American Academy of
Orthopaedic Surgeons Annual Meeting
Day 3 - March 17, 2000
Issues in Wear: Bearing Surfaces
William J. Hozack, MD and
Fabio Orozco, MD
Wear debris, be it polyethylene (PE), metal or ceramic, continues to determine the ultimate longevity of total hip arthroplasty (THA). Orthopaedic surgeons still do not have adequate clinical data to determine which bearing surface is "the best." Various papers presented here on the third day of sessions underscore the existing controversy.
Figure 1. Migration of polyethylene particles. Credit - © W. HeimAlloys of cobalt and chromium have traditionally been preferred for metal-metal bearings in THA. Little is known about the rates of in vivo production, lymphatic transport and systemic dissemination of metallic particles. The metal wear particles are even smaller than the PE wear particles. Levels of metal ions in blood and urine of patients who have undergone metal-metal THA are generally higher in the initial "running in phase" corresponding to the first 2 years after surgery. The clinical importance of this trace metal elevation has yet to be established.
Cobalt and Chromium Blood Concentrations in Patients With Metal-Metal Pairings
Lhotka and colleagues[1] compared metal blood levels of patients who had undergone a metal-metal total hip arthroplasty with metal-metal pairings from different manufacturers. In this study, blood cobalt and chromic levels were determined in 219 patients (mean age, 54 years). Patients were assigned to 1 of 3 groups. The first group was the control group (31 patients). A second group had received metal-metal articulating surfaces from manufacturer I (94 patients). The third group received metal-metal articulating surfaces from manufacturer II (94 patients).The analysis was performed with Graphite Tube Atom Absorption Spectrometry (GFAAS). Patients were divided into 4 groups: a postoperative group; a 3- to 6-month group; a 12- to 15-month group; and a 35- to 38-month group. During the observation period, the different material combinations were associated with different blood cobalt and chromium concentrations. Metal levels in group 3 (manufacturer II) had a lower baseline value, but significantly increased within 15 months (P<.018). At 35 to 38 months, patients in groups 2 and 3 showed a statistically significant difference for cobalt and chromium (P<.012). The authors concluded that there was strong evidence of a time- and product-related metal abrasion pattern in hip implants with metal-metal articulating surfaces.
Conclusions
The levels of metal ions in blood and urine are elevated in patients who have undergone metal-metal THA.
These levels are generally higher in the initial "running in phase" and it is during this period in which the greatest variability among designs and manufacturers is seen.
The clinical significance (toxicity and carcinogenesis) of high blood levels of cobalt and chromic after metal-metal total hip arthroplasty has not been established and therefore long-term studies are needed.
Wear Between Polethylene Lines Articulating With Ceramic or Cobalt-Chrome Femoral Heads
Christi J. Sychterz, MD, and colleagues[2] compared in vivo wear of PE liners articulating with ceramic and cobalt-chrome femoral heads. The authors radiographically compared the wear performance between 81 alumina ceramic femoral heads and a matched group of 43 cobalt-chrome (CoCr) femoral heads. The ceramic group included 81 hips followed for a mean 7.0 years. The CoCr group included 43 hips followed for a mean of 6.8 years. Head penetration into the PE at annual intervals was measured.At 7 years, the PE wear rate of the ceramic group was slightly greater (0.09 ± 0.07 mm/year) than that of the CoCr group (0.07 ± 0.04 mm/year). Sychterz affirmed that although ceramic heads typically outperform CoCr heads during in vitro testing, the in vivo wear performance of both heads was surprisingly similar in this study. The multifactorial nature of the PE wear process in vivo may explain this finding. The authors concluded that ceramic femoral heads offer a little advantage over CoCr heads in decreasing in vivo PE wear.
Implications for Clinical Practice
Most in vitro laboratory and hip simulator studies have demonstrated a clear advantage of ceramic heads over metal heads.
Long-term studies are needed to confirm that the in vitro performance of femoral heads can be achieved in vivo circumstances.
These clinical studies need to control for the multiple patient factors that might influence wear rates, specifically patient activity
Prosthetic Offset and PE Wear in THA
Polyethylene wear is a multifactorial process after THA. Patient-related factors, including age, gender, weight and level of activity and implant related factors, including quality and thickness of the PE insert, composition of the articulation, size of the femoral head, cup angulation and amount of offset, all can influence PE wear.Sakalkale and colleagues[3] evaluated the correlation between offset reconstruction and PE wear in THA. Seventy-five consecutive primary cementless THAs using 28 mm, CoCr heads were grouped as follows:
Group I included 40 hips with difference in offsets (preoperative offset minus postoperative offset) less than 5 mm. Group II included 35 hips with difference in offsets of at least 5 mm. The average follow-up periods were 60 months for group I hips and 68 months for group II hips. The mean cup angle was 47° for group I and 49.8 mm for group II. The mean rate of linear PE wear was 0.16 mm/yr for group I and 0.25 mm for group II. This was a statistically significant difference (P =.02).
Preoperative offset minus postoperative offset and the PE wear rate were positively correlated, with a correlation coefficient of +0.47.
Group I, having postoperative offset more closely restored to the preoperative offset, had reduced PE wear. Group II, which failed to restore preoperative femoral offset, had greater PE wear rate influenced by suboptimal hip mechanics.
The authors concluded that reconstruction of appropriate offset in THA in an effort to restore the natural biomechanics of the hip is associated with a lower rate of PE wear.
