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Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty
The Journal of Arthroplasty xxx (2015) xxx–xxx
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Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty R. Michael Meneghini, MD a,b, Luke R. Lovro, BS b, Joseph M. Wallace, PhD c, Mary Ziemba-Davis b a b c
Department of Orthopaedic Surgery, Indiana University School of Medicine, Fishers, Indiana Orthopedics and Sports Medicine, Indiana University Health Physicians, Fishers, Indiana Department of Biomedical Engineering, Purdue School of Engineering and Technology at Indiana University–Purdue University, Indianapolis, Indiana
a b s t r a c t Purpose: Trunnionosis has reemerged in modern total hip arthroplasty for reasons that remain unclear. Bearing frictional torque transmits forces to the modular headneck interface, which may contribute to taper corrosion. The purpose of this study is to compare frictional torque of modern bearing couples in total hip arthroplasty. Methods: Mechanical testing based on in vivo loading conditions was used to measure frictional torque. All bearing couples were lubricated and tested at 1 Hz for more than 2000 cycles. The bearing couples tested included conventional, highly crosslinked (XLPE) and vitamin E polyethylene, CoCr, and ceramic femoral heads and dual-mobility bearings. Statistical analysis was performed using Student t test for single-variable and analysis of variance for multivariant analysis. P ≤ .05 was considered statistically significant. Results: Large CoCr metal heads (≥36 mm) substantially increased frictional torque against XLPE liners (P = .01), a finding not observed in ceramic heads. Vitamin E polyethylene substantially increased frictional torque compared with XLPE in CoCr and ceramic heads (P = .001), whereas a difference between conventional and XLPE was not observed (P = .69) with the numbers available. Dual-mobility bearing with ceramic inner head demonstrated the lowest mean frictional torque of all bearing couples. Conclusion: In this simulated in vivo model, large-diameter CoCr femoral heads and vitamin E polyethylene liners are associated with increased frictional torque compared with smaller metal heads and XLPE, respectively. The increased frictional torque of vitamin E polyethylene and larger-diameter femoral heads should be considered and further studied, along with reported benefits of these modern bearing couples. Article history: Received 28 July 2015 Accepted 22 September 2015 Available online xxxx Keywords: trunnionosis, frictional torque, total hip arthroplasty, highly crosslinked polyethylene, vitamin E polyethylene
© 2015 Elsevier Inc. All rights reserved.
Developed by Sir John Charnley as “low-friction” arthroplasty, total hip arthroplasty (THA) is one of the most successful procedures in medicine [1,2]. Consistent with autogenous low friction between living bone and articular cartilage, Charnley [3,4] recognized that reducing resistance to the movement of prosthetic femoral heads within acetabular cups would reduce surface wear and torsional forces on the interface between implants and native bone. Interest in improving upon Charnley's results [5] and the need to remediate component loosening, osteolysis, and polyethylene wear have led to the development of many implant designs and improvements over the last 50 years. A number of biomechanical and biomaterial changes have targeted the most common complications of instability and polyethylene wear-related osteolysis, and include modular large-diameter femoral heads and
One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements, refer to http://dx.doi.org/10.1016/j.arth.2015.09.020. Reprint requests: R. Michael Meneghini, MD, Department of Orthopaedic Surgery, Indiana University Health Physicians, Indiana University School of Medicine, 13100 136th St, Suite 2000, Fishers, IN 46037.
polyethylene innovations such as increasing crosslinking and infusing vitamin E to reduce oxidation. Mechanically assisted crevice corrosion (MACC) has been reported as a reemerging failure mode in THA with modular metal heads [6]. Mechanically assisted crevice corrosion between the femoral head and stem has been documented in both mixed-metal and similar-metal modular tapers, with fretting, pitting, and electrochemical-mechanical interactions underlying the corrosion [7]. Because biomechanical factors are known to play a role in the initiation of crevice corrosion, it is postulated that increased frictional torque imparted at the modular taper junction may contribute to MACC at the head-neck junction. Metalon-metal THA with larger heads greater than 32 mm has been shown to result in higher revision rates [8] and larger-diameter heads have shown increased wear at the taper junction with otherwise minimal wear at the bearing [9–11], which may implicate frictional torque forces at the trunnion. A recent in vitro analysis using increasing offsets to simulate the increasing torque produced by 28-, 36-, and 50-mm heads found significant correlations between increasing offsets and fretting corrosion at the head-stem interface [12], further supporting the role of biomechanical forces in MACC. The use of large femoral heads, the development of highly crosslinked polyethylene (XLPE) acetabular liners, and infusion of
http://dx.doi.org/10.1016/j.arth.2015.09.020 0883-5403/© 2015 Elsevier Inc. All rights reserved.
