- Email: [email protected]
S-38 Is Immediate Full Mobilisation Possible After Medial Ulnar Collateral Ligament Reconstruction at the Elbow?
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Oral and Poster Presentations / Journal of Biomechanics 43S1 (2010) S23–S74
patients were compared to those from the asymptomatic cohort (n = 881). Retrieved explants underwent analysis using a coordinate measuring machine. Results: There were 4201 patients (follow up 10–142 months). There were 51 failures secondary to ARMD. Median Cr and Co concentrations in the failed group were significantly higher than in the control group (p < 0.001). Metal ion levels in the ASR patients were significantly higher than in the B and C groups. Kaplan Meier survival analysis showed a failure rate in the ASR patient group of 9.8% at five years, compared to <1% at five years for the C+ group and 1.5% at ten years for the BHR group. Three ARMD patients had relatively low wear of the retrieved components. Conclusion: Increased wear from the MoM bearing surface was associated with an increased failure rate secondary to adverse tissue reactions. We believe the sub hemispheric cup of the ASR predisposes it to increased articular wear and this was reflected in the increased failure rates. A minority of patients (<1%) may be sensitive to relatively low levels of metal debris. S-36 UHMWPE for Hip Bearings: Past, Present and Future R. Chiesa. Materials and Chemical Engineering ‘G. Natta’ Politecnico di Milano, Via Mancinelli 7, Italy This study aims to provide updated information on the use of Ultra High Molecular Weight Polyethylene (UHMWPE) as bearing material for artificial hip joints. The study considers and discusses the UHMWPE failure reasons, especially associated to degradation and oxidation of the material, and focuses on the innovations recently introduced and currently clinically applied. Moreover, the current trend on biomaterials research for the development of the future UHMWPE generation will be presented and discussed. It is today generally recognized that UHMWPE degradation through oxidation is related to the gamma or beta irradiation sterilization processes; high irradiation dose may cause the breakage of the polymeric chains, generating free radicals that may react with oxygen and oxidize UHMWPE. UHMWPE oxidation is the responsible for the mechanical properties and wear resistance decrease. In this study we assess the quality and state of some shelf aged and retrieved UHMWPE components. UHMWPE mechanical properties, wear resistance and oxidation level were evaluated directly on retrieved components through specific tests and experiments 1]. Moreover, wear and mechanical characterization was performed on some crosslinked UHMWPE, currently used in clinical [2]. A comparison of the performance of the past and present UHMWPE is experimentally shown. Finally, some experimental results on the new generation of vitamin E stabilized UHMWPE are shown and discussed. The experiments and tests presented in this study underlined the oxidation problems of the past generation of UHMWPE. Mechanical and wear tests developed on specimens obtained from shelf aged and retrieved UHMWPE components, confirmed that irradiation induced UHMWPE modification is the main reason of its degradation over time, leading to the prosthesis failure. The tests and experiments performed on crosslinked UHMWPE showed the improvements of the new materials currently used in clinics, but also evidenced some limits of such innovative materials. The use of biocompatible stabilizer in UHMWPE, such as vitamin E, seems to improve the mechanical and wear problems of UHMWPE. Chemicalphysical, mechanical and wear tests and experiments performed on such new generation of stabilized UHMWPE indicate that these materials have the potentiality to successfully enter in clinic use. Reference(s) [1] M. Moscatelli, S. Fare, ´ E. Delvecchio, A. Ferretto, F. D’Angelo, M. Giudici, R. Chiesa. Structural, mechanical and wear resistance assessment of UHMWPE orthopedic components. Journal of Applied Biomaterials & Biomechanics 2006; Vol. 4: no. 3: 165–171.
[2] R. Chiesa, M. Moscatelli, C. Giordano, F. Siccardi, A. Cigada. Influence of heat treatment on structural, mechanical and wear properties of crosslinked UHMWPE. Journal of Applied Biomaterials & Biomechanics, 2004; Vol 2: N. 1, pp. 20–28.
