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Wing-augmentation reduces femoral head cutting out of dynamic hip screw
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Wing-augmentation reduces femoral head cutting out of dynamic hip screw Chih-Yu Chen a,b, Shu-Wei Huang d, Jui-Sheng Sun c,f,∗, Shin-Yiing Lin c, Chih-Sheng Yu e, Hsu-Pin Pan e, Ping-Hung Lin e, Fan-Chun Hsieh e, Yang-Hwei Tsuang b, Feng-Huei Lin d, Rong-Sen Yang c, Cheng-Kung Cheng a,∗∗ a
Department of Biomedical Engineering, National Yang-Ming University, Linong St, Beitou District, Taipei City, Taiwan Department of Orthopedics, Shuang Ho Hospital, Taipei Medical University, Wuxing St, Xinyi District, Taipei City, Taiwan c Department of Orthopedic Surgery, National Taiwan University & Hospital, No. 7, Zhongshan S Rd, Zhongzheng District, Taipei City, Taiwan d Institute of Biomedical Engineering, National Taiwan University, No. 1, Section 4, Roosevelt Rd, Da’an District, Taipei City, Taiwan e Instrument Technology Research Center, National Applied Research Laboratories 20, R&D Rd. VI, Hsinchu Science Park, Hsinchu 300, Taiwan f Biomimetic Systems Research Center, National Chiao Tung University, No. 1001, Daxue Rd, East District, Hsinchu City, Taiwan b
a r t i c l e
i n f o
Article history: Received 6 January 2016 Revised 1 February 2017 Accepted 22 February 2017 Available online xxx Keywords: Femoral intertrochanteric fracture Cut-out Lag screw Dynamic hip screw
a b s t r a c t The dynamic hip screw (DHS) is commonly used in the treatment of femoral intertrochanteric fracture with high satisfactory results. However, post-operative failure does occur and result in poor prognosis. The most common failure is femoral head varus collapse, followed by lag screw cut-out through the femoral head. In this study, a novel-designed DHS with two supplemental horizontal blades was used to improve the fixation stability. In this study, nine convention DHS and 9 Orthopaedic Device Research Center (ODRC) DHSs were tested in this study. Each implant was fixed into cellular polyurethane rigid foam as a surrogate of osteoporotic femoral head. Under biaxial rocking motion, all constructs were loaded to failure point (12 mm axial displacement) or up to 20,0 0 0 cycles of 1.45 kN peak magnitude were achieved, whichever occurred first. The migration kinematics was continuously monitored and recorded. The final tip-to-apex distance, rotational angle and varus deformation were also recorded. The results showed that the ODRC DHS sustained significantly more loading cycles and exhibited less axial migration in comparison to the conventional DHS. The ODRC DHS showed a significantly smaller bending strain and larger torsional strain compared to the conventional DHS. The changes in tip-to-apex distance (TAD), post-study varus angle, post-study rotational angle of the ODRC DHS were all significantly less than that of the conventional DHS (p < 0.05). We concluded that the ODRC DHS augmented with two horizontal wings would increase the bone–implant interface contact surface, dissipate the load to the screw itself, which improves the migration resistance and increases the anti-rotational implant effect. In conclusion, the proposed ODRC DHS demonstrated significantly better migration resistance and anti-rotational effect in comparison to the conventional DHS construct. © 2017 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction Femoral intertrochanteric fractures account for a large proportion of proximal femoral fractures in the elderly with osteoporo∗ Corresponding at Department of Orthopedic Surgery, National Taiwan University & Hospital, No. 7, Zhongshan S Rd, Zhongzheng District, Taipei City, Taiwan ∗∗ Corresponding author. E-mail addresses: [email protected] (C.-Y. Chen), [email protected] (S.-W. Huang), [email protected], [email protected] (J.-S. Sun), [email protected] (S.-Y. Lin), [email protected] (C.-S. Yu), [email protected] (H.-P. Pan), [email protected] (P.-H. Lin), [email protected] (F.-C. Hsieh), [email protected] (Y.-H. Tsuang), [email protected] (F.-H. Lin), [email protected] (R.-S. Yang), [email protected] (C.-K. Cheng).
sis. These fractures are one of the most important health care issues faced by orthopedic surgeons today. Many people who experience such fractures rapidly deteriorating in health status with a significant physical and functional impairment and require substantial financial resources during the perioperative and rehabilitative care [1]. In the 1950s, operative treatment for femoral intertrochanteric fractures was introduced to improve the functional outcome and reduce the complications associated with longterm immobilization and prolonged bed rest [2–4]. Later, a variety of different extra-medullary or intra-medullary implants have been developed since the 1950 s. The most commonly used extramedullary implant is the dynamic hip screw (DHS) with side plate. Intramedullary nails may be used for the surgical fixation of extracapsular hip fractures in adults; however, there is limited
http://dx.doi.org/10.1016/j.medengphy.2017.02.015 1350-4533/© 2017 IPEM. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: C.-Y. Chen et al., Wing-augmentation reduces femoral head cutting out of dynamic hip screw, Medical Engineering and Physics (2017), http://dx.doi.org/10.1016/j.medengphy.2017.02.015
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evidence to date and it is insufficient to determine whether there are important differences in outcome between different designs of intramedullary nails used in the internal fixation of extracapsular hip fractures. Further studies comparing different designs of intramedullary nails are not a priority. DHS is still currently considered to be the gold standard for extra capsular hip fracture fixation as well as the implant that any new design should be compared to [5–8]. Since its introduction, the DHS has been shown to produce good results; however, complications are frequent, particularly in unstable fractures [9–11]. Post-operative implant-related complications have been reported in recent meta-analysis studies [7]. The most common cause of failure is reported to be varus collapse of host bone and cutting-out of the lag screw through the femoral head [9,12,13]. For the failure mechanisms about varus collapse and cuttingout of the lag screw through the femoral head, previous mechanical studies have relied on static or dynamic uniaxial loading regimens to induce construct failure. However, the hip is loaded in a multiplanar, dynamic manner during normal gait. In the insertwearing study for prosthetic hip joints, the biaxial rocking motion (BRM) technology has been chosen to simulate dynamic multiplanar forces loading during level walking [14,15]. The hip implant performance simulator (HIPS) developed by Ehmke et al. can reproduce the dynamic multi-planar hip forces seen during level walking [16]. By using multi-planar forces with a loading protocol designed for hip ab-adduction, flexion–extension, and a double peak load history, BRM technology could simulate much closer to the normal physiologic hip joint motion. In this study, the biomechanical behavior and cut-out performance of a novel wing-augmented DHS and the conventional DHS implant were investigated under physiologic multi-planar loading model. Our hypothesis is that the additional wings would significantly dissipate the stress on the lag screw and enhance the anti-rotational effect. These factors would decrease femoral head cutting out incidence under cyclic load dynamic testing. 2. Materials and methods 2.1. Implants Nine dynamic hip screws (DHS, Synthes, Oberdorf, Switzerland) made of stainless steel were tested as the gold-standard for single lag screw implants. The DHS lag screws had a shaft diameter of 7.8 mm, an outer thread diameter of 12.5 mm and a total length of 110 mm. The wing-augmented ODRC-DHS implants [ODRC: Orthopaedic Device Research Center, National Yang-Ming University, Taipei City, Taiwan] made of titanium were used for comparison. Besides the two grooved structure and the additional two horizontal wings, the ODRC DHS had the exact mechanical parameters derived from the control DHS implant (Fig. 1). To investigate if the newly designed implant can provide greater migration resistance and better biomechanical behavior, nine winged ODRC-DHS implants were tested for comparison. 2.2. Surrogate specimens To yield higher reproducibility and consistent cutout failure, surrogate specimens were used as a cancellous bone substitute [16,17]. The stability of lag screw fixation was tested in surrogate femoral head and neck specimens machined from cellular polyurethane foam (50 mm in diameter, #1522-11, Pacific Research Inc., Vashon, Washington, USA). As validated in a previous study, these specimens had an elasticity modulus (E-modulus) of 48 MPa with 4 MPa compressive strength and a density of 12.5 pcf (0.2 g/cm3 ) to simulate mildly osteoporotic bone [17]. These material properties correspond to the osteoporotic range of human
Fig. 1. Orthopedic device research center (ODRC) DHS design scheme. Upper: Lag screw profile and its cross section view. Middle: Blade profile and its lateral view. Lower: estimated increases in contact surface area both in the vertical and horizontal plane.
