Numerical evaluation of the mechanical properties of a novel expandable intramedullary nailing: A new alternative to standard interlocking nailing

Highlights

• In expandable nail, wedges are responsible for distal fixation by compressing the medullary canal in the radial direction.

• Tightening torque of wedge locking mechanism is directly effective in maintaining the nail position constant in the canal.

• The safety of the fixation is better ensured with increase of the tightening torque but stress states on bone must be carefully evaluated.

• Expandable nail provided comparable results to the standard nails in fixation stiffness, stress, and contact forces.

• The expandable nailing may be evaluated as an alternative in the fractures of long bones.

ABSTRACT

Introduction

: There has been a great effort in preventing the disadvantages of distal locking in intramedullary nailing to date. From this scope, a novel expandable nail fixation eliminating distal locking screws has been designed. The primary aim of this numerical parametric study is to investigate mechanical behavior of expandable nail fixation on the fractured femur model under different contact parameters which are effective in maintaining the nail position and to specify the appropriate values of these contact parameters for a safe fixation. The second aim is to compare mechanical behavior of the expandable nail fixation with the standard interlocking nail fixation.

Materials and Methods

: The expandable nail has three wedges which are responsible for distal fixation by compressing the medullary canal in the radial direction. 4th generation Sawbones femur model was used as bone model. A transverse osteotomy with 20 mm gap was created to simulate a subtrochanteric fracture. The fixations have been examined under axial compression with 1200 N and torsion with 7 Nm. In the parametric study, the tightening torque and static friction coefficient in wedge-canal contact were selected as contact parameters. The outputs were stiffnesses of the fixations, equivalent von-Mises stress distribution on the models, and load sharing between the canal and distal locking elements.

Results

: The results of the parametric study showed that the model with the tightening torque of 3 Nm and friction coefficient of 0.7 was the safest. The load borne by wedges is generally prone to increase with increased tightening torque and friction coefficient. The both fixations showed close stiffness and stress values.

Conclusion

: The tightening torque of the wedge locking mechanism is directly effective in maintaining the nail position constant in canal, and the safety of the fixation is better ensured with increased tightening torque but stress states on bone must be carefully evaluated. The expandable nail provided comparable results to standard interlocking nails with respect to the fixation stiffness, stress, and contact forces. The expandable nailing may be evaluated as an alternative in the fractures of long bones in the case that the numerical results are supported by future experimental studies.

Introduction

Intramedullary nailing has an important manner in the treatment of long bone fractures. Although the primal fixations like intramedullary nailing were seen in Aztecs’ records [1], the first successful intramedullary nailing was introduced by Smith-Petersen and colleagues in 1931 [2]. The first modern intramedullary nail design [3] was introduced in 1940 by Gerhard Küntscher who is accepted as the father of the intramedullary nailing by some. Although his studies [3,4] have been quite criticized by other surgeons at the beginning, the intramedullary nailing has become the gold standard in the treatment of long bone fractures.

The closed locked intramedullary nailing is highly preferred because it provides sufficient stability, allows early load bearing, walking, and physical exercises [5]. Additionally, it is advantageous to a certain extent in infection risk, hospital stay, non-union, or mal-union rates compared to some other methods [6]. Immediately after implantation, the most of the physiologic loads is borne by intramedullary nails and then the load is transferred to the bone with healing [7]. According to Bong et al., the fixation stability at the fracture site directly affects the load borne by intramedullary nails. If a good bone and nail construct is achieved, the load bearing capacity of the nail is increased.

The intramedullary nailing is generally performed with proximal and distal locking screws or bolts which restrict the motion of the nail in the medullary canal. This is the standard procedure in the interlocking intramedullary nailing. Locking the nail provides axial and rotational stability which are essential for the bone healing and preventing the further fractures. Socci et al. have stated the necessity of the distal locking in providing the axial and rotational stability of the unstable fractures or in the osteoporotic femurs [8]. On the other hand, Ska´la-Rosenbaum et al. have concluded that distal locking is required to provide the sufficient stability in the treatment of stable fractures with the short nails otherwise the postoperative femoral fractures may be occurred [9].

In despite of its advantages, interlocking intramedullary nails have several disadvantages. Kreb et al. have showed that intramedullary nailing without interlocking has decreased operational time and complication rate when compared to interlocking nailing [10]. With interlocking, the risk of infection may increase in consequence of extra wounds which are necessary for interlocking. Besides all these, interlocking intramedullary nailing has been associated with high radiation exposure in both patient's extremity and surgeon's dominant hand. Levin et al. have evaluated the radiation exposure on the surgeon hand during interlocking intramedullary nailing in their study [11]. They have showed that the amount of radiation exposure on the surgeon during distal locking was nearly the same as that the total amount of radiation exposure during the nail insertion and proximal locking. Beyond this, also the operating team is under radiation exposure during the intramedullary nailing; so several studies have been conducted to minimize the amount of the radiation exposure during intramedullary nailing [12], [13], [14].

From the mechanical point of view, Bong et al have stated that intramedullary nails bear compression, tension, and torsion loads combined in the physiologic conditions. This is similar to the physiological loading of the intact femur. The compression and tension on the intramedullary nail originate by the distance between the center of the femoral head to the shaft which causes the bending moment in the femoral shaft. If the contact of fracture is achieved, the cortical bone bears most of the compressive load. Otherwise, the compressive load acts to the locking screws and generates four point bending on the screws. In this situation, delayed fracture healing or nonunion may result in implant failure. The implant failure may occur as the breakage of the locking screw or breakage of the nail at the screw hole site [7]. Bucholz et al have reported that the fracture of the nail was commonly seen at upper distal screw hole [15]. Additionally, there are many studies reporting the breakage of the distal locking screws in the literature [16], [17], [18], [19], [20], [21]. The breakage of the distal screws may be sometimes associated with fatigue [16] or not [17,18]. The breakage of distal locking screw may be problematic because the removal of a broken screw must be sometimes very difficult [16].

There has been a great effort in preventing the disadvantages of distal locking of the intramedullary nailing to date. Some authors have studied the effects of the orientations, combinations, or the number of the distal screws with the idea of preventing the breakage of the distal locking screws [17,[22], [23], [24], [25], [26], [27], [28], [29]]. On the other hand, different nails or locking designs that totally eliminate the distal locking screws have been put into use in some studies [30], [31], [32], [33], [34]. From this scope of view, a novel expandable nail fixation eliminating the distal locking screws has been designed by the authors of the present study. It is thought that the expandable nail may become an alternative of the standard interlocking nails. The present study is a numerical parametric study that focuses on the medullary canal-nail contact that mainly effective in the load bearing capacity of the novel expandable nail fixation. The primary aim of this study is to investigate the mechanical behavior of the novel expandable nail fixation on the fractured femur model under different contact parameters which are effective in maintaining the nail position and to specify the appropriate values of these contact parameters for a safe fixation. The second aim of this study is to compare mechanical behavior of the expandable nail fixation with the standard interlocking nail fixation. In the analyses, the fixation models have been examined under the axial compression and torsional loads.