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
Studies have indicated that the shallow Ultra High Molecular Weight Polyethylene (UHMWPE) acetabular socket or the socket with no head center inset can significantly increase the risk of hip joint dislocation. A previous study suggested the rim loading model in UHMWPE socket and metal femoral head can generate an intrinsic dislocating force component pushing head out of socket. Recently there has been renewed interest in dual mobility articulations due to the excellent stability. The outer bearing couple of the dual mobility articulations are comprised of the UHMWPE femoral head and metal acetabular socket while inner bearing is the locked conventional metal-poly construct. The acetabular socket is also featured by an anatomically shaped head inset wall. The purpose of this study was to theoretically compare the intrinsic dislocating force between conventional metal head on UHMWPE socket articulations and the poly head on metal socket articulations used in the dual mobility cup under direct loading. The 3-D finite element analysis (FEA) models were same as previous study but with different material combinations. Sixty FEA model assemblies were consisted of CoCr or UHMWPE femoral heads and their corresponding 10mm thick generic UHMWPE or CoCr acetabular sockets. There were five different head center insets of 0, 0.5, 1, 1.5 and 2mm for each of six bearing diameters of 22, 28, 32, 36, 40 and 44mm for either sockets. The joint load of 2,446N was applied through the femoral head center as the same fashion as previous study. The dislocating force generated by the joint loading force intrinsically pushed femoral head out of socket. FEA results were verified with two data points of physical testing of actual UHMWPE 28mm ID liners with 0 and 1.5mm head center insets. The highest dislocating force was 1,269N per 2,446N of rim loading force for the 0mm head center inset in poly cup with 22mm CoCr femoral head or the case of easiest to dislocate. The lowest dislocating force was 17.7N per 2,446N force for the 2mm inset in CoCr socket with 44mm poly head which therefore was the least likely to dislocate. The average dislocating force decreased by 78% from metal head- poly cup couple to poly head - metal cup couple. The dislocating force decreased as the head center inset and head size increased in all material cases. The study suggests that not only the head center inset and head size but also the bearing material combinations can affect the intrinsic dislocating force component. The dual mobility poly head and metal socket couple generates less intrinsic dislocating force in all comparable conditions for conventional metal head and poly socket couple. During the hip separation and vertical placement of the cup, all variables found in this study may play the important rules to maintain joint stability. The stiffened cup rim reduces the deformation and thus reduces the potential cup wedge effect to generate dislocating force. The result of this study should provide the guidance to improve acetabular cup design for better joint stability.
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.
Taper locking connection has been widely used in orthopedic implant devices. The long term successful clinical results indicated it is a safe and effective structural component. The common materials used are solid titanium and cobalt chromium alloys. Recently, foam metal materials showed promising results of bony in-growth characteristics and became the excellent choices for the orthopedic implants. Clinically it is desirable to taper lock the foam metal component to other structural components. To date there is no data for the foam metal being used directly in taper connection. The purpose of this study was to investigate the static locking strength of the taper junctions made of titanium foam metal comparing to that of conventional solid titanium material. (5) 43mm long and 4mm thick sleeve were machined internally with 17mm major diameter and 3° included taper angle for each 70% porosity CP titanium foam metal and solid Ti6AL4VELI alloy materials. (10) Solid Ti6AL4VELI alloy stems were machined with OD geometry matching the ID of the sleeves. All components were inspected, cleaned and assembled to (5) pairs of each sleeve material combinations with 2224N axial compression force. Each assembled specimen was mounted on MTS Bionix test machine for torque resistance test. The angular displacement at 0.1 degree/sec was applied to the stem when sleeve was rotationally locked. The maximum torque resistance was recorded. The specimen was then re-assembled with 2224N axial compression force. Axial push out test was performed by loading at smaller end of the stem when the opposite end of sleeve was supported. The maximum push out force was recorded. Procedures were repeated for all foam metal and solid metal specimens. The taper interface surfaces were visually inspected to compare two types of sleeve materials. The average torque resistance for foam metal and solid tapers were 20.4Nm (SD=3.68) and 21.7Nm (SD=3.72) respectively (p=0.59). The average axial locking forces were 2035.7N (SD=201.11) for foam metal taper and 1989.3N (SD= 451.84) for solid taper (p=0.839). There was no visual difference observed for tested stem outer and sleeve inner surfaces of foam metal and solid metal pairs. This study suggested that the foam metal sleeve is capable to have comparable taper locking strength as the conventional solid taper components under dry static condition. The study indicated that the contact area does not significantly influence the friction locking. This is in agreement with the friction force definition which depends only on the coefficient of friction and normal contact force.