cancellous bone, with 5–104 MPa E-modulus and 2–21 MPa compressive strength [18]. For delivery of dynamic loading, the surrogate specimens were placed in a 6 mm thick, polished steel shell to provide a rigid, spherical interface. 2.3. Implant insertion In this study, DHS lag screw surrogate specimens were reamed but not tapped. The lag screw was placed centrally within the femoral head surrogate and advanced to a depth leaving 20 mm tip-to-apex distance (TAD) [19]. This measurement, the TAD, is the sum of the distance from the tip of the lag screw to the apex of the femoral head both on an anteroposterior radiograph and lateral radiograph, after controlling for magnification. This corresponds to a 10 mm distance of the screw tip to the femoral head apex in both views. The ODRC DHS were inserted using the same protocol as the control; after the lag screw was inserted to the intended depth, the ODRC-DHS blade trench was created to pre-determined length, and then the ODRC-DHS blade was inserted and fixed with compression screw. All implants were inserted according to the manufacturer’s guidelines. 2.4. Experimental setup The constructs were then tested in the HIPS-mode to reproduce the dynamic multi-planar hip forces during level walking. This model has been validated for simulation of lag screw migration and cut-out in a most serious clinical condition, that is, combination of an unstable femoral intertrochanteric fracture (OTA classification 31-A.2) in an osteoporotic bone and gait cycle loading [16]. The testing system is fixed at a solid base which simulated the physiologic situation that anatomic axis of femoral shaft aligned perpendicular to the horizontal plane. The proximal aspect of the base was fixed to simulate a pertrochanteric fracture with a fracture line inclination of 40° to the anatomic axis of the femoral shaft (Fig. 2). A bipolar-designed steel shell femoral head back plate with a 40 mm diameter hole was used to ensure unconstrained shear translation of the lag screw in the surrogate femoral neck. This back plate sit against a polyethylene bolster attached to the base plate to reproduce the constraints characteristic of a reduced, but unstable pertrochanteric fracture with deficient posteromedial neck support. This bolster construct simulated fracture
Please cite this article as: C.-Y. Chen et al., Wing-augmentation reduces femoral head cutting out of dynamic hip screw, Medical Engineering and Physics (2017), http://dx.doi.org/10.1016/j.medengphy.2017.02.015
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Fig. 2. The material test system setup for application of biaxial rocking motion representing the hip loading during level walking.
surface abutment in clinical scenario that femoral head varus collapse and rotation occurred after lag screw sliding completion. To replicate clinically relevant sliding conditions, the clamping part in the base plate incorporated the section of the wing-augmented DHS with the addition of two slits for the ODRC DHS, or the barrel and side-plate in the case of DHS. In order to retrieve bending and axial rotational strain of the implants throughout the experiment, two types of strain gages (bending strain gage: KFG® series; torsional strain gage: KFRS® series, Kyowa Electronic Instruments Co., Sapporo, Japan) were glued onto the lag screw without interfere the bending and rotation of the construct. The data was then collected and analyzed by computer. 2.5. Loading To reproduce level walking simulation, biaxial rocking motion (BRM) was used to produce concurrent rotational displacement and axial loading controlled by a biaxial material test system (ElectroForce® 3510-AT, Bose Corporation – ElectroForce Systems Group, Eden Prairie, Minnesota, USA). The output load that applied to the steel shell through the polyethylene meniscus was a dynamic (at 1 Hz), double-peak loading regimen (of 1.45 kN peak load; approximating two times bodyweight). The path of meniscus traced on the femoral head was to simulate the path of resultant force vectors during level walking. The BRM chosen to simulate level walking consisted of concurrent flexion–extension and abduction–adduction motion superimposed with sinusoidal rotation on a 23° inclined block affixed to the actuator (Fig. 2), which resulted in an 18° joint load vector and 5° valgus of the femoral shaft axis [16]. Exaggerated walking kinematics of the limb (i.e., 45° arc of flexion–extension and a 17° arc of ab-adduction) was simulated using ±75° rotation of the actuator. Implants were exercised to the preset failure point (12 mm axial displacement) or 20,0 0 0 load cycles, which achieved first. 2.6. Outcome measures Eight dependent outcome variables were reported, two of which describe the axial displacement (Daxial ), and the stress on the base plate (Sbase ). Another two describe the bending strain (STbend ) and the rotational strain (STrotation ) on the lag screw. While the remaining four variables (TADdiff , α varus , α horizontal , α neck ) describe the migration behavior of the bone-implant construct. The number of loading cycles to implant failure, axial displacement (Daxial ), and the stress on the base plate, Sbase , was registered by the material test system. The bending strain (STbend ) and rotational
Fig. 3. The displacement–cycle curve and the load–cycle curve. (Solid line: conventional DHS; Dashed line: ODRC DHS). The conventional DHS failed at earlier cycles than that of the ODRC DHS failed; while the average final displacement of the conventional DHS were higher than that of ODRC DHS (p < 0.001). The load of conventional DHS at 60 0 0 cycles was also significantly higher than that of ODRC DHS load (p = 0.0 0 05). (∗ : p < 0.05; ∗ ∗ : p < 0.01; ∗ ∗ ∗ : p < 0.001).
strain (STrotation ) were recorded concurrently through the study by strain gages. Cut-out failure was defined by reaching 12 mm axial load on the material test system, which triggered the system to stop and preserve the cut-out stage. At this point, the migrations of femoral head were analyzed in terms of varus collapse (α varus ), horizontal plane rotation (α horizontal ), and rotation around the neck axis (α neck ). TAD changes before and after the study (TADdiff ) was also recorded. 2.7. Statistical analysis Differences in the axial displacement and stress distribution between implants were tested at discrete time points during the load history. Statistical analysis was performed by using two-tailed Student’s t-tests for unpaired samples at a confidence level of α = 0.05. 3. Results 3.1. Axial displacement and load distribution The ODRC DHS showed significantly less axial migration and more loading cycles in comparison to the conventional DHS. The representative displacement/load–cycle curve is shown in Fig. 3. The conventional DHS failed at 6200 ± 2375 cycles, while the ODRC DHS failed at 11,0 0 0 ± 1221 cycles (p < 0.001). The average final displacement of the conventional DHS and ODRC DHS were 11.68 ± 2.20 mm and 9.13 ± 1.39 mm (p < 0.001), respectively. The conventional DHS and ODRC DHS load at 60 0 0 cycles was 914.67 ± 106.13 N and 253.57 ± 27.16 N (p = 0.0 0 05), respectively. Table 1 shows the cycle-changes on displacement and load throughout the study.
