The effect of locked distal screws in retrograde nailing of osteoporotic distal femur fractures: a laboratory study using cadaver femurs

OBJECTIVES: To examine the effects of locked distal screws in retrograde nails used in unstable osteopenic distal femur fractures. DESIGN: Biomechanical testing of paired human cadaveric femurs. INTERVENTION: Seven matched pairs of embalmed, moderately osteopenic cadaver femurs were instrumented with 12-mm intramedullary nails in a statically locked, retrograde fashion. One femur of each pair had locked distal screws and the other femur had unlocked distal screws. A 2.5-cm gap of bone was cut nine centimeters from the distal condyles to simulate an unstable fracture. The locked distal screw nails were compared to unlocked distal screw nails for collapse of the fracture gap, medial-lateral and anterior-posterior translation of the nail within the fracture site, and fracture angulation. The femurs were axially loaded, cycled, and then loaded to failure. MAIN OUTCOME MEASURES: Motion at the fracture site with axial cyclic loading and site of failure when loaded to failure. RESULTS: After cycling, both locked distal screw and unlocked distal screw nails demonstrated several millimeters medial and anterior translation within the fracture site and approximately 1 mm collapse of the fracture gap. Although no statistically significant differences were found, the locked distal screw nails had less anterior and medial translation, angulation, and collapse of the fracture gap after cycling. Loads to failure were similar for both locked distal screw and unlocked distal screw nails. It was noted that proximal femur failure occurred at the level of the proximal screw hole in the nail at the subtrochanteric level in 7 (4 locked distal screws and 3 unlocked distal screw groups) of the 14 samples. Four other samples failed through the intertrochanteric region (2 locked distal screw and 2 unlocked distal screw groups) and the remainder within the distal fragment by fracture of the femur along the medial cortex. CONCLUSIONS: Although most differences in fixation stability were not significant, the locked distal screw nails exhibited less fracture collapse and anterior and medial translation of the nail at the fracture site than the unlocked distal screw nails. The degree of varus angulation after cyclic loading was also less for the locked distal screw nails. The length of the nail chosen should avoid having proximal locking screws distal to the lesser trochanter, thus averting proximal femur stress risers and fractures.     

Use of Huckstep nail in the periimplant femoral shaft fracture

87-year-old female underwent open reduction of distal femoral fracture and internal fixation with locking compression plate and bone graft. She was operated for ipsilateral proximal femoral fractures and stabilized by intramedullary interlocked nail 5 years ago. She developed stress fracture proximal to locked plate. We inserted Huckstep nail after removal of the previous operated proximal femoral nail without removing the remaining plate and screws. At 15 month followup the fractures have united. The Huckstep nail has multiple holes available for screw fixation at any level in such difficult situations.     

Treatment of femoral shaft fractures with a titanium intramedullary nail

Ninety-nine femoral shaft fractures were treated with locked intramedullary nails made from titanium alloy. One of the distal interlocking screws failed in six fractures (6%) and both screws failed in two fractures (2%). Delayed union was associated with all of the eight fractures that had locking screw failure. Young, heavier patients who had nails of small diameter had an increased risk of screw failure. Additional surgery was needed when both screws failed. The authors still use this nail, but currently prefer to ream the medullary canal more so that larger nails can be inserted. Decisions concerning weightbearing are made on an individual basis for each patient, and currently full weightbearing is delayed for young, active, and heavy patients. Two distal interlocking screws should be inserted for treatment of femoral shaft fracture when a Ti locked intramedullary nail is used.     

