Abstract
Pseudotumour formation is being reported with increasing frequency in failing metal-on-metal hip resurfacings and replacements. This mode of failure complication has also been reported with metal-on-polyethylene bearing bearings when it is usually associated with evidence of surface corrosion and no apparent wear at the head–neck taper.
We present a case with evidence of taper wear and damage secondary to corrosion in an uncemented total hip replacement with a metal on polyethylene articulation (TMZF (Titanium, Molybdenum, Zirconium and Ferrous) Accolade® stem, Trident® HA coated acetabular shell, Low Friction Ion Treatment (LFIT™) Cobalt-Chrome anatomic head (40 mm), X3® polyethylene liner)
Case
A 69 year old woman had a THR in 2008. A year later she started to complain of lateral based hip pain. Clinical examination and initial imaging indicated trochanteric bursitis and heterotopic bone formation. The symptoms became worse over the next 3 years and the patient was listed for exploration and excision of heterotopic bone.
Surgical findings
Extensive pseudotumour was encountered deep to bursa and adherent to capsule. Abductors, external rotators and vastus lateralis were spared. There was minimal calcar osteolysis and marginal erosion in superior acetabulum. There was no obvious wear on the articulating surfaces of the femoral head and polyethylene inlay of the socket nor was there any evidence of neck-to-rim impingement or edge loading. There was visible blackening of both the taper and trunnion after femoral head removal.
Results
Peripheral blood samples taken at the time of surgery recorded Cobalt 107 nmol/L, Chromium 9 nmol/L.
Wear Measurements
These were performed on the articulating surfaces and the head taper using the RedLux Artificial Hip Profiler. No discernable wear was noted at the articulating surfaces.
Qualitative 3-D surface mapping demonstrated a trunnion imprint on head taper of 10.1 mm in length. At the distal end of the taper this was incomplete indicating possible taper/trunnion incongruity. The majority of the material loss was demonstrated at the proximal end on the polar opposite side to incomplete imprint (Figure 1).
Corrosion analysis
The head was sectioned for more detailed surface analysis of areas of interest. This demonstrated a polished region just inside the taper (G), followed by a deep pit (F), a region of extensive pitting (E) and running along the taper length were longitudinal needle like pits (D) indicative of a path of fluid ingress (D-G ref Fig 2.). These findings were more suggestive of crevice corrosion than mechanical wear.
Histology
There was evidence of aseptic lymphocytic vasculitis-associated lesion (ALVAL) scoring 8 out of 10 (as per Campbell et al).
With the continuing increase in the use of larger head sizes the findings of pseudotumour formation and taper damage in a commonly used hip replacement raises considerable concern. Wear analysis has highlighted that although there is evidence of motion occurring at the taper / trunnion interface, corrosion appears to be the dominant mechanism of damage to the taper. In addition to the use of larger head sizes this case also raise concerns about the compatibility of certain metal alloy combinations.
