A Seven-Zone Rating System for Assessing Bone Mineral Density after Hip Resurfacing Using Implants with Metaphyseal Femoral Stems

Abstract

The seven Gruen zones are widely accepted for the assessment of periprosthestic bone mineral density (BMD) following traditional total hip arthroplasty, but no comparable rating system exists for hip resurfacing arthroplasty (HRA). We describe a seven-zone rating system around the femoral component of a typical HRA implant with a short metaphyseal stem and its intra- and inter-observer reliability. A cohort of 23 selected male patients underwent bone mineral density (BMD) measurements by dual-energy X-ray absorptiometry (DEXA) two years after HRA. After development of the new seven-zone rating system, reliability was assessed using intraclass correlation coefficients (ICC). The coefficient of variation was also determined. The new rating-system proved high reliability with ICCs for the intra-and inter-observer reliability ranging from 0.92 to 1.0. The coefficient of variation ranged from 1.4% to 3.3%. Mean BMD values were highest in the medial zones 5 to 7 and lowest in the lateral zones 1 to 3 and below the stem tip, corresponding to zone 4. We observed that BMD after HRA can be measured reliably in seven zones around the femoral component. The use of this easily applicable method in future investigations might enhance the comparability of study results. However, the rating system described may have limitations in HRA implants with very short metaphyseal femoral stems.