Despite the development of implants, devices, and surgical techniques, complications related to the distal locking screw of the PFN system remain [2]. These include insufficient dissection of the fascia, excessive tightening of the distal locking screw, and loosening of the targeting device between the nails [14]. The most common initial complication is unstable coupling. When a distal locking screw is inserted in an incorrect position, it creates an additional hole at the anterior or posterior femur.
The following reasons may explain possible occurrence of fracture around the hole. First, fractures occur because of cortical bone loss caused by an additional hole. The cortical bone is critical for maintaining the mechanical strength of the bone. In particular, a 3-mm hole in the cortical long bone decreases the bending strength by 40%, while the torsional strength reduces by approximately 12% [15]. However, the PFNA-II uses a 4.9-mm screw, and it is expected that these decreases in bending and torsional strength are even greater. Therefore, the authors estimated that repeated loads may induce fractures when such a large amount of cortical bone loss negatively affects the mechanical strength of the femur.
Second, the type of additional holes may affect the incidence of fracture. Additional screw holes can be classified into unicortical, bicortical, half-bicortical, and transcortical penetrations [16]. Most additional screw holes that are directed anteriorly and posteriorly to avoid nails are of the transcortical type. It is supposed that the highest stresses result from transcortical penetration, and the fracture risk ratio was dramatically elevated under both axial and torsional loads [17]. If an additional screw hole occurs in a patient with a large proportion of the nail in the medullary cavity, such as in an Asian with a small body frame, the hole follows a transcortical shape and is pushed outward from the center by the nail, which can generate a risk condition for fracture. In addition, thermal bone necrosis that occurs during transcortical penetration may further increase the risk of fracture [18].
Third, the stress concentration around the additional holes may influence the occurrence of fractures. The standard walking condition results in the bending of the bone during the stance phase. This generates tensile stress in the lateral femur [19]. The peak stress concentration of the femurs in trochanteric fractures was mainly located in the site where the distal lock screw was making contact [20]. Robinson et al. [21] suggested that cortical bone hypertrophy around the distal nail, observed on a simple radiograph during the healing process of intertrochanteric fractures using an intramedullary nail, was a radioactive hallmark, suggesting that stress concentration occurred around the distal nail [21]. Stress concentration can reduce the mechanical integrity of a bone, making it more susceptible to sudden brittle fracture during trauma or to gradual fatigue failure over time (stress fractures). A distally locked construct bears most of the load, which is gradually transferred to the distal cortex as the fracture heals. With good cortical apposition of the fracture, the bone cortices support most of the compressive load. Without cortical contact of the fracture, the entire load is transferred to the distal screw through the nail until the fracture heals [22]. Therefore, efforts should be made to increase the stable cortical bone contact surface of the fracture through possible anatomical reduction before nail insertion.
Fourth, the mechanical influence should be considered. Additional holes allow the movement of the nail at its junction in the medulla. This movement may cause the nail to slip anteriorly or posteriorly and lead to the eccentric position of the nail in the medullary canal, which will direct mechanical stimulation to the anterior or posterior side of the femur and may cause a fracture [23, 24]. In particular, Asian women have shorter femoral necks and smaller femoral neck angles and increased anterior bowing of the shaft than Western women do. Therefore, the nail could be located eccentrically in the intramedullary space [25], leading to a higher risk of fracture. Notably, biomechanical loading tests and FEA studies have shown that AH was more susceptible to fracture than PH. The femur with anterior bowing is subjected to increased posteromedial compressive force and anterolateral tensile force. Considering its physical properties, which are more vulnerable to tensile strength, the bone may be more susceptible to fracture when an AH is formed. Therefore, if an AH occurs in older patients with osteoporosis and severe bowing, the risk of fracture would be further increased.
This study had some limitations. First, this study did not consider various forms of load from the human body and did not use real human bones. The authors used a composite femoral bone model to overcome this problem, determined ten identical conditions for each experimental condition, investigated the stress change according to the load using FEA, and further investigated the effect of bone density. Second, the study did not reflect various fracture types and reduction statuses. The authors applied a fracture model with the highest severity among fracture types, excluding reverse oblique intertrochanteric fracture for which the prognosis is known to be poor. The experiment was conducted assuming that stable reduction was obtained. Third, various types of additional holes, such as unicortical, bicortical, half-bicortical, and transcortical penetrations, were not tested. However, the authors chose the transcortical type to focus on in this study because this type is the most common and the most susceptible to fracture. Finally, the effects of the various diameters of the medulla and nail and the position of the distal end of the intramedullary nail were not considered, and various designs of the PFN system were not tested. Future biomechanical studies incorporating many cadaveric bone models under various conditions and nail systems are recommended.