Cancellous Bone Strength Measurements with the Osteopenetrometer

Abstract

An instrument designed to measure cancellous bone strength at the tibial and femoral condyles during total knee replacement is described and evaluated with respect to the relationship of measurements to conventional compression tests on machined specimens in a material testing machine. The instrument, referred to as an osteopenetrometer, indirectly measures the force and depth of penetration of a needle that is advanced into the cancellous bone at constant speed. A small computer controls the measuring procedure and corrects the force readings for resistance in the hydraulic system. The relationship between osteopenetrometer measurements and compression tests is best described by a power law, with correlation coefficients of approximately 0.90 when ultimate stress data were compared. A 2.5 mm pointed needle with the shaft milled to 2.3 mm to avoid friction along the needle shaft is favoured for clinical measurements since readings from different depths beneath the resection surface may then be directly compared. The error of measurement was assessed by comparison of observed and smoothed data from measurements across tibial and femoral condylar resection surfaces. Ninety-nine per cent tolerance limits of ± 3.0 MPa (compared to a range of 0–40 MPa in units peculiar to the osteopenetrometer) were found.