Total hip arthroplasty remains one of the most effective and safe orthopaedic reconstructive procedures. A major focus over the past year has been the controversies regarding the safety of metal-on-metal articulation couplings and the associated adverse tissue reactions. Another area of focus has been on the outcome measures and practice protocols developed with evidence-based criteria. We have also included selected citations from the most recent published and presented data on the clinical outcomes and treatment of complications following total hip arthroplasty.
Clinical Outcome
Over the past year, there were many publications and abstracts focusing on the actual and potential deleterious effects of metal-on-metal bearings. Inferior outcomes have been reported in association with current large-head total hip replacements with metal-on-metal bearings. Smith et al.1 analyzed the National Joint Registry of England and Wales, which included 402,051 hips. Of these, 31,171 hips received a stemmed implant with a metal-on-metal bearing. The risks for failure were larger femoral head size and female sex. The five-year failure rate associated with larger heads was 5.1% for men and 6.1% for women, compared with 3.2% and 1.6%, respectively, for 28-mm heads. The failure rates for patients with ceramic-on-ceramic couplings were actually lower for larger heads (2.0%) than for 28-mm heads (3.3%). The authors recommended against the use of stemmed implants with metal-on-metal bearings. They also recommended that asymptomatic patients should undergo at least annual clinical and radiographic evaluations. In the study by Molli et al.2, 1589 hip arthroplasties (in 1352 patients) with large-head metal-on-metal bearings were compared with 779 arthroplasties (in 693 patients) with metal-on-polyethylene bearings. The revision rate was higher in the metal-on-metal group (4.0% versus 2.2%).
Discrepant data, however, have been reported. Meding et al.3 reviewed 681 arthroplasties (in 611 patients) that were performed with use of the same implant system as was used by Molli et al. The mean duration of follow-up was three years. The rate of cup failure was only 0.4%. Nine patients (1.5%) had hip pain. Greater failure rates may be observed with longer follow-up. Good clinical outcomes have been reported in patients with hip resurfacing arthroplasties and in those with total hip arthroplasties performed with smaller-head metal-on-metal bearings. Beaulé et al.4 reported only a 0.10% prevalence of pseudotumors in a study of 3432 hip resurfacing arthroplasties from nine Canadian centers. The mean duration of follow-up was 3.4 years. In the study by Migaud et al.5, thirty-nine hips with a Metasul implant (Zimmer, Warsaw, Indiana) were compared with thirty-nine hips with a ceramic-on-polyethylene bearing implant. The mean duration of follow-up was thirteen years. In the Metasul group, there were no revisions and the prevalence of osteolysis was 3%. In the ceramic-on-polyethylene group, the rate of revision because of bearing failure was 28% and the prevalence of osteolysis was 46%. The twelve-year survival rates in the Metasul and ceramic-on-polyethylene groups were 100% and 70%, respectively (p = 0.003). In the Metasul group, the median cobalt and chromium levels were 0.95 and 1.2 μg/L, respectively. The high rate of success and durability of the Metasul bearings were attributed to the metallurgy (wrought high-carbide cobalt-chromium alloy).
Cause of Failure
Most investigators believe that the adverse reactions seen in association with metal-on-metal bearings are due to bearing wear and corrosion at the taper junction. Glyn-Jones et al.6 examined thirty-six retrieved hip resurfacing implants. Half of these implants were retrieved from patients with a confirmed pseudotumor, whereas the other eighteen implants were retrieved from patients who did not have any soft-tissue lesions. The mean linear wear rate was significantly greater in the pseudotumor group (8.4 versus 2.9 μm/yr; p = 0.01). The mean volumetric wear rate was also higher in the pseudotumor group (3.3 versus 0.8 mm3/yr; p = 0.009). Moreover, edge wear was apparent in seventeen of the eighteen cups from the pseudotumor group, compared with six of eighteen cups from the group without pseudotumor (p < 0.001). However, there was no difference between the groups with regard to the cup abduction angle, anteversion angle, cup diameter, or femoral head diameter. Kinkel et al.7 analyzed forty-six retrieved metal-on-metal bearings from patients with failed total hip arthroplasties. They found scratching in 98% of the cups and 93% of the heads. Pitting was observed in 43% of the cups and 67% of the heads. Wear patches were seen in 5% of the cups and 40% of the heads. Some of the pitting was determined to be due to corrosion, particularly at the cup rim (three specimens).
