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Accuracy of radiographic wear measurements in a cementless modular cobalt-chrome acetabular component
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The Journal of Arthroplasty Vol. 13 No. 2 February 1998 PAPER # 7
*THE EFFECT OF CLEARANCE AND DIAMETER ON DEBRIS GENERATION IN A METAL-ON-METAL HIP Michael A. Jacobs, MD, Baltimore, MD, Mary Beth Schmidt, PhD, Richard Farrar Despite some successful clinical results with early metal-on-metal articulations, concerns about poor mechanical behavior and the effects of metal debris caused this concept to be abandoned. The purpose of this study is to reassess the potential application of metal-on-metal bearings for total hip arthroplasty, using a systematic approach to begin optimizing the wear performance of the articulating surfaces. The influence of clearance and diameter of the bearing couple on the amount of wear debris generated are specifically addressed. Test samples consisted of rigid hemispherical cups, pohshed to implantgrade finish, and standard femoral heads with approximate diameters of 22 ram, 28 mm, and 35 ram. All components were fabricated from wrought CoCrMo (ASTM F-1537). The clearance between the heads and the cups was adjusted to provide up to twelve separate clearances for each size. For the 28 m m samples, diametral clearances ranged from 0.014 m m - 0.322 ram. The range narrowed to 0.01 m m - 0.15 m m for the 22 m m and 35 m m samples. Wear studies were performed using a multi-station MMED hip simulator. The simulator produces a biaxial rocking motion that is synchronized with variable compressive loads. All tests were run at 1.1 Hz, using the Paul loading curve, with a peak load of 2000 N, for two million cycles. Samples were completely immersed in calf serum with 0.2% W / V sodium azide and 20 rnM EDTA during testing. At specific intervals, the heads and the cups were cleaned and the wear for each component was measured on the basis of weight loss. Total wear at each interval was calculated by adding the weight losses for the head and cup. For all diameters, the smallest clearances (<0.015 ram) resulted in increased wear which may be caused by minor asphericities. Test results for the 28 m m samples indicate that total wear after two million cycles is positively correlated to clearance (P = 0.0002, r 2 = 0.95) over the range 0.033 - 0.322 nhm. Total wear did not correlate as strongly with clearance for the 22 and 35 m m samples, over the decreased range of 0.02 - 0.15 m m (22 mm: p = 0.48, r 2 = 0.075; 35 ram: p = 0.05, r 2 = 0.58). The amount of wear debris generated for samples in the clearance range 0.02- 0.15 m m was not statisticany different for all 3 head sizes. The results of this study indicate that clearance between the head and the cup plays a critical role in determining the wear performance of metal-onmetal articulations. In contrast, diameter does not appear to be as important. The failure of early metal-on-metal designs to identify the proper range of clearances and the inability to generate successful articulation couples within these demanding tolerances may have lead to increased wear in these designs. With the tighter manufacturing control afforded by current precision machining technology, the small clearances required to minimize debris generation can be consistently produced. Therefore, defining the relationship between clearance and wear is an essential step in the development of an optimal metal-on-metal design for total hip arthroplasty applications.
PAPER # 8 ACCURACY OF RADIOGRAPHIC WEAR MEASUREMENTS IN A CEMENTLESS MODULAR COBALT-CHROME ACETABULAR COMPONENT Robert L. Barrack, MD, New Orleans, LA, Ed Szuszczewicz, MD, Carlos Lavernia, MD A number of techniques have been described for estimating the amount of polyethylene wear based on plain radiographs. A study was undertaken to determine the accuracy of various radiographic techniques for measuring polyethylene wear in a cementless modular cobalt-chrome acetabular component.
* D e n o t e s t h a t s o m e t h i n g of v a l u e w a s received P r e s e n t e r s are boldface
Twenty consecutive retrieved acetabular components formed the study group. The indications for revision included osteolysis, infection and component malposition. All acetabular components were full hemispheres and fabricated of cobalt-chrome with a sintered bead coating. The amount of linear polyethylene wear was measured from the polyethylene inserts directly by creating molds of the articular surfaces using vinyl polysiloxane (Imprint, 3M Corporation, Minneapohs, MN). An optical comparator was used to assess linear wear using the shadowgraph technique described by Kabo et al. Postoperative and pre-revision radiographs were then reviewed to measure the amount of linear wear radiographically. Seven radiographic methods described in the literature were utilized: 5 were manual techniques and 2 techniques utilized a computer assisted digitizer (Research Metrics Orthographics, Inc., Salt Lake City, UT). Linear regression analysis showed that there was a statistically significant correlation between the radiographic measurements compared to the direct measurement for 4 of the 5 manual techniques. The Livermore technique showed a strong statistical trend (p = 0.1) but did not achieve statistical significance. The 2 computerized techniques also showed statistical trends (p =. 12 and p =. 15) but did not achieve statistical significance. A strong correlation existed for the Dorr technique (r = 0.72, p < .0001). This was followed by the Unlradiographic technique (r = 0.67, p < .0001); the Duoradiographic techrdque (r = 0.53, p = .014); and the tangential technique (r = 0.46, p = .036). The present study demonstrates that there is a fairly good correlation between most radiographic techniques and actual wear measurements with a modular cementless acetabular component. The technique described by Dorr et al was most accurate with the highest correlation coefficient and the Livermore technique was the least accurate and did not achieve a statistically significant correlation with the actual wear measurements. In the present study, the addition of a computer digitizer did not improve the accuracy of wear measurements. Based on these results, radiographic wear measurements of cementless, modular components should be considered qualitative rather than quantitative. There is a significant difference in the measurements obtained among various published techniques. The addition of computer digitization does not necessarily improve accuracy.
