Elsevier

Clinical Biomechanics

Volume 27, Issue 9, November 2012, Pages 929-935
Clinical Biomechanics

Influence of time in-situ and implant type on fixation strength of cemented tibial trays — A post mortem retrieval analysis

https://doi.org/10.1016/j.clinbiomech.2012.06.008Get rights and content

Abstract

Background

Loosening of the tibial tray is cited as the most common cause of failure in total knee arthroplasty but the mechanism remains unclear. Post mortem specimens provide a unique opportunity to investigate the clinical condition.

Methods

Twenty two cemented components were serially retrieved in situ at autopsy from a university clinic. They were investigated for mechanical stability by pull-out, which was related to cement morphology and bone quality from CT scans, and to polyethylene wear by score analysis. Implants were grouped into three types: a particular fixed bearing design (n = 8), a particular rotating platform design (n = 5) and other mixed designs (n = 9).

Findings

Trends were observed for pull-out force to decrease with time in situ and increase with cement penetration but was unrelated to bone density or polyethylene wear. For the fixed bearing implants decreasing pull-out strength was related to an increasing proportion of failure at the bone–cement interface. For the mixed designs the opposite was observed. The rotating platform implants failed at the implant–cement interface.

Interpretation

The analysis demonstrated that interface failure is dependent on the implant design, but that both the stem and the bone interfaces weaken with time in situ. Published findings for laboratory implantations have demonstrated that greater cement penetration improves fixation and this was reflected for clinical samples in this study.

Introduction

The most common treatment for a diseased knee joint is replacement by total knee arthroplasty (TKA). National revision rates of less than 5% at 10 years (Swedish Knee Register, 2010), in combination with relief from discomfort and relatively low cost, make this surgical procedure one of the most successful (Lavernia et al., 1997). However, absolute numbers of global implantations are around one million annually and are steadily increasing, and the corresponding absolute number of revisions amounts to thousands (Kurtz et al., 2007).

Currently, the majority of TKAs is cemented and in national registries a quarter to a third of primary TKA revisions are attributed to loosening of the cemented tibial component (Australian Hip and Knee Register, 2010, Kurtz et al., 2010, National Joint Registry for England and Wales, 2010, Swedish Knee Register, 2010). Cementing technique, as well as bone quality, have been related to implant loosening (Guha et al., 2008, Minoda et al., 2010, Rea et al., 2007, Swedish Knee Register, 2010). In various studies the depth of penetration of the cement into the cancellous bone has been shown to increase the strength of the bone–cement interface, leading to improved implant anchorage (Askew and Lewis, 1981, MacDonald et al., 1993, Schlegel et al., 2010). However, there is little documentation relating the long-term clinical status of the cement mantle to cement penetration. Neither is it clear whether the bone–cement or the implant–cement interface becomes the weaker link.

Polyethylene (PE) insert wear has been identified to be an indication for revision in about 10% of the clinical cases (Hossain et al., 2010, Lu et al., 2010, Swedish Knee Register, 2010). Little is known about the clinical relationship between PE insert damage and implant stability over time in situ for different implant designs. Material properties of tibial inserts have been related to clinical performance, but other factors such as surgical technique, sterilisation method, ligament tension and patient-specific variables in particular, also play an important role (Collier et al., 1991, Collier et al., 2006, Collier et al., 2007, Collier et al., 2008, Edidin et al., 2000, Kurtz et al., 2000).

The purpose of this study was to assess the mechanical stability of tibial trays that had been in service in vivo. This was undertaken by measuring the anchorage capacity of post-mortem retrieved tibial trays and relating it to the failure mode, implant design, cement penetration, bone quality and wear of the polyethylene liner. We expected anchorage capacity to decrease with time, with less cement penetration, with lower bone density and higher polyethylene wear.

Section snippets

Specimens

Twenty-two knees implanted with TKAs were retrieved serially over a period of 6 months from 17 donors post mortem at the Department of Legal Medicine at the University Clinic in Hamburg. Consent of the relatives was given, as well as ethical approval from the Medical Association of the State of Hamburg. All joints had been functioning asymptomatically, although a detailed description of activity was not available. Each tibia was sectioned approximately 15 cm distally to the knee joint, excised,

Specimens

Of the 22 retrievals, 12 patients had unilateral TKA, and 5 bilateral TKA. 16 of the implants were from female donors and 6 from male donors. The mean donor age per implant at the time of death was 80.1 years (68.8–98.3 years). The mean BMI per implant retrieved was 30.5 kg/m2 (19.6–43.0 kg/m2). Specimens were retrieved after a mean of 5.3 years in situ (0.0–11.2 years). The majority (n = 15) of implants was of fixed bearing design (Table 1). Nine of these were titanium, and 6 cobalt-chrome (CoCr). The

Discussion

The aim of this study was to better understand the mechanical failure of total knee replacements. For this purpose randomly retrieved post-mortem specimens were available, which had been in situ over periods of 0 to 11.2 years. A general decrease in anchorage strength with time in situ was observed for all implants combined and also within the PFC implant subgroups, despite variations in failure modes (failure interface). Expected effects, such as increased polyethylene wear score with time in

Conclusion

Autopsy retrieval studies allow implant systems to be studied that have been in service in vivo. In this relatively large knee retrieval study an even distribution of the input variables for a random sample allowed observations that give insight into tibial component anchorage. In contrast to a group of mixed retrieved implants, two popular contemporary bearing designs demonstrated minimal polyethylene delamination and pull-out failure was predominantly at the stem–cement interface,

Acknowledgements

This study was funded by a grant from The State of Hamburg, Germany.

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