Modularity provides versatility by allowing the surgeon to adjust limb lengths and offset and to use different femoral bearings during surgery. Increasing evidence has been reported on wear and corrosion at the taper junction. Takamura et al.8 analyzed the wear characteristics of 130 implants that had been retrieved following the failure of hip replacements with metal-on-metal bearings. All of the implants had been revised because of an adverse tissue reaction. There was wear in all of the taper junctions. Wear depth in some cases exceeded 100 μm. Wear volume loss was <4 mm3. Taper wear was observed in association with all of the stem designs. Elevated metal ion levels were found in 50% of the patients. Engh et al.9 analyzed corrosion at the taper junction in twenty-nine retrieved femoral heads from hips with metal-on-metal bearings. All of the heads were mated to the identical cobalt-chromium stem with a 12/14 taper. Eighty-six percent of the heads had some corrosion, with 42% showing severe to extreme corrosion. Corrosion extending beyond the head-neck taper junction area was correlated with adverse tissue reaction (p < 0.01). The length of in situ usage was correlated with the severity of corrosion (p = 0.012).
Imaging
Imaging assessment can be done with use of ultrasound, computed tomography (CT), or magnetic resonance imaging (MRI). Ultrasound is less sensitive in obese patients and for detecting lesions in the medial aspect of the hip joint. Williams et al.10 reported on the use of ultrasound to monitor three groups: (1) thirty-one patients with a metal-on-metal total hip arthroplasty, (2) twenty patients with a metal-on-metal hip resurfacing arthroplasty, and (3) twenty-four patients with a metal-on-polyethylene total hip arthroplasty. Pseudotumors were detected in all three groups. The overall incidence of pseudotumor and fluid collection was 43%, 30%, and 12% for the three groups, respectively. The difference between the first and third groups was significant (p = 0.015). Given the number of patients, there was no significant difference between the first and second groups (p = 0.755) or between the second and third groups (p = 0.077). There was no correlation between the serum metal ion levels and the occurrence or size of the pseudotumors. Hart et al.11 evaluated thirty symptomatic and twenty-eight asymptomatic patients with metal-on-metal bearings with use of metal artifact reduction sequence (MARS) MRI. There was no difference between the symptomatic and asymptomatic groups in terms of the prevalence of pseudotumors (57% compared with 61%). Moreover, there was no difference between the groups with regard to the characteristics of the pseudotumors or with regard to the cup position on CT scans. Bosker et al.12 used CT scans to evaluate 649 patients (732 hips) after a mean duration of follow-up of 2.7 years (range, one to 6.5 years). Pseudotumors were found in 31% of the hips. Revision was performed in 6% of the hips. Risk factors for pseudotumors included female sex, hip pain, clicking sensation, and the anterolateral surgical approach. Metal ion levels were only marginally higher in the patients with pseudotumors (cobalt, 10 versus 4 μg/L; chromium, 8 versus 4 μg/L). These reports further underscore the lack of specific evidence-based data with regard to establishing precise clinical protocols to monitor patients with metal-on-metal bearings.
