An extensometer for global measurement of bone strain suitable for use in vivo in humans

TECHNOLOGIES FOR STRAIN ASSESSMENT FROM WHOLE BONE TO MINERALIZED OSTEOID LEVEL: A CRITICAL REVIEW

Bone has distinctive structures and mechanical properties at the whole bone, perilacunar and mineralized osteoid levels. A systematic understanding of bone strain magnitudes at different anatomical levels and their internal interactions is the prerequisite to advances in bone mechanobiology. However, due to the intrinsic shortcomings of the strain-measuring technologies, the systematic assessment of bone strain at different anatomical levels under physiological conditions and a deep understanding of their internal interactions are still restricted. To promote technological advances and provide systematic and valuable information for mechanical engineers and bone biomechanical researchers, the most useful methods for measuring bone strain at different anatomical levels are demonstrated in this review, and suggestions for the future development of the technologies and their potential integrated applications are proposed.

From conventional sensors to fibre optic sensors for strain and force measurements in biomechanics applications: a review

In vivo measurement, not only in animals but also in humans, is a demanding task and is the ultimate goal in experimental biomechanics. For that purpose, measurements in vivo must be performed, under physiological conditions, to obtain a database and contribute for the development of analytical models, used to describe human biomechanics. The knowledge and control of the mechanisms involved in biomechanics will allow the optimization of the performance in different topics like in clinical procedures and rehabilitation, medical devices and sports, among others. Strain gages were first applied to bone in a live animal in 40's and in 80's for the first time were applied fibre optic sensors to perform in vivo measurements of Achilles tendon forces in man. Fibre optic sensors proven to have advantages compare to conventional sensors and a great potential for biomechanical and biomedical applications. Compared to them, they are smaller, easier to implement, minimally invasive, with lower risk of infection, highly accurate, well correlated, inexpensive and multiplexable. The aim of this review article is to give an overview about the evolution of the experimental techniques applied in biomechanics, from conventional to fibre optic sensors. In the next sections the most relevant contributions of these sensors, for strain and force in biomechanical applications, will be presented. Emphasis was given to report of in vivo experiments and clinical applications.

A new method for measurement of bone deformation by echo tracking

No method has been available to noninvasively detect bone deformation or strain under loading in vivo. We focused on ultrasonic measurement of the displacement at a certain point on a bone using the echo-tracking method (ET). To develop a method that can noninvasively detect bone deformation in vivo, a preliminary investigation was performed. We investigated the accuracy of measuring displacement with our echo tracking system by using a flat metal panel and found that the method could measure displacement with a precision of a few microns. A three-point bending test of a porcine tibia with both ends fully constrained was performed to measure bone surface displacement, and simultaneous measurement of the surface strain was done using two strain gauges. The correlation between the displacement measured by ET and the strain gauge readings was completely linear (r=0.999), showing that the method could precisely detect bone deformation. The loads versus displacement curves obtained with cyclic loading were typical hysteresis loops that showed viscoelastic properties of the measured bone. We also improved a multi-ET system capable of simultaneously tracking multiple points to detect deformation of the bone surface. Measurement by this echo tracking system was also compared with strain gauge readings during a three point bending test with both ends of the tibia supported. The linearity of both methods was very high (r=0.998). Our ET method might have considerable potential for noninvasive measurement of bone viscoelasticity and plasticity.

Stress and strain distribution in the intact canine femur: finite element analysis

Information regarding the stresses and strains in the canine femur during various activities is important for veterinary orthopaedic surgeons, engineers designing implants for dogs, and researchers of human orthopaedics who use dogs as models. Nevertheless, such information is currently unavailable. The objective of this study is to determine the stress and strain distribution in the canine femur during mid-stance, for two loading scenarios. Three-dimensional finite element models of the canine femur were created. Two loading cases were considered: the hip joint reaction force alone, and the hip joint reaction force with all muscle forces acting on the femur. Force directions and magnitudes were obtained from the literature. Analyses were performed with NASTRAN for Windows software. When all muscle forces were considered, stresses and strains were significantly reduced, peak compressive stresses were found to occur in the medial diaphysis, and peak tensile stresses occurred in the lateral diaphysis. While the canine femur seems to be loaded primarily in bending when only the hip joint reaction force is considered, the bending moment is significantly decreased when all muscle forces are considered as well. Further in vivo and in vitro experiments are needed to validate the results of the calculations described in this paper. It is expected that future studies will be carried out, in which the stress and strain distributions in femora with different types of implants and stems will be compared to those in the normal femur.

Posterior-anterior glide of the femoral head in the acetabulum: a cadaver study

STUDY DESIGN

Descriptive study employing cadaver dissection and measurement of posterior-anterior (PA) glide of the femoral head in the acetabulum.

OBJECTIVE

To quantify PA glide of the femoral head in the acetabulum in a cadaveric sample.

BACKGROUND

Posterior-anterior glide of the femoral head within the acetabulum is a joint mobilization procedure described in orthopaedic physical therapy texts, yet there is no published evidence that the joint structures of the hip allow such movement. This study attempted to quantify PA glide of the femoral head in the hip joints of embalmed cadavers.

METHODS

Twelve hips, 3 male and 9 female, from 8 embalmed cadavers were employed in this study. Hips were dissected to the level of the joint capsule and a metal rod inserted through the femoral neck served as a mobilizing handle. A load cell was installed into this handle so that mobilizing forces could be monitored. A dial gauge, which recorded displacement of the femoral head, was mounted to the pelvis via bone pins and an external fixator.

RESULTS

Using mobilizing forces of 89, 178, 267, and 356 N, mean femoral head displacements of 0.57, 0.93, 1.20, and 1.52 mm were recorded. Within the 89-N trials, PA displacement ranged from a minimum of 0.04 mm to a maximum of 1.54 mm. Within the 356-N trials, PA displacement of the femoral head ranged from a minimum of 0.25 mm to a maximum of 2.90 mm.

CONCLUSION

In an embalmed cadaveric model, measurable PA glide of the femoral head within the acetabulum does exist and it is highly variable between individuals.