Abstract
Background
Large-head metal-on-metal (MoM) total hip arthroplasties (THA) are associated with high failure rates and possible pseudotumour formation. This study reports the first results of 160 Biomet Magnum M2 large-head MoM total hip articulations.
Patients and methods
From 2006 to 2010 the Reinier de Graaf Hospital implanted 160 large-head Magnum M2 MoM THAs (Biomet Inc. Warsaw, Indiana, USA) in 150 patients. These patients were recalled after a warning from the Dutch Orthopaedic Association. Patients were offered a clinical and radiographic assessment of the hip prosthesis, serum control on cobalt and chromium ions, and an ultrasound of the hip. If indicated, additional MARS-MRI or CT scan was performed. Descriptive statistical analysis, correlations, t-tests, non-parametric tests and implant survival were calculated.
Results
The mean follow-up was 6.1 years (4.8-8.4). A cumulative survival rate of 93.1% (95% CI: 88.3-98%) was found after 5 years. Reasons for revision were loosening, pain, infection and pseudotumour formation. The prevalence of pseudotumour formation around the prostheses was 8.75%.
Conclusions
This study reports the first results of 160 MoM THAs implanted in our clinic from 2006-2010. In total, 13 (8.1%) of the THAs were eligible for revision after the recall. In most patients the reason for revision was pseudotumour formation. A total of 14 (8.75%) pseudotumours were diagnosed at the first recall. These results show that a comprehensive follow-up strategy is essential for MoM THAs to promptly identify and manage early complications.
Introduction
Metal-on-metal (MoM) total hip arthroplasties (THA) were introduced as long-lasting total hip prostheses, especially for young patients. Better function, decreased implant wear and higher survival rates were expected (1, 2). In England, more than 60,000 patients have received a MoM THA since 2003 (3). However, since the safety alert from the Medicines and Healthcare product Regulatory Agency (4), the use of these articulations has decreased both in relative and absolute terms.
Multiple studies have shown high revision rates with the MoM resurfacing prostheses as well as with the MoM THAs by early failures and pseudotumour formation (2, 5-6-7). Metal debris due to high wear can lead to elevated serum levels of cobalt and chromium which may result in local tissue reactions around the prostheses causing the formation of pseudotumours. Histological characteristics of the pseudotumour tissue are described as ‘aseptic lymphocyte dominated vasculitis associated lesion’ (ALVAL) (5). Pseudotumour formation can results in early failure of the MoM articulations, their prevalence however, is still unknown (8). Design characteristics, component size, surgical implant factors, such as inclination of the acetabular component and female gender have already been documented as risk factors for pseudotumour formation and early failure of the MoM articulations (9, 10).
In 2011 the Dutch Orthopaedic Association (Nederlandse Orthopedische Vereniging) began a recall of all MoM articulations in The Netherlands. Their advice included an active recall of all MoM articulations as well as active follow-up.
This study reports the first results of this recall of all large-head MoM THA implanted in our clinic from 2006 to 2010.
Materials and methods
Patient demographics
In the Reinier de Graaf Hospital (Delft, The Netherlands) 160 primary large-head MoM THA were implanted in 150 patients between 2006 and 2010. The THAs were performed by 2 surgeons and implant selection was based on surgeon experience.
Implants and operative technique
All patients received the Biomet Magnum (M2a-Magnum) prostheses with Recap cup and Taperloc (Taperloc®Hip Stem) or Mallory stem (Mallory®) (all Biomet Inc. Warsaw Indiana, USA). Surgery was performed through either an anterior supine intermuscular (ASI) approach or through a straight lateral approach. All ASI-patients were study patients for a prospective ASI-study while the Mallory stems were used in an RSA Mallory study. During the first 24 hours postoperatively antibiotic prophylaxis was given. Low-molecular-weight heparin was given for 6 weeks postoperatively.
Metal ion analysis
Whole blood samples were obtained from all patients in trace-element free tubes. Whole blood cobalt (Co) and chromium (Cr) ion levels (nmol/l) were measured by mass spectrometry. Advised by the Dutch Orthopaedic Association, the cobalt ranges were set as normal <40 nmol/l (<2 mmg/L), normal high 40-85 nmol/l (2-5 mmg/L), high 85-170 nmol/l (5-10 mmg/L) and extreme high >170 nmol/l (>10 mmg/L). Patients with a bilateral MoM THA were excluded in measurements.
