Engineering Functional Partial Joint Replacements: A Soft Solution To A Hard Problem

Arthritis often leads to joint replacement, where metal and plastic or ceramic components are used to restore function. While effective, these replacements can fail over time, leading to complex and unpredictable revision surgeries. In many cases, arthritis affects only one side of the joint, making partial joint replacement (hemiarthroplasty or focal cartilage repair) a less invasive alternative. However, replacing soft cartilage with hard metallic surfaces in current hemiarthroplasty devices often results in poor outcomes, as the stiff implants reduce contact area and increase stress on the remaining cartilage, potentially causing further degeneration.

This thesis explores the use of polyelectrolyte functionalised biomaterials as cartilage interfacing surfaces, focusing on SPMK-g-PEEK — a biomimetic interface composed of 3-sulfopropyl methacrylate potassium salt (SPMK) tethered to a polyetheretherketone (PEEK) substrate, inspired by the natural biopolyelectrolytes in synovial fluid. SPMK-g-PEEK surfaces form a highly hydrated, compliant layer (~ 5 μm thick) due to their dense coverage of hydrophilic sulphonic acid groups, which supports aqueous boundary lubrication and promotes cartilage interstitial fluid recovery.

Under aqueous conditions, SPMK-g-PEEK exhibits ultra-low friction coefficients (μ < 0.02), consistent across physiologically relevant speeds (0.1 – 200 mm/s) and contact pressures (0.25 – 2 MPa), mimicking the tribological properties of natural cartilage. Additionally, these surfaces facilitate a novel mechanism of polyelectrolyte-enhanced tribological rehydration (PETR), promoting cartilage interstitial fluid recovery even in static contact areas. This mechanism supports continuous lubrication and contrasts with conventional theories that attribute cartilage rehydration to hydrodynamic fluid entrainment facilitated by convergent cartilage contact geometries. PETR is attributed to the combined effects of fluid confinement within the contact gap and the enhanced elastohydrodynamic behaviour of surface tethered polyelectrolytes.

This work not only enhances the understanding of cartilage tribology but also offers a promising strategy for developing joint replacement materials that more effectively replicate the natural function of cartilage. The implications extend to advancing the design of next-generation implants for focal cartilage repair, offering new potential for improved patient outcomes in orthopaedic applications.