aSchool of Medicine, Nankai University, Tianjin 300071, China
bDepartment of Orthopedic Surgery of Hebei Province, The Third Hospital of Hebei Medical University, Shijiazhuang 050051, China
cKey Laboratory of Biomechanics of Hebei Province, Orthopaedic Research Institute of Hebei Province, Shijiazhuang 050051, China
dNational Health Commission (NHC) Key Laboratory of Intelligent Orthopeadic Equipment, The Third Hospital of Hebei Medical University, Shijiazhuang 050051, China
eChinese Academy of Engineering, Beijing 100088, China
With the aging population, intertrochanteric femur fracture in the elderly has become one of the most serious public health issues and a hot topic of research in trauma orthopedics. Due to the limitations of internal fixation techniques and the insufficient mechanical design of nails, the occurrence of complications delays patient recovery after surgical treatment. Design of a proximal femur bionic nail (PFBN) based on Zhang’s N triangle theory provides triangular supporting fixation, which dramatically decreases the occurrence of complications and has been widely used for clinical treatment of unstable intertrochanteric femur fracture worldwide. In this work, we developed an equivalent biomechanical model to analyze improvement in bone remodeling of unstable intertrochanteric femur fracture through PFBN use. The results show that compared with proximal femoral nail antirotation (PFNA) and InterTan, PFBN can dramatically decrease the maximum strain in the proximal femur. Based on Frost’s mechanostat theory, the local mechanical environment in the proximal femur can be regulated into the medium overload region by using a PFBN, which may render the proximal femur in a state of physiological overload, favoring post-operative recovery of intertrochanteric femur fracture in the elderly. This work shows that PFBN may constitute a panacea for unstable intertrochanteric femur fracture and provides insights into improving methods of internal fixation.
Hip fractures in older people are becoming increasingly common as the world’s population ages [1], [2], [3], [4]. Epidemiological surveys show that there were 700 000 cases of hip fracture in China in 2013, and it is estimated that the number will increase to 4.5 million by 2050 [5], [6]. Hip fractures seriously affect the vitality of frail older people, with high disability and mortality rates [7], [8], and are therefore known as the last fracture of life [9]. In addition, the cost of medical and nursing care for hip fractures in older people represents a major health and financial burden for countries around the world [10], [11]. The situation is even more serious in Asian countries, most of which are not yet wealthy and are already aging. Effective treatment of hip fracture in the elderly is therefore an urgent medical and social challenge [12].
More than half of the cases of hip fracture in elderly patients are intertrochanteric femur fractures [13], [14]. The recognized best clinical approach to treating this is to perform efficient surgery as early as possible to relieve pain and allow patients to recover in a timely manner [15], which can avoid bed-ridden-related complications as much as possible [16], thereby increasing patient survival rates and reducing the economic burden on society and the individual [17]. There are currently a number of internal fixation options for surgery depending on the classification of intertrochanteric femur fracture [18]. However, due to defects in the mechanical design of these existing internal fixation systems, postoperative failure of fixation such as hip varus, internal fixation incision, nail removal, nail fracture, and even bone nonunion often occur [19], [20]. This greatly delays the patient’s rehabilitation. Therefore, optimal design of internal fixation systems is needed to provide better clinical treatment options.
1.2. Anatomical and biomechanical characteristics of the proximal femur
The proximal femur belongs to the metaphyseal-diaphyseal junction [21]. In terms of anatomical features, the proximal femoral neck is slender and composed entirely of hard cortical bone; the distal trochanteric region is larger with thin and weak cortical bone but abundant cancellous bone [22]. This results in asymmetry in thickness and density between the two ends of the fracture. Due to the presence of the neck-shaft angle, anteversion angle, and torsion angle, transmission of the gravitational load exhibits an eccentric characteristic, leading to a natural tendency for inward instability in the proximal femur. In addition, the muscles in the hip region are powerful, and even in a resting state, their contraction strength is considerable. Therefore, proximal femur fracture exhibits inherent instability, including axial instability, rotational instability, and lateral instability [23].
