Patellar Instability: Diagnostic Workup

Abstract

Background:

The patella is a sesamoid bone that comprises a vital component of the extensor mechanism of the knee. It has static and dynamic restraints that differ throughout range of motion of the knee. Tracking, and potentially maltracking, are impacted additionally by the bony anatomy of the pelvis, femur, and tibia.

Indications:

Evaluation of patients with patellofemoral instability warrants consideration of each of these nuanced arenas of tracking. The diagnostic workup must be based on a thorough understanding of anatomy, which guides focused physical examination maneuvers that are not always completed in the standard knee examination of the adult or pediatric patient.

Technique Description:

A complete patellofemoral instability examination requires evaluation of femoral anteversion (while prone), tibial external torsion (thigh foot angle), knee range of motion (J sign), patellar evaluation (to assess apprehension and translation), standing examination, and a gait and foot progression analysis.

Results:

The key components of a patellofemoral physical examination allow a provider to assess all parameters involved in the patellofemoral pathology.

Discussion/Conclusion:

In conjunction with these anatomy-informed physical examination maneuvers, the provider can also evaluate radiographic and cross-sectional imaging, which are vital to informed surgical decision-making.

Patient Consent Disclosure Statement:

The author(s) attests that consent has been obtained from any patient(s) appearing in this publication. If the individual may be identifiable, the author(s) has included a statement of release or other written form of approval from the patient(s) with this submission for publication.

Graphical Abstract

This is a visual representation of the abstract.

Video Transcript

Today, we will discuss the diagnostic workup of patellar instability.

We will begin with a discussion on diagnostic workup, specifically anatomy, risk factors, and related physical examination findings. Of note, chronic patellar instability is different than recurrent (episodic) instability. Chronic patellofemoral instability requires more extensive correction and added attention to rotational and coronal alignment, contracted lateral tissue, extensor mechanisms alignment, and trochlear dysplasia.

Background

The patella is a sesamoid bone. It lies within the extensor mechanism of the knee. Between 0° and 30°, the patella is restrained by soft tissue. After 30° of flexion, the patella is restrained by bony anatomy and soft tissue restraints. This means it is entirely reliant on the soft tissues and underlying bony anatomy for normal tracking. Static restraints to the abnormal tracking include the medial patellofemoral ligament (MPFL), whereas dynamic restraints include the vastus medialis obliquus muscle.

Source for above: ² for the visual.

Changed: Anatomy of the medial knee is important to understand and has been extensively studied. The medial epicondyle should be an immediately palpable structure, the MPFL proximal and posterior to the medial epicondyle, while the adductor tubercle is proximal and posterior to the MPFL insertion.

Indications

As we gain some perspective and look to bony anatomy outside of, but impactful to, the knee, we must consider the contribution of bilateral lower extremity alignment. First, from a coronal plane perspective, there are anatomical variations that we expect to see based on sex, as well as patient age, early in life. Pelvic skeletal anatomy differs between the narrower male pelvis and the wider female pelvis, which is capable of childbirth. The impact of that pelvic anatomy is the placement of the acetabulum in space. The lateralization of the acetabulum and, therefore, the femoral head in space creates a relative valgus alignment of the knee. The quadriceps, or Q angle, is defined as the angle between the anatomic axis of the femur/quadriceps and a longitudinal line through the patella. The potential consequence of this increased Q angle is a relative deviation from the normal “mechanical” axis of the femur based on a line drawn from the center of the femoral head to the center of the talus. This mechanical axis deviation can be seen here in this patient with an increased Q angle and bilateral valgus knee alignment. Notably, there is a physiologic progression of coronal alignment, where children transition from genu varum at a young age to genu valgum and then normalize as they continue to grow.

Average femoral anteversion varies by up to 30°. With femoral anteversion, there is often a “compensatory, dynamic internal rotation of the hip” that occurs to improve femoral coverage and maximize the stretch on the abductors and the hip musculature. Therefore, isolated femoral aversion (FA) can lead to in-toeing in those without tibial deformity.

Technique Description

To better appreciate FA, a prone position is best for examination because it places the hip into extension. Testing rotation with the hip in flexion almost always results in less internal rotation and more external rotation, potentially masking the underlying deformity. If there is any concern for rotational malalignment, this examination is invaluable. Generally, you need your patients to relax well. You can place one hand on their sacrum to ensure their pelvis remains flat during the examination. Many patients tend to tense their lower back and glute muscles, so having your hand in this position allows for accurate detection. Some patients will experience apprehension as you internally rotate. First, hip internal rotation can be completed one side at a time by flexing the knee to 90° and pushing the foot away from the midline. This allows for some side-to-side comparison in internal rotation of the femur. Then, the external rotation of each side can be measured one at a time by bending the knee to 90° and bringing the heel to the contralateral side. Qualitative diagnosis of FA is with ≥70° of internal rotation or large asymmetry in rotation. While in the prone position, the examination can shift focus to tibial torsion (TT).

