Patient Outcomes Following Implantation of Modular Neck Hip Prostheses in Primary Total Hip Arthroplasty

Bimodular femoral neck components were introduced to optimize implant size and offset. Failures have been reported and long-term outcome of modular necks must be assessed. The purpose of this study was to investigate the results of a series of modular neck implants and catalog the causes of failure of them. Our series included 277 hips (244 patients) with Profemur-E and Profemur-Z femoral stems (Wright Medical Technology Inc., now MicroPort Orthopaedics Arlington, TN). Data for this cohort of patients included demographics, implant dimensions, and the type of articular surface. Correlation of these variables with failure was analyzed to estimate the effect of each on survivorship. Implant failure was defined as cases requiring revision surgery. Seven cases were revised for implant neck fracture (2.5 %), and four were revised for adverse local tissue reactions (ALTR; 1.4 %). Seven required replacement of fractured neck with a cobalt-chromium (CoCr) modular neck. Regression analysis showed that a 1-unit increase in body mass index (BMI) increased the probability of neck fracture by a factor of 1.19 and every 4-mm increase in head diameter increased the chance of neck fracture by a factor of 3.2. Increase in neck offset is associated with a 1.83 times increased likelihood of neck fracture. Co levels correlated positively with neck fracture, and titanium levels correlated positively with gas in the joint. Cr and titanium levels were significantly higher in patients who had aseptic revision surgery. These data suggest that failure of modular neck junctions may be potentiated by higher BMI, greater offset, and larger head diameters. It is suspected that these factors contribute to modular neck failure by creating a stronger moment arm about the neck's insertion into the stem. A lower level of titanium than 7 ng/dL might be considered as the threshold. These data may help identify a set of factors that contraindicate the use of modular neck junctions.