Patellar Dislocation
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Direct answer
A pivoting teenager feels the kneecap escape laterally, the knee buckles into a strange flexed pseudo-locked posture, and by the time imaging happens the patella has usually flipped back on its own — first-time lateral patellar dislocation, the commonest acute patellar problem. The medial patellofemoral ligament, avulsed from the femur or patella, is the restraint lost; risk factors gather around trochlear dysplasia, patella alta, an increased tuberosity-trochlear groove distance and generalised laxity, clustering in adolescent girls. A first episode is managed conservatively — brief immobilisation followed by quadriceps, especially vastus medialis, rehabilitation — while recurrent instability or an osteochondral fragment loose in the joint moves management toward arthroscopy and medial patellofemoral ligament reconstruction, with tubercle realignment for marked malalignment.
What you must remember
- Direction and structure: virtually always lateral; the medial patellofemoral ligament (MPFL) is the primary medial restraint in early knee flexion and is torn in nearly every acute dislocation.
- Who: adolescent and young adult females, ligamentously lax, often with a family history; occasionally habitual (displacing with every flexion cycle) in children, a different mechanical problem.
- The risk-factor quartet: trochlear dysplasia (the dominant anatomical factor), patella alta, tuberosity-trochlear groove distance above about 20 millimetres, and an increased Q angle with weak vastus medialis.
- The named test: the patellar apprehension test — laterally directed pressure on the patella at 20 to 30 degrees of flexion makes the patient seize the thigh in anticipation of dislocation; positive in instability, pathognomonic enough for exams.
- Imaging: skyline or Merchant views for congruence and fragments; magnetic resonance imaging confirms the MPFL tear and the osteochondral shearing injury off the medial patellar facet and lateral trochlea.
- First-episode management: conservative — a short period of bracing in extension, then structured physiotherapy emphasising vastus medialis, hip abductors and core; the exceptions are an osteochondral loose body and extensor-mechanism disruption, which are operated.
- Recurrent dislocation: MPFL reconstruction, typically with a gracilis tendon graft, adding tibial tubercle medialisation or distalisation when the TT-TG distance is markedly increased.
The kneecap that won't stay
A 16-year-old netballer plants her foot, twists, and feels the kneecap shift out; the knee swells within hours with a haemarthrosis, and the story of it "going back with a clunk" as she straightened the leg completes the classical history. She is tender along the medial retinaculum, apprehensive under lateral patellar pressure — the examination's decisive moment — and the skyline view shows a small osteochondral fragment lying in the lateral gutter, sheared from the medial patellar facet. This first episode breaches the conservative rule at one point: the loose body. Arthroscopy removes or fixes the fragment, the haemarthrosis is decompressed, and she still completes the standard rehabilitation, because her first dislocation earns her ligament a chance to heal.
Eight months later she dislocates stepping off a bus — the natural history of neglected risk factors. The pathway now measures the anatomy: magnetic resonance imaging shows trochlear dysplasia and an MPFL scarred in laxity, and the TT-TG measures 21 millimetres. Surgery addresses both — MPFL reconstruction with a gracilis graft to rebuild the checkrein, and a medialising tubercle osteotomy to realign the extensor corridor — after which rehabilitation is slower and squatting returns late. Indian counselling has one extra paragraph: deep squatting and cross-legged sitting load the patellofemoral joint heavily and are cultural daily functions, so return-to-activity guidance is negotiated with the family, not merely prescribed.
How the exam frames it
Four stems carry the marks. The direction — lateral, always. The structure — MPFL, whose name examiners now expect rather than the old vague "medial retinaculum". The test — apprehension, described with the patient's protective grasp as the positive endpoint. The management fork — first episode conservative, recurrent or fragment-bearing episodes surgical, with TT-TG above 20 millimetres as the number that adds a tubercle procedure. Trochlear dysplasia earns its own question as the strongest anatomical predictor. The habitual dislocation of childhood — the patella tracking out with every flexion — appears as a contrast stem: a different entity, often with tight lateral retinaculum or quadriceps contracture, treated by addressing that pathology rather than reconstruction.
Frequently asked questions
Which ligament is torn in acute patellar dislocation?
The medial patellofemoral ligament, the primary medial stabiliser of the patella in early flexion, avulsed from its femoral or patellar attachment.
What is the patellar apprehension test?
Laterally directed pressure on the patella with the knee at 20-30 degrees of flexion; the patient's reflex seizing of the thigh from fear of dislocation constitutes a positive result.
How is a first-time dislocation managed?
Conservatively — brief bracing, then structured quadriceps and vastus medialis rehabilitation — unless an osteochondral loose body or extensor-mechanism injury mandates surgery.
What TT-TG distance is considered abnormal?
Above about 20 millimetres (with 15-20 borderline), indicating lateralisation of the tibial tuberosity that may warrant realignment alongside ligament reconstruction.
When is MPFL reconstruction performed?
For recurrent dislocation or persistent instability after adequate rehabilitation, typically with a gracilis tendon graft, combined with tubercle transfer when malalignment is severe.