Finite Element Analysis of Necessity of Reduction and Selection of Internal Fixation for Valgus-impacted Femoral Neck Fracture

doi:10.21203/rs.3.rs-88627/v1

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Finite Element Analysis of Necessity of Reduction and Selection of Internal Fixation for Valgus-impacted Femoral Neck Fracture

https://doi.org/10.21203/rs.3.rs-88627/v1

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Background

Although valgus-impacted fractures don’t displace obviously, a larger valgus angle and posterior tilt of the femoral head can cause hip joint dysfunction and femoral head necrosis. The optimal surgical approach for valgus-impacted femoral neck fractures remains controversial. This study compared the biomechanical characteristics of different treatment strategies based on finite element analysis.

Methods

Ten valgus-impacted femoral neck fractures were analyzed in terms of posterior tilt and valgus angle measured on X-ray radiographs. Using these data, we generated 7 finite element models that were compared in terms of von Mises stress distribution and displacement.

Results

In the intact femur, von Mises stress was concentrated at the medial and inferior sides of the femoral neck. In valgus-impacted femoral neck fractures, von Mises stress was at the same locations but was 5.66 times higher than that in the intact femur. When 3 cannulated screws were used for internal fixation, anatomic reduction diminished the stress at the fracture end from 140.6 to 59.14 MPa, although displacement increased from 0.228 to 0.450 mm. When the fracture was fixed with a sliding hip screw (SHS) + cannulated screw, there was less stress at the fracture end and greater displacement with anatomic reduction than that without reduction (stress: 15.9 vs 37.9 MPa; displacement: 0.329 vs 0.168 mm).

Conclusions

The SHS + cannulated screw has superior biomechanical stability than 3 cannulated screws, and is recommended following anatomic reduction to treat valgus-compacted femoral neck fractures.

Orthopedic Surgery

Valgus-impacted femoral neck fracture

Internal fixation

Finite element analysis

Sliding hip screw

Cannulated screw

A valgus-impacted femoral neck fracture classified as Garden type I[1] has a posteriorly tilted femoral head, is relatively stable, associated with mild symptoms, and easily misdiagnosed. The optimal treatment approach for this type of fracture is controversial. Because of their stability (minimal displacement) they are treated conservatively sometimes. However, there is a high risk of further displacement and the relatively short femoral neck can lead to complications such as hip dysfunction and pain.[2-4] The necessity of reduction and method of internal fixation are also debated. Some studies suggest that for Garden I and II fractures—in particular, valgus-impacted femoral neck fractures—intraoperative reduction does not influence the success of the surgery.[5, 6] It has also been argued that as valgus-impacted fractures can destabilize after reduction, there is no need for anatomic reduction.[7] Others recommend anatomic reduction because the fact that malreduction significantly increases the risk of long-term complications.[8] Additionally, valgus impaction can cause femoral neck shortening resulting in lower limbs of unequal length, which could negatively affect hip function and gait.[2, 3] The most commonly used types of internal fixation are as follows: 1. Inverted triangle fixation of 3 semi-threaded cannulated screws; and 2. Sliding hip screw (SHS) with antirotation cannulated screw.[9, 10] In the present study, we compared the biomechanical stability of different types of internal fixation using models generated by finite element (FE) analysis, and investigated whether reduction can improve surgical outcome.

Measurement of displacement in valgus-impacted femoral neck fractures

In order to observe the actual displacement and measure the angle of rotation of valgus-impacted femoral neck fracture，We observed fracture morphology, measured displacement, and established the FE analysis model based on data from 10 patients with obvious valgus-impacted femoral neck fractures. The posterior tilt was measured from archived lateral X-ray radiographs as previously described.[11] Briefly, we drew a straight line perpendicular to the femoral neck cortex at the narrowest part of the femoral neck, followed by parallel lines at 5 mm on either side; the midpoints of the 3 lines were connected as the mid-collum line (MCL). A circle was drawn along the edge of the femoral head; the intersection of this circle with the MCL was taken as the radius collum line (RCL). The angle between the MCL and RCL was taken as the posterior tilt（Figure 1）. The valgus angle was measured according to the Garden index, which is defined as the angle between the medial femoral head pressure trabeculae and medial cortex of the femoral shaft[7] and normally has a value of 160°. In the event of a fracture, the valgus angle is determined as the increase in Garden index.