To adapt to a complex mechanical environment [24], the trabeculae of the proximal femur exhibit a specific distribution with biomechanical significance [25]. Specifically, the cancellous bone of the proximal femur is divided into two trabecular systems, compression trabeculae and tension trabeculae, aligned with the line of force, as shown in Fig. 1. The compression trabeculae include the supporting bundle and trochanteric bundle. The supporting bundle intersects with the arcuate bundle of the tension trabeculae at the neck and head of the femur, receiving strong support from the thicker cortex beneath the femur neck and the femoral calcar. The trochanteric bundle intersects with the arcuate bundle of the tension trabeculae at the surface of the greater trochanter and the intertrochanteric line. The bone in the cross-sectional area is relatively dense and strong, but the gravitational load system of the medial column in the latter cross area becomes weaker due to age-related osteoporosis [26]. The compression trabeculae, the tension trabeculae, and the femoral calcar form a stable trigonum at the center of the femoral neck, as shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4; this is named Ward’s triangle [27], which plays a significant role in understanding the biomechanical characteristics of the proximal femur and guiding the design of internal fixation. Based on Ward’s triangle, Zhu et al. [9] proposed several macroscale triangles and many microscale triangles, which is called Zhang’s N triangle theory. These triangular truss structures are important for maintaining the stability of the bony structure of the proximal femur and are also of great clinical value for the design and development of internal fixation after fracture, as well as for the implantation of internal fixation.
The integrity of the femoral calcar in intertrochanteric fracture determines the inherent stability of the fracture [23]. In the classification, if the femoral calcar remains intact or maintains a normal alignment, it is considered a stable fracture; if the femoral calcar is fractured, dislocated, or the lesser trochanter is torn, it is classified as an unstable fracture. For unstable intertrochanteric fractures, the lesser trochanter on the posterior and medial side often separates and becomes displaced along with the femoral calcar, resulting in a block of the lesser trochanter and femoral calcar [28]. This leads to loss of alignment and mutual support in the posterior and medial cortex, undermining the foundation for proper positioning in intertrochanteric fractures. For this situation, it is necessary to promptly reduce the intertrochanteric fracture; after applying instrumentation for fixation, it is then necessary to transmit the load of the hip joint between the implant and the bone. If the reduction quality is excellent, the sliding movement between the femoral head and neck fragment and the cortical bone of the femoral shaft allows them to interlock tightly. This enables them to bear more load force. Thus, the force borne by the internal fixation instrumentation is correspondingly reduced, making the implant less susceptible to fatigue fracture or displacement from the bone [13].
Biomechanical studies of internal implant instrumentation involve various issues encountered in clinical work, such as the degree of osteoporosis, the quality of fracture reduction, design parameters of instruments, comparisons between different implants, placement of femoral head implants, timing of weight-bearing, and progress of rehabilitation exercises [23]. Below is a brief overview on the research progress of internal fixation treatment for intertrochanteric femur fractures.
1.3. Related work
Modern surgical treatment of hip fractures began with invention of the three-flanged nail by Smith-Petersen in the early 20th century [29], [30]. Johansson [31] and Wescott [32] independently employed intraoperative radiography techniques to guide insertion of three-blade nails, later evolving into minimally invasive surgery for fracture. Thornton [33] is credited with the first attachable side plate bolted to a Smith-Petersen nail, which allowed for surgical treatment of intertrochanteric fractures of the femur. However, that connection was unstable, and the affected limb could not withstand gravity and was prone to complications such as nail plate loosening, hip inversion deformity, and nonunion, which led to many new devices within ten years. Jewett [34] designed a fixed-angle blade plate with a triple-edged nail that could provide stable fixation, but compression of the fracture site can cause the triple-edged nail to cut through the femoral head or breakage of the nail can occur during the patient’s early movement out of bed. Evans [35] reported on use of a Capener-Neufeld nail type technique with open reduction, which greatly improved the effectiveness of treatment. Circa the 1970s, the blade plate system was gradually replaced by the dynamic hip screw (DHS, or named as sliding hip screw, SHS) [36]; as represented by Richards nail, it is considered the classical method of treating intertrochanteric fractures of the femur. Its pivotal concept is to convert the inversion force of the proximal fracture block into an axial force along the direction of the head nail by means of the head nail and the main plate sleeve structure, which changes the mechanical conduction to enhance the fracture section pressure, thus improving the fracture healing rate. However, further research suggested that due to defects of the DHS structure, there was a significant stress-concentration effect at the nail-board joint, resulting in a higher incidence of problems such as screw cutout and loosening [37].
