Intermediate-Term Survivorship of Total Hip Arthroplasty With a Proximally Coated Tapered-Wedge Femoral Stem: A Retrospective, Multi-Center Registry Review

Background: Short tapered-wedge stems have been used frequently over the past decade, but long-term follow-up data are not readily available in the literature. Methods: A retrospective outcomes review was conducted to assess survivorship and clinical outcomes for the TRI-LOCK® Bone Preservation Stem (TRI-LOCK BPS; DePuy Synthes, Warsaw, IN, USA), a proximally coated, tapered-wedge femoral stem. Results: In a cohort of 2,040 hips, Kaplan-Meier survivorship estimates (95% CI {confidence interval}; N with further follow-up, where N is the number of hips remaining at each post-operative interval), with survivorship defined as no revision of any component for any reason were 96.6% (92.8%,98.4%; 45) at eight years under the clinical assumption and 98.6% (97.9%,99.1%; 90) at 14 years under the registry assumption. With survivorship defined as stem revision for any reason, estimates were 97.7% (93.7%,99.2%; 45) at eight years under the clinical assumption and 99.2% (98.6%,99.5%; 90) under the registry assumption. Mean Harris Hip Scores and WOMAC scores were 90.08 and 21.98, respectively, at 10 years postoperatively. Conclusion: Our evaluation demonstrates excellent construct and stem survivorship and clinical outcomes at intermediate-term postoperative follow-up.


Introduction
Total hip arthroplasty (THA) is one of the most successful surgeries in the world [1]. Successful THA helps patients regain mobility and range of motion and reduce pain, which often greatly enhances quality of life [2][3][4]. A large number of THAs today are performed as cementless procedures, and appropriate implant design is required for adequate rotational stability and biological bone ingrowth and on-growth [5,6]. Additionally, within the last decade there has been increasing interest in minimally invasive techniques [7][8][9] and short stems [10][11][12][13][14][15] that are often optimized for use in these procedures. Many published reports have shown success for short stems regarding survivorship, particularly related to aseptic loosening [11], but there are conflicting reports regarding complications with use of short stems in minimally invasive THA. Some studies have shown short tapered stems to be helpful in reducing complications [16][17][18], while other studies have reported no difference [19] or increased complications [20,21].
The original TRI-LOCK® stem was introduced in 1981 as the first proximally coated tapered-wedge hip stem. Released in 2008, the TRI-LOCK® Bone Preservation Stem (TRI-LOCK BPS; DePuy Synthes, Warsaw, IN, USA) was designed to help provide consistent implant seating based on a simple reproducible surgical technique and to achieve initial fixation and allow long-term, durable fixation. The stem leverages the metaphyseal geometry of its predecessor along with GRIPTION® Porous Coating (DePuy Synthes, Warsaw, IN, USA), decreased M/L width, and a shorter distal length. A retrospective outcomes review was conducted to provide further data on the use of this femoral stem in primary cementless THA.

Materials And Methods
A retrospective outcomes review was conducted using data collected from an ongoing, standard-of-care, multi-center, company-sponsored joint replacement registry. All participating centers were in the United States. All subjects that received a TRI-LOCK BPS stem with a PINNACLE® Hip Solutions cup (DePuy Synthes, Warsaw, IN, USA) were included. Metal-on-metal articulations were excluded. Preoperative and postoperative clinical assessments included Harris Hip Score (HHS) [22], and Western Ontario and McMaster Universities Arthritis Index (WOMAC) score [23], and a registry-specific hip evaluation questionnaire.

Ethical considerations
Written informed consent was collected for all subjects prior to participation and Institutional Review Board (IRB) review and approval (approval number 20120963) were obtained for the registry and all participating sites. The central IRB from which the registry approval was obtained is WCG IRB (formerly Western Institutional Review Board {WIRB}; https://www.wcgirb.com).

Statistical analysis
Clinical assessments were summarized with sample size, mean, and standard deviation for numeric scores and with sample size and percentages for categorical responses. It is recognized that sites within the registry have different standards of care regarding clinical follow-up visits. Therefore, standardized registry visit windows were established. If an individual subject had multiple visits within the standardized window, then only the latest visit was included in the summaries.
Kaplan-Meier survivorship was performed with revision of the stem and revision of any component as endpoints. For each endpoint two survivorship analyses were performed with differing censoring assumptions. First, unrevised subjects were censored at the last clinical follow-up (clinical assumption {CA}), making no assumptions on implant survivorship beyond the patient's last visit. Second, unrevised subjects were censored at the date of database extract (registry assumption {RA}), assuming that if a revision or death has not been reported then the devices remain implanted. In all cases survival estimates were truncated at 40 hips remaining at risk. For the survivorship of the stem component subjects were censored at the time of removal of other components.
Incidence of thigh pain after cementless total hip arthroplasty has been reported by Brown et al. as highly variable, present in 1.9% to 40.4% of cases across a wide range of stem designs [30]. The occurrence of thigh pain for the legacy TRI-LOCK® stem was reported at 2% by Khanuja et al. [12] and 9% by Healy et al. [31]. There are also variable reports of thigh pain for the TRI-LOCK® BPS, with incidence ranging from 1.6% to 27.8% [24,[32][33][34]. Thigh pain in our cohort was reported in 19.8% of post-operative visits. Even though most of the recorded thigh pain was "slight" or "mild" (66.6%), the overall levels of reported thigh pain from our evaluation are at the high end of the range that has been previously reported in the literature.

Limitations
Limitations of this study include a non-randomized, non-controlled design. Another limitation is the standard-of-care follow-up schedule for registry subjects, which typically does not allow the same follow-up compliance as a clinical study. Additional long-term follow-up from large national registries and clinical studies would be beneficial to continue to assess the performance of this stem.