Elasto-plastic contact analysis of an ultra-high molecular weight polyethylene tibial component based on geometrical measurement from a retrieved knee prosthesis

Polyvinyl alcohol hydrogel irradiated and acetalized for osteochondral defect repair: mechanical, chemical, and histological evaluation after implantation in rat knees

Polyvinyl Alcohol (PVA) hydrogel plugs were implanted in artificial osteochondral defects on the trochlear groove of rat knees. After 0, 3, 6, 12, and 24 weeks of followup, samples containing the implants were mechanically evaluated by creep indentation test, chemically, and histologically by optical microscopy. The mechanical test pointed towards an increase of the implant creep modulus and the chemical analysis exhibited an increasing concentration of calcium and phosphorus within the implants over time. Optical microscopy showed no foreign body reaction and revealed formation, differentiation, and maintenance of new tissue at the defect/implant interface. The absence of implant wear indicated that the natural articular lubrication process was not disturbed by the implant. The performance of the irradiated and acetalized PVA was considered satisfactory for the proposed application.

Polyethylene damage and deformation on fixed-bearing, non-conforming unicondylar knee replacements corresponding to progressive changes in alignment and fixation

BACKGROUND

Deviations from nominal alignment of unicondylar knee replacements impact knee biomechanics, including the load and stress distribution at the articular contact surfaces. This study characterizes relationships between the biomechanical environment, distinguished by progressive changes in alignment and fixation, and articular damage and deformation in a consecutive series of retrieved unicondylar knee replacements.

METHODS

Twenty seven fixed-bearing, non-conforming unicondylar knee replacements of one design were retrieved after 2 to 13 years of in vivo function. The in vivo biomechanical environment was characterized by grading component migration measured from full-length radiographs and grading component fixation based on intraoperative manual palpation. Articular damage patterns and linear deformation on the polyethylene inserts were measured using optical photogrammetry and contact point digitization.

FINDINGS

Articular damage patterns and surface deformation on the explanted polyethylene inserts corresponded to progressive changes in component alignment and fixation. Component migration produced higher deformation rates, whereas loosening contributed to larger damage areas but lower deformation rates. Migration and loosening of the femoral component, but not the tibial component, were factors contributing to large regions of abrasion concentrated on the articular periphery.

INTERPRETATION

Classifying component migration and fixation at revision proved useful for distinguishing common biomechanical conditions associated with the varied polyethylene damage patterns and linear deformation for this fixed-bearing, non-conforming design. Pre-clinical evaluations of unicondylar knee replacements that are capable of reproducing variations in clinical alignment and predicting the observed wear mechanisms are necessary to better understand the impact of knee biomechanics and design on unicondylar knee replacement longevity.

Comparison of polyethylene tibial insert damage from in vivo function and in vitro wear simulation

Function and wear of total knee arthroplasties were compared by analysis of damage patterns on polyethylene tibial inserts retrieved from patients (Group R) with inserts obtained after in vitro force-controlled knee joint wear simulation. Two simulator input profiles were evaluated, including standard walking (Group W), and combined walking and stair descent (Group W + S), simulating varied activities and a more severe physiological environment. Damage regions on all inserts were quantitatively assessed. On average, inserts in all groups had internally rotated damage patterns and the greatest articular deformation in the lateral compartment. These patterns were more pronounced in Group W + S compared to Group W. Deformation rates of simulated inserts were analogous to about six years of physiologic function. However, both groups of simulated inserts generally underestimated the magnitude of damage area and extent observed on retrieved inserts, consistent with differences in the simulator's tibiofemoral contact mechanics and those known to occur in patients during functional activities. Modification of simulator inputs, such as the increased anteroposterior excursion and more severe loading conditions in Group W + S, can generate greater wear volume, larger damage areas, and increased surface deformation rates compared to standard inputs.