Mechanical study of the safe distance between distal femoral fracture site and distal locking screws in antegrade intramedullary nailing

OBJECTIVE: To determine the safe distance for distal femoral fractures relative to the distal locking screws in antegrade intramedullary femoral nailing using a currently available titanium alloy nail design. DESIGN: Cyclic (fatigue) mechanical testing study. SETTING: Biomechanics laboratory. INTERVENTION: Intramedullary nailing of left synthetic fiberglass composite femora with type 32/33-C fractures at 1, 2, 3, and 4 cm from the more proximal of the distal locking screws. MAIN OUTCOME MEASUREMENT: The number of loading cycles to failure of the nail. RESULTS: A load level of 700 N through the femoral mechanical axis was validated as adequate to cause fatigue failure within 200,000 cycles in slotted stainless- steel nails. In the nonslotted titanium alloy nails, this load level caused failure in only 1 of 3 nails with a fracture at 2 cm from the more proximal of the 2 distal locking screws and in 2 of 3 nails with a fracture at 1 cm from the more proximal of the 2 distal locking screws. All of the other nails did not fail >1 million cycles. CONCLUSIONS: Under laboratory conditions, it is safe to assume that an antegrade titanium alloy nail will survive 1 million compression/bending cycles when the fracture is > or = 3 cm from the more proximal of the 2 distal locking screws.     

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

IntroductionIntramedullary 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 MethodsIn 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.ResultsExperimental 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.DiscussionIn 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.     

Fracture strain and stability with additional locking screws in intramedullary nailing: a biomechanical study

BACKGROUND: We studied the effect of additional locking screws on fracture strain and stability in tibial intramedullary nailing. METHODS: We drilled an additional diaphyseal locking hole into 8-mm solid tibial nails 185 mm from the proximal end of the nail, and locked it proximally and distally. An osteotomy was produced 4.5 cm distal to the additional hole, and the construct loaded axially, in flexion, extension, and torsion. The nails were also tested for their fatigue strength. RESULTS: With the additional locking screw, strain increased proximally during loading in neutral and flexion. Strain decreased on loading in extension. The extra locking screw decreased strain close to the osteotomy site in all loading positions. A significant reduction in angular motion at the osteotomy site occurred with the addition of the extra locking screw. The nails survived the fatigue test, although the stress increased around the additional locking hole CONCLUSIONS: Nails with additional locking options, by altering strain and motion at the fracture site, may have the clinical potential to affect fracture healing.     

Femoral shaft fractures in the elderly treated with Grosse-Kempf slotted locked intramedullary nail.

Thirty-one femoral shaft fractures in patients over 60 years of age were treated with the Grosse-Kempf slotted, locked intramedullary nail and followed for a median of 24 months. The patients tolerated the operation reasonably well and the mortality was not higher than the mortality connected with femoral neck fractures. Three reoperations were performed due to intra- and postoperative complications. The main reason for the less satisfactory result was shortening > 2 cm after unstable dynamical locking. This occurred in 8 of 15 dynamically nailed fractures with locking screw(s) in one end of the nail only. A 9 cm shortening among these was the reason for one of the two poor results in the series. The other poor result was a malalignment of the distal fragment in a statically locked fracture. We conclude that locked intramedullary nailing is a good way to treat femoral shaft fractures in the elderly, the high subtrochanteric, midshaft and infraisthmic fractures.     

Infra-isthmal fracture is a risk factor for nonunion after femoral nailing: a case–control study

Background

The rates of nonunion after femoral nailing are currently reported to be 4.1–12.5 %. The purpose of this study was to identify the risk factors of noninfected nonunion after femoral nailing, focusing in particular on the effects of the length of the distal main fragment.

Methods

A case–control study was conducted with 105 patients, with a case (nonunion group)–control (control group) ratio of 1:2. The nonunion group (n = 35) comprised patients with consecutive symptomatic nonunions after femoral nailing who were treated in our institute; the control group (n = 70) were matched by age to the nonunion group. Type of fracture, soft tissue injury, length of femur and nail length, incidence of screw breakage, nail diameter, mean length of distal main fragment, and any episode of dynamization were retrospectively examined. Univariate and multivariate analyses were performed to elucidate the risk factors of nonunion after femoral nailing.

