Abstract
Background
Body Mass Index (BMI) is used to quantify generalised obesity, but does not account for variations in soft tissue distribution.
Aims
To define an index quantifying the knee soft tissue depth, utilising underlying bony anatomy, and compare with BMI as a measure of individual patient's knee soft tissue envelopes. We performed a practicality and reproducibility study to validate the Bristol Knee Index for future prospective use.
Method
Femoral trans-epicondylar axis, and the proximal tibial plateau width were measured on 225 antero-posterior pre-operative knee radiographs. Corresponding measurements of soft tissue were performed at both levels. These were expressed as a ratio: Soft tissue width (mm)/Bone width (mm) = BKI. Time taken performing each measurement was recorded, and inter- and intra-observer variability was assessed.
Results
Average BMI was 32 (18-54). Measuring femoral and tibial BKI averaged 35 seconds. Inter-observer interclass correlation coefficient (ICC) for femoral and tibial BKI was 0.994 and 0.997 respectively. Intra-observer ICC was 0.996 for both. Correlation of BKI to BMI was 0.64 (for both femoral and tibial BKI). When divided into BMI subgroups (normal, overweight, obese, morbidly obese), the correlation was poor. BMI cannot be used to predict the amount of knee soft tissue in the individual patient. Tibial measurement was the most reproducible method.
Conclusions
BKI is a fast, reproducible measurement to assess knee soft tissue depth. BMI cannot be used to assess individual patient's knee soft tissue. We plan to correlate BKI to ‘surgical’ orthopaedic complications such as malalignment, wound complications and infection.
