A patellar dislocation is a knee injury in which the patella (kneecap) slips out of its normal position. Often the knee is partly bent, painful and swollen. The patella is also often felt and seen out of place. Complications may include a patella fracture or arthritis. A patellar dislocation typically occurs when the knee is straight and the lower leg is bent outwards when twisting. Occasionally, it occurs when the knee is bent and the patella is struck directly. Commonly associated sports include soccer, gymnastics, and ice hockey. Dislocations nearly always occur away from the midline. Diagnosis is typically based on symptoms and supported by X-rays. Reduction is generally done by pushing the patella towards the midline while straightening the knee. After reduction, the leg is generally splinted in a straight position for a few weeks. This is then followed by physical therapy. Surgery after a first dislocation is generally of unclear benefit. Surgery may be indicated in those cases where a fracture occurs within the joint or where the patella has repeatedly dislocated. Patellar dislocations occur in about 6 per 100,000 people per year. They make up about 2% of knee injuries. It is most common in those 10 to 17 years old. Rates in males and females are similar. Recurrence after an initial dislocation occurs in about 30% of people.
Signs and symptoms People often describe pain as severe and being "inside the knee cap". The leg tends to flex even when relaxed. In some cases, the injured ligaments involved in patellar dislocation do not allow the leg to flex.
Risk factors A predisposing factor is tightness in the tensor fasciae latae muscle and iliotibial tract in combination with a quadriceps imbalance between the vastus lateralis and vastus medialis muscles can play a large role, found, mainly, in women involved in sports. Moreover, women with patellofemoral pain may show increased Q-angle compared with women without patellofemoral pain. Another cause of patellar symptoms is lateral patellar compression syndrome, which can be caused from lack of balance or inflammation in the joints. The pathophysiology of the kneecap is complex, and deals with the osseous soft tissue or abnormalities within the patellofemoral groove. The patellar symptoms cause knee extensor dysplasia, and sensitive small variations affect the muscular mechanism that controls the joint movements. 24% of people whose patellas have dislocated have relatives who have experienced patellar dislocations.
Athletic population Patellar dislocation occurs mainly in youths (under age 20) engaged in sports that may involve accidental rotation of the knee while in flexion, a movement clinically called valgus, which is the cause of some 93% of patellar dislocation cases. It is more common in females than males and in young in-training military personnel who have a high incidence of patellar dislocation in relation to young athletes and the general population. Direct trauma to the knee displacing the patella is rare. Displacement of the patella laterally out of its groove strains the medial stabilizing connective tissues, particularly the medial patellofemoral ligament (supporting 50–80% of the knee mechanisms in lateral patellar glide), which is torn usually at its femoral attachment. Traumatic patellar dislocation may cause bleeding into the joint space, ligament and muscle attachment tearing, and fracture of the medial wing of the patella. Fracture of the weight-bearing portion of the lateral femoral condyle occurs in 25% of traumatic patellar dislocations. Surgical repair of the patellar stabilizing structures – the medial patellofemoral ligament and vastus medialis muscle – may be needed for athletes.
Anatomical factors People who have larger Q angles tend to be more prone to having knee injuries such as dislocations, due to the central line of pull found in the quadriceps muscles that run from the anterior superior iliac spine to the center of the patella. The range of a normal Q angle for men ranges from <15 degrees and for females <20 degrees, putting females at a higher risk for this injury. An angle greater than 25 degrees between the patellar tendon and quadriceps muscle can predispose a person to patellar dislocation. In patella alta, the patella sits higher on the knee than normal. Normal function of the VMO muscle (VMO) stabilizes the patella. Decreased VMO function results in instability of the patella.
Forces When there is too much tension on the patella, the ligaments will be susceptible to tearing due to shear force or torsion force, which then displaces the patella from its groove. Patellar dislocation may also occur when the trochlear groove is shallow, a condition defined as trochlear dysplasia.
Mechanism of injury
Patellar dislocations occur by:
A direct impact that knocks the patella out of joint A twisting motion of the knee, or ankle A sudden lateral cut
Anatomy of the knee The patella is a triangular sesamoid bone that is embedded in tendon. It rests in the patellofemoral groove, an articular cartilage-lined hollow at the end of the thigh bone (femur) where the thigh bone meets the shin bone (tibia). Several ligaments and tendons hold the patella in place and allow it to move up and down the patellofemoral groove when the leg bends. The top of the patella attaches to the quadriceps muscle via the quadriceps tendon, the middle to the vastus medialis obliquus and vastus lateralis muscles, and the bottom to the head of the tibia (tibial tuberosity) via the patellar tendon, which is a continuation of the quadriceps femoris tendon. The medial patellofemoral ligament attaches horizontally in the inner knee to the adductor magnus tendon and is the structure most often damaged during a patellar dislocation. Finally, the lateral collateral ligament and the medial collateral ligament stabilize the patella on either side. Any of these structures can sustain damage during a patellar dislocation.
Diagnosis
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