Hip resurfacing is a bone sparing approach to treating arthritis in younger or more active patients. Accurate positioning of the femoral component in the hip resurfacing procedure is essential for the success of the operation [1-2]. An alignment guide assisting the operator in accurately positioning the resurfacing implant may increases the success rate of the operation. This study focuses on the effectiveness of a CT based resurfacing alignment guide, shown in Figure 1. Four full fresh frozen human cadaveric specimens were CT scanned to reconstruct bone models of the femoral head/neck geometries with no cartilage included in the segmentation. Femoral head resurfacing alignment guides were then created through computer aided design (CAD) modeling using landmarks from the reconstructed bone models for proper seating. A total of 12 resurfacing alignment guides (3 for each specimen) were prepared. After the exposure of the hip joints, the first two out of three resurfacing alignment guides were used to asses the fit, stability, and visual assessment of valgus and version alignments. The third resurfacing alignment guide for each specimen was placed on the femoral head/neck region and the guide wire was drilled into the femur. A fluoroscopy image was taken to assess and measure the valgus and version alignment. The acceptance criteria for valgus alignment, as shown in Figure 2, is set to be ±2.5° from a line parallel to the medial calcar of the femoral neck, Similarly, the acceptance criteria for the version alignment was set to be ±2.5° from a line passing through the neutral axis of the femoral neck.Introduction
Materials and Methods
Proper positioning of the components of a knee prosthesis for obtaining post-operative knee joint alignment is vital to obtain good and long term performance of a knee replacement. Although the reasons for failure of knee arthroplasty have not been studied in depth, the few studies that have been published claim that as much as 25% of knee replacement failures are related to malpositioning or malalignment [x]. The use of patient-matched cutting blocks is a recent development in orthopaedics. In contrast to the standard cutting blocks, they are designed to fit the individual anatomy based on 3D medical images. Thus, landmarks and reference axes can be identified with higher accuracy and precision. Moreover, stable positioning of the blocks with respect to the defined axes is easier to achieve. Both may contribute to better alignment of the components. The objective of this study was to check the accuracy of femoral component orientation in a cadaver study using specimen-matched cutting blocks in six specimens; first for a bi-compartmental replacement, and then for a tri-compartmental replacement in the same specimen. Frames with infrared reflective spherical markers were fixed to six cadaveric femurs and helical CT scans were made. A bone surface reconstruction was created and the relevant landmarks for describing alignment were marked using 3D visualisation software (Mimics). The centres of the spherical markers were also determined. Based on the geometry of the articular surface and the position of the landmarks, custom-made cutting blocks were designed. One cutting block was prepared to guide implantation of a bi-compartmental device and another one to guide implantation of the femoral component of a total knee replacement. The knee was opened and the custom-made cutting block for the bi-compartmental implant was seated onto the surface. The block was used to make the anterior cut, after which it was removed and replaced with the conventional cutting block using the same pinning holes to ensure the same axial rotational alignment. The other cuts were made using the conventional cutting block and the bi-compartmental femoral component was implanted. Afterwards, a similar procedure was used to make the extra cuts for the total knee component. The position of the components with respect to the reflective markers was measured by locating three reference points and “painting” the articular surface with a wand with reflective markers. The position of all marker spheres was continuously recorded with four infrared cameras and Nexus software.Purpose
Materials and Methods