S-37 In-Vivo like Testing of Ceramic Ball Heads T. Pandorf, R. Preuss. CeramTec AG, Medical Division, Germany Ceramic hip components are known for their superior material properties concerning the in-vivo loading situation. In comparison to other commonly used materials, ceramics have a very low friction coefficient and a high fracture load. However, there are a few reported occasions of in-vivo fracture of ceramic ball heads. An experimental set-up imitating the in-vivo loading situation is used to analyze different scenarios that may lead to the fracture of the ball heads, such as dynamic loading, edge loading and the metal taper condition. Ceramic ball heads made of pure alumina have been loaded until fracture under various conditions. The angle between the loading direction and the metal taper equals 35°, the ceramic ball is mounted in an alumina insert. Parameters under investigation were the inclination of the insert, the loading rate, and the condition of taper and ball head. To resemble the position of the human acetabulum during walking and standing up, the inclination of the insert was chosen to differ between 45° (walking) and 80° (standing up). A variation of the loading speed is also tested. The behavior of the ball heads for the different scenarios shows a great variation: If the inclination of the insert equals 45°, it is not possible to break the ceramic ball head at all because of the high plastic deformation of the metal taper. In case of edge loading, the fracture load of the ceramic ball heads drops significantly. The loading rate has no measurable influence on this value. A large effect on the fracture load has a contamination with osseous tissue. The fracture load decreases to 32% compared to the value measured without the contamination. A minimal fracture load of approximately 8 kN (KK 28–12/14 L) was measured. It has been shown that even the worst-case set-up does not lead to critical fracture loads if the interface between ceramic ball head and metal taper is clean and dry. In contrast, certain disturbances/impurities of this interface can cause a further reduction of the fracture load. Therefore, diligence is recommended during the implantation of the ceramic hip components in order to avoid disturbances of this interface. S-38 Is Immediate Full Mobilisation Possible After Medial Ulnar Collateral Ligament Reconstruction at the Elbow? Q. Vanat1 , M. Chizari2 . 1 St. Peters and Ashford Hospitals, UK; 2 Brunel University West London, UK Knowledge about the mechanical behaviour of tendon grafts can determine whether immediate post operative mobilisation is possible in valgus stress injuries to the medial aspect of the elbow where rupture of the medial ulnar collateral ligament causes instability and associated sequelae. Tendon is used to reconstruct the ligament (usually Palmaris longus), this is looped through predrilled channels (within the distal humerus and proximal ulnar) and sutured together where the ends meet. Postoperative regimes usually restrict arc of motion to protect the tendon loop. The hypothesis examined in this study is, that suturing of the tendon graft may affect the material properties of the corresponding tendon-tendon-suture interfaces by affecting the local tendon matrix composition. To investigate this mechanical behaviour, loadto-failure, and cyclic loading tests were applied to tendon graft sutured with a whipstitch. Using a bovine tendon model the tendon-suture junction was tested. Moist specimens were armed with a whipstitch using an Arthrex Speed Whip® 2.0 suture. Maximum tensile load of suture, followed
Oral and Poster Presentations / Journal of Biomechanics 43S1 (2010) S23–S74
by suture-tendon junction and finally tendon-tendon-suture were tested. Initially loop sutures clamped rigidly in the test rig and loaded at a rate of 5 mm/min tensile load showed load-displacement and the mode of failure, visually. Next a whip stitched prepared tendon specimen with lengths of 160 mm and average cross section of 3.90 mm2 were loaded to failure at a rate of 5 mm/sec. The displacement, force, slippage of the suture and deformation of the tendon were monitored. Stiffness was determined by using a linear regression employing data. Finally looped tendons, where the overlapping ends were whip stitched for 20 mm (total loop 50 mm) were load-to-failure and cyclic loading tested. Placed in a clamp with the sutured portion visibly in the middle a preload of 10 N was applied for 1 minute. The resulting displacement was set to zero. Initial lengths of sutured and non-sutured portions of the loop were measured. Loading protocols of 100 cycles from 10–75 N at 1 Hz were applied. Tested samples showed immediate stretching and most failure occurred at the suture/tendon interface as sutures cut into