Please cite this article as: Meneghini RM, et al, Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.09.020
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R.M. Meneghini et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
XLPE liners with vitamin E (α-tocopherol) are among modern-day changes that may affect mechanical performance of the bearing couple, including frictional torque. Large femoral heads were introduced to reduce intraprosthetic impingement and subsequent instability with considerable success [13–16]. Increased crosslinking and the addition of vitamin E to ultra-high-molecular-weight polyethylene liners have been associated with decreased particle wear [17–23]. The potential consequences of frictional resistance and torsional stresses on the taper junction associated with these modifications remain largely unknown. The purpose of this study was to assess whether various bearing couples were associated with different frictional torque magnitudes. A mechanical testing model simulating in vivo hip loading conditions was used to address 3 questions: Does THA bearing couple frictional torque vary with (1) femoral head size; (2) femoral head material; conventional, XPLE, and vitamin E polyethylene liners; (4) dual-mobility bearings; and (5) XLPE liners infused and not infused with vitamin E?
Materials and Methods A Mini-Bionix 858 MTS testing machine (Eden Prairie, MN) was used to generate and measure the frictional torque (Nm) of THA bearing couples (Fig. 1). The bearing couples included ceramic (Biolox Delta; Stryker, Mahwah, New Jersey) and CoCr (LFIT; Stryker) femoral heads against conventional polyethylene (N2Vac; Stryker), highly XLPE (X3; Stryker), and vitamin E polyethylene (Vivacit-E; Zimmer, Warsaw, Indiana) acetabular liners. Because of manufacturer discontinuation, conventional liners were only available for 28-mm femoral heads. Because of limited availability, vitamin E–infused polyethylene liners were only available for 32-mm femoral heads. A large-head dualmobility bearing with highly XLPE outer ball (against a CoCr metal acetabular liner) ball and 22-mm inner diameter ball of either ceramic or CoCr (MDM; Stryker) were also tested to assess the effect of the additional interface on frictional torque. In total, testing materials included 1 metal femoral head in 4 sizes (28 mm, 32 mm, 36 mm, and 40 mm), 1 ceramic femoral head in 3 sizes (28 mm, 32 mm, and 36 mm), 6 conventional liners for 3 tests each with 28-mm metal and ceramic heads, 6 XLPE liners for 3 tests each with 28-mm metal and ceramic heads, 6 XLPE liners for 3 tests each with 32-mm metal and ceramic heads, 6 vitamin E–infused liners for 3 tests each with 32-mm metal and ceramic heads, 6 XLP liners for 3 tests each with 36-mm metal and ceramic heads, 3 XLP liners for 3 tests each with 40-mm metal heads, and 6
Fig. 1. Test setup showing CoCr femoral head in polyethylene liner loaded in MTS machine.
XLP liners for 3 tests each with metal and ceramic dual-mobility bearings. Acetabular components were press fit into a metal test block for the MTS platform, and polyethylene liners or the metal dual-mobility liners were then inserted into the shells. Five drops of calf serum (100% diluted Alpha Calf Fraction Serum; Hyclone, Logan, Utah) was added to the liner surface to lubricate bearings during testing. The femoral heads were attached to the actuator side of the MTS, inserted into the receiving acetabular liner, and held at a static compressive force of 650 N during 2000 cycles of torsional testing at 1-Hz rotation speed. Three tests were performed under cyclic loading conditions through a 40 ° angular displacement range for each combination of bearing couples. Testing was conducted at room temperature of 25°. The MTS data acquisition system was used to collect the frictional torque, measured in Nm, required to articulate through the given angular (rotational) displacement (Fig. 2A, B). The amount of frictional torque required for the femoral head to articulate from one extreme of rotation (20° internal rotation) to the other extreme of rotational displacement (20° external rotation) within the acetabular cup was recorded during the 500th and 1500th cycles of rotation for each test. To more accurately assess frictional torque throughout a simulated “patient step or cycle,” we calculated the total torque of the cycle by adding the absolute values of the frictional torque at 20° internal rotation and the torque at 20° external rotation. This absolute additive value represents the total amount of resistance to rotation from one extreme
Fig. 2. Depiction of frictional torque relative to the angle of rotation within the acetabular cup from 20° internal to 20° external (A) and values of frictional torque vs angle in 1 rotation cycle from 20° internal to 20° external (B).