Due to their superior wear characteristics, oxidized Zr-2.5Nb heads are used with hip stems made of conventional orthopaedic alloys. Galvanic interactions between Zr-2.5Nb (Zr) and Ti-6Al-4V (Ti), cobalt-chromium (CoCr), and 316L stainless steel (SS) alloys were evaluated. Galvanic current density was measured for Zr/Ti,Zr/CoCr, Zr/SS, CoCr/Ti, and CoCr/SS couples under static conditions in aneutral Ringer’s solution and in an acidic (1.7 pH) solution. To simulate fretting, one or both coupled alloys in the neutral solution subsequently were abraded by a bone cement pin (82 MPa Hertzian stress). An extended(7-day) static test in the acidic solution was performed for Zr/SS and CoCr/Ti to simulate crevice conditions. The dissolved metal ion concentration was determined using direct coupled plasma emission spectrometry. Mean initial current densities of the Zr/SS, SS/CoCr,Zr/CoCr, CoCr/Ti, and Zr/Ti couples were 3.0, 0.36, 0.16, 0.05, and 0.04μA/cm2, respectively, in the neutral solution, and 0.57, −0.29, 0.04, 0.02, and 0.03 μA/cm2, respectively, in the acidic solution (positive when first alloy was anode). Within 30 minutes, all values decreased below 0.02μA/cm2. The current densities increased by orders of magnitude under fretting conditions. When both alloys were abraded, the highest values were minus;677 and 464 μA/cm2 for CoCr/Ti and Zr/SS, respectively. In the extended static test of Zr/SS, the mean total metal ion concentration decreased from 8.15 mg/L when the alloys were uncoupled to 4.50 mg/L(p=0.007) when they were coupled. For CoCr/Ti, the change from 1.28 to 1.72mg/L when the alloys were coupled was not statistically significant(p=0.22). With its strong tendency to passivate, the Zr alloy produced galvanic interactions within the range observed with conventional alloy couples. Its anodic characteristic helped protect SS in a galvanic couple.
An instrumented extensively porous coated stem was implanted in composite femur models (n=3) and mechanically tested. The stem stresses resulting from proximal overbroaching, ETO, cable grips, and various cable and strut constructs were determined.
Stem stresses increased 98 when a proximally loose stem was combined with an ETO using laboratory tests. This stress was decreased by up to 37 percent when a long trochanteric plate was utilized.
An instrumented extensively porous coated stem was implanted in composite femur models (n=3) and mechanically tested. The stem stresses resulting from proximal overbroaching, ETO, cable grips, and various cable and strut constructs were determined.
Stem stresses increased 98 when a proximally loose stem was combined with an ETO using laboratory tests. This stress was decreased by up to 37 percent when a long trochanteric plate was utilized.
Oxidized Zirconium (OxZr), metallic zirconium alloy oxidized to form a ceramic surface, was developed as an alternative bearing material to cobalt-chrome (CoCr) alloy for improvements in roughening resistance, frictional behavior, and biocompatibility without a risk of brittle fracture. Knee simulator testing without intentional addition of abrasives demonstrated that the ultra-high molecular weight polyethylene (UHMWPE) wear rate was 85% less with OxZr than with CoCr femorals. The relative performance of articulating materials can change when tested under abrasive conditions, so test protocols were investigated with abrasives added directly to the simulator test lubricant. Testing was conducted on a six-station, four-axis, physiological knee simulator. OxZr and CoCr medium-sized, cruciate-retaining, femoral components were tested against UHMWPE tibial inserts sterilized by ethylene oxide. Alumina powder was mixed into 50% bovine serum lubricant at a concentration of 0.2 mg/cm3. Tests were conducted with different powder sizes in the range of 0.3 to 150 μm. Measurements included tibial insert weight and femoral surface roughness. The lubricant in CoCr tests became opaque with gray debris while the femoral condyles became scratched. In contrast, the lubricant in OxZr tests remained normal (as in tests without abrasives), and femoral condyle scratching was much less severe. Despite these obvious effects, the UHMWPE wear produced by each material did not increase appreciably over that of tests without abrasives, with OxZr maintaining a wear rate about 85% less than for CoCr. It was noted that the scratches were aligned, or became realigned, with the translation motion and had little evidence of the swirls or cross-hatching often observed on retrieved components. Previous testing indicates that UHMWPE wear increases significantly only if scratches are oblique to the sliding direction. Thus, a test technique that produces scratches with more clinically relevant orientations is needed for a performance comparison between femoral materials under abrasive conditions.
