The Reverse Dorr: Preoperative radiographic evaluation of bone quality correlates to bone mineral density in total knee arthroplasty

Background

There has been an increase in cementless total knee arthroplasty (TKA) across the country. Although radiographic classification systems of the knee exist, these algorithms have not been correlated to bone mineral density (BMD). We aimed to correlate radiographic measurements of the knee with preoperative BMD and develop a model predictive of osteopenia.

Materials and methods

We prospectively enrolled 100 patients, scheduled to undergo elective TKA, to obtain a preoperative dual energy x-ray absorptiometry (DEXA) scan. Four independent surgeons measured cortical indices of the knee on preoperative radiographs and correlated these ratios with local BMD. Intraclass correlations (ICC) were used to assess interobserver reliability and Pearson correlation coefficients (PCC) to describe the strength of relationship between BMD and radiographic ratios. A “Reverse Dorr” model was created to evaluate the ratio of the distal femur cortical indices. Logistic regression was used to predict the odds of having osteopenia.

Results

We found several cortical ratios of the knee correlated with bone mineral density, but the lateral radiograph had the highest correlation. All measurements showed, at a minimum, fair interobserver reliability with most achieving an ICC >0.81. The proximal femur, distal femur ratio, “Reverse Dorr” model, was found to be a strong predictor of osteopenia with an ROC of 0.7335.

Conclusions

This radiographic assessment demonstrates that cortical thickness of the knee is correlated with bone mineral density—most notably on the lateral radiograph. Utilization of the “Reverse Dorr” model was strongly predictive of osteopenia and may help guide implant selection.