Please cite this article as: Meneghini RM, et al, Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.09.020
R.M. Meneghini et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
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Table 1 Example of Frictional Torque Data Collection for 2 Test Specimens. Nm at 20° Internal Rotation Ceramic 32-mm femoral head with vitamin E liner Test 1 500th cycle 0.77 Test 1 1500th cycle 0.78 Test 2 500th cycle 0.64 Test 2 1500th cycle 0.66 Test 3 500th cycle 0.78 Test 3 1500th cycle 0.75 Mean (SD) 0.72 (0.05) Cobalt chromium 32-mm femoral head with vitamin E liner Test 1 500th cycle 0.63 Test 1 1500th cycle 0.68 Test 2 500th cycle 0.66 Test 2 1500th cycle 0.69 Test 3 500th cycle 0.60 Test 3 1500th cycle 0.60 Mean (SD) 0.64 (0.04)
angle of rotation to the other. Table 1 illustrates data collection for one test specimen. Minitab 17 (State College, Pennsylvania) was used for data analysis. Student t test (t) and analysis of variance (F) were used to compare group means. Post hoc statistical power for 2-tailed hypotheses was calculated based on observed means and SDs, group sample number, and α ≤ .05. Results Frictional torque for each bearing couple tested is shown in Table 2. When paired with XLPE liners, the mean frictional torque of 36-mm (0.95 Nm) and 40-mm (0.94 Nm) CoCr femoral heads was significantly greater than 28-mm (0.77 Nm) and 32-mm (0.78 Nm) CoCr femoral heads (F = 4.49, P = .014). Mean frictional torque did not statistically vary based on the head diameter within the group of ceramic femoral heads (F = 1.00, P = .393), although resistance incrementally increased with increasing head size (Table 2). With respect to the dual-mobility bearings, mean frictional torque was significantly higher in the CoCr (0.85 Nm) inner femoral head group compared with ceramic femoral head group (0.70 Nm) within outer XLPE spherical liners articulating with a CoCr metal acetabular liner (t = 3.41, P = .008). Vitamin E polyethylene liners demonstrated significantly greater frictional torque compared with XLPE liners without vitamin E measured in 32-mm diameter femoral head couples (Table 2). This difference was statistically significant for both CoCr (1.25 Nm vs 0.78 Nm, t = 8.19, P = .001) and ceramic (1.42 Nm vs 0.80 Nm, t = 16.85, P = .001) femoral heads. Statistical power for these comparisons was greater than 90%. Figs. 3 and 4 illustrate that the largest values of average frictional torque between CoCr and ceramic test specimens were associated
Nm at 20° External Rotation
Nm Difference
−0.66 −0.72 −0.66 −0.71 −0.71 −0.69 −0.69 (0.02)
1.39 1.51 1.30 1.37 1.49 1.44 1.42 (0.07)
−0.54 −0.60 −0.67 −0.69 −0.55 −0.56 −0.60 (0.06)
1.17 1.29 1.33 1.38 1.15 1.16 1.25 (0.10)
with vitamin E polyethylene liners. Analysis of variance was statistically significant for both CoCr (F = 15.52, P = .001) and ceramic (F = 24.28, P = .001) femoral heads, indicating that one or more of the group means were significantly different. Pairwise comparisons [24] with tolerance for type I errors conservatively set at ≤0.01% indicated that the frictional torque of CoCr and ceramic implants with 32-mm femoral heads and vitamin E liners were significantly different from all other test specimens. There was no statistically significant difference in frictional torque when XLPE liners were compared with conventional polyethylene liners in CoCr (0.77 Nm vs 0.76 Nm, t = −0.16, P = .876) and ceramic (0.77 Nm vs 0.79 Nm, t = 0.42, P = .687) femoral heads of 28-mm diameter (Table 2). Statistical power for both comparisons was low (≤7%) given the observed means and SDs. Discussion Human synovial joints such as the ball and socket of the hip have one of the lowest known static and kinetic coefficients of friction (μs ≤ 0.01, μk ≤ 0.005). Sir John Charnley [3] recognized that resistance to implant movement increases wear and shearing stresses at the hip joint. The frictional torque of Charnley's low-friction hip arthroplasty was 0.4 to 1.2 Nm under a loading force of 890 N [25]. Today, most of the available combinations of femoral stems, femoral neck trunnions, femoral heads, and acetabular cups and liners deviate from the low-friction hip arthroplasty first used in 1962. Because it is widely agreed that biomechanical forces and stresses are a contributing factor in trunnionosis at
Table 2 Mean (SD) Frictional Torque (in Nm) for Each Test Specimen. Femoral Head Size 28 mm Ceramic Conventional liner XLP liner Vitamin E liner Cobalt chromium Conventional liner XLP liner Vitamin E liner
32 mm
36 mm
40 mm
MDM X3
0.79 (0.11) 0.77 (0.13) 0.80 (0.04) 0.90 (0.25) 1.42 (0.08)
0.70 (0.07)
0.76 (0.11) 0.77 (0.07) 0.78 (0.10) 0.95 (0.14) 0.94 (0.13) 0.85 (0.09) 1.25 (0.10)
Fig. 3. Boxplot illustrating the range (rectangle), mean (circle), and median (diamond) of frictional torque values for CoCr test specimens (P = .000).
Please cite this article as: Meneghini RM, et al, Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.09.020
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R.M. Meneghini et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Fig. 4. Boxplot illustrating the range (rectangle), mean (circle), and median (diamond) of frictional torque values for ceramic test specimens (P = .000).
