Screw-blade fixation systems in Pauwels three femoral neck fractures

Screw-blade fixation systems in Pauwels three femoral neck fractures: a biomechanical evaluation Matthias Knobe, Simon Altgassen, Klaus-Jürgen Maier, Gertraud GradlDietsch, Chris Kaczmarek, Sven Nebelung, Kajetan Klos, et al. International Orthopaedics ISSN 0341-2695 International Orthopaedics (SICOT) DOI 10.1007/s00264-017-3587-y

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Author's personal copy International Orthopaedics (SICOT) DOI 10.1007/s00264-017-3587-y

ORIGINAL PAPER

Screw-blade fixation systems in Pauwels three femoral neck fractures: a biomechanical evaluation Matthias Knobe 1 & Simon Altgassen 1 & Klaus-Jürgen Maier 2 & Gertraud Gradl-Dietsch 1 & Chris Kaczmarek 1 & Sven Nebelung 3 & Kajetan Klos 4 & Bong-Sung Kim 5 & Boyko Gueorguiev 6 & Klemens Horst 1 & Benjamin Buecking 7

Received: 21 February 2017 / Accepted: 11 July 2017 # SICOT aisbl 2017

Abstract Objectives To reduce mechanical complications after osteosynthesis of femoral neck fractures, improved fixation techniques have been developed including blade or screw– anchor devices. This biomechanical study compares different fixation systems used for treatment of unstable femoral neck

fractures with evaluation of failure mode, load to failure, stiffness, femoral head rotation, femoral neck shortening and femoral head migration. Methods Standardized Pauwels type 3 fractures (AO/OTA 31-B2) with comminution were created in 18 biomechanical sawbones using a custom-made sawguide. Fractures were

Matthias Knobe and Simon Altgassen both contributed equally to this work. Electronic supplementary material The online version of this article (doi:10.1007/s00264-017-3587-y) contains supplementary material, which is available to authorized users. * Matthias Knobe [email protected]

Klemens Horst [email protected] Benjamin Buecking [email protected]

Simon Altgassen [email protected] Klaus-Jürgen Maier [email protected]

1

Department of Orthopaedic Trauma, University of Aachen Medical Center, 30 Pauwelsstreet, 52074 Aachen, Germany

Gertraud Gradl-Dietsch [email protected]

2

Department of Surgery, RoMed Hospital Bad Aibling, Bad Aibling, Germany

Chris Kaczmarek [email protected]

3

Department of Radiology, University of Aachen Medical Center, Aachen, Germany

Sven Nebelung [email protected]

4

Department of Foot and Ankle Surgery, Catholic Hospital Mainz, Mainz, Germany

5

Department of Plastic Surgery, Reconstructive and Hand Surgery, University of Aachen Medical Center, Aachen, Germany

6

AO Research Institute Davos, Davos, Switzerland

7

Department of Trauma, Hand and Reconstructive Surgery, University Hospital Gießen and Marburg GmbH, Campus Marburg, Marburg, Germany

Kajetan Klos [email protected] Bong-Sung Kim [email protected] Boyko Gueorguiev [email protected]

Author's personal copy International Orthopaedics (SICOT)

stabilized using either SHS-Screw, SHS-Blade or Rotationally Stable Screw-Anchor (RoSA). Femurs were positioned in 25 degrees adduction and ten degrees posterior flexion and were cyclically loaded with an axial sinusoidal loading pattern of 0.5 Hz, starting with 300 N, with an increase by 300 N every 2000 cycles until bone–implant failure occurred. Results Mean failure load for the Screw-Anchor fixation (RoSA) was 5100 N (IQR 750 N), 3900 N (IQR 75 N) for SHS-Blade and 3000 N (IQR 675 N; p = 0.002) for SHSScrew. For SHS-Screw and SHS-Blade we observed fracture displacement with consecutive fracture collapse as the main reason for failure, whereas RoSA mainly showed a cut-out under high loadings. Mean stiffness at 1800 N was 826 (IQR 431) N/mm for SHS-Screw, 1328 (IQR 441) N/mm for SHS-Blade and 1953 (IQR 617) N/mm for RoSA (p = 0.003). With a load of 1800 N (SHS-Screw 12° vs. SHS-Blade 7° vs. RoSA 2°; p = 0.003) and with 2700 N (24° vs. 15° vs. 3°; p = 0.002) the RoSA implants demonstrated a higher rotational stability and had the lowest femoral neck shortening (p = 0.002), compared with the SHS groups. At the 2700 N load point, RoSA systems showed a lower axial (p = 0.019) and cranial (p = 0.031) femoral head migration compared to the SHS-Screw. Conclusions In our study, the new Screw-Anchor fixation (RoSA) was superior to the comparable SHS implants regarding rotational stability and femoral neck shortening. Failure load, stiffness, femoral head migration, and resistance to fracture displacement were in RoSA implants higher than in SHS-Screws, but without significance in comparison to SHS-Blades. Keywords Sliding hip screw . Helical blade . Rotationally stable screw-anchor . Biomechanical testing . Femoral neck fracture . Cut-out . Migration . Rotation . Fragment displacement . Failure

