Linear and Volumetric Polyethylene Wear Patterns after Primary Cruciate-Retaining Total Knee Arthroplasty Failure: An Analysis Using Optical Scanning and Computer-Aided Design Models

Abstract

(1) Background: Analyses of retrieved inserts allow for a better understanding of TKA failure mechanisms and the detection of factors that cause increased wear. The purpose of this implant retrieval study was to identify whether insert volumetric wear significantly differs among groups of common causes of total knee arthroplasty failure, whether there is a characteristic wear distribution pattern for a common cause of failure, and whether nominal insert size and component size ratio (femur-to-insert) influence linear and volumetric wear rates. (2) Methods: We digitally reconstructed 59 retrieved single-model cruciate-retaining inserts and computed their articular load-bearing surface wear utilizing an optical scanner and computer-aided design models as references. After comprehensively reviewing all cases, each was categorized into one or more of the following groups: prosthetic joint infection, osteolysis, clinical loosening of the component, joint malalignment or component malposition, instability, and other isolated causes. The associations between volumetric wear and causes of failure were estimated using a multiple linear regression model adjusted for time in situ. Insert linear penetration wear maps from the respective groups of failure were further processed and merged to create a single average binary image, highlighting a potential wear distribution pattern. The differences in wear rates according to nominal insert size (small vs. medium vs. large) and component size ratio (≤1 vs. >1) were tested using the Kruskal-Wallis test and the Mann-Whitney test, respectively. (3) Results: Patients with identified osteolysis alone and those also with clinical loosening of the component had significantly higher volumetric wear when compared to those without both causes (p = 0.016 and p = 0.009, respectively). All other causes were not significantly associated with volumetric wear. The instability group differentiated from the others with a combined peripheral antero-posterior wear distribution. Linear and volumetric wear rates showed no significant differences when compared by nominal insert size (small vs. medium vs. large, p = 0.563 and p = 0.747, respectively) or by component (femoral-to-insert) size ratio (≤1 vs. >1, p = 0.885 and p = 0.055, respectively). (4) Conclusions: The study found increased volumetric wear in cases of osteolysis alone, with greater wear when combined with clinical loosening compared to other groups. The instability group demonstrated a characteristic peripheral anterior and posterior wear pattern. Insert size and component size ratio seem not to influence wear rates.

Keywords: articular surface wear distribution; endoprosthesis; failure analysis; implant retrieval analysis; orthopedic surgery; polyethylene wear; total knee reconstruction; volumetric wear.

Figure 1

Flow diagram of studied cohort identification. Abbreviation: TKA, total knee arthroplasty.

Figure 2

(A) Process of metrology data collection from an insert with an optical scanner. (B) Example of a penetration wear map of the articular load-bearing surface with a point of maximal linear wear indicated with an arrow. Abbreviations: Max. dev., maximal deviation.

Figure 3

Image processing steps of a penetration wear map before superimposition with other images within the group. (A) Cropped and horizontally mirrored image (inserts of left knees only). (B) Areas with no or less damage removed (shown in black); more damaged areas with depth still visible. (C) Binarized more damaged areas represented in yellow.

Figure 4

(A) Long-leg standing AP view. (B) Enlarged image (A): A = femoral mechanical axis; B = tibial mechanical axis; a = distal horizontal tangent to the femoral component; b = proximal horizontal tangent to the tibial component; (1) hip-knee-ankle (HKA) angle; (2) mechanical lateral distal femoral angle (mLDFA) = angle within [A] and [a]; (3) medial proximal tibial angle (MPTA) = angle within [B] and [b]. (C) Lateral knee view: X = femoral anatomical axis in sagittal plane; y = horizontal tangent to the femoral component; Y = perpendicular axis to [x]; (4) distal femoral flexion angle (DFFA) = angle within [X] and [Y]; Z = tibial anatomical axis in sagittal plane; z = perpendicular horizontal to Z; w = proximal tangent to the tibial component; (5) tibial slope (TS) = angle within [w] and [z].

Figure 5

Illustration of the relationship between volumetric wear (left) and linear wear (right) with time in situ deduced using the linear regression model with regression lines (dark blue) and 95% confidence intervals (grey area). The light blue dots represent individual cases.

Figure 6

The attribute plot of identified possible causes of TKA failure. Abbreviations: loosening, clinical loosening of the component; malposition, joint malalignment or component malposition; other, other isolated causes; PJI, confirmed prosthetic joint infection. Note: Cases with identified osteolysis and clinical loosening of the component are shown in blue.

Figure 7

Wear distribution within groups of causes of TKA failure. In the instability group, areas of higher frequency of wear are located in the peripheral anterior and posterior areas of the articular load-bearing surface.

Figure 8

Microscopic images showing the predominant visual wear patterns within the cohort. (A) Pitting. (B) Scratching. (C) Burnishing. (D) Embedded debris at the bottom of a pit.

Figure 9

Insert with articular load-bearing surface protuberances. (A) Photo. (B) Penetration wear map with resulting volumetric wear of −72.46 mm³ (note; a negative value indicates an increase in volume). (C) Microscopic view of the black rectangle shown in (A) exhibiting discoloration and layer separations observed using a digital microscope. (D) Scanning electron microscope image of a section through a protuberance showing rough layer separation. (E) Scanning electron microscope image of a section through an area adjacent to a protuberance showing delamination. (F) Enlarged view of the red rectangle shown in (E). Abbreviations: det, detector; ETD, Everhart-Thornley detector; HV, high voltage; mag, magnification; SE, secondary electrons; vac, vacuum; WD, working distance.

Figure 10

Insert showing a significant discrepancy from its reference CAD model. (A) Photo. (B) Penetration wear map. (C) Microscopic view of the black rectangle shown in (A), with arrows indicating the direction of parallel machining marks.