the tendon. Failures were due to a progressive breakage of the tendon fibers beginning with slippage of the suture at the first suture throw and tearing of the first knot through the most distal portion of the tendon. The time of suture rupture was identical with the time of maximum force. Cyclical testing showed elongation, either of the suture or the tendon more in the first few cycles than compared to the fifth cycle. Importantly failure occurred with less force when testing the non-looped (160 mm) model and whether single lengths or loops permanent viscoplastic elongation occurred after loading around the whipstitch. Hence the importance of preloading is to avoid postoperative laxity. The conclusion is that failure does occur and this occurs at the tendon-tendon-suture interface, but that the loads required to cause failure are still higher than those that a patient would deliver postoperatively. It is therefore recommended to fully mobilise without restriction, achieving full range of motion early on. S-39 Tribological Testing of Implants under More Physiological Conditions in Vitro A. Kamali1 , J. Pamu1 , J.T. Daniel2 , A. Hussain1 , C. Li1 . 1 Smith & Nephew Orthopaedics Ltd, UK; 2 University of Cambridge, Cambridge, UK Researchers have been using hip simulators for decades to determine the tribological performance of hip implants. It has been reported that there is no significant difference between the wear generated by various cobalt chromium (CoCr) alloy microstructures in hip simulator studies [1,2]. However, higher wear, metal ion levels and failure rates have been reported in heat treated and consequently low carbide (high carbon) metal-on-metal (MoM) devices compared to the as cast CoCr devices in vivo [3–5]. The continuous and fast cycles in hip simulator studies may artificially protect the bearing surfaces, masking the effect of microstructure on wear. On the contrary, the extensive range of kinetics and kinematics, stop/start motion and various walking frequencies in vivo could break down the fluid film in MoM bearings resulting in the implants operating in less favourable lubrication regimes. The aims of this study were to develop a more physiologically relevant hip simulator test protocol and to investigate the effect of microstructure on the tribological performance of MoM devices under the new test condition. Patients’ activities were monitored using Step Activity Monitor (SAM) devices. The data showed that the patients were walking at a slower pace than that used in traditional hip simulator studies. This information was then used to develop a more physiologically relevant hip simulator test protocol. Three pairs of 50 mm as cast (AC) and four pairs of 50 mm double heat treated (DHT) CoCr MoM devices were tested in a ProSim hip simulator under the newly developed conditions, including stop/start motion, relevant walking frequency and various kinetic and kinematic profiles.
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Under the newly developed physiologically relevant test conditions the DHT CoCr devices generated 40% higher wear than the AC CoCr devices. Metal ion analysis also showed similar trends, however, the difference between the AC and DHT devices was further increased. It has been reported that the DHT devices generate smaller size particles and in much larger numbers compared to those generated by the AC devices [6]. This would contribute to a larger net surface area of particles exposed to corrosion explaining the comparatively higher elevation of metal ion levels with the DHT devices compared to AC devices. These in vitro results correlate well with the in vivo results, where higher wear, metal ion levels and failure rates have been observed with double heat treated CoCr devices compared to as cast CoCr. Reference(s) [1] Nevelos J, Shelton JC, Fisher J. Metallurgical considerations in the wear of metal-on-metal hip bearings. Hip Int 2004;14:1–10. [2] Bowsher JG, Nevelos J, Williams PA, Shelton JC. ‘Severe’ wear challenge to ‘as-cast’ and ‘double heat-treated’ large-diameter metal-on-metal hip bearings. Proc Inst Mech Eng [H] 2006;220:135–43. [3] Daniel J, Ziaee H, Kamali A, Pradhan C, Band T, McMinn DJW. Ten-year results of a double-heat-treated metal-on-metal hip resurfacing. J Bone Joint Surg [Br] 2010;92-B:20–7. [4] Clarke MT, Lee PT, Arora A, Villar RN. Levels of metal ions after smalland large diameter metal-on-metal hip arthroplasty. J Bone Joint Surg [Br] 2003;85-B:913–7. [5] Khan M, Kuiper JH, Richardson JB. The exercise-related rise in plasma cobalt levels after metal-on-metal hip resurfacing arthroplasty. J Bone Joint Surg [Br] 2008;90-B:1152–7. [6] Kinbrum A, Unsworth A, Kamali A. The wear of high carbon metal-onmetal bearings after different heat treatments. ORS, 2008:33.
