Conflict of interest: None.
A computational study on the effect of fracture intrusion distance in three- and four-part trochanteric fractures treated with Gamma nail and sliding hip screw
Version of Record online: 10 OCT 2013
© 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.
Journal of Orthopaedic Research
Volume 32, Issue 1, pages 39–45, January 2014
How to Cite
Goffin, J. M., Pankaj, P. and Simpson, A. H. (2014), A computational study on the effect of fracture intrusion distance in three- and four-part trochanteric fractures treated with Gamma nail and sliding hip screw. J. Orthop. Res., 32: 39–45. doi: 10.1002/jor.22469
- Issue online: 19 NOV 2013
- Version of Record online: 10 OCT 2013
- Manuscript Accepted: 22 JUL 2013
- Manuscript Received: 5 JAN 2013
- finite element analysis;
Using finite element analysis, the behaviors of the Gamma nail and the sliding hip screw (SHS) were compared in an osteoporotic bone model for the fixation of three- and four-part trochanteric fractures (31-A2 in the AO classification, types IV and V in Evans' classification). The size of the medial fragment was varied based on clinical data, and the case of a fractured greater trochanter was also considered. Our results showed that for Evans' type V stabilized with a Gamma nail and for Evans' types IV and V with the SHS, cancellous bone around the lag screw is susceptible to yielding, thus indicating a risk of cut-out. The volume of bone susceptible to yielding increases with an increase in size of the medial fragment. Conversely, Evans' type IV with a Gamma nail was not predicted to cut out. Our findings suggest that future clinical trials investigating fixation of unstable proximal fractures should include the size of the medial fragment and the integrity of the greater trochanter as covariables and be powered to evaluate whether intramedullary devices are superior to SHSs for Evans' type IV fractures and inferior/equivalent for type V. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:39–45, 2014.
In the latest Cochrane review comparing intramedullary nails with extramedullary implants for the fixation of extracapsular hip fractures, Parker et al. concluded that the sliding hip screw (SHS) was superior to intramedullary devices due to lower complication rates. The following outcomes were considered: operative fracture of the femur, later fracture of the femur, cut-out, non-union, reoperation, and deep wound infection. Conversely, a recent prospective randomized trial reported no significant difference between the two implants in reoperation rates (revision surgery was indicated in case of screw cut-out, implant failure, late fracture, and deep infection) or any secondary outcome measure (e.g., mortality, transfusion, length of hospital stay, and mobility). The authors concluded that SHS remains the gold standard for stabilization of fractures labeled as 31-A2 in the AO classification because it is associated with similar outcomes at a lower cost.
Unfortunately, previous studies do not discriminate among patients with respect to the size of the medial fragment or greater trochanter integrity. In fact, this fracture group (31-A2), defined by the lack of medial support at the level of the lesser trochanter, can be subdivided into two subgroups according to Evans' classification, based on the absence (Type IV) or the presence (Type V) of a fractured greater trochanter (Fig. 1). The Cochrane review also suggested that for subtrochanteric and some unstable trochanteric fractures, it might be worth investigating further whether intramedullary devices such as the Gamma nail have advantages that outweigh those of the SHS.
Cut-out is the most frequent failure mode of these devices. Born et al. reported that the cut-out rate can be as high as 8% for hip screws. Therefore, our first aim was to use finite element (FE) modeling to investigate a range of fracture configurations (Evans' type IV and V with an intrusion distance up to 60%) based on clinical data from our unit and to determine whether bone-implant constructs with higher intrusion distances are at greater risk of cut-out. Our second aim was to compare the biomechanical response of the SHS with an intramedullary Gamma nail. We hypothesized that in terms of load sharing, the Gamma nail is superior to a SHS for fractures in which the greater trochanter remains intact.
MATERIALS AND METHODS
The shape of a 3rd generation composite femur was taken from the BEL repository (www.biomedtown.org). Isotropic elastic properties corresponding to osteoporotic bone were assigned to bone components: elastic modulus was 77 MPa for cancellous bone and 12.4 GPa for cortical bone; both were assigned a Poisson's ratio of 0.3.
