Excessive or abnormal joint loading that leads to cartilage degeneration has been associated with hip osteoarthritis (OA). Before preventative measures for OA can be designed, such as physiotherapy techniques, braces, or surgical interventions, the connection between load-bearing and cartilage degeneration needs to be validated experimentally. As a first step towards such a validation, a method of measuring the load distribution across the hip joint is needed; ideally that can be used in vivo and can detect changes in the load distribution during an applied load. The objective of this study was to assess the accuracy of using biplanar radiography combined with CT imaging to estimate hip cartilage strain across the joint as an indication of the load distribution.
Estimating cartilage strain using biplanar radiography and CT imaging is a multi-device multistep measurement protocol that has error associated with each step. While biplanar radiography systems are commonly assessed on their ability to measure radio-opaque bead locations, to the author’s knowledge no studies have quantified errors in the additional steps of estimating cartilage strain. The present study used a phantom hip joint to quantify the errors in measuring bone displacement with biplanar radiography, segmenting 3D joint surfaces from a CT image, and measuring the relative proximity of joint surfaces in the biplanar radiography coordinate frame. The quantified errors were much lower than ex vivo hip cartilage deformation results in the literature, which demonstrated the potential for using this technique to estimate cartilage strain in the hip.
As a proof of concept, cartilage strain was estimated in the ex vivo hip joint during a compressive load. Two hemi-pelvis/proximal femur specimens, with radio-opaque beads inserted in each bone, were loaded in compression in a materials testing machine, with biplanar radiographs acquired throughout. A small amount of cartilage deformation (0.1mm) was detected across the hip joint; however, due to the low load applied the deformation results were not comparable to the literature. The largest cartilage strains were identified in the anterior and superior regions, which was consistent with the literature. Future studies using higher loads are needed to further assess the capabilities of our system.
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