With the promotion of minimally invasive surgical concepts, intramedullary fixation has gradually become the mainstream method of treating intertrochanteric fractures in clinical practice [38]. Enders nails were used since around the 1970s [39]. The idea of its mechanical conduction has been followed by subsequent generations, though many complications such as hip varus and rotational instability during clinical treatment with Enders nail have been reported. The Gamma nail is a representative intramedullary fixation of the intertrochanteric femur [40]. Compared with DHS, the Gamma nail has the advantages of less injury, less incision, easier surgical operation, and lower incidence of complications such as hip impingement and screw cut out and loosening [41]. The proximal femoral nail (PFN) [42], designed by the Association for the Study of Internal Fixation (AO/ASIF), added a parallel nail to the Gamma nail to increase anti-rotation, which led to an improvement in the stress concentration problem, but it still could not solve the problems of screw regression, cutting, and fixation failure. The further developed proximal femoral nail antirotation (PFNA) [43] adds a helical design to the head nail structure to enhance the stability of the head nail to improve the anchoring force to the proximal bone [44]. By comparing with DHS based on three-month of follow-up and the satisfaction of patients at six months after surgery, clinical reports show that the slight superiority of PFNA may be principally related to its mechanical advantages [45]. Indeed, PFNA has become one of the most widely used and effective tools available. Nevertheless, the problems of screw withdrawal, fracture, and cutout remain [46]. For example, postoperative complications of PFNA still reach 2.5%-12.5% due to the instability of intertrochanteric femur fracture [47], [48]. The InterTan nail [49], with two screws of different diameters interlocking with each other, can effectively enhance the stability and antirotation, but it might lead to a higher loss of bone volume in the femoral head and also requires a higher level of surgical operation technique. Failure of internal fixation is related to factors such as osteoporosis, premature weight bearing, and improper placement of internal fixation in elderly patients, but more importantly, it is closely related to the structural characteristics of internal fixation and its mechanical distribution and conduction properties after placement and the proximal femur as a whole [50].
1.4. Proximal femur bionic nail (PFBN)
The incidence of postoperative screw breakage, screw withdrawal, cutting, and bone nonunion of traditional endoprosthesis ranges 5%-12%, with more than 50% of cases requiring a secondary surgery. The fundamental reason is that the traditional endoprosthesis is incompatible with the structure of Ward’s triangle of the proximal femur and the mechanical conduction path and is not able to provide a stable and effective mechanical support. To address this problem, Yingze Zhang originally developed the PFBN, as shown in Fig. 2(a), with the main nail, compression nail, and tension nail forming an interlocking whole that mimics the structure of the proximal femur to achieve mechanical conduction.
