The combination of using a buttress plate with three parallel, inverted, and isosceles triangular (PIIT) screws offers an alternative approach for stabilizing fractures with increased resistance to shear forces, with early satisfactory results [6, 7, 9]. For displaced vFNFs, especially those with comminution, this construct can prevent the inferior translation along the fracture line. Open approaches also provide the best opportunity for achieving anatomical reduction, and reduction of the apical fracture spike may dramatically increase the stability [3]. Moreover, capsulotomy allows decompression of the intra-capsular hematoma [6]. As Fig. 3 & Fig. 4 show, in comparison with fixation using three parallel cannulated screws, the buttress plate augmented the stiffness of the structure and reduced the maximal displacement of the internal fixation and the femur, the maximal relative displacement of the fracture fragments (MIFD, MBD, MID) and the shear stress on cannulated screws (MCS) and surrounding bone (MBS). Consequently, the buttress plate improves construct mechanics, helps to resist shear force and prevents varus collapse.
However, historical failure rates still exist [9]. We found that the detailed surgical technique of the buttress plate is obscured without biomechanical evidence, for instance, whether the proximal screw is used [6, 7, 9] and whether the proximal screw is unlocked [9] or locked [7]. In the present study, we aimed to address this uncertainty by comparing four fixed vFNF models that used two types of buttress plates over the apex of the fractures with or without proximal screws on the plates. Our pilot study found that a proximal screw locked to the buttress plate was a key technical feature contributing to transmitting force from the head to the shaft of femur and could reduce stress and displacements on the cannulated screws and bone, as well as reduce the relative displacement of fracture fragments. However, more complicated questions in terms of the “unlocked and locked” problem were associated with static and dynamic fixation concepts and could hardly be answered biomechanically, which only reflects initial stability rather than the healing process. Therefore, this problem should be left out of further studies, and “whether the proximal screw is used” should be the focus, following the fixation model of Kunapuli SC’s study [7].
The stiffness of PIIT (1511.0 N/mm) was found to be similar to that reported in previous biomechanical tests (1418 ± 88 N/mm [20] and 1469.0 ± 113.5 N/mm [21]), indicating that our modelling method is appropriate for the evaluation of vFNFs stability. In terms of the construction choice of CSs, we used PIIT rather than a specific construction in Giordano et al.’s study [11], as PIIT is considered to provide good biomechanical stability and is more commonly used in clinical practice [9, 15]. As our aim was to investigate the biomechanics of the proximal screw, all the buttress plates in our models were placed in a standard medial position [7, 9, 10] different from the previous study [11].
Since the buttress plate only resists vertical shear stress and protects the cannulated screw structure from failure before fracture union, Mir and Collinge did not provide any hypotheses regarding the usage of the proximal screws [6] and proximal screws are sometimes omitted in clinical cases to reduce the dynamic compression of the cannulated screws (Fig. 5). However, other studies [7, 9] used proximal screws to improve the reduction of shear stress. Our results are in line with the latter, showing that proximal screws are important for the augmentation of vertical shear stress in vFNFs. We observed that a greater maximal stress on buttress fixation was associated with a smaller maximal stress on the cannulated screws and bone (Table 2). The maximal stress of the models that involved a proximal screw (PIIT+4HI, PIIT+6HI) was concentrated on the junction of the plate and the proximal screw, while the maximal stress of the models without a proximal screw was concentrated on the junction of the plate and distal screw (Fig. 3). This indicates that proximal screws can help to transmit force from the head to the shaft of the femur, release stress on the cannulated screws and bone, and resist shearing.
Ye et al. reported one case of implant breakage at the screw-plate junction, representing one of three cases of implant failure [9]. Our results can explain this phenomenon. There was an obvious stress concentration at the screw-plate junction in all models (Fig. 3). The MPS of the models with a proximal screw at the proximal screw-plate junction (PIIT+4HI & PIIT+6HI, 795.6 & 947.2 MPa) was even close to the yield strength of the plates (Ti-6Al-4 V-alloy, 889–921 MPa) [23]. When a proximal screw was not used, the MPS values of the buttress models (PIIT+4HI-1 & PIIT+6HI-1) were reduced and the values (294.9 & 556.2 MPa) were inferior to the yield strength. It seems that non-use of proximal screws is beneficial to prevent implant breakage. However, considering the whole structure (proximal femur, three cannulated screws, and the buttress plate), the increased MPS at the proximal screw-plate junction in the models with a proximal screw in the buttress group resulted from improved resistance of the femoral head to shear and varus displacement. This is supported by the fact that the other biomechanical parameters of the same models, such as MBS, MCS, MIFD, MBD and MID, were all reduced compared with those in the models without a proximal screw in the buttress group (Table 2). Consequently, the proximal screw on the buttress plate, acting as a load-bearing implant [24], exhibited better fixational ability in terms of stress bearing and reduction of stress and displacement on bones and cannulated screws as well as reduction of relative displacement of fracture fragments. Furthermore, maximal MPS was observed in extreme conditions where the load applied on the femoral head was three times the body weight (2100 N). This is the peak theoretical load acting on the hip joint [25], which would not occur in daily life, especially for patients who have undergone internal fixation. Therefore, the stress at the proximal screw-plate junction should be treated carefully, but should not be a complete denying factor, and proximal screws are still a good choice for the initial stability of vFNFs [7].
