Optical interferometry analyzes the body

April 1, 2001
The study of mechanical stresses on the human body assists in the design of useful medical solutions. Speckle interferometry is one method that gives engineers something to work with.
The study of mechanical stresses on the human body assists in the design of useful medical solutions. Speckle interferometry is one method that gives engineers something to work with.

John R. Tyrer

It is a fascinating and challenging task to try to understand how the human body works. Unfortunately, the body does not easily reveal its biomechanical performance in a way that helps engineers and surgeons alleviate the aging process. Optical interferometry, traditionally the toolbox of the research laboratory, can assist in the study of several parts of the body, including the hip, jaw, and cornea.

Speckle interferometry, initially developed for the study of engineering structures, has revealed the subtle deformation of various parts of the human anatomy. A multidisciplinary approach is necessary to enable scientists and engineers to assist their counterparts in the medical professions in modifying the structural behavior of the human.

Hip replacementTotal hip replacement (THR) surgery is a popular and successful procedure in orthopedic surgery. Nearly a million such operations are performed every year. It is interesting to note that since its inception in the early 1960s, with more than 100 different designs, the life expectancy of a replaced joint has remained only 10 to 12 years. Understanding how natural bone behaves under the complex loading of the human body would allow design modifications that would increase the life expectancy of the prosthesis.
FIGURE 1. Prototype loading frame provides a representation of the knee and pelvis joints.
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Most biomechanical representations of this arrangement regard the entire femur as a single compressive strut and impose the compressive loads at the knee and hip joints. The loading frames we have developed incorporate tensile simulators of the muscles that attach to tendons of the femur. The task we faced was to accurately model the loading of the prosthetic implant in cadaveric femora and develop suitable optical measuring techniques for generating useful engineering data. In conjunction with surgeons at the Department of Orthopaedics, Glenfield University Hospital (Leicester, England) we devised a loading regime that required femurs with the key tendons intact, to which we attached our loading frame (see Fig. 1).

Modeling implantsHolographic interferometry is one method that can record three-dimensional deformation over an entire object surface. Extracting data from single holographic-interferograms using polar or Cartesian coordinate systems is very difficult and extremely error-prone. While academically interesting, comprehending the overall displacement of such highly complex structures requires a more practical instrument.

The speckle interferometer, which can be thought of as a derivative of the holographic system, possesses the advantage that, with dual-beam illumination, the instrument is sensitized to motion purely along the axis of illumination. This enables us to extract a single in-plane axis of motion from within the general movement of the bone. If two in-plane dual-beam interferometers are used, one orientated along the vertical axis of the bone and the other in the horizontal direction, then both sets of data can be combined to provide a complete quantitative two-dimensional map of displacement (see Fig. 2).

FIGURE 2. Electronic speckle-pattern interferometry reveals 2-D displacement in the human femur bone.
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Within the speckle interferometer, a television camera stores the initial digital image of the bone at some rest state and subtracts it from the image in real time. The result is a fringe pattern providing the displacement of the object along the optical axis of the two illuminating beams. Such an arrangement is known as electronic speckle pattern interferometry (ESPI), a noncontact displacement-measuring instrument developed at Loughborough University in the early 1970s. The lens aperture of the television camera governs the size of the speckle pattern. By varying this aperture, a unique random pattern covering the entire surface of the bone is imaged by the camera. The fringe pattern sensitivity is determined by the laser wavelength and the angle between the two illuminating beams. Each fringe represents a displacement, d, given by l/2sinq, where l is the wavelength of the laser and q is the beam separation angle (see Fig. 3).

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FIGURE 3. In electronic speckle-pattern interferometry, two laser beams combine to create a fringe pattern highlighting the displacement on an object, which is then imaged by a CCD camera

Two human cadaveric femurs were used for the studies. These retained the necessary tendons onto which tensile rods were attached that had previously been strain-gauged and calibrated to enable us to determine the loads applied during testing. The femurs are mounted upside down to facilitate data acquisition. The experimentation studied the bones in both their natural and implanted form. Loads were applied to a hydraulic ram, which simulated those generated during walking. Without any load on the abductor group, the fringes were skewed significantly and it was possible to adjust the abductor load and rotate the fringes, thus ensuring the main shaft was in pure compression.

