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Functional reconstruction after Enneking Type I + II resections of the pelvis (those involving both the ilium and the acetabulum) is challenging, especially if resection of part of the sacrum is included. To assess the clinical outcomes of a newly designed modular pedicle-hemipelvic endoprosthesis, we performed a preliminary retrospective study on its clinical use in a small group of patients.The purposes of this study were (1) to evaluate in a small case series whether the new endoprosthesis restored lower limb function and lumbopelvic stability in the short term; (2) to identify the complications associated with use of the new prosthesis; and (3) to assess the 5-year cumulative survival, the cumulative incidence of a major postoperative event, and the cumulative incidence of implant failure in this group of patients.Between August 2012 and August 2014, our center performed 274 internal hemipelvectomies for oncologic indications. Among these, 20 were treated with the new endoprosthesis, which was designed for fixation both to the residual sacrum as well as the lumbar spine. An earlier version of the device had been removed from the market because of an unacceptable risk of serious complications. All of the 20 tumors were sarcomas necessitating en bloc resection. The implant is modular and can meet the different-sized defects in each patient. The general indication for use of the new implant was a total acetabular defect with extensive iliac involvement or total loss of the sacroiliac joint and/or hemisacrum. All 20 patients were followed up for a minimum of 24 months or until death in those patients who survived < 2 years (median, 36 months; range, 6-60 months). The clinical data were retrieved from the database and the study endpoints (function according to the Musculoskeletal Tumor Society [MSTS] score, complications, and survivorship of patients and implants) were ascertained by chart review. Lumbopelvic stability was defined as an excellent or good rating according to the International Society of Limb Salvage radiologic implant evaluation system. The cumulative survival of patients was estimated using the Kaplan-Meier approach. The cumulative incidence of major postoperative events including local recurrence, metastasis, and reoperation was estimated using a competing events analysis; the cumulative incidence of implant failure, including mechanical failure or deep infection, in patients who underwent reoperation was also estimated using a competing events analysis.In the 16 patients who survived > 12 months, the median MSTS score was 19 of 30 (range, 5–26). Radiographic assessments demonstrated lumbopelvic stability in all of the 16 patients. Twelve of 20 patients developed postoperative complications, primarily including deep infection (one), hip dislocation (two), and local recurrence (three). Major revision surgery was performed in five of 20 patients. The estimated 5-year Kaplan-Meier patient survival rate was 69% (95% confidence interval [CI], 59%-79%), whereas the cumulative incidence of major postoperative events and implant failure using the competing risk estimator was 42% (95% CI, 23%-60%) and 15% (95% CI, 4%-34%) at 5 years, respectively.Preliminary results with hemipelvic reconstruction using this new endoprosthesis achieved fair functional results and the complications that were observed appeared comparable to other reconstruction options at short-term followup. Longer-term surveillance is called for to see whether this implant will be durable compared with other available reconstructive alternatives such as a custom-made megaendoprosthesis or an autograft/allograft-prosthetic composite.Level IV, therapeutic study.