Introduction Treatment strategies for femoral neck fractures are either arthroplasty or fracture fixation, dependent on age, functional requirements and bone quality [1]. For young adults as well as older patients with good bone quality and undisplaced fractures, head preserving therapy is preferred as it is less invasive and associated with good functional results [1]. Whereas screws offer advantages in terms of torsional stability, preservation of blood supply to the femoral head and a less invasive approach, the SHS provides higher stability especially in osteoporotic bone [2, 3]. However, while in undisplaced fractures failure rate is about 10% [4], insufficient fracture fixation with failure rates of up to 37% within two years remains a common complication in the treatment

of displaced femoral neck fractures [5]. The main biomechanical complication in young adults is leg shortening caused by subsidence, e.g. excessive sintering and shortening or varus collapse [6]. Biomechanical studies have shown that the pertinent benefit of a helical blade lies in its rotational stability [7–10]. However, its resistance to pullout forces was rather low [9], restricting the possibility of intra-operative compression. The Rotationally Stable Screw-Anchor (RoSA) is supposed to combine pullout strength and compression capability of a lag screw with the improved load ability and rotational stability of a blade [11, 12]. The aim of this biomechanical study was to compare implant anchorage and overall performance of three fixation systems (SHSScrew, SHS-Blade and Screw-Anchor [RoSA]) (Fig. 1) for femoral neck fractures using an unstable Pauwels type 3 fracture model. Two central questions were addressed in the present study: 1. Is there any difference in failure mode, load to failure and stiffness between the tested implants? 2. Can we detect differences in implant anchorage characterized by fragment displacement, femoral head rotation, screw migration, and tendency for femoral neck shortening?

Materials and methods A total of 18 left Femur Biomechanical Sawbones, 4th Generation (www.sawbones.com, medium size) were assigned to one of three test groups (n = 6). One group received RoSA fixation (Rotationally Stable Screw-Anchor, 3-hole side-plate 136°, Koenigsee Implants, Allendorf, Germany), the second group received an SHS system (DePuy Synthes, Zuchwil, Switzerland) with femoral neck screw (SHS-Screw, 4-hole side-plate 135°), while the third group received treatment with an SHS system with a blade (SHS-Blade, 4-hole side-plate 135°) (Fig. 2). Osteotomy and surgical technique We simulated an unstable mediocervical fracture Pauwels type 3 (AO/OTA 31-B2) using a custom-made sawguide box made of gypsum. Osteotomies were performed according to the protocol of Windolf et al. [7] with an osteotomy axis of 20° angulation and an additional 30° distal wedge and 15° posterior wedge resembling comminution (Fig. 3). The implant position was chosen centrally in the femoral head with a tip-apex distance (TAD) of less than 25 mm and was controlled radiographically during implantation [13].


Fig. 2 Photograph of the different implant constructions investigated in this study. SHS-Screw (top), SHS-Blade (middle) and Screw-Anchor (bottom)

Fig. 1 Radiographs of clinical cases with different implant constructions investigated in this study. Screw fixation (SHS-Screw; a Anteriorposterior; b Axial), blade fixation (SHS-Blade; c Anterior-posterior; d Axial) (both DePuy Synthes, Zuchwil, Switzerland) and Screw-Anchor fixation (RoSA; e Anterior-posterior; f Axial) (Koenigsee Implants, Allendorf, Germany)

Embedding and mechanical testing Specimens were embedded (Technovit 4006, Heraeus Kulzer GmbH, Wehrheim, Germany) with 25° adduction and 10° posterior flexion to provoke bending moments in the sagittal as well as in the frontal plane and to ensure rotational moments under axial load [7, 11, 12, 14–20]. Femurs were exposed to vertical loading using a biomechanical testing machine (Zwick/Roell, BZ1-MM14450.ZW04, Force transducer

