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.

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.

[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.

BOP: biocompatible osteoconductive polymer: an experimental approach.

BOP (biocompatible osteoconductive polymer) is a material proposed for osteosyntheses and for filling of bone defects in orthopaedics, neurosurgery and stomatology. It is a composite made of a copolymer of N-vinylpyrrolidone and methylmethacrylate, of polyamide-6 fibers and of calcium gluconate. The histological investigation includes the study of 30 intact rabbit femurs instrumented with a BOP rod, as well as the study of organs of the reticuloendothelial system. The currently available results show the absence of toxicity on hematopoietic tissue. Zones of osteoblastic activity surround the rods, coupled with an osteoclastic reaction which may result in the partial fragmentation of the polyamide fibers and its incorporation in the newly formed bone. We also observed the encapsulation of the material. The biomechanical approach investigated the mechanical properties of the material in bending and in shear. The radiological aspects of the investigation consisted of computerized axial tomography of the implanted femurs to measure density at the bone-implant interface.

Osteosynthesis of the proximal femur anchorage of a cervical nail.

One of the factors determining the stability of osteosynthesis is the mechanical strength of the bone fragments required for the anchorage of the implant. The aim is to study the driving of a Thornton nail in the proximal epiphysis of a human femur as a way to measure the strength of the trabecular bone and to predict the stability of the implanted system.

Morphometric and physical investigations of segmental cortical bone autografts and allografts in canine ulnar defects.

Cortical bone grafts were implanted for six months in mature dogs using an osteoperiosteal 3-cm defect in the ulna to evaluate their respective morphometric and physical values compared with autografts. The bone-grafting material included fresh auto- and allografts, frozen and thimerosal preserved allografts, and partially demineralized bone allografts. The grafts were evaluated by roentgenograms, microradiograms, photon absorptiometry, porosity, fluorescence labeling measurements, and torsional loading at failure. Autografts achieved a better union score than the allografts, but intracortical bone porosity, percentage of cumulative new bone, and mineral apposition rate were not variables with statistical significance. Lamellar bone was found earlier and in greater quantity in autografts. Within the graft, new bone was deposited at a slower rate than in the recipient bone. Autografts showed less peripheral resorption and a greater torsional resistance than allografts. Photon absorptiometry demonstrated that nondemineralized allografts underwent a substantial loss of peripheral bone. This marked reduction in the outer diameter of the graft had more influence on torsional resistance than did the intracortical porosity of the graft. Demineralized allografts were osteoinductive in only 28% of the cases and appeared to respond in an all-or-nothing pattern. Frozen and thimerosal preserved allografts were the most acceptable substitutes to autografts.

Cancellous bone and mechanical strength of the femoral neck.

Twenty pairs of dried macerated femora were submitted to progressive, physiologically oriented loading. The aim of this work was to determine the role of trabecular bone, the importance of the bone mineral density of the femoral shaft, and the importance of the Singh index in the mechanical strength of the femoral neck. By means of an original technique, the influence of both the principal tensile and secondary compressive trabecular groups on the mechanical strength to bending stress has been demonstrated. The artificial destruction of these trabecular groups is responsible for a loss of strength of more than 50% in varus angulation while it has practically no effect in valgus angulation. On the other hand, the mechanical strength of the femoral neck is better correlated with the bone mineral density of the femoral shaft (r = 0.74) than with the Singh index (r = 0.50) or with age (r = 0.15).

Pharmacodynamic profile of the selective beta 1-adrenoceptor antagonist bisoprolol.

The pharmacodynamic activity of (+/-)-1-[4-(2-isopropoxyethoxymethyl)-phenoxy]-3-isopropylamino-2- propranol- hemifumarate (bisoprolol, EMD 33 512) has been investigated under in vitro and in vivo conditions. Bisoprolol was found to be an effective beta-adrenoceptor antagonist, the pA2 values determined against isoprenaline in guinea pig atria and tracheal muscle being 7.45 and 6.41, respectively. Thus, the selectivity ratio of bisoprolol in favour of beta 1-adrenoceptors is 11. Inhibition of the isoprenaline-induced tachycardia in guinea pigs indicated a long duration of action for bisoprolol. The compound was devoid of intrinsic sympathomimetic activity as shown by the lack of effect on heart rate in anaesthetized and reserpine pretreated rats. Studies in rabbits and guinea pigs revealed a local anaesthetic activity of bisoprolol at high concentrations. Bisoprolol protected the hearts of anaesthetized dogs against the sequelae of intermittent coronary occlusions, as judged by the reduction of the ST-segment elevation in the epicardial ECG. Bisoprolol exerted a blood pressure lowering effect in conscious renal hypertensive dogs after oral administration of 30 micrograms/kg. There was no indication of any action on the CNS in monkeys following an oral dose of up to 8 mg/kg.

Canine cortical bone autograft remodeling in two simultaneous skeletal sites.

The morphological and physical aspects of cortical bone autografts implanted in dogs for 1-9 months in two differently located skeletal defects are reported with a twofold aim: to provide a reference system for further comparison with various allografts and to delineate a general pattern of cortical bone graft healing. A 3-cm osteoperiosteal gap was created in the diaphyseal segment of the ulna and fibula of mature dogs. The grafts, freed from periosteum and bone marrow, were then inverted and replaced for the autografts in the left limb bone without internal fixation or external splints. On the right side, different allografts were tested. A group of three animals also had an unfilled segmental resection on the right as control. Dogs were observed for 1, 2, 3, 6, and 9 months and were able to bear weight within 3 days. Twenty-eight ulnae and 27 fibulae were available for this autograft study. Fluorochromes were injected at mid-term and at the end of the observation. All the grafts were assessed morphologically by cross-section microradiographs and ultraviolet light microscopy, and a morphometric analysis for porosity and fluorescence was done. To evaluate the physical aspects of graft healing, the recovered ulnar autografts, when available, were submitted to photon absorptiometry and to torsional loading. Morphologically, resorption was found to invade the cortical bone graft transversely through radial tunnels, and in addition to the host-bone-graft junction, the entire transplant surface provided another way for revascularization. The highest porosity level was achieved 2 months after surgery for both ulna and fibula, while new bone formation, as assessed by fluorochromes, was most important at 3 months. At 9 months, porosity remained above the normal range as determined in a set of five nongrafted dogs. While the lack of correlation for porosity between the two grafts suggests that local factors are more important in graft resorption, the observed correlation for fluorescence indicates that new bone deposition is more dependent upon skeletal metabolic activity. Within each graft, porosity and new bone formation were not well correlated. In the ulna, the bone mineral content (BMC) reflected the graft volumetric variations during the remodeling, with the lowest mean value at 3 months. For each graft, BMC was well correlated with the torsional stiffness. When torsionally loaded, the maximal tangential shear stress at failure of the graft was negatively related to its cortical porosity.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.