In vitro and in vivo studies of pressurization of femoral cement in total hip arthroplasty

Abstract

Improvements in cementing techniques in the absence of pressurization of the cement have led to major increases in the long-term success rate of fixation of the femoral components of cemented total hip arthroplasty (THA). The strength of the cement-bone interface is strongly related to cement intrusion into the bone. The depth of cement intrusion, in turn, is correlated with the cement-intrusion pressure. Thus, adding cement pressurization to those current techniques that have already been validated may further increase the long-term durability of fixation of the femoral component of cemented THA. To assess cement pressurization in the proximal femur for THA, the authors compared in vitro the efficacy of three existing pressurization systems (the Johnson and Johnson system [New Brunswick, NJ], the Miller system [Zimmer, Warsaw, IN], and the Zimmer system [Zimmer]) in cadaver femurs using pressure transducers and evaluated their ease and optimization for clinical use. The authors then selected one (the Zimmer system) for use in studies in vivo to quantify the actual pressures achieved in the medullary canal in vivo under surgical conditions using pressure transducers placed throughout the femoral cortex. Each of the three commercially available femoral cement pressurization systems has its own advantages and disadvantages. All three systems were shown to produce average peak cement-intrusion pressures in vitro of over 21 N/cm 2 (30 psi) throughout the cement mantle including, importantly, in the proximal portion of the femur. Under laboratory conditions all three systems produced adequate pressurization for optimal cement penetration into cancellous bone. Because the repeated application of pressure is more valuable than the single thrust provided by the Johnson and Johnson cement compactor, the Miller and Zimmer systems seemed preferable. Because the Miller system does not pressurize the most proximal portion of the canal (the volume occupied by the silicone conical seal) and because the proximal portion of the cement mantle is important to longterm fixation of the femoral component, the Zimmer system was selected for the in vivo study. The cement-intrusion pressures in the proximal portion of the femurs obtained in vivo in the operating room during the 10 primary and 5 revision THAs had a mean value from all 15 patients of 19 ± 8 N/cm 2 (27 ± 11 psi). The primary cases had a significantly higher intrusion pressure (22 ± 7 N/cm 2 [32 ± 10 psi]) than the revision cases (13 ± 6 N/cm 2 [19 ± 9 psi]). In a prior study from the authors' laboratory using bovine cancellous bone, which is more dense than human cancellous bone, cement pressures of 14 N/cm 2 (20 psi) were shown to produce a cement-intrusion depth of 5 mm. An intrusion depth of 3–4 mm has been shown to be optimal for the strength of the cement-bone interface. Therefore, 10 N/cm 2 (15 psi) is likely to be adequate to achieve optimal cement-intrusion depth in humans. The results of this study show that a pressure of 10 N/cm 2 (15 psi) was obtained in 9 of 10 primary cases and 3 of 5 revision cases.