This 1980 landmark monograph by Reimers established the Migration Percentage (MP) as the standard radiographic measure of hip containment in children. It defined subluxation and dislocation thresholds using MP and validated normal values across childhood. The study then applied this measure to assess the effect of adductor muscle surgery on hip stability in cerebral palsy.
Every hip surveillance program for children with cerebral palsy traces back to this paper. Before Reimers, the radiographic language for CP hip instability was inconsistent — "dislocation," "subluxation," and "dysplasia" were applied without shared definitions, making outcome comparison impossible.
This paper gives you the tool: obtain a neutral-rotation AP pelvis, draw Perkins' line, and calculate what fraction of the femoral head lies lateral to it. That number is the Migration Percentage.
When MP crosses 33%, the hip is subluxated — this is your threshold for escalating intervention. When MP reaches 100%, the hip is fully dislocated. Serial MP tracking is how you decide whether adductor surgery worked or whether bony reconstruction is needed.
The clinical pearl worth internalizing: the normal pediatric hip never spontaneously subluxates. Any child with cerebral palsy whose MP is rising is not experiencing a normal developmental variant — that migration is driven by adductor/abductor imbalance and will progress without treatment.
This 1980 landmark monograph by Reimers established the Migration Percentage (MP) as the standard radiographic measure of hip containment in children. It defined subluxation and dislocation thresholds using MP and validated normal values across childhood. The study then applied this measure to assess the effect of adductor muscle surgery on hip stability in cerebral palsy.
Every hip surveillance program for children with cerebral palsy traces back to this paper. Before Reimers, the radiographic language for CP hip instability was inconsistent — "dislocation," "subluxation," and "dysplasia" were applied without shared definitions, making outcome comparison impossible.
This paper gives you the tool: obtain a neutral-rotation AP pelvis, draw Perkins' line, and calculate what fraction of the femoral head lies lateral to it. That number is the Migration Percentage.
When MP crosses 33%, the hip is subluxated — this is your threshold for escalating intervention. When MP reaches 100%, the hip is fully dislocated. Serial MP tracking is how you decide whether adductor surgery worked or whether bony reconstruction is needed.
The clinical pearl worth internalizing: the normal pediatric hip never spontaneously subluxates. Any child with cerebral palsy whose MP is rising is not experiencing a normal developmental variant — that migration is driven by adductor/abductor imbalance and will progress without treatment.