Kneecap Dislocation (Patellar Dislocation)
What is the function of the kneecap?
The kneecap or patella is a bone enveloped by the great extensor tendon of the thigh muscles. Its posterior surface slides up and down a groove on the end of the thigh bone. Due to its congruent triangular-shape, the kneecap normally fits perfectly into the groove (trochlea). The muscle contraction is transferred directly to the lower leg and extends and straightens the knee.
What is kneecap dislocation?
Basically, there are two different types of patellar dislocation: one is induced by the bodily frame (habitus), the second occurs as an injury, due to a trauma or accident and can become a chronic and recurrent problem. In most cases, the kneecap slips sideways and around to the outside of the knee (patellar maltracking). Dislocated kneecaps occur slightly more frequently in women than men, with the first incident, leading to the recurrent or habitual dislocation of the patella, usually occurring before the age of twenty. After the initial, very painful, dislocation, the patella usually repositions itself spontaneously, i.e. jumps back into its groove, however, the ligaments securing the patella to the inside of the knee, are often torn and in 24% of all cases, cartilage and bones are injured (so-called shear-fractures or osteochondral lesions occur). The cartilage damage which develops in this context is often overlooked and results in a chondropathy, a disease of the cartilage which progresses gradually.
Are there risk factors leading to kneecap dislocation?
There are several factors implicated in the cause of kneecap dislocation, such as kneecap dysplasia (the shape of the kneecap and the groove it sits on do not match or the lower surface of the kneecap is too flat), knock-knees (Valgus Malalignment) or if there is an increased pathologic Q angle, i.e. the patellar tendon's insertion is displaced outwardly. A general weakness of the connective tissue, an imbalance of the thigh muscles, and an abnormally high patella in relation to the femur (patella alta) due to a congenital predisposition or a trauma, are further risk factors.
Trochlear dysplasia (developmental abnormality of the femoral trochlear groove of the kneecap) and a misalignment of the kneecap on the opposite side can be implicated in the cause of patellar subluxation and often lead to further dislocations.
Diagnosis of kneecap (patella) dislocation
The basic requirements for the successful treatment of kneecap dislocation, are an extensive case history (anamnesis) and the thorough physical examination of the patient. X-rays of the knee joint may provide indications of a possible predisposition or existing injuries to the bones. A spot film of the patella, taken while continuously bending the knee, helps track the grade of patellar malalignment. At this point it is already possible to make an estimate of how severe the chondropathy, i.e. the cartilage damage, most probably is.
When clinical findings are ambiguous, an examination under anaesthesia may be recommended or an arthroscopic diagnosis may be necessary.
During the arthroscopy it would also be possible to remove detached bone or cartilage fragments or to smooth damaged cartilage.
MRI-examinations (magnetic resonance imaging) just like EMG (electromyography test checking the health of the leg's muscles and nerves) help to establish a more precise and complete diagnosis of the condition of the knee cartilage and soft tissue.
Why is it so important to treat kneecap dislocation?
Each patella dislocation can cause irreparable damage to the patella's cartilage surface. In many cases the dislocation results in osteochondral shear fractures. Loose bone or cartilage fragments may lock the joint, which may lead to further cartilage damage at other parts of the joint. There is also the risk of tearing the intra-articular ligaments of the articular capsule.
After an initial acute kneecap dislocation, about 50% of patients develop recurrent patellar instability if they are not treated properly. The probability of retropatellar chondral damage or a patellar chondropathy increases considerably with each dislocation. That is why the treatment's aim is to stabilise the kneecap and avoid further patellar subluxations, to provide pain relief and, in the long term, to avoid arthritis developing in the knee joint.
What are the possible treatments for patellar subluxation?
After a first subluxation without further risk factors, patients usually undergo non-surgical treatments such as physical therapy, but if there are risk factors which may cause further dislocations or recurrent dislocations, surgery may be needed.
In the case of a shear fracture (a detached osteochondral fragment) the main priority is to reattach the fragment. If this is not possible, the fragment should be removed, in order to prevent further cartilage damage.
If there is an isolated tear of the proximal intra-articular ligaments of the kneecap without the presence of further risk factors, the suture and plication of the tissue concerned (use of a medial double-plating technique) are necessary.
There are different surgical reconstruction techniques, but all make use of an autogenic tendon graft, in most cases a semitendinosus graft. Usually the tendon is attached to the kneecap via a borehole to the femur, by means of a resorbable screw, or on the adductor magnus tendon without a screw. Results for all methods are good, but more extensive cartilage lesions may have a negative influence on the operation.
Performing a lateral release, i.e. cutting through the thigh retinaculum to allow the kneecap to sit properly within its groove, is an another option for patients suffering from chronically recurrent dislocation. Lateral release is a minimally invasive procedure, where the lateral retinaculum of the knee is cut, reducing the force which is pulling the kneecap to the outside. Another advantage of this type of operation is the reduction of the pressure applied to the external cartilage of the kneecap. Chondropathy, i.e. the development of severe cartilage damage, particularly to the external part of the kneecap, can therefore be reduced. If there is a distal transfer of the the tibial tuberosity (Tuberositas Tibiae), which can provoke a lateral dislocation of the kneecap, the treatment may consist of tendon displacement. To ensure the correct therapy, it is necessary to determine certain clinical parameters, such as the Q angle and radiological parameters such as the TTTG distance (tibial tuberosity - trochlear groove distance).
What are the long-term prospects after the treatment of kneecap dislocations?
Better sooner than later should be the motto. In most cases, however, the reaction generally comes too late rather than too early. The majority of patients, particularly younger ones, tend to consult their doctor only when the dislocations have become a recurrent problem. If they did not wait so long, seeking early treatment instead, success rates for a stable kneejoint without any discomfort would be over 80 percent! When the diagnosis and resultant therapy are delayed, the chance to be symptom-free falls to under 20 percent and the development of arthritis in the kneejoint (femoropatellar joint) is generally the consequence. Chondropathies and severe cartilage damage are also often associated with this condition.
Recent studies confirm excellent clinical results with reconstructive surgery. This method also has a reduced risk of trauma, however, long term studies do not as yet exist.
The knee joint is often - and rightly - referred to as the most complex joint in the body. The knee's ingenious biomechanical structure, consisting of the patella, meniscus, capsule, ligaments, muscles, and tendons, gives it a remarkable load-bearing capacity. With each step we take, the knee smartly distributes and absorbs compressive forces of around twice our body weight! 
The menisci (the medial meniscus and the lateral meniscus) play a number of important roles in proper knee functioning. They act as shock absorbers between the bearing surfaces in the knee, they evenly distribute the compressive forces from the femur onto the tibial plateau, and they provide both lubrication and stability for the knee joint. While meniscus injuries may be acute or degenerative in nature, they often require prompt surgical intervention to avoid later damage to the cartilage in the knee joint.