Two subtypes of pertrochanteric fractures were considered: 31-A2 according to the Müller AO classification of fractures or types IV and V according to Evans' classification. The anatomy of the fracture complex was defined by several parameters. One was the angle of the fracture line lateral to the medial fragment, whose mean value was 43° (unpublished clinical data from our unit). Another was the intrusion distance of the medial fragment into the fracture complex, which was defined as the ratio of the length of the medial wedge in the AP view to the length of the fracture line (expressed as a percentage). This second parameter was varied from 15% to 60%. To simulate a type V fracture, the greater trochanter was cut so that no bone supported the top of the Gamma nail. Since the integrity of the trochanter should not affect the mechanics of the SHS construct, the SHS models applied to both types IV and V. The validity of this assumption was confirmed for 40% intrusion distance, since it is a typical example of the fracture fixation model, which exhibits a large volume of bone susceptible to yielding and large stresses in the implant.
FE modeling was used to compare the mechanical environment for the fixation of the aforementioned fractures with the Gamma nail and the SHS. Our FE models were developed using Abaqus 6.8-1 (Simulia, Providence, RI) and Autodesk Inventor Professional 2011 (Autodesk, San Rafael, CA). Boolean operations were applied to the composite femur and 3D models provided by Stryker Osteosynthesis (Schönkirchen, Germany) to reproduce the drilling and reaming employed in the operative management of pertrochanteric fractures.
The Gamma3 Trochanteric Nail 180 (Stryker) had material properties of Ti-6Al-4V alloy, that is, 113.8 GPa for elastic modulus and 0.34 for Poisson's ratio. Locking of the nail was performed distally. According to a study of >3,000 cases of Gamma nail fixation (including 35% classified as 31-A2), distal locking was performed in ∼96% of patients. Distal locking was therefore considered adequate to represent clinical practice. A “set screw”, sometimes used for proximal locking, was not employed. A frictional interaction was assumed between the lag screw and the nail. The angle between the nail and the lag screw was 125°.
The Omega3 Compression Hip Screw (Stryker) had properties of 316 LVM stainless steel with an elastic modulus of 195 GPa and a Poisson's ratio of 0.3. A 130° 4-hole standard barrel plate was locked with Asnis III screws to the distal femur.
The femoral head was subjected to a load of 1866N, corresponding to the maximum load on the hip during a walking cycle for an 80 kg person. The force vector pointed laterally in the coronal plane with an angle of 13° from the axis of the femoral shaft. In the sagittal plane, it pointed posteriorly and was characterized by an angle of 8° with the shaft.
For boundary conditions, the point of load application on the head was constrained in the plane orthogonal to the loading vector while the medial femoral condyle was constrained in the three translational degrees of freedom at a point located in the coronal plane 23 mm away from the axis of the shaft. The bone could rotate about this point about the frontal and the sagittal axis.
Contacts between bone and implant and between bone fragments were considered to be frictional, except for the threaded surfaces of the lag screw and of the distal locking screws, for which tie constraints (bone bonded to the screw) were used. Friction coefficients were taken from the literature: 0.46 between bone fragments, 0.23 between Ti-6Al-4V parts, 0.3 between bone and Ti-6Al-4V, 0.2 between stainless steel components, and 0.42 between bone and stainless steel.
The models were meshed using 4-node linear tetrahedral elements. Convergence tests were performed on the Gamma nail 3-part fracture model with 15% intrusion distance to ensure a sufficiently fine element discretization. The displacement at the point of load application in the direction of the load was computed as a function of the total number of elements.
Von Mises stresses in implants and minimum principal strains in cancellous bone were reported; the former is an indicator of yielding of metals and the latter an indicator of possible cut-out due to compression.[11, 12] A minimum principal strain of −1.3% was taken as a cut-off (yield strain) value below which bone was susceptible to yielding in accordance with previously published data. Regions characterized by strains smaller (i.e., more compressive) than this value were assigned a gray color to emphasize regions where bone was susceptible to yielding.
