M. Pan: Study design, data collection, analysis and interpretation, and drafting of the manuscript.
L. Chai: Study design, data collection, analysis and interpretation, and drafting of the manuscript.
F. Xue: Study design, data analysis and interpretation, and critical revisions.
L. Ding: Study design, data interpretation, and critical revisions.
G. Tang: Study design, data collection, and critical revisions.
B. Lv: Study design, data collection, and critical revisions.
The aim of this study was to compare the biomechanical stability and clinical outcome of external fixator combined with limited internal fixation (EFLIF) and open reduction and internal fixation (ORIF) in treating Sanders type 2 calcaneal fractures.
Two types of fixation systems were selected for finite element analysis and a dual cohort study. Two fixation systems were simulated to fix the fracture in a finite element model. The relative displacement and stress distribution were analysed and compared. A total of 71 consecutive patients with closed Sanders type 2 calcaneal fractures were enrolled and divided into two groups according to the treatment to which they chose: the EFLIF group and the ORIF group. The radiological and clinical outcomes were evaluated and compared.
The relative displacement of the EFLIF was less than that of the plate (0.1363 mm to 0.1808 mm). The highest von Mises stress value on the plate was 33% higher than that on the EFLIF. A normal restoration of the Böhler angle was achieved in both groups. No significant difference was found in the clinical outcome on the American Orthopedic Foot and Ankle Society Ankle Hindfoot Scale, or on the Visual Analogue Scale between the two groups (p > 0.05). Wound complications were more common in those who were treated with ORIF (p = 0.028).
Both EFLIF and ORIF systems were tested to 160 N without failure, showing the new construct to be mechanically safe to use. Both EFLIF and ORIF could be effective in treating Sanders type 2 calcaneal fractures. The EFLIF may be superior to ORIF in achieving biomechanical stability and less blood loss, shorter surgical time and hospital stay, and fewer wound complications.
The study aimed to design a new treatment for calcaneal fractures and compare the biomechanical stability and clinical outcome of external fixator combined with limited internal fixation and open reduction and internal fixation in treating Sanders type 2 calcaneal fractures.
This new technique could fix Sanders type 2 calcaneal fractures.
The patients were satisfied with the technique.
The rate of wound complications was low.
A strength of the study was the evaluation of outcomes in a series of patients with a comparison group.
As a strength, both the stability and clinical outcomes were evaluated.
As a limitation, the study lacked biomechanical analysis.
Calcaneal fractures are the most common type of tarsal bone fractures in clinical practice. Almost 75% of these fractures are intra-articular, and their management is still controversial.
A GE 64-row spiral CT scanner (Siemens, Munich, Germany) supported by the Southern Medical University Fengxian District Central Hospital was used to conduct the helical scan of the male volunteer’s right lower limbs (173 cm height and 64 kg weight, the trial had been approved by the hospital’s ethic committee, and an informed consent had been signed), and the data were saved in Digital Imaging and Communications in Medicine format. The scanning conditions were as follows: 155 mAs at 120 kV. The thickness from 15 cm above the ankle to the planta pedis was 2 mm. The size of the pixel matrix density in each scanning layer was 512 pixels × 512 pixels.
The data were inputted into the Mimics 15.01 software (Materialise NV, Leuven, Belgium). A computer-aided design (CAD) model (
The CAD model of the EFLIF for Sanders type 2 calcaneal fractures.
The CAD model of the plate for Sanders type 2 calcaneal fractures.
Finite element models were constructed using linear tetrahedrons. In this study, there were 765 848 elements with 163 617 nodes in the EFLIF model after meshing, and 1 323 960 elements with 254 345 nodes in the plate model.
