No abstract

In this study we describe the use of ultrashort echo time (UTE) magnetic resonance imaging (MRI) to evaluate short and long T2* components as well as the water content of cortical bone. Fourteen human cadaveric distal femur and proximal tibia were sectioned to produce 44 rectangular slabs of cortical bone for quantitative UTE MR imaging, micro computed tomography (μCT), and biomechanical testing. A two-dimensional (2D) UTE pulse sequence with a minimal nominal TE of 8 μs was used together with bi-component analysis to quantify the bound and free water in cortical bone using a clinical 3T scanner. Total water concentration was measured using a 3D UTE sequence together with a reference water phantom. UTE MR measures of water content (total, free and bound), T2* (short and long), and short and long T2* fractions were compared to porosity assessed with μCT, as well as elastic (modulus, yield stress and strain) and failure (ultimate stress, failure strain and energy) properties, using Pearson correlation. Porosity significantly correlated positively with total (R2=0.23; P<0.01) and free (R2=0.31; P<0.001) water content as well as long T2* fraction (R2=0.25; P<0.001), and negatively with short T2* fraction and short T2* (R2=0.24; P<0.01). Failure strain significantly correlated positively with short T2* (R2=0.29; P<0.001), ultimate stress significantly correlated negatively with total (R2=0.25; P<0.001) and bound (R2=0.22; P<0.01) water content, and failure energy significantly correlated positively with both short (R2=0.30; P<0.001) and long (R2=0.17; P<0.01) T2* values. These results suggest that UTE MR measures are sensitive to the structure and failure properties of human cortical bone, and may provide a novel way of evaluating cortical bone quality.

Biological tissues usually contain distinct water compartments with different transverse relaxation times. In this study, two-dimensional, multi-slice, ultrashort echo time spectroscopic imaging (UTESI) was used with bi-component analysis to detect bound and free water components in musculoskeletal tissues. Feasibility studies were performed using numerical simulation. Imaging was performed on bovine cortical bone, human cadaveric menisci and the Achilles' tendons of volunteers. The simulation study demonstrated that UTESI, together with bi-component analysis, could reliably quantify both T(2)* and fractions of the short and long (2)* components. The in vitro and in vivo studies each took less than 14 min. The bound water components showed a short T(2)* of ~0.3 ms for bovine bone, ~1.8 ms for meniscus and ~0.6 ms for the Achilles' tendon. The free water components showed about an order of magnitude longer T(2)* values, with ~2 ms for bovine bone, ~14 ms for meniscus and ~8 ms for the Achilles' tendon. Bound water fractions of up to ~76% for bovine bone, 50% for meniscus and ~75% for the Achilles' tendon were measured. The corresponding free water components were up to ~24% for bovine bone, 50% for meniscus and ~25% for the Achilles' tendon by volume. These results demonstrate that UTESI, combined with bi-component analysis, can quantify the bound and free water components in musculoskeletal tissues in clinically realistic times.

Recent proton magnetic resonance (MR) spectroscopy studies have shown that cortical bone exists as different components which have distinct transverse relaxation times (T2s). However, cortical bone shows zero or near zero signal with all conventional MR sequences on clinical scanners and the different water components cannot be assessed with this approach. In order to detect signal in this situation a two-dimensional (2D) non-slice selective ultrashort echo time (UTE) pulse sequence with a nominal TE of 8 μs was used together with bi-component analysis to quantify bound and free water in bovine cortical bone at 3T. Total water concentration was quantified using a 3D UTE sequence together with a reference water phantom. 2D and 3D UTE imaging were performed on 14 bovine bone samples which were subjected to sequential air-drying to evaluate free water loss, followed by oven-drying to evaluate bound water loss. Sequential bone weight loss was measured concurrently using a precision balance. Bone porosity was measured with micro computed tomography (μCT) imaging. UTE measured free water loss was higher than the volume of cortical pores measured with μCT, but lower than the gravimetric bone water loss measured during air-drying. UTE assessed bound water loss was about 82% of gravimetric bone water loss during oven-drying. On average bovine cortical bone showed about 13% free water and 87% bound water. There was a high correlation (R = 0.91; P < 0.0001) between UTE MR measured free water loss and gravimetric bone weight loss during sequential air-drying, and a significant correlation (R = 0.69; P < 0.01) between UTE bound water loss and gravimetric bone weight loss during oven-drying. These results show that UTE bi-component analysis can reliably quantify bound and free water in cortical bone. The technique has potential applications for the in vivo evaluation of bone porosity and organic matrix.

