Unexplained hip pain and ALTR after MOM THA can result in an unacceptably high rate of early cup loosening and revision [3
]. In response to these concerns, both the Medicines and Healthcare products Regulatory Agency in the United Kingdom and the US Food and Drug Administration have issued warnings that patients with unexplained hip pain or prosthetic malposition after MOM THA should be monitored closely with clinical and radiographic evaluation and metal ion testing. Due to these early failures, we evaluated LD-MOM THA by determining the incidence of (1) hip pain, (2) function, (3) osteolysis, and (4) complications in patients with LD-MOM THA.
There are limitations to this study. First, the followup is still relatively early. However, in a review of 39 failed MOM THAs, Browne et al. [9
] noted 20 failures (51%) occurred before 24 months. No failures were evident at 24 months in our study. Second, we did not use advanced imaging techniques, such as ultrasound, MRI, or CT, to establish or rule out the presence of a soft tissue mass. While the inclusion of these studies would be of substantial scientific interest, routine use of these studies in asymptomatic patients would be questionable at this time. Third, we did not obtain serum or whole-blood metal ion levels. Although the etiology and diagnosis of the nine patients with regional hip pain may have been accurate, one cannot rule out with 100% certainty that each of these patients did not have an intraarticular hip problem. Although such laboratory information is useful in symptomatic hips, we believe it premature to routinely measure metal ion levels in asymptomatic patients with MOM THAs [35
]. Fourth, surgeons did not implant all patients with this particular prosthesis during this time interval. Indications varied with the surgeon’s confidence to obtain adequate fixation of the cup at the time of implantation. In general, however, this implant is not used in women of childbearing age and patients with end-stage renal disease, known metal allergy, and inadequate bone stock to support a press-fit cup with no screw supplementation. Fifth, we measured cup anteversion intraoperatively. Although accurate intraoperative determination of the frontal plane of the pelvis can be difficult, we did not obtain CT scans to measure or validate the accuracy of the anteversion. This variable would be of extreme importance when considering the relationship between cup malposition and failure in MOM THA [8
]. Finally, while we cannot definitely state the reasons for groin pain, the incidence was low.
The reasons for our successful early results may be both surgeon and implant related. Surgeon factors include cup positioning and seating. Early failures of MOM articulations are associated with cup malposition, including cup abduction of 50° to 55° and cup anteversion of greater than 45° to 50° [8
]. The average abduction and anteversion in this study are not as extreme and may be contributing factors. While the adequacy of cup seating is not specifically measured in this report, it is noteworthy the seating and initial press fit of all three loose cups were determined to be inadequate. When using a monoblock shell with no screw holes, we recognize the native acetabular anatomy may preclude the use of this design due to bone stock deficiency. In some cases, this scenario may not be recognized until cup insertion. Implant factors associated with less wear in MOM articulations include surface finish [27
], clearance (affected by head size tolerance, sphericity, and cup deformation) [23
], carbon content [8
], and casting process [11
]. Additionally, closed backing of the femoral head can diminish metal ion release compared to an open-head design [55
In contrast to 28- and 32-mm MOM THAs, there are relatively few studies reporting clinical results of LD-MOM THA (Table ). Malviya et al. [43
] reviewed 28 small-diameter MOM THA studies and specifically noted the importance in differentiating between large- and small-diameter heads. Unfortunately, of nine published LD-MOM studies, only two reported results beyond 24 months [5
]. The other six reported results at 13 months or less.
Small- and large-diameter metal-on-metal THA
In our study, there were no cases of regional hip pain that could be considered intraarticular in nature. Of the MOM THA studies reporting pain results [5
], only Lavigne et al. [39
], Peters et al. [48
], and Stuchin [52
] reported no cases of pain attributed to the hip.
Most authors reported functional results as well [5
]. Average HHSs of at least 90 have been noted [5
]. Stuchin [52
] reported an average HHS of 88 in 40 LD-MOM THAs. Delaunay [18
] and Delaunay et al. [19
] reported 97% and 98% good or excellent results, respectively.
Several authors reported dislocation rates in their MOM THA series [5
]. Of the LD-MOM series, only Peters et al. [48
] reported any dislocations (two of 99 THAs). Not unexpectedly, the dislocation rate was higher in the MOM series using smaller head sizes: two of 98 THAs (2%) [18
], one of 83 THAs (1.2%) [19
], 19 of 652 (3%) THAs [21
], and four of 94 (4%) THAs [46
Of the seven studies reporting radiographic cup osteolysis [5
], only Neumann et al. [46
] and Park et al. [47
] reported cases of osteolysis (4% and 17%, respectively).
The highest rates of loose or failed cups were noted in the 28-mm Ultima®
] (Table ). Of the LD-MOM cups, Berton et al. [5
] reported on 92 Durom®
LD-MOM THAs at an average followup of 43 months and noted loosening in five of 92 cups (5%). Peters et al. [48
] reported on 99 Magnum™ LD-MOM THAs with an average followup of 36 months and noted one loose cup. Specifically, these authors noted no cases of unexplained hip pain or synovitis.
In conclusion, although we remain concerned over the issues of metal ion release, ALTR, and unexplained hip pain, the clinical manifestations of these reported problems are not clinically evident in our study. Further study is needed to determine whether our early observations will be confirmed.