Overview of Patellofemoral Joint Instability

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Original Editor - Jess Bell based on the course by Claire Robertson
Top Contributors - Jess Bell, Kim Jackson, Wanda van Niekerk and Ewa Jaraczewska

Introduction

Patellofemoral instability is a complex orthopaedic condition that frequently affects children and adolescents. It is estimated that the incidence of patellofemoral instability in paediatric patients ranges from 23 to 43 per 100,000 person-years. The highest rates are in teenagers aged between 14 and 18 years.

The management of patellofemoral instability is complex. Vellios et al. note that first-time dislocation may be managed conservatively (i.e. through rehabilitation, bracing, activity modification), but as many as 36 percent of patients will have recurrent instability on the same leg.

Definitions

Patellofemoral stability is defined as “constraint by passive soft tissue tethers and chondral/bony geometry that, with muscular forces, guide the patella into the trochlear groove and keep it engaged within the trochlear groove as the knee flexes and extends.”

Conversely, patellofemoral instability is defined as “symptomatic deficiency of the aforementioned passive constraint (patholaxity) such that the patella may escape partially or completely from its asymptomatic position with respect to the femoral trochlea under the influence of displacing force.”

Dislocation - patella comes completely out of the trochlea. Dislocation might be caused by a:

• Trauamatic event (e.g. rugby tackle)
  • In these cases, the knee has been structurally intact prior to injury
  • These individuals tend to have a better prognosis as there are no underlying structural morphological elements to predispose them to instability
• Minor trauma - a slight tap / knock to the knee causes dislocation
  • These individuals tend to have a structural element which predisposes them to instability

Subluxation - a “halfway" point between dislocation and an in situ patella. Patients with a subluxing patella may complain of a painful knee / anterior knee pain.

Recurrent Patellofemoral Instability

Recurrent patellofemoral instability is relatively common. Individuals who experience recurrent patellofemoral instability often have specific pathoanatomical features which predispose them to patellar dislocation. Migliorini note the following risk factors:

• Bony conformation abnormalities (e.g. trochlear dysplasia)
• Lower limb mal-alignment syndromes (e.g. tibial extra-rotation)
• Soft tissue abnormalities (e.g. patella alta)

Jaquith and Parikh found that the following are risk factors for multiple dislocations in children and adolescents:

• Aged under 14 years at first dislocation
• History of contralateral dislocation
• Any type of trochlea dysplasia
• Skeletal immaturity (i.e. the physis in the distal femur or proximal tibia are still partially / fully open)
• Long patella tendon

The number of risk factors a child / adolescent has affects their risk of recurrent dislocations:

• Children who have the later four risk factors are 88 percent likely to have recurrent dislocations
• Having any three risk factors is associated with a 75 percent risk of recurrent dislocations
• Having any two risk factors is associated with a predicted risk of around 55 percent
• 97 percent of recurrences occur within three years

The mean age of first dislocation for children who experienced re-dislocation was 12.9 years and for those who did not re-dislocate, the mean age of first dislocation was 13.8 years. Thus, 13 years is an important milestone when considering patellofemoral dislocation.

Morphology and Instability

This is an area of growth:

• Imaging is improving
• More sophisticated surgeries are available
• Our understanding of the subtleties associated with this condition is improving
• There is an awareness that individuals with patellofemoral instability are more likely to have accelerated wear leading to premature patellofemoral arthritis

Dysplasia Assessment

The following are important features to consider when assessing individuals with potential / confirmed patellofemoral instability:

• Morphology of the trochlear
• Position of the tibial tubercle
• Length of the patella tendon

These features will be discussed in detail below.

Morphology of the Trochlea

The patella is essentially a floating bone which needs to stay in the trochlea (i.e. the groove of the distal femur). It is well recognised that trochlear dysplasia is associated with patella instability.

Trochlea Depth

Ideally, the trochlea will be deep, with a sharp inclination on the lateral side. It is important to look out for a “shallow trochlea” on an MRI report. Patients may also have a long lateral facet and / or shallow inclination.

If imaging is not available, you can assess the following:

• Feel for passive glide, especially laterally (see video on left below)
• Look for J sign (see video on right below) for subluxation (i.e. increased lateral patellar deviation when moving from flexion to extension) and poor engagement at the entrance to the trochlea

There is a spectrum of trochlea dysplasia. Ideally, a trochlea will be shaped liked a soup bowl. Individuals who have a saucer-shaped trochlea (i.e. shallow) are more likely to experience subluxation or dislocation. They may report a sense of the patella “skidding around” or “slipping”. At the more extreme end, patients can have a domed trochlea. These persons are most likely to dislocate and are unlikely to stabilise without surgery.

“High-grade trochlear dysplasia is characterized by the combination of a flat and/or prominent trochlea proud of the anterior femoral cortex, which offers inadequate tracking during flexion and leads to patellar subluxation.”

Trochleoplasty - Prevention of Rapid Wear

• Addresses trochlea, Q angle and TT-TG distance (see below)
• Addresses the effect of a long patella tendon (see below)
• Addresses the over-stretching of medial structures

Please note that this is major surgery and best done with pristine chondral surfaces (i.e. in late teens).