A clinical research of mechanical analysis of tumor type knee prosthesis

GENG Lei; WANG Zhen; GUO Zheng; FAN Hong-bin; LI Xiang-dong; LI Jing; CHEN Guo-jing; FU Jun; SHI Lei; SUN Chang-ning; WANG Ling

doi:10.3969/j.issn.2095-252X.2015.10.014

Abstract：Objective To investigate the matching relationship between femur and prosthesis alignment in sagittal plane, to seek causes for early prosthesis loosening and to provide a scientiifc proof for the installation, design and manufacture of clinical bone tumor prosthesis.Methods One knee tumor type prosthesis was collected from a patient to establish a 3 D mathematical model with a ifnite element analysis to obtain clinical data. The angle between femur and prosthesis of 86 patients on post-operative coronal and sagittal X-rays were analyzed to investigate the inlfuence of this bias on implant loosening. Physiological curvature of the femur in all age groups was measured. Results ( 1 ) Intramedullary stem bias angle in all four directions had inlfuence on cancellous bone deformation. At a bias of 3°, the maximum mean deformation value was ( 0.590±0.001 ) mm; at a bias of 6°, the maximum mean deformation value was ( 1.163 ± 0.002 ) mm; at a bias larger than 3°: the maximum value of cancellous bone deformation exceeded the limit ( 0.58 mm ). Statistical signiifcance was found in the effects of the bias angle on enforced deformation of intramedullary substantia ossea (P<0.05 ). ( 2 ) Out of 86 cases, 78 cases were followed up for 2 years and 67 cases for 5 years postoperatively. In coronal plane, at 2 years post-op 2 cases had a bias angle >3°, no loosening of the prosthesis was found; while at 5 years postoperatively, 4 cases ( 4.7% ) had coronal bias angle >3°, 2 of them ( 2.3% ) developed aseptic loosening. In sagittal plane, 8 cases ( 9.3% ) had bias angle >3°. At 2 years post-op, 4 of them developed aseptic loosening; while at 5 years post-op, 13 cases ( 15.1% ) had a bias angle >3° and loosening occurred in 13 cases, including 4 cases with aseptic loosening at 2 years postoperatively. Results of analyses on the constituent ratio of the effects of coronal and sagittal bias angles on the loosening of prosthesis showed statistical signiifcance (P<0.01 ). Sagittal bias angle had more signiifcant effects on prosthetic loosening. As for physiological curvature of the femur in sagittal plane, the distal arc was smaller than the proximal arc.Conclusions ( 1 ) When the bias angle of tumor type prosthetic stem is larger than 3°, the stress on the cancellous bone of the femoral canal exceeds the allowed deformation limit. Sagittal malalignment is an important factor in early aseptic loosening of the prosthesis. ( 2 ) Femoral curvature in sagittal plane varies between different patients. Individualized prosthesis that ifts the canal morphology should be applied.