S-40 Edge Loading and Wear of Metal on Metal Hip Resurfacing Devices A. Hussain, K. Packer, C. Li, A. Kamali. Smith & Nephew Orthopaedics Ltd, UK The in vivo performance of a hip implant depends upon many factors such as good design and material, but it also depends on surgical technique and correct implant orientation. Mal-positioned devices may result in contact between the femoral head and the rim of the acetabular cup (edge loading) which in turn may result in excessive wear and failure of the implant. This study was conducted in order to determine the effect of edge loading on retrieved implants through linear wear analysis. Linear wear of 55 BHR retrieved acetabular cups with known time in vivo were examined using a Taylor-Hobson Talyrond 290 roundness machine. Multiple measurements were taken to locate the area of wear. The maximum deviation of the profile from an ideal circle was taken as the maximum linear wear. Edge loaded cups were classified as cups which showed the maximum area of wear crossing over the edge of the cup. Non-edge loaded devices were devices with the wear area within the articulating sphere of the cup. Among the 55 retrieved BHR cups examined, 32 were classed as edge loaded and 23 classed as non-edge loaded. Edge loaded cups displayed a greater range of linear wear rates than the non-edge loaded cups. The edge loaded cups also generated a significantly higher wear rate than the non-edge loaded cups, at 24.02 (±22.72 SD) and 1.48 (±1.01 SD) mm/year respectively. The difference between the two groups was statistically significant (p = 0.00001). The majority of hip resurfacing implants examined in this study had experienced edge loading, which is the prominent factor in the generation of a high linear wear rate. The edge loaded cups in this study generated over 16 times higher linear wear rate than that generated by the non-edge loaded cups. All the non-edge loaded (wear area within the articulating sphere) cups generated low amount of wear. High metal ion levels in patients have been closely linked with high wear of devices [1]. This study has given us an insight into the adverse effect of edge loading resulting in
Oral and Poster Presentations / Journal of Biomechanics 43S1 (2010) S23–S74
patients were compared to those from the asymptomatic cohort (n = 881). Retrieved explants underwent analysis using a coordinate measuring machine. Results: There were 4201 patients (follow up 10–142 months). There were 51 failures secondary to ARMD. Median Cr and Co concentrations in the failed group were significantly higher than in the control group (p < 0.001). Metal ion levels in the ASR patients were significantly higher than in the B and C groups. Kaplan Meier survival analysis showed a failure rate in the ASR patient group of 9.8% at five years, compared to <1% at five years for the C+ group and 1.5% at ten years for the BHR group. Three ARMD patients had relatively low wear of the retrieved components. Conclusion: Increased wear from the MoM bearing surface was associated with an increased failure rate secondary to adverse tissue reactions. We believe the sub hemispheric cup of the ASR predisposes it to increased articular wear and this was reflected in the increased failure rates. A minority of patients (<1%) may be sensitive to relatively low levels of metal debris. S-36 UHMWPE for Hip Bearings: Past, Present and Future R. Chiesa. Materials and Chemical Engineering ‘G. Natta’ Politecnico di Milano, Via Mancinelli 7, Italy This study aims to provide updated information on the use of Ultra High Molecular Weight Polyethylene (UHMWPE) as bearing material for artificial hip joints. The study considers and discusses the UHMWPE failure reasons, especially associated to degradation and oxidation of the material, and focuses on the innovations recently introduced and currently clinically applied. Moreover, the current trend on biomaterials research for the development of the future UHMWPE generation will be presented and discussed. It is today generally recognized that UHMWPE degradation through oxidation is related to the gamma or beta irradiation sterilization processes; high irradiation dose may cause the breakage of the polymeric chains, generating free radicals that may react with oxygen and oxidize UHMWPE. UHMWPE oxidation is the responsible for the mechanical properties and wear resistance decrease. In this study we assess the quality and state of some shelf aged and retrieved UHMWPE components. UHMWPE mechanical properties, wear resistance and oxidation level were evaluated directly on retrieved components through specific tests and experiments 1]. Moreover, wear and mechanical characterization was performed