Xforce Type HP, 10KN nominal force, software testXpert® II; Zwick GmbH and Co. KG, Ulm, Germany). Before testing, a preload of 100 N was applied and maintained throughout the test series to ensure permanent contact between the fragments corresponding with physiological conditions during the swinging phase [21]. Specimens were then loaded with 300 N over 2000 (sinusoidal) cycles with a frequency of 0.5 Hz. After a five minutes relaxation phase, the load was increased stepwise by 300 N (600 N, 900 N, 1200 N, 1500 N, 1800 N, etc.) every 2000 cycles (sinusoidal, 0.5 Hz) until failure. Radiographs in two planes (ap and lateral view) were taken (Philips BV 300, Philips Medical Systems, 5680 DA Best, the Netherlands) before testing and after every load step, and the sample was inspected and measured macroscopically with regard to fracture movement and rotation. Failure was defined as fracture of the femoral shaft, and/ or cut-out/cut-through, and/or implant failure, more than 15 mm fracture displacement and/or sudden decrease in the recorded force [11, 12]. Bergmann et al. studied the in-vivo forces acting on the hip joint and found maximum gait loads (4 km/h) of up to 350% body weight and mean loads of 238% body weight [22]. In the light of that, we studied the behaviour of the bone-implant constructs up to failure at loads corresponding to average every-day loads (1800 N) [11, 12, 22] and at the maximum load (2700 N).

Author's personal copy International Orthopaedics (SICOT) Fig. 3 Osteotomy model. The left photograph shows our sawguide for creating the Pauwels 3 fracture. The draft at the right illustrates the performed cuttings: Osteotomy was performed according to Windolf et al. [6] with an osteotomy axis of 20° angulation and an additional 30° distal wedge and 15° posterior wedge resembling comminution

Measurements Measurement of any displacement occurred in three planes as shown in prior publications [11, 12], e.g. fragment displacement in the frontal plane (vertical displacement and femoral neck shortening) (Fig. 4a), migrational behaviour in the frontal and transverse plane (Fig. 4b), and fragment rotation (static rotation macroscopically evaluated after every load step) (Fig. 4c). For measurement of rotation and before radiographic evaluation, a line was drawn at the greater trochanter across the section plane, parallel to the radiographic plane using a standard permanent marker (tip diameter, 0.4 mm) and a plain guidance (femur was fixed). The distance between the screw axis and the marker was measured on the radiographs Fig. 4 Displacement measurements in the frontal plane (a), measurements of transverse migration (b), and illustration of measurements of rotation (c). a Femoral neck shortening along the sliding direction of the implant (white arrow), vertical displacement of the femoral head with respect to the shaft (black arrow), cranial and axial head migration (grey arrows); b Ventro-dorsal head migration (grey arrow); c Distance measured by a digital calliper (marker A versus marker A’, as a result of rotation of the headneck-fragment), static rotation α = arcos (1-(a2/2b2))

(distance = b). Using a standard digital calliper (Mitutoyo, Digital Absolut IP67 150 mm, accuracy = 0.01 mm), static rotation was evaluated macroscopically after every load step by measuring the distance (distance = a) between the generated lines. The resulting rotational angle α was computed with the law of cosines: α = arcos [1 - (a2/2b2)] (Fig. 4c).

Stiffness For each single loading cycle, the software registered deformation and the applied force. From these data, the stiffness as N/mm was calculated and the median of these 2000 single measurements was taken for every load step.

Author's personal copy International Orthopaedics (SICOT) Table 1

Results of the mechanical test series at failure point (median [IQR])

Parameter

SHS-Screw

SHS-Blade

RoSA

Failure load (N)

3000 (675)

3900 (75)

5100 (750)

0.002

n.s.

Significant

n.s.

Failure cycle

20,000 (4115)

26,000 (500)

33,147 (5126)

0.001

n.s.

Significant

n.s.

Failure modus Trochanteric fracture (lateral wall)

n.s.

Significant

n.s.

1

1

2

Displacement / fracture collapse Implant failure

4 1

4 0

0 0

Cut-out

0

1

4

Displacement (mm) Rotational angle (°)

14.0 (1.4) 22° (18°)

13.1 (2.6) 21° (14°)

8.5 (2.7) 11° (5°)

0.002 0.03

n.s. n.s.

