Analysis of Serial Foot Radiographs to Determine Foot Height Multipliers.

Background: The multiplier method is an arithmetic calculation that estimates the amount of growth remaining until skeletal maturity. When predicting lower limb length discrepancy, differences in foot height are added to femur and tibia discrepancies. Foot height multipliers have not been calculated using radiographic measurements, so it is unclear whether foot height develops at the same pace as the femur and tibia. This study used serial images to calculate foot height multipliers and compared them to published lower limb and foot length multipliers.

Methods: The Bolton Brush radiograph collection was used to measure foot height on the lateral foot view. Multipliers were calculated for ages with at least 10 serial study visits. 212 patients (2195 radiographs) were included in the study (102 female, 110 male patients). Foot height multipliers were calculated for ages 0 to 17 years (females) and 0 to 18 years (males).

Results: Multipliers decreased with age, but qualitatively plateau at age 13 (females) and age 15 (males). Lower extremity multipliers have a more dramatic growth curve, indicating comparatively greater lower extremity growth after birth. However, when comparing the limb length discrepancy calculation using the lower extremity multiplier versus the foot height multiplier for the foot portion, the difference was negligible.

Conclusions: This paper provides a database of foot height multipliers. Foot height seems to grow on a different trajectory than other lower limb components, confirming that one should consider separate multiplier values. The difference created by the foot height multiplier versus the lower extremity multiplier appears to be modest. Separate use of the foot height multiplier may only be necessary for young children with large foot height discrepancies, but further study to confirm the lack of impact of the foot height multiplier on limb length discrepancy calculations is needed. Our data were derived from normal children, so it is unknown if the presence of a talo-calcaneal coalition would affect foot height on the involved side.

Level of evidence: Level III-retrospective comparative study.