What Are the Functional, Radiographic, and Survivorship Outcomes of a Modified Cup-cage Technique for Pelvic Discontinuity?
Authors: Mu Wenbo , Xu Boyong , Wahafu Tuerhongjiang , Wang Fei , Guo Wentao , Zou Chen , Cao Li 
Published: July 2024

Background
Pelvic discontinuity (PD) presents a complex challenge in revision hip arthroplasty. The traditional cup-cage construct, which involves a screw-secured porous metal cup and an overlying antiprotrusio cage, has shown promising mid- to long-term results. However, there is limited information on the outcomes of modifications to the original technique. Our study aims to evaluate a modified technique in which the cup position is determined by the placement of the overlying cage, allowing for adjustments to achieve optimal orientation.

Questions/purposes
Among patients treated for PD with a cup-cage construct in which the cup position was dictated by the position of the cage: (1) What are Harris hip scores achieved at a minimum of 2 years of follow-up? (2) What is the Kaplan-Meier survivorship free from aseptic loosening or component migration? (3) What is the Kaplan-Meier survivorship free from revision for any reason? (4) What surgical complications are associated with the procedure?

Methods
Between October 2013 and January 2022, we performed 805 acetabular revisions. Among these, 33 patients with PD confirmed intraoperatively were considered potentially eligible for a cup-cage construct; no other method of surgical management was used. We performed 64% (21 of 33) of these procedures from October 2013 to January 2018, with 6% (2 of 33) of patients lost to follow-up before the minimum study follow-up of 2 years; these 19 patients were monitored over a period ranging from 70 to 115 months. A further 12 patients underwent this procedure from January 2018 to January 2022, with one lost to follow-up before the minimum study follow-up of 2 years; the other patients met the minimum 2-year follow-up requirement. The remaining 30 patients with data analyzed here (10 men, 20 women) had a mean ± SD age of 61 ± 12 years and a median BMI of 29 kg/m2 (range 20 to 33 kg/m2) at the time of revision surgery. Twenty-one patients underwent revision due to aseptic loosening, and nine due to periprosthetic joint infection (PJI). The causes of PD in our patients were as follows: cup aseptic loosening without significant osteolysis in 20% (6 of 30), where the loose cup caused erosion of the host bone, leading to PD; PJI in 30% (9 of 30); intraoperative iatrogenic PD in 3% (1 of 30); and osteolysis in 47% (14 of 30), which also resulted in aseptic loosening. The median follow-up time was 79 months (range 25 to 115 months). The Harris hip score was used to evaluate clinical outcomes, with preoperative values compared with the most recent follow-up. Radiographs were reviewed by two experienced surgeons at each follow-up visit to assess component loosening (defined as migration > 5 mm or the presence of circumferential radiolucent lines) or clear migration. PD was considered healed if bridging callus or trabecular bone was visible across the site of the discontinuity. Complications were assessed through a comprehensive review of electronic medical records. Kaplan-Meier analysis was used to estimate implant survivorship and radiographic loosening, with aseptic loosening or component migration as the endpoint, as well as survivorship free from any reoperation.

Results
The Harris hip score improved from a median of 39 (range 30 to 66) preoperatively to a median of 76 (range 30 to 90) postoperatively (median difference 33 [range 2 to 48]; p < 0.01). Within the limitations of two-dimensional (2D) radiographic imaging, successful bone graft integration and the healing of PD were noted in 83% (25 of 30) of patients. Kaplan-Meier survivorship free from radiographic signs of aseptic loosening or component migration was 100% (95% CI 100% to 100%) at 115 months. When any revision related to the acetabular component was considered the endpoint, survivorship free from acetabular component revision at 115 months after revision surgery was 100% (95% CI 100% to 100%). When the need for any reoperation was considered the endpoint, survivorship free from needing reoperation at 115 months after revision surgery was 85% for all patients (95% CI 73% to 100%). When including only patients with a follow-up time of > 4 years (20 of 30), survivorship free from needing reoperation at 115 months after revision surgery was 90% (95% CI 78% to 100%). Postoperative complications during the follow-up period included one early dislocation on the fifth day after surgery, treated with closed reduction and 6 weeks of abduction bracing. One femoral stem loosening occurred at 56 months postoperatively, although the acetabular component remained securely fixed; this patient declined revision surgery. One patient experienced a dislocation 5 months after surgery but refused treatment and opted for prolonged bed rest. Additionally, one patient underwent a debridement, antibiotics, and implant retention procedure 1 week after the revision surgery and subsequently showed no signs of infection at the latest follow-up, 38 months postoperatively.

Conclusion
Our study highlights the effectiveness of a modified cup-cage technique in complex hip revisions, showing promising results in terms of construct survivorship and low complication rates. Surgeons could consider delaying screw fixation until after positioning the cage within the porous cup to allow for optimal adjustment and using metal augments for severe bone defects to achieve better alignment. Surgeon experience with the cup-cage technique is crucial for achieving optimal outcomes. Future studies should focus on long-term follow-up visits to assess the durability and effectiveness of these modifications and explore the comparative effectiveness versus other methods, such as custom triflange components and jumbo cups with distraction.

Level of Evidence
Level III, therapeutic study.