Conventional bicortical pin substitution with a novel unicortical pin in external fixation: A biomechanical study

INTRODUCTION

As most patients with polytrauma or open fractures are converted from temporary external fixation to definite stabilization, the prevention of complications such as infection is especially important. To overcome the high risk of infection associated with the use of the conventional bicortical pin for temporary external fixation, the authors developed a novel unicortical pin and analyzed it in a biomechanical study.

METHODS

The unicortical pin consisted of an inner screw, purchasing the cortical bone, and an outer sleeve with 6 spikes. A bicortical pin was used for the purpose of comparison. A fracture gap model was stabilized using a monoplanar configuration. Both the unicortical pins (Uni group) and bicortical pins (Bi group) underwent axial compressive and torsional load testing using a servo-hydraulic testing machine. Stiffness, load to failure, and mode of failure were documented.

RESULTS

Stiffness and load to failure of the Uni group (average, 40.5 N/mm and 1098.4 N, respectively) were greater than that of the Bi group (average, 33.7 N/mm and 968.6 N, respectively) in the axial compressive load test (P = 0.008 and 0.032). Stiffness and load to failure of the Uni group (average, 1.2 Nm/degree and 1.7 Nm, respectively) were also significantly higher than those of the Bi group (average, 0.8 Nm/degree and 0.6 Nm, respectively) in the torsional load test (P = 0.008 and 0.016). All pins in the Bi group were bent at the pin-synthetic bone interface without synthetic bone failure. Contrarily, the Uni group did not show any pin bending or failure. However, in the axial compression test, partial cracks in the synthetic bone were found at the interface with spikes in the outer shell. In addition, in the torsion test, incomplete fractures were seen through the inner screws' holes.

CONCLUSION

Compared with the conventional bicortical pin, the newly designed unicortical pin significantly increased fracture stability under both axial compressive and torsional loads. The unicortical pin can be considered an alternative biomechanical solution to obtain adequate stability when performing external fixation of fractures.

Risk factors for refracture of the femoral shaft in children after removal of external fixation

Background

External fixation is the primary treatment option in children for femoral shaft fractures, such as open femoral or multiple fractures. One complication is refracture, which is the biggest limitation of fixation devices. This study aims to investigate the risk factors associated with refracture after the removal of external fixation devices and decrease the frequency of refracture.

Materials and methods

Retrospectively reviewed clinical data of 165 patients treated at our hospital for fresh femoral shaft fractures with external fixation between May 2009 and February 2018 were included in this study. Patients with pathological fractures, fractures of the femoral neck, fractures that were fixed using plates or elastic stable intramedullary nailing, and old fractures, as well as those who underwent postoperative femoral surgery were excluded. Potential risk factors included: patient age, gender, and weight, fracture sides, open or closed fracture, fracture sites, reduction methods, operation time, perioperative bleeding, number and diameter of the screws, and immobilization time. These factors were identified by univariate and logistic regression analyses.

Results

Femoral shaft refracture developed in 24 patients. Univariate analysis revealed that refracture was not statistically significantly associated with any of the above factors, except AO Pediatric Comprehensive Classification of Long Bone Fractures (PCCF) classification type 32-D/4.2 and L2/L3 ratio (L2, length of femur fixed by the two screws farthest from the fracture line; L3, the total length from the greater trochanter to the distal end of femur; P < 0.001 and P = 0.0141, respectively). Multivariate analysis showed that PCCF classification type 32-D/4.2 and L2/L3 ratio were also independent risk factors for femoral refracture.

Conclusions

Femoral shaft refracture is relatively common in children treated with external fixation. Because of the limited number of cases in this study, we cautiously concluded that the PCCF classification type 32-D/4.2 and L2/L3 ratio were independent risk factors for femoral shaft refracture in these patients.

Level of evidence

IV

External fixation of the lower extremities: Biomechanical perspective and recent innovations

The concept of supporting fractured long bones externally with mechanical fixation has been evidentially applied for over 2000 years, and since been expanded on in the mid-19th century by percutaneous bone fixation. Surgical techniques, external fixator systems, and materials have made continued progress since. The benefits of traditional external fixation have been enhanced in recent years with the introduction of hexapod-style fixators, innovative configurations, and pin modifications, among other things. It is generally agreed upon that biomechanical testing of advancements in external fixation must be inclusive of transverse or torsional loading to simulate construct behaviour in realistic scenarios. Biomechanical studies indicate that hexapod-style fixators show comparable axial stiffness to Ilizarov-style systems and improved performance under torsional and transverse forces. The addition of configuration elements to fixators, inclusion of certain carbon fibre chemical compositions, and techniques intended to augment ring thickness have also been investigated, in hopes of increasing construct stiffness under loading. Novel external fixators attempt to broaden their applications by rethinking bone mounting mechanisms and either expanding on or simplifying the implementation of 3D bone segment transport for corrective osteotomy. Older and seemingly unconventional fixation techniques are being rediscovered and evolved further in order to increase patient comfort by improving everyday usability. The development of new pin coatings can potentially enhance the pin-bone interface while lowering infection rates typically expected at thicker soft tissue envelopes. Although complication, malunion, and nonunion rates have decreased over the past 50 years, the clinical results of external fixation today can still be optimized. Unsatisfactory healing in the lower extremities has especially been reported at locations such as the distal tibia; however, advancements such as osteoinductive growth hormone treatment may provide improved results. With the current progression of technology and digitization, it is only a matter of time before 'smart', partly-autonomous external fixation systems enter the market. This review article will provide a versatile overview of biomechanically proven fixator configurations and some carefully selected innovative systems and techniques that have emerged or been established in the past two decades.