Overview of Patellofemoral Joint Instability

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

Introduction[edit | edit source]

Patellofemoral instability is a complex orthopaedic condition that frequently affects children and adolescents.[1][2] 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[2], with the first episode of dislocation typically occurring at the age of 15.[3]

The management of patellofemoral instability is complex.[4] Vellios et al.[2] note that first-time dislocation may be managed conservatively (i.e. through rehabilitation, bracing, and activity modification), but as many as 36 percent of patients will have recurrent instability on the same leg.

Definitions[edit | edit source]

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.” [5]

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.”[5]

Comparison of a patella in situ, subluxed and dislocated.

Dislocation - the patella comes completely out of the trochlea (see image). Dislocation might be caused by a:[6]

  • Traumatic event (e.g. rugby tackle)
    • In these cases, the knee has been structurally intact prior to injury[7]
    • 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 a patella in situ and dislocation (see image).[6] Patients with a subluxing patella may complain of a painful knee / anterior knee pain.[6][8]

Recurrent Patellofemoral Instability[edit | edit source]

Recurrent patellofemoral instability is relatively common.[9] Individuals who experience recurrent patellofemoral instability often have specific pathoanatomical features which predispose them to patellar dislocation. Migliorini et al.[9] note the following risk factors:

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

Jaquith and Parikh[4] found that the following are risk factors for multiple dislocations in children and adolescents:

  1. Aged under 14 years at first dislocation
  2. History of contralateral dislocation
  3. Any type of trochlear dysplasia
  4. Skeletal immaturity (i.e. the physis in the distal femur or proximal tibia are still partially / fully open)
  5. Long patellar tendon

The number of risk factors a child / adolescent has impacts their risk of recurrent dislocations:[4]

  • Children who have risk factors 2-5 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[6]

Jaquith and Parikh[4] found that 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.[6]

Morphology and Instability[edit | edit source]

This is an area of growth:[6]

  • 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[edit | edit source]

The following are important features to consider when assessing individuals with potential / confirmed patellofemoral instability:[6]

  • Morphology of the trochlea
  • Position of the tibial tubercle
  • Length of the patellar tendon

Morphology of the Trochlea[edit | edit source]

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 patellofemoral instability.[10]

Trochlear Depth[edit | edit source]

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.[6]

If imaging is not available, you can assess the following:[6]

  • 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[11]) and poor engagement at the entrance to the trochlea

There is a spectrum of trochlear 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 that it feels like their patella is “skidding around” or “slipping”. At the more extreme end of the spectrum, patients can have a domed trochlea. These persons are most likely to dislocate and are unlikely to stabilise without surgery (e.g. trochleoplasty).[6]

“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.”[10]

Trochleoplasty - Prevention of Rapid Wear[edit | edit source]

  • Addresses trochlear morphology, Q angle and TT-TG distance (see below)
  • Addresses a long patellar tendon (see below)
  • Addresses the over-stretching of medial structures (see below)

Please note that trochleoplasty is major surgery and outcomes are better when this procedure is performed on pristine chondral surfaces (i.e. on individuals in their late teens).[6]

Non-Operative Rehabilitation[edit | edit source]

Individuals with patellofemoral instability may have multi-directional instability, but they may not always dislocate:[6]

  • Look out for patients using descriptors such as ‘slipping’
  • Robertson[6] notes that patellofemoral instability is often associated with small patellae and hypermobility
  • It is important to try to establish if the individual is subluxing, dislocating or has general excess mobility
  • If the problem is with engagement, look at rehabilitating in the early range of motion, avoiding pain
  • Isometrics are very useful
  • Robertson[6] also notes that individuals with multi-directional instability are often very puffy in their parapatellar soft tissues
  • They often have medial and lateral fat pad oedema
  • Can consider bracing (see below)
  • Not always isolated to terminal extension instability

Bracing[edit | edit source]

It is essential to use clinical reasoning when considering bracing. Ask the following questions:[6]

  • Is there any chondral damage or bone oedema? If yes where?
    • Lateral facet / lateral trochlea, then consider Q brace[14]
    • If no damage, then consider a patellar stabilising brace
Q Brace[edit | edit source]

Callaghan et al.[14] explored the use of a Bioskin Patellar Tracking Q Brace on patients with painful patellofemoral joint osteoarthritis (PFJOA):[14]