Results

Increased risk of nonunion after femoral nailing was associated with (1) open fracture, (2) screw breakage, (3) shorter length of a distal fragment, and (4) any episodes of dynamization. Receiver operating characteristic analysis showed that a distal fragment length of <43 % of the total femur length was the cutoff level for nonunion after nailing. The odds ratio for nonunion was 6.40 (95 % CI 2.70–15.2) when the length of the distal main fragment was <43 % of the femur length. Multivariate logistic analysis revealed that the risk of nonunion after femoral nailing increased (1) with breakage of locking screws (p = 0.0021), (2) with dynamization (p = 0.0029), (3) with a shorter distal fragment length (p = 0.0379), and (4) with an open fracture (p = 0.0397).

Conclusion

The elucidated risk factors of nonunion after femoral nailing were identified as open fracture, infra-isthmal femoral fracture, breakage of locking screw, and inappropriate dynamization. We believe that the surgeon should be consciously aware of the need for additional surgical fixation for the distal fragment when performing femoral nailing of infra-isthmal femoral fractures.     

The Gamma nail. A preliminary note

The Gamma nail is a new device used in the treatment of pertrochanteric fractures in the elderly in which the principles of locked intramedullary nailing are applied. It is constituted by an intramedullary nail crossed in the proximal segment by a cervico-cephalic screw, by which compression of the fracture segments may be obtained. It is furthermore possible to obtain distal locking by means of two screws, such as for Grosse-Kempf nailing. It constitutes an alternative to Ender nailing, screw-plate, and nail-plate. Our experience with 31 cases did not reveal any differences as compared to what occurs with Ender nailing with regard to surgical stress. Instead, a shorter amount of time seems to be required for functional recovery, as the stability of the assembly allows for early weight-bearing.     

Treatment of femoral shaft aseptic nonunion associated with broken distal locked screws and shortening

BACKGROUND: This retrospective study investigated the treatment of femoral shaft aseptic nonunions associated with broken distal locked screws and shortening. METHODS: In this study, 11 femoral shaft aseptic nonunions associated with both broken distal locked screws and shortening in 11 consecutive adult patients were treated. All nonunions were associated with at least 1.5 cm (range, 1.5-3.5 cm) shortening. These nonunions were treated by removal of locked nails, one-stage femoral lengthening, static locked nail stabilization, and corticocancellous bone graft supplementation. Postoperatively, ambulation with protected weightbearing was encouraged as early as possible. RESULTS: Ten nonunions were followed up for a median of 4.1 years (range, 1.8-5.5 years), and nine fractures healed at a median of 4 months (range, 3-6 months). The nonunion case had broken locked screws again at 5 months and was treated with exchange nailing. The fracture healed uneventfully at 4 months. No other complications occurred. CONCLUSIONS: The key to removal of broken screws is withdrawal of the nail slightly to release the incarcerated broken screw end. The screw end then is pushed out with a used Knowles' pin or a smaller screwdriver under image intensifier guidance. Concomitant one-stage femoral lengthening to treat nonunion with shortening has a high success rate.     

Gamma钉治疗股骨转子间骨折远端是否锁钉疗效比较

目的探讨Gamma钉内固定治疗股骨转子间骨折远端是否锁钉对疗效的影响。方法对144例稳定性股骨转子间骨折患者采用Gamma钉内固定,根据远端锁钉与否将患者分为远端未采用锁钉组(非锁钉组,70例)和远端采用锁钉组(锁钉组,74例)。观察两组患者手术时间、透视时间、输血量、住院时间、骨折复位质量、骨折愈合时间及并发症。结果两组均获得12个月随访。两组骨折复位质量、住院时间、骨折愈合时间比较差异无统计学意义(P>0.05)。手术时间、术中透视时间、输血量锁钉组明显长(多)于非锁钉组(P<0.001)。两组患者并发症发生率比较差异无统计学意义(P>0.05)。结论采用Gamma钉治疗稳定性股骨转子间骨折远端可以不行锁钉固定,不影响骨折愈合和临床疗效,还具有减少透视时间、手术时间、术后并发症等优点。     

