Abstract
Software to segment and to analyse connective CT-scan images of the bone-cement-stem complex was developed and validated. Parameters assessed included: volumes (cortical bone, cancelous bone, cement, stem, air in bone and air in cement), cement mantle thickness, cortical & cancelous bone thickness, contact surface area between cement and bone, degree of centralisation (stem in cement, stem and cement in cancelous and in cortical bone). To validate and assess intra- and interob-server reliability two models were implanted in two dried macerated cadaver femurs using a third generation cementing technique. In the first a polished tapered stem (CPT, Zimmer) was cemented and removed after cement curing. The air filled cavity within the cement mantle could be identified as implant, avoiding metallic scatter artefacts. The second model (SLA) used a plastic stem prototype produced by computer design and a rapid prototyping stereolithographic technique. This model does not need to be removed before CT-scanning and allows assessment of whatever femoral implant. Validation occurred by comparing 41 manually segmented femoral cross-sections (25 CPT, 16 SLA) with data of corresponding CT-scan slices. Inter-observer reliability was assessed by having the same person performing the CT-scan and the analysis of both models four times. To assess intra-observer reliability, four different observers segmented 97 representative CT-images (46 CPT, 51 SLA). The average accuracy for surfaces areas (bone, cement, stem) within CT-images was 7.47 mm2 (1.80%), bone & cement mantle thickness: 0.51 mm (9.39%), distances between centroids (stem-cement, stem-bone, cement-bone): 0.38 mm (18.5%) and contours (bone, cement): 0.27 mm (2.57%). The intra- and interobserver reliability of air content in bone and cement was suboptimal (intraclass-correlation coefficient (ICC) as low as 0.54, average ICC: 0.85). All other variables assessed were reliable (ICC > 0.81, average ICC: 0.96). Validity and reliability were comparable when assessed separately for the proximal, middle and distal third of both models. This in vitro technique can assess characteristics of cement mantles produced by different cementing techniques, centralizers and existing femoral implants or stem prototypes.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