Biomechanical experiments by Zhang et al. [50] and Wang et al. [51] showed that the triangle supporting fixation is more effective in fixing intertrochanteric fracture than DHS, which not only significantly improves fixation stability but also restores the conductive properties of proximal femoral mechanics to some degree. By using finite element analysis, the maximum stresses σmax of four different kinds of internal fixations including the Gamma nail, PFNA, InterTan, and the PFBN were investigated [52], [53]. The results indicated that the PFBN has superior advantages in stress distribution and construct stability compared with the Gamma nail, PFNA, and InterTan fixation for intertrochanteric femur fracture. Chen et al. [54] also supported that the PFBN leads to better biomechanical stability in treatment of patients with intertrochanteric fracture than does PFNA and DHS, as based on their numerical results. Cheng et al. [55] claimed that compared with DHS and PFNA, the PFBN has advantages in stress distribution and biological stability. Their findings also illustrate that the concept of triangle fixation is helpful to reduce the femoral neck shortening associated with DHS and PFNA, thus improving the prognosis of basicervical femoral neck fracture. Clinical treatments show that the PFBN with a rectangular top modification can effectively decrease the occurrence of bone nonunion, which is called the Yingze nail. Its Chinese name has been engraved on the implant, “英泽钉,” as shown in Fig. 2(b). One possible explanation for the ability of the Yingze nail to improve bone nonunion is that it effectively improves the local mechanical environment as well as reduces the fracture gap, allowing for a further increase in the efficiency of fracture repair. A quantitative study on the mechanism is ongoing.
Along these lines, we describe an equivalent biomechanical model to investigate regulation of the local mechanical environment of the proximal femur by using the PFBN. Then, we quantitatively analyze how the PFBN improves the process of bone remodeling for elderly patients with intertrochanteric femur fracture. Finally, we discuss the advantages of the PFBN for treatment of intertrochanteric femur fracture and the future direction of internal fixation systems.
2. Equivalent biomechanical theory
To investigate regulation by the PFBN on local mechanical environment of the proximal femur, we developed an equivalent biomechanical model based on the assumption of elastic deformation. First, we assume that the whole force from external section on the proximal femur can be effectively conducted by internal fixation. Moreover, with the external loading increasing, the contact area A between the proximal femur and the internal fixation changes slowly, that is, in a quasi-static state. In this case, both the proximal femur and the screws experience elastic deformation under the external force interaction, as shown in our previous analysis [50] and numerical simulations [53]. We consider the stresses σ of the screws within the internal fixation linearly changes with the external force Fext, that is, σ∼Fext. Our previous experiments showed that different parts of the proximal femur experience elastic deformation after fixed by the triangle support fixation nails [50]. As for the screws in the system, if we take these nails as a whole object, the corresponding strain ε of the screws can also reveal a linear relationship with the loading force based on the elastic deformation assumption, that is, ε=σ/Escrew∼Fext/(Ascrew-eqEscrew). The maximum strain corresponding to the maximum stress can be described as εmax=σmax/Escrew∼Fext/(Ascrew-eqEscrew). Fig. 3 shows the validation of our equivalent biomechanical model with three different types of internal fixations including PFNA, InterTan, and the PFBN. The data were obtained from a previous finite element analysis [53], in which the Young’s modulus of screws is Escrew=113800 MPa and the Poisson’s ratio is 0.342. Ascrew-eq represents the equivalent loading area of the internal fixation. All the maximum strains in the three types of internal fixation experience linear relationships with external forces. Moreover, the PFBN can effectively decrease the maximum strain of internal fixation, which supports the previous study [56] about the advantages of avoiding the occurrence of complications such as screw withdrawal and loosening, among others. The numerical results are analyzed by ANSYS Workbench 2020R2 (ANSYS Canonsburg, PA, USA) [53], and the theoretical prediction below are analyzed by a homemade python code.
