Microscale Mechanics and Composition of Articular Cartilage Repair Tissue Following Microdrilling

Introduction: Articular cartilage injuries, which cause discomfort and swelling, are present in 65% of knees that undergo arthroscopy [1,2]. However, articular cartilage is limited in its capacity to undergo repair due to its avascular nature, thus increasing the risk for osteoarthritis [2]. Microfracture and microdrilling repair techniques promote self-healing within cartilage by stimulating the bone marrow; perforations placed in the subchondral bone recruit stem cells to the defect region that differentiate and form new cartilage tissue [3]. While microfracture is a common, relatively simple and cost-effective surgical method, it is not effective over the long-term as the defect becomes infilled by a rough, ‘fibrocartilage-like’ tissue [4,5]. In contrast, the microdrilling (DRL) approach results in comparably stiffer repair tissue and improved patient outcomes at 6 months post-surgery [6,7]. This study seeks to evaluate the structure, composition, and mechanical properties of both repair cartilage and uninjured surrounding cartilage in a large animal model one year after microdrilling within the femoral condyle.

Materials and

Methods: Microdrilling was performed in the load-bearing region of medial femoral condyles from 6 female skeletally mature Boer-cross goats. Chondral defects were made (6.6 mm diameter end-mill), then five holes were drilled (1.1 mm drill bit) in the subchondral bone. One-year post-surgery, 8 mm diameter articular cartilage plugs were obtained following sacrifice. The microdrill repair region (DRL) was visually distinguished from the surrounding articular cartilage.

Microindentation was performed to evaluate DRL repair and surrounding articular cartilage tissues. 4x4 arrays (175 µm spacing) of indents (Hysitron TI 950, R = 250 µm) were performed with a 15 µm/s displacement rate and a 45 s hold at 12.5 µm max depth. Hertzian contact analysis evaluated for contact modulus (Ec) and permeability, equilibrium modulus (E∞) was determined after the 45 s hold, and fluid load fraction was calculated as the relative difference between Ec and E∞ [8].

Tissue biochemical composition of DRL and surrounding cartilage was evaluated using Raman spectra (Renishaw InVia, 785 nm laser) in a 4×4 array (175 µm spacing). Cosmic rays were removed, linear baseline subtracted, intensity was vector normalized, and spectra were smoothed. Spectral acquisitions for each region were averaged and analyzed for relative proteoglycan content, as the ratio of 1377 cm-1/1245 cm-1 peaks, and collagen content was reported as the sum of normalized peak intensities: 814 + 855 + 919 + 1245 + 1271 cm-1.

Results, Conclusions, and Discussions: Microindentation testing revealed that repair tissue following microdrilling had a larger contact modulus than the surrounding articular cartilage (+136%, p < 0.0001). Permeability was decreased in the DRL repair region (-87%, p < 0.001); however, fluid load fraction did not significantly differ between the repair tissue and surrounding cartilage (p = 0.078). Averaged Raman spectra demonstrated an increase in relative proteoglycan content in DRL repair (+26%, p < 0.05) with no significant effect on collagen content.

The mechanical behavior and biochemical composition of repair tissue following microdrilling differed from the surrounding articular cartilage. The tissue repair had higher contact modulus and decreased permeability, indicating a greater resistance to both compressive loading and fluid flow. These changes occurred concomitant with increased proteoglycan content. However, these changes did not correspond with a difference in fluid load fraction; further evaluation of the distribution of proteoglycans and other ECM constituents is needed to understand the mechanical behavior observed. This study demonstrated that repair tissue following microdrilling may be superior to the less stiff repair tissue formed following microfracture [9].

In this ongoing study, we will evaluate how the composition and structure of repair tissue differs from that of surrounding cartilage and compare the repair tissue and surrounding cartilage to articular cartilage in uninjured knees. In addition, we will evaluate how other tissues within the synovial joint (i.e., medial meniscus and synovial fluid) are negatively affected by microdrilling.

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