No abstract

Objective. To characterize cartilage shear strain during articulation, and the effects of lubrication and degeneration.Methods. Human osteochondral cores from lateral femoral condyles, characterized as normal or mildly degenerated based on surface structure, were selected. Under video microscopy, pairs of osteochondral blocks from each core were apposed, compressed 15%, and subjected to relative lateral motion with synovial fluid (SF) or phosphate buffered saline (PBS) as lubricant. When cartilage surfaces began to slide steadily, shear strain (E xz ) and modulus (G) overall in the full tissue thickness and also as a function of depth from the surface were determined.Results. In normal tissue with SF as lubricant, E xz was highest (0.056) near the articular surface and diminished monotonically with depth, with an overall average E xz of 0.028. In degenerated cartilage with SF as lubricant, E xz near the surface (0.28) was 5-fold that of normal cartilage and localized there, with an overall E xz of 0.041. With PBS as lubricant, E xz values near the articular surface were ϳ50% higher than those observed with SF, and overall E xz was 0.045 and 0.062 in normal and degenerated tissue, respectively. Near the articular surface, G was lower with degeneration (0.06 MPa, versus 0.18 MPa in normal cartilage). In both normal and degenerated cartilage, G increased with tissue depth to 3-4 MPa, with an overall G of 0.26-0.32 MPa.Conclusion. During articulation, peak cartilage shear is highest near the articular surface and decreases markedly with depth. With degeneration and diminished lubrication, the markedly increased cartilage shear near the articular surface may contribute to progressive cartilage deterioration and osteoarthritis.

Biological tissues frequently contain different water compartments, and these often have distinct transverse relaxation times. Quantification of these may be problematic on clinical scanners because spin echo sequences usually have initial echo times that are too long to accurately quantify shorter relaxation time components. In this study, an ultrashort echo time pulse sequence was used together with bicomponent analysis to quantify both the short and long T 2 components in tissues of the musculoskeletal system. Feasibility studies were performed using numerical simulation, and on phantoms and in vitro tissues including bovine cortical bone, ligaments, menisci, tendons, and articular cartilage. The simulation and phantom studies demonstrated that this technique can quantify T 2 * and fractions of the short and long T 2 components. The tissues studies showed two distinct components with short T 2 *s ranging from 0.3 ms for bovine cortical bone to 2.1 ms for menisci, and long T 2 *s ranging from 2.9 ms for bovine cortical bone to 35.0 ms for articular cartilage. The short T 2 * fraction ranged from 18.5% for patella cartilage to 80.9% for ligaments. The results show that ultrashort echo time imaging with bicomponent analysis can quantify the short and long T 2 water components in vitro in musculoskeletal tissues. Magn Reson Med 67:645-649,

Background Osteochondral allografts are currently stored at 4°C for 2–6 weeks before implantation. At 4°C, chondrocyte viability, especially in the superficial zone, deteriorates starting at 2 weeks. Alternative storage conditions could maintain chondrocyte viability beyond 2 weeks, and thereby facilitate increased graft availability and enhanced graft quality. Purpose Determine effects of prolonged 37°C storage compared to traditional 4°C storage on chondrocyte viability and cartilage matrix content. Study Design Controlled Laboratory Study Methods Osteochondral samples from humeral heads of adult goats were analyzed (i) fresh, or after storage in medium for (ii) 14d at 4°C including 10% FBS, (iii) 28d at 4°C including 10% FBS, (iv) 28d at 37°C without FBS, (v) 28d at 37°C including 2% FBS, or (vi) 28d at 37°C including 10% FBS. Portions of samples were analyzed by microscopy after LIVE/DEAD® staining to determine chondrocyte viability and density, both en face (to visualize the articular surface) and vertically (overall and in superficial, middle, and deep zones). The remaining cartilage was analyzed for sulfated-glycosaminoglycan and collagen. Results 37°C storage maintained high chondrocyte viability compared to 4°C storage. Viability of samples after 28d at 37°C was ~80% at the cartilage surface en face, ~65% in the superficial zone, and ~70% in the middle zone, which was much higher than ~45%, ~20%, and ~35%, respectively, in 4°C samples after 28d, and slightly decreased from ~100%, ~85%, and ~95%, respectively, in fresh controls. Cartilage thickness, glycosaminoglycan content, and collagen content were maintained for 37°C and 4°C samples compared to fresh controls. Conclusion 37°C storage of osteochondral grafts supports long-term chondrocyte viability, especially at the vulnerable surface and superficial zone of cartilage. Clinical Relevance Storage of allografts at physiological temperature of 37°C may prolong storage duration, improve graft availability, and improve treatment outcomes.