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C.-Y. Chen et al. / Medical Engineering and Physics 000 (2017) 1–6 Table 1 Cyclic axial displacement and stress data between conventional DHS and ODRC DHS. Cycles
500 10 0 0 30 0 0 60 0 0 10,0 0 0
Axial displacement (mm)
p value
Conventional DHS
ODRC DHS
4.38 ± 0.91 6.88 ± 1.44 8.55 ± 1.68 11.68 ± 2.20
1.57 ± 0.62 2.98 ± 1.27 4.89 ± 1.24 7.01 ± 1.28 9.13 ± 1.39
<0.0 0 01 <0.0 0 01 <0.0 0 01 <0.0 0 01
Load (N)
p value
Conventional DHS
ODRC DHS
1003.2 ± 57.43 936.03 ± 57.51 970.5 ± 79.82 914.67 ± 106.13
675.2 ± 9.97 519.36 ± 37.74 276.3 ± 30.05 253.57 ± 27.16 273.2 ± 7.11
0.0 0 06 0.0 0 05 0.0 0 01 0.0 0 05
3.3. Specimen analyses The final varus (α varus ), horizontal (α horizontal ), and neck rotation (α neck ) angles for the conventional DHS were 10.61 ± 2.19°, 1.88 ± 0.89°, and 46.11 ± 1.76°, respectively. On the other hand, the final varus (α varus ), horizontal (α horizontal ), and neck rotation (α neck ) angles for the ODRC DHS were 0.50 ± 0.61°, 0.50 ± 0.66°, and 1.61 ± 1.96°, respectively. The final TAD changes (TADdiff ) for the conventional DHS and ODRC DHS were 1.96 ± 0.95 mm and 0.31 ± 0.16 mm, respectively (Fig. 5). These data were also shown in Table 2.
4. Discussion
Fig. 4. The bending and torsional strain. The bending strain of conventional DHS was significantly higher than that of the ODRC DHS (p < 0.001); while for the torsional strain, the conventional DHS significantly lower than that of ODRC DHS (p < 0.001). (∗ : p < 0.05; ∗ ∗ : p < 0.01; ∗ ∗ ∗ : p < 0.001).
Table 2 Strain gages data and final translation and rotation ankle data of conventional DHS and ODRC DHS.
Strain STbend STrotation Translation TADdiff Rotation
α varus α horizontal α neck
Conventional DHS
ODRC DHS
p value
744.00 ± 69.64 um/m 188.11 ± 107.16 um/m
331.00 ± 125.01 um/m 942.40 ± 152.63 um/m
<0.001 <0.001
1.96 ± 0.95 mm
0.31 ± 0.16 mm
0.04
10.61 ± 2.19° 1.88 ± 0.89° 46.11 ± 1.76°
0.50 ± 0.61° 0.50 ± 0.66° 1.61 ± 1.96°
0.02 <0.001 <0.001
3.2. Strain gages analyses The ODRC DHS showed a significantly smaller bending strain and larger torsional strain compared to the conventional DHS. The bending strain of conventional DHS was 744.00 ± 69.64 um/m, while the ODRC DHS was 331.00 ± 125.01 um/m (p < 0.001). For the torsional strain, the conventional DHS and ODRC DHS were 188.11 ± 107.16 um/m and 942.40 ± 152.63 um/m (p < 0.001), respectively (Fig. 4). These data were shown in Table 2.
Fractures of the proximal femur are one of the greatest challenges in the present medical community, which also constitutes a heavy socioeconomic burden worldwide. Although, the dynamic hip screw (DHS) has been recognized as the standard surgical outcome comparison device for the treatment of femoral intertrochanteric fracture [19,20], it still has a reported failure rate of 8% to 13% [21,22]. Controversy does exist regarding the optimal treatment for independent patients with displaced intra-capsular fractures of the proximal femur. The most common mechanical failure in the internal fixation of trochanteric hip fractures is the cut-out of the sliding screw through the femoral head [21]. Previous reports showed that osteoporotic bone is one of the risk factors for increasing rate of cut-out [23,24]. Several ingenious designs have been developed to reduce the incidence of cut-out complications [25–29]. Unfortunately, some of these designs were too expensive or mechanically too complicated. Our study introduces a newly designed wing-augmented dynamic hip screw (ODRC DHS) that could significantly improve the biomechanical performance of implant, resulting in reduced femoral head cut-out rate. The proposed approach is a simple, inexpensive design and that is also technically easy to implant. Our study confirmed that the ODRC DHS with 2 added wings at the horizontal plane along the lag screw that would largely increase the contact surface and also increase the torsional resistance along the lag screw; the wings would also improve implant anchorage in the femoral head. The ODRC DHS had much better biomechanical performance than the conventional DHS in both stress distribution and total displacement within the surrogate bone. The ODRC DHS stress distribution was significantly less than the conventional DHS (914.67 ± 106.13 N vs. 253.57 ± 27.16 N (60 0 0 cycles), p = 0.0 0 05); on the other hand, the total surrogate bone displacement of the ODRC DHS was also much less than that of the conventional DHS (1.96 ± 0.95 mm vs. 0.31 ± 0.16 mm, p < 0.001) (Fig. 3 and Table 1). In addition to the better load distribution and displacement, ODRC DHS also showed smaller bending strain compared to the conventional DHS (331.00 ± 125.01 um/m vs. 744.00 ± 69.64 um/m, p < 0.001) (Fig. 4 and Table 2). With the added wings, the ODRC DHS increases the contact surface at the bone–implant interface, decreases the strain taken by the lag screw itself by dissipating the load through the wings, and eventually
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Fig. 5. The final TAD translation and rotation of specimen. The final varus (α varus ), horizontal (α horizontal ), and neck rotation (α neck ) angles for the conventional DHS were all significantly higher than that of the ODRC DHS. The final TAD changes (TADdiff ) for the conventional DHS was also significantly higher than that of the ODRC DHS. (∗ : p < 0.05; ∗ ∗ : p < 0.01; ∗ ∗ ∗ : p < 0.001).
