Based on the complexity of proximal femoral geometry, osteotomies in the coronal plane may change femoral version, while osteotomies in the plane of the femoral neck should not. Theoretically, this relationship can lead to a systematic error when measuring rotational correction in the proximal part of the femur.Methods:
We developed a mathematical model to predict the error with three different techniques for rotational measurement: (1) a pin along the axis of the femoral neck and a lateral pin across the femoral condyles, (2) a proximal lateral pin perpendicular to the femur and a lateral pin across the femoral condyles, and (3) two anterior pins representing a score mark on the proximal part of the femur. Varus derotational osteotomies were performed on twenty-four Sawbone femora, and femoral anteversion and the angles between each set of pins were measured.Results:
Our mathematical model predicted no error with the neck-pin technique regardless of the type of osteotomy performed. With the lateral pin technique, there was no predicted error with the neck plane osteotomy but a large error with the coronal plane osteotomy. The score-mark technique had a small amount of error with the neck plane osteotomy and a large error with the coronal plane osteotomy. Our Sawbones model measurements correlated well with the mathematical model, with intraclass correlation coefficients of 0.96, 0.89, and 0.91 for the neck-pin, lateral pin, and score-mark techniques, respectively.Conclusions:
This study describes and validates a model for predicting the error of three different techniques for measuring femoral rotation. It demonstrates that inadvertent systematic error can occur with lateral pin and score-mark techniques, particularly when the osteotomy is out of plane with the femoral neck.Clinical Relevance:
Although the mathematics of this study are complex, the conclusions are clinically straightforward: rotational error is minimized by performing an osteotomy in the plane of the femoral neck or by using a pin in the femoral neck to judge rotation. If neither of these techniques is employed, then systematic error can be introduced and can be substantial.