It is more accurate to compare the thigh with the transmalleolar (TM) axis because this negates foot deformities or inaccurate foot positioning. The foot can be lined up with the TM axis for easier visualization. Some patients, especially those with excessive TT, have adapted their foot to resemble metatarsus adductus—the medial structures are noticeably tight, but the foot is flexible. Normal TT is between 10° of internal rotation and 15° of external rotation.

 Obvious differences in TT can also be observed during a supine examination. Now that we have acknowledged both femoral version and TT in isolation, we can consider the entire lower extremity as a whole.

When evaluating patients in the clinic, it is important to have them stand in front of you. We have a couple of variables to consider while standing.

These variables are the version and the torsion. If the patient naturally in-toes, either a large component of FA could be at play, or tibial internal torsion could be the leading cause. It is not always that simple, and it is important to obtain a more granular examination.

What is more common in the patellofemoral population is FA and/or external TT. When completing your standing examination, our team has found that this sequence of steps is the most valuable.

First, have your patients stand with their feet straight toward the examiner. From here, you can look to see where their knees are pointing. A normal examination would be that both the knees and feet point forward (adjusting for a small “normal” amount of TT). If, when asked to put feet pointing toward the examiner, the patient's knees point in, this is at least a sign of external TT.

Then, ask the patient to face forward with their knees together. From here, the true impact of TT can be seen.

Before moving to the imaging workup of rotation, it is worth mentioning that gait should be observed in these patients with close attention to foot progression angle and dynamic valgus.

The foot progression angle is estimated as the angle between the axis of the foot and the line of direction of gait. It helps to look specifically at foot progression while walking away from you and then the knee progression as they walk toward you. While watching gait, look for the presence of intermittent internal or external rotation of the foot. While FA has both static and dynamic contributions to patellar stability, external TT is primarily a dynamic problem because you tend to rotate the femur to get the foot straight over-externally. You should also evaluate for a stiff-appearing hip and a genu valgum appearance.

This visual effectively demonstrates the impact of bony rotational malalignment and its potential effect on patella tracking.

There are various equations used to measure rotation on a computed tomography scan. Essentially, when conceptualizing the anteversion, specific calculations depend on whether the condylar axis is internally or externally rotated. In this particular scenario, the head neck angle (angle between the femoral neck and neutral) is added to the posterior condylar axis.

Measurement of TT depends on whether the ankle is internally or externally rotated. In this clinical scenario, this is completed by adding the posterior plateau axis from the transmalleolar axis. One should consider rotating the knee condyles to a flat position and how that affects the distal segment.

Tibial rotation has been reported^1,3,4 to range from 0° to 47°. External TT has been documented as a risk factor for patellofemoral instability.^1,5 Also, ≥30° is a usual indication for treatment. ¹

Zeroing back in on the patellar anatomy, we have to consider the position of the patella as well as its groove. First, there are multiple measurements of the height of the patella in relation to the tibia and joint line while the knee is flexed to ~30°. The Caton-Deschamps index is most frequently used at our institution and fell within normal limits for our patient. Examples of normal ranges and measurement techniques for all indices can be visualized here.

Next, we consider the trochlear groove. There are varying degrees of dysplasia, ranging from a shallow trochlea all the way to a large “spur” or “bump” in the area of the trochlea that is supposed to form the depressed groove. Our patient demonstrated a DeJour D trochlea with a double contour sign and supratrochlear spurring.

Trochlear dysplasia can also be seen on sunrise knee films. Patellar tilt can be demonstrated here. Additionally, a chronic dislocation can be seen here in the smaller inset visual.

Finally, the tibial tubercle to trochlear groove (TTTG) distance can be measured on axial imaging. This patient demonstrates a TTTG of 18. Normal TTTG ranges vary in the literature. While some reports of >20 abnormal, others report >15 to be concerning.

Patellar translation should be evaluated and measured in quadrants, with <2 quadrants to be normal. Apprehension with patellar translation should also be considered.

Finally, the “J sign” should be checked. The knee is taken from full extension to flexion. The laterally translated patella dramatically shifts medially as it begins to engage the bony groove. The reverse is observed as the knee is taken from flexion to extension, and the patella jumps laterally as soon as the trochlea no longer constrains it. The final visual demonstrates a flexion dislocation pattern.

Thresholds

There are several physical examination findings and ranges of normal throughout the reported literature. These are our general considerations for abnormal findings prompting management.

Surgical options for chronic patellar instability are numerous because of the many variables involved in patellar instability.

Discussion/Conclusion

In summary, a combination of risk factors increases a patient's risk for patellar instability. An appropriate workup should also include physical examination and imaging studies. This is the first step in transitioning a patient from this examination to the next one.