Significant Significant

Significant n.s.

Axial migration (mm) Cranial migration (mm)

1.2 (1.2) 2.1 (0.9)

1.5 (1.5) 2.4 (2.0)

3.7 (3.2) 4.0 (3.5)

0.162 0.338

Ventro-dorsal migration (mm)

0.6 (1.2)

0.9 (2.1)

2.4 (1.4)

0.005

n.s.

Significant

Significant

Stiffness (N/mm) Femoral neck shortening (mm)

809 (189) 6.1 (0.9)

1062 (335) 5.5 (1.1)

1174 (534) 3.1 (3.5)

0.039 0.013

n.s. n.s.


n.s. n.s.

Medial/caudal fracture gap (mm)

3.7 (2.8)

5.8 (1.5)

4.3 (6.4)

0.072

Statistics After testing for normality (Shapiro-Wilk test), Kruskal-Wallis Test was chosen to compare the three test groups, followed by a pairwise comparison with adjusted significance according to Dunn-Bonferroni as post-hoc test. Fisher’s exact test was used to assess differences for categorical variables (failure mode). Data was evaluated with SPSS® (IBM, Armonk, NY, USA). Results are presented as median and interquartile range. All tests were two-tailed and assessed at the 5% significance level.

Results Radiographic controls showed that all implants were placed in centre-centre position with a tip-apex distance of 14.0 mm (IQR 5.5 mm). For the SHS systems, the required length of

Fig. 5 Illustration of the predominant failure mechanisms in this study. The femora instrumented with SHS-Screw (left) tended to show an increasing displacement, eventually leading to fracture collapse and loss of reduction. The failure mechanism with SHS-Blade implants (middle) was

p-value

Screw vs. Blade

Screw vs. RoSA

Blade vs. RoSA

the neck screw was measured at 90 mm, the corresponding equivalent for RoSA was an 80 mm blade/70 mm screw combination.

Failure load and failure mode In the SHS-Screw group, failure first occurred with the 2700 N load step (fragment displacement in the frontal plane of more than 15 mm — failure of osteosynthesis). This was the main reason for failure in this group and in the SHS-Blade group as well (with four in six cases; Table 1). In contrast, the main fracture pattern of RoSA was a cut-out in four cases, while a fracture in the trochanteric region was observed in the remaining two cases. Before cut-out of the RoSA implants occurred a more parallel displacement of the head fragment was seen (Fig. 5).

similar. For femora treated with RoSA, we observed a more parallel and decelerated displacement which finally caused a cut-out of the load carrier through the cranial side of the femoral head


Displacement

Fig. 6 Failure loads. The tolerated maximum loading of each group is shown as a boxplot. The box accounts for the first and third quartile, with the bold line showing the median. Additionally, minimum and maximum failure load is marked

The average failure load of the RoSA system was 5100 N (IQR 750 N), whereas that for the SHS-Screw was 3000 N (IQR 675 N) and for the SHS-Blade 3900 N (IQR 75 N), respectively (p = 0.002; Fig. 6). Regarding the total number of cycles, this corresponded to 33,147 (IQR 5126) load cycles for the RoSA, 20,000 (IQR 4115) for the SHS-Screw and 26,000 (IQR 500) for the SHS-Blade (p = 0.001; Table 1). Post-hoc testing revealed significant differences between SHS-Screw and RoSA for failure load and cycle count. Differences between both SHS systems and SHS-Blade vs. RoSA were not significant.

Over all load steps, fragment displacement within the RoSA group was significantly lower than in the SHSScrew or the SHS-Blade group, with the SHS-Screw showing the largest displacement (Fig. 7a). After 1800 N we observed a displacement of 8.9 mm (IQR 2.9) for the SHS-Screw, compared to 4.9 (IQR 2.0) mm for the SHS-Blade and 2.4 (IQR 1.2) mm for the femora treated with RoSA (p = 0.001; Table 2). Pairwise comparison showed significance between SHS-Screw and the RoSA system. In addition, at the 2700 N load step displacement in the RoSA group was significantly lower than in the SHS-Screw (p < 0.001; Table 3).

