CHAPTER 2. THE EFFECT OF MECHANICAL FACTORS ON THE INITIAL STABILITY
2.5 SUMMARY
This chapter describes the use of finite element analyses to investigate the mechanism of screw fixation for the acetabular cup under multiple loading modes.
Based on the simulated results, it could be concluded that: The screw provides only a
on the initial stability of the acetabular cup. The friction coefficient of the acetabular cup resists relative micromotion in the cup–pelvis interface, but it plays a smaller role than the bone quality.
Chapter 3.
Effects of Screw Eccentricity on the Initial Stability of the Acetabular Cup
3.1 Introduction
Inserting the bone screws should theoretically enhance the initial stability of the acetabular cup (Egol et al., 2004 ). Most of the relevant published papers have focused on the effects of screw numbers and their position on the initial stability of the acetabular cup (Hadjari et al., 1994; Heiner et al., 1994; Kwong et al., 1994; Won et al., 1995; Spears et al., 2001), and to our knowledge no paper has reported the effects of the surgical technique used to insert screws on the initial stability. This chapter discusses the effects of eccentric screwing on the initial stability of a hemispheric acetabular cup.
3.2 Materials and Methods
3.2.1 Types of Screw Eccentricity
A commercial hemispherical acetabular cup (58-mm outer diameter) with 12 screw holes (PinnacleTM, Depuy, Warsaw, IN, USA) was used in this study. Three screw holes were selected (labeled A, B and C) as the target screw holes (Figure 3.1 and Figure 3.2), and screw eccentricity was classified into offset eccentricity (Figure 3.3a) and angular eccentricity (Figure 3.3b). A special apparatus was developed for producing the desired eccentric angles and offset used in this study (Figure 3.4). This study evaluated the effects of two angular eccentricities (15o and 25o) and one offset eccentricity (1 mm) (Table 3.1). All of the bone screws (6.5-mm inner diameter and 25-mm long) were tightened to a torque of 4 Nm (Figure 3.4).
Figure 3.1 Positions of the three screws in the acetabular cup, and the bone screw used.
Figure 3.2 Positions of screw holes A, B, and C: (a) top view and (b) side view.
(a)
(b)
Figure 3.3 Types of screw eccentricity during fixation: (a) offset and (b) angular.
Figure 3.4 Instruments for precisely inserting the bone screws: (a, b) apparatus for controlling the rotation angle, (c, d) apparatus for controlling the tilt angle, (e) making
a guide hole with the drilling machine, and (f) the torque meter used to control the screw tightness.
Table 3.1 Screw configurations in this study. The parenthesized values in column 2 correspond to the eccentricity offset and angles.
Number of
In principal the use of frozen fresh human cadaveric pelvises would have been preferred, but obtaining the necessary number of specimens with identical bone quality and acetabulum size would have been problematic, if not impossible.
Therefore, in this study we used foam bone (Sawbones, Vashon, WA, USA) whose elastic modulus (380 MPa) and density (0.38 g/cm3) were similar to those of pelvic cancellous bone. Blocks of foam bone with dimensions of 140 mm × 100 mm × 76 mm were prepared, and a surgical reamer attached to a drill press was used to ream the foam bone to a diameter of 57 mm so as to obtain a 1-mm press-fit (Figure 3.5).
Figure 3.5 A block of foam bone and the inserted acetabular cup.
A compressive force of 1500 N was applied to ensure seating of the acetabular cup in the foam bone prior to testing. Fluoroscopy was used to confirm that the cups were in intimate contact with the surface of the cavity (Figure 3.6). Linear loads of 0 N to 1000 N were applied using a testing machine (MTS, Minneapolis, MN, USA) at a speed of 0.05 mm/s to the flat rim of the acetabular cup located at the opposite side of the screw region so as to impart a shear load to the interface between the cup and the foam bone (Figures 3.7 and 3.8). The motion of the cups were monitored with a laser-based displacement sensor (ANR1251, NAIS, Kadoma, Osaka, Japan) from the opposite side of the rim. Each configuration was measured five times. The data were analyzed using Duncan’s multiple-range test to ascertain the effect of screw eccentricity on the initial stability of the acetabular cup.
