Abstract
Background: Inadequate proximal femoral pressures obtained during a cemented, primary hip replacement may lead to poor stem fixation. Proximal occlusion during stem insertion,may help in achieving a uniform and sustained rise in intra-medullary pressures, distally and proximally. High intra-medullary pressures correlate with better cement penetration and increased cement-bone interface push-out strength.
Methodology: An In-vitro analysis of femoral pressures was performed. A femoral medullary cavity was created in plaster of Paris constrained in an aluminium cylinder. Intramedullary pressures were measured via pressure transducers. High viscosity bone cement (Palacos-R) was gunned into the medullary cavity. No.3 Exeter stem was inserted with no proximal occlusion, with thumb occlusion over the calcar and with the Exeter Horse-collar. Experiments were repeated by delaying the timing of insertion and with lower viscosity cement (Simplex-P). A small series of experiments were done to ensure that that the stem insertion was performed at standard cement viscosity. The experiments were carried out with the same viscosity of Palacos-R at 4 minutes and Simplex-P at 6 minutes. Palacos-R at 4 minutes 30 seconds had a higher viscosity.
Results: A total of 54 experiments were performed. Of these 18 experiments were done with Palacos R cement, with the stem inserted early on in the curing phase and 18 with a delayed time of insertion. The last 18 experiments were performed with Simplex P cement with the stem inserted early on in its curing phase.
Intramedullary pressures were better in all zones, for all cement modes, with proximal occlusion. The highest pressures were seen with Palacos-R at 4 minutes 30 seconds with proximal thumb occlusion. Stem insertion into Palacos-R at 4 minutes or 4 minutes 30 seconds, gave higher pressures than Simplex-P, with or without any form of occlusion. With Simplex-P, intramedullary pressures were higher, with Collar rather than thumb occlusion.
Conclusion: Occluding the medial cal car area during stem insertion, is an effective way of achieving and sustaining high-pressures in the proximal and distal femur. The highest pressures are obtained with stem inserted into Palacos-R at 4 minutes 30 seconds, with proximal thumb occlusion. Collar occlusion may be better in achieving higher pressures, with lower viscosity, Simplex-P.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