The mesh convergence study (Fig. 2) showed that the increase in displacement was <2% when the number of elements increased from 684,813 to 1,367,831. Therefore, numbers of elements from 1 to 1.5 million for different models were employed, resulting in meshes much finer than those used in previous studies.[7, 8]
Analyses with Evans' type IV and type V fractures were conducted to confirm the assumption that mechanics of the SHS construct is similar for both fracture types. The stress and strain variations were similar for the two cases (Table 1). With the SHS, all subsequent analyses were therefore confined to Evans' type IV fractures.
|SHS 3-Part/Evans' Type IV||SHS 4-Part/Evans' Type V|
|von Mises stress (lag screw)||1,881 MPa||2,052 MPa|
|Bone susceptible to yielding||4,499 mm3||4,501 mm3|
In the implant, the largest von Mises stresses were at the medial side of the aperture for the lag screw (Figs. 3 and 4). Stresses with almost similar magnitude were found on the lateral side. For the SHS, the location of the largest von Mises stresses was on the superior surface of the lag screw (Fig. 5). Their peak values increased for a given fracture/implant type with increase in the intrusion distance of the medial fragment into the fracture complex (Fig. 6). For a particular distance, the peak stress was higher in the SHS than in the Gamma nail for both fracture types.
In cancellous bone, contour plots showing minimum principal strains in a cross-section through the head and neck are shown in Figure 7, since failure of fixation is likely to occur due to compressive strains superior to the screw. Quantitative values of the bone volume with a minimum principal strain below −1.3% (Fig. 8) increased as the size of the medial fragment increased for Gamma nail 4-part fractures and for the SHS. This region was larger for the Gamma nail 4-part fracture (Evans' type V) than for the SHS (Evans' types IV and V). Regarding the Gamma nail 3-part fracture (Evans' type IV), the volume susceptible to yielding was small for most intrusion distances, except for 60% where the volume was higher, but still much smaller than the volumes corresponding to the Gamma nail 4-part fracture and the SHS.
The displacement of the femoral head along the axis of the shaft showed that the Gamma nail construct was stiffer than the SHS system (Fig. 9). It increased with increasing intrusion distances; comminution of the greater trochanter decreased even further the stiffness of the Gamma nail constructs.
Our results show that as the size of the lesser trochanter fragment becomes larger the likelihood of failure increases as demonstrated by increasing stresses in the implant and larger compressive strains in the bone at higher intrusion distances. Our results also show that failure with the SHS is more likely than with an intramedullary device for Evans' type IV fractures. However, this apparent advantage of intramedullary devices over the SHS seems less obvious when treating Evans' type V fractures, where the integrity of the greater trochanter is lost. In this case, although the peak stresses in the SHS are higher, the region of bone susceptible to cut-out is higher with an intramedullary device. Currently the SHS is the gold standard for fixation of trochanteric fractures.[1, 2] As randomized control trials looking at unstable trochanteric fractures are not equipped to examine differences in the subgroup 31-A2, the trials to date would have missed a possible benefit of intramedullary devices in Evans' type IV trochanteric fractures with large lesser trochanteric fragments.
The yield strength of Ti-6Al-4V (Gamma nail) is 880 MPa. As a consequence, for most intrusion distances in 3-part fractures, the Gamma nail is not predicted to yield. For models with an intrusion distance >25% and 4-part fractures, von Mises stresses exceeded the yield stress slightly, so the nail may yield in localized regions with load redistribution. The yield strength of grade 316 LVM stainless steel (SHS) is around 792 MPa, a value which is slightly less than half of the peak von Mises stresses seen for the case of the SHS.
The results obtained for von Mises stresses in the implant parts can be explained by the mechanics of load sharing. The Gamma nail construct is stiffer than the SHS. Furthermore, when the greater trochanter is intact, the top of the nail contributes to resisting bending across the fracture complex. This is not the case with the SHS model in which the fracture complex is held together primarily by the lag screw. With increase in fracture intrusion distance, the fracture fixation systems are required to carry higher loads, which result in higher peak stresses.
The SHS does yield or fail with a tendency to bend in trochanteric fractures with low stability, which is a clinical fact supported by the increasing trend in peak von Mises stresses shown in this study. For the Gamma nail, the peak stresses were on the medial side of the nail at the level of the aperture. This hole is a typical location of implant breakage. But stresses with almost similar magnitude were found on the lateral side; von Mises stress is indeed independent of compression/tension. The different location of the large von Mises stresses at the nail aperture can be explained by the fact that at low intrusion distances (Nail 3-part), the aperture mainly undergoes bending with compression on the medial side, while for high intrusion distances (Nail 3-part) and for 4-part fractures, compression imparted by the screw at the inferior side appears to dominate.
