Abstract
Introduction: It is not uncommon situation, in a hip fracture patient treated with dynamic hip screw(DHS) system, that the hip arthroplasty should be done after removal of DHS. However multiple screw holes and postplating osteopenia under the barrel plate will be created in the proximal femur resulting adverse mechanical effects.
Purpose: The authors analysed the micromotion of femoral stem and the stress concentration of proximal femur in hip replacement performed after removal of DHS using finite element analysis.
Methods: For simulation of femoral cortical defects after removal of 4-holed DHS system, four Φ4.5 mm cortical screw holes on medial and lateral cortices of the femur and one Φ12mm lag screw. One 20mmx90mm weakened cortical bone area on lateral cortex was made for simulation of the postplating osteopenia created under the barrel plate. After meshing with eight node linear hexahedron, nonlinear contact analysis was done using ABAQUS 5.8 package system. For the postplating osteopenia we decreased the bony strength of cortical bone up to 20%.
Results: In one leg stance, the maximal micromotions at metal to bone interface were around 150& #13211; (142.3-160.6& #13211;) even in the osteoporotic femur. However, in stair climbing, it increased over 150& #13211; (170.1-191.1& #13211;) even in the non-osteoporotic intact femur. The maximal micromotions were 170.1& #13211; in intact non-osteoporotic femur and 191.1& #13211; in osteoporotic DHS removed femur in a stair climbing. The pattern of stress distribution on the surface of the femur was changed showing distal transfer of the point with maximal stress from the proximal medial area to the stem tip area. The maximal stress increased up to 89% at the lag screw hole.
Conclusion: This study suggests that the femoral stem for primary cementless hip replacement could be used in the DHS removed femur regardless of bone quality, if it is long enough to pass the screw holes and also if the post-operative rehabilitation is strictly controlled.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
