A majority of the acetabular shells used today are designed to be press-fit into the acetabulum. Adequate initial stability of the press-fit implant is required to achieve biologic fixation, which provides long-term stability for the implant. Amongst other clinical factors, shell seating and initial stability are driven by the interaction between the implant's outer geometry and the prepared bone cavity. The goal of this study was to compare the seating and initial stability of commercially available hemispherical and rim-loading designs. The hemispherical test group (n=6) consisted of 66mm Trident Hemispherical shells (Stryker, Mahwah NJ) and the rim-loading test group (n=6) consisted of 66mm Trident PSL shells (Stryker, Mahwah NJ). The Trident PSL shell outer geometry is hemispherical at the dome and has a series of normalizations near the rim. The Trident Hemispherical shell outer geometry is completely hemispherical. Both shells are clinically successful and feature identical arc-deposited roughened CpTi with HA coatings on their outer geometry. Hemispherical cavities were machined in 20pcf polyurethane foam blocks (Pacific Research Laboratories, WA) to replicate the press-fit prescribed in each shell's surgical protocol. The cavity for the hemispherical design was machined to 65mm (1mm-under ream) and the cavity for the rim-loading design was machined to 67mm (1mm- over ream). Note that the rim-loading design features ∼2mm build-up of material at the rim when compared to the hemispherical design. The shells were seated into the foam blocks using a drop tower (Instron Dynatup 9250G, Instron Corporation, Norwood, MA) by applying 7 impacts of 6.58J/ea,. The number and energy of impacts are clinically relevant value obtained from surgeon data collection through a validated measurement technique. Seating height was measured from the shell rim to the cavity hemispherical equator (top surface foam block) using a height gage, thus, a low value indicates a deeply seated shell. A straight torque out bar was assembled to the threads at the shell dome hole and a linear load was applied with a MTS Mechanical Test Frame (MTS Corporation, Eden Prairie, MN) to create an angular displacement rate of 0.1 degrees/second about the shell center. Yield moment of the shell-cavity interface, representing failure of fixation, was calculated from the output of force, linear, displacement, and time. Two sample T-tests were conducted to determine statistical significance.Introduction
Materials and Methods
During activities of daily living (ADL), varus moments are experienced in the knee, which can result in frontal plane rotation, or liftoff, of the lateral femoral condyle with respect to the tibial plateau. An understanding of this rotation is valuable as it could potentially lead to contact between the femoral component and polyethylene post of a total knee replacement (TKR). Therefore, the purpose of this study was 1) to assess how much frontal plane rotation was achieved due to varus moments imposed on a total stabilized (TS) TKR from the stair ascent activity, and 2) to determine whether a TS TKR could withstand the contact stresses imposed by the varus loading for 1 million cycles without the post fracturing or plastically deforming. A PS femoral component paired with a TS polyethylene insert and baseplate (Triathlon, Stryker, Mahwah, NJ) were aligned on a multi-axis testing system (MTS Systems Corp, Eden Prairie, MN) (Figure 1). Size 1 components were used as they represented the worst-case size for testing. The femoral component was fixed at 60 degrees of flexion, representing an angle of peak varus moment during stair ascent [1]. The peak varus moment used in this study was determined by scaling the data from In order to evaluate the frontal plane rotation achieved due to the varus moment with minimal influence from other loads, an FEA model of the physical test setup was used to determine the lowest joint compressive load that would allow testing to be stable. Given this, testing was completed with a constant joint compressive load of 1500 N (33% of that reported by Lastly, a validation test was run on a component with the polyethylene post notched at the medial distal aspect. The post fractured during testing indicating that the test could induce the clinical failure mode of interest.INTRODUCTION
METHODS
Frictional torque is generated at the hip joint during normal gait loading and motion [1]. This study investigated the effect of shell deformation due to press-fit on frictional torque generated at the articulating surfaces of cementless acetabular shells that incorporated fixed and dual mobility bearing designs. Figure 1 lists the study groups (minimum of n = 5). All groups were tested with a 50 mm Trident PSL shell (Stryker Orthopaedics, NJ) and a Ti6Al4V trunnion. Metal-on-Metal specimens were custom designed and manufactured, and are not approved for clinical use. The remaining groups consisted of commercially available products (Stryker Orthopaedics, NJ). All groups were tested with the shells in deformed and undeformed states. Deformed Setup: A two-point relief configuration was created in a polyurethane foam block (Figure 2) with a density of 30 lb/ft3 to replicate shell deformation due to press-fit [2]. The blocks were machined to replicate the press-fit prescribed in the shell's surgical protocol. Each shell was assembled into the foam block by applying an axial force at 5 mm/min until it was completely seated. Undeformed Setup: Each shell was assembled in a stainless steel block with a hemispherical cavity that resulted in a line-to-line fit with the shell OD. Frictional torque was measured using a physiologically relevant test model [3]. In this model, the specimen block was placed in a fixture to simulate 50° abduction and 130° neck angle (Figure 2). A 2450N side load was applied and the femoral head underwent angular displacement of ± 20° for 100 cycles at 0.75 Hz. The articulating surfaces were lubricated with 25% Alpha Calf Fraction Serum. Peak torque was observed towards the end or the beginning of each cycle where the velocity of the femoral head approaches 0 and the head changes direction. This torque is referred as maximum Introduction
Materials and Methods
Many tests have been published which measure frictional torque [1–4] in THR. However, different test procedures were used in those studies. The purpose of this study was to determine the effect of test setup on the measured friction torque values.