Fragment rotation In addition to the general medial–caudal movement, an external rotation of the head–neck fragment was regularly observed. With a load of 1800 N (SHS-Screw 12° vs. SHS-Blade 7° vs. RoSA 2°; p = 0.003, significant for RoSA vs. SHS-Screw) and with 2700 N (24° vs. 15° vs. 3°; p = 0.002, significant for RoSA vs. SHS-Screw and RoSA vs. SHS-Blade) the RoSA implants demonstrated a higher rotational stability compared with the SHS groups (Tables 2 and 3) (Fig. 7b).

Migration behaviour Stiffness Mean stiffness at 1800 N was 826 (IQR 431) N/mm for the SHS-Screw, 1328 (IQR 441) N/mm for the SHS-Blade and 1953 (IQR 617) N/mm for the RoSA (p = 0.003; Table 2). RoSA also demonstrated the highest stiffness at the 2700 N load step (Table 3). Post-hoc testing showed significance only between SHS-Screw and RoSA.

Table 2

Although no significant differences in multidirectional migration were observed up to a load-level of 1800 N (Table 2), the SHS-Screw implants showed significantly greater axial (1.2 (IQR 0.3) mm vs. 0.6 (IQR 1.4) mm vs. 0.3 (IQR 0.6) mm; p = 0.019) and cranial migration (1.8 (IQR 1.5) mm vs. 0.9 (IQR 1.4) mm vs. 0.3 (IQR 0.8) mm; p = 0.031; Fig. 7c; Table 3) at the 2700 N load point compared to the RoSA implants.

Results of the mechanical test series after 1800 N load step (median [IQR])

Measure

SHS-Screw

SHS-Blade

RoSA

p-value

Screw vs. Blade

Screw vs. RoSA

Blade vs. RoSA

Displacement (mm) Rotational angle (°)

8.9 (2.9) 12° (8°)

4.9 (2.0) 7° (9°)

2.4 (1.2) 2° (2°)

0.001 0.003

n.s. n.s.


n.s. n.s.

Axial migration (mm) Cranial migration (mm) Ventro-dorsal migration (mm) Stiffness (N/mm) Femoral neck shortening (mm) Medial/caudal fracture gap (mm)

0.6 (0.8) 0.3 (1.2) 0.6 (0.8) 826 (431) 4.6 (1.2) 3.4 (1.5)

0.0 (1.0) 0.0 (1.2) 0.3 (1.1) 1328 (441) 3.7 (1.5) 4.9 (0.8)

0.0 (0.2) 0.0 (0.2) 0.0 (1.4) 1953 (617) 1.5 (0.8) 1.5 (0.9)

0.257 0.347 0.274 0.003 0.003 0.001

n.s. n.s. n.s.

Significant Significant n.s.

n.s. Significant Significant

Author's personal copy International Orthopaedics (SICOT) Table 3

Results of the mechanical test series at 2700 N load step (median [IQR])

Measure

SHS-Screw

SHS-Blade

RoSA

p-value

Screw vs. Blade

Screw vs. RoSA

Blade vs. RoSA

Displacement (mm)

14.6 (3.1)

9.2 (1.1)

3.7 (1.8)

0.001

n.s.

Significant

n.s.

Rotational angle (°)

24° (17°)

15° (6°)

3° (3°)

0.002

n.s.

Significant

Significant

Axial migration (mm) Cranial migration (mm)

1.2 (0.3) 1.8 (1.5)

0.6 (1.4) 0.9 (1.4)

0.3 (0.6) 0.3 (0.8)

0.019 0.031

n.s. n.s.


n.s. n.s.

Ventro-dorsal migration (mm) Stiffness (N/mm)

0.9 (1.4) 840 (222)

0.0 (1.4) 1235 (362)

0.6 (1.1) 1635 (656)

0.153 0.002

n.s.

Significant

n.s.

Femoral neck shortening (mm)

5.8 (2.0)

5.2 (1.8)

2.1 (1.4)

0.002

n.s.

Significant

Significant

Medial/caudal fracture gap (mm)

3.1 (1.5)

5.2 (1.5)

2.1 (1.5)

0.003

n.s.

n.s.

Significant

Femoral neck shortening

Discussion

At 1800 N (4.6 (IQR 1.2) mm vs. 3.7 (IQR 1.5) mm vs. 1.5 (IQR 0.8) mm; p = 0.003) and 2700 N (5.8 (IQR 2.0) mm vs. 5.2 (IQR 1.8) mm vs. 2.1 (IQR 1.4) mm; p = 0.002), RoSA treated fractures showed significantly less femoral neck shortening than the SHS treated groups, blades and screws respectively (Tables 2 and 3) (Fig. 7d).

