Between July 2002 and February 2007, 39 consecutive CUT prostheses were implanted in 32 consecutive patients (12 male, 20 female; 7 bilateral) for symptomatic osteoarthritis who were either less than 55 years of age or had an anatomical anomaly preventing implantation of a regular diaphyseal stem. Mean age was 37 (17–58) years. The preoperative diagnosis in the patients younger than 55 years was primary osteoarthritis in 9 hips and secondary osteoarthritis in 29 hips. Osteoarthritis was secondary to osteonecrosis of the femoral head (9), rheumatoid arthritis (6), juvenile idiopathic arthritis (5), developmental dysplasia (4), morbus Morquio (2), epiphysiolysis (1), septic monoarthritis (1) and monoarthritis of unknown cause (1). In 1 patient, the anatomical appearance necessitated placement of this short-stemmed prosthesis: a 58-year-old woman with osteoarthritis secondary to congenital arthrogryposis multiplex who had a fork-deformed femur.
The CUT prosthesis is made of a CoCrMo alloy. The femoral component consists of a body and a modular conus with various angles and lengths in order to restore leg length and offset. The surface of the prosthesis has a macroporous spongious metal surface structure to facilitate bone ingrowth. 2 experienced orthopedic surgeons (RGHHN and HJO) operated on the patients using a lateral approach in the lateral decubital position. The senior author learned the technique from one of the prosthesis designers. In 37 cases, the CUT prostheses was combined with the standard press-fit cup (ESKA Implants, Lübeck, Germany) and in 2 cases it was combined with a press-fit Mallory Head cup (Biomet, Warsaw, IN). In all cases, a polyethylene liner was combined with a ceramic head.
For RSA analysis, 6–10 1-mm tantalum balls (Industrial Tectonics, Ann Arbor, MI) were inserted into the proximal femur during surgery. Furthermore, the implant manufacturer attached 1 marker at the distal tip of the hook-shaped end of the prosthesis (). Attachment of additional markers to the prosthesis without fundamentally altering the design of the prosthesis was attempted, but proved unsuccessful. Also, usage of the femoral head as an additional prosthesis marker, which is common in RSA of femoral components, was not feasible due overprojection of the metal-backed acetabular component. Thus, only translations of the tip could be determined. The first RSA radiographs were taken in the first postoperative week before ambulation (median 4 (1–7) days postoperatively). The patients were allowed minimal weight bearing using 2 crutches in the first 3 postoperative weeks, partial weight bearing using 1 crutch during the next 3 postoperative weeks, and full weight bearing thereafter.
The CUT femoral neck prosthesis. Note the RSA marker at the distal tip of the prosthesis.
Patients were evaluated preoperatively and postoperatively at 6 weeks, 3 months, 6 months, 1 year, and annually thereafter. At each evaluation, the Harris hip score and RSA radiographs were obtained. Conventional anteroposterior and lateral radiographs were obtained at 6 weeks, 1 year, 2 years and 5 years of follow-up. None of the patients were lost to follow-up, but 1 patient was unable to attend the most recent follow-up due to pregnancy. Mean length of follow-up was 7.0 (4.8–9.5) years. 37 5years prostheses had at least 5 years of follow-up.
Marker-based RSA measurements were performed (MB-RSA software; Medis Specials, Leiden, the Netherlands) (Kaptein et al. 2003
). The first RSA examination served as the reference for all further examinations; all evaluations are related to the position of the prosthesis relative to the bone at that time. Migration is expressed along the longitudinal, transverse, and sagittal axes. Accuracy of individual RSA measurements was given by the limits of the 95% prediction interval of the accuracy of zero motion (Valstar et al. 2005
), calculated as 1.96√(∑d2
/2n) (Ranstam et al. 2000
) using 25 double examinations obtained at 1-year follow-up. Individual measurement accuracy was ± 0.08mm for medial-lateral translation, ± 0.08 mm for cranial-caudal translation, ± 0.22 mm for anterior-posterior translation, and ± 0.25 mm for total translation.
In 3 of 39 prostheses, there were insufficient or incorrectly placed RSA markers and these patients were unsuitable for RSA analysis. In addition, 1 patient refused further RSA examinations after the 6-week postoperative RSA radiograph and this patient was excluded from migration analysis. In these 4 patients, routine clinical and radiographic follow-up was performed. Thus, RSA follow-up was complete in 35 of 39 prostheses. For all examinations, the rigid body error was below 0.35 and the condition number was below 56; these values satisfy the marker stability and distribution criteria according to the RSA guidelines of Valstar et al. (2005)
. No examinations had to be excluded.
Preoperative offset and caput collum diaphyseal angle (CCD angle) were measured on conventional preoperative pelvic radiographs. On conventional postoperative pelvic radiographs, the amount of femoral neck resection (no femoral neck engagement, less than 50% resection, or more than 50% resection), alignment of the stem, postoperative offset and CCD angle, distance of the proximal stem to the medial femoral cortex, and distance of the distal stem to the lateral femoral cortex were determined. The presence of radiolucent lines and stress shielding was assessed in 5 zones around the stem ().
5 assessment zones around the CUT femoral neck prosthesis (modified according to Gruen).
Informed consent from the patients and approval of the institutional review board were obtained for the study.
Values are reported as mean (SD) and range, or as median (range). Estimates are reported as mean with 95% confidence interval (CI).
The Kaplan-Meier method was used to estimate survival of the prostheses. Initial (0–2 years) and steady-state (≥ 2 years) migration were analyzed using a linear mixed model with random slope and random intercept, which accounts for the repeated measurements of migration over time and the correlation of these measurements in patients. Variables in the analysis were: days to the first postoperative RSA examination, alignment of the stem, postoperative CCD angle, change in CCD angle and offset, categorized distance (0–2, 2–4, 4–6, > 6 mm) to the medial and lateral cortex, the amount of femoral neck resection, and the occurrence of stress shielding (steady-state migration only). Any p-value of < 0.05 was considered statistically significant. We used SPSS statistical software version 17.0.