Evaluation of the mechanical behaviour of the expandable wedge locked nail fixation in retrograde use: A finite element study.

INTRODUCTION: In literature, there have been many studies conducted to research the alternatives of standard interlocking intramedullary nailing. The expandable wedge locked nail fixation, which is thought as a new alternative to the standard interlocking nailing, has been presented in previous numerical studies. The antegrade usage of the wedge locked nail fixation has provided promising results. From this point, the aim of the study is to evaluate mechanical behavior of its retrograde usage on femur models. Additionally, another aim of the study is to investigate the effect of fracture level on mechanical properties of the fixation. MATERIALS AND METHODS: The mechanical behaviors of the wedge locked nail and standard interlocking nail fixations were compared by finite element methods. Sawbones femurs having osteotomies at five different levels to simulate different fractures were fixed with wedge locked nail or interlocking nail by using retrograde approach. With respect to the fracture level, two different nail lengths were used. Axial compression load was applied to fixations. The mechanical behaviors of the fixations were evaluated with respect to stiffness of the fixations and stresses occurred on both implants and bones. RESULTS: Any of the wedge locked nail fixation did not slip at canal. The stress and stiffness results were mostly close with each other for both nail types. The maximum stresses at locking elements or bones contacting these elements increased with decreased distance between the fracture and relevant locking elements. DISCUSSION: The wedge locked nail fixation showed comparable results to the standard interlocking nail fixation with respect to the stiffness and stress. Under axial loading, wedge locked nail provided a secured fixation without any slippage and preserved its position inside the medullary canal. It may be thought as a safe alternative to the standard interlocking nail fixation for retrograde usage. Additionally, according to stress results, it is advised to the surgeons to avoid a close locking to the fracture line.

Mechanical comparison of standard interlocking, clawed, and expandable wedge locked nail fixations: An experimental and numerical study.

INTRODUCTION: Intramedullary nailing has been used as a standard in the treatment of the long bone fractures with its clinical and mechanical advantages. However, using distal locking screws has been associated with longer operative times, higher radiation exposure rates, and complications like breakages of distal screw or nail at the screw hole level. Therefore, attempts to eliminate distal locking screws has been always present for intramedullary nail fixation. With a similar purpose, the present study has been carried out to compare mechanical behaviors of intramedullary nail fixations with different distal locking elements. MATERIALS AND METHODS: In this study, mechanical behaviors of standard interlocking and clawed nail fixations were compared experimentally in the first part. Six fourth generation Sawbones femurs, which have a simulated subtrochanteric fracture, were divided equally and were fixed with standard interlocking and clawed nails. During axial compression tests, experimental strain measurements were taken from all fixations. After validation of numerical models with using experimental strains and stiffnesses, mechanical behaviors of standard interlocking, clawed, and wedge locked nail fixations were compared numerically under axial compression loads. In numerical comparison, the stress-strain distributions were evaluated. RESULTS: Experimental results showed that although that there was no significant difference in stiffnesses, standard nail fixation bore two times higher loads than clawed nail fixations. Under loading, decrease in the distance between fracture surfaces was approximately seven times higher in the clawed nail fixation when compared to standard nail fixations. Numerical results showed that wedge locked nail fixation provided equivalent mechanical behavior to standard interlocking nail. DISCUSSION: In experiments of clawed nail fixation, the high decrease in the distance between fracture surfaces was evidence of the slippage of nail in the medullary canal. For a safe fixation, claws should be deployed when they are completely in contact with the cortical bone, they should be stuck into the bone in a fair amount, and the deployment in the distal third of the femur should be avoided. According to experimentally validated numerical analyses, wedge locked nail fixation may be an alternative for standard interlocking nail fixation if experimental studies support the present results.

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

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.

Expandable self-locking nail in the management of closed diaphyseal fractures of femur and tibia.

BACKGROUND: Intramedullary fixation is the treatment of choice for closed diaphyseal fractures of femur and tibia. The axial and rotational stability of conventional interlocking nails depends primarily on locking screws. This method uses increased operating time and increased radiation exposure. An intramedullary implant that can minimize these disadvantages is obviously better. Expandable intramedullary nail does not rely on interlocking screws and achieves axial and rotational stability on hydraulic expansion of the nail. We analyzed 32 simple fractures of shaft of femur and tibia treated by self-locking expandable nail. MATERIALS AND METHODS: Intramedullary fixation was done by using self-locking, expandable nail in 32 patients of closed diaphyseal fractures of tibia (n = 10) and femur (n = 22). The various modes of injury were road traffic accidents (n = 21), fall from height (n = 8), simple fall (n = 2), and pathological fracture (n = 1). Among femoral diaphyseal fractures 16 were males and six females, average age being 33 yrs (range, 18- 62 yrs). Seventeen patients had AO type A (A1 (n = 3), A2 (n = 4), A3 (n = 10)) and 5 patients had AO type B (B1 (n = 2), B2 (n = 2), B3 (n = 1)) fractures. Eight patients having tibial diaphyseal fractures were males and two were females; average age was 29.2 (range, 18- 55 yrs). Seven were AO type A (A1 (n = 2), A2 (n = 3), A3 (n = 2)) and three were AO type B (B1 (n = 1), B2 (n = 1), and B3 (n = 1)). We performed closed (n = 27) or open reduction (n = 5) and internal fixation with expandable nail to stabilize these fractures. The total radiation exposure during surgery was less as no locking screws were required. Early mobilisation and weight-bearing was started depending on fracture personality and evidences of healing. Absence of localised tenderness and pain on walking was considered clinical criteria for union, radiographic criteria of union being continuity in at least in three cortices in both AP and lateral views. Patients were followed for at least one year. RESULTS: The average operative time was 90 min (range, 55-125 min) for femoral fractures and 53 min (range, 25-115 min) for tibial fractures. Radiation exposure was minimum, average being 84 seconds (range, 54-132) for femoral fractures and 54 seconds (range, 36-78) for tibial fractures. All fractures healed, but few had complications, such as infection (one case with tibial fracture) bent femoral nail with malunion (n = 1), and delayed union (n = 3; 2 cases in femur and 1 case in tibia). Mean time of union was 5.1 months (range, 4-10(1/2) months) for femoral fractures and 4.8 months (range, 3-9 months) for tibial fractures. CONCLUSION: We found the nail very easy to use with effective fixation in AO type A and B fractures in our setting. Less surgical time is required with minimum complications. The main advantage of the expandable nail is that if affords. satisfactory axial, rotatory, and bending stability with decreased radiation exposure to operating staff and the patient.