Increased initial cement-bone interlock correlates with reduced total knee arthroplasty micro-motion following in vivo service

Abstract

Aseptic loosening of cemented tibial components in total knee arthroplasty (TKA) has been related to inadequate cement penetration into the trabecular bone bed during implantation. Recent postmortem retrieval work has also shown there is loss of interlock between cement and bone by resorption of trabeculae at the interface. The goal of this study was to determine if TKAs with more initial interlock between cement and bone would maintain more interlock with in vivo service (in the face of resorbing trabeculae) and have less micro-motion at the cement-bone interface. The initial (created at surgery) and current (after in vivo service) cement-bone interlock morphologies of sagittal implant sections from postmortem retrieved tibial tray constructs were measured. The implant sections were then functionally loaded in compression and the micro-motion across the cement-bone interface was quantified. Implant sections with less initial interdigitation between cement and bone and more time in service had less current cement-bone interdigitation (r(2)=0.86, p=0.0002). Implant sections with greater initial interdigitation also had less micro-motion after in vivo service (r(2)=0.36, p=0.0062). This work provides direct evidence that greater initial interlock between cement and bone in tibial components of TKA results in more stable constructs with less micro-motion with in vivo service.

Keywords: Knee arthroplasty; Loosening; Micro-motion; Postmortem; Radiolucent lines.

Fig. 1

Tibial tray constructs were sectioned in the sagittal plane to create medial and lateral test specimens (A). The center of the contact patch (COC) on the polyethylene insert was used to define the load application point on the test specimens. Specimens were imaged at high resolution on interior and exterior faces to document interface morphology (B).

Fig. 2

The cement–bone interface morphology of a postmortem retrieved tibial component shows trabecular bone (b) interdigitated with cement (c) and areas of resorbed (r) bone (A). The initial interdigitation depth (inID) and current interdigitation depth (curID) were determined. Radiographic interdigitation depth (radID) was calculated as the distance from the rim on the tibial tray to the maximum depth of cement into the bone (B). Cement–bone contact fraction (CF) was determined using a random ray stereology method to document contact (black squares) or non-contact (white circles) at the cement–bone interface (C).

Fig. 3

Tibial tray sections (A) were loaded axially in compression to one body weight (BW) equivalent loading through a load transfer block that was positioned coincident with the polyethylene insert center of contact (COC). Digital image correlation imaging was used to measure the relative cement–bone micro-motion at five locations along the interface per side of the specimen (black squares). The distance between cement and bone measurement points with 0.01 BW loading (B) and with 1.0 BW loading (C) was used to determine micro-motion (L1–L2).

Fig. 4

A lab-prepared specimen (A) with substantial interdigitation and contact between cement and bone had virtually no interface micro-motion (<1 µm). A postmortem retrieval with some remaining interdigitation (B) had interface micro-motions of 4 µm. A postmortem retrieval with no interdigitation along the specimen surface (C) had 12 µm of micro-motion with a non-linear response to the applied load. A postmortem retrieval with extensive soft tissue (D) between the cement and the bone had 110 µm of micro-motion. The micro-motion response of these four representative sections of the tibial tray constructs is shown (E).

Fig. 5

Contour plot of current cement–bone interdigitation depth (curID) as a function of time in service and initial interdigitation depth (inID) for 12 tibial tray constructs. Implants with more time in service and less initial interdigitation had the least amount of current interdigitation.

Fig. 6

There was a power-law response between current contact fraction (CF) and cement–bone interface micro-motion (MM) (A) where interfaces with greater CF had less micro-motion. A similar relationship between current interdigitation depth (curID) and micro-motion (MM) was also found (B).

Fig. 7

There was no correlation between polyethylene insert wear score (PE) and micro-motion (MM) at the cement–bone interface.