Various factors such as patient age, comorbidities, fracture classification (often unreliable) and in-hospital algorithms for hip fracture surgery influence decision-making in femoral neck fractures [23]. Metaanalyses and randomized controlled studies demonstrate that there is a strong trend towards arthroplasty [5, 24, 25]. However, for young adults as well

Fig. 7 Essential parameters measured in this study at increasing load steps. a Fragment displacement. b Femoral head rotation. c Cranial migration. d Femoral neck shortening


as older patients with less compromised bone quality, or in undisplaced fractures, head-preserving therapy is preferred as it is less invasive and associated with good functional results [26, 27]. Commonly accepted fixation constructs are a threescrew technique or the sliding hip screw (SHS) [28]. In terms of re-operation rates the sliding hip screws showed no advantage, but some groups of patients (smokers and those with displaced or base of neck fractures) might do better with a sliding hip screw than with cancellous screws [29]. The main biomechanical complication in young adults is leg shortening caused by subsidence (e.g. excessive sintering and shortening or varus collapse) [26]. Patients with osteoporosis often show femoro-acetabular penetration or cut-out following a rotational movement of the femoral head fragment [26]. For fracture fixation, the anchorage of the lag screw within the femoral head plays a crucial role depending on the implant’s design [30]. One approach for the improvement of implant anchorage was the development of a blade as a load carrier instead of the traditional lag screw [7]. A recent approach was the combination of a blade and a screw in one single implant, which was placed on the market as Brotationally stable screwanchor (RoSA)^ [11, 12]. However, a biomechanical evaluation of this innovative implant in femoral neck fractures is still missing. In this study, we tested the performance of different screw and blade fixation systems in unstable AO/OTA 31-B2 (Pauwels 3) femoral neck fractures using biomechanical composite sawbones and an incremental weight bearing simulation with sinusoidal cyclic axial loading. Under these conditions, the new Screw-Anchor fixation (RoSA) was superior to the comparable SHS implants regarding rotational stability and femoral neck shortening. Failure load, stiffness, femoral head migration, and resistance to fracture displacement were in RoSA implants higher than in SHS-Screws, but without significance in comparison to SHS-Blades. The cycle count in this setting was enormously high with a median ranging from 20,000 to 33,147 cycles. This is much more than most comparable study designs investigated, thus fatigue of surrounding bone due to repeated load change is more likely to play a notable role [7, 14, 20, 21, 31–33]. However, we observed different failure patterns for the implants tested in this study: the SHS-Screw and SHS-Blade both showed an increasing fragment displacement under loading, which finally led to fracture collapse and failure of osteosynthesis. In contrast, the more rigid Screw-Anchor system was not so prone to displacement, but failed due to cut-out at the cranial side of the femoral head under high loading forces (Fig. 5). Superiority of the RoSA in comparison to the SHS-Screw in terms of stiffness and failure load has already been observed in a previous study, which also reported the tendency for cut-out with RoSA implants [11]. However, the 5100 N failure load of the RoSA corresponds to almost