Clinical analysis
All patients were asked if they experienced pain in the groin, suffered from deafness, dizziness, fear behaviour/depression or neurological problems after the surgery. Moreover, the Harris Hip Score was used to evaluate all hip disabilities. The HHS was divided into 5 categories (90>100 excellent, 80>90 good, 70>80 fair, 60>70 poor, <60 really poor). Anteroposterior pelvic and lateral hip radiographs were obtained and reviewed by a specialist radiologist. Radiographs were assessed for osteolysis, bone resorption, radiolucent areas and component migration. The angle of inclination of the acetabular component was measured by 2 authors (MCK, NM) using the transischial line and a second line drawn across the rims of the cup. Also, all patients received an ultrasound scan of the hip by a specialist radiologist. Features of liquid in or around the joint as well as space occupying masses were measured.
Additional analysis
If the patient experienced hip pain, elevated serum cobalt levels >40 nmol/l or if radiographic imaging and/or ultrasound analysis were abnormal, a metal artefact reduction sequence (MARS)- magnetic resonance imaging (MRI) or computed tomography (CT) was obtained. The diagnosis pseudotumour was based on a combination of clinical presentation, ultrasound, MARS-MRI and/or CT (Fig. 1).
Flowchart created in the Reinier de Graaf Hospital for control and follow-up of MoM articulations. The yearly control include anamnesis, physical exam, radiographic control and serum tests. Ultrasound was only done during our recall. If any anomalies were found we suggest a MARS-MRI and with suspicion of pseudotumour we advise revision surgery.
Statistical analysis
Descriptive statistical analysis, correlations, t-test, non-parametric tests and an implant survival were conducted using statistical software (PSAW 18.0, Chicago, Illinois). Confidence intervals for the Kaplan-Meier method were set on 95%. Revision for any reason was defined as implant failure, and calculated as the time between ‘date of operation’ and ‘date of revision’. Patients who died without revision were censored at the date of death.
Results
Patients and demographics
In total, 160 MoM THAs were evaluated of which 10 patients had bilateral MoM articulations. 9 patients died of unrelated causes, 1 had a bilateral MoM articulation. 4 patients were lost to follow-up or refused to participate. A total of 152 hips were diagnosed with primary osteoarthritis, other demographic features are shown in Table I. The cohort contained 82 women (89 hips) and 68 men (71 hips) with a mean age at surgery of 62 years (range 22-85 y). The mean follow-up was 6.1 years (4.8-8.4) with an overall survival of 90.6% (CI: 82.9-98.7%).
Patient demographic features and radiological parameters
As of July 2014, a total of 15 hips (9.4%) were revised (13 being revised only after the recall). The mean time to revision was 3.7 years (range 0.5-5.4 y). The 2 earlier performed revisions were due to infection and loosening. Of the later 13 revised prostheses, 7 (53.8%) patients experienced pain, 2 (22.2%) were revised due to loosening, and a total of 10 (76.9%) patients were positive for pseudotumour formation. Within the revised prostheses, there was no significant difference in component size (p = 0.29).
4 patients with a possible pseudotumour refused revision surgery and are closely monitored. 1 patient died, unrelated to the pseudotumour. After the recall, a total of 12 patients, 14 prostheses (8.75%), were diagnosed with pseudotumour formation with a mean implantation time of 4.9 years (2.8-6.7). The cumulative survival rate of the large-head Biomet MoM THA at 5 years is 93.1% (CI: 88.3-98%). There was no significant difference in survival between the 2 used stems nor in gender (data not shown).
Clinical analysis
The patients with revised hip prostheses scored a mean Harris Hip Score in the group ‘fair’ (70>80).
The mean HHS for the total group was excellent. Pearson Chi-square tests showed patients with pseudotumour formation had a significantly (p = 0.007) lower HHS compared to patients without pseudotumour. 7 of the 13 revised prostheses (53.8%) experienced pain in the groin, compared to 21 (18.3%) of the non-revised patients (p<0.001).
Of the 12 patients (14 hips) diagnosed with pseudotumour, 6 (46.2%) experienced pain in the groin, compared to 20% of the patients not diagnosed with pseudotumour formation (p = 0.054).
No significant differences were found between the revised patients or pseudotumour formation group and the total cohort concerning deafness, dizziness, fear behaviour or neurological problems.
