A patient specific finite element simulation of intramedullary nailing to predict the displacement of the distal locking hole
Introduction
Intramedullary nailing is a common operative method for treatment of fractures of femoral and tibial shafts [1]. In this method, a metallic tubular rod or nail is inserted in the medullary canal to hold the bone fragments together. In order to provide a stable fixation, which is critical for the healing process, interlocking screws are used at the proximal and distal ends of the nail, to maintain the length, alignment and rotation of the bone.
The procedure of intramedullary nailing can be technically challenging due to the difficulties encountered in the insertion of the interlocking screws. For proper insertion of screws, it is necessary to drill the bone at the proximal and distal sites such that the resulting holes are exactly coaxial with the interlocking holes on the nail. In general, proximal locking might be easily performed using a mechanical guide which is mounted on the proximal end of the nail. However, such method is not applicable for distal locking due to the deformation of the nail after insertion. Considering the requisite of entering into the canal, the intramedullary nail has usually a different curvature from the medullary canal; the average radius of curvature of the femoral medullary canal is about 722 mm and that of the nail in the range of 1500–3000 mm [2]. This substantial difference results in a relatively large deformation in the distal nail, reaching up to 18 mm in the lateral plane [3]. As a result, the position of the distal hole is changed during the insertion procedure and cannot be located using a pre-designed proximally mounted targeting device that does not compensate for nail deformation [4], [5].
The typical method of finding the position of the distal hole is the freehand technique. In this method, a large number of fluoroscopic shots are taken from the inserted nail, at different intensifier adjustments in the coronal and sagittal planes, until perfect circles are obtained for the holes. Obviously, this technique exposes the patient and operating room staff to excessive radiation [6], and prolongs the operation time [4], [7], [8], with magnitudes depending heavily on the surgeon's experience and skill.
There have been several proposed solutions to facilitate the distal locking procedure [9], based on electromagnetic systems [7], [8], [10], stereo and virtual fluoroscopy [11], [12], mechanical devices mounted on the image intensifier [13], [14], and proximally mounted radiation dependent mechanical guides [15], [16], [17]. Such solutions, however, are technically demanding and require extra equipment and trained staff that might not be affordable for many operating rooms. There have been also attempts to develop improved image-based techniques in order to recover the axis of a distal hole using two [18], [19] or even one fluoroscopic shot [20], [21]. Although these techniques are claimed to reduce the dosage of exposure, their efficacy for practical use has not been verified yet.
Patient-specific finite element (PSFE) modeling has been extensively used in recent years to predict the biomechanical behavior of muscelo-skeletal systems during medical interventions [22]. In particular, several studies have used PSFE to predict the mechanical response of long bones, i.e., stress, strain and displacement fields, after nail insertion [23], [24], [25]. However, to the best of our knowledge, there is no previous attempt on the estimation of the deformation of an inserted intramedullary nail, using simulation method.
The objective of this study is to propose and validate a PSFE model of intramedullary nailing, based on the preoperative data, and in order to predict the 3D deflection of the nail before surgery. Using such model, it will be possible to estimate the altered position of the distal hole of the nail, preoperatively, and provide the required compensatory information for tuning the proximally mounted targeting systems during surgery. Besides, this model can help nail manufacturers to optimize the design of the nail by analyzing the sensitivity of the nail deformations or stress distribution with respect to its geometrical parameters.
Section snippets
Modeling
A fresh frozen human femur from a 63-year-old male cadaver was obtained with the approval of the local ethics commission. The approximate length of the femur specimen was 380 mm. The bone was cleaned from soft tissues and scanned in air using a CT machine (Brilliance 64, Philips, Germany; intensity: 100 mA; voltage: 120 kV) with a voxel size of 0.33 × 0.33 × 1 mm3 and a slice thickness of 0.67 mm. A bone density calibration phantom (QRM-BDC/3 H200, QRM Gmbh, Germany) with three reference
Results
Typical results of the PSFE model for the intact and distally fractured femoral models with the 11 mm nail (tests No. 2 and No. 6, respectively) are illustrated in Fig. 8. In all cases, the maximum stress occurred in the middle third of the femur and nail, around the femoral canal isthmus. The maximum von Mises stress ranged between 18 and 40 MPa for the femur, and 212 and 320 MPa for the nail. In one case, i.e., test No. 2, the stress in the nail exceeded the yield stress and the nail
Discussion
Proximally mounted targeting systems are often used in operation theaters to reduce the potentially harmful radiation burden during intramedullary nailing. These systems, however, suffer from low accuracy in locating the distal hole of the nail, considering the fact that they do not compensate for the insertion-induced nail deformations. In this paper, a patient specific finite element model was proposed to predict the altered position of the distal hole preoperatively, in order to provide the
Acknowledgment
The authors would like to thank Osveh Asia Medical Instruments Company for supplying the implants, and Fanavaran Jarahyar Sharif Company for providing the segmentation software used in this research. This work was supported by a research grant from Iranian National Science Foundation.
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2019, Medical Engineering and PhysicsCitation Excerpt :In spite of the good performance claimed for some of these methods, none has gained widespread clinical acceptance due to being technically demanding, i.e., imposing extra equipment and specially trained staff to the operating room. In a recent study, we proposed a pre-planning methodology, based on patient-specific finite element modeling, to estimate the deformation of the nail during surgery [18]. In spite of the good accuracy shown in a cadaver experiment (max error: 1.5 mm), this methodology requires high resolution CT data of the bone before surgery, which is costly and not clinically indicated for long bone fractures.