PAPER # 9 RESULTS OF UNCEMENTED ACETABULAR COMPONENTS WITH SCREW FIXATION IN PATIENTS OVER SEVENTY-FIVE James T. Ninomiya, MD, Milwaukee, WI, Victor M. Goldberg, MD Although the use of uncemented acetabular cups in total hip replacements (THR) is widely accepted, the use of these components in elderly patients can still be questioned. Compared to younger patients, this population has an increased incidence of osteoporosis, as well as nutritional and other metabohc abnormalities that m i g h t adversely affect the fixation of an uncemented component. In general, the decreased quality of the underlying bone could lead to early failures due to aseptic loosening. Due to these concerns, we elected to compared the results of uncemented acetabular components in an elderly population with the results from a younger group. These results of 207 consecutive THR utilizing uncemented acetabular components with a minimum follow-up of seven years were reviewed. At the time of the index operation, 31 of these patients were 75 years of age or older (ave. 78, range 75-87); the remaining 178 hips had an average age of 56 (range 17-74). All of the operations were performed by a single surgeon. During the time of the study, all of the acetabulae were prepared by hemispheric reaming, followed by insertion of the cup without under-reaming using an average of three screws for each component. Harris hip scores (HHS) and radiographic assessments were carried out at regular intervals, and the presence of radiolucencies, osteolysis, cup migration, and polyethylene wear were recorded for each component. At last follow-up all of the patients had obtained excellent clinical results. Average pre-operative HI-IS were 37 and 41 in the elderly (>75) and younger groups (<75) with average post-operative scores being 92 and 93 respectively. There were three revisions for aseptic loosening of uncemented stems and one for a fractured femur. Radiohicencies were seen in both groups, but in the elderly population 70% did not have any radiolucent lines, while in the younger patients 58% were free of radiolucencies. In both groups none of the radiolucent lines were assessed as being progressive. In the elderly group 61% were free of polyethylene wear, while this was the case for only 51% of the younger population.
The Journal of Arthroplasty Vol. 13 No. 2 February 1998 PAPER # 7
*THE EFFECT OF CLEARANCE AND DIAMETER ON DEBRIS GENERATION IN A METAL-ON-METAL HIP Michael A. Jacobs, MD, Baltimore, MD, Mary Beth Schmidt, PhD, Richard Farrar Despite some successful clinical results with early metal-on-metal articulations, concerns about poor mechanical behavior and the effects of metal debris caused this concept to be abandoned. The purpose of this study is to reassess the potential application of metal-on-metal bearings for total hip arthroplasty, using a systematic approach to begin optimizing the wear performance of the articulating surfaces. The influence of clearance and diameter of the bearing couple on the amount of wear debris generated are specifically addressed. Test samples consisted of rigid hemispherical cups, pohshed to implantgrade finish, and standard femoral heads with approximate diameters of 22 ram, 28 mm, and 35 ram. All components were fabricated from wrought CoCrMo (ASTM F-1537). The clearance between the heads and the cups was adjusted to provide up to twelve separate clearances for each size. For the 28 m m samples, diametral clearances ranged from 0.014 m m - 0.322 ram. The range narrowed to 0.01 m m - 0.15 m m for the 22 m m and 35 m m samples. Wear studies were performed using a multi-station MMED hip simulator. The simulator produces a biaxial rocking motion that is synchronized with variable compressive loads. All tests were run at 1.1 Hz, using the Paul loading curve, with a peak load of 2000 N, for two million cycles. Samples were completely immersed in calf serum with 0.2% W / V sodium azide and 20 rnM EDTA during testing. At specific intervals, the heads and the cups were cleaned and the wear for each component was measured on the basis of weight loss. Total wear at each interval was calculated by adding the weight losses for the head and cup. For all diameters, the smallest clearances (<0.015 ram) resulted in increased wear which may be caused by minor asphericities. Test results for the 28 m m samples indicate that total wear after two million cycles is positively correlated to clearance (P = 0.0002, r 2 = 0.95) over the range 0.033 - 0.322 nhm. Total wear did not correlate as strongly with clearance for the 22 and 35 m m samples, over the decreased range of 0.02 - 0.15 m m (22 mm: p = 0.48, r 2 = 0.075; 35 ram: p = 0.05, r 2 = 0.58). The amount of wear debris generated for samples in the clearance range 0.02- 0.15 m m was not statisticany different for all 3 head sizes. The results of this study indicate that clearance between the head and the cup plays a critical role in determining the wear performance of metal-onmetal articulations. In contrast, diameter does not appear to be as important. The failure of early metal-on-metal designs to identify the proper range of clearances and the inability to generate successful articulation couples within these demanding tolerances may have lead to increased wear in these designs. With the tighter manufacturing control afforded by current precision machining technology, the small clearances required to minimize debris generation can be consistently produced. Therefore, defining the relationship between clearance and wear is an essential step in the development of an optimal metal-on-metal design for total hip arthroplasty applications.
