Performance parity in cartilage repair: SPMK-g-PEEK versus cartilage–cartilage interfaces

Effective fluid exudation and rehydration are essential for the low-friction function of healthy articular cartilage, facilitating interstitial fluid pressurisation, solute transport, and aqueous lubrication. However, current metallic biomaterials used in focal cartilage repair or hemiarthroplasty compromise this fluid-pressure dependent load support, leading to the erosion of the interfacing cartilage. This study investigates bioinspired hydrophilic 3-sulfopropyl methacrylate potassium salt (SPMK) polymer grafted onto a PEEK substrate (SPMK-g-PEEK) as a potential solution. SPMK-g-PEEK aims to mimic the natural tribology of cartilage by providing an aqueous low friction interface and polyelectrolyte-enhanced tribological rehydration (PETR), supporting fluid recovery and interstitial fluid pressurisation during cartilage sliding. We compare the tribological characteristics of physiological cartilage-cartilage interfaces, which rely on osmotic swelling and hydrodynamic tribological rehydration, with PETR enabled by SPMK-g-PEEK interfaces. This study introduces a bespoke Fuzzy-PI controlled biotribometer. Employing a dual-phase testing method, static compression followed by sliding, allows simultaneous measurement of friction and cartilage strain recovery, indicative of interstitial fluid recovery following compressive exudation. Cartilage condyle, unfunctionalised PEEK, and SPMK-g-PEEK surfaces were investigated against flat cartilage plugs, which provide no hydrodynamic entrainment zone for tribological rehydration, and convex cartilage plugs, which create a convergent hydrodynamic zone for tribological rehydration. Matched cartilage-cartilage contacts exhibited low friction coefficients of ∼ 0.04 and strain recovery of up to ∼ 14% during the sliding phase. SPMK-g-PEEK surfaces sliding against convex cartilage plugs demonstrated similar strain recovery of ∼ 13% and reduced friction coefficients of ∼ 0.01, due to the combined effects of PETR and hydrodynamic tribological rehydration. In contrast, unfunctionalised PEEK surfaces, similar to current hard biomaterials employed in cartilage resurfacing, showed significantly higher friction and inhibited rehydration. SPMK-g-PEEK effectively mimics the physiological rehydration of connatural articular cartilage surfaces, highlighting its potential as a biomimetic material for cartilage resurfacing.