This study demonstrates the notable improvements in compressive, torsional, and bending strength associated with partially opened C-shaped SMA implants (C-clip) for fixation of the proximal segment compared to the traditional cerclage cables. Although the C-clip has less wrapping area to the bone than the cerclage cable, the holding stiffness appears stronger than cerclage cable. We think that this is because the shape memory effect of SMA is reformed to the shape that was originally designed. Although it depends on the degree, almost all metals have a ductile property, so they tend to loosen slightly over time from the initial fixation. In particular, cerclage wiring has a natural tendency to slip and loosen when any part of the bone tapers 13. However, although the effect of underneath C-clip was not evaluated yet, this made of SMA is still holding the bone and plate tightly.
Cerclage wiring has advantages and disadvantages from wrapping the cable around the bone. Tension in the cable can pull radially displaced fragments together, an advantage that neither nails nor screws can offer. The disadvantage of the 360-degree wrap is the potential for nerve and blood vessel damage. However, because of the 360-degrees wrapping method, it has several demerits and make problems. Contrary, the C-clip that did not wrap 360-degrees get several merits than cerclage. First, it can reduce the possibility of a large vessel and nerve injury; Surgeons dissect the femur in PFF from the safe lateral or anterolateral side which does not exist anatomical fatal structure such as large vessel and nerve, advanced and penetrated the wire passer to a medial and posterior direction which located anatomical fatal structure without direct vision. This is entirely dependent on the surgeon’s skill, experience, and sense. Although most cases are without any accident, there are reported serious complication about cerclage wiring 14. Second, it enables early rehabilitation. Most muscles and tendons acting on the thigh are attached to the linea aspera. The 360-degree wrapping method inevitably has to dissect the linea aspera to some extent and be penetrated the tendon attached area in several places. If we could preserve the linea aspera (if we could fix femur and implant with similar stiffness without penetrating through the linea aspera), it will be very helpful for the movement of thigh muscles and will allow for early rehabilitation. Third, it enables less skin incision and dissection. The method of full cerclage wrapping inevitably requires more incisions and soft tissue dissection than the method of using an internal fixation that can use only when the front is visible. When the dissection site increased, prolonged surgical time, increased bleeding, and an increased likelihood of surgical site infection are accompanied naturally. Although some recently developed wire clamp for minimal invasive aimed was used, it has to be used under the fluoroscopy and influenced by the surgeon's feeling 15. Therefore, it is not possible to be free from radiation hazards and the complication of large vessel injury 16,17. Of course, C-clip has not only advantages. In order to obtain the effect of shape memory effect, it is necessary to have high and low-temperature conditions. In this regard, it will be necessary to develop a heat conduction tray using electricity in the future.
Treatment of PFF following hip arthroplasty presents a major clinical challenge in orthopaedic surgery 18. Patients with PFF are typically elderly individuals with varying degrees of osteoporosis and medical comorbidity. The fixation is very difficult because of the hip stem and poor bone quality. Fixation of the proximal portion, which contained a huge hip stem, was limited with a standard screw technique. Moreover, when the host bone is in osteoporotic condition, the screw does not work and may be pulled out easily. Our authors contemplated whether a partially wrapped implant could have enough holding power, like a 360-degree wrapped cerclage cable. The present study shows that the partially wrapped implant may have the same or better holding power as the 360-degree cerclage methods.
There are some limitations to the present study. First, we did not insert a hip stem. Although not to be used as a hip stem, this study has clinical implications for the treatment of unstable VB1-type PFF. Many other biomechanical tests of PFF have been conducted using cemented hip stem 5,11,19,20. Using a hip stem would introduce more variables, such as femoral stem position and cementing technique, and increase the cost. This study compared the stiffness of cable and C-clip under the same conditions. We think that the presence or absence of a hip stem would affect both groups equally. This is supported by another study in which biomechanical tests were performed without inserting the hip stem 21,22.
Second, synthetic bones were used in this study in order to avoid the discrepancies associated with cadaveric tissues. This poses an inherent limitation to this study. Nonetheless, synthetic models have been proven to be valuable in biomechanical studies involving fracture fixations because of the relatively lower cost involved and the reduced level of variability between the specimens23. They are designed and developed to represent the average geometry and more accurately match the ideal bone properties24. In fact, synthetic bones generally show lower standard deviations in the parameters measured as compared to cadaveric specimens25.
Third, our study did not use the proximal uni- or bi-cortical screw as a control group. Recent biomechanical tests commonly compare the experimental method and control as a screw fixation method. However, clinically, cerclage cable systems are still widely used implants in PFF.
Fourth, the present study used a small sample size. While previous authors have typically used more than 5 specimens per test group, the current study is limited to three (n=3) specimens per each construct group. This could yield low statistical power to detect all statistical differences between the test groups. Contrarily, the small sample size of the present study suggests that the various statistical differences detected were, in fact, present 11.
Fifth, we were unable to accurately simulate all of the physiologic force components in femur when standing or walking. So, present study did not test physiologic loading but isometric loading modes such as axial compression, pure bending, and torsion. This makes it easier to identify if specific modes cause instability, but it may be that these cases do not occur in vivo. 25.
Sixth, in a previous study, to simulate torsional load, the hip stem was oriented parallel to the floor and the MTS 858 universal testing machine was used to apply a downward force to the femoral ball. In this configuration, the applied force was offset from the axis of the femoral shaft creating a twisting moment thereby simulating a torsion test 12.
An ideal implant should not only maintain prosthesis stem stability but also provide stable fixation of poor-quality bone around the prosthetic stem 6. This study shows that partially opened C-shaped implants made with SMA (C-clip) have superior strength in axial compression, bending, and torsion than traditional cerclage cables. Although the C-clip may require many further studies to be used clinically, it is encouraging to show superior stiffness to traditional cerclage wiring in biomechanical tests.