Figure 3.6 Using fluoroscopy to confirm that the cups were in perfectly contact with the surface of the cavity.
Figure 3.7 Experimental setup for offset loading and the position of the laser displacement sensor.
Figure 3.8 Schematic cross-section of the experimental setup.
3.3 Results
No gross failure of the fixation occurred during loading of any of the cups up to 1000N. The experimental results given in Table 3.2 and Figure 3.9 indicate that the most unstable case was when the cup was fixed without additional screws. For the ideal configurations (i.e., in the absence of eccentricity), increasing the number of screws enhanced the cup stability (p<0.01); with single-screw fixation as the baseline, adding the second and third screws decreased the cup displacement by 17.6% and 42.7%, respectively.
Table 3.2 Displacement data of the acetabular cup for different screw configurations obtained in three trials.
Trial number
Figure 3.9 Displacement of the acetabular cup for different screw configurations.
Error bars indicate SD values.
The presence of 1-mm offset eccentricity significantly reduced the cup stability relative to the ideal configurations irrespective of the number of screws (p<0.01).
However, for angular eccentricity, there was no significant difference between A and A(15o) or between A-B and A(15o)-B. However, significant reductions in cup stability occurred for A(15o)-B-C(15 o) and for all configurations with 25o of angular eccentricity (p<0.01).
3.4 Discussion
This study used blocks of foam bone so as the ensure uniform material properties between sample (Litsky and Pophal, 1994; Baleani et al., 2001; Markel et al., 2002).
Although the offset loading here is not a common physiological loading it is thought to provide a more stringent test of initial fixation than axial loading or torsion (Lachiewicz et al., 1989). Moreover, there are many reports that offset loading is
experiments, such as the different loading type and the variations in the quality of their cadaver pelvic bones. Moreover, screw eccentricity may have occurred in their in vitro experiments, which is supported by the results in Table 3.2 and Figure 3.3 that indicate that inserting extra eccentric screws does not enhance the cup stability. For example, the displacement in the case of a single ideal screw is less than when eccentricity is present with either two or three screws.
Inserting a single screw with 1 mm offset eccentricity increased the cup displacement by 36.9% compared with the ideal configuration (Table 3.1). Moreover, when using two and three screws, the presence of a single eccentric screw and two eccentric screws increases the displacement by 48.4% and 185.2%, respectively.
These results indicate that the impact of screw eccentricity increases with the number of screws used for fixation. This may be due to the cup being slightly rotated to a new well-fitted position in the case of single-screw fixation, whereas for multiple screws the cup cannot adjust its position due to it being locked by the additional screw(s).
In general, the head of a commercial bone screw is shaped as a “fisheye” so as to tolerate a small degree of angular eccentricity. The tolerance of the screws used in this study was 15o (Figure 3.10), and is consistent with our finding that the presence of 15o angular eccentricity did not produce adverse effects for both one-and two-screw fixations and that an eccentricity angle of 25o had significant effects on the cup stability.
Figure 3.10 The screw angle could vary by up to 30o for the cup used in this study (http://www.ocdnordic.com).
This study demonstrates that both the skill of screw insertion and the number of screws have significant effects on cup stability. After removing the screws, we observed some metal fragments in the screw-head–hole interfaces in the groups with
offset or angular eccentricity. There are many reports that fragmentation or debris can induce acetabular osteolysis (Perona et al., 1992; Huo et al., 1993; Bi et al., 2001), indicating that screw eccentricity not only reduces the cup stability, but also promotes osteolysis.
