Congenital Dislocation of the Knee

Congenital dislocation of the knee (CDK) is a rare but important neonatal condition where the knee presents in hyperextension or frank anterior dislocation at birth. It sits on a spectrum from simple genu recurvatum to irreducible anterior dislocation. Early recognition is critical because outcomes are excellent with prompt serial casting, but delayed or inadequate treatment leads to progressive deformity, growth disturbance, and the need for complex surgery. The high association with DDH means every CDK baby must have hip screening.

The patella is frequently abnormal in CDK and is a source of later morbidity that is easy to overlook in the neonate. The neonatal patella is unossified, but the underlying cartilaginous patella is often small (hypoplastic), laterally positioned, or even absent, and the extensor mechanism may be laterally subluxed — features that are especially pronounced in Larsen syndrome. After the tibiofemoral dislocation is reduced, persistent patellar maltracking or recurrent lateral patellar instability may emerge as the child grows and the patella ossifies, sometimes requiring later extensor-mechanism realignment (lateral release, medial reefing, or distal realignment). Examine and document patellar position at presentation and follow it through growth, because a reduced tibiofemoral joint does not guarantee a stable patellofemoral joint.

A positional hyperextended knee is soft and passively correctable to full flexion without resistance. The tibiofemoral joint is congruent on radiographs. True CDK has a firm endpoint to passive flexion, the tibia is anteriorly displaced, and radiographs show anterior subluxation or dislocation. If you can flex the knee to 90 degrees easily with no firm resistance, it is likely positional and will resolve without casting.

The diagnosis of CDK is primarily clinical. Plain radiographs confirm the degree of tibial displacement and guide grading. The most important investigation is hip ultrasound at 6 weeks, because DDH coexists so frequently and has its own treatment window. Do not delay casting while waiting for advanced imaging.

When CDK and DDH coexist (as they frequently do), the knee is reduced first. The reason is mechanical: the standard devices for DDH — the Pavlik harness and the hip spica — hold the hip in flexion and abduction, which requires the knee to be flexed. A knee fixed in hyperextension or anterior dislocation cannot be positioned in a harness or spica, so treating the hip first is futile and may even displace it. Serial casting (or surgery) is therefore directed at restoring knee flexion first; once the knee flexes past roughly 90 degrees, a Pavlik harness can be applied and may treat a reducible knee and a dysplastic hip together. In bilateral Grade III disease this sequencing — knees, then hips — also dictates the order of any staged surgery.

Best prognosis: Grade I or II, isolated CDK (no syndrome), treatment started within first week of life, compliant serial casting

Poor prognosis: Grade III, bilateral involvement, underlying arthrogryposis or Larsen syndrome, delayed treatment initiation beyond 3 months

Key threshold: 3 months of age is the practical cutoff for casting. Beyond this, surgical success rates decline and residual deformity becomes more likely.

There is no dedicated registry for congenital knee dislocation. The evidence base consists entirely of retrospective case series and expert opinion. No randomised controlled trials exist comparing casting protocols or surgical techniques. Management is therefore guided by clinical grading (Tarek), institutional experience, and the consistent principle that early, gradual serial casting gives the best outcomes for Grades I and II.