the patella from dislocating laterally. In trochlear dysplasia the medial and lateral trochlear facets are the same hight, thus facilitating dislocation.
A common mechanism by which the patella can be dislocated is bending the knee during valgus (while the knee is turned in).
Symptoms of patellar instability
Patellar instability is a cause of anterior knee pain. Some say it is just another way of looking at the same theme. Other symptoms of patellar instability include knee “giving way” when climbing up and down stairs, pain while sitting, knee stiffness, knee crackles and intermittent knee swelling.
Physical examination of patellar dislocation
Because patellar dislocation can be a result of a local problem in the knee or a problem in the hip or foot, the physical examination for patellar dislocation should be thorough:
- Hip, knee and foot examination
- Observation of walking
- J sign – Ask the patient to sit with the legs over a chair. While the leg is straightened, the patella escapes laterally.
- Apprehension test – The patellar apprehension test involves pressing on the patella while the leg is straight.
- Patellar tilt test – Move the patella from side to side and observe a tilt on the medial side.
- Patellar grinding test – Move the patella while the leg is slightly flexed (at aprox. 10 degrees). If there is grinding and squeaking, that indicates a osteoarthritis or chondromalacia.
- Active patellar grinding test – Press the patella from above, toward the tibia. This causes the quadriceps to contract. If there is pain, it is a positive test.
- Hyper-laxity should be looked for because hyper-laxity is a risk factor for patellar dislocation.
Imaging patellar dislocation
The first imaging study to choose when suspecting dislocation of the patella, is a regular x-ray. Knee images should be taken in four positions: antero-posterior (standing), antero-posterior (sitting – Tunnel view), lateral and skyline views.
Knee tunnel view - AP while sitting
On the x-ray the following findings may be seen:
- Bipartite patella – A bipartite patella may be an incidental finding or be a cause of pain in athletes. Most bipartite patellas are lateral. A medial bipartite patella should raise the suspicion for dislocation:
- High sitting patella – A patella situated higher than normal in the trochlea is prone to dislocation.
Treatment for patellar dislocation
Treatment for patellar dislocation consists of the following:
- Rest
- Physiotherapy – One of the reasons for knee instability after injury is not actual mechanical instability but rather functional instability. This means that even after injury the knee still has all the components it “needs” in order to function properly, but it does not. This is because the knee “feels” as though something is missing. This is where physiotherapy kicks in, allowing “reprogramming” of the knee sensory-motor pathways.
- Knee stabilization – This can be done by Taping, a specialized strap or brace or an elastic bandage. Sometimes a cast is used. Aside for the mechanical advantages, knee fixation helps re-institute the knee’s sensory-motor function.
- Insoles to improve foot positioning
- Surgery – Surgery for patellar dislocation is not commonly performed. Surgery can be one of two types:
- Proximal – In this type of surgery, the patella is moved by releasing the lateral head of the quadriceps and tightening the medial head
- Distal – The patella is moved medially by repositioning the tibial tuberosity
- Sometimes surgery is needed after traumatic patellar dislocation, to remove foreign bodies. Otherwise, even for traumatic patellar dislocation, surgery is not warranted. Blood is evacuated from the joint and the knee is cooled and stabilized.
