Although total hip arthroplasty (THA) has been one of the most successful, reliable and common prosthetic techniques since the introduction of cemented low-friction arthroplasty by Charnley in the early 1960s, aseptic loosening due to stem-cement and cement-bone interface failures as well as cement fractures have been known to occur. To overcome this loosening, the stem should be mechanically retentive and stable for long term repetitive loading. Migration studies have shown that all stems migrate within their cement mantle, sometimes leading to the stem being debonded from the cement [1]. If we adopt the hypothesis that the stems debond from the cement mantle, the stem surface should be polished. For the polished stem, the concept of a double taper design, which is tapered in the anteroposterior (AP) and mediolateral (ML) planes, and a triple-tapered design, which has trapezoidal cross-section with the double tapered, have been popularized. Both concepts performed equally well clinically [2]. In this study, we aimed to analyze stress patterns for both models in detail using the finite element (FE) method. An ideal cemented stem with bone was made using three dimensional FE analyses (ANSYS 13). The cortical bone was 105 mm long and 7 mm thick and the PMMA cement mantle was 5 mm in thickness surrounding the stem. Young's modulus was set at 200 GPa for the bone and 2.2 GPa for the cement. Poisson's ratio was 0.3 for both materials. The bone-cement interface was completely bonded and cement-stem interface was not bonded in cases where a polished stem surface was used. The two types of stems were compared. One being the double tapered (Fig 1 left) and the other the triple tapered (Fig 1 right). The coefficient of friction (μ) at the stem-cement interface was set at 0 for both models. The distal ends of the stems were not capsulated by the PMMA and therefore the stems were free to subside. All materials were assumed to be linearly isotropic and homogeneous. The distal ends of the bone were completely constrained against any movements and rotations. An axial load of 1200 N and a transverse load of 600 N were applied at the same time simulating the bending condition [3].Introduction
Methods
Dislocation continues to be a common complication of total hip arthroplasty (THA) [1]. Although many factors affect the prevalence of dislocation, achieving proper intraoperative soft tissue tension is one of the main surgical goals to reduce this risk. However, a sensor to measure the soft tissue of ball joints i.e. hip and shoulder has not yet been developed. The sensor enables surgeons to adjust the size or position of the implants depending on soft tissue tension. Hence, we have developed a sensor-instrumented modular femoral head for THA to measure soft-tissue tension intraoperatively [2]. This study demonstrates the possibility of a soft tissue tension and joint angle data connection using a wireless system. The sensor-instrumented modular femoral head that we developed was made of epoxy resin with linear strain gauges (BTM-1C, Tokyo Sokki, Japan) inside the head and a triple-axis gyroscope (MPU-6500). Strain outputs and angle data from the gyroscope were transferred to a computer via a 2.4 GHz wireless link (RN42, Bluetooth Module). Data logging was performed by a custom program using C++ (Microsoft Visual Studio 2012) via both wired and wireless link. The strain gauges were embedded inside the head. For the calibration study, the sensor was fixed in a clamping block of an angle vice to permit changes in the direction of force. The calibration jig with the angle vice was placed on top of a low-friction two-dimensional translation table that eliminated horizontal constraints. A constant vertical force was applied using a vertical die set. The experimental setup is shown in Fig. 1. Instead of a portable battery, a DC electric power supply is used (bottom left). A picture of the Gyroscope and the radio module is inserted (bottom right). The force values and applied angles were changed recording strain gauge and angle outputs.Introduction
Materials and Methods