on some crosslinked UHMWPE, currently used in clinical [2]. A comparison of the performance of the past and present UHMWPE is experimentally shown. Finally, some experimental results on the new generation of vitamin E stabilized UHMWPE are shown and discussed. The experiments and tests presented in this study underlined the oxidation problems of the past generation of UHMWPE. Mechanical and wear tests developed on specimens obtained from shelf aged and retrieved UHMWPE components, confirmed that irradiation induced UHMWPE modification is the main reason of its degradation over time, leading to the prosthesis failure. The tests and experiments performed on crosslinked UHMWPE showed the improvements of the new materials currently used in clinics, but also evidenced some limits of such innovative materials. The use of biocompatible stabilizer in UHMWPE, such as vitamin E, seems to improve the mechanical and wear problems of UHMWPE. Chemicalphysical, mechanical and wear tests and experiments performed on such new generation of stabilized UHMWPE indicate that these materials have the potentiality to successfully enter in clinic use. Reference(s) [1] M. Moscatelli, S. Fare, ´ E. Delvecchio, A. Ferretto, F. D’Angelo, M. Giudici, R. Chiesa. Structural, mechanical and wear resistance assessment of UHMWPE orthopedic components. Journal of Applied Biomaterials & Biomechanics 2006; Vol. 4: no. 3: 165–171.
[2] R. Chiesa, M. Moscatelli, C. Giordano, F. Siccardi, A. Cigada. Influence of heat treatment on structural, mechanical and wear properties of crosslinked UHMWPE. Journal of Applied Biomaterials & Biomechanics, 2004; Vol 2: N. 1, pp. 20–28.
S-37 In-Vivo like Testing of Ceramic Ball Heads T. Pandorf, R. Preuss. CeramTec AG, Medical Division, Germany Ceramic hip components are known for their superior material properties concerning the in-vivo loading situation. In comparison to other commonly used materials, ceramics have a very low friction coefficient and a high fracture load. However, there are a few reported occasions of in-vivo fracture of ceramic ball heads. An experimental set-up imitating the in-vivo loading situation is used to analyze different scenarios that may lead to the fracture of the ball heads, such as dynamic loading, edge loading and the metal taper condition. Ceramic ball heads made of pure alumina have been loaded until fracture under various conditions. The angle between the loading direction and the metal taper equals 35°, the ceramic ball is mounted in an alumina insert. Parameters under investigation were the inclination of the insert, the loading rate, and the condition of taper and ball head. To resemble the position of the human acetabulum during walking and standing up, the inclination of the insert was chosen to differ between 45° (walking) and 80° (standing up). A variation of the loading speed is also tested. The behavior of the ball heads for the different scenarios shows a great variation: If the inclination of the insert equals 45°, it is not possible to break the ceramic ball head at all because of the high plastic deformation of the metal taper. In case of edge loading, the fracture load of the ceramic ball heads drops significantly. The loading rate has no measurable influence on this value. A large effect on the fracture load has a contamination with osseous tissue. The fracture load decreases to 32% compared to the value measured without the contamination. A minimal fracture load of approximately 8 kN (KK 28–12/14 L) was measured. It has been shown that even the worst-case set-up does not lead to critical fracture loads if the interface between ceramic ball head and metal taper is clean and dry. In contrast, certain disturbances/impurities of this interface can cause a further reduction of the fracture load. Therefore, diligence is recommended during the implantation of the ceramic hip components in order to avoid disturbances of this interface. S-38 Is Immediate Full Mobilisation Possible After Medial Ulnar Collateral Ligament Reconstruction at the Elbow? Q. Vanat1 , M. Chizari2 . 1 St. Peters and Ashford Hospitals, UK; 2 Brunel University West London, UK Knowledge about the mechanical behaviour of tendon grafts can determine whether immediate post operative mobilisation is possible in valgus stress injuries to the medial aspect of the elbow where rupture of the medial ulnar collateral ligament causes instability and associated sequelae. Tendon is used to reconstruct the ligament (usually Palmaris longus), this is looped through predrilled channels (within the distal humerus and proximal ulnar) and sutured together where the ends meet. Postoperative regimes usually restrict arc of motion to protect the tendon loop. The hypothesis examined in this study is, that suturing of the tendon graft may affect the material properties of the corresponding tendon-tendon-suture interfaces by affecting the local tendon matrix composition. To investigate this mechanical behaviour, loadto-failure, and cyclic loading tests were applied to tendon graft sutured with a whipstitch. Using a bovine tendon model the tendon-suture junction was tested. Moist specimens were armed with a whipstitch using an Arthrex Speed Whip® 2.0 suture. Maximum tensile load of suture, followed