Limitation:
First, the sample size was small and the loading type cannot reflect all physiology loadings. Secondly, we investigated only three configurations of screw eccentricity:
one offset and two angular. That is not enough to represent all the eccentric screwing in the real surgery. However, these limitations would not affect the main conclusion of this study, that the presence of screw eccentricity affects the initial stability of the acetabular cup.
3.5 Summary
In this study, the hemispherical cups were fixed into the blocks of foam bone with zero to three screws. The effects of three types of screw eccentricity (a 1-mm offset, and angular eccentricities of 15o and 25o) on the initial stability of the acetabular cup were evaluated. The experimental results indicate that increasing the number of screws enhances the cup stability in the case of ideal screwing (i.e., with no eccentricity). An angular eccentricity of 15o did not affect the cup stability for fixation with one or two screws. However, the presence of 25o of angular eccentricity significantly reduced the stability of the cup, while 1 mm of offset eccentricity and an even greater impact.
Chapter 4.
Conclusions
The results from our finite element analyses (Chapter 2) of the effects of screw numbers on the initial stability of the acetabular cup reveal that a screw has only a localized effect in reducing the relative micromotion between cup and acetabulum.
Therefore, inserting several screws close together is no better than inserting a single screw. The analyses also revealed that loading mode is an important factor affecting the micromotion. Since it is difficult to control this in a patient, multiple-screw placement is recommended to cover different load scenarios and provide a sufficiently stable region as well as sufficient torque stability. Moreover, reducing the elastic modulus of the pelvis nonlinearly increases the peak micromotion and reduces the stable region. Therefore, extra screws may be needed in patients with poor bone quality to enhance the initial stability of the acetabular cup. Finally, the peak micromotion increases linearly and the size of the stable region decreases linearly as the friction coefficient decreases, although these effects are much smaller than those resulting from variations in the bone quality. However, in clinical practice we can choose the type of acetabular cup but we cannot choose the patient. We therefore recommend using a cup with a high friction coefficient, such as a plasma-sprayed titanium acetabular cup to enhance the initial stability of the acetabular cup. The experiments described in Chapter 3 indicate that the presence of screw eccentricity affects the initial stability of the acetabular cup. For the particular commercial acetabular cup and screws used in this study, an offset eccentric of 1 mm and an angular eccentricity of 25o adversely affect the initial stability of the acetabular cup.
Surgeons should bear this in mind when performing screw insertions in THRs.
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附錄：發表之期刊論文
1. Jui-Ting Hsu, Chih-Han Chang, Kai-Nan An, Heng-Li Huang, Zi-Ling Liu, and Kuo-An Lai, “Mechanism of Screw Fixation on the Initial Stability of Acetabular Cup”, Journal of Medical and Biological Engineering, Accepted, 2006. (EI)
2. Jui-Ting Hsu, Chih-Han Chang, Heng-Li Huang, Kuo-An Lai, Mark E. Zobitz, and Kai-Nan An, “The Number of Screws, Bone Quality, and Friction Coefficient Affect Acetabular Cup Stability”, Medical Engineering and Physics, First Review. (SCI)
3. Jui-Ting Hsu, Kuo-An Lai, Qingshan Chen, Kai-Nan An, Heng-Li Huang, and Chih-Han Chang, “The Relation between Micromotion and Screw Fixation in Acetabular Cup”, Computer Methods and Program in Biomedicine, Accepted.
(SCI)
4. Jui-Ting Hsu, Chih-Han Chang, Kai-Nan An, Mark E. Zobitz, Rapin Phimolsarnti, Ronald R. Hugate, Kuo-An Lai, “Effects of Screw Eccentricity on the Initial Stability of the Acetabular Cup”, International Orthopaedics, Accepted,
4. Jui-Ting Hsu, Chih-Han Chang, Kai-Nan An, Mark E. Zobitz, Rapin Phimolsarnti, Ronald R. Hugate, Kuo-An Lai, “Effects of Screw Eccentricity on the Initial Stability of the Acetabular Cup”, International Orthopaedics, Accepted,