All the studies described in this thesis are aimed at validating and refining a pre\dously proposed theoretical boundary friction model [14] and hence improve our understanding of the fundamental mechanisms governing cartilage lubrication. This model showed that even under the limiting case of pure boundary friction (in the absence of a fluid film and boundary lubricants) between joint surfaces, the fluid phase of cartilage pressurizes considerably and shifts most of the applied load (W) from the solid phase to the interstitial water. Through this “fluid load” (W^p), the frictional resistance felt by the solid matrix is considerably reduced. According to this model, the transient or effective friction coefficient (μ_eff) is dependent on three quantities, (i) interstitial fluid load support (W^p/W), (ii) “equilibrium” friction coefficient (μ_eq);​ this is the friction coefficient when all the interstitial fluid pressurization has subsided, (i.e. W^p/W=0) and (iii) fraction of the total contact area where solid to solid contact occurs (φ).

In previous studies [139-141], it has been shown that interstitial fluid pressurization contributes to over 90% of total applied stress for several hundred seconds after loading which can affect the tribological response of the tissue [14, 52,105,110].

The following questions have been addressed in this dissertation:​ (a) Is there an inhomogeneity in cartilage material properties that would maximize the fluid load support at the articular surface and hence enhance its fiictional resistance? (b) Is the previously proposed biphasic boundary friction model valid? (c) Is Superficial Zone Protein an effective boundiuy lubriccmt? (d) During cyclical compressive loading (a more physiological loading configuration) will the fluid load support in the tissue ever subside? Correspondingly, will μ_eff rise to undesirable values as in the case of static loading? (e) Does the frictional response vary as a function of strain? (f) How does the fluid load support (W^p/W) and solid to solid contact area fraction (φ) (both the variables directly influencing μ_eff), vary at large strain levels?