the head-taper junction, it is logical that frictional torque at the bearing surface interface may be transmitted to the taper junction and if excessive contribute to MACC as a failure mode in THA [6,7,26]. The etiology and resurgence of clinically relevant trunnionosis remains poorly understood. Factors that have been implicated in promoting trunnion corrosion include large-diameter femoral heads, surface topography of trunnions, taper geometry, flexural rigidity of femoral stem tapers, and environmental conditions during taper assembly [27–34]. The biomechanical factor most relevant to this study is whether large-diameter heads contribute to increased taper corrosion and although there are numerous reports implicating the larger the diameter of metal femoral head, the greater the mechanical forces and subsequent trunnion corrosion at the taper junction [11,30,33], a recent Hip Society award paper refuted that concept and reported that fretting and corrosion were related to taper design and mixed-metal alloy taper-head combinations rather than femoral head diameter [27]. The contribution of taper design has been recently reported by other authors and appears to have a significant affect on trunnion corrosion [28,31,32]. The study findings reported here demonstrated that the mean frictional torque was greater in the larger 36- and 40-mm diameter femoral heads articulating against XLPE liners compared with smaller-diameter femoral heads. Using an in vitro model, Schmidig et al [35] similarly observed that frictional torque in CoCr implants significantly increased as head size increased, in both spherical and artificially deformed acetabular shells. Burroughs et al [36] reported that 40-mm CoCr femoral heads articulating against aged XLPE liners resulted in greater frictional torque than 40-mm CoCr heads articulating against aged conventional liners. Greater frictional torque with increasing head sizes was consistently reflected in the data from Burroughs et al [36] for both XLPE and conventional liners at several different levels of loading force. In another study under simulated walking conditions, Bishop et al [37] observed that frictional torque was greatest for large-diameter metal-on-metal, followed by metal-on-polyethylene, and lowest for small-diameter ceramic-ceramic and ceramic-metal bearings. Similar to our findings, several studies have observed that the a positive linear relationship between frictional torque and femoral head size begins to level off for femoral heads size 36 mm and greater [35,36]. Several studies have examined wear properties of vitamin E polyethylene liners [21–23,38–41]; however, to the best of our knowledge, published studies on the frictional characteristics of vitamin E polyethylene do not exist. Our study findings document that mean frictional torque was significantly higher in XLPE liners infused with vitamin E paired with both metal and ceramic femoral heads when compared with XLPE without vitamin E. Thirty-two-millimeter heads with vitamin E polyethylene liners resulted in a minimum 58% and 31% increase in
frictional torque in ceramic and CoCr femoral heads, respectively (Table 2). Analysis of all available combinations of implant type, femoral head size, and acetabular liner type demonstrated that the mean frictional torque of CoCr and ceramic 32-mm femoral heads and vitamin E liners was significantly higher than all other test specimens (Figs. 3 and 4). In contrast to vitamin E liners, we found no difference in the amount of frictional torque in XLPE and conventional polyethylene liners paired with 28-mm femoral heads. It is possible that the small number of observations for each test specimen (n = 6) may have influenced our ability to detect a significant difference. Burroughs et al [36] compared the average of 5 observations of frictional torque in artificially aged conventional and XLPE liners, and reported that frictional torque was nearly universally higher in aged XLPE liners [36]. Our testing was not performed with artificially aged specimens, which may have also contributed to our results conflicting with those of Burroughs et al. Our study had limitations. First, the extent of generalizability between a mechanical testing model and the human model remains unknown. To the extent possible, however, we simulated in vivo loading conditions during walking for in vitro testing. Furthermore, although more complex patterns of hip motion are encountered and applicable for mechanical testing, we attempted to simplify and standardize our testing methods to isolate the variable of frictional torque between bearing couples of differing materials. In addition, 2000 cycles of testing was chosen for testing each bearing couple based on 2 well-conducted studies on frictional torque, both of which used only 120 cycles to test frictional torque [36,37], where Burroughs and colleagues [36] used only 120 cycles to asses frictional torque on XLPE compared with 12 million cycles to test wear. It is therefore reasonable that 2000 cycles was a sufficient cycles for testing the isolated mechanical property of frictional torque based on the current accepted literature. Second, our simulated compressive force was applied perpendicular to the bearing surface and this direction of force (or force vector) is only one of the multidirectional forces that occur in vivo in a dynamic state. A third limitation was our inability to test all possible combinations of implant designs and anatomical positions for the acetabular cup, both of which are known to interact with femoral head size and joint stability [42]. Finally, although our sample size (6 observations for each bearing couple tested) is consistent with similar biomechanical studies, the fact that α ≤ .05 was used as our criterion for statistical significance and that power for some comparisons was at least 90% addresses this concern. Many THA bearing couples comprising different materials have been developed and integrated in the quest to make a successful surgical procedure a perfect surgical procedure. It is commonly understood that material composition changes come with resultant consequences in mechanical properties, but long-term evidence for the efficacy and safety of implant innovations is compiled after, rather than before, adoption. Recent commentary warns that this process has potentially harmed many patients and placed others at high risk [43]. Although important advances have reduced serious clinical concerns such as dislocation and osteolysis, taper corrosion has reemerged as a significant challenge in the last decade [43]. Although vitamin E polyethylene was developed to reduce oxidation and extend the life of hip implants, the clinical implications of increased frictional torque against vitamin E polyethylene should be monitored and studied closely to address whether these biomechanical findings potentially result in any clinical deleterious consequences. References 1. Hansson T, Hansson E, Malchau H. Utility of spine surgery: a comparison of common elective orthopaedic surgical procedures. Spine (Phila Pa 1976) 2008;33(25):2819. 2. Wylde V, Blom AW, Whitehouse SL, et al. Patient-reported outcomes after total hip and knee arthroplasty: comparison of midterm results. J Arthroplasty 2009;24(2):210. 3. Charnley J. Surgery of the hip-joint: present and future developments. Br Med J 1960; 1(5176):821. 4. Wroblewski BM, Siney PD, Fleming PA. The principle of low frictional torque in the Charnley total hip replacement. J Bone Joint Surg (Br) 2009;91(7):855.