  • MRI was used to monitor bone marrow lesions in lateral PFJOA
  • Participants either wore a brace or had no brace (control)
  • Participants in the brace group wore their braces for a mean of 7.4 hours per day
  • At the six-week follow-up, Callaghan et al.[14] found that subjects using the Q brace had decreased pain, as well as a reduction in the volume of patellofemoral bone marrow lesions

Taping[edit | edit source]

  • McConnell taping can be useful for lateral pathology (see video below)
  • K tape can be used to stabilise the knee[6]


Position of the Tibial Tubercle[edit | edit source]

Ideally, the tibial tubercle will be positioned directly under the trochlea. However, some people present with a lateralised tibial tuberosity. When the quadriceps works, the position of the tibial tubercle creates an overall lateralising force, which can have an impact on stability.[6]

TT-TG distance (tibial tuberosity-trochlear groove) is measured on MRI to accurately determine the position of the tibial tubercle in relation to the trochlea.[6][16] The greater the distance, the more lateralised the tibial tuberosity.[16]

To measure TT-TG distance:[6]

  • A line is made through the trochlea and tibial tuberosity
  • The distance between these two lines is measured[6]
    • Up to 15 mm is considered normal
    • 15-20 mm may start to affect stability and potentially create pain
    • ≥20 mm indicates individuals at risk for future instability[16]

When MRI is not available, it is possible to conduct a visual examination. You can then ask the patient to perform a static quadriceps hold and look to see if the patella moves laterally. While this is not an accurate measure, it can be useful when considered alongside the overall clinical picture.[6]

Non-Operative Management of a Large TT-TG[edit | edit source]

A lateralised tibial tuberosity causes a lateral valgus movement, which overloads the lateral facet and provides a lateral force. Thus, it is important for patients to avoid the following femoral positions:[6]

  • Adduction
  • Internal rotation
  • Adduction with internal rotation

Check also if the patient is anteverted (using Craig's test - see video below). Other points to consider include:[6]

  • Are patients aware of any issues with dynamic control (i.e. do they knock their knees when running?)
  • Do they have weakness or poor neuromusculoskeletal (NMS) control?
  • Can they correct this? What cues work best?
  • Consider at what angle of hip flexion the rotation problem occurs
  • Is the “medial collapse” driven from the foot?
    • Care with orthotics is required for this group


The following exercises can be useful in the rehabilitation of individuals with a large TT-TG:[6]

Length of the Patellar Tendon[edit | edit source]

A patella alta is essentially a “high-riding” patella.[6] It can occur when individuals are born with long patellar tendons[6] and it is known to be associated with patellofemoral instability.[18]

The Insall-Salvati ratio is most often used to measure the height of the patella.[19] It compares patellar tendon length with patellar length.[19] An ideal ratio is 1:1. This can also be informally assessed in the clinic.[6]

Non-Operative Management of Patella Alta[edit | edit source]

In individuals with patella alta, the patella takes longer to enter the trochlea. Typically, the patella will engage in the trochlea at around 15 degrees of knee flexion. But this is delayed in individuals with patella alta.[18] Thus, their knee is relatively more unstable for the first 30 degrees of knee flexion.[6]

It is, therefore, important to target 0-30 degrees of knee flexion with dynamic stability. Conservative management should focus on making the Vastus Medialis Oblique (VMO) architecturally favourable - i.e. increase the fibre angle and the insertion ratio.[6][20]

Medial Patellofemoral Ligament[edit | edit source]

The medial patellofemoral ligament (MPFL) is the primary restraint to the patella, so it can have a significant impact on stability. While people do not tend to have a congenital problem with this ligament, it is disrupted after dislocation (either very stretched or ruptured).[6]

If a patient presents after a confirmed or potential dislocating event, look for:[6]

  • Bruising
  • Swelling
  • Medial tenderness (just medial to the patella)

These will increase your suspicion of a MPFL injury.[6]

Outcome Measures[edit | edit source]

The following outcome measures can be useful in this client group:[6]

References[edit | edit source]