Anterograde femoral nail for the treatment of femoral metastases

Introduction  Intramedullary nailing is a common technique for the treatment of impending and pathological fractures of the femur due to bone metastases when diaphysis or metadiaphysis is involved. Reconstructive nailing is currently used in the treatment of subtrochanteric involvements for reducing the risk of fracture above the nail. The aim of this study is to assess the results of a consecutive series of 13 impending or complete femoral fractures due to metastastic localization treated with anterograde femoral nail (Synthes). Patients and methods  Thirteen cases of femoral metastases in patients afflicted by multiple bone localization were considered in this series. Indications were femoral fracture or prophilactic stabilization in the case of osteolysis with a fracture risk defined according to Mirels’ criteria. Titanium anterograde femoral nail was used in all the cases. The proximal part was 17 mm in diameter in all the cases; nail diameter ranged from 10 to 12 mm, and nail was distally locked with two screws inserted in a static mode. Biopsy for confirming the diagnosis was routinely performed. Nail was locked with two distal screws inserted in a static mode and two proximal full screws with recon mode. Discussion  The improvement of the quality of life, with no mechanical problems (screw breakage or implant failure), was observed in this series. Two patients died within 6 months after operation; the others were alive at the time of follow-up (maximum follow-up of 16 months). Results  Results confirmed that intramedullary reconstructive-locked nailing is the treatment of choice in plurimetastatic patients afflicted by impending/or complete femoral fracture. These results suggest that reconstructive nailing using an AFN may be considered a useful strategy for the treatment of diaphyseal and metaphyseal femoral metastatic lesions.     

Immediate weight-bearing after treatment of a comminuted fracture of the femoral shaft with a statically locked intramedullary nail

BACKGROUND: The purpose of this two-part investigation was to test the feasibility, safety, and efficacy of immediate weight-bearing after treatment of fractures of the shaft of the femur with a statically locked intramedullary nail. METHODS: In the first part of the investigation, a biomechanical study was performed to determine the fatigue strength of eleven different statically locked intramedullary nail constructs. Segmentally comminuted midisthmal fractures were simulated with use of sections of polyvinyl chloride pipe; each construct was cyclically loaded in compression with use of physiologically relevant loads in a materials testing machine at eight hertz. The fatigue tests were conducted according to the so-called staircase method, and the construct was considered to have run out (exceeded its anticipated service life) if it had not failed after 500,000 cycles. In the second part of the study, a clinical investigation of immediate weight-bearing after treatment of comminuted fractures of the femoral shaft with a Russell-Taylor (RT-2) construct was performed. Complete follow-up data were available for twenty-eight of the thirty-five patients (thirty-six fractures) entered into the study. RESULTS: In Part I of the study, two constructs, a statically locked twelve-millimeter-diameter Russell-Taylor femoral nail with two distal locking screws (RT-2) and a statically locked twelve-millimeter-diameter Zimmer femoral nail with two distal locking screws (Z-2), had significantly higher mean fatigue strengths (2171 and 2113 newtons, respectively) than all other constructs tested (p<0.001), but the strengths of these two constructs were not significantly different from each other. Constructs with only one distal locking screw demonstrated significantly lower (p<0.05) fatigue strengths than the two-screw constructs. These results suggest that full weight-bearing during the weeks immediately after insertion of the nail may be possible, even for patients who have a comminuted fracture of the femoral shaft. In Part II of the study, twenty-six of the twenty-eight patients were bearing full weight on the fractured limb or limbs at the six-week follow-up visit. All fractures united; only one of these needed an additional procedure (the removal of the screws five months after the insertion of the nail) to stimulate union. No loss of fixation, such as back-out or breakage of a locking screw or breakage or bending of the intramedullary nail, occurred. CONCLUSIONS: We concluded from this two-part investigation that immediate weight-bearing after stabilization of a comminuted fracture of the femoral shaft with a statically locked intramedullary nail is safe when the construct has a relatively high fatigue strength. Immediate weight-bearing after stabilization of a fracture of the femoral shaft permits patients who have multiple fractures of the extremity to walk and to participate in physical therapy earlier, possibly decreasing the duration of the hospital stay or reducing the need for prolonged rehabilitation on an inpatient basis.     

