Abstract
A 35-year-old female (age 35Yrs) had primary MOM total hip arthroplasty (THA) in 2008. At 8 months this patient postoperatively developed headaches, memory loss, vertigo, and aura-like symptoms that progressed to seizures. At 18 months review, she complained of progressive hip pain, a popping sensation and crepitus with joint motion. This patient weighed 284lbs with BMI of 38.5. Radiographs revealed the cup had 55° inclination, 39° anteversion (Fig. 1). Metal ion concentrations were high (blood: Co=126 mcg/L, Cr= 64mcg/L). Revision was performed in November 2010 A dark, serous fluid was observed, along with synovitis. The implants were well fixed and the femoral head could not be removed; thus the stem was removed by femoral osteotomy. With the head fused on this femoral stem, for the 1st time it was possible to precisely determine the habitual patterns of MOM wear relative to her in-vivo function. We investigated (1) size and location of wear patterns and (2) signs of cup-stem impingement to help explain her symptoms developed over 32 months follow-up.
The retrieved MOM was a Magnum™ with head diameter 50mm and 50×56mm cup (Biomet). This was mounted on a Taperloc™ lateralized porous-coated stem. Components were examined visually and wear damage mapped by stereo-microscopy, interferometry, CMM, SEM, and EDS. Main-wear zone (MWZ) areas were calculated using standard spherical equations1 and centroidal vectors determined.
The head-cup mismatch was 427um with the cup revealing a form factor of 228um. The cup showed wear area of 1275mm² that extended up to the cup rim over 150°arc. The cup rim was worn thin over a 90° arc with loss of cup bevel. The head showed an elliptical wear area of 2200mm2 located centrally on the superior-medial surface (ellipsoidal ratio ×1.2). Compared to the hemispherical surface (50mm: hemi-area = 3927mm2), the worn area represented hemi-area ratio of 56%. The centroidal vectors measured 8° anterior and 24° superior to the head's polar axis (Fig. 2). Stripe wear damage revealed multiple impingement sites. SEM and EDS revealed stripes were contaminated by metal transfer from the stainless-steel instruments used at revision. The main impingement position was identified (Fig. 3) indicating the site of repetitive subluxations whereby the subluxing head thinned the cup, i.e. “edge wear”.
Cup and head wear patterns corresponded well, reinforcing our definition of the MWZ locations in vivo. The femoral MWZ was centrally located superiorly and medially with respect to the polar axis of the femoral neck and head. The noted impingement position indicated this patient had experienced repetitive subclinical subluxations (RSS).2 The taper inside the fused head may also have been a contributory factor that we cannot ignore. Nevertheless her excessive cup thinning was likely a result of a steep cup and considerable anteversion allowing the femoral head to sublux over the cup rim, thus thinning the cup and wearing the rim bevel, and producing MOM wear debris.