resulting in less breakage of bony trabeculae and better biomechanical performance. When the human cadaveric hips loaded in a multiplanar, dynamic manner during normal gait, the degree of varus collapse (8.5 ± 7.7°) and rotation (7.2 ± 6.4°) were comparable to that in surrogate specimens (varus collapse: 5.4 ± 2.9°; rotation 7.2 ± 2.8°), with the surrogate specimens showing significantly less variability [16]. In this study, the final varus (α varus ), horizontal (α horizontal ), and neck rotation (α neck ) angles for the conventional DHS were significantly higher than those for the ODRC DHS (Fig. 5; p < 0.05). On the contrary, the rotational strain of the ODRC DHS was significantly larger to the conventional DHS (942.40 ± 152.63 um/m vs. 188.11 ± 107.16 um/m, p < 0.001) (Fig. 5 and Table 2). This finding suggested that the added wings resulted in larger torsional strain of the ODRC DHS by increasing the lever-arm of the implant. Larger torsional strain implied that most of the load was scattered through the metallic implant, instead of the osteoporotic bone, and also lead to much lesser bone trabecular damage at the bone–implant contact surface. These findings explained the significantly better anti-torsional capability of the ODRC DHS device. Similar to this results, the DHS blade® (DePuy Synthes, West Chester, Philadelphia, U.S.A.) and X-bolt® dynamic plating system (X-Bolt Orthopaedics, Dublin, Ireland) also had a similar design concept and also resulted in better biomechanical performance and better clinical outcomes than the conventional DHS [17,25,30]. As mentioned above, lag screw cut-out failure following fixation of unstable intertrochanteric fractures in osteoporotic bone remains an unsolved challenge. In a study tested for improving cut-out resistance, Kouvidis et al. demonstrated that the double screw construct provided significantly greater resistance against varus collapse and neck rotation in comparison to a standard DHS single lag screw implant [31]. Although, double screw construct does provide significantly greater resistance for cut-out; however, the insertion of double screw in the actual clinical scenario is both technical demanding and time consuming. In our study, the ODRC DHS also demonstrated significantly better migration resis-
tance and anti-rotational effect in comparison to the conventional DHS construct; the ODRC DHS is relatively less technical demanding and can save the operation time. This is an important issue to lower operation risk when taking surgery on an elderly and relatively high morbidity senile patient. Although our study results were promising, there were several limitations in this study. First, this was an in vitro study. Although we used the HIPS system to simulate the natural dynamic hip loading conditions, this approach could not reproduce the natural hip motion during human ambulation; further studies are necessary to access the ODRC DHS biomechanical activities under a clinical scenario. Second, we used only one type of fracture pattern in this experimental setup to simulate the unstable femoral intertrochanteric fracture. It was unclear that whether different fracture patterns would affect the experiment results. Third, the ODRC DHS design used 2 horizontal wings along the lag screw axis to increase the contact surface, we could not evaluate the biomechanical performance if the 2 wings were not level. In a preliminary study of eleven female osteoporotic patients (mean age 82 yearsold) with intertrochanteric fractures, the follow-up results (90– 255 days) show that this ODRC-DHS system did reduce lag-screw cutout complications in patients with osteoporosis [32]; however, further experiments will be necessary to answer these questions. In conclusion, our study presented a novel DHS design with two horizontal wings to increase the contact surface of bone–implant interface. When compared to the conventional DHS, the ODRC DHS would decrease the stress along the implant, decrease the displacement of total bone-implant construct, decrease the rotational deformity, and eventually result in better cut-out resistance. Although the results were promising, further studies will be necessary to validate the potential benefits and limitations of this novel implant.
Conflict of interest None
Please cite this article as: C.-Y. Chen et al., Wing-augmentation reduces femoral head cutting out of dynamic hip screw, Medical Engineering and Physics (2017), http://dx.doi.org/10.1016/j.medengphy.2017.02.015
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Funding Ministry of Science and Technology, ROC. Ethical approval Not applicable. Acknowledgment We thank the Ministry of Science and Technology, ROC for their financial support for this experiment. References [1] Coughlan T, Dockery F. Osteoporosis and fracture risk in older people. Clin Med 2014;14(2):187–91. [2] Massie WK. Fractures of the hip. J Bone Joint Surg Am 1964;46:658–90. [3] Schumpelick W, Jantzen PM. A new principle in the operative treatment of trochanteric fractures of the femur. J Bone Joint Surg Am 1955;37-A(4):693–8. [4] Pugh WL. A self-adjusting nail-plate for fractures about the hip joint. J Bone Joint Surg Am 1955;37-A(5):1085–93. [5] Radford PJ, Needoff M, Webb JK. A prospective randomised comparison of the dynamic hip screw and the gamma locking nail. J Bone Joint Surg Br 1993;75(5):789–93. [6] Saudan M, Lübbeke A, Sadowski C, Riand N, Stern R, Hoffmeyer P. Pertrochanteric fractures: is there an advantage to an intramedullary nail?: a randomized, prospective study of 206 patients comparing the dynamic hip screw and proximal femoral nail. J Orthop Trauma 2002;16(6):386–93. [7] Audigé L, Hanson B, Swiontkowski MF. Implant-related complications in the treatment of unstable intertrochanteric fractures: meta-analysis of dynamic screw-plate versus dynamic screw-intramedullary nail devices. Int Orthop 2003;27(4):197–203. [8] Parker MJ, Handoll HH. Intramedullary nails for extracapsular hip fractures in adults.. Cochrane Database Syst Rev 2014;9:CD004961. [9] Davis TR, Sher JL, Horsman A, Simpson M, Porter BB, Checketts RG. Intertrochanteric femoral fractures. Mechanical failure after internal fixation. J Bone Joint Surg Br 1990;72(1):26–31. [10] Leung KS, So WS, Shen WY, Hui PW. Gamma nails and dynamic hip screws for peritrochanteric fractures. A randomised prospective study in elderly patients. J Bone Joint Surg Br 1992;74(3):345–51. [11] Haynes RC, Pöll RG, Miles AW, Weston RB. Failure of femoral head fixation: a cadaveric analysis of lag screw cut-out with the gamma locking nail and AO dynamic hip screw. Injury 1997;28(5-6):337–41. [12] Jensen JS, Tøndevold E, Mossing N. Unstable trochanteric fractures treated with the sliding screw-plate system. A biomechanical study of unstable trochanteric fractures. III. Acta Orthop Scand 1978;49(4):392–7. [13] Ahrengart L, Törnkvist H, Fornander P, Thorngren KG, Pasanen L, Wahlström P, et al. A randomized study of the compression hip screw and Gamma nail in 426 fractures. Clin Orthop Relat Res 2002;401:209–22. [14] Pedersen DR, Brown TD, Maxian TA, Callaghan JJ. Temporal and spatial distributions of directional counterface motion at the acetabular bearing surface in total hip arthroplasty. Iowa Orthop J 1998;18:43–53.