The related material properties of a calcaneal fracture finite element model were set up to establish a 3D finite element model similar to the practical model in material parameters and mechanical behaviour. The ligaments were set up as linear elastic materials sustaining tensile stress only. The material parameters of the fixation devices were obtained from Gefen et al
Material parameters for the bone and fixation systems
Component name | Young’s modulus (MPa) | Poisson’s ratio | Yield strength (MPa) |
---|---|---|---|
Schanz screw | 193 400 | 0.33 | 225 |
External fixator connecting rod | 69 400 | 0.31 | 135 |
Plate and screw | 2 000 000 | 0.28 | 225 |
Cortical bone | 7300 | 0.3 | |
Cancellous bone | 1100 | 0.26 | |
Cartilage | 1 | 0.1 | |
Fracture line | 5 | 0.4 |
Simkin and Stokes
A total of 50 consecutive patients with closed Sanders type 2 calcaneal fractures were included in this study from January 2010 to December 2013, and divided into two groups according to the treatment (as chosen by the patient): the EFLIF group and the ORIF group. The trial had been approved by the hospital’s ethic committee, and all patients signed informed consent. The inclusion criteria were as follows: patients with unilateral Sanders type 2 calcaneal fractures, diagnosed by radiograph and CT images; ages from 18 to 58 years; and patients with a clear trauma history, and without any other treatment. The exclusion criteria were as follows: age less than 18 years or more than 58 years; Sanders types 1, 3, and 4 calcaneal fractures; bilateral calcaneal fractures or combined with other damage; open or pathological fracture, and infection near the heel combined with serious medical diseases, cannot tolerate surgery or actively co-operate with the post-operative function of exercisers.
All patients were positioned supine with continuous epidural anaesthesia. In the EFLIF group, the use of a tourniquet was not preferred. Two 5 mL syringe needles were used to position the medial side of the non-fractured area of the calcaneal tuberosity. Two Schanz screws were then inserted through a small incision to replace the syringe needle after the most appropriate location was verified by a lateral radiographic view. A Schanz screw was inserted into the tibia and navicular bones in the same way. The fixator rods and clamps were assembled to allow for a full visualisation of the subtalar joint on a lateral radiographic view. The clamps were then locked temporarily under traction. A 2.5 cm incision was made under the lateral malleolus to expose the surface of the subtalar joint. The fracture could be reduced under direct vision. Two cannulated screws were inserted to fix the fractures. Finally, the clamps were locked permanently after the reduction was confirmed by fluoroscopy.
In the ORIF group, the patients underwent a standard procedure as described by Benirschke and Sangeorzan.
Three months after healing, two examiners assessed pain, walking ability, gait, ankle activity, ankle stability, and alignment, respectively. Hospital stay, blood loss, operation time, intra-operative fluoroscopy times, subtalar joint range of movement, return to previous work, patient satisfaction, and subtalar arthritis were recorded and compared between the two groups.
Means (and standard deviations) were calculated for each variable of interest, and
In a neutral loading of 160 N, the relative displacement was as follows: 0.1363 mm for the EFLIF in treating a Sanders type 2 calcaneal fracture; 0.1808 mm for the plates and less than 1 mm for both fixation systems.
There was no fixation failure in either fixation system. The highest value of von Mises stress of the external fixator was 30.88 MPa on the Schanz screw (
External fixator stress nephogram.
Screw stress nephogram.
Plate stress nephogram.
Age, gender, type of accident, accompanying injuries and comorbidities were similar for the two groups. Follow-up time in the ORIF group was 23.66 months and 25.03 months in EFLIF group, and no significant differences were observed between the two groups (t = 1.686, p = 0.096). The external fixator and the plate were removed at the last follow-up. The timing of surgery, operation time, blood loss, and hospital stay were significantly less in the EFLIF group (p < 0.05). The intra-operative fluoroscopy times were greater in the EFLIF group (4.43 to 6.75, t = 8.070, p = 0.000). The Böhler angle was reconstructed anatomically (
A patient with Sanders type 2 calcaneal fractures treated with EFLIF. From left to right: a) pre-operatively in the lateral view; b) CT pre-operatively; one week post-operatively in the c) lateral and d) axial view; and e) one year post-operatively in the lateral view.