Radiographic analysis
None of the antero-posterior pelvic and lateral hip radiographs showed signs of bone resorption, lysis or fractures. Loosening of the cup was seen in 1 patient. The lateral cup inclination had a mean (SD) angle of 44.3° (9.4) in the revised patients and 40.9° (7.3) in the total group (Tab. II). Patients diagnosed with pseudotumour formation scored a mean inclination angle of 45.6° (7.0) (p = 0.005 and p = 0.061 respectively). The intra-observer and inter-observer reliability was evaluated using the intra-class correlation coefficient (ICC) and calculated 98.5% (95% CI: 98.0-98.8%) within the measurements.
Clinical outcome; all patients and all revised patients
Metal ion analysis
The mean (SD) level of serum cobalt was 42.3 nmol/l (127.7) and of serum chromium 43.7 nmol/l (52.8) (Tab. II). The median (IQR) level of serum cobalt was 23.8 nmol/l (14.1-39.0) and of serum chromium 31.7 nmol/l (19.3-54.6).
There was a significant increase in cobalt level in the pseudotumour patients (p<0.05), however no significant difference was found in chromium level (Tab. II). There were no differences in the revised group.
The median cobalt level for males was 16.7 nmol/l (8.5-32.6) and a median chromium level of 22.2 nmol/l (15.4-37.8). For females, the median level of cobalt in serum was 26.7 nmol/l (19.4-44.6) and the median chromium serum was 38.0 nmol/l (23.4-4.2). No significant differences in cobalt or chromium level between men and women were found.
Ultrasound analysis
In 19 (14.1%) of the 135 performed ultrasounds aspects of possible pseudotumour formation were seen. A total of 26 (19.3%) patients was diagnosed after ultrasound with fluid around the joint or capsule. After performing an MRI, 2 of the 19 diagnosed pseudotumours were positive and revised.
8 patients (10 hips) did not have ultrasound, but MRI or CT was performed immediately. The ultrasound was not conclusive in 3 patients because of obesity (Tab. II).
CT and MARS-MRI
A total of 47 MARS-MRIs and 28 CT-scans were obtained in 72 hips. 11 hips (23.4%) were positive for pseudotumour formation on the MARS-MRI of which 6 prostheses have been revised. 4 patients refused revision and 1 patient died during follow-up. In 13 patients (27.7%) the MARS-MRI showed ‘fluid around the joint’, 2 of them were revised and appeared to be positive for pseudotumour tissue. In 23 patients (48.9%) the MRI showed no sign of pseudotumour formation. CT scan investigation of 28 hips showed a pseudotumour in 2 (7.1%) patients and underwent revision. In 24 hips (85.7%) no pseudotumour was found and 2 (7.1%) hips showed fluid around the joint or capsule.
Table III shows the MARS-MRI findings of the revised and monitored patients in relation to the mean serum levels of cobalt and chromium (nmol/l). The groups are too small for any significant differences.
MARS-MRI findings of the revised and monitored patients. Results of the MARS-MRI are presented in relation to the mean serum levels of cobalt and chromium. The 5 patients positive for pseudotumour and no revision are closely monitored. 1 patient died, unrelated to the tumour
Discussion
Multiple studies on survival of the large-head MoM THA have shown early failures, pseudotumour formation and high revision rates (2, 4, 6). Since the warning of the National Joint Registry of England and Wales in 2010 MoM articulations have been closely monitored (2). The Dutch Orthopaedic Society advised centres in the Netherlands to recall all patients who had received a MoM articulation. Clinical assessment of the hip, measurement of serum inflammatory markers, serum cobalt and chromium levels as well as a radiographic evaluation and ultrasound of the hip were advised.
Results of survival studies of the Biomet MoM prostheses vary widely. Puolakka et al showed a poor survival of Biomet cementless prostheses, mostly related to a poor cup survival (11). The Mallory head cup had a survival of 98% after 5 years. Bosker et al showed a survival of 88% of the M2a-Magnum articulation after a mean follow-up of 3.6 years (12). In comparison, Meding et al and Sturup et al both showed a low incidence of groin pain, adverse reactions and no early failure of the prostheses (13, 14). However they do not define survival rate clearly.
A study of Bolland et al reported a cumulative survival rate (with revision for any reason) of 92.4% at 5 years for a large hybrid MoM THA (15), a survival rate comparable to the survival rate of metal-on-polyethylene bearings and our results.