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Please cite this article as: Meneghini RM, et al, Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.09.020
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Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty R. Michael Meneghini, MD a,b, Luke R. Lovro, BS b, Joseph M. Wallace, PhD c, Mary Ziemba-Davis b a b c
Department of Orthopaedic Surgery, Indiana University School of Medicine, Fishers, Indiana Orthopedics and Sports Medicine, Indiana University Health Physicians, Fishers, Indiana Department of Biomedical Engineering, Purdue School of Engineering and Technology at Indiana University–Purdue University, Indianapolis, Indiana
a b s t r a c t Purpose: Trunnionosis has reemerged in modern total hip arthroplasty for reasons that remain unclear. Bearing frictional torque transmits forces to the modular headneck interface, which may contribute to taper corrosion. The purpose of this study is to compare frictional torque of modern bearing couples in total hip arthroplasty. Methods: Mechanical testing based on in vivo loading conditions was used to measure frictional torque. All bearing couples were lubricated and tested at 1 Hz for more than 2000 cycles. The bearing couples tested included conventional, highly crosslinked (XLPE) and vitamin E polyethylene, CoCr, and ceramic femoral heads and dual-mobility bearings. Statistical analysis was performed using Student t test for single-variable and analysis of variance for multivariant analysis. P ≤ .05 was considered statistically significant. Results: Large CoCr metal heads (≥36 mm) substantially increased frictional torque against XLPE liners (P = .01), a finding not observed in ceramic heads. Vitamin E polyethylene substantially increased frictional torque compared with XLPE in CoCr and ceramic heads (P = .001), whereas a difference between conventional and XLPE was not observed (P = .69) with the numbers available. Dual-mobility bearing with ceramic inner head demonstrated the lowest mean frictional torque of all bearing couples. Conclusion: In this simulated in vivo model, large-diameter CoCr femoral heads and vitamin E polyethylene liners are associated with increased frictional torque compared with smaller metal heads and XLPE, respectively. The increased frictional torque of vitamin E polyethylene and larger-diameter femoral heads should be considered and further studied, along with reported benefits of these modern bearing couples. Article history: Received 28 July 2015 Accepted 22 September 2015 Available online xxxx Keywords: trunnionosis, frictional torque, total hip arthroplasty, highly crosslinked polyethylene, vitamin E polyethylene
© 2015 Elsevier Inc. All rights reserved.
Developed by Sir John Charnley as “low-friction” arthroplasty, total hip arthroplasty (THA) is one of the most successful procedures in medicine [1,2]. Consistent with autogenous low friction between living bone and articular cartilage, Charnley [3,4] recognized that reducing resistance to the movement of prosthetic femoral heads within acetabular cups would reduce surface wear and torsional forces on the interface between implants and native bone. Interest in improving upon Charnley's results [5] and the need to remediate component loosening, osteolysis, and polyethylene wear have led to the development of many implant designs and improvements over the last 50 years. A number of biomechanical and biomaterial changes have targeted the most common complications of instability and polyethylene wear-related osteolysis, and include modular large-diameter femoral heads and
One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements, refer to http://dx.doi.org/10.1016/j.arth.2015.09.020. Reprint requests: R. Michael Meneghini, MD, Department of Orthopaedic Surgery, Indiana University Health Physicians, Indiana University School of Medicine, 13100 136th St, Suite 2000, Fishers, IN 46037.
polyethylene innovations such as increasing crosslinking and infusing vitamin E to reduce oxidation. Mechanically assisted crevice corrosion (MACC) has been reported as a reemerging failure mode in THA with modular metal heads [6]. Mechanically assisted crevice corrosion between the femoral head and stem has been documented in both mixed-metal and similar-metal modular tapers, with fretting, pitting, and electrochemical-mechanical interactions underlying the corrosion [7]. Because biomechanical factors are known to play a role in the initiation of crevice corrosion, it is postulated that increased frictional torque imparted at the modular taper junction may contribute to MACC at the head-neck junction. Metalon-metal THA with larger heads greater than 32 mm has been shown to result in higher revision rates [8] and larger-diameter heads have shown increased wear at the taper junction with otherwise minimal wear at the bearing [9–11], which may implicate frictional torque forces at the trunnion. A recent in vitro analysis using increasing offsets to simulate the increasing torque produced by 28-, 36-, and 50-mm heads found significant correlations between increasing offsets and fretting corrosion at the head-stem interface [12], further supporting the role of biomechanical forces in MACC. The use of large femoral heads, the development of highly crosslinked polyethylene (XLPE) acetabular liners, and infusion of
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Please cite this article as: Meneghini RM, et al, Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.09.020
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XLPE liners with vitamin E (α-tocopherol) are among modern-day changes that may affect mechanical performance of the bearing couple, including frictional torque. Large femoral heads were introduced to reduce intraprosthetic impingement and subsequent instability with considerable success [13–16]. Increased crosslinking and the addition of vitamin E to ultra-high-molecular-weight polyethylene liners have been associated with decreased particle wear [17–23]. The potential consequences of frictional resistance and torsional stresses on the taper junction associated with these modifications remain largely unknown. The purpose of this study was to assess whether various bearing couples were associated with different frictional torque magnitudes. A mechanical testing model simulating in vivo hip loading conditions was used to address 3 questions: Does THA bearing couple frictional torque vary with (1) femoral head size; (2) femoral head material; conventional, XPLE, and vitamin E polyethylene liners; (4) dual-mobility bearings; and (5) XLPE liners infused and not infused with vitamin E?