Oral and Poster Presentations / Journal of Biomechanics 43S1 (2010) S23–S74
by suture-tendon junction and finally tendon-tendon-suture were tested. Initially loop sutures clamped rigidly in the test rig and loaded at a rate of 5 mm/min tensile load showed load-displacement and the mode of failure, visually. Next a whip stitched prepared tendon specimen with lengths of 160 mm and average cross section of 3.90 mm2 were loaded to failure at a rate of 5 mm/sec. The displacement, force, slippage of the suture and deformation of the tendon were monitored. Stiffness was determined by using a linear regression employing data. Finally looped tendons, where the overlapping ends were whip stitched for 20 mm (total loop 50 mm) were load-to-failure and cyclic loading tested. Placed in a clamp with the sutured portion visibly in the middle a preload of 10 N was applied for 1 minute. The resulting displacement was set to zero. Initial lengths of sutured and non-sutured portions of the loop were measured. Loading protocols of 100 cycles from 10–75 N at 1 Hz were applied. Tested samples showed immediate stretching and most failure occurred at the suture/tendon interface as sutures cut into the tendon. Failures were due to a progressive breakage of the tendon fibers beginning with slippage of the suture at the first suture throw and tearing of the first knot through the most distal portion of the tendon. The time of suture rupture was identical with the time of maximum force. Cyclical testing showed elongation, either of the suture or the tendon more in the first few cycles than compared to the fifth cycle. Importantly failure occurred with less force when testing the non-looped (160 mm) model and whether single lengths or loops permanent viscoplastic elongation occurred after loading around the whipstitch. Hence the importance of preloading is to avoid postoperative laxity. The conclusion is that failure does occur and this occurs at the tendon-tendon-suture interface, but that the loads required to cause failure are still higher than those that a patient would deliver postoperatively. It is therefore recommended to fully mobilise without restriction, achieving full range of motion early on. S-39 Tribological Testing of Implants under More Physiological Conditions in Vitro A. Kamali1 , J. Pamu1 , J.T. Daniel2 , A. Hussain1 , C. Li1 . 1 Smith & Nephew Orthopaedics Ltd, UK; 2 University of Cambridge, Cambridge, UK Researchers have been using hip simulators for decades to determine the tribological performance of hip implants. It has been reported that there is no significant difference between the wear generated by various cobalt chromium (CoCr) alloy microstructures in hip simulator studies [1,2]. However, higher wear, metal ion levels and failure rates have been reported in heat treated and consequently low carbide (high carbon) metal-on-metal (MoM) devices compared to the as cast CoCr devices in vivo [3–5]. The continuous and fast cycles in hip simulator studies may artificially protect the bearing surfaces, masking the effect of microstructure on wear. On the contrary, the extensive range of kinetics and kinematics, stop/start motion and various walking frequencies in vivo could break down the fluid film in MoM bearings resulting in the implants operating in less favourable lubrication regimes. The aims of this study were to develop a more physiologically relevant hip simulator test protocol and to investigate the effect of microstructure on the tribological performance of MoM devices under the new test condition. Patients’ activities were monitored using Step Activity Monitor (SAM) devices. The data showed that the patients were walking at a slower pace than that used in traditional hip simulator studies. This information was then used to develop a more physiologically relevant hip simulator test protocol. Three pairs of 50 mm as cast (AC) and four pairs of 50 mm double heat treated (DHT) CoCr MoM devices were tested in a ProSim hip simulator under the newly developed conditions, including stop/start motion, relevant walking frequency and various kinetic and kinematic profiles.
