The sclerotic line: Why it appears under knee replacements (a study based on the Oxford Knee)
Introduction
Radiolucent lines and sclerotic margins are present on as many as 96% (Tibrewal et al., 1984) of knee radiographs taken a year or longer after surgical implantation of Oxford Unicompartmental Knee Replacements (OUKRs) (Biomet, Swindon, UK) (Goodfellow and O’Connor, 1978). A radiolucent line is a relatively dark line observed on radiographs adjoining the layer of bone cement under the tibial component. A sclerotic margin is a white line adjoining the radiolucent line. Histology of material from beneath OUKRs shows that the radiolucent line represents predominantly fibrocartilage and the sclerotic margin indicates lamellar bone (Tibrewal et al., 1984). Histological studies of post-mortem hip implants too have found fibrocartilage and fibrous tissue in the cement bone interface (Charnley and Crawford, 1968). The reasons for the formation of these types of tissue are not clearly understood although several theories exist. Radin et al. (1982) speculated that the tissues result from macrophage induced osteoclasis of bone and its fibrous replacement. Charnley and Crawford (1968) suggested that bone initially in contact with cement is damaged by thermal and chemical trauma and is removed and replaced by fibrous tissue. Others think that the bone at the cement interface undergoes resorption with subsequent fibrosis in the resulting gap (Freeman et al., 1982).
Despite these descriptions and conjectures, there is still no adequate explanation for the formation of the sclerotic line. Giori et al. (1995) proposed that bone necrosis occurs at the bone implant interface soon after implantation and that new tissue is formed at the interface. They used a two-dimensional finite element (FE) model of a proximal tibia implanted with a tibial component of a unicompartmental knee replacement and suggested that the type of tissue formed at the interface is related to the initial (soon after implantation) mechanical conditions to which the tissue is subjected. However, this study only involved a single loading iteration and therefore was unable to model the evolution of tissue over a time period. Furthermore, only the tissue in the radiolucent region was analysed, with no attention paid to the sclerotic line. Further limitations of the study were that it used a two-dimensional model together with a single set of material properties to represent bone.
The aim of the current study was to apply established rules for bone remodelling and tissue differentiation to a validated three-dimensional FE model of a human tibia, and use an iterative process to simulate the in vivo remodelling that occurs during the 1st year of post-OUKR surgery, and thus to investigate if and why the model predicts the radiolucent line and the sclerotic margin.
Section snippets
The FE model
A three-dimensional FE model of the proximal 75 mm of the medial half of a human tibia based on the geometry and material properties from a previously validated implanted full tibia model (Gray et al., 2008) was used in the current study. A reduced tibia model was used in this study in order to decrease the time taken for each iterative solution of the model (Fig. 1). The maximum and minimum principal strains for nodes in the bone within 3 mm of the bone–cement interface were compared using a
Results
The ‘synthetic radiographs’ were able to reproduce the radiolucencies and the sclerotic lines shown on knee radiographs taken a year or longer after surgical implantation of OUKRs (Fig. 3). Analysis revealed that the application of smaller loads resulted in less radiolucency in the 2 mm zone than did the application of larger loads. This is shown by the histograms in Fig. 4a and b, with the highest ARD% values occurring for the half load (load case 2) and the lowest for the double load (load
Discussion
The current study applied known rules for bone remodelling to a three-dimensional FE model of a human tibia in order to simulate the in vivo remodelling that occurs during the 1st year of post-OUKR surgery in order to understand the formation of the radiolucent line and the sclerotic line commonly seen on 1-year post operative knee radiographs.
The model was able to predict the radiolucent line as well as the sclerotic line and give some insight into factors that may have a bearing on the amount
Conclusion
Based on the findings of the current study the following theory was formulated to explain the formation of the radiodense layer of bone which is observed adjacent to the layer of soft tissue which produces radiolucency under the OUKR. Due to resecting of bone and the exothermic reaction of curing cement during implantation of an OUKR, necrosis of bone occurs. Mechanical stimuli have a significant role to play in the type of tissue formed subsequently under the cement layer. When the mechanical
Acknowledgments
NIHR Biomedical Research Unit at the Nuffield Orthopaedic Centre. Felix Scholarship for funding Hans Gray’s D.Phil. at the University of Oxford.
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