The sliding mechanism allows linear intraoperative and postoperative compression in the treatment of vFNFs and may facilitate fracture healing [26]. Based on this principle, the dynamic fixation, such as paralleled cannulated screws, sliding hip screws and buttress plate fixation without proximal screws, was used. However, the dynamic treatment of vFNFs is also accompanied by femoral neck shortening. As a previous study reported [27], severe fracture shortening was the most common complication identified (61%) in all failure patterns. This complication is caused by excessive resorption of bone around the fracture [28]. Severe neck shortening was classified as mechanical failure [29], as it could reduce the abductor moment and decrease the functional scores, and it may lead to local soft tissue irritation due to screws back-out [30, 31] and may even increase the risk of femoral head collapse [32]. To prevent neck shortening, one type of fixation, with the used of fully threaded screws, has been suggested by some authors [33] as a length-stable strategy. Nevertheless, it had a high risk of cutting into the acetabulum because of the absence of a sliding mechanism [29]. Another type of fixation, the use of three cannulated screws plus a buttress plate using a proximal screw, is beneficial for preventing the femoral neck shortening without the risk of cutting. Most importantly, the dominant effect of the buttress plate with a proximal screw is to resist shear force across the fracture site, which is the main biomechanical problem of vertical femoral neck fractures and may also be a key factor in femoral neck shortening. Consequently, proximal screws are a reasonable consideration for preventing femoral neck shortening in vFNFs.
When buttress plates are used in vFNFs, some drawbacks should be highlighted. First, buttress plates placed on the medial side result in a possibility of hip impingement and cases have been described in previous studies [9, 34]. Careful attention to intra-operative plate placement and avoiding the placement of buttress plates too superiorly or too anteriorly onto the femoral neck can avoid this iatrogenic impingement. However, buttress plate application should be avoided in subcapital femoral neck fractures; otherwise, the position of the plate should be as distal as possible. Transcervical and basicervical fracture patterns are more amenable patterns to be considered for the application of buttress plates, as they offer a larger footprint for hardware placement farther from the hip joint [34]. Second, for placement of the plate, an additional incision is needed, which may lead to a destruction of the blood supply. Although proper placement does not endanger blood supply to the femoral head [8], the location of the buttress plates still results in potential damage to inferior retinacular artery. Finally, the stress concentration at the screw-plate junction leads to a risk of fixation breakage [9], but if the implant is strong enough, it should not be problematic. There is no need for plate removal unless infection or nonunion necessitates total hip arthroplasty [7].
In conclusion, buttress plates improve construct mechanics, help to resist shear force and prevent varus collapse. The use of a buttress plate combined with a proximal screw shows better transmission of force and bears more stress, leading to the stress and displacement on cannulated screws and bone reduced, and finally improves overall stability of vFNFs.
There are some limitations in this study that should be acknowledged. First, the inserted parts of the screws were tied to the bone; thus, the screws could not detach from the bone under the load, which may have led to the overestimation of stiffness and the underestimation of MID. However, since all models in this study were set in the same way, between-sample comparisons would not have been affected. Second, due to the simplification of omitting the pressure capacity of the plate and the difficulty in contouring the buttress plate to the bone surface in actual practice, the fixation stability of the buttress plate was likely underestimated. The maximal improved stiffness determined in this study is smaller than that of Kunapuli SC et al. [7], in which the stiffness improved by 35% on average. However, the stiffness reported by Kunapuli SC et al. was only 959 ± 257 N/mm, which is much smaller than our study and two others [20, 21], indicating that their specific boundary condition may have overestimated the augmentation effects of the buttress plate. The purpose of our study was to analyse the function of proximal screws, therefore, these underestimations will not affect our conclusions. Third, Sawbone composite bone rather than cadaveric bone were used to represent young patients with good bone quality. Nevertheless, the stiffness of our constructs should not be considerably different than that of cadaveric bone models, as shown in the paper by Topp et al. [35]. Fourth, our models can only reflect initial stability, and the more complicated biomechanical conditions during bone healing process, such as secondary stability can hardly be addressed via current biomechanical methods. Finally, all the conclusions should be validated by further clinical studies.