Modular to Monoblock: Difficulties of Detaching the M2 a-MagnumTM Head Are Common in Metal-on-metal RevisionsMäntymäki, Heikki1,5,a; Mäkelä, Keijo, T.1; Vahlberg, Tero2; Hirviniemi, Joni3; Niinimäki, Tuukka4
Background Modern hip implants typically feature modular heads, which allow for easy exchange and removal from the femoral stem at the time of revision. However, owing to fretting, corrosion, or cold welding, the modular head may be difficult or impossible to separate from the underlying trunnion, especially if the implant has titanium interfaces between the head and the stem. We have repeatedly encountered difficulty removing the titanium sleeve adapter in the M2a-MagnumTM implant. Although the manufacturer warns about this complication and cases with these difficulties have been reported to the United States FDA, we believed this topic is important to study, because the frequency of difficulties in head removal is unknown and the complications related to this event have not been characterized.
Questions/purposes We asked: (1) Do revisions of M2a-MagnumTMimplants differ from those of M2a-38TM implants in terms of ease of removal of the femoral head? (2) In cases where difficulty with M2a-MagnumTM head removal occurred, was the operative time, bleeding, risk of periprosthetic fracture, or joint infection increased compared with cases where the M2a-MagnumTM head was removed without difficulties?
Methods Between 2004 and 2014, we revised 296 THAs with metal-on-metal implants that involved M2a-MagnumTM (123) or M2a-38TMheads (88); of those, 84 were planned to include a femoral stem revision and insufficient data were available for three operations, so they were excluded from this analysis, leaving 124 THAs in the current retrospective study (70 THAs with M2a-MagnumTM and 54 THAs with M2a-38TM heads).The method of modular head removal, any difficulties removing the femoral head from the trunnion, operation time, and complications were recorded based on chart review.
Results All the observed problems of detaching the head or taper adapter were among M2a-MagnumTM heads; there were no problems detaching the head in revisions of the M2a-38TM implant. In 29% (20 of 70) of revisions of the M2a-MagnumTM implant, the modular head could not be detached by knocking it with a punch and a mallet. Seventeen percent (12 of 70) of hips needed an unplanned stem revision owing to difficulties with head removal. In revisions of the M2a-MagnumTM implant that experienced head-removal problems, the median operative time was longer (144 minutes; range, 75-274 minutes) and bleeding was greater (725 mL; range, 300-2200 mL) compared with revisions of the M2a-MagnumTM implant without head removal problems (77 minutes, range, 33-197 minutes, p < 0.001; 475 mL, range, 50-1500 mL, p = 0.004). With the numbers available, we did not see differences in terms of the proportion of patients experiencing major complications (periprosthetic fracture or postoperative infections) between the groups (difficult versus easy; 25% [five of 20] versus 8% [four of 50]; odds ratio, 3.8 [95% CI, 0.9-16.2], p = 0.067).
Conclusions The titanium-titanium taper junction can be very difficult to separate during revision THAs, and if not anticipated, this problem can result in larger and more complicated revision procedures in patients who have the M2a-MagnumTM implant. Although the global use of metal-on-metal implants in THAs has decreased dramatically during the last several years, many thousands remain in service and therefore still might require revision. It is crucial to be prepared with special tools, including a femoral head extraction tool and diamond saw. The patient has to be informed of the possibility of a more extensive operation than preoperatively planned.
Level of Evidence Level III, therapeutic study.