Poster No. 627 • 54th Annual Meeting of the Orthopaedic Research Society

Cartilage Shear Strain During Femoral-Tibial Articulation: Effect of Acute Joint Injury on Synovial Fluid Lubrication

Benjamin L. Wong1, Seung Hyun C. Kim1, Jennifer M. Antonacci1, C. W. McIlwraith2, Robert L. Sah1

1University of California-San Diego, La Jolla, CA; 2Colorado State University, Fort Collins, COrsah@ucsd.edu

Introduction: As the knee bends, the cartilage of the femoral condyle (FC) andtibia plateau (TP) articulate. Synovial fluid (SF) lubricates the sliding cartilage sur-faces to maintain low friction and wear.1 Following acute joint (AI) injury, the fric-tion-lowering boundary lubrication function of SF is diminished but can berestored by addition of hyaluronan (HA).2 Since SF normally acts to decreases car-tilage shear strain (Exz) during articulation,3 and applied shear markedly regulateschondrocyte metabolism,4,5 injury-related changes in SF lubricant function maymodulate cartilage mechanobiology through effects on cartilage Exz. Past studieshave examined articulation mechanics in a simplified geometry, with the apposingcartilage samples both from the FC.3 The objectives of this study were to deter-mine, for articulating human FC and TP cartilage, (1) Exz, (2) the effects of AI onSF lubricant function, and (3) the ability of HA addition to AI-SF to restore lubri-cant function.

Materials and Methods: Samples. Human cartilage exhibiting normal age-related features were harvested from the lateral aspects of cadaveric knees, from theFC (LFC) and uncovered region of the TP (LTP) as osteochondral blocks (each,~3x8x7mm3, WLH). The donors were 48±2yr in age (range 45-50yr, n=4). LFC-LTP samples were stained at 4C with PBS+propidium iodide (20μg/ml)+pro-teinase inhibitors (PI) for 2h, and then submerged in lubricant+PI for ~12h.Lubricants & Experimental Design. Equine SF was aspirated from n=4 animals, 2-4yr old (with IACUC approval) during arthroscopic surgery within 3wk after acuteinjury. Lubricant samples, each with added PI, were (1) PBS, (2) SF from theinjured joint (AI-SF), confirmed by friction tests,2 (3) AI-SF to which was added800 kDa HA to 1mg/ml (AI-SF+HA), and (4) SF from the contralateral non-injured joint (NL-SF). Lubricants from the four groups were tested sequentially inmicroscopic shear, with rinsing, reswelling, and reincubation for ~4h at 4C inbe-tween. Microscopic Shear Testing. Each apposed sample pair (Fig. 1, schematic) wastested in a bi-axial loading chamber and viewed with an epi-fluorescence micro-scope.3 Cartilage was compressed by 1−Λz=15%. After stress-relaxation for 1h, lat-eral displacements were applied, one for preconditioning, and then two for tests(each Δx of + 1 then -1 mm at 100 μm/s), one each for LFC and LTP. Images(FOV~3x2mm2) contained the full thickness of LFC or LTP cartilage and a par-tial view of the apposing surface (Fig. 1A-H). Data Analysis. Cells (~500/image)were tracked using custom software to determine maximum displacement andLagrangian Exz, which was averaged depth-wise and overall. The effects of location(LFC, LTP), and lubricant (PBS, AI-SF, AI-SF+HA, NL-SF) were assessed byrepeated-measures ANOVA. Within LFC and LTP, differences between lubricantswere assessed by Tukey post-hoc testing.

Results: For all cases, Exz increased with applied lateral displacement, reachinga maximum when the surfaces slid (Fig. 1A-H). Local Exz was always highest atthe surface and diminished with depth (Fig. 2A,B). Surface Exz was 3-4x higher inLTP than LFC (p<0.05) with an interactive effect of lubricant (p<0.001, Fig. 3A).In the LTP, surface Exz with NL-SF was lower than that with PBS and AI-SF, butindistinguishable from that with AI-SF+HA; surface Exz with AI-SF+HA waslower than that of AI-SF. In the LFC, similar trends, at lower absolute values ofsurface Exz were evident. Also, for overall Exz (Fig. 3B), similar trends were evidentfor both the LTP and LFC.

Discussion: These results provide the first micro-scale information on femoro-tibial cartilage articulation. The higher Exz in LTP than LFC cartilage is consistentwith their relative shear moduli.6 After injury, an increased surface Exz associatedwith decreased SF lubrication function may induce chondrocyte responses thateither help maintain homeostasis, or contribute to cartilage wear and further dete-rioration. The restitution of injury SF by added HA suggests that biomechanicallubricants could be a therapeutic counter to aberrant mechanobiological effects.

Figure 1. Micrographs of LFC (A-D) sliding against LTP (E-H) cartilage when lubricatedwith (A,E) PBS, (B,F) AI-SF, (E,G) AI-SF+HA, and (D,H) NL-SF. Dashed lines and colormaps (max shear strain, Exz) are areas before and after applied shear.

Figure 2. Effect of surface lubricants on local shear strain vs. depth for (A) LFC and (B) LTPwhen micro-shear tested. Mean±SEM, n=4.

Figure 3. Effect of surface lubricants on (A) surface and (B) overall shear strain magnitudes.Mean±SEM, n=4.

References: 1 Buckwalter+, J Bone Joint Surg Am 79-A:612-32, 1997. 2Antonacci+, Trans Orthop Res Soc 32:156, 2007. 3 Wong+, Trans Orthop Res Soc32:100, 2007. 4 Jin+, Arch Biochem Biophys 395:41-8, 2001. 5 Nugent+, ArthritisRheum 54:1888-96, 2006. 6 Arokoski+, J Biomed Mater Res 48:99-107, 1999.

Acknowledgements: HHMI, NIH, NSF, Cal-IT2