Clinical Outcomes of Primary Cementless Total Hip Arthroplasty Using the Collarless Actis Stem

The collarless Actis stem is a cementless femoral component featuring a proximal DUOFIX hydroxyapatite coating and a triple-tapered geometry, intended to enhance proximal bone remodeling, promote favorable load distribution, and limit stress shielding. Favorable short-term outcomes have been reported by the manufacturer and in registry-based analyses, and a comparative series has shown results similar to those of the Corail stem. These data, however, derive almost entirely from Western populations, and the majority of published series have evaluated the collared version of the implant. No study has reported the outcomes of the collarless Actis stem in an East Asian population, in whom osteonecrosis of the femoral head is a more frequent indication for arthroplasty and the mean age at the time of surgery is lower than in Western cohorts.

All participants undergo primary cementless ceramic-on-ceramic total hip arthroplasty using the collarless Actis femoral stem and the Pinnacle acetabular cup, with a fourth-generation BIOLOX delta ceramic head and liner. Hybrid use of implants or ceramic bearings is precluded in order to avoid bias. All procedures are performed through a posterior approach with the patient in the lateral decubitus position. Component positioning follows a combined anteversion technique using anatomical landmarks, with a target cup anteversion of 37.3 degrees minus 0.7 times the stem anteversion.

At the routine preoperative medical clearance, participants are assessed for age, sex, body mass index, ambulatory function graded by the Koval classification, diagnosis, comorbidities, and operative risk according to the American Society of Anesthesiologists classification. Preoperative imaging comprises a three-dimensional computed tomography scan of the hip for planning, together with hip anteroposterior, hip cross-table lateral, pelvis lateral, and lumbosacral spine radiographs. Operative data include operative time, type of anesthesia, estimated blood loss, implant fixation, cup position, intraoperative complications, and length of hospital stay.

Postoperative mobilization begins on the first or second day after surgery, and participants ambulate with partial weight bearing using a walking aid for six weeks. Follow-up is scheduled at 6 weeks, 3 months, and 12 months after surgery and annually thereafter until 5 years, corresponding to the institution's routine postoperative schedule. No additional examinations are performed for research purposes beyond those obtained in routine care.

Serial hip anteroposterior and cross-table lateral radiographs are assessed for implant fixation, ceramic wear, osteolysis, implant loosening, and heterotopic ossification according to previously established criteria. Radiographic evaluation is performed by two independent observers who did not participate in the operations, with the six-week radiographs serving as the baseline for comparison. Patient-reported outcome measures are obtained preoperatively and at one year postoperatively and annually thereafter, and comprise the modified Harris Hip Score, the Hip Disability and Osteoarthritis Outcome Score, the EQ-5D, the University of California Los Angeles activity score, and the Forgotten Joint Score, the last of which is acquired postoperatively only.

The target enrollment of 210 hips was derived from an a priori power analysis. Assuming a complication rate of 6.5 percent, based on the investigators' previously reported 10- to 12-year outcomes of fourth-generation ceramic-on-ceramic total hip arthroplasty using the Pinnacle cup, 178 hips were required to detect a 2 percent difference with 80 percent power at a two-sided alpha of 0.05. Allowing for a 15 percent dropout rate, enrollment was set at 210 hips.

Data are recorded in an electronic case report form using a web-based REDCap platform and are monitored every six months for safety and data quality. Categorical data are summarized as counts and percentages and continuous data as means with standard deviations. Implant survivorship is analyzed using the Kaplan-Meier method with reoperation or revision as the endpoint.