Image data acquisition and fracture modeling

An intact femur model was generated from computed tomography (CT) images of a 55-year-old healthy adult female volunteer. CT data were collected with a 64-row imaging system (Siemens, Munich, Germany) with a layer thickness of 0.625 mm and pixel size of 512 × 512. A total of 726 DICOM images were imported into Mimics v20.0 software (Materialise, Southport, Australia) to construct a 3-dimensional model of the femur. After conversion to stl format, Geomagic software (3D Systems, Rock Hill, SC, USA) was used to remove noise, optimize parameters, and generate a model of the curved surface of the femur, which was saved in stp format. The file was imported to Unigraphics NX software (Siemens) to obtain a 3-dimensional model of the femur; a cut was made in the subhead area of the femoral neck, with the position adjusted to simulate valgus-impacted femoral neck fractures. The limiting case of acceptable reset[7] was simulated: valgus angle of 20° (Garden Index of 180°) and posterior tilt of 20° (lateral Garden Index of 160°). The cancellous bone in the area of overlap between the femoral head and femoral neck was compressed to simulate an actual fracture. Internal fixation assembly and simulation of fracture reduction were performed on the model.（Figure 2）

Assembly of the internal fixation

The most commonly used internal fixation methods for femoral neck fractures are cannulated screws and SHS + cannulated screw. DePuy Synthes (Raynham, MA, USA) product specifications were used for the cannulated screws and SHS. The following models were compared: Model 1, intact femur; Model 2, valgus-impacted femoral neck fracture (without reduction or fixation); Model 3, valgus-impacted femoral neck fracture (with reduction but without fixation); Model 4, valgus-impacted femoral neck fracture (without reduction but fixed with 3 cannulated screws); Model 5, valgus-impacted femoral neck fracture (with reduction and fixed with 3 cannulated screws); Model 6, valgus-impacted femoral neck fracture (without reduction and fixed with SHS + cannulated screw); and Model 7, valgus-impacted femoral neck fracture (with reduction and fixed with SHS + cannulated screw).

Tetrahedral 10-node 3D meshing of the models was performed using Workbench software (Ansys, Canonsburg, PA, USA). The number of nodes and elements in the fracture model and for the internal fixation (Table 1) were counted. The models were validated and the mesh quality was >98%.

Table 1. Number of nodes and elements in the femur and internal fixation screws

Model	Nodes	Elements
Femoral head	81,043	54,000
Femoral shaft	1021	557
Cannulated screws	25,687	13,451
Sliding hip screw	300,175	165,829

The properties of bone and the titanium alloy internal fixation materials (Table 2) are reported elsewhere.[12, 13]

Table 2. Properties of bone and internal fixation

Material	Young’s Modulus (MPa)	Poisson ratio
Cortical bone	16,800	0.3
Cancellous bone	840	0.2
Internal fixation	105,000	0.35

Boundary and loading conditions

The femur was adducted by 15°[14], the lower end of the femur was fixed, and a pressure of 700 N[15] was applied directly above the femoral head to simulate the situation of an adult standing on 1 leg.（Figure 3）

Measurement of femoral head displacement in valgus-impacted fractures

From the measurement based on X-ray of 10 patients with valgus-impacted femoral neck fractures, the femoral head showed valgus and posterior tilting (18° and 19°, respectively).

