Functional dissatisfaction following total knee replacement (TKR) is recorded as high as 20%. The majority of these patients report anterior knee pain (AKP) as the main source of dissatisfaction. Elevated patellofemoral compression forces and soft tissue extensor hood strain have been implicated in the generation of significant AKP. A novel method of assessing and measuring patellofemoral compression forces dynamically in the native and resurfaced patella for TKR in four different quadrants of the patella is described. Results are reported from an in vitro model and cadaveric studies in the native and resurfaced knee. Patellofemoral compression forces are shown to be characteristic and consistent over repeated assessments in the native knee. Placement of a TKR significantly alters this pattern. Furthermore, over-stuffing or under-stuffing the resurfaced patella also significantly alters the nature and magnitude of patellofemoral compression forces. These studies may lead to an improved understanding of the nature of AKP following TKR, and using this assessment tool presents an opportunity to more effectively balance the third space, reproduce the native patellofemoral forces, and subsequently reduce AKP following TKR.

Introduction: Patellofemoral joint loading is influenced by implant design and surgical execution during knee replacement. Altered patellofemoral mechanics have been implicated in postoperative symptoms, yet the magnitude and variability of intraoperative load changes across different replacement procedures and implant systems remain incompletely characterised. The aim of this registry study was to describe and quantify intraoperative patellofemoral joint load changes following total knee replacement (TKR) and patellofemoral joint resurfacing (PFJR), focusing on cumulative and compartment-specific load redistribution. Materials and methods: This observational registry study analysed prospectively collected intraoperative patellofemoral joint load measurements from 291 knee replacement procedures. Load was recorded in medial, lateral, superior, and inferior compartments before and after trial component implantation. Absolute and percentage changes in compartmental and cumulative load were calculated. Descriptive analyses examined the distribution and variability of load change across procedure type and implant system, with implant comparisons restricted to TKR procedures. No formal hypothesis testing was pre-specified, and no adjustment for potential confounding variables was undertaken. Results: Cumulative patellofemoral load change demonstrated substantial inter-individual variability. While most cases clustered around small percentage changes, a distinct subset of patients exhibited large cumulative load increases exceeding + 70%. TKR procedures demonstrated a broader distribution and higher observed frequency of large cumulative load increases compared with PFJR. Implant-specific analysis of TKR procedures revealed descriptive differences in cumulative load distributions between implant cohorts, with certain implant systems showing a higher observed frequency of large load increases within this dataset. Compartmental magnitude analysis demonstrated that lateral and superior compartments accounted for most of the load redistribution, whereas medial and inferior compartments contributed minimally. Conclusions: Intraoperative patellofemoral load redistribution following knee replacement is highly variable and was observed to differ descriptively between procedure types and implant systems. Load changes were predominantly observed in the lateral and superior compartments, with a meaningful subset of patients experiencing large cumulative increases. Registry-based intraoperative load measurement allows detailed characterisation of patellofemoral biomechanics beyond cumulative metrics alone and may inform future investigations into implant design and surgical optimisation. Given the observational registry design and absence of confounder adjustment, these findings should be interpreted as descriptive associations rather than causal relationships.