Patella alta, or a high riding patella, is a situation where the position of the patella is considered high in relation to distal femur. It may be idiopathic or may result secondary to an injury or disease process.
The patella is up and above the knee joint instead of normal position in the groove.
As the patella is out of grove and there is a very small barrier [which normally groove offers] to keep the high-riding patella in place, a strong contraction of the quadriceps muscle can easily pull the patella over the edge and out of the groove, leading to a patellar dislocation.
Apart from patellar dislocation, chondromalacia patella and patellofemoral osteoarthritis are also associated with patella alta.
It is considered a predisposing factor for the development of patellofemoral pain.
The knee joint consists of three bones, the femur, the tibia, the fibula. The patella is a sesamoid bone which develops in quadriceps tendon. On one side patella attaches to quadriceps and the other side is called patellar tendon.
The patella articulates with femoral condyle within its groove with the knee movements.
For detailed anatomy please consider following articles
Effects of Patella Alta on Biomechanics
Instability and patellofemoral arthritis are two major occurrences in patella alta.
Normally on knee flexion, patella glides into a reciprocal groove in the front of the femur, called the trochlear groove. The groove keeps the patella stable in the middle of the knee [sidewise barrier] as it moves up and down.
The patella in straight knee sits above this groove, entering the trochlear groove at about 20 or 30 degrees of knee flexion.
Till the time it enters the groove, the patella has the potential to slide sideways especially laterally.
If the patella is sitting to high then the patella will only engage in the trochlear groove later in the flexion arc (i.e. >20-30 degrees). This means that the patella is less stable for a greater percentage of time.
Another thing is greater ‘windscreen wiper’ effect.
The shorter the patellar tendon the more restricted the potential side-to-side movement of the patella will be.
The longer the patellar tendon by virtue of the longer the radius of the arc of potential movement can lead to more side-to-side movement.
Causes of Patella Alta
Though there is no exact etiology or cause for the existence of the patella alta. Following factors can be considered in relation to this condition
- Thin built.
- Patellofemoral Pain
- Knee Injuries
- Congenital /developmental
- Abnormally long patellar tendons (>52 mm).
The patient may not have complaint for a long period. Anterior knee pain especially in the patellar portion can be present.
There could be instability of the gait. Or the patient may present with recurrent dislocation of patella.
It can be present in patella-femoral osteoarthritis as well.
Symptoms may be slowly progressive or there may be acute presentation.
Activities which increase joint reaction force are ambulation on incline, squatting, prolonged sitting, and going up and down stairs.
Often, the knee may be normal and most comparable to other side as it is difficult to appreciate patella alta clinically especially in mild cases.
An effusion though may indicate a chronic pathology.
For clinical examination of patellar height, the patient sits on the edge of the examination table with the feet on the ground, knees bent at 90 degrees, and the thighs horizontally positioned.
The position of the patella is best observed from lateral side.
In patella alta, a partially tilted patella protrudes above the level of the thigh, more remarkably so in unilateral cases.
Normally, the patella points straight forward. And there is only one prominence noted and that is of tibia tuberosity.
In patients with a high-riding patella, the patella points upward.
This is better appreciated in bilateral patella alta. Some patients may have externally rotated patellae consistent with patella alta and lateral tilt. This is known as the “grasshopper eyes” sign.
In case of patellar instability, Patellar apprehension test would be positive.
Following conditions may be associated with patella alta and There are a lot of conditions that are known to be associated with patella alta. For example
- Patellofemoral instability
- Recurrent patellofemoral dislocation
• Neuromuscular diseases like poliomyelitis
• spastic cerebral palsy
• Osgood Schlatter disease
• Sinding Larsen Johanssen disease
• Patella tendo-lateral femoral condyle friction syndrome
• Chondromalacia patella
Imaging and Confirmation of Patellar Height
X-rays of knee are basic investigations. Various ratios have been devised to confirm patellar height. These ratios vary on various parameters and are measured on lateral views of knee x-rays. On radiography, it is important that standard positioning is used, with the knee flexed at 30 degrees. An off-angle x-ray beam or nonstandard positioning may result in a spuriously abnormal ratio.
Several methods are used to determine the presence of patella alta.
Following methods are used commonly
- Insall-Salvati ratio and modified Insall-Salvati ratio
- Caton-Deschamps index
- Blackburne Peel Method
- Blemensaat Method
|Index Name||Normal Patellar Height||Patella Alta||Patella Baja|
|Insall Salvati Ratio||1.0||>1.2||0.8|
|Modified Insall Salvati Ratio||1.25||>2.0|
Treatment of Patella Alta
Tibial tubercle transfer
With this surgery they move the attachment of the patellar ligament downwards to the tibia. The patella is also attached to this ligament, so the patella moves downwards.
Other Surgeries which may be considered considering the different conditions associated with patella alta are
- Lateral release to loosened lateral tightness
- Vastus Medialis Oblique advancement surgery
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