The error of DEXA reproducibility for the assessment of projection BMD in the periprosthetic zone of a polished wedge-shaped femoral stem component for cemented fixation

Abstract

Aim: To determine differentially with in vitro model the influence of positioning non-observation and that of the proper error of apparatus-programming complex on the results of projection BMD (PBMD) study using DEXA method for the hip endoprosthetics with cemented fixation (CPT model, Zimmer). It has been revealed that in case of incorrect positioning with femoral rotation ±5o the error average values in the Gruen zones increased in the following sequence: the Gruen zone 4, 1, 3, 5, 2, 6, 7. The maximal error of DEXA complex reproducibility was observed in zone 6, amounting to 6,2%. To take it down to clinically insignificant values (±1%) the following algorithm has been suggested. At first two consecutive measurements should be made without changing the position, and after that, according to the results obtained, PBMD Mav1 should be calculated in every Gruen zone. If both results are within the boundaries of Mav1 (±1%), there is no need in further measurements. Otherwise one more measurement should be performed. Then, on the basis of the all three measurements Mav2 is calculated in each Gruen zone, and it is compared with Mav1. If the difference between them is less 1% the number of measurements is considered sufficient. If it exceeds 1% one more procedure of PBMD estimation is necessary, and so on until the difference between Mav of the previous and that of the last measurement amounts to ≥ 1%. This algorithm allows a physician to control the amount of reproducibility error of the apparatus-programming complex.