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
Dislocation continues to be a common complication of total hip arthroplasty (THA). Many factors affect the prevalence of dislocation after THA, including soft tissue laxity, surgical approach, component position, patient factors, and component design [1]. Achieving proper intraoperative soft tissue tension is one of the surgical goals to reduce the risk of the dislocation. However, reports of the intraoperative soft tissue tension measurements have not been enough yet. One way to quantify the intraoperative soft tissue tension is to measure joint forces using an instrumented prosthesis. Hence, we have developed a sensor-instrumented modular femoral head of THA to measure the soft-tissue tension intraoperatively. The goal of this study was to design and calibrate the sensor. The sensor-instrumented modular femoral head that we developed was made of polycarbonate with four linear strain gauges (BTM-1C, Tokyo Sokki Kenkyujo Co., Ltd., JP). To fabricate the sensor, four penetrant holes (1.6 millimeter in diameter), parallel to the coordinate axes were produced (Fig1). The strain gauges were embedded on inside wall of these holes. Finally, the holes were filled by epoxy resin (A-2 adhesive, Tokyo Sokki Kenkyujo Co., Ltd., JP). For calibration study, the sensor was fixed in a clamping block of an angle vice to permit change of force directions. The calibration jig with the angle vice was placed on top of a low-friction x-y translation table that eliminated horizontal constrains. Known forces (Fi) were applied by a standard material testing machine (Instron4204, INSTRON, Norwood, MA) through a polyethylene insert (Fig. 2). Two different series of forces were applied. One is that force values were increased from zero to 600 N on the z axis. And the other force pattern is 600 N forces were applied by changing force angles. The external force vector (Fi) can be expressed in terms of the strain gauge outputs as follows: Fi = T Si where T is a calibration matrix and Si corresponds to the outputs of the strain gauges. Calibration errors were calculated according to well-established methods [2].Introduction
Materials and Methods
Modular femoral stems of Total Hip Arthroplasty (THA) have been designed to fit the metaphysis and diaphysis separately. Clinical results with modular femoral stems are reported to be satisfactory, but there exists several concerns with modular implant connections, including fretting corrosion, fracture of implant, and dissociation the stem from the proximal sleeve. Recently, we have become aware of another potential consequence of the modular design: sleeve deformation secondary to forces encountered during insertion. In our patients, we noted that the stems would not fully seat in the machined taper of the sleeve, indicating that some type deformation to the sleeve had occurred. We began an in vivo study to characterize this phenomenon. The objectives of this study were (1) Does deformation occur by impacting the sleeve into the metaphysis? (2) If so, quantify the sleeve deformation in hip arthroplasty patients. One man and 7 women undergoing primary THA were enrolled. This project was approved by IRB. This modular system (4-U CLS; Nakashima Medical Co., Japan) consists of a metaphyseal sleeve that connects with the diaphyseal stem via a Morse taper. The sleeve was impacted into the metaphysis first, followed by the stem. A custom taper gauge for each size of sleeve (Figure 1A) was inserted into the sleeve before and after impacting the sleeve into the metaphysis, and the distance between the top of the sleeve and the top of the gauge was measured using a caliper (* in Figure 1B). Deformation was defined as the difference in distance between the before and the after impacted dimensions. Preoperative femoral morphology, assessed using Dorr classification system, was type A in 2 hips, type B in 5 hips, and type C in 1 hip.INTRODUCTION:
MATERIALS AND METHODS:
It is said that the mechanical stress is a main factor to advance degenerative osteoarthritis. Therefore, to keep the joint stability is very important to minimize mechanical stress. Methods to evaluate bone-related factor are almost established, especially in hip dysplasia. On the other hand, it is unclear how much each soft tissue contribute to the joint stability. In this study we evaluated the soft tissue contribution for hip joint stability by distraction testing using MTS machine. We used seven fresh frozen hips from four donors, whose race was all western and reason of death was not related to hip disease in all cases. Average age of them at death was 83 years old. Mean average weight and height were each 52 kg and 162 cm. We retrieved hemi pelvis and proximal femur which kept hip joint intact. We removed all other soft tissue except iliofemoral ligament, pubofemoral ligament, ischiofemoral ligament and capsule. The hemi-pelvis mounted on angular-changeable fixator and the femur fixed to MTS machine (Figure 1). XY sliding table was used to minimize the horizontal direction stress during distraction. MTS machine was set to pull the femur parallel to its shaft by 0.4 mm/sec velocity against pelvis after 10N compression and to keep 5 mm distance for 5 seconds. We measured the force at 1 mm, 3 mm, 5 mm distraction. In case the joint was dislocated, the maximum force just before dislocation was recorded. The specimen was changed its posture as neutral (flexion0° abduction0° external rotation0°), flexion (flexion60° abduction0° external rotation0°), abduction (flexion0° abduction30° external rotation0°) and extension (extension20° abduction0° external rotation0°). Each position was measured in six sequential conditions, which are normal, Incised iliofemoral ligament, Circumferentially incised capsule, resected capsule, labral radial tear and resected labrum. After measurement joint surface was observed to evaluate the joint condition.[Introduction]
[Materials & Methods]
