Custom Femur and Tibia BMD Precision in Elective Total Knee Arthroplasty Patients

Purpose/Aims

To compare bone mineral density (BMD) precision of knee custom regions of interest (ROI) with and without total knee arthroplasty (TKA).

Rationale/Background

TKA is a common procedure that results in 10 to 15% BMD loss at the distal femur. This could contribute to complications such as periprosthetic fracture, especially if osteoporosis is present at the time of TKA. Prior work supports measuring BMD around the knee using custom ROIs, this study investigates precision error of such an approach.

Methods

Thirty participants from a study evaluating BMD pre- and post-TKA had duplicate posteroanterior (PA) and lateral (LAT) scans in TKA and non-TKA knees with repositioning between. Scans were acquired on a Lunar iDXA with the orthopedic knee feature (GE enCORE software v18). Custom ROIs were manually placed on PA and LAT scans at the distal femur condyle (ROI 1), metaphysis (ROI 2) and shaft (ROI 3), and the proximal tibia (ROI 4) and tibial shaft (ROI 5) (Figure 1). The prosthesis was identified as artifact by the software. Precision error was calculated using the ISCD Advanced Precision Calculator and differences between TKA vs non-TKA legs were assessed by F-test.

Results

Study participants (n = 30; 6M, 24F) with mean (SD) age and BMI of 69.2 (6.5) years and 31.6 ± 4.9 kg/m2 respectively were included. Precision at various ROIs (Table 1) on non-TKA legs ranged from 1.2 - 3.8% on PA and 2.5 – 5.6% on LAT projections. Similarly, TKA leg ROI %CV ranged from 1.5 - 5.4% and 1.0 – 4.1% on PA and LAT respectively. PA precision differed (p < 0.001) between TKA and non-TKA legs at the distal femur condyle and tibia shaft. LAT precision differed between legs (p < 0.05) at the femur metaphysis, shaft, and tibia shaft. In the non-TKA leg, lateral positioning precision was numerically poorer at all ROIs; a generally similar pattern was observed in the TKA leg. The bone area post-TKA was small in the most distal femur and proximal tibia ROIs due to implant artifact. Tibial PA shaft reproducibility was confounded by fibular overlap in 23% of non-TKA scans but none post-TKA. However, fibula overlap was present on LAT view in 30% and 43% of non-TKA and TKA legs respectively.

Implications

Distal femur and proximal tibia BMD measurement may have utility for surgical planning and is best assessed in the PA projection. Based on precision, monitoring is best at the PA femur shaft and postoperatively at the tibial shaft. It is reasonable to expect precision improvement with automated ROI placement.