Materials and Methods A Mini-Bionix 858 MTS testing machine (Eden Prairie, MN) was used to generate and measure the frictional torque (Nm) of THA bearing couples (Fig. 1). The bearing couples included ceramic (Biolox Delta; Stryker, Mahwah, New Jersey) and CoCr (LFIT; Stryker) femoral heads against conventional polyethylene (N2Vac; Stryker), highly XLPE (X3; Stryker), and vitamin E polyethylene (Vivacit-E; Zimmer, Warsaw, Indiana) acetabular liners. Because of manufacturer discontinuation, conventional liners were only available for 28-mm femoral heads. Because of limited availability, vitamin E–infused polyethylene liners were only available for 32-mm femoral heads. A large-head dualmobility bearing with highly XLPE outer ball (against a CoCr metal acetabular liner) ball and 22-mm inner diameter ball of either ceramic or CoCr (MDM; Stryker) were also tested to assess the effect of the additional interface on frictional torque. In total, testing materials included 1 metal femoral head in 4 sizes (28 mm, 32 mm, 36 mm, and 40 mm), 1 ceramic femoral head in 3 sizes (28 mm, 32 mm, and 36 mm), 6 conventional liners for 3 tests each with 28-mm metal and ceramic heads, 6 XLPE liners for 3 tests each with 28-mm metal and ceramic heads, 6 XLPE liners for 3 tests each with 32-mm metal and ceramic heads, 6 vitamin E–infused liners for 3 tests each with 32-mm metal and ceramic heads, 6 XLP liners for 3 tests each with 36-mm metal and ceramic heads, 3 XLP liners for 3 tests each with 40-mm metal heads, and 6
Fig. 1. Test setup showing CoCr femoral head in polyethylene liner loaded in MTS machine.
XLP liners for 3 tests each with metal and ceramic dual-mobility bearings. Acetabular components were press fit into a metal test block for the MTS platform, and polyethylene liners or the metal dual-mobility liners were then inserted into the shells. Five drops of calf serum (100% diluted Alpha Calf Fraction Serum; Hyclone, Logan, Utah) was added to the liner surface to lubricate bearings during testing. The femoral heads were attached to the actuator side of the MTS, inserted into the receiving acetabular liner, and held at a static compressive force of 650 N during 2000 cycles of torsional testing at 1-Hz rotation speed. Three tests were performed under cyclic loading conditions through a 40 ° angular displacement range for each combination of bearing couples. Testing was conducted at room temperature of 25°. The MTS data acquisition system was used to collect the frictional torque, measured in Nm, required to articulate through the given angular (rotational) displacement (Fig. 2A, B). The amount of frictional torque required for the femoral head to articulate from one extreme of rotation (20° internal rotation) to the other extreme of rotational displacement (20° external rotation) within the acetabular cup was recorded during the 500th and 1500th cycles of rotation for each test. To more accurately assess frictional torque throughout a simulated “patient step or cycle,” we calculated the total torque of the cycle by adding the absolute values of the frictional torque at 20° internal rotation and the torque at 20° external rotation. This absolute additive value represents the total amount of resistance to rotation from one extreme
Fig. 2. Depiction of frictional torque relative to the angle of rotation within the acetabular cup from 20° internal to 20° external (A) and values of frictional torque vs angle in 1 rotation cycle from 20° internal to 20° external (B).