No significant differences were noted in the Böhler angle between the two groups pre-operatively, post-operatively, or at last follow-up (p > 0.05); nor were any significant differences found in the American Orthopedic Foot and Ankle Society (AOFAS) ankle hindfoot scale (83.83 to 84.40, t = 0.994, p = 0.324) or the Visual Analogue Scale (1.63 to 1.56, t = 0.389, p = 0.698) between the two groups. The subtalar joint range of movement, return to previous work, patient satisfaction, and subtalar arthritis were similar between the two groups (
Clinical outcomes
ORIF group (mean, |
EFLIF group (mean, |
p-value | ||
---|---|---|---|---|
Age (yrs) | 41.92 ( |
43.80 ( |
0.512 | |
Gender (male/female) | 29/7 | 30/5 | 0.753 | |
Type of accident | ||||
Fall from height | 27 | 24 | 0.605 | |
Traffic accident | 9 | 11 | ||
Accompanying injuries | 2 lumbar fractures | 4 lumbar fractures | - | |
Comorbidities | 1 diabetes mellitus | 2 diabetes mellitus | - | |
Timing of surgery (days) | 7.60 ( |
3.08 ( |
0.000 | |
Hospital stay (days) | 12.83 ( |
9.14 ( |
0.000 | |
Operation time (mins) | 74.77 ( |
65.22 ( |
0.000 | |
Mean blood loss (mL) | 79.14 ( |
33.33 ( |
0.000 | |
Intra-operative fluoroscopy time (n) | 4.43 ( |
6.75 ( |
0.000 | |
Böhler’s angle (°) | ||||
Pre-operatively | 4.01 ( |
3.20 ( |
0.672 | |
Post-operatively | 31.04 ( |
31.81 ( |
0.310 | |
Last follow-up | 30.64 ( |
30.85 ( |
0.793 | |
Follow-up (mths) | 23.66 ( |
25.03 ( |
0.096 | |
Range of movement (º) | 26.57 ( |
25.81 ( |
0.188 | |
Return to previous work | 16 | 17 | 0.814 | |
Patient satisfaction | 32 | 34 | - | |
Subtalar arthritis | 3 | 2 | - | |
AOFAS (max.; worst pain; 100) | 83.83 ( |
85.58 ( |
0.324 | |
Visual analogue score (max.; worst pain; 10) | 1.63 ( |
1.56 ( |
0.698 |
AOFAS, American Orthopaedic Foot and Ankle Society Ankle Hindfoot Scale
One external fixation pin tract infection (2.85%) occurred in the EFLIF group, while six superficial infections and one deep infection (22.22%) occurred in the ORIF group, complications in the EFLIF group was significantly lower than ORIF group (p = 0.028, chi-squared). Administration of oral antibiotics and dressing changes were performed to treat the external fixation by pin tract and superficial infections. Implant removal, operative debridement and administration of intravenous antibiotics were performed to treat the deep infection at the sixth week post-operatively.
ORIF is a standard technique for calcaneal fractures that can effectively restore the anatomy and allows for early weight-bearing and functional exercise.
Many scholars have reported the use of an external fixator in treating calcaneal fractures,
With significant development in computer technology, the finite element analysis is widely used in the field of orthopaedic biomechanical research, especially in the foot.
The stability of the medical apparatus and instruments used in fracture fixation is an important index to evaluate the effect. In this study, the EFLIF and plate for Sanders type 2 calcaneal fracture models were simulated. Under 160 N loading in the direction of the Achilles tendon, the displacement of both fixation systems were less than expected: separating or shifting 1 mm after the treatment of intra-articular calcaneal fractures.
The stress distribution of the fixation system is also an important index to evaluate its effect. The stress distribution in the ideal state should be evenly distributed on the medical apparatus and instruments, and should not be overly focused on any single area. To this end, the stress distribution of the external fixator, screws and plate was tested after constraint and loading on the model. The maximum stress values of the two fixation systems were less than the yield strength of medical equipment. Compared with the plate, the maximum stress value on the EFLIF was lower. Although a stress concentration area was found on the two fixation systems, the value of stress was still less than the yield strength of medical devices. As the actual stress in a clinical scenario should be less than the experimental values, failure should not happen on the EFLIF.
Owing to the limitations of the study, we could not compare the finite element model with the cadaver model. While treating calcaneal fractures surgically, the ligaments, muscles and other soft tissues may be damaged during operation and repaired post-operatively. Due to the lack of thin soft-tissue slices of the foot, the reconstruction of ligaments could only be obtained from Gefen et al.
In conclusion, the EFLIF technique is better, and hence preferable, to the ORIF due to the following advantages: it is more stable, there are lower levels of blood loss, a shorter surgical time and hospital stay, and fewer wound complications.
This work was supported by Shanghai Key Medical Subject Construction Project grant number ZK2012A09 and Shanghai Municipal Health Bureau grant number 2007045.
These authors contributed equally to this work.
None declared