Bone yielding likely contributes to fracture fixation failure via cut-out of the tip of the lag screw through the surface of the hip or via implant loosening if the surrounding bone is weakened by yielding. Cut-out is primarily due to the large compressive strains (or minimum principal strains) exerted on the cancellous bone. For the Gamma nail 3-part fracture (Evans' type IV), the contour plots predicted almost no regions of bone yielding in the region superior to the thread of the lag screw. Conversely, for the Gamma nail 4-part fracture (Evans' type V) and for the SHS (both fracture types) a large gray region occurred around the lag screw thread emphasizing the region of cancellous bone likely to yield and therefore at high risk of being involved in cut-out.
Linear elasticity was used to model the mechanical behavior of the different materials. As long as models are compared within the scope of the study, use of a linear elastic model is adequate. The 4-node tetrahedron is an acceptable element type for the purpose of comparing different models and outlining general trends. Furthermore, the 4-node tetrahedron is capable of producing closely matched predictions to those produced by 10-node tetrahedra if the mesh employed is sufficiently fine.
A load of 1866N was applied to the model, thereby assuming full weight-bearing in patients. In the period following implantation, the actual load on the femoral head is likely lower, thus decreasing the bone volume susceptible be yielding. Only a single load case was considered without including muscle forces. Other load cases would be unlikely to change the trends provided by our results. A recent study with a methodology similar to ours concluded that additional muscle loading as suggested by Heller et al. had no effect on the stresses in the implant.
The implant was assumed to be perfectly positioned, that is, middle position of the lag screw in the AP view and middle position in the lateral view together with a reasonable tip-apex distance. This position is supported by a large clinical study on 937 patients that showed that the lowest percentage of cut-out cases had been obtained for a middle-middle position.
Consistent with previous studies, a bonded interaction was employed between the lag screw thread and the cancellous bone since the screw does not readily permit relative movement. Bonded contact, however, can transfer tensile forces, which can result in a slight underestimation of compressive strains superior to the screw. Rotation of the head is indeed more likely with an anterior or posterior screw but less likely here since the lag screw lies in the center of the head.
Potential limitations of the anatomy of the fracture complex include the facts that the fracture surfaces were considered to be smooth and the influence of the angle of the wedge removed at the level of the lesser trochanter was not considered.
Femoral heads obviously feature anisotropy and variable density. However, homogeneous and isotropic material properties were used to prevent specific assumed anisotropy/inhomogeneities from influencing the results. Previous numerical[7, 8] and experimental[21-23] studies also used homogeneous material properties for bone.
No experimental validation was conducted, which clearly is a limitation. However, our aim was to examine trends rather than absolute values and to show that some fracture/implant configurations are less prone to cut-out than others. In this respect, the lack of experimental validation is justified. A previous experimentally validated numerical study, which employed the same loading and boundary conditions as our study found a peak von Mises stress of 623 MPa for a Gamma nail model without lack of medial support (AO 31-A1.1). In our study, a higher von Mises stress of 700 MPa was computed for the same construct with 15% intrusion distance.
In conclusion, the behaviors of two fixation devices used to stabilize pertrochanteric fractures in osteoporotic bone were compared using FE modeling. For 3-part fractures (Evans' type IV), superior results were obtained by stabilizing the fracture with an intramedullary device such as the Gamma nail instead of the more unstable SHS. This superiority is relinquished for 4-part fractures (Evans' type V) when the integrity of the greater trochanter is lost. These conclusions contrast with the recommendations of the last Cochrane review on the subject and provide a rationale for a prospective controlled trial to examine this clinically.
The first author thanks Finn Donaldson and Noel Conlisk for helpful discussions and is indebted to Stryker Osteosynthesis (Schoenkirchen, Germany) and in particular to Christian Lutz, Geert von Oldenburg, and Claus Gerber, who generously provided him with 3D CAD models of the Gamma nail and of the SHS. The first author was supported by a Principal's Career Development Scholarship awarded by The University of Edinburgh.
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