Our data were derived from the Swedish Hip Arthroplasty Register (SHAR Annual Report 2010
), initiated in 1979. All public and private orthopedic units report demographic data, laterality, surgical approach, and detailed information on implants and fixation to the Register. The SHAR has been validated, and the completeness has been found to be about 99% for primary THA (SHAR Annual Report 2010
) and 94% for revision hip arthroplasties (Söderman 2000
Femoral head sizes above 28 mm and dual-mobility cups have only been in frequent use in Sweden since 2005. Thus, only primary THA procedures registered in the SHAR from January 1, 2005 to December 31, 2010 were extracted. An extension of the selection criteria to earlier time periods would only have resulted in the addition of more head sizes below 28 mm and of more historical cup designs that are no longer in clinical use.
To reduce the influence of cup designs used in small numbers, we included only cups that had been inserted in at least 1,000 hips—with the exception of the dual-mobility design, which had only been used in 287 THAs. In Sweden, this type of cup has only recently been introduced, and, with few exceptions, only one design has been inserted (Avantage; Biomet, Warsaw, IN). Hip resurfacings and implants with rarely used head diameters (24, 26, 30, and 40 mm; n = 140), 20 hips with missing data on head diameter, and 487 hips with uncertain information on the type of surgical approach were excluded. Thus, we identified and analyzed 78,098 procedures in 61,743 patients.
The term “revision” was defined as an intervention where 1 or more components of the prosthesis were exchanged, where an augmentation device was added, or where the whole prosthesis was removed. Thus, other types of reoperations in which the implant was left in situ, e.g. closed reductions or incision and drainage, were disregarded.
The covariate “diagnosis” was divided into primary osteoarthritis (pOA), inflammatory joint disease, femoral neck fracture, osteonecrosis of the femoral head (FHN), previous pediatric hip diseases such as developmental dysplasia of the hip, Perthes’ disease and slipped femoral epiphysis, and other diagnoses. The category “other diagnoses” included secondary osteoarthritis due to previous trauma or malignancy. The covariate “surgical approaches” was divided into lateral, posterior, and minimally invasive. The category “lateral approaches” included the approaches described by Hardinge, Gammer, or Bauer, with or without trochanteric osteotomy. “Posterior approaches” included approaches based on the description by Moore, with or without trochanteric osteotomy. “Minimally invasive approaches” included lateral (n = 604), posterior (n = 100), and 2-incision approaches (n =30).
Continuous descriptive statistics used means, median values, ranges, and 95% confidence intervals (CIs) where appropriate. Cox proportional hazards models were used to analyze the influence of various covariates on the relative risk (RR) of revision due to dislocation. Follow-up started on the day of primary THA and ended on the day of revision, death, emigration, or December 31, 2010, whichever came first. The covariates age (< 50, 50–59, 60–75, > 75), sex, primary diagnosis, head size (22, 28, 32, 36, dual-mobility) and surgical approach were initially investigated as singular covariates resulting in a crude RR with 95% CI. All covariates mentioned above were subsequently entered into the model, and adjusted RRs were calculated using the Breslow method for handling ties. A separate analysis was performed for the first 6 months after the index procedure. The assumption of proportional hazards was investigated by calculating the correlation coefficient between transformed survival time and the scaled Schoenfeld residuals. The level of significance was set at p < 0.05 in all analyses, and we used the R software package (version 2.14.1).
The inclusion of both joints in bilaterally operated patients does not appear to lead to dependency issues in registry studies of this size, and both joints in bilaterally operated patients were thus included in the analysis (Hailer et al. 2010
). To confirm these previous observations, separate analyses excluding the second THA in bilaterally operated patients were performed without affecting parameter estimates.
Characteristics of the study population
The population studied consisted of 36,968 females (60%) and 24,775 males. The distribution of age and diagnosis groups is given in and . 35,460 THAs (45%) had been performed using a lateral approach, 41,904 (54%) using a posterior approach, and 734 THAs (1%) had been inserted using minimally invasive approaches. 28 mm was by far the most widely used head diameter (). 71,094 cups (91%) were cemented and 7,004 (9%) were uncemented. 2 different types of Avantage cups had been used: the cemented, polished version (n = 275) and the uncemented, hydroxyapatite-coated version (n = 12). Cross-tabulations for diagnosis or surgical approach by femoral head size are summarized in . The Lubinus cup was the most commonly used acetabular implant (). Cups had been combined with a wide variety of stems, both cemented (n = 64,808; 83%) and uncemented (n = 13,251; 17%, with missing data for 39 stems), creating both cemented, hybrid, and uncemented systems. Mean follow-up time was 2.7 (0–6) years.
Head diameters inserted at index THA
Head size by diagnosis or by surgical approach
Cup types inserted during index THA