Implications for Clinical Practice
Appropriate restoration of the offset in THA to resemble the natural biomechanics of the hip is associated with a lower rate of PE wear. Reduction in Wear: Highly Crosslinked PE Acetabular Liners vs Ceramic/Ceramic Bearings
Manley and colleagues[4] compared the wear as measured in a hip simulator with highly crosslinked PE / metal bearings and modern ceramic / ceramic bearing couples. This hip simulator study demonstrated a reduction in wear of about 90% with the highly crosslinked PE bearings and 99% with the ceramic / ceramic bearings when compared with conventional PE cup liners.The material properties of the ceramic / ceramic bearings met the standards contained in the regulatory guidance document for ceramic material. The material properties of the crosslinked PE were similar to those of conventional PE. If the crosslinked PE was melted and re-crystallized during the manufacturing process, however, a substantial reduction in toughness and creep properties was developed.
The clinical results at 2 years indicated that first-generation crosslinked PE bearings wearing at the rate predicted in the hip simulator. Clinical findings at 2 and 3 years' follow-up with the ceramic / ceramic bearings showed no measurable wear and no catastrophic ceramic failures.
According to Manley, the volume removed in 5 million wear cycles was 177.6 mm3 for CoCr/UHMWPE and 0.1 mm3 for Alumina/Alumina, a reduction of 1700x. The authors concluded that both bearing couples have acceptable material properties and will significantly reduce wear in the hip.
Implications for Clinical Practice
Short-term results with the use of highly crosslinked PE and ceramic bearings are promising, but long-term clinical trials are necessary to elucidate the ultimate outcome with the use of these materials.
Ceramic bearings have demonstrated the lowest in vitro/in vivo rates to date of any bearing combination. New Polys for Old: Contribution or Caveat?
Greenwald and colleagues[5] presented a scientific exhibit evaluating the history and current state of affairs in PE use. The authors reviewed the currently available PE types, structure, sterilization and storage mechanism of common PE bearing surfaces used today.
Results
For the past 30 years, UHMWPE have been predominantly sterilized by gamma irradiation in air and have shown remarkable overall resilience in clinical function.
Irradiation in an environment where oxygen is present encourages UHMWPE component oxidation resulting in embrittlement and a decrease in wear performance. This process continues when components are shelf-stored in air or in permeable packing for prolonged periods before use.
Sterilization in oxygen-free environments with barrier packing and shelf dating reduce the prospect of material compromise.
The use of ethylene oxide and gas plasma as alternative sterilization methods avoid oxidative degradation but do not realize potential benefits with respect to polymer wear reduction derived from cross-linking.
Recently "improved" polymers have emerged, with increased cross-linking, minimizing oxidation. Hip simulator models suggest a significant reduction in wear with these new polymers.
Currently, there is no direct clinical experience to support the promised long-term in vivo integrity suggested by in vitro performance of cross-linked PE. Implications for Clinical Practice
Recently "improved" polymers have emerged, with increased cross-linking, minimizing oxidation. Hip simulator models suggest a significant reduction in wear with these new polymers.
Currently, there is no direct clinical experience to support the long-term in vivo integrity suggested by laboratory studies of cross-linked PE. Long-term trials are needed to evaluate in vivo performance of these new materials. Zirconia Ceramic vs Alumina Ceramic in THA
On the last day of this year's AAOS annual meeting, Hernigou [6] will be presenting on a study in which 40 hips with 2 different ceramics were followed for 10-years. In half the cases the femoral head was made of dense alumina (Al2 O3) while in the other half, the femoral head was made of yttrium oxide partially stabilized zirconia. Hopefully his study will shed some light on the in vivo wear rate of zirconia in THA.
References
- Lhotka C, Steffan I, Zhuber K, Zweymuller. Determination of cobalt and chromium blood concentrations in patients with difference metal-metal pairings in total hip joint endoprostheses. In: Program and abstracts of the 67th annual meeting of the American Academy of Orthopaedic Surgeons; March 15-19, 2000; Orlando, Fla. Paper No. 68.
- Sychterz CJ, Engh CA Jr, Yang AM, Hopper RH, Engh CA Sr. Comparison of in vivo wear between polethylene lines articulating with ceramic and cobalt-chrome femoral heads. In: Program and abstracts of the 67th annual meeting of the American Academy of Orthopaedic Surgeons; March 15-19, 2000; Orlando, Fla. Paper No. 201.
- Sakalkale DP, Lombardi Jr. AV, Mallory TH, Kefauver CA. Correlation between prosthetic offset and polyethylene wear in THA. In: Program and abstracts of the 67th annual meeting of the American Academy of Orthopaedic Surgeons; March 15-19, 2000; Orlando, Fla. Paper No. 198.
- Manley MT, Capello WN, D'Antonio JA, Taylor SK, Wang A. Reduction in wear in total hip replacement: highly crosslinked polyethylene acetabular liners versus ceramic/ceramic bearings. In: Program and abstracts of the 67th annual meeting of the American Academy of Orthopaedic Surgeons; March 15-19, 2000; Orlando, Fla. Scientific Exhibit No. SE022.
- Greenwald AS, Ries MD, Duncan CP, Jacobs JJ, Stulberg BN. New polys for old: contribution or caveat? AAOS committees on biomedical engineering and hip & knee arthritis. In: Program and abstracts of the 67th annual meeting of the American Academy of Orthopaedic Surgeons; March 15-19, 2000; Orlando, Fla. Scientific Exhibit No. SE057.
- Hernigou P, Bahrami T. Osteolysis and acetabular waer of polyethylene against zirconia ceramic in comparison to alumina ceramic in total hip arthroplasty. In: Program and abstracts of the 67th annual meeting of the American Academy of Orthopaedic Surgeons; March 15-19, 2000; Orlando, Fla. Paper No. 277.
