A simple assessment of bone quality prior to hip arthroplasty: cortical index revisited

To determine from routine clinical radiographs the diversity of cortical bone structural quality, i.e. the cortical index, of the proximal femur in 117 hips prior to hip arthroplasty and to evaluate its variance associated with patient demographic variables (gender, age at the time of surgery, height, weight, body mass index (BMI), and preoperative diagnosis).

A cross-sectional study was conducted using preoperative anteroposterior radiographs of the hip from 110 consecutive patients with 117 hip arthroplasty procedures, in which the patients at the time of surgery had an average age of 69.9 years (range, 19 to 92 years). The primary diagnosis leading to hip surgery was either degenerative joint disease (68 hips) or femoral neck fracture (49 hips). The cortical index, as a measure of cortical bone structural quality, was determined by one experienced reader for the proximal femur of each hip from conventional preoperative anteroposterior radiographs. Correlation analysis was performed between the cortical index and patient demographic variables to assess factors associated with the cortical index.

For the entire cohort the average femoral cortical index was 47.9% (range, 25.8% to 64.8%). The hips in female patients had a lower average cortical index than in male patients (mean value of 46.9% and 50.0% respectively). Statistical analysis demonstrated highest significant positive correlation between cortical index and BMI (r = +.441, p < 0.0001) and with body weight (r = +0.396, p < 0.0001). Significant negative correlation was also found between cortical index and age at the time of hip surgery (r = -0.423, p < 0.0001). Significant differences were found between the cortical index and preoperative diagnosis with lower values among the hips diagnosed with femoral neck fractures (mean value, 43.9%) compared to hips with degenerative joint disease (mean value, 50.9%).

A simple radiogrammetric parameter obtained from routine radiographs, i.e. the femoral cortical index, was found to demonstrate diversity of cortical bone structural quality among patients prior to hip arthroplasty. The cortical index decreased with older patients and increased with heavier (weight) and obese (BMI) patients. It was significantly lower among the hips with femoral neck fractures in patients who were predominantly fragile older females with lower body mass index, which would corroborate published studies associating lower bone mass with risk of hip fracture.