AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
Post-traumatic arthritis (PTA) is a debilitating joint disease which occurs as a result of direct trauma to a joint. The pathological changes throughout the process have been well documented, but the initiating factors remain unknown. One theory suggests that upon impact, the cartilage is irreversibly damaged, initiating the cascade of destruction leading ultimately to PTA. It is conceivable that impact stresses over a given threshold initiate the PTA cascade. This possible etiology has not been studied extensively, partly because no adequate animal model exists.
One of the difficulties in applying an impact of given stress is that a fundamental assumption must be made: -the stress over the given area must be uniform. Given that joint surfaces are of complex shapes, it is necessary to apply impacts with a device which contours precisely to the shape of the area to be impacted. The purpose of this research was to develop a methodology to impact femoral condyles with blows of uniform, quantifiable stress. Furthermore, a method to estimate the impact area was developed and tested.
Impactors were developed using dead adult New Zealand White rabbits. The rabbit femur was mounted on an impaction jig, with exposing the medial femoral condyle. Polymethyl methacrylate (PMMA) bone cement was mixed and used to fill cups of 7 mm diameter and 5 mm depth when the PMMA achieved a doughy consistency. Then, the exposed medial femoral condyle was pressed into the impactor cup, leaving a contoured imprint on the curing PMMA. After the PMMA hardened, Fuji pressure sensitive film was placed between the impactor and condyle. The impactor was pressed against the condyle with a 425 g weight, imprinting the pressure sensitive film. The uniformity of contact pressure between the impactor and condyle was assessed visually. The impact area was estimated by painting the impactor contour with a wax marker, then transferring the mark onto paper. The area imprinted on the paper was measured using a Digitizing tablet (GTCO, MD) with Research Metrics Software. The validity of this area measurement technique was validated by making 10 impressions of a 12 mm marble on impactor cups in the same manner that the femoral condyles were imprinted. The 10 imprints were made at different depths, to give a range of areas. These areas were measured in the same manner as the femoral condyles. The depth and diameter of the imprints was measured by micrometer, and these measurements plugged into the analytical expression for the surface area of a spherical base.
Figure 1 shows the comparison between the indentation area as estimated through the marker and calculated through the above equation. The mean difference between estimated and calculated areas was 0.007 cm2, which amounted to 2% deviation. The maximum difference was 0.04 cm2, which was 12% of that specific area. The excellent fit between these shows that the marker technique gives accurate determinations of the imprint area, despite the small distortions caused by representing a three-dimensional area on a two dimensional surface. Similarly, the pressure sensitive film shows that relatively uniform pressure is found throughout the impact area, as all of the film is uniformly developed, with no saturation within the imprinted area.
The technique developed to produce this custom impactor produces a contour which closely matches the condyle surface. Impacts are delivered after the PMMA hardens fully, which ensures that the impactor surface does not change during the course of the impact. The bone cement is also sterile, which makes it particularly well suited for survival experiments. Following the surgical procedure, the impactor can be removed, imprinted with a wax marker, and its area measured post-operatively. The excellent relationship given by comparing imprints of a known area with that measured by the digitizing tablet enables us to accurately measure the area impacted in the surgical procedure. Given this technique, and an impactor cup holder instrumented with force sensing strain gages, it is possible to deliver blows of quantifiable stress to ther femoral condyles of live rabbits. Consequently, a live animal model of the development of post-traumatic arthritis can be developed.
Moskowitz, st al., "Osteoarthritis Diagnosis and Management," W.B. Sanders, Philadelphia, 1984.
Repo, R.U., Finley, J.B. JBJS, 59A:1068-1076, 1975.
Supported by a Grant from the AO Foundation
