Polyethylene wear is influenced by manufacturing technique in modular TKA

Abstract

Polyethylene insert backside surface wear is implicated in osteolysis and failure of total knee arthroplasty. Manufacturing and sterilization methods reduce articular-sided wear. We questioned whether manufacturing technique influences the severity of backside wear. We examined 39 explanted tibial bearings in a blinded fashion using visual, stereomicroscopic, and scanning electron microscopic techniques. We examined 26 direct compression molded components and 13 nondirect compression molded components and applied a new backside wear severity score. The score characterized the magnitude of the various modes of wear with severity ranging from 0 (no wear) to 27 (severe wear). Time in vivo, tibial baseplate material, and manufacturing technique were used as variables for comparison. Backside wear was related to polyethylene manufacturing process with direct compression molded implants having a wear score of 2.3 and nondirect compression molded a score of 5.7. Time in vivo influenced backside wear, although direct compression molded predicted decreased backside wear independent of time in vivo. The data suggest manufacturing technique influences backside wear in total knee arthroplasty polyethylene inserts.

Figures

Fig. 1

Thirty-nine retrieved bearings were examined. Twenty-six bearings were direct compression molded (DCM) and 13 bearings were non-DCM. Of the DCM bearings, there were 19 retrieved from nonmodular (Non-Mod) cobalt-chromium baseplates. Seven DCM bearings were retrieved from modular titanium tibial baseplates (Modular). Of the Non-Mod DCM bearings, nine were from Vanguard (V) and were all cruciate retaining (CR). Ten Non-Mod DCM bearings were from Maxim articulations (M). Of these, two were CR and eight were posterior stabilized (PS). Of the modular baseplate DCM bearings, one was M-PS, one V posterior stabilized-constrained (PSC), and five M-PSC. All 13 non-DCM bearings were retrieved from modular baseplates. Of these, there were three M-CR, eight M-PS, and two M-PSC.

Fig. 2

The Maxim and Vanguard Complete Knee System tibial baseplate incorporates a novel polyethylene locking mechanism. The anterior locking bar is designed to force molding of the polyethylene bearing into the locking mechanism, thus reducing backside wear.

Fig. 3

Backside wear severity scores were calculated in a blinded fashion. The bearing is divided into six areas and the type and severity of wear recorded.

Fig. 4A–B

(A) An example of a direct compression molded (DCM) retrieval is shown. Stereomicroscopic images (stereo) and scanning electron microscopic images (SEM) demonstrating burnishing and scratching are shown. (B) An example of a nondirect compression molded component is shown. Burnishing and discoloration were prevalent in the non-DCM bearings. Examples of these wear types are given.

Fig. 5

Grouped linear regression analysis with time as the covariant demonstrates a difference in wear between direct compression molded (DCM) and non-DCM bearings. Adequate power to detect a difference was present.