Please cite this article as: Meneghini RM, et al, Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.09.020
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Table 1 Example of Frictional Torque Data Collection for 2 Test Specimens. Nm at 20° Internal Rotation Ceramic 32-mm femoral head with vitamin E liner Test 1 500th cycle 0.77 Test 1 1500th cycle 0.78 Test 2 500th cycle 0.64 Test 2 1500th cycle 0.66 Test 3 500th cycle 0.78 Test 3 1500th cycle 0.75 Mean (SD) 0.72 (0.05) Cobalt chromium 32-mm femoral head with vitamin E liner Test 1 500th cycle 0.63 Test 1 1500th cycle 0.68 Test 2 500th cycle 0.66 Test 2 1500th cycle 0.69 Test 3 500th cycle 0.60 Test 3 1500th cycle 0.60 Mean (SD) 0.64 (0.04)
angle of rotation to the other. Table 1 illustrates data collection for one test specimen. Minitab 17 (State College, Pennsylvania) was used for data analysis. Student t test (t) and analysis of variance (F) were used to compare group means. Post hoc statistical power for 2-tailed hypotheses was calculated based on observed means and SDs, group sample number, and α ≤ .05. Results Frictional torque for each bearing couple tested is shown in Table 2. When paired with XLPE liners, the mean frictional torque of 36-mm (0.95 Nm) and 40-mm (0.94 Nm) CoCr femoral heads was significantly greater than 28-mm (0.77 Nm) and 32-mm (0.78 Nm) CoCr femoral heads (F = 4.49, P = .014). Mean frictional torque did not statistically vary based on the head diameter within the group of ceramic femoral heads (F = 1.00, P = .393), although resistance incrementally increased with increasing head size (Table 2). With respect to the dual-mobility bearings, mean frictional torque was significantly higher in the CoCr (0.85 Nm) inner femoral head group compared with ceramic femoral head group (0.70 Nm) within outer XLPE spherical liners articulating with a CoCr metal acetabular liner (t = 3.41, P = .008). Vitamin E polyethylene liners demonstrated significantly greater frictional torque compared with XLPE liners without vitamin E measured in 32-mm diameter femoral head couples (Table 2). This difference was statistically significant for both CoCr (1.25 Nm vs 0.78 Nm, t = 8.19, P = .001) and ceramic (1.42 Nm vs 0.80 Nm, t = 16.85, P = .001) femoral heads. Statistical power for these comparisons was greater than 90%. Figs. 3 and 4 illustrate that the largest values of average frictional torque between CoCr and ceramic test specimens were associated
Nm at 20° External Rotation
Nm Difference
−0.66 −0.72 −0.66 −0.71 −0.71 −0.69 −0.69 (0.02)
1.39 1.51 1.30 1.37 1.49 1.44 1.42 (0.07)
−0.54 −0.60 −0.67 −0.69 −0.55 −0.56 −0.60 (0.06)
1.17 1.29 1.33 1.38 1.15 1.16 1.25 (0.10)
with vitamin E polyethylene liners. Analysis of variance was statistically significant for both CoCr (F = 15.52, P = .001) and ceramic (F = 24.28, P = .001) femoral heads, indicating that one or more of the group means were significantly different. Pairwise comparisons [24] with tolerance for type I errors conservatively set at ≤0.01% indicated that the frictional torque of CoCr and ceramic implants with 32-mm femoral heads and vitamin E liners were significantly different from all other test specimens. There was no statistically significant difference in frictional torque when XLPE liners were compared with conventional polyethylene liners in CoCr (0.77 Nm vs 0.76 Nm, t = −0.16, P = .876) and ceramic (0.77 Nm vs 0.79 Nm, t = 0.42, P = .687) femoral heads of 28-mm diameter (Table 2). Statistical power for both comparisons was low (≤7%) given the observed means and SDs. Discussion Human synovial joints such as the ball and socket of the hip have one of the lowest known static and kinetic coefficients of friction (μs ≤ 0.01, μk ≤ 0.005). Sir John Charnley [3] recognized that resistance to implant movement increases wear and shearing stresses at the hip joint. The frictional torque of Charnley's low-friction hip arthroplasty was 0.4 to 1.2 Nm under a loading force of 890 N [25]. Today, most of the available combinations of femoral stems, femoral neck trunnions, femoral heads, and acetabular cups and liners deviate from the low-friction hip arthroplasty first used in 1962. Because it is widely agreed that biomechanical forces and stresses are a contributing factor in trunnionosis at
Table 2 Mean (SD) Frictional Torque (in Nm) for Each Test Specimen. Femoral Head Size 28 mm Ceramic Conventional liner XLP liner Vitamin E liner Cobalt chromium Conventional liner XLP liner Vitamin E liner
32 mm
36 mm
40 mm
MDM X3
0.79 (0.11) 0.77 (0.13) 0.80 (0.04) 0.90 (0.25) 1.42 (0.08)
0.70 (0.07)
0.76 (0.11) 0.77 (0.07) 0.78 (0.10) 0.95 (0.14) 0.94 (0.13) 0.85 (0.09) 1.25 (0.10)
Fig. 3. Boxplot illustrating the range (rectangle), mean (circle), and median (diamond) of frictional torque values for CoCr test specimens (P = .000).
Please cite this article as: Meneghini RM, et al, Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.09.020
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Fig. 4. Boxplot illustrating the range (rectangle), mean (circle), and median (diamond) of frictional torque values for ceramic test specimens (P = .000).