Using phase-shifted speckle interferometry, it was possible to acquire the horizontal and vertical in-plane images of the head and shaft of the femur. A pre-load of the rig was necessary to establish stability of all components. Images were then recorded within the frame store and loading was then increased to simulate forces naturally occurring on the femur during walking. Identical loading arrangements were applied to both bones, in both their natural and implanted conditions. In this way a reasonable comparison of the implanted performance is possible. While differences can be observed between the original and implanted bones, it is perhaps more useful when the two data fields are subtracted from one another to reveal quantified changes within the structure.

Analysis of the human jawOrthognactic surgeons require measurement of the stress distribution within the human lower jaw to aid and improve surgical procedures. Using ESPI to study the dry human mandible under load, progressively greater forces were applied on the lower border of the chin, in the upwards (impact) direction with biting boundary conditions included in the experimental model. The mandible deformation was examined again using an in-plane ESPI system. The out-of-plane examination revealed negligible deformation contribution within the mandible in this load case. By performing two series of experiments, each using one orthogonal direction for the in-plane dual-beam laser illumination, it was possible to assess the whole in-plane deformation of the jaw (see Fig. 4).
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FIGURE 4. Studies of stress distribution on the human jaw can be useful for improving surgical procedures.

A cast of the teeth made with surgical cement was manufactured to allow the simulation of the biting boundary condition, providing uniform biting force among the teeth. In addition, the condyles of the mandible were mounted on a similar cement cast, which restricted movement of the mandible. The mandible was mounted in an inverted arrangement with the load acting vertically downward onto the "chin" section of the bone. Single-static mass-loading was used because of the delicate nature of the human mandible. The analysis of the fringe patterns indicates that the deformation of the bone takes place only in the chin's basal bone, while the alveolar bone, where incisors and canines are fixed, is not deformed at all. This has demonstrated the manner in which the jaw transfers load around the bone without damaging the teeth.

Study of cornea strainDuring the last 10 years, laser corneal refractive surgery has become increasingly popular for the treatment of myopia. Originally phototherapeutic keratechtomy (PRK) ablated less than 5% of the corneal thickness. More recently, laser in-situ keratomileusis (LASIK) uses a planar cut performed at approximately 160 µm from the corneal surface to generate a thin flap, which is hinged open to enable an excimer laser to ablate up to 60% of the corneal bed. The flap is then repositioned over the residual corneal wall. Although the flap angles downward, the fibers within the tissue may not grow back across the cut.
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FIGURE 5. Initial results obtained from use of ESPI on the corneal surface of a pig's eyeball, which suitably approximates that of a human eyeball, shows response to strain

This procedure may induce structural weakening of the cornea, which could lead to outward movement and subsequent degradation of the performance of the eye. The cornea is an elastic membrane. However, its deformation under an applied force is nonlinear and does not obey Hook's Law, which states that, for relatively small deformations of an object, the displacement is directly proportional to the deforming force or load. The cornea behaves like an anisotropic membrane, in which its response depends upon the orientation of the force supplied, similar to steel banding in a radial car tire. We have been working with St. Thomas' Department of Ophthalmology (London, England) to examine the performance of cornea deformation (see Fig. 5).

Another practical applicationIn another use of this technology, our study of live human soft tissue has resulted in a practical application—the determination of the flawed structural performance of the women's support garment, the bra. Some may argue that you don't have to be an optical engineer to determine this, but not only have we proven the bra does not work, but in reverse engineering the load case, we have been able to design support garments that genuinely do work.

John Tyrer recently received the Queen's Award for Innovation in Laser Technology and Optical Engineering. JOHN R. TYRER is a professor in the Department of Mechanical Engineering at the University of Loughborough, Leicestershire, England; e-mail: [email protected].

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