S45
Under the newly developed physiologically relevant test conditions the DHT CoCr devices generated 40% higher wear than the AC CoCr devices. Metal ion analysis also showed similar trends, however, the difference between the AC and DHT devices was further increased. It has been reported that the DHT devices generate smaller size particles and in much larger numbers compared to those generated by the AC devices [6]. This would contribute to a larger net surface area of particles exposed to corrosion explaining the comparatively higher elevation of metal ion levels with the DHT devices compared to AC devices. These in vitro results correlate well with the in vivo results, where higher wear, metal ion levels and failure rates have been observed with double heat treated CoCr devices compared to as cast CoCr. Reference(s) [1] Nevelos J, Shelton JC, Fisher J. Metallurgical considerations in the wear of metal-on-metal hip bearings. Hip Int 2004;14:1–10. [2] Bowsher JG, Nevelos J, Williams PA, Shelton JC. ‘Severe’ wear challenge to ‘as-cast’ and ‘double heat-treated’ large-diameter metal-on-metal hip bearings. Proc Inst Mech Eng [H] 2006;220:135–43. [3] Daniel J, Ziaee H, Kamali A, Pradhan C, Band T, McMinn DJW. Ten-year results of a double-heat-treated metal-on-metal hip resurfacing. J Bone Joint Surg [Br] 2010;92-B:20–7. [4] Clarke MT, Lee PT, Arora A, Villar RN. Levels of metal ions after smalland large diameter metal-on-metal hip arthroplasty. J Bone Joint Surg [Br] 2003;85-B:913–7. [5] Khan M, Kuiper JH, Richardson JB. The exercise-related rise in plasma cobalt levels after metal-on-metal hip resurfacing arthroplasty. J Bone Joint Surg [Br] 2008;90-B:1152–7. [6] Kinbrum A, Unsworth A, Kamali A. The wear of high carbon metal-onmetal bearings after different heat treatments. ORS, 2008:33.
S-40 Edge Loading and Wear of Metal on Metal Hip Resurfacing Devices A. Hussain, K. Packer, C. Li, A. Kamali. Smith & Nephew Orthopaedics Ltd, UK The in vivo performance of a hip implant depends upon many factors such as good design and material, but it also depends on surgical technique and correct implant orientation. Mal-positioned devices may result in contact between the femoral head and the rim of the acetabular cup (edge loading) which in turn may result in excessive wear and failure of the implant. This study was conducted in order to determine the effect of edge loading on retrieved implants through linear wear analysis. Linear wear of 55 BHR retrieved acetabular cups with known time in vivo were examined using a Taylor-Hobson Talyrond 290 roundness machine. Multiple measurements were taken to locate the area of wear. The maximum deviation of the profile from an ideal circle was taken as the maximum linear wear. Edge loaded cups were classified as cups which showed the maximum area of wear crossing over the edge of the cup. Non-edge loaded devices were devices with the wear area within the articulating sphere of the cup. Among the 55 retrieved BHR cups examined, 32 were classed as edge loaded and 23 classed as non-edge loaded. Edge loaded cups displayed a greater range of linear wear rates than the non-edge loaded cups. The edge loaded cups also generated a significantly higher wear rate than the non-edge loaded cups, at 24.02 (±22.72 SD) and 1.48 (±1.01 SD) mm/year respectively. The difference between the two groups was statistically significant (p = 0.00001). The majority of hip resurfacing implants examined in this study had experienced edge loading, which is the prominent factor in the generation of a high linear wear rate. The edge loaded cups in this study generated over 16 times higher linear wear rate than that generated by the non-edge loaded cups. All the non-edge loaded (wear area within the articulating sphere) cups generated low amount of wear. High metal ion levels in patients have been closely linked with high wear of devices [1]. This study has given us an insight into the adverse effect of edge loading resulting in