Comparison of von Mises stress distribution between intact femur and valgus-impacted femoral neck fractures

In all models, von Mises stress was concentrated at the medial and inferior sides of the femoral neck, with a higher stress distribution for the former than for the latter. von Mises stress was 5.66 times higher in the valgus-impacted fracture (Model 2; 67.3 MPa) than that in the intact femur (Model 1; 11.9MPa) (Figure 4). In the reduction-only model (Model 3), without fixation, the fracture end is very unstable. With a little external force, there is a great displacement. So， no further calculations or processing were performed.

Valgus-impacted femoral neck fracture fixed with 3 cannulated screws

Fixation followed by anatomic reduction reduced von Mises stress at the fracture end to 42% of the value for fixation without reduction (59.14 vs 140.6 MPa; Figure 5). In the reduction with fixation model, cannulated screws bore more stress at the fracture end, yielding a stress value that was 2 times more than that in the non-reduction model (Figure 6).

Valgus-impacted femoral neck fracture fixed with SHS + cannulated screw

After reduction, stress distribution at the fracture end was significantly reduced (from 15.921 to 37.9 MPa; Figure 7), but there was little change in the stress on the internal fixation device (Figure 8).

Comparison of the 2 types of internal fixation

The above comparisons indicate that using SHS + cannulated screw reduced von Mises stress distribution at the fracture end compared to 3 cannulated screws, regardless of whether the fracture was reduced.

Comparison of displacement

In all models, the femoral head showed the greatest displacement; Model 2 had a maximum displacement closest to that of the intact femur (0.332 and 0.382 mm, respectively). The displacement was significantly reduced in Model 4 (0.228 mm) and the head was the most stable after fixation with the SHS + cannulated screw without reduction (Model 6), with a displacement of just 0.168 mm. In the 2 models with reduction, the maximum displacements were 0.45 mm (Model 5) and 0.329 mm (Model 7) (Figure 9).

Treatment of valgus-impacted femoral neck fractures

Femoral neck fractures are clinically challenging because of the high risk of complications. Internal fixation and arthroplasty are the most commonly used surgical techniques for treating femoral neck fractures, but there is disagreement over which one is superior. Internal fixation can significantly reduce the occurrence of complications. A systematic review of 1120 cases treated conservatively and 3951 treated with surgery revealed bone healing rates of 68.8% and 92.6%, respectively, and femoral head necrosis rates of 10.3% and 7.7%, respectively.[16] However, femoral head necrosis and fracture nonunion cannot be completely avoided with internal fixation, which has complication rates of 15.9%–34.6%[17]^,[18] and a high rate of reoperation, especially in elderly patients. Kain et al.[19] reported that 10% of patients over the age of 65 years with stable femoral neck fractures underwent conversion arthroplasty at a mean of 9 months after internal fixation. In a recent multicenter randomized controlled trial involving elderly patients (≥70 years) with undisplaced femoral neck fracture, there was no difference in hip function between those with screw fixation compared to arthroplasty, but the former was a risk factor for reoperation.[20] As such, doctors are increasingly recommending arthroplasty over screw fixation for elderly patients, even for undisplaced fractures.[18-20] However, in another study, internal fixation of stable femoral neck fractures achieved good results in patients >65 years of age; at the 1-year follow-up, 24/25 cases achieved fracture union and only 3 cases experienced complications (1 nonunion, 1 osteonecrosis, and 1 subtrochanteric fracture following minor trauma).[21] Thus, appropriate internal fixation and surgical technique can reduce the risk of complications and improve outcome.