Similarly, we extend the equivalent biomechanical theory to build the corresponding linear relationship between the external force and the strain of proximal femur, which can be described as:
where εcor is the corresponding strain of the cortical bone in proximal femur, εcan is the corresponding strain of the cancellous bone in proximal femur, Ecor is the Young’s modulus of the cortical bone in proximal femur, Ecan is the Young’s modulus of the cancellous bone in proximal femur, Acor-eq is the equivalent loading area of the cortical bone proximal femur, and Acan-eq is the equivalent loading area of the cancellous bone in proximal femur. Then, the maximum strain of the cortical bone can be derived as εcor-max=σcor-max/Ecor∼Fext/(Acor-eqEcor). The maximum strain of the cancellous bone can be derived as εcan-max=σcan-max/Ecan∼Fext/(Acan-eqEcan) by the interpolation fitted method [57] and the previous numerical analysis [53], where Young’s modulus and the Poisson ratio of cortical bone are Ecor=17000 MPa and 0.3 and of the cancellous bone are Ecan=445 MPa and 0.2, respectively, as shown in Fig. 4. Overall, the PFBN provides a low maximum strain in both the cortical and cancellous bone of the proximal femur as well as a strain closest to that of normal bone in the cortical portion (Fig. 4(a)).
3. Analytical results
The equivalent biomechanical model can be adopted to analyze the improvement on bone remodeling by using the PFBN. We considered that the weight of an elderly person is approximately 50−70 kg, with g = 9.8 m∙s-2. Every hip joint will bear 1/3 of the body weight when a person stands bipedally [23]. During the unipedal stance phase during slow walking, the force on the unilateral hip joint is calculated by geometrical mechanics to be three times the weight due to the shift in the center of gravity [58], which may increase up to four times the weight with rapid walking. In addition, a cane can reduce the force to approximately one times the weight for unipedal standing and two times the weight for slow walking [59]. Thus, when a man stands bipedally, the force loading on the femur is approximately [163,229] N, whereas during slow walking, the force is approximately [1470,2058] N. For standing and walking with a cane, the forces are approximately [490,686] and [980,1372] N, respectively. Then, we can further obtain the maximum strain of the proximal femur based on our equivalent biomechanical model, as shown in Table 1.
The analytical results in Table 1 show that the PFBN can significantly reduce the maximum strain of both the cortical bone and cancellous bone induced by different practice motions. In particular, the external force on one hip joint is approximately equal to the body weight when a patient stands with a cane. In this case, the maximum strain of the cortical bone with the PFBN is approximately 3134.5 με (yellow region in Table 1), which illustrates that most of the proximal femur probably changes from a pathologic microdamage region to a physiological overload region based on Frost’s mechanostat theory [60], as shown in Fig. 5. With PFNA and InterTan, both maximum strains are respectively 3926.5 and 3739.0 με, which is overloaded and might increase the risk of a second bone fracture [50]. In addition, the maximum strain in cancellous bone is greatly decreased in patients standing bipedally (blue region in Table 1). However, the corresponding values of the maximum strain in cortical bone are all close to the minimum effect strain threshold modeling (MESm), which may be not a proper range for bone remodeling [61], [62].
4. Discussion
China has more than 300 000 cases of intertrochanteric femur fracture every year, accounting for 1/5-1/4 of the worldwide cases, and the medical expenditure is about 20 billion CNY [4]. The basic reason is that the traditional endoprosthesis is incompatible with the structure of Ward’s triangle and the mechanical conduction pathway of the proximal femur and cannot provide stable and effective mechanical support [9]. The incidence of post-operative screw fracture, screw withdrawal, cutting, and bone nonunion of the traditional endoprosthesis is 5%-12%, and more than 50% of patients require secondary surgery, which imposes a heavy burden on patients, their families, the healthcare system, and society [13]. To solve this problem, Zhang et al. [50], [63], [64] developed the PFBN, which combines the main nail, pressure screw, and tension screw into an interlocking whole.