[15] Saikko V, Calonius O, Keränen J. Effect of extent of motion and type of load on the wear of polyethylene in a biaxial hip simulator. J Biomed Mater Res B Appl Biomater 2003;65(1):186–92. [16] Ehmke LW, Fitzpatrick DC, Krieg JC, Madey SM, Bottlang M. Lag screws for hip fracture fixation: Evaluation of migration resistance under simulated walking. J Orthop Res 2005;23(6):1329–35. [17] Sommers MB, Roth C, Hall H, Kam BC, Ehmke LW, Krieg JC, et al. A laboratory model to evaluate cutout resistance of implants for pertrochanteric fracture fixation. J Orthop Trauma 2004;18(6):361–8. [18] Lindahl O. Mechanical properties of dried defatted spongy bone. Acta Orthop Scand 1976;47(1):11–19. [19] Saudan M, Lübbeke A, Sadowski C, Riand N, Stern R, Hoffmeyer P. Pertrochanteric fractures: is there an advantage to an intramedullary nail?: a randomized, prospective study of 206 patients comparing the dynamic hip screw and proximal femoral nail. J Orthop Trauma 2002;16(6):386–93. [20] Parker MJ, Handoll HH. Gamma and other cephalocondylic intramedullary nails versus extramedullary implants for extracapsular hip fractures in adults. Cochrane Database Syst Rev 2010(9):CD0 0 0 093. [21] Baumgaertner MR, Curtin SL, Lindskog DM, Keggi JM. The value of the tip-apex distance in predicting failure of fixation of peritrochanteric fractures of the hip. J Bone Joint Surg Am 1995;77(7):1058–64. [22] Wu CC, Shih CH, Chen WJ, Tai CL. Treatment of cutout of a lag screw of a dynamic hip screw in an intertrochanteric fracture. Arch Orthop Trauma Surg. 1998;117(4-5):193–6. [23] Hsueh KK, Fang CK, Chen CM, Su YP, Wu HF, Chiu FY. Risk factors in cutout of sliding hip screw in intertrochanteric fractures: an evaluation of 937 patients. Int Orthop 2010;34(8):1273–6. [24] Bojan AJ, Beimel C, Taglang G, Collin D, Ekholm C, Jönsson A. Critical factors in cut-out complication after Gamma nail treatment of proximal femoral fractures. BMC Musculoskeletal Disord 2013;14:1. [25] Griffin XL, McArthur J, Achten J, Parsons N, Costa ML. The Warwick hip trauma evaluation one—an abridged protocol for the WHiTE One Study: an embedded randomised trial comparing the X-bolt with sliding hip screw fixation in extracapsular hip fractures. Bone Joint Res 2013;2(10):206–9. [26] Gotfried Y. The Gotfried percutaneous compression plate compared with the conventional classic hip screw for the fixation of intertrochanteric fractures of the hip. J Bone Joint Surg Br 2003;85(1):148. [27] Parker MJ, Das A. Extramedullary fixation implants and external fixators for extracapsular hip fractures in adults. Cochrane Database Syst Rev 2013;2:CD0 0 0339. [28] Takemoto RC, Lekic N, Schwarzkopf R, Kummer FJ, Egol KA. The effect of two different trochanteric nail lag-screw designs on fixation stability of four-part intertrochanteric fractures: a clinical and biomechanical study. J Orthop Sci 2014;19(1):112–19. [29] Strauss E, Frank J, Lee J, Kummer FJ, Tejwani N. Helical blade versus sliding hip screw for treatment of unstable intertrochanteric hip fractures: a biomechanical evaluation. Injury 2006;37(10):984–9. [30] Lustenberger A, Bekic J, Ganz R. [Rotational instability of trochanteric femoral fractures secured with the dynamic hip screw. A radiologic analysis. Unfallchirurg 1995;98(10):514–17 [Article in German]. [31] Kouvidis GK, Sommers MB, Giannoudis PV, Katonis PG, Bottlang M. Comparison of migration behavior between single and dual lag screw implants for intertrochanteric fracture fixation. J Orthop Surg Res 2009;4:16. [32] Chen THA. Novel dynamic hip screw design for femoral intertrochanteric fracture: the excellent outcome in osteoporosis patients. In: The Tenth International Congress of Chinese Orthopaedic Association, Beijing, China,; 2015.
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Wing-augmentation reduces femoral head cutting out of dynamic hip screw Chih-Yu Chen a,b, Shu-Wei Huang d, Jui-Sheng Sun c,f,∗, Shin-Yiing Lin c, Chih-Sheng Yu e, Hsu-Pin Pan e, Ping-Hung Lin e, Fan-Chun Hsieh e, Yang-Hwei Tsuang b, Feng-Huei Lin d, Rong-Sen Yang c, Cheng-Kung Cheng a,∗∗ a
Department of Biomedical Engineering, National Yang-Ming University, Linong St, Beitou District, Taipei City, Taiwan Department of Orthopedics, Shuang Ho Hospital, Taipei Medical University, Wuxing St, Xinyi District, Taipei City, Taiwan c Department of Orthopedic Surgery, National Taiwan University & Hospital, No. 7, Zhongshan S Rd, Zhongzheng District, Taipei City, Taiwan d Institute of Biomedical Engineering, National Taiwan University, No. 1, Section 4, Roosevelt Rd, Da’an District, Taipei City, Taiwan e Instrument Technology Research Center, National Applied Research Laboratories 20, R&D Rd. VI, Hsinchu Science Park, Hsinchu 300, Taiwan f Biomimetic Systems Research Center, National Chiao Tung University, No. 1001, Daxue Rd, East District, Hsinchu City, Taiwan b
a r t i c l e
i n f o
Article history: Received 6 January 2016 Revised 1 February 2017 Accepted 22 February 2017 Available online xxx Keywords: Femoral intertrochanteric fracture Cut-out Lag screw Dynamic hip screw
a b s t r a c t The dynamic hip screw (DHS) is commonly used in the treatment of femoral intertrochanteric fracture with high satisfactory results. However, post-operative failure does occur and result in poor prognosis. The most common failure is femoral head varus collapse, followed by lag screw cut-out through the femoral head. In this study, a novel-designed DHS with two supplemental horizontal blades was used to improve the fixation stability. In this study, nine convention DHS and 9 Orthopaedic Device Research Center (ODRC) DHSs were tested in this study. Each implant was fixed into cellular polyurethane rigid foam as a surrogate of osteoporotic femoral head. Under biaxial rocking motion, all constructs were loaded to failure point (12 mm axial displacement) or up to 20,0 0 0 cycles of 1.45 kN peak magnitude were achieved, whichever occurred first. The migration kinematics was continuously monitored and recorded. The final tip-to-apex distance, rotational angle and varus deformation were also recorded. The results showed that the ODRC DHS sustained significantly more loading cycles and exhibited less axial migration in comparison to the conventional DHS. The ODRC DHS showed a significantly smaller bending strain and larger torsional strain compared to the conventional DHS. The changes in tip-to-apex distance (TAD), post-study varus angle, post-study rotational angle of the ODRC DHS were all significantly less than that of the conventional DHS (p < 0.05). We concluded that the ODRC DHS augmented with two horizontal wings would increase the bone–implant interface contact surface, dissipate the load to the screw itself, which improves the migration resistance and increases the anti-rotational implant effect. In conclusion, the proposed ODRC DHS demonstrated significantly better migration resistance and anti-rotational effect in comparison to the conventional DHS construct. © 2017 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction Femoral intertrochanteric fractures account for a large proportion of proximal femoral fractures in the elderly with osteoporo∗ Corresponding at Department of Orthopedic Surgery, National Taiwan University & Hospital, No. 7, Zhongshan S Rd, Zhongzheng District, Taipei City, Taiwan ∗∗ Corresponding author. E-mail addresses: [email protected] (C.-Y. Chen), [email protected] (S.-W. Huang), [email protected], [email protected] (J.-S. Sun), [email protected] (S.-Y. Lin), [email protected] (C.-S. Yu), [email protected] (H.-P. Pan), [email protected] (P.-H. Lin), [email protected] (F.-C. Hsieh), [email protected] (Y.-H. Tsuang), [email protected] (F.-H. Lin), [email protected] (R.-S. Yang), [email protected] (C.-K. Cheng).