Hauptfleisch et al.13 proposed an MRI-based classification system for pseudotumors around metal-on-metal bearings that included three types of lesions: thin-walled cystic masses (cyst wall, <3 mm) (Type I), thick-walled cystic masses (cyst wall, >3 mm) (Type II), and predominantly solid masses (Type III). Type-I lesions were the most prevalent and were more commonly located in the posterior aspect of the hip joint. Type-III lesions were generally larger and were more commonly located in the anterior aspect of the joint. The severity of symptoms was the least with Type-I lesions. Revision was most commonly associated with Type-III lesions. This classification system could serve as a predictor for revision and treatment recommendations, provided that it can be validated with larger patient populations and by other centers. It could also be of importance for predicting the outcome of revision surgery for the treatment of adverse tissue reactions around metal-on-metal bearings. Mehmood et al.14 reported on the outcome of revision following bearing failure in a study of thirty-three hip resurfacing and four total hip arthroplasties. Nine hips (24%), all in female patients, required another revision because of disease progression. All had progression of the pseudotumors, which were all of the solid type. One significant difference between the groups was the length of time between the index procedure and the revision (thirty-four months for those who required repeat revision, compared with 60.5 months for those who did not; p < 0.01). No metal ion levels were correlated with the clinical course.
Metal Ions
Controversies remain with regard to the upper limits of normal for the analysis of metal ions. The Medicines and Healthcare products Regulatory Authority (MHRA) of the United Kingdom has suggested a cutoff level of 7 ppb (parts per billion). Hart et al.15 reported on the metal ions in a group of eighty-eight symptomatic patients who were awaiting revision surgery. They also analyzed the metal ions in a matched group of asymptomatic patients with non-metal-on-metal bearings. Their data demonstrated 5 ppb to be the optimal upper limit for both cobalt and chromium levels. Malek et al.16 evaluated 209 consecutive symptomatic patients with metal-on-metal bearings with use of both MARS MRI and serum metal ion levels. Pseudotumors were present in 40% of the patients. The median cobalt level was 4.2 μg/L, and the median chromium level was 4.9 μg/L. The sensitivity was only 57% and the specificity was 64% with use of >7 μg/L as the upper limit. The receiver operating characteristic (ROC) curve analysis demonstrated 90% sensitivity at a cutoff value of 2 μg/L for both cobalt and chromium and demonstrated 90% specificity at a cutoff value of >16 μg/L for cobalt and of >11 μg/L for chromium. Griffin et al.17 analyzed the metal ion levels in eighty-four patients who underwent revision of failed arthroplasties with metal-on-metal bearings. The positive predictive values for cobalt and chromium were 46% and 26%, respectively. There was a trend of higher metal ion levels correlating with greater histological tissue damage grading (cobalt, p = 0.50; chromium, p = 0.99). The length of time to revision was correlated with the severity of tissue damage (p = 0.05).
Several studies focused on the metal ions following revision surgery. Ebreo et al.18 reported that the metal ion levels decreased to below the upper limit at one year following revision surgery. Brown et al.19 reported on the metal ion levels in sixteen patients at three to six months following revision surgery. The mean cobalt level decreased from 28.7 to 6.4 μg/L (p = 0.004). The mean chromium level decreased from 16.9 to 12.2 μg/L (p = 0.009). Takamura et al.20 compared the pre-revision and post-revision metal ion levels in twenty-eight patients with either hip resurfacing arthroplasty (seventeen patients) or total hip arthroplasty (eleven patients). The mean serum cobalt level decreased from 67.2 to 2.74 μg/L (p < 0.0001). The mean chromium level decreased from 43.3 to 9.5 μg/L (p < 0.0001). The reduction was observed an average of 4.3 months following revision.
Presently, there is no consensus with regard to the optimal cutoff level for serum metal ions in the evaluation of symptomatic patients with metal-on-metal bearings. Moreover, nearly half of the patients with documented pseudotumors do not have elevated metal ion levels. Further research is necessary to determine the clinical utility of metal ion analysis in the follow-up of asymptomatic patients.