The exact incidence and prevalence of pseudotumours around MoM articulations is still unknown and varies widely (8, 16, 17). Some believe early revision of the MoM THA is recommended when diagnosed with pseudotumour formations (7, 18). In contrast, Hart el al state that pseudotumour formation alone is not an absolute indication for revision. In their cohort, the prevalence of pseudotumour formation in patients with a painful hip is 57% compared to a prevalence of 61% in the control group (19, 20). Kwon et al found a prevalence of pseudotumour formation of 5% in screened asymptomatic patients with a hip resurfacing with a mean follow-up of 61 months (20).
Our study showed a prevalence of pseudotumour formation of 8.75% (14 hips) after a mean follow-up of 6.1 years (4.8-8.4). The first clinical signs of pseudotumour formation are pain, mostly starting in the groin, and discomfort (7, 21). Other possible signs are deafness, fear or dizziness (16).
In our cohort, the number of patients experiencing pain in the groin was significant higher (p<0.001) in patients with a pseudotumour (46.2%) when compared to patients without pseudotumour formation (20%). Patients with pseudotumour formation had a significant (p = 0.007) lower HHS and experienced significantly more pain in the groin. We did not find any relation between pseudotumour formation and deafness, fear or dizziness. However, only 47 (31.3%) MARS-MRIs and 28 (18.7%) CT scans were obtained in 72 (48%) patients, meaning possible asymptomatic patients with pseudotumour formation can be missed. Of the 13 revised prostheses in our cohort, 10 (77%) were positive for pseudotumour formation.
Pseudotumour formation seems also to be associated with a significant elevated cobalt and chromium serum level (22-23-24). The high levels of cobalt and chromium are especially found in large-diameter MoM articulations, suggesting that pseudotumour formation is associated with high wear and corrosion (25-26-27). The high wear and edge loading might be the result of suboptimal positioning or poor design of the components (16, 28). Excessive inclination, with an inclination angle greater than 55° and a small size, increases edge loading and may result in high wear and local debris (29, 30). An inclination of the acetabular component of more than 55° is related to increased serum cobalt and chromium levels. A study of Kwon et al report in all revised prostheses due to pseudotumour formation signs of edge loading (31). Pandit et al claimed that metal wear particles were detected in every case, even with well positioned implants (7).
The effects of metal wear particles and elevated serum metal have been documented but are still not understood (8). High serum levels of cobalt and chromium were known to have evolved during the running-in phase of the prostheses (32). However, these elevated levels still persist at long-term follow-up (33). During the steady state phase, when wear rate is constant, metal ion levels seem to reach a steady state (8). Healthy controls have a mean serum cobalt level of 4.07 nmol/L and a mean serum chromium level of 5.38 nmol/L (34). We only found a significant increased (p<0.05) serum cobalt in patients with a pseudotumour. However no relation between increased metal ion levels and risk of revision has been reported (8). Moreover, a high serum level of cobalt and chromium is not by definition caused by pseudotumour formation (35).
Glyn-Jones et al reported women are more at risk of developing pseudotumours than men (17). Mont et al showed that pseudotumours develop more often in women after hip resurfacing arthroplasty (36). The exact reason is still unknown, however the naturally small acetabulum in women requires small sized components with a greater change of malposition and thereby a greater change in wear and elevated ion levels. Venditolli et al reported higher serum levels cobalt and chrome in women (37), other articles also relate the high levels of cobalt and chromium with increased pain (26). Our cohort showed no relation between the serum level of cobalt and chromium between gender or with increased pain. There is also no difference in HHS.
We described the first results of large-head Biomet Magnum M2 MoM THA, including the short term survival, complications and failures during the first recall. It seems that cobalt and chromium are important metal ions relating to the formation of pseudotumours and are useful markers for follow-up. Although pseudotumour formation in normal functioning hip prostheses is not a clear statement for revision, we do recommend considering revision in order to prevent future complications. The patients in our cohort, diagnosed with pseudotumour formation and a poor HHS, were offered revision surgery. Patients with pain only were offered active physiotherapy and close follow-up (Fig. 1). We also recommend close follow-up in other hard-on-hard bearings as they can also result in adverse local tissue responses as seen in MoM THA. Concerning MoM articulations, we recommend future implantation of these prostheses only in a research setting.
Footnotes
Financial support: None.
Conflict of interest: None.