During insertion of the femoral stem, the intramedullary pressures are increased. Fat and marrow elements can become embolized into the bloodstream at this point resulting in ventilation perfusion mismatch in the lungs.
Kim et al prospectively followed 156 total hip arthroplasties including bilateral and unilateral procedures as well as cemented and uncemented procedures. They found no difference in fat embolization amongst any of the groups. However, they did find that if patients had evidence of bone marrow cells in the right atrium on the first postoperative day, they developed diffuse encephalopathy with confusion and agitation that lasted for about twenty-four hours.
Kim YH, Oh SW, Kim JS: Prevalence of fat embolism following bilateral simultaneous and unilateral total hip arthroplasty performed with or without cement: A prospective, randomized clinical study. J Bone Joint Surg Am 2002;84:1372-1379.
PMID: 12177267 (Link to Abstract)