T1ρ and T2 quantitative magnetic resonance imaging analysis of cartilage regeneration following microfracture and mosaicplasty cartilage resurfacing procedures

Traumatic knee injuries, often seen in otherwise healthy, active patients, commonly result in focal cartilage defects (CDs) of the weight-bearing articular surfaces of the knee. These defects can be asymptomatic, but swelling, pain, and instability of the joint may occur (1). In addition, chronic degeneration of the cartilage due to altered joint mechanics and loading may predispose the patient to the development of premature osteoarthritis (1). Several surgical options for repair of CDs exist, including microfracture (MFx) and autologous osteochondral mosaicplasty (MOS) surgery. MFx is performed by inducing multiple penetrating injuries into the subchondral bone. Clot formation occurs as well as migration of pluripotent mesenchymal progenitor cells into the area of acute injury, which subsequently differentiate and generate a new articular surface (2). MOS involves transplantation of small cylindrical osteochondral hyaline articular cartilage grafts harvested from the nonweight-bearing surface of the knee into the focal area of cartilage damage (3,4). Autologous chondrocyte implantation and subchondral drilling are other available surgical options but are not the focus of this study. Ideally, any cartilage repair procedure would result in the regeneration of hyaline cartilage that has morphologic and mechanical properties similar to those of native cartilage (5). In practice, fibrocartilage is produced following MFx (6–9), and a mixture of hyaline and fibrocartilage are produced after MOS (3,9,10). Cartilage biopsy and second-look arthroscopy are currently the gold standards for the evaluation of regenerated tissue (RT) following MFx and MOS (11–13). Second-look arthroscopy allows for direct visualization of the graft surface as well as assessment of the RT’s mechanical integrity through indentation measurements (14). Despite these advantages, arthroscopic biopsies are invasive procedures with significant associated surgical morbidity, making routine use impractical and uncommon. In contrast, magnetic resonance imaging (MRI) is a noninvasive method for nonmorbid visualization and evaluation of articular cartilage in the knee (10,14–21). dGEMRIC (delayed gadolinium-enhanced MRI of cartilage), which has been utilized to monitor cartilage repair in humans, requires intravenous or intraarticular injection of gadolinium contrast (14). In this study, T1ρ (T1rho) mapping is utilized in conjunction with T2 mapping to examine cartilage regeneration following MFx and MOS cartilage resurfacing procedures. These MRI sequences were chosen because they do not require contrast injection and have significant potential for clinical application as additive sequences to traditional qualitative MRI. Several studies have used T2 MRI to characterize the biochemical and morphologic properties of normal cartilage (NC) and RT following cartilage repair procedures (10,14–21). Quantitative T2 mapping has been used specifically for this purpose because it is sensitive to the orientation, concentration, and integrity of collagen, as well as the water content in articular cartilage (22–25). T1ρ imaging describes the spin-lattice relaxation time in the rotating frame and has been proposed as an alternative candidate to measure and assess macromolecular properties and biochemical changes in cartilage content (22,26–30). In addition, T1ρ is sensitive to trypsin-induced proteoglycan loss in bovine cartilage and correlates with proteoglycan content in human cartilage specimens with early degeneration (27,28,31). While T2 quantitative MRI has been extensively studied in cartilage repair (10,14–21), no study has examined T1ρ and compared the efficacy of T1ρ and T2 quantitative MRI in evaluating cartilage regeneration after MFx and MOS cartilage resurfacing procedures. Here we present a nonrandomized prospective cross-sectional study utilizing quantitative T1ρ and T2 mapping at clinically relevant timepoints in humans following two different and methodologically distinct cartilage resurfacing procedures to assess cartilage regeneration and maturation.