sis. These fractures are one of the most important health care issues faced by orthopedic surgeons today. Many people who experience such fractures rapidly deteriorating in health status with a significant physical and functional impairment and require substantial financial resources during the perioperative and rehabilitative care [1]. In the 1950s, operative treatment for femoral intertrochanteric fractures was introduced to improve the functional outcome and reduce the complications associated with longterm immobilization and prolonged bed rest [2–4]. Later, a variety of different extra-medullary or intra-medullary implants have been developed since the 1950 s. The most commonly used extramedullary implant is the dynamic hip screw (DHS) with side plate. Intramedullary nails may be used for the surgical fixation of extracapsular hip fractures in adults; however, there is limited
http://dx.doi.org/10.1016/j.medengphy.2017.02.015 1350-4533/© 2017 IPEM. Published by Elsevier Ltd. All rights reserved.
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evidence to date and it is insufficient to determine whether there are important differences in outcome between different designs of intramedullary nails used in the internal fixation of extracapsular hip fractures. Further studies comparing different designs of intramedullary nails are not a priority. DHS is still currently considered to be the gold standard for extra capsular hip fracture fixation as well as the implant that any new design should be compared to [5–8]. Since its introduction, the DHS has been shown to produce good results; however, complications are frequent, particularly in unstable fractures [9–11]. Post-operative implant-related complications have been reported in recent meta-analysis studies [7]. The most common cause of failure is reported to be varus collapse of host bone and cutting-out of the lag screw through the femoral head [9,12,13]. For the failure mechanisms about varus collapse and cuttingout of the lag screw through the femoral head, previous mechanical studies have relied on static or dynamic uniaxial loading regimens to induce construct failure. However, the hip is loaded in a multiplanar, dynamic manner during normal gait. In the insertwearing study for prosthetic hip joints, the biaxial rocking motion (BRM) technology has been chosen to simulate dynamic multiplanar forces loading during level walking [14,15]. The hip implant performance simulator (HIPS) developed by Ehmke et al. can reproduce the dynamic multi-planar hip forces seen during level walking [16]. By using multi-planar forces with a loading protocol designed for hip ab-adduction, flexion–extension, and a double peak load history, BRM technology could simulate much closer to the normal physiologic hip joint motion. In this study, the biomechanical behavior and cut-out performance of a novel wing-augmented DHS and the conventional DHS implant were investigated under physiologic multi-planar loading model. Our hypothesis is that the additional wings would significantly dissipate the stress on the lag screw and enhance the anti-rotational effect. These factors would decrease femoral head cutting out incidence under cyclic load dynamic testing. 2. Materials and methods 2.1. Implants Nine dynamic hip screws (DHS, Synthes, Oberdorf, Switzerland) made of stainless steel were tested as the gold-standard for single lag screw implants. The DHS lag screws had a shaft diameter of 7.8 mm, an outer thread diameter of 12.5 mm and a total length of 110 mm. The wing-augmented ODRC-DHS implants [ODRC: Orthopaedic Device Research Center, National Yang-Ming University, Taipei City, Taiwan] made of titanium were used for comparison. Besides the two grooved structure and the additional two horizontal wings, the ODRC DHS had the exact mechanical parameters derived from the control DHS implant (Fig. 1). To investigate if the newly designed implant can provide greater migration resistance and better biomechanical behavior, nine winged ODRC-DHS implants were tested for comparison. 2.2. Surrogate specimens To yield higher reproducibility and consistent cutout failure, surrogate specimens were used as a cancellous bone substitute [16,17]. The stability of lag screw fixation was tested in surrogate femoral head and neck specimens machined from cellular polyurethane foam (50 mm in diameter, #1522-11, Pacific Research Inc., Vashon, Washington, USA). As validated in a previous study, these specimens had an elasticity modulus (E-modulus) of 48 MPa with 4 MPa compressive strength and a density of 12.5 pcf (0.2 g/cm3 ) to simulate mildly osteoporotic bone [17]. These material properties correspond to the osteoporotic range of human
Fig. 1. Orthopedic device research center (ODRC) DHS design scheme. Upper: Lag screw profile and its cross section view. Middle: Blade profile and its lateral view. Lower: estimated increases in contact surface area both in the vertical and horizontal plane.
cancellous bone, with 5–104 MPa E-modulus and 2–21 MPa compressive strength [18]. For delivery of dynamic loading, the surrogate specimens were placed in a 6 mm thick, polished steel shell to provide a rigid, spherical interface. 2.3. Implant insertion In this study, DHS lag screw surrogate specimens were reamed but not tapped. The lag screw was placed centrally within the femoral head surrogate and advanced to a depth leaving 20 mm tip-to-apex distance (TAD) [19]. This measurement, the TAD, is the sum of the distance from the tip of the lag screw to the apex of the femoral head both on an anteroposterior radiograph and lateral radiograph, after controlling for magnification. This corresponds to a 10 mm distance of the screw tip to the femoral head apex in both views. The ODRC DHS were inserted using the same protocol as the control; after the lag screw was inserted to the intended depth, the ODRC-DHS blade trench was created to pre-determined length, and then the ODRC-DHS blade was inserted and fixed with compression screw. All implants were inserted according to the manufacturer’s guidelines. 2.4. Experimental setup The constructs were then tested in the HIPS-mode to reproduce the dynamic multi-planar hip forces during level walking. This model has been validated for simulation of lag screw migration and cut-out in a most serious clinical condition, that is, combination of an unstable femoral intertrochanteric fracture (OTA classification 31-A.2) in an osteoporotic bone and gait cycle loading [16]. The testing system is fixed at a solid base which simulated the physiologic situation that anatomic axis of femoral shaft aligned perpendicular to the horizontal plane. The proximal aspect of the base was fixed to simulate a pertrochanteric fracture with a fracture line inclination of 40° to the anatomic axis of the femoral shaft (Fig. 2). A bipolar-designed steel shell femoral head back plate with a 40 mm diameter hole was used to ensure unconstrained shear translation of the lag screw in the surrogate femoral neck. This back plate sit against a polyethylene bolster attached to the base plate to reproduce the constraints characteristic of a reduced, but unstable pertrochanteric fracture with deficient posteromedial neck support. This bolster construct simulated fracture
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Fig. 2. The material test system setup for application of biaxial rocking motion representing the hip loading during level walking.