the head-taper junction, it is logical that frictional torque at the bearing surface interface may be transmitted to the taper junction and if excessive contribute to MACC as a failure mode in THA [6,7,26]. The etiology and resurgence of clinically relevant trunnionosis remains poorly understood. Factors that have been implicated in promoting trunnion corrosion include large-diameter femoral heads, surface topography of trunnions, taper geometry, flexural rigidity of femoral stem tapers, and environmental conditions during taper assembly [27–34]. The biomechanical factor most relevant to this study is whether large-diameter heads contribute to increased taper corrosion and although there are numerous reports implicating the larger the diameter of metal femoral head, the greater the mechanical forces and subsequent trunnion corrosion at the taper junction [11,30,33], a recent Hip Society award paper refuted that concept and reported that fretting and corrosion were related to taper design and mixed-metal alloy taper-head combinations rather than femoral head diameter [27]. The contribution of taper design has been recently reported by other authors and appears to have a significant affect on trunnion corrosion [28,31,32]. The study findings reported here demonstrated that the mean frictional torque was greater in the larger 36- and 40-mm diameter femoral heads articulating against XLPE liners compared with smaller-diameter femoral heads. Using an in vitro model, Schmidig et al [35] similarly observed that frictional torque in CoCr implants significantly increased as head size increased, in both spherical and artificially deformed acetabular shells. Burroughs et al [36] reported that 40-mm CoCr femoral heads articulating against aged XLPE liners resulted in greater frictional torque than 40-mm CoCr heads articulating against aged conventional liners. Greater frictional torque with increasing head sizes was consistently reflected in the data from Burroughs et al [36] for both XLPE and conventional liners at several different levels of loading force. In another study under simulated walking conditions, Bishop et al [37] observed that frictional torque was greatest for large-diameter metal-on-metal, followed by metal-on-polyethylene, and lowest for small-diameter ceramic-ceramic and ceramic-metal bearings. Similar to our findings, several studies have observed that the a positive linear relationship between frictional torque and femoral head size begins to level off for femoral heads size 36 mm and greater [35,36]. Several studies have examined wear properties of vitamin E polyethylene liners [21–23,38–41]; however, to the best of our knowledge, published studies on the frictional characteristics of vitamin E polyethylene do not exist. Our study findings document that mean frictional torque was significantly higher in XLPE liners infused with vitamin E paired with both metal and ceramic femoral heads when compared with XLPE without vitamin E. Thirty-two-millimeter heads with vitamin E polyethylene liners resulted in a minimum 58% and 31% increase in
frictional torque in ceramic and CoCr femoral heads, respectively (Table 2). Analysis of all available combinations of implant type, femoral head size, and acetabular liner type demonstrated that the mean frictional torque of CoCr and ceramic 32-mm femoral heads and vitamin E liners was significantly higher than all other test specimens (Figs. 3 and 4). In contrast to vitamin E liners, we found no difference in the amount of frictional torque in XLPE and conventional polyethylene liners paired with 28-mm femoral heads. It is possible that the small number of observations for each test specimen (n = 6) may have influenced our ability to detect a significant difference. Burroughs et al [36] compared the average of 5 observations of frictional torque in artificially aged conventional and XLPE liners, and reported that frictional torque was nearly universally higher in aged XLPE liners [36]. Our testing was not performed with artificially aged specimens, which may have also contributed to our results conflicting with those of Burroughs et al. Our study had limitations. First, the extent of generalizability between a mechanical testing model and the human model remains unknown. To the extent possible, however, we simulated in vivo loading conditions during walking for in vitro testing. Furthermore, although more complex patterns of hip motion are encountered and applicable for mechanical testing, we attempted to simplify and standardize our testing methods to isolate the variable of frictional torque between bearing couples of differing materials. In addition, 2000 cycles of testing was chosen for testing each bearing couple based on 2 well-conducted studies on frictional torque, both of which used only 120 cycles to test frictional torque [36,37], where Burroughs and colleagues [36] used only 120 cycles to asses frictional torque on XLPE compared with 12 million cycles to test wear. It is therefore reasonable that 2000 cycles was a sufficient cycles for testing the isolated mechanical property of frictional torque based on the current accepted literature. Second, our simulated compressive force was applied perpendicular to the bearing surface and this direction of force (or force vector) is only one of the multidirectional forces that occur in vivo in a dynamic state. A third limitation was our inability to test all possible combinations of implant designs and anatomical positions for the acetabular cup, both of which are known to interact with femoral head size and joint stability [42]. Finally, although our sample size (6 observations for each bearing couple tested) is consistent with similar biomechanical studies, the fact that α ≤ .05 was used as our criterion for statistical significance and that power for some comparisons was at least 90% addresses this concern. Many THA bearing couples comprising different materials have been developed and integrated in the quest to make a successful surgical procedure a perfect surgical procedure. It is commonly understood that material composition changes come with resultant consequences in mechanical properties, but long-term evidence for the efficacy and safety of implant innovations is compiled after, rather than before, adoption. Recent commentary warns that this process has potentially harmed many patients and placed others at high risk [43]. Although important advances have reduced serious clinical concerns such as dislocation and osteolysis, taper corrosion has reemerged as a significant challenge in the last decade [43]. Although vitamin E polyethylene was developed to reduce oxidation and extend the life of hip implants, the clinical implications of increased frictional torque against vitamin E polyethylene should be monitored and studied closely to address whether these biomechanical findings potentially result in any clinical deleterious consequences. References 1. Hansson T, Hansson E, Malchau H. Utility of spine surgery: a comparison of common elective orthopaedic surgical procedures. Spine (Phila Pa 1976) 2008;33(25):2819. 2. Wylde V, Blom AW, Whitehouse SL, et al. Patient-reported outcomes after total hip and knee arthroplasty: comparison of midterm results. J Arthroplasty 2009;24(2):210. 3. Charnley J. Surgery of the hip-joint: present and future developments. Br Med J 1960; 1(5176):821. 4. Wroblewski BM, Siney PD, Fleming PA. The principle of low frictional torque in the Charnley total hip replacement. J Bone Joint Surg (Br) 2009;91(7):855.
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Please cite this article as: Meneghini RM, et al, Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.09.020