Purpose: The purpose of this study is to measure in vivo linear head penetration of a newer generation highly crosslinked liner (X3, Stryker Orthopedics) using Radiostereometric Analysis (RSA).

Method: The 12 hips (11 patients) included in this study are a subset from a larger randomized controlled trial comparing Exeter stem migration in cement mantles with and without Tobramycin. Criteria for inclusion in this subset were the use of an uncemented Trident acetabular component containing X3 polyethylene in combination with a 32 mm stainless steel femoral head. The average age was 72.7 years (range 65 to 80), and there was an equal gender distribution. RSA examinations were taken with patients lying supine at six weeks, six months, one year, and two years postoperatively. The six week examinations were used as the reference examinations for measuring head penetration. Radiographic measurements and analyses were performed with the UmRSA software suite version 6.0 (RSA Biomedical, Umea, Sweden). Head penetration was determined via edge-detection measurements of the femoral head and acetabular cup.

Results: The mean cumulative femoral head penetration at 6 months was 0.23 mm; this remained statistically unchanged both at 1 year: 0.20mm (p=0.69, 95% UCL of the difference: +0.15mm) and 2 years: 0.25mm (p=0.77, 95% UCL of the difference: +0.10mm).

Conclusion: It is generally recognized that femoral head penetration of more than 0.1 mm per year can result in osteolysis. The purpose of cross linking polyethylene is to reduce wear to below this level. The results of this study show that after an initial bedding in of approximately 0.2 mm, femoral head penetration is not detectable over the subsequent 18 months. Further follow-up is underway to confirm that this promising reduction in wear is maintained.

Purpose: The purpose of this study is to determine if the addition of Tobramycin antibiotic powder to cement for primary hip replacement surgery increases the risk of long term aseptic loosening. This was accomplished by measurement of implant micromotion with Radiostereometric Analysis (RSA).

Method: Exeter femoral stems and Trident acetabular components were implanted into 33 patients. Stems were cemented in a randomized manner with either Simplex P or Simplex T. Tantalum beads were injected into the femur to serve as reference points for RSA measurements. RSA examinations were taken with supine positioning at six weeks, six months, one year, and two years post-operatively. Radiographic measurements and analyses were performed with the UmRSA software suite version 6.0 (RSA Biomedical, Umea, Sweden). Distal migration of the stem centroid was measured at each follow-up period and the mean migration rates for both groups were determined. Non-inferiority testing of stem migration in Simplex T compared to Simplex P was accomplished using a one sided t-test, with the significance level set at 0.05. A clinically inferior additional amount of distal migration was set at 0.4mm/yr.

Results: Eleven patients were excluded from the study: seven patients either dropped out of the study or missed the six week and/or two year follow-up examinations, two patients had radiographic image quality issues, and two patients had loosening of their tantalum markers in subsequent follow-ups. Of the remaining 22 patients, eight were male and 14 were female, with an average age at time of surgery of 71.2 (range, 63–81) years. The mean total distal migrations for the Tobramycin and non-Tobramycin cement groups at two years were 0.891 and 0.732 mm, respectively; the mean stem migration rates were 0.263 and 0.179 mm/yr, respectively. The differences in total distal migration and stem migration rate were not statistically significant (P = 0.06 and UCL = 0.173, respectively).

Conclusion: The addition of Tobramycin to Simplex cement does not appear to impact the distal migration pattern for a polished tapered cemented hip stem at two years. This finding suggests that Tobramycin does not have any clinically relevant deleterious effects on the in vivo mechanical properties of Simplex cement.