surface abutment in clinical scenario that femoral head varus collapse and rotation occurred after lag screw sliding completion. To replicate clinically relevant sliding conditions, the clamping part in the base plate incorporated the section of the wing-augmented DHS with the addition of two slits for the ODRC DHS, or the barrel and side-plate in the case of DHS. In order to retrieve bending and axial rotational strain of the implants throughout the experiment, two types of strain gages (bending strain gage: KFG® series; torsional strain gage: KFRS® series, Kyowa Electronic Instruments Co., Sapporo, Japan) were glued onto the lag screw without interfere the bending and rotation of the construct. The data was then collected and analyzed by computer. 2.5. Loading To reproduce level walking simulation, biaxial rocking motion (BRM) was used to produce concurrent rotational displacement and axial loading controlled by a biaxial material test system (ElectroForce® 3510-AT, Bose Corporation – ElectroForce Systems Group, Eden Prairie, Minnesota, USA). The output load that applied to the steel shell through the polyethylene meniscus was a dynamic (at 1 Hz), double-peak loading regimen (of 1.45 kN peak load; approximating two times bodyweight). The path of meniscus traced on the femoral head was to simulate the path of resultant force vectors during level walking. The BRM chosen to simulate level walking consisted of concurrent flexion–extension and abduction–adduction motion superimposed with sinusoidal rotation on a 23° inclined block affixed to the actuator (Fig. 2), which resulted in an 18° joint load vector and 5° valgus of the femoral shaft axis [16]. Exaggerated walking kinematics of the limb (i.e., 45° arc of flexion–extension and a 17° arc of ab-adduction) was simulated using ±75° rotation of the actuator. Implants were exercised to the preset failure point (12 mm axial displacement) or 20,0 0 0 load cycles, which achieved first. 2.6. Outcome measures Eight dependent outcome variables were reported, two of which describe the axial displacement (Daxial ), and the stress on the base plate (Sbase ). Another two describe the bending strain (STbend ) and the rotational strain (STrotation ) on the lag screw. While the remaining four variables (TADdiff , α varus , α horizontal , α neck ) describe the migration behavior of the bone-implant construct. The number of loading cycles to implant failure, axial displacement (Daxial ), and the stress on the base plate, Sbase , was registered by the material test system. The bending strain (STbend ) and rotational
Fig. 3. The displacement–cycle curve and the load–cycle curve. (Solid line: conventional DHS; Dashed line: ODRC DHS). The conventional DHS failed at earlier cycles than that of the ODRC DHS failed; while the average final displacement of the conventional DHS were higher than that of ODRC DHS (p < 0.001). The load of conventional DHS at 60 0 0 cycles was also significantly higher than that of ODRC DHS load (p = 0.0 0 05). (∗ : p < 0.05; ∗ ∗ : p < 0.01; ∗ ∗ ∗ : p < 0.001).
strain (STrotation ) were recorded concurrently through the study by strain gages. Cut-out failure was defined by reaching 12 mm axial load on the material test system, which triggered the system to stop and preserve the cut-out stage. At this point, the migrations of femoral head were analyzed in terms of varus collapse (α varus ), horizontal plane rotation (α horizontal ), and rotation around the neck axis (α neck ). TAD changes before and after the study (TADdiff ) was also recorded. 2.7. Statistical analysis Differences in the axial displacement and stress distribution between implants were tested at discrete time points during the load history. Statistical analysis was performed by using two-tailed Student’s t-tests for unpaired samples at a confidence level of α = 0.05. 3. Results 3.1. Axial displacement and load distribution The ODRC DHS showed significantly less axial migration and more loading cycles in comparison to the conventional DHS. The representative displacement/load–cycle curve is shown in Fig. 3. The conventional DHS failed at 6200 ± 2375 cycles, while the ODRC DHS failed at 11,0 0 0 ± 1221 cycles (p < 0.001). The average final displacement of the conventional DHS and ODRC DHS were 11.68 ± 2.20 mm and 9.13 ± 1.39 mm (p < 0.001), respectively. The conventional DHS and ODRC DHS load at 60 0 0 cycles was 914.67 ± 106.13 N and 253.57 ± 27.16 N (p = 0.0 0 05), respectively. Table 1 shows the cycle-changes on displacement and load throughout the study.
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C.-Y. Chen et al. / Medical Engineering and Physics 000 (2017) 1–6 Table 1 Cyclic axial displacement and stress data between conventional DHS and ODRC DHS. Cycles
500 10 0 0 30 0 0 60 0 0 10,0 0 0
Axial displacement (mm)
p value
Conventional DHS
ODRC DHS
4.38 ± 0.91 6.88 ± 1.44 8.55 ± 1.68 11.68 ± 2.20
1.57 ± 0.62 2.98 ± 1.27 4.89 ± 1.24 7.01 ± 1.28 9.13 ± 1.39
<0.0 0 01 <0.0 0 01 <0.0 0 01 <0.0 0 01
Load (N)
p value
Conventional DHS
ODRC DHS
1003.2 ± 57.43 936.03 ± 57.51 970.5 ± 79.82 914.67 ± 106.13
675.2 ± 9.97 519.36 ± 37.74 276.3 ± 30.05 253.57 ± 27.16 273.2 ± 7.11
0.0 0 06 0.0 0 05 0.0 0 01 0.0 0 05
3.3. Specimen analyses The final varus (α varus ), horizontal (α horizontal ), and neck rotation (α neck ) angles for the conventional DHS were 10.61 ± 2.19°, 1.88 ± 0.89°, and 46.11 ± 1.76°, respectively. On the other hand, the final varus (α varus ), horizontal (α horizontal ), and neck rotation (α neck ) angles for the ODRC DHS were 0.50 ± 0.61°, 0.50 ± 0.66°, and 1.61 ± 1.96°, respectively. The final TAD changes (TADdiff ) for the conventional DHS and ODRC DHS were 1.96 ± 0.95 mm and 0.31 ± 0.16 mm, respectively (Fig. 5). These data were also shown in Table 2.
4. Discussion
Fig. 4. The bending and torsional strain. The bending strain of conventional DHS was significantly higher than that of the ODRC DHS (p < 0.001); while for the torsional strain, the conventional DHS significantly lower than that of ODRC DHS (p < 0.001). (∗ : p < 0.05; ∗ ∗ : p < 0.01; ∗ ∗ ∗ : p < 0.001).
Table 2 Strain gages data and final translation and rotation ankle data of conventional DHS and ODRC DHS.
Strain STbend STrotation Translation TADdiff Rotation
α varus α horizontal α neck
Conventional DHS
ODRC DHS
p value
744.00 ± 69.64 um/m 188.11 ± 107.16 um/m
331.00 ± 125.01 um/m 942.40 ± 152.63 um/m
<0.001 <0.001
1.96 ± 0.95 mm
0.31 ± 0.16 mm
0.04
10.61 ± 2.19° 1.88 ± 0.89° 46.11 ± 1.76°
0.50 ± 0.61° 0.50 ± 0.66° 1.61 ± 1.96°
0.02 <0.001 <0.001
3.2. Strain gages analyses The ODRC DHS showed a significantly smaller bending strain and larger torsional strain compared to the conventional DHS. The bending strain of conventional DHS was 744.00 ± 69.64 um/m, while the ODRC DHS was 331.00 ± 125.01 um/m (p < 0.001). For the torsional strain, the conventional DHS and ODRC DHS were 188.11 ± 107.16 um/m and 942.40 ± 152.63 um/m (p < 0.001), respectively (Fig. 4). These data were shown in Table 2.