PAPER # 8 ACCURACY OF RADIOGRAPHIC WEAR MEASUREMENTS IN A CEMENTLESS MODULAR COBALT-CHROME ACETABULAR COMPONENT Robert L. Barrack, MD, New Orleans, LA, Ed Szuszczewicz, MD, Carlos Lavernia, MD A number of techniques have been described for estimating the amount of polyethylene wear based on plain radiographs. A study was undertaken to determine the accuracy of various radiographic techniques for measuring polyethylene wear in a cementless modular cobalt-chrome acetabular component.
* D e n o t e s t h a t s o m e t h i n g of v a l u e w a s received P r e s e n t e r s are boldface
Twenty consecutive retrieved acetabular components formed the study group. The indications for revision included osteolysis, infection and component malposition. All acetabular components were full hemispheres and fabricated of cobalt-chrome with a sintered bead coating. The amount of linear polyethylene wear was measured from the polyethylene inserts directly by creating molds of the articular surfaces using vinyl polysiloxane (Imprint, 3M Corporation, Minneapohs, MN). An optical comparator was used to assess linear wear using the shadowgraph technique described by Kabo et al. Postoperative and pre-revision radiographs were then reviewed to measure the amount of linear wear radiographically. Seven radiographic methods described in the literature were utilized: 5 were manual techniques and 2 techniques utilized a computer assisted digitizer (Research Metrics Orthographics, Inc., Salt Lake City, UT). Linear regression analysis showed that there was a statistically significant correlation between the radiographic measurements compared to the direct measurement for 4 of the 5 manual techniques. The Livermore technique showed a strong statistical trend (p = 0.1) but did not achieve statistical significance. The 2 computerized techniques also showed statistical trends (p =. 12 and p =. 15) but did not achieve statistical significance. A strong correlation existed for the Dorr technique (r = 0.72, p < .0001). This was followed by the Unlradiographic technique (r = 0.67, p < .0001); the Duoradiographic techrdque (r = 0.53, p = .014); and the tangential technique (r = 0.46, p = .036). The present study demonstrates that there is a fairly good correlation between most radiographic techniques and actual wear measurements with a modular cementless acetabular component. The technique described by Dorr et al was most accurate with the highest correlation coefficient and the Livermore technique was the least accurate and did not achieve a statistically significant correlation with the actual wear measurements. In the present study, the addition of a computer digitizer did not improve the accuracy of wear measurements. Based on these results, radiographic wear measurements of cementless, modular components should be considered qualitative rather than quantitative. There is a significant difference in the measurements obtained among various published techniques. The addition of computer digitization does not necessarily improve accuracy.
PAPER # 9 RESULTS OF UNCEMENTED ACETABULAR COMPONENTS WITH SCREW FIXATION IN PATIENTS OVER SEVENTY-FIVE James T. Ninomiya, MD, Milwaukee, WI, Victor M. Goldberg, MD Although the use of uncemented acetabular cups in total hip replacements (THR) is widely accepted, the use of these components in elderly patients can still be questioned. Compared to younger patients, this population has an increased incidence of osteoporosis, as well as nutritional and other metabohc abnormalities that m i g h t adversely affect the fixation of an uncemented component. In general, the decreased quality of the underlying bone could lead to early failures due to aseptic loosening. Due to these concerns, we elected to compared the results of uncemented acetabular components in an elderly population with the results from a younger group. These results of 207 consecutive THR utilizing uncemented acetabular components with a minimum follow-up of seven years were reviewed. At the time of the index operation, 31 of these patients were 75 years of age or older (ave. 78, range 75-87); the remaining 178 hips had an average age of 56 (range 17-74). All of the operations were performed by a single surgeon. During the time of the study, all of the acetabulae were prepared by hemispheric reaming, followed by insertion of the cup without under-reaming using an average of three screws for each component. Harris hip scores (HHS) and radiographic assessments were carried out at regular intervals, and the presence of radiolucencies, osteolysis, cup migration, and polyethylene wear were recorded for each component. At last follow-up all of the patients had obtained excellent clinical results. Average pre-operative HI-IS were 37 and 41 in the elderly (>75) and younger groups (<75) with average post-operative scores being 92 and 93 respectively. There were three revisions for aseptic loosening of uncemented stems and one for a fractured femur. Radiohicencies were seen in both groups, but in the elderly population 70% did not have any radiolucent lines, while in the younger patients 58% were free of radiolucencies. In both groups none of the radiolucent lines were assessed as being progressive. In the elderly group 61% were free of polyethylene wear, while this was the case for only 51% of the younger population.