Anatomy
The large anterior thigh muscle (musculus quadriceps femoris) consists of four muscles (MM vastus lateralis, vastus medialis, vastus intermedius and rectus femoris). The muscles all attach to the upper edge of the kneecap. The patella tendon (ligamentum patellae) connects the lower edge of the patella to the upper, anterior part of the tibia (tuberositas tibiae). The patella is held in place by the structures that attach to the patella, especially the anterior thigh muscle and patella tendon, as well as the joint capsule and several ligaments (retiaculum patellae mediale and retiaculum patellae laterale).
Knee from the front:
A. Tendo m. adductoris magni
B. Retinaculum patellae mediale
C. Meniscus medialis
D. Ligamentum collaterale mediale/tibiale
E. Bursa anserina
F. Bursa subtendinea m. sartori
G. Ligamentum patellae
H. Patella
Cause
Acute kneecap dislocation occurs primarily in children and adolescents (and young adults) during sports activity, where there is a twist on a slightly bent knee while the foot is on the ground. The kneecap can be displaced to the outside of the knee, causing the ligaments holding the kneecap to rupture. Often the kneecap bumps against the femur and in about half of them, cartilage damage occurs on the back of the kneecap (chondromalacia patellae).
In almost all cases of patellar dislocation, there are anatomical changes in the normal anatomy that reduce the load that can trigger a dislocation. In 35% of patellar dislocations, there is a familial predisposition such as a raised patella or flattened patellar groove on the femur (trochlear dysplasia) (Samelis PV, et al. 2023)
Symptoms
Sudden onset of severe pain that makes continued sports activity impossible. In some cases, the kneecap may dislocate completely on the outside of the knee, locking the knee in a bent position (total dislocation) until the kneecap suddenly slips back into place, which often happens when the knee is stretched. In other cases, the patella is only partially displaced on the outside of the knee (partial dislocation = subluxatio patellae) and spontaneously moves back into place.
Examination
If the kneecap is in place, diagnosis can be difficult and anyone with a suspected dislocated kneecap should always be medically examined to ensure diagnosis and proper treatment. During the examination, severe pain will typically be triggered when the patella is pressed outwards (laterally) while bending the knee (Apprehension test).
The kneecap will often extend further outwards on the injured knee than on the healthy knee. In 30% of cases, X-rays will show bone tearing and can also reveal any anatomical variants. MRI scans can provide information about the cartilage conditions in the knee after total and partial kneecap dislocations. (Sahin E, et al. 2024).
Treatment
Acute treatment includes cancelling the kneecap dislocation. Partial joint slips should primarily be treated with offloading and rehabilitation. There is no consensus on the treatment of total kneecap dislocation, where 30-50% will have a recurrence (Lee DY, et al. 2023).
For joint slips without bone damage, bandages are often applied for 2 weeks, after which rehabilitation is intensified. Surgery is often recommended, especially for repeated joint slips and for major bone and cartilage damage, where the medial patellofemoral ligament (MPFL) is restored. The documentation of the various treatment measures is not clear-cut (Parikh SN, et al. 2024). Sport-specific training can often be resumed after 1-2 months. Resumption of non-contact sports (e.g. volleyball, badminton, tennis) can often be resumed after 3-4 months and contact sports (basketball, handball, football) after 6-8 months.
Rehabilitation
Rehabilitation of non-surgical kneecap dislocation primarily includes thigh muscle strengthening and balance training. If surgery has been performed, rehabilitation should be organised in collaboration with the department where the surgery was performed.
Bandage
Tapes and bandages do not have a convincingly documented preventive effect in previous total or partial kneecap dislocations (Vermeulen D, et al. 2019), but experience shows that they are still widely used. You can buy bandages that are specially designed to keep the kneecap in place. These can be used during sports, but not in everyday life. See tape.
Complications
If no progress is made, you need to consider whether the diagnosis is correct.
In particular, the following should be considered:
- Cartilage damage
- Bone fracture in the knee
- Meniscus lesion
- Outer collateral ligament rupture
- Rupture of the posterior cruciate ligament
- Periosteal avulsion (periosteal avulsion)
