Surface Damage in Machined Ram-Extruded and Net-Shape Molded Retrieved Polyethylene Tibial Inserts of Total Knee Replacements

Background: Polyethylene wear has emerged as a major determining factor in the long-term clinical performance of total knee replacements. This study addresses the in vivo wear performance of two types of polyethylene tibial inserts used in similar total knee arthroplasty designs.

Methods: A surface damage assessment of retrieved specimens was performed for twenty-six net-shape molded tibial inserts manufactured from H1900 resin without calcium stearate additive (Miller-Galante) and forty-three machined ram-extruded tibial inserts manufactured from GUR 4150 resin with calcium stearate additive (Miller-Galante II). Stereomicroscopic inspection and digital image analysis were used to quantify the extent and severity of pitting, dimensional change, and delamination.

Results: Pitting and dimensional change were the most common modes of damage in both groups, with the prevalence ranging from 77% to 92% for pitting and from 51% to 81% for dimensional change. Delamination was the least common mode of damage, with the prevalence ranging from 21% to 35%. The severity of pitting was higher in association with the cemented implant-bone interface. The extent and severity of delamination increased with implantation time. No severe delamination was observed before sixty months after implantation in the net-shape molded group, whereas severe delamination was present as early as ten months after implantation in the machined ram-extruded group. The time between surgery and the discovery of damage was longer in the net-shape molded group for all modes of damage except for medial dimensional change.

Conclusions: On the basis of the components available in our implant retrieval pool, we found that at equivalent levels of surface damage, the net-shape molded H1900 resin tibial inserts demonstrated longer service life than did the machined ram-extruded GUR 4150 components. The superior performance of the net-shape molded components may be related to the resin type, the absence of calcium stearate, the consolidation method, or the method of final geometry shaping. This superior damage resistance is expected to contribute to superior long-term clinical performance of net-shape molded ultra-high molecular weight polyethylene in total knee arthroplasty.