INTRODUCTION: Acetabular erosion following hemi-arthroplasty is well known. We compared acetabular erosion in patients with comorbid diseases that incur pathologically soft bone to controls, and compare conversion rates between groups. 
METHODS: This retrospective cohort study examined erosion in 40 unipolar hemiarthroplasties (HA) performed between 1998 to 2013 with comorbid diagnoses of rheumatoid and inflammatory arthritis, severe osteoporosis, renal osteodystrophy, or chronic steroid use, and compared them to 54 control HA without these comorbidities. No patients were excluded. There were no differences in BMI or gender between groups, but the control groups were mean 19.0 years older (74.5 vs 55.5, p<0.01). Migration was measured from standing AP pelvis x-rays with magnification-correction using OrthoView software (Materialise HQ, Belgium). The distance between the center of the femoral head and a perpendicular point on the transischial line was compared from the earliest standing post-op radiograph to the most recent radiograph. The mean interval between radiographs was 2.9 years (SD 2.8 years). We reviewed both groups for later revision or conversion to total hip (THA), at mean follow-up of 9.37 years (SD 3.6 years). A 2-tailed Students t-test was used to compare differences. 
RESULTS SECTION:  HA in pathologic bone was associated with significantly more acetabular erosion (P=0.0034). Mean migration of the femoral head was 3.03 mm (±3.14), or 0.27mm/yr. 14 HA in the pathologic group (35%) had conversion THA at mean 5.5 years, compared to 5 in the control group (9.3%, p<0.01). 
DISCUSSION:  At early follow-up of 3 years, acetabular erosion was nearly four-times greater in these disease states than controls. These patients were significantly more likely to need conversion THA. These extent of migration has not been found for THA with well-fixed acetabular components. Therefore, THA is recommended when medically possible for patients with these comorbidities. 
SIGNIFICANCE/CLINICAL RELEVANCE:  Clinically we theorize the increased acetabular erosion rate in patients with pathologic bone who undergo HA is a major contributor for undergoing revision surgery to THA in the future. Therefore, performing THA as the initial surgery instead of HA has the potential to decrease the need for revision operations in this patient population.

1450 - Early Acetabular Erosion Following Unipolar Hemiarthroplasty in Patients with Pathologic Bone