  1. Bailey MEA, Metcalfe A, Hing CB, Eldridge J; BASK Patellofemoral Working Group. Consensus guidelines for managing patellofemoral instability. Knee. 2021;29:305-12.
  2. 2.0 2.1 2.2 Vellios EE, Trivellas M, Arshi A, Beck JJ. Recurrent Patellofemoral Instability in the Pediatric Patient: Management and Pitfalls. Curr Rev Musculoskelet Med. 2020;13(1):58-68.
  3. Kim HK, Parikh S. Patellofemoral Instability in Children: Imaging Findings and Therapeutic Approaches. Korean J Radiol. 2022 Jun;23(6):674-687.
  4. 4.0 4.1 4.2 4.3 Jaquith BP, Parikh SN. Predictors of recurrent patellar instability in children and adolescents after first-time dislocation. J Pediatr Orthop. 2017;37(7):484-90.
  5. 5.0 5.1 Post WR, Fithian DC. Patellofemoral instability: a consensus statement from the AOSSM/PFF Patellofemoral Instability Workshop. Orthop J Sports Med. 2018;6(1):2325967117750352.
  6. 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17 6.18 6.19 6.20 6.21 6.22 6.23 6.24 6.25 6.26 6.27 6.28 6.29 6.30 6.31 6.32 Robertson C. Patellofemoral Joint Instability Course. Plus. 2022.
  7. Duthon VB. Acute traumatic patellar dislocation. Orthop Traumatol Surg Res. 2015;101(1 Suppl):S59-67.
  8. Monk AP, Doll HA, Gibbons CL, Ostlere S, Beard DJ, Gill HS, Murray DW. The patho-anatomy of patellofemoral subluxation. J Bone Joint Surg Br. 2011;93(10):1341-7.
  9. 9.0 9.1 Migliorini F, Oliva F, Maffulli GD, Eschweiler J, Knobe M, Tingart M, Maffulli N. Isolated medial patellofemoral ligament reconstruction for recurrent patellofemoral instability: analysis of outcomes and risk factors. J Orthop Surg Res. 2021;16(1):239.
  10. 10.0 10.1 Batailler C, Neyret P. Trochlear dysplasia: imaging and treatment options. EFORT Open Rev. 2018;3(5):240-47.
  11. Hayat Z, El Bitar Y, Case JL. Patella Dislocation. [Updated 2022 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538288/
  12. www.sportsinjuryclinic.net. Patellar Glide Test. Available from: https://www.youtube.com/watch?v=0VXNx4yuWEo [last accessed 14/6/2022]
  13. Claire Patella. J Sign Claire Patella. Available from: https://www.youtube.com/shorts/4dHF6LxhAMw [last accessed 14/6/2022]
  14. 14.0 14.1 14.2 14.3 Callaghan MJ, Parkes MJ, Hutchinson CE, Gait AD, Forsythe LM, Marjanovic EJ, et al. A randomised trial of a brace for patellofemoral osteoarthritis targeting knee pain and bone marrow lesions. Ann Rheum Dis. 2015;74(6):1164-70.
  15. McConnell Physiotherapy Group. MCCONNELL KNEE TAPING (OFFICIAL). Available from: https://www.youtube.com/watch?v=WbHXYnwUwws [last accessed 14/6/2022]
  16. 16.0 16.1 16.2 Heidenreich MJ, Camp CL, Dahm DL, Stuart MJ, Levy BA, Krych AJ. The contribution of the tibial tubercle to patellar instability: analysis of tibial tubercle-trochlear groove (TT-TG) and tibial tubercle-posterior cruciate ligament (TT-PCL) distances. Knee Surg Sports Traumatol Arthrosc. 2017;25(8):2347-51.
  17. Clinical Physio. Craig's Test for Hip | Clinical Physio Premium. Available from: https://www.youtube.com/watch?v=Qi41LYsVy1E [last accessed 14/6/2022]
  18. 18.0 18.1 Patel RM, Gombosh M, Polster J, Andrish J. Patellar Tendon imbrication is a safe and efficacious technique to shorten the patellar tendon in patients with patella alta. Orthop J Sports Med. 2020;8(10):2325967120959318.
  19. 19.0 19.1 Gaillard F, Weerakkody Y. Insall-Salvati ratio. Reference article, Radiopaedia.org. Available from: https://radiopaedia.org/articles/insall-salvati-ratio?lang=us (accessed 14 June 2022).
  20. Khoshkhoo M, Killingback A, Robertson CJ, Adds PJ. The effect of exercise on vastus medialis oblique muscle architecture: An ultrasound investigation. Clin Anat. 2016;29(6):752-8.