Quality outcome measures for both hospitals and orthopaedic surgeons have been developed and evaluated for total joint arthroplasty. As total hip and knee arthroplasties combined account for the largest procedural cost to the Medicare budget of the Centers for Medicare and Medicaid Services (CMS), they are a prime focus for the identification of quality measures.
CMS has developed two hospital-level quality outcome measures for patients undergoing elective primary total hip and knee arthroplasty: (1) the risk-standardized complication rate and (2) the risk-standardized thirty-day all-cause readmission rate21. The measures are calculated with use of Medicare claims data (Parts A and B). The measures use hierarchical logistic regression modeling to adjust for differences in patient case-mix and to account for the clustering of patients within a hospital. Variables in the risk-standardized model include age, procedure type (hip versus knee), number of procedures (one versus two), and comorbid conditions. These variables may help to predict the rates of complications and/or readmission.
The measures only include primary hip or knee arthroplasties, not arthroplasty for the treatment of hip fractures, revision arthroplasties, hemiarthroplasties, resurfacing arthroplasties, or bilateral arthroplasties. The resultant complication and readmission rates following risk adjustment are compared with national norms. The complication measure identifies complications coded with use of ICD-9 (International Classification of Diseases, Ninth Revision) codes during the index hospital admission or subsequent admissions within seven to ninety days after discharge. Specifically, it assesses quality on the basis of the outcome of one or more of the following complications: acute myocardial infarction, pneumonia, or sepsis/septicemia/shock within seven days after admission; surgical site bleeding requiring reoperation, pulmonary embolism, or death within thirty days after admission; or mechanical complications, periprosthetic joint infection, or wound infection requiring debridement in the operating room within ninety days after admission. The readmission measure captures readmissions to acute-care hospitals for any reason within thirty days after the discharge date following the index operation. This measure does not include planned readmissions for another elective arthroplasty.
Unadjusted complications and readmission rates are relatively high for patients who undergo joint arthroplasty. The mean complication rate based on Medicare claims data in 2008 was 4.90%. After adjustment for patient and clinical characteristics, the mean hospital-level complication rate was 4.23% (range, 2.20% to 8.88%). The unadjusted mean readmission rate was 6.78%, and the adjusted hospital-level mean readmission rate was 6.30% (range, 3.06% to 50.94%).
The measures were developed with extensive expert and public input. Yale New Haven Health Services Corporation/Center for Outcomes Research and Evaluation (CORE) led the development of the measures. CORE obtained expert input via an advisory group comprising orthopaedic surgeons and experts in orthopaedic quality measurement. It also convened a national technical expert panel of clinicians, consumers, hospitals, purchasers, and experts in quality improvement and obtained public comment on the measures. Last, CORE modified the measure cohort exclusions and specifications on the basis of a medical record validation study of the ICD-9 codes used to identify complications. The measures are endorsed by the National Quality Forum (NQF).
By identifying hospitals with high rates of complications and/or readmissions relative to their peers, quality measures can be used to target quality-improvement efforts. Hospitals can identify and address specific problems in their arthroplasty protocols or in their approaches to transitioning patients to the outpatient setting. Even though these hospital measures do not assess individual surgeon performance, they should provide highly accurate and insightful information on how the surgeons’ practices and hospital outcomes compare with national norms. The measures will place increased emphasis on potentially helpful systems-based changes in hospital coding, physician and physician extender documentation in the medical record, preoperative and postoperative medical evaluation and management, and close surgical follow-up. The ultimate goals are improved patient care and cost savings.
The editorial staff of The Journal reviewed a large number of recently published research studies related to the musculoskeletal system that received a Level of Evidence grade of I. Over 100 medical journals were reviewed to identify these articles, which all have high-quality study design. In addition to articles published previously in this journal or cited already in this Update, five Level-I articles were identified that were relevant to total hip arthroplasty. A list of those titles is appended to this review after the standard bibliography. We have provided a brief commentary about each of the articles to help to guide your further reading, in an evidence-based fashion, in this subspecialty area.