Introduction
Methods
The femoral head/stem taper modular junction has several advantages; it also has the potential to result in fretting [1]. Stability of the taper junction is critical in reducing the risk associated with fretting. The purpose of this test was to measure the strength of various commercially available head-stem taper combinations under torsional loads to determine the effect of taper geometry and material on the strength of this taper junction. CoCr femoral heads were tested with trunnions that were machined with both a large and small taper geometry, replicating commercially available stem taper designs, V40 (small) and C (large) (Table-1, Stryker Orthopaedics, NJ). The femoral heads were assembled onto the trunnions with a 2 kN axial force. A multi-axis test frame (MTS Corp, MN) was used to test the head-trunnion combination by dynamically loading with a torque of ± 5Nm and a constant axial load of 2450N for 1000 cycles at 1.5 Hz (Figure 1). Samples were submerged in 25% diluted Alpha Calf Fraction Serum (Hyclone, UT). Upon completion of the dynamic test, a static torque to failure test was performed where the axial force of 2450N was maintained and the trunnion was rotated to 40° at a rate of 3°/sec. The torque required to rotate the trunnion by 1° was determined for each specimen. Also, the torsional resistance, defined as change in torque/change in angle in the linear region of the torque-angular displacement data curve, was calculated for all the specimens. A limitation associated with the static test was that at 1° rotation it was difficult to differentiate between rotation of the trunnion inside the femoral head and physical twisting of the trunnion. Specimen groups were compared with a single-factor ANOVA test and a Tukey post hoc test at 95% confidence level.Introduction
Methods and Materials
Traditional instrumentation relies on rigid IM rods to determine the distal femoral resection which influences size and orientation of the femoral component. Anterior femoral bowing may unexpectedly affect implant sizing. The purpose of this study was to determine the sensitivity of a flexible rod to the femoral anterior bow versus a traditional rod. A database of 93 Asian bone models from CT images was utilized. The bones were subdivided into those having proximal third, distal third, or overall femoral bows. Only the latter group was selected for further analysis, which consisted of 54 with an average bow of 98cm (±20cm). The rigid and flexible rods were placed iteratively so that the proximal portion of the rod touched the anterior cortical-cancellous boundary and no portion of the rod protruded through that boundary. The flexible rod was allowed to flex, as a substantially thin central portion flexes exclusively in the sagittal plane. The relative angle difference between the position of the flexible and rigid rod were calculated. Three femura were chosen from the subset with bows of 123cm, 100cm and 78cm. The femura showed differences between the rigid and flexible rod of 7.5°, 4.5° while no significant angle measured for the smallest bow. Implants were virtually assembled onto the bones and the greatest bowed femur's component reduced one size from the rigid to the flexible rod orientation. The results of this study show that higher bowed femura yielded larger angular deviations between rigid and flexible rods. For higher bowed femura, the flexible rod allows smaller components to be implanted. The flexible rod serves the same purpose as a conventional rod by defining the distal valgus orientation but allows component orientation in the sagittal plane closer to the femoral bow.
