Abstract
Aims Cemented acetabular components remain the standard for many surgeons around the world, however the main draw back of this technology is that of aseptic loosening. It has been suggested that loosening is initiated when mechanical failure occurs at the cement/bone interface. Successive generations of cementation techniques have evolved over the years in order to address this problem by improving the mechanical integrity of the cement bone construct. Negative pressure intrusion cementation techniques (NPI) represent a more recent phase of this evolution. These techniques involve the introduction of vacuum into the peri-acetabular bone immediately prior to cement application with the aims of decreasing the deleterious effects of the systemic bleeding pressure, removing fat and debris from the path of the advancing cement and causing deeper cement ingress through the direct effects of negative pressure. There exists in the literature very little scientific information relating to this technique; therefore our aim is to assess the quality of the cement bone interface in constructs created using a specially constructed rig.
Methods: Samples of screened, fresh frozen, human femoral head are machined to create a cylindrical core of cancellous bone measuring 24 x 40 mm. These samples are carefully stratified for porosity using a method of combing DEXA scan bone mineral density findings with microCT (Scanco 40, Bassendorf, Switzerland) histomorphological parameters. These cores are then introduced into the vacuum chamber of the rig and are subjected to a negative pressure of −30 kPa using a clinical suction machine (Cherion, Czech Rep). Simplex P (Stryker, Mahwah, US) polymethylmethacrylate cement is applied to the exposed cancellous bone within the rig and subjected to a constant positive external pressure via the vertical actuator of a servohydraulic materials testing machine (Instron 8873, Mass. US). The cement is allowed to set and the constructs are removed en bloc and frozen. The constructs produced are cylindrical and consist of three distinct zones; cement, bone and that of the cement bone interface. The quality of the cement bone interface is assessed in two distinct ways:
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MicroCT is used to produce both quantitative and qualitative data relating to the cement bone interface. This data is processed using the 3D reconstruction software (Scanco, Bassendorf, Switzerland) to give values for intrusion depth and the integrity of cement bone interlock indicated using a previously published method of recording incidence and size of vacuolation within the cement bone interface.
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The mechanical integrity of the cement bone interface is tested in shear, torsion and tension. Control is provided by repeating the identical procedure in porosity controlled bone in the absence of vacuum.
Results: Early results indicate a tendency toward deeper and ‘tighter’ cement interdigitation within the cement bone interface in the samples created using the NPI method. These samples also tend to be mechanically stronger than controls.
Discussion This series of in-vitro experiments provides important information about this accepted but poorly understood technique. The model accurately mimics the operative technique and the use of microCT in this way is a novel application, allowi ng the digital assessment of cement intrusion depth and quality without having to physically section the constructs. It also attempts to relate these properties to mechanical strength.
The abstracts were prepared by Emer Agnew, Secretary to the IOA. Correspondence should be addressed to him at Irish Orthopaedic Association Secretariat, c/o Cappagh National Orthopaedic Hospital, Finglas, Dublin 11, Ireland.
