Patellar instability is the clinical syndrome due to morphologic abnormalities in the patellofemoral joint where the patella is prone to recurrent lateral dislocation.
Most patients with patellar instability are young and active individuals, especially females in the 2nd decade. Prevalence is 6-77 per 100,000 population 2.
The normal patellofemoral joint has two kinds of stabilisers - active stabilisers (extensor muscles), and passive stabilisers (bones and ligaments):
- bone stabilisers: deep femoral sulcus, high lateral trochlea
- ligament stabilisers: medial patellofemoral retinaculum, medial patellofemoral ligament
Three major morphologic abnormalities which predispose to patellar instability are:
- trochlear dysplasia: trochlear joint surface is flattened
- patella alta
- lateralisation of the tibial tuberosity: femorotibial malrotation, genu recurvatum
The most common mechanism of first-time patellar dislocation is internal rotation of the femur relative to the knee (i.e. exorotation of the tibia relative to the femur) while the foot is planted and the knee is flexed.
The "crossing sign" represents abnormally elevated floor of the trochlear groove rising above the top of the wall of one of the femoral condyles, assessed on lateral radiographs. "Double contour sign" is a double line at the anterior aspect of condyles, and seen if medial condyle is hypoplastic. Skyline view can show decreased trochlear depth and large sulcus angle (>144º).
Lateral trochlear inclination, trochlear facet asymmetry, and trochlear depth are used to judge trochlear dysplasia.
Classification of trochlear dysplasia (Dejour et al)
- type A: normal shape of the trochlea, but a shallow trochlear groove
- type B: markedly flattened or even convex trochlea
- type C: trochlear facet asymmetry, with too high lateral facet, and hypoplastic medial facet
- type D: type C features and a vertical link between facets ('cliff pattern') 3
Lateral trochlear inclination
The most superior section showing trochlear cartilage is selected for this measurement. The inclination is the angle formed between the plane of the lateral trochlear facet subchondral bone and a tangential line through posterior femoral condyles. An angle of <11º is considered abnormal.
Trochlear facet asymmetry
This is the ratio of medial trochlear facet width to lateral trochlear facet width and measured in the axial plane. A ratio of <0.4 is considered abnormal (i.e. it is abnormal if the medial facet is <40% the width of the lateral facet).
This measures the inset depth of the trochlear groove (sulcus) relative to the mean of the medial and lateral femoral condyle outsets. It is determined by axial imaging at the same level as the trochlear facet asymmetry. A trochlear depth of <3 mm is considered abnormal.
Patella alta (or a high riding patella) describes a situation where the position of the patella is considered high. See patella alta article for a description of the Insall-Salvati ratio.
Patellar translation, known as the TT-TG distance (tibial tubercle to trochlear groove distance) is currently advocated as an accurate measure of patellar instability. The distance from the tibial tuberosity to the trochlear groove is measured parallel to the tangential line through posterior femoral condyles:
- <15 mm is considered normal
- 15-20 mm is considered borderline
- >20 mm is considered abnormal
The distance has been shown to be slightly less when measured on MRI than CT 4.
Associated radiological features of patellar instability
- knee joint effusion
- medial patellofemoral ligament tear
- bone contusion in patella and lateral condyle
- osteochondral defects in patella
- edema/hemorrhage of vastus medialis
- intra-articular loose bodies
- internal derangement of knee
Treatment and prognosis
Nearly half of the patients with first-time dislocation will sustain recurrent dislocation after conservative management. Chronic patellar instability, if not treated, may lead to severe arthritis and chondromalacia patellae. The aim of the surgery is to repair the knee damage caused by patellar dislocation and to correct the primary anatomical anomaly. Surgical modalities are:
- medial patellofemoral ligament reconstruction
- medialisation of tibial tuberosity
- medial capsular plication
- 1. Diederichs G, Issever AS, Scheffler S. MR imaging of patellar instability: injury patterns and assessment of risk factors. Radiographics. 30 (4): 961-81. doi:10.1148/rg.304095755 - Pubmed citation
- 2. Fithian DC, Paxton EW, Stone ML et-al. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med. 32 (5): 1114-21. doi:10.1177/0363546503260788 - Pubmed citation
- 3. Dejour H, Walch G, Neyret P et-al. [Dysplasia of the femoral trochlea]. Rev Chir Orthop Reparatrice Appar Mot. 1990;76 (1): 45-54. - Pubmed citation
- 4. Hinckel BB, Gobbi RG, Filho EN et-al. Are the osseous and tendinous-cartilaginous tibial tuberosity-trochlear groove distances the same on CT and MRI?. Skeletal Radiol. 2015;44 (8): 1085-93. doi:10.1007/s00256-015-2118-4 - Pubmed citation
The knee is a complex synovial joint that can be affected by a range of pathologies:
- bone and cartilage
- distal femoral condyle fracture
- tibial plateau fracture (classification)
- patella fracture
avulsion fractures of the knee
- Segond fracture
- reverse Segond fracture
- anterior cruciate ligament avulsion fracture
- posterior cruciate ligament avulsion fracture
- arcuate complex avulsion fracture (arcuate sign)
- biceps femoris avulsion fracture
- iliotibial band avulsion fracture
- semimembranosus tendon avulsion fracture
- Stieda fracture (MCL avulsion fracture)
- patella fracture
- chronic avulsion injuries
- chondromalacia patellae
- osteoarthritis of the knee
- osteochondral defects
- osteochondritis dissecans of the knee
- pattern of bone contusion in knee injuries
- knee fractures
- meniscal lesions
- synovial lesions
- fat pad
- popliteal fossa