Implant breakage is an uncommon complication of patients treated with cephalomedullary nails for intertrochanteric/pertrochanteric fractures. However, some studies have reported breakages of another type of nail such as the Gamma nail (Stryker, Mahwah, NJ) for proximal hip fractures, including subtrochanteric fracture.[14–18] However, until now there have been no studies investigating the breakage of PFNA implants for intertrochanteric/pertrochanteric fractures. To the best of our knowledge, this is the first study to report the frequency of breakage after PFNA for intertrochanteric/pertrochanteric fracture. We found that the overall frequency of implant breakages after PFNA for intertrochanteric/pertrochanteic fracture was 28.0%, among patients with mechanical complications and failure.
Moreover, a longer horizontal offset of the blade was associated with breakage of the PFNA nail. All breakages of the nail went through the proximal aperture for the helical blade. We can hypothesize that the horizontal offset of the helical blade can influence breakage of the PFNA nail. Stress forces on the medial tip of helical blade made it act as a first-class lever. The lower margin of the medial aperture of the blade then acts as the fulcrum. This applies a tensile stress/load to the upper margin of the lateral aperture of the blade. Therefore, a longer horizontal offset of the blade creates a longer lever arm of effect and can increase the tensile stress at the lateral aperture of the blade. Certain conditions can enlarge the horizontal offset of the blade. It might be a good example that distraction of the fracture site can require longer blade for adequate tip apex distance.
Considering the first-class lever action, the distance between the fulcrum and the lateral aperture of the blade can also influence the stress at the lateral aperture of the blade. The shorter lever arm for the load will increase the stress forces at the lateral surface.
The manufacturer does not provide any exact information on the distance between the fulcrum (the lower margin of medial aperture for blade) and the point of load (the upper margin of lateral aperture for the blade). We calculated the distance through reverse design technique using the limited available information that the manufacturer provides to the public (the proximal diameter of 16.5 mm and diameter of helical blade of 11.5 mm). Through this reverse design technique, we could estimate the distance of the lever arm to determine that the load point is 1.1757 cm in the 125° nail and 1.1648 cm in the 130° nail (Fig. 2).
Our reverse design technique suggested that a PFNA with a CCD of 130ᵒ has a shorter lever arm for load than that of the 125ᵒ PFNA. This means that a CCD of 130ᵒ has a higher risk of breakage; however, there was no association between the CCD angle and breakage of the nail in our study. The small sample number of events (breakages) could be a reason for the lack of statistical significance between CCD angles in our study. When the horizontal offset of the blade was fixed or constant to obtain optimal fixation with adequate TAD, PFNA with a CCD of 130° has a shorter lever arm of load, which results in increased stress and a higher risk of breakage in the case of nonunion events. A previous finite element analysis study showed that a higher CCD angle had larger stress in the direction of the sliding blade and it induced greater medial rotation of the proximal fragment.
To overcome the risk of breakage of the nail, the manufacturer has recently developed a new design of nail. This new nail encompassed the improvement of not only the mechanical properties but also the design of the device. This new device named the TFN-ADVANCED® system (TFNA, Synthes, Solothurn, Switzerland) has been introduced. According to the manufacturer, the TFNA nail consists of a higher strength titanium alloy, thereby improving its mechanical properties, and has a different design of the aperture for the blade named as having a “bump cut design” to increase resistance of the mechanical stresses at the hole. However, there was a report of 16 implant breakages of TFNA in 13 patients. The study suggested cautious surveillance of patients with unstable hip fracture who were treated with a TFNA implant. They did not evaluate the CCD angle of the broken nails.
There were several limitations in this study. First, our study was retrospective, and the number of patients was too small to determine associated factors. However, the breakage of fixation devices is a rare event. Considering this rarity of breakage, a well-designed, larger, multicenter study will be needed in the future to improve on our findings. Second, we could not evaluate other factors including fracture type, reduction quality, the tip-apex-distance (TAD) and position of blade at the immediate postoperative, because many patients underwent the index surgery elsewhere in this retrospective study. For example, intertrochanteric fractures, poor reduction, larger TAD and anterosuperior position of blade have been known to be more prone to complications like implant breakage. Third, we did not use the real distance between the fulcrum and the load point, because the manufacturer did not provide this information. However, we used the reverse design technique to obtain an estimated real distance, by using the available information that the manufacturer reveals to the public. Reverse design technique is useful and valid method in this situation. Fourth, we calculated the distance of the lever arm of the load just in the coronal plane. Stress forces act as both torsion and tension; therefore, the stress forces would need to be investigated in other planes.
Despite the limitations, our results showed the frequency of implant breakage after PFNA for intertrochanteric/pertrochanteric fracture. When combined with the results of previous finite element method studies, we noticed that a higher horizontal offset and a higher CCD angle can increase the risk of breakage of the PFNA nail at the aperture for the helical blade.