Ultrasound palpation for fast in-situ quantification of articular cartilage stiffness, thickness and relaxation capacity

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Most current cartilage testing devices require the preparation of excised samples and therefore do not allow intra-operative application for diagnostic purposes. The gold standard during open or arthroscopic surgery is still the subjective perception of manual palpation. This work presents a new diagnostic method of ultrasound palpation (USP) to acquire applied stress and strain data during manual palpation of articular cartilage. With the proposed method, we obtain cartilage thickness and stiffness. Moreover, repeated palpations allow the quantification of relaxation effects. USP measurements on elastomer phantoms demonstrated very good repeatability for both, stage-guided (97.2%) and handheld (96.0%) applications. The USP measurements were compared with conventional indentation experiments and revealed very good agreement on elastomer phantoms (\(r = 0.98\)) and good agreement on porcine cartilage samples (\(r = 0.76\)). Artificially degenerated cartilage samples showed reduced stiffness, weak capacity to relax after palpation and an increase of stiffness of approximately 50% with each single palpation. Intact cartilage was measured by USP directly at the patella (in situ) and after excision and removal of the subchondral bone (ex situ), leading to stiffness values of \(12.1\pm 5.5\) and \(8.5\pm 5.9\,\hbox {MPa}\) (\(p<0.05\)), respectively. The results demonstrate the potential of the USP system for cartilage testing, its sensitivity to degenerative changes and as a method for quantifying relaxation processes by means of repeated palpations. Furthermore, the differences in the results of in-situ and ex-situ measurements are of general interest, since such comparison has not been reported previously. We point out the limited comparability of ex-situ cartilage with its in-situ biomechanical behavior.

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The authors thank the team of GAMPT mbH for the collaboration within this project and the provision of the ultrasound hardware. The authors thank Dag Wulsten (Julius Wolff Institute) for technical support with the material testing machine.

Funding This study was funded by the German Federal Ministry for Economic Affairs and Energy (BMWi) as a ZIM cooperation project (KF2744004LW3). RMS was funded by the Free State of Saxony within the framework of research funding of biotechnology and life sciences (Grant 100243759).

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Correspondence to K. Raum.

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Schöne, M., Schulz, R.M., Tzschätzsch, H. et al. Ultrasound palpation for fast in-situ quantification of articular cartilage stiffness, thickness and relaxation capacity. Biomech Model Mechanobiol 16, 1171–1185 (2017).

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  • Articular cartilage
  • In-situ
  • Biomechanical testing
  • Stiffness
  • Ultrasound palpation
  • Indentation
  • Degeneration