Displacement and prognosis of valgus-impacted femoral neck fractures

Most Garden I and II fractures (79%) have varying degrees of posterior displacement in lateral X-ray radiographs.[22] In this study, the main valgus angle was 18° and the average posterior tilt angle was 19°, similar to previous reports[22-24] and close to the limit of acceptable reduction.[7] A preoperative posterior tilt ≥20° or anterior tilt >10° increases the risk of fixation failure;[6, 24] therefore, some doctors consider a neck-shaft angle between 130° and 150° and 0–15° of anteversion as acceptable reduction for displaced femoral neck fractures.[25, 26] Mild valgus (<15°) is tolerable but varus angulation and posterior tilt are not as they increase the probability of complications.[7, 25, 27] A more stringent standard for reduction has been proposed as valgus angulation >5° following reduction is thought to significantly affect long-term prognosis, the incidence of severe hip osteoarthritis at the 10-year follow-up was 55.6%, which was 2 times higher than that in cases of valgus angulation <5° (21.2%).[8] However, it is reported that for Garden I and II femoral neck fractures, good reduction doesn’t reduce the failure rate of surgery, and the reoperation rate is related only to preoperative valgus angulation.[5] But another study showed that preoperative posterior tilt measurements on lateral radiographs could not be used to detect healing complications in these fractures.[11]

Internal fixation of femoral neck fractures

Cannulated screws and SHS are the most commonly used materials for fixation of femoral neck fractures, although there is controversy over which one is superior.

Characteristics of cannulated screws

3 cannulated screws arranged in an inverted triangle is widely accepted as the surgical treatment for femoral neck fractures. [12]^,[13] However, cannulated screws have a high failure rate (up to 38.7%)[28] and places a high technical requirement on the surgeon. Drilling frequency must be limited to achieve better outcomes.[12] A triangle formed by one inferior partially threaded and 2 superior fully threaded cannulated screws is more stable and can bear more stress while maintaining the femoral neck at an appropriate length.[13]

Characteristics of SHS + cannulated screw

SHSs have greater stability than cannulated screws and are preferred for unstable femoral neck fractures such as the Pauwel III and cervical basal types.[29-32] When using SHSs, an antirotation cannulated screw is needed to counteract rotation, and the tip apex distance must be <25 mm to prevent the screws from cutting out.[33-35]

Comparison between cannulated screws and SHS + cannulated screw

There is ongoing debate over whether more stable fixation can be achieved with SHS + cannulated screw vs 3 cannulated screws. A retrospective analysis of 65 patients found that the former was associated with greater fracture shortening than the latter.[10] However, a prospective randomized controlled study with a follow-up of 4 years reported no significant differences in fracture healing and femoral head necrosis rates and postoperative hip function score between the two types of fixation.[36]

The most common complication after internal fixation with cannulated screws is femoral neck shortening and subtrochanteric soft-tissue irritation caused by prominent cancellous screws.[37] The outcome is better with SHS, with no further displacement or soft tissue stimulation. However, the incidence of osteonecrosis was higher in patients treated with SHS compared to cannulated screws, possibly due to poor control over the rotation of the large-diameter screw.[38] Some studies have found no difference in rates of postoperative complications and hip function between the two types of internal fixations, although SHSs were associated with greater intraoperative bleeding and operation time.[39]^,[40]

Biomechanical characteristics of internal fixation for valgus-impacted femoral neck fractures

Our results showed that von Mises stress was concentrated at the medial and inferior sides in the intact femur, which explains why the femoral neck fractures so easily. All 4 surgical methods increased the stress at the medial and inferior sides of the femoral neck in the rank order of Model 7 < Model 6 < Model 5 < Model 4. The stress in Model 7 (15.921 MPa) and displacements in Models 2 and 7 (0.332 and 0.329 mm, respectively) were closest to the values for the intact femur (Model 1: stress, 11.911 MPa; displacement, 0.38 mm). The latter result indicates that the valgus-impacted fracture was stable in Models 2 and 7, and explains why some patients can move the hip joint and feel no pain. Fixing the fracture by cannulated screws after reduction (Model 5) significantly increased the displacement (0.45 mm), making it the most unstable treatment option. Taken together, our results indicate that fracture reduction is necessary and that the SHS + cannulated screw yields an outcome that is closest to the intact femur in terms of stress distribution and displacement, and is therefore a better choice for fixation.

