Patellaluxation
A dislocated kneecap (patella) is one of the most unpleasant instabilities of the knee joint.
The patellar dislocation is a relatively common problem in children and adolescents. Approximately 43 children per 100,000 per year report such an accident of the knee extensor apparatus. In half of all cases, the dislocation repeats and becomes recurrent. The earlier in age the first dislocation occurs, the more probable is its recurrent course.
Symptoms of patellar dislocation
Patients report intense pain after the first dislocation. The pain usually reduces if the kneecap repeatedly dislocates. Other symptoms become more pronounced.
Instability
Often an extension of the knee combined with rotational movements are responsible for the instability. A complete dislocation, in which the patella lies completely lateral outside the trochlea, and subluxation with only a partially lateral positioned patella can distinguish. Both of them occur suddenly. However, some patients report that they had anticipated the dislocation.
In many cases, pain is not the most significant problem, but fear of further instability. Sometimes, anxiety can negatively influence work or leisure activities.
Fear
Patients notice that the kneecap is unstable in different situations. Some patients are only insecure during more intense sports such as football or volleyball, while for others feel the instability in everyday life.
Adaptation of gait pattern
Our research presents a changed gait pattern after a first dislocation. Usually, a healthy knee joint flexes up to 25° during the stance phase (red line). Patients with patellar dislocations bend their knee up to 10° less (blue line). This unfavourable gait leads to higher loads on the knee joint.
One step recorded in gait analysis. The red line shows a regular gait pattern, the blue one a gait pattern typical for a patient with patellar dislocation. The knee is significantly less flexed.
Blockades
The patella dislocates with forces of up to half a ton. The patella slams against the outer wall of the thigh and a piece of the patellar cartilage or the femur can break off. This piece of cartilage usually lies within the knee joint (arrow) and does not interfere with a normal function. However, it can get stuck between the bones and cause pain. A piece of cartilage can measure several cms. Large parts of cartilage have to be refixed to restore cartilage continuity.
Axial MRI of the knee. An approx. 2.5 cm large piece of gnarled bone has broken off the patella during dislocation.
Causes & risk factors
Trochlear dysplasia
The trochlea is the sliding bearing of the patella. This part of the femur is the most important stabilizer for the kneecap. It is comparable to a bobsleigh track in which the kneecap slides down like a bob during flexion of the knee joint. The bone and the cartilage above define the structure of the trochlea. In case of a dysplastic trochlea, there is no bobsleigh track or a canal, but a flat area, or in the worst case, even a hill. Trochlear dysplasia (white arrow) is found in over 90% of patients who suffer from recurrent patellar luxation. Therefore, other structures have to stabilize the kneecap. The most important is the medial patellofemoral ligament (MPFL, red arrow), a ligament between the kneecap and the femur. After the first dislocation, this ligament gets usually torn. However, it heals with scars and is somewhat elongated. The kneecap drifts further lateral and becomes even more unstable.
Trochlear surgery corrects dysplasia. However, in children and adolescents, great caution is necessary, since the growth plate in the area of the operation are at risk to be injured.
Axial MRI of the knee. The trochlea is not channel-shaped, but flat (white arrow). The MPFL (red arrow) is very thin compared to the lateral patellofemoral band (green arrow).
Patella alta - the high riding kneecap
Patella alta is a kneecap, which is located more proximally. The patellar ligament, which runs from the patella to the lower leg, is longer. The kneecap enters the trochlea significantly later when the knee gets flexed. Therefore, even a normal trochlea would not stabilize the kneecap.
The treatment of high riding kneecap should not be underestimated. Various techniques are particularly suitable for children and adolescents.
Valgus malalignment
With a straight leg, the mechanical axis runs through the middle of the knee joint. In the case of a valgus alignment, the mechanical axis is shifted laterally. The kneecap is also pushed laterally. This position can further worsen the stability of the patella.
In children, the valgus improves as it grows. A valgus malalignment is usually still healthy for people under the age of ten.
Anteversion of the femur - rotation of the thigh
The thigh is usually twisted about 15-25 °.
If this torsion of the thigh is over 35 °, it can affect the stability of the kneecap. The femur is turned further inwards, and the kneecap thus positions itself also laterally. Therefore, structures on the side of the kneecap are tense and can loosen.
As with the valgus, the anteversion improves with growth. Nevertheless, an increased anteversion must be considered when treating an unstable kneecap.
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Diagnosis
During a consultation, it becomes clear how severe the symptoms are. These symptoms can vary widely from patient to patient. Some patients may experience daily dislocations. However, they can deal with the situation. Others experience significant restrictions already in their everyday life to such an extent that measures must be taken. The individual perception of instability also points the way forward in therapy.
Also, the clinical examination is an integral part of finding the right therapy. The patella can be assessed, as well as compensation mechanisms or malpositioning of the leg.
The decisive diagnostic examination is still the physical examination of the knee joint (video). Besides, magnetic resonance imaging (MRI) is indispensable nowadays. This is where a trochlear dysplasia shows up best. Rotation problems or the condition of the cartilage can also be optimally assessed.
