Experimental and computational studies on the femoral fracture risk for advanced core decompression
Introduction
Core decompression (CD) represents an established technique for treatment of early stage osteonecrosis and most commonly used for disease that affects the hip joint. The procedure is designed to decrease pressure within the bone by restoring blood flow to the bone (Steinberg, 2000, Veillette et al., 2006). CD consists of drilling one or more small channels into the dead bone (necrotic lesion) from the lateral subtrochanteric region of femur (Mont et al., 2004). This is associated with a lack of structural support of the bone. Subtrochanteric stress fractures at the surgical entrance point of the core track were regularly described as a complication of conventional core decompression with a rate of about 1–2% (Schneider et al., 2000, Smith et al., 1995, Steinberg, 1995, Steinberg et al., 1995). Camp and Colwell (1986) found an even higher fracture rate. That's why patients normally are requested to be partial weight bearing for several, normally six weeks due to the risk of fracture (Shuler et al., 2007, Veillette et al., 2006). Stronach et al. (2010) stated that a more proximal entrance point might prevent subtrochanteric fracture. However, these recommendations are mainly based on clinical experience. So there is a need for rigorous studies to determine specific indications for this kind of treatment.
The so-called “advanced core decompression” (ACD) is a modified technique of core decompression that may allow better removal of the necrotic tissue by using a new percutaneous expandable reamer and refilling of the drill hole and the defect with an injectable, hard-setting, composite calcium sulfate (CaSO4)–calcium phosphate (CaPO4) bone graft substitute (PRO-DENSE®, Wright Medical Technology™, Inc., Arlington, TN, USA) (Civinini et al., 2012). In a former study it was shown by biomechanical in-vitro tests that the load-bearing capacity of femur treated by ACD was the same as that of the untreated femur of the contralateral side from the same body donor (Landgraeber et al., 2012). These tests were performed using the standard entrance point at the tuberculum innominatum, under the assumption that this is the optimal entrance point. In the current study it should be evaluated what influence variations of the entrance point may have on the structural support of the bone. For that purpose finite element method is used in addition to biomechanical testing.
The finite element (FE) method has recently become a gainful and powerful technique for numerical simulation in orthopedic biomechanics (Lee et al., 2006, Lian et al., 2008, Volokh et al., 2006, Yang et al., 2002). Given the structural nature of the femur, finite element model derived from the reconstruction of CT or MRI images may help to effectively simulate the influences of CD on the mechanics of femur. Floerkemeier et al. (2011) performed a finite element analysis of CD using a 10-mm drill and CD via multiple small drillings without bone grafting as well as CD combined with the insertion of a tantalum rod. They suggested that CD using small drillings should be preferred since it was associated with a lower fracture risk compare to conventional CD using a 10-mm drill.
The aim of this study was to develop a model for bones treated by core decompression to answer two questions which are often addressed by orthopedic surgeons regardless if bone substitutes are used or not: 1) Is the core decompression procedure associated with a considerable lack of structural support of the bone? and 2) Is there an optimal region for the surgical entrance point for which the fracture risk would be lowest. The results of the model were verified by comparing with data from an established biomechanical in-vitro test. As core decompression in combination with bone substitutes becomes more and more common, we performed this study for the ACD.
Section snippets
Finite element simulation
A three-dimensional computer aided design model of a full left femur of a male donor was generated from a MRI scan data set of about 350 slices using the software Avizo® (VSG). The procedure included first a segmentation process that assigns the bone region to pixels of the image. A triangular surface model was then extracted from segmentation results based on surface reconstruction techniques. The last step was the generation of the model volume enclosed by the surface. A necrosis domain was
Finite element simulation
In this study, the bone failure criterion based on principal stresses was adopted to estimate fracture risk. Therefore, the maximum values of the first principal stress given in percent of the tensile strength of cortical bone were summarized in Table 4 for all models. For normal walking, the maximum principal stress in all models was smaller than the tensile strength of cortical bone. In the untreated model, the maximum stress was found on the superior domain of the neck and distal region of
Discussion
In this work, we used MRT scan-based linear finite element (FE) model of the femur to investigate the effect of the advanced core decompression (ACD) procedure and the entrance point position on the femoral fracture risk in the early postoperative state. Ten postoperative CD models with different entrance point positions and a preoperative (untreated) model were analyzed and compared. For each model, peak loads of hip joint forces for normal walking and walking downstairs have been applied.
To
Conflict of interest
One of the authors is a consultant of Wright Medical.
Acknowledgments
This study is supported by a research grant from the Bundesministerium für Wirtschaft und Technologie (BMWi), Germany and from the German Research Foundation (DFG), Germany (LA 2619/6-1 and 3396/9-1).
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