Internal fixation stress concentration and stress masking are the main reasons for the occurrence of internal fixation failure. Therefore, it is extremely important to decrease the postoperative stress distribution of endoprostheses and the maximum strain of internal fixation. The above analytical results suggest that use of the PFBN for intertrochanteric femur fracture in the elderly can greatly improve the local biomechanical environment of the proximal femur, which plays a significant role in bone remodeling during postoperative recovery. Our analysis about maximum deformation might be slightly overestimated due to the assumption linear elastic deformation of both the cortical bone and cancellous bone during the process of external loading. This illustrates that the regulated strain region might be shifted a little bit left overall, which might be better for bone remodeling according to Frost’s mechanostat theory [60]. As for pathologic overload of the proximal femur for patient slow walking, further optimization in the next generation of internal fixation systems is needed. In our recent clinical study [65], we conducted close follow-up of 12 patients after surgery. At 12 months of follow-up, the Harris and Parker-Palmer scores showed good results. Regarding complications, fracture union complications occurred in two cases (16.6%) including one case of nonunion (8.3%) and one of delayed union (8.3%) based on statistical analysis by statistical product and service solutions (SPSS). However, the number of cases in these studies was small, which had a certain influence on the evaluation of the results; furthermore, as the follow-up time of the patients was short, long-term efficacy needs to be further explored. Overall, the PFBN demonstrates both stability and safety and has advantages in treatment of unstable intertrochanteric femur fractures.
The comparison analysis from our equivalent biomechanical model has been also supported by recent clinical studies. One clinical report [66] showed that compared with the PFNA group, the postoperative pain score and complication rate of the PFBN group were significantly reduced, and the postoperative walking ability score was significantly improved, indicating that PFBN fixation of intertrochanteric fracture can obtain better initial stability, effectively relieve postoperative pain, meet the patients’ needs for early postoperative functional exercise, shorten bed-riddance time, and reduce the occurrence of complications, which is conducive to the recovery of walking ability and hip function. In addition, another clinical study [67] claimed that the fracture healing time in the PFBN group was shorter than that in the InterTan group and that the cervical stem angle was greater than that in the InterTan group at six months postoperatively, with a statistically significant difference (P < 0.05).
There are still some limitations about the application of our equivalent biomechanical model. Based on our assumption, the equivalent biomechanical model can be used for quasi-static situations where the external load is low and the equivalent area does not change much. In the event of complications such as severe screw withdrawal or loosening, the model would not be applicable. In a real situation, the localized forces on the proximal femur would be more complex due to its structural characteristics. For example, torsion, bending, and their composite loading forms with tension and compression often occur [65]. Moreover, elderly hip fracture patients have varying degrees of osteoporosis, as well as sex differences. In our recent clinical study [65], one patient with intertrochanteric femur fractures had delayed union after surgery, which might be attributed to age-related osteoporosis [68], [69], [70]. In light of these limitations, further improvements to our theoretical model are needed to extend its applicability.
5. Conclusions
In this study, we developed an equivalent biomechanical model to investigate improvement by the PFBN for bone remodeling during treatment of intertrochanteric femur fracture in the elderly. Based on our analysis, the PFBN can effectively decrease the maximum strain on the proximal femur and regulate much of the proximal femoral biomechanical environment into a better remodeling region than PFNA and InterTan. This also supports that in clinical treatment of intertrochanteric femur fractures in the elderly, patients with the PFBN may be able to avoid long times of bed-riddance and exercise, preventing the occurrence of complications. This model might be extended to analyze other types of fractures based on elastic deformation. In addition, further improvement of the local biomechanical environment to increase the efficiency of bone remodeling is a promising direction for development of the next generation of internal fixation systems.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (32130052, 82072447, and 82272578) and the Fundamental Research Funds for the Central Universities, Nankai University (730-C02922112 and 730-DK2300010314). Kaixuan Zhang would like to acknowledge M.D. Zhongzheng Wang for helpful discussions.
Authors' contribution
Kaixuan Zhang, Wei Chen, and Yingze Zhang designed the project, drafted the original manuscript, drew the figures, revised and polished the manuscript, reviewed and approved the final version of the revised manuscript. Kaixuan Zhang and Wei Chen developed the model and analyzed the data. Wei Chen and Yingze Zhang designed and funded the project.
Compliance with ethics guidelines
Kaixuan Zhang, Wei Chen, and Yingze Zhang declare that they have no conflict of interest or financial conflicts.
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