Early experience with the gamma interlocking nail for peritrochanteric fractures of the proximal femur.

Surgical fixation, early weight-bearing, and bony union remain a challenge in the treatment of peritrochanteric femur fractures, especially if the fractures are comminuted or unstable. Preliminary experience with the Gamma locking nail, a short intramedullary nail connected to a sliding compression screw augmented with distal locking screws, is presented. In a consecutive series of 29 patients, all fractures were adequately reduced and immediate weight-bearing was begun regardless of fracture configuration (13/27 fractures classified as unstable). Twenty-seven patients were reviewed at 6 months. At follow-up, all patients continued to be ambulatory and all fractures healed. Major complications included screw migration in the femoral head (two patients), difficulty in securely placing the distal screws (eight patients), and a femoral shaft fracture through the distal locking screws following a fall. The technical problems inherent in the device and its instrumentation are discussed. In this early experience, the Gamma nail appears to allow for early patient ambulation regardless of the fracture configuration with excellent clinical results.     

Biomechanical performance of short and long cephalomedullary nail constructs for stabilizing different levels of subtrochanteric fracture

IntroductionThe aim of this study was to assess biomechanical performance of short and long Cephalomedullary nail constructs consisting of different number of distal screw for stabilizing different levels of subtrochanteric fracture.Materials and methodsThe femur obtained from computed tomography scanner was used to create a transverse fracture at 15 mm (level A), 35 mm (level B), and 55 mm (level C) below the lesser trochanter. Short and long Cephalomedullary nails were virtually inserted to the fractured femur. Four-node tetrahedral element was used to build up finite element (FE) models for biomechanical analysis. The analysis focused on post-operative stage of partial weight-bearing.ResultsStress on the implant localized at the surface between lag screw/nail and distal screw/nail. Short Cephalomedullary nail exhibited higher stress than long Cephalomedullary nail. The stress in short Cephalomedullary nail could be reduced by using two distal screws fixation and the fracture at level A produced less stress than that of level B and C. Either short or long nail with two distal screws is sufficient to withstand the stress magnitude produced from the physiologic load. When single dynamic distal screw was used, stress on implant, elastic strain at fracture gap, and bone stress reached the high values. Elastic strain of the fracture gap at level C were less than that of level A and B, but no statistically significant difference. There was no proximal cancellous bone damage observed from the FE analysis.ConclusionsLong Cephalomedullary nail with at least two distal locking screws remains a proper implant for subtrochanteric fracture fixation in overall locations. However, short Cephalomedullary nail with two distal screws may be a candidate for a high subtrochanteric fracture. Single dynamic screw insertion is strongly not recommended with either short or long nail regarding implant failure.     

Distal locking in short hip nails: Cause or prevention of peri-implant fractures?

ObjectivesThe most common cause of femoral fractures after osteosynthesis of trochanteric fractures with short nails is weakening of the femoral cortex via distal locking and stress concentrations at the tip of the nail. The aim of the study was to verify whether the incidence of peri-implant fractures is dependent upon the distal locking technique.MethodsWe prospectively analysed a group of 849 pertrochanteric fractures (AO/ASIF 31-A1 + 2) managed with short nails from 2009 to 2013. Unlocked nailing was performed in 70.1% and distal dynamic locking was performed in 29.9%. The mean age was 82.0 years. Peri-implant fractures were divided into 3 groups according to the height of the fracture in relation to the tip of the nail.ResultsIn total 17 fractures (2.0%) were detected. One peri-implant fracture occurred after locked nailing, whereas 16 cases occurred after unlocked nailing (p = 0.037). Patients without distal locking had an 85.7% greater risk of peri-implant fracture. Fractures of the proximal femur (Type I) occurred significantly earlier than fractures at the tip of the nail (Type II) (p = 0.028).ConclusionUnlocked nails do not guarantee sufficient stability. Distal locking serves to prevent postoperative femoral fractures. We recommend the routine use of distal locking when utilizing short nails.     

