Microindentation sensor system based on an optical fiber Bragg grating for the mechanical characterization of articular cartilage by stress-relaxation

- A new indentation sensor based on fiber Bragg grating for cartilage stiffness characterization is presented.
- In vitro stress-relaxation indentation measurements up to 300 μm depth were conducted on two bovine condyle cartilage samples.
- A viscoelastic Maxwell-Wiechert model with five parameters was found to fit well to the relaxation behavior of the cartilage at all indentation depths.
- The potential of the sensor in distinguishing the mechanical characteristics of superficial and middle layer of cartilage was demonstrated.
- The potential of the sensor for detecting spatial stiffness variations was confirmed.

A novel microindentation sensor system with the potential to be integrated into arthroscopic instrumentation is presented to assess biomechanical properties of articular cartilage. The piezoelectric driven indentation device was realized by using the tip of an optical fiber with an integrated fiber Bragg grating for force sensing. The performance of the system was demonstrated by ex vivo stepwise indentation experiments performing 15 steps each 20 μm in depth on explants of bovine femoral condyle cartilage. Parameters describing the relaxation were extracted using a viscoelastic model, which includes two relaxation time constants, one in the order of 0.2-5 s and the other in the order of 10-40 s. All model parameters showed increasing values with indentation depth. The plots of the model parameters against the indentation depth suggested that the faster relaxation process was mainly influenced by the superficial zone of the articular cartilage while the middle zone cartilage contributed predominantly to the slower relaxation process. The small indenter size allowed the assessment of the spatial distribution of the mechanical characteristics of the cartilage, which was determined within a frame of 3 × 5 mm2 on each sample. A stronger dependence of the model parameters along the medial-lateral axis than the posterior-anterior direction of the knee was found. In conclusion the device presented showed the potential to discriminate between the superficial and middle zone of the articular cartilage and to assess with high resolution changes in the constitution of the tissue.