Factors contributing to complications

Blood supply

An inadequate blood supply to the femoral neck is the main reason for nonunion and osteonecrosis. Blood is mainly supplied to the proximal femur by the retinacular arteries, which are derived from the medial femoral circumflex artery.[41-43] We previously reported that the microstructure of the Weitbrecht retinacula consists of loose structures containing vessels and nerves sandwiched between and protected by dense structures.[44] Retinacula (and the different parts thereof) vary in terms of the extent of attachment to bone. The anterior retinaculum is more strongly affixed than the superior retinaculum, whereas the inferior retinaculum is anchored to the femur at either end but has no connection to the bone along the middle portion. Thus, the inferior retinacular artery is more mobile than other retinacular vessels and is more likely to escape injury, while the anterior retinaculum may be easily injured in the event of fracture. For valgus-impacted femoral neck fractures, the superior retinaculum can be clamped at the fracture end, which would block the blood supply.

Open reduction alters the blood supply to the femoral neck to a greater degree than closed reduction, which can in theory increase the incidence of nonunion and osteonecrosis. One study reported that for femoral neck fractures with significant displacement, the rate of necrosis was higher after open vs closed reduction but there was no difference in healing rate.[45]

Use of stable-length implants

In valgus-impacted fractures, the femoral head is usually valgus and posteriorly tilted, with shortening of the femoral neck. During the bone healing process, the femoral neck may be further shortened as a result of bone resorption at the fracture end and continuous compression from the internal fixation, irrespective of the type of screw that is used. In the past, femoral neck shortening was considered acceptable, but it is now known to negatively affect hip function and gait, decrease patients’ quality of life, and increase the rate of reoperation.[2, 3, 46, 47] Therefore, some doctors recommended stable-length implants (for example, full-thread cannulated screws, crossed cannulated screws, and proximal femoral locking plates) to fasten and prevent shortening of the femoral neck, leading to a better prognosis.[48-51]

Effect of intraarticular pressure

There is no evidence that intraarticular pressure affects the prognosis of femoral neck fractures. Our previous research showed that an increase in intracapsular pressure reduced but did not completely obstruct blood flow. Vessel twisting and stretching has a greater negative impact on blood flow than intravascular pressure does, underscoring the importance of fracture reduction.[52]

Although valgus-impacted femoral neck fractures are relatively stable, we still recommend surgical treatment to avoid the risk of further displacement and to restore hip function. In the present study, FE analysis revealed that SHS + antirotation cannulated screw fixation after anatomic reduction showed the highest biomechanical stability, making it the ideal surgical approach. Anatomic reduction is necessary as it can decrease stress on the fracture end, improve postoperative hip joint function, and reestablish the blood supply to the Weitbrecht retinacula; it may also decrease nonunion and osteonecrosis rates, although additional studies are required to confirm this possibility.

MFCA: medial femoral circumflex artery; SHS:sliding hip screw; MCL:mid-collum lines; RCL:radius collum line; MIMICS:Materialise's interactive medical image control system

Ethics approval and consent to participate

Approval was not required, as neither human participants nor animals were involved in this study.

Consent for publication

Not applicable.

Availability of data and materials

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests

Funding

This study was supported by the National Natural Science Foundation of China (Grant No. 81271991), and the general project of Shanghai Municipal Health Burean (201840137).

Authors’ contributions

Y. Dai performed the literature search, drafted the manuscript, generated the 3D models, and analyzed the data. M. Ni carried out the FE analysis and assisted with manuscript preparation. B. Dou collected data and critically revised the manuscript. Z. Wang analyzed the data and prepared the figures. Y. Zhang and X.Cui assisted with manuscript translation. W. Ma, and T. Qin, collected the X-rays and did the measurement. X.Xu developed the 3D model. J. Mei designed the study and edited, reviewed, and revised the manuscript.

Acknowledgements

Not applicable.

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Finite Element Analysis of Necessity of Reduction and Selection of Internal Fixation for Valgus-impacted Femoral Neck Fracture

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