Revision total hip replacements are likely to have higher complication rates than primary procedures due to the poor quality of the original bone. This may be constrained to achieve adequate fixation strength to prevent future “aseptic loosening” [1]. A thin, slightly flexible, acetabular component with a three dimensional, titanium foam in-growth surface has been developed to compensate for inferior bone quality and decreased contact area between the host bone and implant by better distributing loads across the remaining acetabulum in a revision situation. This is assumed to result in more uniform bone apposition to the implant by minimizing stress concentrations at the implant/bone contact points that may be associated with a thicker, stiffer acetabular component, resulting in improved implant performance.[2] To assemble the liner to the shell, the use of PMMA bone cement is recommended at the interface between the polyethylene insert and the acetabular shell as a locking mechanism configuration may not be ideal due to the flexibility in the shell [3]. The purpose of this study was to quantify the mechanical integrity of a thin acetabular shell with a cemented liner in a laboratory bench-top total hip revision condition. Two-point loading in an unsupported cavity was created in a polyurethane foam block to mimic the contact of the anterior and posterior columns in an acetabulum with superior and inferior defects. This simulates the deformation in an acetabular shell when loaded anatomically [4]. The application has been extended to evaluate the fatigue performance of the Titanium metal foam Revision Non-Modular Shell Sequentially Cross Linked PE All-Poly Inserts and its influence on liner fixation.
Neutron beam irradiation is currently being explored as an alternative modality to improve local control of sarcomas. The purpose of this study was to investigate the effects of a sarcoma-dose fast neutron therapy on the wear properties of standard and highly cross-linked polyethylene total hip arthroplasty liners Two groups of 28 mm I.D. polyethylene liners were used in this study – conventional polyethylene liners (N2vac: 3Mrads innitrogen, Howmedica Osteonic, Allendale, NJ), and highly cross-linked liners (Crossfire: 10.5MRads total radiation dose, Howmedica Osteonics, Allendale, NJ). All liners were sterilized in a oxygen free environment and stored in inert nitrogen packages. The plastic cups were sandwiched between two tissue-equivalent blocks to simulate the human hip region and brought to the fast neutron therapy unit. The neutron beam is produced in a super conducting cyclotron by bombarding an internal beryllium target with 48.5 MeV deuterons [d(48.5)+BE]. The cups were exposed to a dose of 15 Gy represented a typical neutron-dose given to a sarcoma patient. Wear testing was then performed utilizing a hip simulator (MTS, EdenPrairie, MN) with matched 28 mm diameter CoCr femoral heads. Physiologic loading was simulated with biaxial cross-path motion and peak loads of 2450 N. All tests were performed in 50 percent diluted alpha-calf serum(Hyclone Laboratories, Logan, UT) to simulate human serum exposure. Every 250,000 cycles the serum was changed and samples were removed from the machine, cleaned and weighed. The volume loss measurement shown below used the weight loss to calculate the wear rate. The wear rate was converted to volume loss by dividing by the density. The value is given as millimeter scubed per million cycles (mm3/mc). Phase one cups were tested within one month of radiation. Phase two cups served as soaked controls, and spent 7months in calf serum prior to wear testing. A total of five million wear cycles were performed for each cup to simulate five years worth of use. The Averaged volumetric wear loss data demonstrated significantly less wear in CrossfireÒ compared to N2vac in both neutron irradiated and non-irradiated samples. This suggests that in sarcoma cases of the hip involving adjuvant fast neutron therapy, highly crosslinked poly-ethylene should be utilized. Averaging all data there was no statistically significant difference between the neutron radiation and non-treated components for both material conditions (N2VacÒ and CrossfireÒ). A trend towards decreasing wear in phase two samples was noted which may represent a material change in the liners exposed to serum over time. In addition, larger than normal variability in wear rates was seen within each group. Further testing of these liners is planned to elucidate these phenomenon. Table 1: – Volumetric wear loss per group. UHMWPE Material Volumetric wear Loss (mm3/mc) Standard Deviation N2VacÒ Phase 1 29.6 1.6 N2VacÒ Phase 2 14.0 N/a Neutron treated N2VacÒ Phase 1 52.2 18.1 Neutron treated N2VacÒ Phase 2 20.9 0.8 CrossfireÒ Phase 1 3.0 0.9 CrossfireÒ Phase 2 2.0 0.6 Neutron treated CrossfireÒ Phase 1 2.5 0.5 Neutron treated CrossfireÒ Phase 2 1.9 0.03 Graph 1– Averaged volumetric wear loss values Sarcoma-dose (15Gy) fast neutron therapy adversely affects the wear of standard polyethylene acetabular cups. The wear rate of the conventional gamma-inert sterilized polyethylene increased by more than 50% following a 15Gy fast neutron treatment. The highly cross-linked polyethylene (Crossfire), on the other hand, was immune to the effect of neutron treatment at the same dose. The mechanisms responsible forth is difference are unclear and warrant further investigation. The clinical implication of this study is that for sarcoma patient receiving total hip replacement, highly crosslinked poly-ethylene rather than standard polyethylene should be used for the acetabular cup.
