Chitranjan S. Ranawat Award: Motion During Total Knee Cementing Significantly Decreases Tibial Implant Fixation Strength

BACKGROUND

Aseptic tibial loosening following primary total knee arthroplasty persists despite technique and device-related advancements. The mechanisms for this mode of failure are not well understood. We hypothesized that knee movement while the cement was curing dispersed lipids at the implant-cement interface and would result in decreased tibial fixation strength.

METHODS

A cadaveric study was performed utilizing 32 torso-to-toe specimens (64 knees). Four contemporary total knee arthroplasty designs were evaluated. Each implant design was randomly assigned to a cadaveric specimen pair with side-to-side randomization. Specimen densitometry was recorded. Each tibial implant was cemented using a standard technique. On one side, the tibial component was held without motion following impaction until complete cement polymerization. The contralateral knee tibial implant was taken through gentle range of motion and stability assessment 7 minutes after cement mixing. Axial tibial pull-out strength and interface failure examination was performed on each specimen.

RESULTS

The average pull-out strength for the no motion cohort (5,462 N) exceeded the motion cohort (4,473 N) (P = .001). The mean pull-out strength between implant designs in the no motion cohort varied significantly (implant A: 7,230 N, B: 5,806 N, C: 5,325 N, D 3,486 N; P = .007). Similarly, the motion cohort inter-implant variance was significant (P ≤ .001). Intra-implant pull-out strength was significantly higher in implant A than D. The average pull-out strength was significantly lower in specimens that failed at the implant-cement interface vs bone failures (4,089 ± 2,158 N vs 5,960 ± 2,010 N, P < .0025).

CONCLUSION

Knee motion during cement polymerization is associated with significant decreases in tibial implant fixational strength. Reduction in implant pull-out strength was identified with each implant design with motion and varied between designs. Across all tested designs, we recommend limiting motion while cementing the tibial implant to improve fixation strength.

Varus Collapse in Total Knee Arthroplasty: Does Fixation or Bone Fail First?

INTRODUCTION

Aseptic tibial loosening is now considered the most common reason that total knee arthroplasties (TKA) fail long term. There are unique subsets of patients that fail into varus alignment of the tibial tray with collapse of the medial proximal tibia. It is currently unknown if the implant fixation fails first or if the proximal medial tibia collapses first.

MATERIALS

We performed a retrospective analysis of 88 patients that were revised at our institution secondary to aseptic varus collapse of the proximal tibia. Two fellowship-trained arthroplasty surgeons performed a retrospective analysis on sequential precollapse radiographs in each patient to determine which failed first: the implant fixation (implant-cement or cement-bone interface) or the medial proximal tibia.

DISCUSSION

36/88 (40.9%) patients had a series of precollapse radiographs that could be reviewed. Failure at the implant-cement interface before varus collapse in 23 vs 22 patients, failure at the implant-cement and cement-bone interface before varus collapse in two patients, and contemporaneous failure at the implant-cement interface and varus collapse in 11 vs 12 patients were identified by reviewers one and two, respectively.

CONCLUSION

The most frequent mechanism of failure identified was failure of the implant-cement interface followed by subsequent medial tibial varus collapse. Improving implant fixation may decrease the incidence of this unique failure mechanism. We advocate the use of supplemental stem fixation in high-risk patients and optimal cement techniques for all patients as methods of potentially avoiding tibial varus collapse, one of the most frequent modes of long-term failure.

Where Is the "Weak Link" of Fixation in Contemporary Cemented Total Knee Replacements?

BACKGROUND

Aseptic tibial loosening is a frequent cause of long-term failure following primary cemented total knee replacement. Failure of the tibial implant can occur at the implant-cement interface or at the cement-bone interface. Currently, it is unknown at which interface failure occurs in cases of aseptic tibial loosening. The following study was designed to determine which interface represents the "weak link" for tibial implant fixation.

METHODS

We performed a retrospective analysis of 149 patients who were revised secondary to aseptic tibial loosening at our institution from 2005 to 2017. Operative reports and radiographs were reviewed on each patient to determine the location and pattern of fixation failure.

RESULTS

Implant failure was more prevalent at the implant-cement than cement-bone interface, 140/149 (94.0%) vs 9/149 (6.0%). Additionally, we noted 2 distinct patterns of failure in patients that loosened at the implant-cement interface. Ninety of 140 (64.3%) patients developed varus collapse pattern of failure. Forty-nine of 140 (35.0%) patients developed failure between the implant-cement interface without angulation. All 149 patients had heterotopic bone formation anterior to the tibial baseplate, which was consistent regardless of which interface failed.

CONCLUSION

The most frequent interface failure identified in our study was at the implant-cement interface, 140/149 (94.0%). This finding has substantial clinical ramifications. Because failure was predominantly at the implant-cement interface there may be design opportunities for increasing implant fixation to cement. Implants with improved undersurface tibial tray features may be necessary to mitigate the risk of failure at this interface, especially in overly active patients or those with elevated body mass indices.