Removal of a broken intramedullary femoral nail with an unusual pattern of breakage: a case report

To the best of our knowledge, only 3 cases, including the present case, have been reported with a three part broken pattern. However, this is the first case associated with a distal locking screw broken. We report the case of a 31-year-old patient who sustained an open femoral shaft fracture . The fracture was stabilized with a Kuntcher femoral nail. After 7 months of the initial surgery he presented with a three part broken intramedullary nail and the distal locking screw broken. We used a combined technique for the removal of the nail through the nonunion fracture site; we used a pull out technique for the middle fragment and a curved thin hook for the distal fragment. Then we applied bone allograft and stabilized with a cannulated intramedullary femoral nail (Synthes, Oberdorf, Switzerland). After 2 years of follow up the nonunion was consolidated and the patient presented a good clinical outcome. This is of particular interest because it is a unique case and the association with a broken distal locking screw is reported for the first time in this study. A combination of methods through the nonunion site approach and an alternative instrumental is a good method for the removal of a hollow femoral intramedullary nail with this unusual pattern of breakage.     

Effectiveness of completely round nails with both-ends-threaded locking screws for tibial shaft fractures

BACKGROUND: Completely round nails, in contrast to conventional locked nails with surface grooves, for postoperative endosteal revascularization may increase the nail rigidity and decrease the manufacture cost. Both-ends-threaded screws with higher fatigue strength require smaller nail holes and can further increase the mechanical strength of the nails. METHODS: In this study, both-ends-threaded locking screws were used to treat 68 tibial fractures, including 56 acute fractures in 54 patients and 12 nonunions in 12 patients. There were 41 men and 25 women with a mean age of 39.6 years, and the average follow-up was 24 months. For acute fractures, there were 11 type I, 9 type II, 5 type IIIA, and 3 type IIIB open fractures. Acute fractures were treated with closed nailing. Five hypertrophic nonunions were treated with closed exchanged nailing, and seven oligotrophic nonunions were treated with open nailing and iliac bone grafting. RESULTS: With a single nailing procedure, 53 acute fractures and all nonunions achieved union with mean times of 17.4 weeks and 18.4 weeks, respectively. Three patients underwent exchange nailing and bone grafting and had eventual fracture union. Three fractures with compartment syndrome were treated with fasciotomy. Deep infection occurred in two open fractures but was successfully treated. Nail breakage occurred in one distal fracture and screw backout occurred in another, but fracture union was not affected. Recovery of joint motion was essentially normal in patients without knee or ankle injury. CONCLUSIONS: This study showed that completely round nails with both-ends-threaded locking screws could effectively treat tibial fractures. Completely round nails have the advantages of high mechanical strength and low manufacturing cost.     

Antegrade locked nailing for humeral shaft fractures

Treatment results of antegrade locked nailing of acute humeral shaft fractures, including union rate and recovery of shoulder function, have been inconsistent. This led the current authors to hypothesize that implant design and surgical techniques might account for this inconsistency. In the current study, 47 fractures (38 acute; nine pathologic) in 47 patients achieved union with the techniques of closed nailing, short to long segment nailing, and fracture compression. Satisfactory recovery of shoulder function occurred because of minimal surgical trauma, prevention of impingement by the nail or locking screws, and prevention of axillary nerve injury or comminution of the humeral head. Forty-seven patients with 38 acute fractures and nine pathologic fractures were treated with humeral locked nails. Mean followup time was 21.4 months. With a single operation, all 38 acute fractures proceeded to eventual union; the average time to union was 7.8 weeks. Thirty-five patients had excellent or satisfactory recovery of shoulder function. Complications included slipout of the proximal screw, nail breakage, fragment displacement, and transient postoperative radial nerve palsy. All nine patients with pathologic fractures had substantial pain relief and increased arm function after surgery. The current study shows the reliability of antegrade locked nailing for proximal and middle third fractures of the humeral shaft.     

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.