Comparative left-right mechanical testing of cancellous bone from normal femoral heads.

In order to assess the potential influence of various physical or chemical treatments on bone grafts, and in particular, on femoral heads to be used as bone grafts, the mechanical symmetry of the femoral head was investigated. Pairs of proximal human femora were harvested and transversally sliced using a coordinate system to orient correctly the head. After being embedded and polished, bone slices were loaded to failure between two steel columns (6 mm of diameter) in a sequential and symmetrical manner for both heads. From ten pairs, 592 mechanical assays on the cancellous bone of femoral heads were performed. There was a high variation in the mechanical properties of the specimens from the different donors. However, no statistical difference could be observed within each pair when side-to-side symmetry was tested for the different mechanical characteristics. Ultimate stress at failure and stiffness were also highly correlated for each mirror location of each pair. Provided that an accurate and reproducible method of slicing femoral head can be achieved, an excellent mechanical symmetry is observed using a static compressive test. Such a method could be used to determine the mechanical influence of any treatment applied to bone.

Concept, design and fabrication of smart orthopedic implants.

Extensive clinical experience has been built up using orthopaedic implants instrumented with strain gauges connected to a Wheatstone bridge by means of percutaneous leads. This research showed the medical relevance of the monitoring of the deformation of implants as a powerful tool to evaluate nursing and rehabilitation exercises, for tracing dangerous overloads and anticipating implant failure and also to observe the healing process. The IMPACT 3500 project focuses on the instrumentation of femoral implants with on board sensors: regular Benoist-Girard implants have been modified, to contain a 'sensing cell', and thoroughly tested in vitro and in vivo. The implant deformations are measured with resistive strain gauges, and the signal is transferred to a personal computer for processing and display, via a hard wired connection, or via a telemetry system. Two fully implantable wireless designs, called Linkstrain and Sealstrain, are powered from the outside by magnetic induction. As Sealstrain contains the whole telemetric system in its cavity, the highest miniaturization was required; this seriously deteriorates the efficiency of the inductive power link.

In vivo bone strain measurements: clinical results, animal experiments, and a proposal for a study of bone demineralization in weightlessness.

An extensive review of the literature of the aetiology of bone demineralization in weightlessness underline the prevalent effect of mechanical stimulations on bone structure and metabolism. The accurate determination of the mechanical environment of bone can be realized using strain gauges. The results of more than 10 years of clinical application of bone strain measurements demonstrate the interest of this technic. We designed an implantable bone strain transducer composed of a strain gauge embedded in epoxy resin and surrounded by a porous shell. The fixation of the transducer is by bone ingrowth. Results on animals and an acute experimentation on man are presented. The objective of our study in space is to compare the mechanical response of bones of an animal or of a human submitted to normal 1-G gravity and to weightlessness. The average amount of bone strains and their variations during specific movements can be compared.

Feasibility of bone healing measurement with external fixation: experimental study.

The measurement of fracture healing is a problem before us now; many techniques are now being developed. One of them is strain gauge measurement, allowing control of the development of the callus stiffness. The mathematical models and the experimental controls always had to deal with "preliminary assumptions." Theoretically, the output of a single strain gauge bonded on the fixation rod drops rapidly accordingly to the quality of the healing. Our aim is to verify experimentally the validity of the method in cases where the "preliminary assumptions" are not strictly true.We investigated the influence of the fixation rod on the loading of the tibia using strain gauges bonded on the bone. The tibial cortex was afterwards weakened by a 1 mm broad saw cut (and later a 5 mm one) in the middle of the bone. We again found the hyperbolic pattern of the theoretical curves.The experiments confirm the theoretical result: the measurement of stress drops rapidly as the degree of healing increases. An asymmetrical healing does not seem to modify significantly the pattern of the curve.

Twenty years experience in fracture healing measurement with strain gauges.

The external fixator provides a mechanical connection between the bone fragments and recording devices or power systems. We have used this characteristic of external fixation since 1965 to record deformations occurring at the fracture site and to measure the mechanical characteristics of fracture healing in more than 500 patients. As in all osteosyntheses, the fixation material (here the external rod) resists the mechanical forces applied to the bone fragments. As healing progresses, the bone participates more and more in the weight bearing of the system and the rod deforms less. Seven types of deformation curves have been defined, characterizing seven different modes of healing: 1) normal healing, 2) slow healing, 3) delayed union, 4) arrest in evolution, 5) pseudarthrosis, 6) resorption of the callus, and 7) breakage of the callus.

[Contribution to the study of pelvic stress during weight-bearing. Role of the pubic branch and trabecular bone]. / Contribution à l'étude des contraintes du bassin en charge. Rôle de la branche pubienne et de l'os spongieux.

INTRODUCTION: Several authors have reported evidence of loosening of the acetabular component in 14 per cent of cases at 10 years follow-up. To understand this phenomena, parametric studies of the acetabulum have involved finite element analysis, photo elastic models or strain gauges. The differing and sometimes contradictory results were due to the way the model was solicited. If the classic "Resultant" is accurate for the study of the forces on the hip, Pauwels described other forces on the pelvis during the gait. The effect of the application of these other forces on the acetabulum are the object of this study. MATERIALS AND METHOD: Four pelvises were loaded with different forces. Nine strain gauges were pasted around the acetabulum. The pelvis was loaded up to 140 Kg and a compressive force was applied to the pubis by 10 Kg increments up to 40 Kg. The stress data with and without the second force were recorded. A cortical window was bored on the anterior inferior iliac spine. Through this patch, the trabecular bone of the acetabular roof was cut up to disturb its mechanical properties. The load was once again applied to the pelvis and the stress data recorded. RESULTS: The compressive force applied to the symphyseal surface decreases the deformation of the posterior acetabular rim and increases deformation in the proximity of the ischio-iliac and iliopubic junctions. The disruption of trabecular bone generates an increase in deformation mainly perpendicularly to the acetabular rim. DISCUSSION: The decrease of the stress on the posterior acetabular rim is interpreted as a reduction of the solicitations on a fragile zone and its increase on the iliopubic and ischio iliac junctions expresses the application of the acetabular horn on the femoral head permitting better settling of the hip during the gait. The constatation of a stress increase around the acetabulum after disruption of the traecbular bone is interesting. The bone behavior with different acetabular cup models on different pelvis may be studied by strain gauges pasted on the cortical bone. The variable quality of the trabecular bone may introduce an error factor in the measurement. CONCLUSION: The deformation of the acetabulum during gait has to be studied following the different forces described by Pauwels. The compressive force on the pubic symphysis during one leg stance permits a decrease of the stress on the acetabular rim and an increase in the setting of the femoral head. Different qualities of trabecular bone change the deformation of the cortical bone for an equivalent load. This incites us, in comparative studies, to be careful in the deduction of the acetabular roof stresses from the recorded cortical bone deformations.