Therapy
Acute patella luxation - the emergency
The first patellar dislocation is usually an impressive experience full of fear and pain. In many cases, the patella does not reduce itself. But even in recurrent patella dislocations, this does not always happen by itself.
The most critical measure to reduce the kneecap without medical staff is relaxation. As a result of the dislocation, the knee joint is immediately flexed, and the patella is stuck on the lateral side. The thigh muscle is tensed to the maximum and fixes the patella this position. Pain and fear cause the muscles to tense even further, so that the knee joint cannot be extended.
The patella can be reduced in almost every case when the thigh muscle relaxes, and the knee joint is then slowly extended again. Close to full extension, the kneecap jumps back into its original position automatical.
Tip in the acute situation: sit on the floor, relax by exhaling each breath as slowly as possible, slowly stretch the knee joint (perhaps a second person should do this).
Due to flexion of the knee joint, the Kneecap is stuck on the lateral side
Conservative therapy
Patients with first-time patellar dislocations can be treated conservatively. A special knee brace to protect the patella and physiotherapy are necessary. Taping the patella also proves to be useful. Under certain circumstances, restrictions of certain activities are required, however, the patient should be able to accept these restrictions - not everybody wants to play football or dance. If the patient does not accept those limitations, and patellar stability cannot be achieved with braces or physiotherapy, further surgical measures are necessary.
Extended tape of the right kneecap.
Operative Therapie
If the conservative means do not satisfy the patient, surgical stabilization of the patella may be an option.
There are two different surgical approaches:
Soft tissue and bony techniques
Soft tissue techniquesThese techniques refer to operations that do not involve the bone, but the muscles and other ligament structures. Over 100 different variants are known. Nowadays, the replacement of the MPFL (medial patellofemoral ligament) is the most common. This procedure stabilizes, i.e. reinforces the ligament on the medial side of the patella with a tendon.
MPFL - plasty (reconstruction) (MPFL: Medial Patello-Femoral Ligament)
The MPFL-plasty has been established as the workhorse in stabilizing the patella. The MPFL is enforced, i.e. replaced by a tendon. Different implants with different grafts (hamstrings, quadriceps tendon, etc.) or fixation methods (interference screws, anchors, suture fixation, etc.) have been described and evaluated. No way seems to perform worse than the other. The redislocation rate is low in the short-term. After two years the percentage of re-dislocation is about 1%. However, there are only a few studies with a follow-up of more than four years. After 4-8 years, the rate of re-dislocation or complaints after MPFL reconstruction is over 20%. In adolescents, this value increases to almost 29%. The long-term suitability of this approach is currently only sparsely proven.
The correct positioning of the ligament on the thigh and the patella is crucially important. In children, the origin of this ligament on the femur is more challenging to determine than in adults. It can be assumed that the origin of the MPFL is located at the growth plate in the femur. If the MPFL implant is positioned anatomically, there is a risk of growth disorders. If it is not implanted anatomically, it is more likely to loosen. Ligaments placed below or above the joint move further away from the origin during growth. It is therefore not surprising that these replacements lead to re-dislocations in children and adolescents more often, and therefore, have worse results than in adults.
Nevertheless, there is still an indication for the MPFL-plasty: a measure to save time until a definitive solution can be found after growth, or to support other interventions (bony or other soft tissue interventions).
Bony techniques
These techniques change the anatomy of the bone, i.e. the axis, attachments of muscles as well as trochleoplasty, which creates a new gliding channel for the patella.
In children and adolescents, bony corrections have the disadvantage that they can interfere with growth. Therefore, these approaches have to be performed carefully with respect to the growth plate.
Trochleoplasty (Trochleaplasty)
Trochleoplasty attempts to correct the leading cause of instability, the trochlear dysplasia. Various techniques are known. As with the MPFL-plasty, studies describe excellent results with a low rate of re-dislocation. The indication for trochleaplasty is made in cases of significant dysplasia. However, the effort requires exceeds by far that of an MPFL replacement. Often trochleoplasty is combined with an MPFL-plasty. The aftercare appears to be more protracted. A major criticism of trochleaplasty is the initial damage to the cartilage caused by the operation itself. Degenerations are observed in 30% of cases. It should be noted here that most studies represent an inhomogeneous patient population and in some cases, report on patients who have undergone multiple operations. In patients, who have undergone trochleaplasty as the first intervention, there is no evidence of osteoarthritis after seven years.
Also, in children and adolescents, trochlear dysplasia is considered the leading cause of instability. Unfortunately, the growth plate runs right along the edge of the cartilage. Trochleoplasty in any technique would also affect the growth plate. In principle, damage to the growth plate can result in growth problems.
In our clinic, we have operated on and published the youngest patients. We have also performed trochleoplasty on premature patients with wide-open growth plates without any growth issues post-operative.
Note:
Statements on this page are all supported by scientific publications.
The literature references can be found here:
