Design and calibration of load sensing orthopaedic implants

Contact forces and moments act on orthopaedic implants such as joint replacements. The three forces and three moment components can be measured by six internal strain gauges and wireless telemetric data transmission. The accuracy of instrumented implants is restricted by their small size, varying modes of load transfer, and the accuracy of calibration. Aims of this study were to test with finite element studies design features to improve the accuracy, to develop simple but accurate calibration arrangements, and to select the best mathematical method for calculating the calibration constants. Several instrumented implants, and commercial and test transducers were calibrated using different loading setups and mathematical methods. It was found that the arrangement of flexible elements such as bellows or notches between the areas of load transfer and the central sensor locations is most effective to improve the accuracy. Increasing the rigidity of the implant areas, which are fixed in bones or articulate against joint surfaces, is less effective. Simple but accurate calibration of the six force and moment components can be achieved by applying eccentric forces instead of central forces and pure moments. Three different methods for calculating the measuring constants proved to be equally well suited. Employing these improvements makes it possible to keep the average measuring errors of many instrumented implants below 1-2% of the calibration ranges, including cross talk. Additional errors caused by noise of the transmitted signals can be reduced by filtering if this is permitted by the sampling rate and the required frequency content of the loads.     

Evaluation of cement stresses in finite element analyses of cemented orthopaedic implants.

Stress analysis of the cement fixation of orthopaedic implants to bone is frequently carried out using finite element analysis. However the stress distribution in the cement layer is usually intricate, and it is difficult to report it in a way that facilitates comparison of implants for pre-clinical testing. To study this problem, and make recommendations for stress reporting, a finite element analysis of a hip prosthesis implanted into a synthetic composite femur is developed. Three cases are analyzed: a fully bonded implant, a debonded implant, and a debonded implant where the cement is removed distal to the stem tip. In addition to peak stresses, and contour and vector plots, a stressed volume and probability-of-failure analysis is reported. It is predicted that the peak stress is highest for the debonded stem, and that removal of the distal cement more than halves this peak stress. This would suggest that omission of the distal cement is good for polished prostheses (as practiced for the Exeter design). However, if the percentage of cement stressed above a certain threshold (say 3 MPa) is considered, then the removal of distal cement is shown to be disadvantageous because a higher volume of cement is stressed to above the threshold. Vector plots clearly demonstrate the different load transfer for bonded and debonded prostheses: A bonded stem generates maximum tensile stresses in the longitudinal direction, whereas a debonded stem generates most tensile stresses in the hoop direction, except near the tip where tensile longitudinal stresses occur due to subsidence of the stem. Removal of the cement distal to the tip allows greater subsidence but alleviates these large stresses at the tip, albeit at the expense of increased hoop stresses throughout the mantle. It is concluded that a thorough analysis of cemented implants should not report peak stress, which can be misleading, but rather stressed volume, and that vector plots should be reported if a precise analysis of the load transfer mechanism is required.     

Digital stereophotogrammetry based on circular markers and zooming cameras: evaluation of a method for 3D analysis of small motions in orthopaedic research

Background  

Orthopaedic research projects focusing on small displacements in a small measurement volume require a radiation free, three dimensional motion analysis system. A stereophotogrammetrical motion analysis system can track wireless, small, light-weight markers attached to the objects. Thereby the disturbance of the measured objects through the marker tracking can be kept at minimum. The purpose of this study was to develop and evaluate a non-position fixed compact motion analysis system configured for a small measurement volume and able to zoom while tracking small round flat markers in respect to a fiducial marker which was used for the camera pose estimation.     

Exocytosis of cytolytic T-lymphocytes studied by cryofracture and stereophotogrammetry

Cryofracture, stereophotogrammetry and three-dimensional reconstruction were used to study the membranes of cytolytic T lymphocytes (CTL) and target cells (TC) conjugates. Circular bulges free of intramembrane granules and ring-shaped structures, analogous to the sites of vacuole release in secretory cells, appear on the surface of the lymphocyte plasmalemma after 30 to 60 minutes of interaction with TC. Polymorphic vacuoles 70 to 140 nm in diameter are observable in the contact space between lymphocytes and TC. Part of the vacuoles are found on the surface or near the CTL plasmalemma. The data confirm an assumption about the secretory mechanism of the cytolytic effect of T killers.     

Preclinical cost analysis of orthopaedic implants: a custom versus standard cementless femoral component for revision total hip arthroplasty

A preclinical cost analysis method was introduced to assess the cost effectiveness of using a custom implant instead of standard “off-the-shelf” implants for revision total hip arthroplasty. Finite element models of proximal femur–implant systems were constructed and an array of environmental factors, including loads and bone properties, was incorporated into a computer experiment to evaluate relative motion between implant and bone. Implant performance related cost was then determined from relative motion measures using a quality loss function. Unit manufacturing cost was added to implant performance cost to determine the cost difference between the two implants. The reduction in relative motion achieved by the custom implant with respect to an equivalent-lengthed standard implant justified its additional unit manufacturing costs. In response to these results and suggestions by surgeons, we increased the length of the standard implant by 50 mm and performed an identical series of analyses. We found that increasing the stem length to 120 mm substantially decreased the relative motion of the standard implant to values less than for the custom implant. This case study provides preliminary evidence that a surgical inventory consisting of longer-stemmed standard implants or modular distal stems is more cost effective than designing custom devices on a case-by-case basis. Additional design studies are warranted before generalizing such a claim.