Background While modular femoral heads have been used in THA for decades, a recent innovation is a second neck-stem taper junction. Clinical advantages include intraoperative adjustment of leg length, femoral anteversion, and easier revision, all providing flexibility to the surgeon; however, there have been reports of catastrophic fracture, cold welding, and corrosion and fretting of the modular junction.
Using sham replacement of the proximal femur in adult mongrel dogs, shear strength at the interface between polymethyhnethdcrylate bone cement and cancellous bone has been found to be linearly dependent on the depth of penetration of the cement into the bone. Shear strength at the interface was increased by 82% and penetration by 74% when distal bone plugging, pressure lavage, and pressurized insertion of cement were employed. Use of a lower-viscosity cement gave a further 18% increase in penetration and shear strength. There was no film of blood at the cement-bone interface with pressurized insertion of Simplex P and Palacos R cements.
Our short-term results for the APG and RP mobile bearing designs are similar and compare more than favorably with reported values for fixed-bearing designs. However, increased backside wear due to multidirectional movement may predispose the APG design to greater wear in the long term.
The high incidence of Poly Implant Prothèse shell rupture is most likely a result of inadequate quality control, with contributory factors being shell thickness variation and manufacturing defects on both inner and outer surfaces of the shell. No evidence of shell degradation with implantation time was determined.
The depth of penetration of five commercial acrylic bone cements into cancellous bone was measured in vitro. Under standard, idealized conditions, cement penetration was found to vary significantly with different cements. Penetration was critically influenced by the coarseness of the cancellous bone and increased directly with the effective volume of the "cells" within the osseous matrix. An inverse correlation was determined between the mean cement viscosity during flow into the bone and final penetration depth. The dough time, set time and working time of each acrylic formulation was found to have no significant effect upon the depth of cement penetration. It is suggested that in addition to the techniques adopted for introduction of cement to the bone, the selection of the bone cement itself may critically influence the incidence of late loosening following total joint replacement.
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