Polyethylene Damage Increases With Varus Implant Alignment in Posterior-stabilized and Constrained Condylar Knee Arthroplasty

Abstract

Background: Implant malalignment in primary TKA has been reported to increase stresses placed on the bearing surfaces of implant components. We used a longitudinally maintained registry coupled with an implant retrieval program to consider whether preoperative, postoperative, or prerevision malalignment was associated with increased risk of revision surgery after TKA.

Questions/purposes: (1) What is the relative polyethylene damage on medial and lateral compartments of the tibial plateaus from revised TKAs? (2) Does coronal TKA alignment affect implant performance, such that TKAs aligned in varus are predisposed to experience increased polyethylene damage? (3) Does TKA alignment differ between postoperative and prerevision radiographs, and if so, what does this difference suggest about the mechanical contact load placed on a knee with a TKA?

Methods: Between 2007 and 2012, we performed 18,065 primary TKAs at our institution. By March 2016, 178 of those TKAs (1%) were revised at our center at least 2 years after primary surgery at our institution. Eighteen of those TKAs were excluded from this analysis because the tibial insert was not explanted during revision surgery, and four more were excluded because the inserts were lost or returned to the patient before the study was initiated, leaving 156 retrieved polyethylene tibial inserts (in 153 patients) revised at greater than 2 years after the primary TKA for this retrospective study. Patients who underwent revision surgery elsewhere were not considered here, since this study depended on having retrieved components. Polyethylene damage modes of burnishing, pitting, scratching, delamination, surface deformation, abrasion, and third-body debris were subjectively graded on a scale of 0 to 3 to reflect the extent and severity of each damage mode. On preoperative, postoperative, and prerevision radiographs, overall alignment, femoral alignment, and tibial alignment in the coronal plane were measured according to the protocol recommended by the Knee Society.

Results: Knees with more overall varus alignment after TKA had increased total damage on the retrieved tibial inserts (Spearman's rank correlation coefficients of -0.3 [95% CI, -0.4 to -0.1; p = 0.001]). We also found revised TKAs tended to drift back into greater varus before revision surgery, with a mean (SD) of 3.6° ± 4.0° valgus for postoperative alignment compared with 1.7° ± 6.4° prerevision (p = 0.04).

Conclusions: Despite surgical efforts to achieve neutral mechanical alignment, remaining varus alignment places an increased contact load on the polyethylene articular surfaces. The drift toward further varus alignment postoperatively is consistent with the knee adduction moment remaining high after surgery.

Clinical relevance: While we found a predisposition toward recurrence of the preoperative varus deformity, we did not find increased medial as opposed to lateral polyethylene damage, which may be explained by the curve-on-curve toroidal design of the articulating surfaces of the TKA implants in this study.

Fig. 1

A flow diagram shows the method for selecting retrieved TKA implants in this study.

Fig. 2A–B

All TKAs in this study used posterior-stabilized implants. The (A) top surface of the polyethylene tibial insert was divided in 10 regions and the (B) backside surface was divided in four zones for damage scoring.

Fig. 3

The same radiograph is shown twice in this figure to show the angles measured for femoral alignment (angle A), tibial alignment (angle B), and overall alignment (angle C) of a TKA. This postoperative AP radiograph is from a 52-year-old woman who had a left TKA, which was revised 3 years later for instability. The femoral component alignment (angle A) was 5° valgus, the tibial component alignment (angle B) was 90°, and overall anatomic knee alignment (angle C) was 5° valgus.

Fig. 4

The bar chart shows the damage scores on different regions of the tibial insert. The TKAs were divided into those that were placed with varus (red), neutral (blue), or valgus (green) postoperative alignment. Implants placed with postoperative varus alignment had higher mean damage scores on the medial compartment (21.9 for varus, 19.3 for neutral, and 16.5 for valgus; p = 0.01), lateral compartment (21.2 for varus, 18.9 for neutral, and 15.4 for valgus; p < 0.01), post (6.9 for varus, 4.5 for neutral, and 4.9 for valgus; p < 0.01), and backside surfaces (12.7 for varus, 10.0 for neutral, and 8.4 for valgus; p = 0.01) of the polyethylene. Maximum damage score is 84 for the medial compartment, lateral compartment, and backside, while the maximum score is 42 for the post.

Fig. 5

A bar chart shows the mean total damage scores on the surfaces of tibial inserts revised for different reasons.

Fig. 6A–D

The bar charts show distribution of (A) preoperative, (B) postoperative, and (C) prerevision overall TKA alignment in patients undergoing TKA. (D) The distribution for preoperative, postoperative, and prerevision alignment changed, with a shift back toward varus alignment from postoperative to prerevision times. PreP = Preoperative; PostP = Postoperative; PreR = Prerevision.

Fig. 7A–B

(A) A scatterplot shows the preoperative, postoperative, and prerevision overall TKA alignment for each patient. On the y-axis, each patient is represented at one mark, such that each horizontal line on the graph should have two dots showing the alignment for the patient postoperatively and before revision surgery. The data were sorted according to postoperative TKA alignment, such that the red dots on the graph represent where the TKA was originally aligned. Most prerevision alignment measurements (yellow dots) are to the left of the postoperative measurements (red dots), showing that most TKAs fall into varus. (B) The scatterplot uses the same patients as those used for Illustration A but the data were sorted according to reason for revision surgery. Most TKAs revised for loosening fall into varus, as seen by the prerevision alignment measurements (yellow dots) being to the left of the postoperative measurements (red dots).