seven times body weight, the 3900 N of the SHS-Blade to five times body weight and the 3000 N load of the SHS-Screw corresponds to four times body weight, whereas everyday hip loads range between 50% and 350% of body weight [22]. Furthermore, our failure loads were higher than those reported in the literature. Kunapuli et al. [34] and Nowotarski et al. [33] used composite sawbones to study femoral neck fracture fixations and reported 1900 N and 2300 N failure loads for SHS constructs. Explanations for the observed differences in failure load could be a different mechanical testing procedure (20,000 cycles, 2 Hz with a load of 350 ± 250 N to measure axial stiffness with subsequent noncyclic load to failure) [33] or differences in the definition of failure itself [34]. In contrast to our study with 15 mm, failure was defined as 5 mm linear or 5 degrees angular displacement at the fracture site [34]. The increased stiffness of the RoSA implant, along with the reduced cranial–caudal displacement of the head–neck fragment (equivalent to fracture displacement) is important for controlled fracture healing in-vivo. In contrast, fracture collapse occurs when reduction is not maintained or when additional fracturing occurs [35]. Reflecting a higher implant-bone anchorage, the rotational resistance of the head fragment in the sagittal and even in the frontal plane (retroversion) was higher in RoSA implants than in SHS implants, with a more parallel displacement (Fig. 5). The SHS systems, on the other hand, showed some degree of angular displacement, together with higher rotation and migration movements at significantly lower failure loads, indicating a fracture collapse with a failure of the osteosynthesis (Table 1; Fig. 5). This typical mode of failure for the SHS constructs with posterior rotation and retroverted varus deviation of the femoral head was described earlier [36]. In line with our findings, Kunapuli et al. reported three modes of failure with SHS systems: varus, torsion, and shear displacement [34]. On the other hand, previous studies showed that the failure mode associated with the SHS was screw bending rather than cut-out [11, 16]. However, the main reasons for fracture collapse in the SHS systems seemed to be the low resistance to tendencies for rotation, migration and sliding. Previous studies described the role of rotational moments and migration tendencies as precursors to cut-out [8, 9, 15]. In contrast, we observed fracture collapse more than cut-out in SHS-Screw systems with the highest rotation and migration tendencies. Furthermore, in RoSA systems with the lowest rotation tendencies and only small migration movements, we recorded four cut-out failures under high loading of seven times body weight. Under these circumstances, we assume that femoral neck shortening which, when excessive, leads to fracture collapse before cut-out occurs, playing a key role in early failure. Therefore, our data suggest that rotation and migration tendencies are precursors to femoral neck shortening with consecutive fracture collapse rather than cut-out.


Biomechanical studies have shown that the pertinent benefit of the helical blade (such as SHS-Blade) lies in its rotational stability [8, 9]. However, in our study SHS-Blade systems showed a remarkable amount of rotational movements and femoral neck shortening resulting in a consecutive fracture displacement. Disadvantages, such as increased femoral neck shortening in the direction of the load carrier were also seen by Windolf et al. [7]. One possible reason for improved anchorage of blade systems is bone compaction around the implant’s surface during insertion, leading to enhanced implant fixation by biological and mechanical mechanisms as proposed by Kold et al. [37]. Yet, this theory seems controversial, as another study could not confirm the potential benefit of bone compaction at least with regard to axial stiffness and cycles to failure [38]. Therefore, the ongoing development of implants that combine the biomechanical effects of both the screw and the blade, like the Screw-Anchor (RoSA), is both innovative and promising. Furthermore, first clinical results showed its stability and usefulness in unstable trochanteric fractures [39].

Conclusion Using biomechanical composite sawbones and an incremental weight bearing simulation with sinusoidal cyclic axial loading in unstable femoral neck fractures, Screw-Anchor fixation (RoSA) showed significantly better performance than SHS fixation regarding rotational stability and femoral neck shortening. Failure load, stiffness, femoral head migration, and resistance to fracture displacement were in RoSA implants higher than in SHS-Screws, but without significance in comparison to SHS-Blades. Acknowledgements The authors thank carpenters Mark Altgassen and Rudi Eigelshoven for design and construction of the sawguide and their assistance with creation of the osteotomy. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest. Funding Koenigsee Implants partially funded this study (student research assistant, sawbones).

Limitations To ensure identical conditions for all three test groups, we used biomechanical sawbones instead of human cadaver femurs [17, 33, 40]. However, the biomechanical properties of fourth generation composite femora are related to femurs of young males with good bone quality and they do not account for fragile bones in postmenopausal women with osteoporosis who predominantly suffer from hip fractures [41]. Thus, results of biomechanical studies do not exactly mirror the clinical setting. However, the big advantage of these synthetic bones is its absolute identical configuration, without changes in CCD angle, and especially bone density. Osteotomies can be performed very precisely using a custom-made sawguide box, and biomechanical parameters depend more on fracture stability and optimal screw placement in the head/neck fragment than on bone configuration. Furthermore, we neglected the role of soft tissue and muscle forces acting on the proximal femur during walking. The absence of stabilization by soft tissue represents a worst-case scenario which is not the usual condition as Hoffmann et al. postulated. It allows for the focused testing of the bone-implant construct, but fracture displacement will probably be smaller in a real scenario with intact soft tissue [42]. Our setting with the uniaxial force applied vertically as an approximation of the resultant force can only partially simulate forces acting on the hip during walking. Although this simplification is a current method reported in the literature, there are other promising studies approximating the human gait with forces applied by a multiaxial loading system, which generates failure modes closer to those of clinical observations [15].

Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors. Informed consent Not applicable.

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