Previous studies suggested that the shallow Ultra High Molecular Weight Polyethylene (UHMWPE) acetabular socket liner or the liner with no head centre inset can significantly increase the risk of hip joint dislocation. Independent to the traditional neck impingement models, the purpose of this study was to investigate an additional dislocation force pushing the femoral head out of UHMWPE acetabular liner bearing under direct hip joint loading and the factors including the head centre inset affecting the magnitude of this force. The 3 D Finite Element Analysis (FEA) models were constructed by (30) 10 mm thick UHMWPE liners with six inner bearing diameters ranging from 22 mm to 44 mm and five head centre insets in each bearing size from 0 mm to 2 mm. A load of 2 446 N was applied through the corresponding CoCr femoral head to the rim of the liner. The DF was recorded as a function of head centre inset and head diameter. The results were verified by the physical tests of two 28 mm head bearing liners with 0 and 1.5 mm head centre insets respectively. The results showed that the highest DF was 1 269N in 0 mm head centre inset and 22 mm head. The lowest DF was 171 N in 2 mm head centre inset and 44 mm head. The DF decreased as the head centre inset and head size increased. When head centre inset increased from 0 mm to 1 mm, the DF was reduced more than 50%. Two experimental data points were consistent with the trend of DF curve found in the FEA. We concluded that the new intrinsic dislocating force DF can be induced by the rim directed joint loading force alone and can reach as high as 51% of the femoral loading force. This can be the addition to the dislocating moment generated by the neck impingement. A head inset above 1mm can effectively reduce DF to less than 25% of the joint force. Furthermore, the larger head diameter generates less DF. The DF is likely caused by the wedge effect between the deformed polyethylene bearing and the femoral head. The inset allows the femoral head to be separated from the spherical bearing surface, thus reducing the wedge effect. Our observation of the stabilizing effect trend of the head centre inset was consistent with reported clinical data. However, the increased height of the capture wall also reduces the range of motion. It is therefore necessary to minimize the inset height with the maximum benefit of the stabilize effect. This study suggested the larger femoral head has the advantage of reducing the DF and the stabilizing effect is more effective when combining with the inset wall. The result of this study should provide the guidance to improve acetabular poly liner design for better joint stability.
Previous studies suggested the lack of capture wall of acetabular Ultra High Molecular Weight Polyethylene (UHMWPE) liner can significantly increase the risk of hip joint dislocation. To date, the dislocation studies have been focused on the femoral neck impingement models. The purpose of this study was to identify a new Dislocating Force (DF) generated by rim directed joint force alone and investigate the factors to affect the magnitudes of the DF. The 3 D Finite Element Analysis (FEA) models were constructed by (30) 10 mm thick UHMWPE liners with six inner bearing diameters ranging from 22 mm to 44 mm and five capture wall heights in each bearing size from 0 mm to 2 mm. A load of 2 446 N was applied through the corresponding CoCr femoral head to the rim of the liner. The DF was recorded as a function of capture wall height and head diameter. The results were verified by the physical tests of two 28 mm head bearing liners with 0 and 1.5 mm capture wall heights respectively. The results showed that the highest DF was 1 269N in 0 mm capture wall and 22 mm head. The lowest DF was 171 N in 2 mm capture wall and 44 mm head. The DF decreased as the capture wall and head size increased. When capture wall increased from 0 mm to 1 mm, the DF was reduced more than 50%. Two experimental data points were consistent with the trend of DF curve found in the FEA. We concluded that the new intrinsic dislocating force DF can be induced by the rim directed joint loading force alone and can reach as high as 51% of the femoral loading force. A capture wall height above 1mm can effectively reduce DF to less than 25% of the joint force. In addition, the larger head diameter also resulted in less DF generation.