Fractures of the proximal femur are one of the greatest challenges in the present medical community, which also constitutes a heavy socioeconomic burden worldwide. Although, the dynamic hip screw (DHS) has been recognized as the standard surgical outcome comparison device for the treatment of femoral intertrochanteric fracture [19,20], it still has a reported failure rate of 8% to 13% [21,22]. Controversy does exist regarding the optimal treatment for independent patients with displaced intra-capsular fractures of the proximal femur. The most common mechanical failure in the internal fixation of trochanteric hip fractures is the cut-out of the sliding screw through the femoral head [21]. Previous reports showed that osteoporotic bone is one of the risk factors for increasing rate of cut-out [23,24]. Several ingenious designs have been developed to reduce the incidence of cut-out complications [25–29]. Unfortunately, some of these designs were too expensive or mechanically too complicated. Our study introduces a newly designed wing-augmented dynamic hip screw (ODRC DHS) that could significantly improve the biomechanical performance of implant, resulting in reduced femoral head cut-out rate. The proposed approach is a simple, inexpensive design and that is also technically easy to implant. Our study confirmed that the ODRC DHS with 2 added wings at the horizontal plane along the lag screw that would largely increase the contact surface and also increase the torsional resistance along the lag screw; the wings would also improve implant anchorage in the femoral head. The ODRC DHS had much better biomechanical performance than the conventional DHS in both stress distribution and total displacement within the surrogate bone. The ODRC DHS stress distribution was significantly less than the conventional DHS (914.67 ± 106.13 N vs. 253.57 ± 27.16 N (60 0 0 cycles), p = 0.0 0 05); on the other hand, the total surrogate bone displacement of the ODRC DHS was also much less than that of the conventional DHS (1.96 ± 0.95 mm vs. 0.31 ± 0.16 mm, p < 0.001) (Fig. 3 and Table 1). In addition to the better load distribution and displacement, ODRC DHS also showed smaller bending strain compared to the conventional DHS (331.00 ± 125.01 um/m vs. 744.00 ± 69.64 um/m, p < 0.001) (Fig. 4 and Table 2). With the added wings, the ODRC DHS increases the contact surface at the bone–implant interface, decreases the strain taken by the lag screw itself by dissipating the load through the wings, and eventually
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Fig. 5. The final TAD translation and rotation of specimen. The final varus (α varus ), horizontal (α horizontal ), and neck rotation (α neck ) angles for the conventional DHS were all significantly higher than that of the ODRC DHS. The final TAD changes (TADdiff ) for the conventional DHS was also significantly higher than that of the ODRC DHS. (∗ : p < 0.05; ∗ ∗ : p < 0.01; ∗ ∗ ∗ : p < 0.001).
resulting in less breakage of bony trabeculae and better biomechanical performance. When the human cadaveric hips loaded in a multiplanar, dynamic manner during normal gait, the degree of varus collapse (8.5 ± 7.7°) and rotation (7.2 ± 6.4°) were comparable to that in surrogate specimens (varus collapse: 5.4 ± 2.9°; rotation 7.2 ± 2.8°), with the surrogate specimens showing significantly less variability [16]. In this study, the final varus (α varus ), horizontal (α horizontal ), and neck rotation (α neck ) angles for the conventional DHS were significantly higher than those for the ODRC DHS (Fig. 5; p < 0.05). On the contrary, the rotational strain of the ODRC DHS was significantly larger to the conventional DHS (942.40 ± 152.63 um/m vs. 188.11 ± 107.16 um/m, p < 0.001) (Fig. 5 and Table 2). This finding suggested that the added wings resulted in larger torsional strain of the ODRC DHS by increasing the lever-arm of the implant. Larger torsional strain implied that most of the load was scattered through the metallic implant, instead of the osteoporotic bone, and also lead to much lesser bone trabecular damage at the bone–implant contact surface. These findings explained the significantly better anti-torsional capability of the ODRC DHS device. Similar to this results, the DHS blade® (DePuy Synthes, West Chester, Philadelphia, U.S.A.) and X-bolt® dynamic plating system (X-Bolt Orthopaedics, Dublin, Ireland) also had a similar design concept and also resulted in better biomechanical performance and better clinical outcomes than the conventional DHS [17,25,30]. As mentioned above, lag screw cut-out failure following fixation of unstable intertrochanteric fractures in osteoporotic bone remains an unsolved challenge. In a study tested for improving cut-out resistance, Kouvidis et al. demonstrated that the double screw construct provided significantly greater resistance against varus collapse and neck rotation in comparison to a standard DHS single lag screw implant [31]. Although, double screw construct does provide significantly greater resistance for cut-out; however, the insertion of double screw in the actual clinical scenario is both technical demanding and time consuming. In our study, the ODRC DHS also demonstrated significantly better migration resis-
tance and anti-rotational effect in comparison to the conventional DHS construct; the ODRC DHS is relatively less technical demanding and can save the operation time. This is an important issue to lower operation risk when taking surgery on an elderly and relatively high morbidity senile patient. Although our study results were promising, there were several limitations in this study. First, this was an in vitro study. Although we used the HIPS system to simulate the natural dynamic hip loading conditions, this approach could not reproduce the natural hip motion during human ambulation; further studies are necessary to access the ODRC DHS biomechanical activities under a clinical scenario. Second, we used only one type of fracture pattern in this experimental setup to simulate the unstable femoral intertrochanteric fracture. It was unclear that whether different fracture patterns would affect the experiment results. Third, the ODRC DHS design used 2 horizontal wings along the lag screw axis to increase the contact surface, we could not evaluate the biomechanical performance if the 2 wings were not level. In a preliminary study of eleven female osteoporotic patients (mean age 82 yearsold) with intertrochanteric fractures, the follow-up results (90– 255 days) show that this ODRC-DHS system did reduce lag-screw cutout complications in patients with osteoporosis [32]; however, further experiments will be necessary to answer these questions. In conclusion, our study presented a novel DHS design with two horizontal wings to increase the contact surface of bone–implant interface. When compared to the conventional DHS, the ODRC DHS would decrease the stress along the implant, decrease the displacement of total bone-implant construct, decrease the rotational deformity, and eventually result in better cut-out resistance. Although the results were promising, further studies will be necessary to validate the potential benefits and limitations of this novel implant.
Conflict of interest None
Please cite this article as: C.-Y. Chen et al., Wing-augmentation reduces femoral head cutting out of dynamic hip screw, Medical Engineering and Physics (2017), http://dx.doi.org/10.1016/j.medengphy.2017.02.015
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Please cite this article as: C.-Y. Chen et al., Wing-augmentation reduces femoral head cutting out of dynamic hip screw, Medical Engineering and Physics (2017), http://dx.doi.org/10.1016/j.medengphy.2017.02.015