Reverse Hip Replacement System: Addressing Instability Concerns or Introducing New Risks? A Scoping Review

Abstract

Background: To address instability after total hip arthroplasty (THA), a reverse hip replacement system (RHRS) has been proposed, inspired by the concept of reverse total shoulder arthroplasty. Objective: to evaluate the early functional, radiological, and complication outcomes of the RHRS. Methods: A systematic search in PubMed and Scopus databases was performed in March 2025 using predetermined search terms, which revealed an initial 656 results, and only four studies were eligible for inclusion (one biomechanical study and three clinical studies (one case series and two case reports)). Results: The three clinical studies included 24 patients aged 60 to 79 years; 13 (54.2%) were male, and all were diagnosed with advanced hip OA. After follow-up ranging from 3 weeks to 24 months, all patients were satisfied with the results, except for 2 (8.3%). Detailed function and PROM outcomes for 22 patients at 24 months of follow up showed significant improvement in WOMAC, HHS, OHS, HOOS, SF-36 physical component summary (PCS), and ED-5D (p<0.05). However, there was a nonsignificant change in the SF-36 mental component summary (MCS) (p=0.272). Radiological outcomes, as determined by RSA analysis, at 24 months for 18 acetabular cups showed a mean migration of 0.087 ± 0.152 mm. Seven (29.2%) complication incidents, one intraoperative calcar crack, one deep PJI, one postoperative traumatic dislocation, one femoral neuropraxia, one occasional pain, and two recalcitrant iliotibial band syndrome. The biomechanical study results showed lower polyethylene wear debris equivalent to other systems, no wear debris due to metal-on-metal contact, and no signs of polyethylene edge or backside wear. Conclusion: Besides the promising biomechanical results, RHRS's early functional and radiological outcomes are encouraging; however, the incidence of complications is considerably high. Moreover, the results should be interpreted cautiously to the small number of included patients and the short follow up period.

Full Text

Introduction:

There is no doubt that total hip arthroplasty is one of the most successful surgical procedures. Nevertheless, surgeons, in collaboration with bioengineers and other relevant parties, are continually seeking improvements to the total hip arthroplasty (THA) prosthesis to minimize potential drawbacks and enhance longevity [1, 2].

One crucial complication, which is a leading cause of revision after THA, is instability or dislocation [3-5]. Where Upfill-Brown et al. reported instability as the most common reason for THA revision in the US between 2012 and 2018, accounting for 20.4% of revision causes [6]. Furthermore, a recent study by Hermansen et al., based on evaluating 5,415 primary THAs from the Danish Hip Arthroplasty register, reported a 1-year dislocation rate of 2.8% (confidence interval (CI) 2.4-3.3), with 37% of these patients having recurrent dislocations [7].

To avoid or minimize THA instability, in addition to surgical approaches and patient selection, various solutions related to implant design and materials have been introduced, such as large heads, dual-mobility articulations, and constrained liners [3]. Moreover, owing to the relative similarity between the hip and shoulder joints and inspired by the successful outcomes of the reverse total shoulder arthroplasty (RTSA) [8, 9], the concept of Reverse Hip Replacement System (RHRS) or reverse THA was proposed where the cobalt-chrome ball is located on a trunion within the acetabular cup rather than the femoral stem and articulates with a polyethylene liner attached to a femoral cup [10].

This device was recently approved for compassionate use by the Food and Drug Administration (FDA), but the clinical results remain insufficient. Therefore, to explore this knowledge gap and determine whether the theoretical stability advantages of RHRS outweigh the added complexity of two tapers, new wear interfaces, and untested long-term behavior. This scoping review aimed to gather reported results, explore early outcomes, and identify potential challenges of the RHRS.

Methods:

Search strategy and selection criteria.

A systematic search was conducted in March 2025, and this scoping review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews (PRISMA-ScR) guidelines (supplementary file 1) to identify studies related to RTHA [11].

A comprehensive literature search was conducted using two databases (PubMed and Scopus) with the following search terms: ((total hip arthroplasty[Title/Abstract]) OR (total hip replacement[Title/Abstract])) AND (reverse[Title/Abstract]).

No restrictions were applied to the study type or language, and the search was conducted from the database's inception through March 2025. The resultant studies were downloaded into EndNote 20, where duplicates were identified and removed. An initial screening for eligibility was then carried out based on the title and abstract.

The full texts of eligible studies were evaluated for inclusion, and the reference lists of included articles were searched for additional eligible studies. The initial search yielded 656 articles eligible for initial assessment, of which only four were ultimately included. These were identified from the databases and a subsequent reference search, comprising three clinical studies [12-14] and one biomechanical study [15]. Details on the number of citations identified and on what was included or excluded at each screening step, along with the justification for exclusions, are presented according to the PRISMA flowchart (Figure 1).

Data Extraction and Critical Appraisal

The following information was extracted from included studies: author(s); date of publication, country of origin, study type; the final number of patients; follow up period, surgical details (including surgical approach, prosthesis fixation, and so on), clinical outcomes, radiological outcomes, and complications. For the biomechanical study, the testing details and variables evaluated were reported. Furthermore, any relevant outcomes reported by any of the studies were collected.

Results:

Clinical studies (Table 1):

Basic characteristics:

The three clinical studies included were one case series from Canada [13], and two case reports from the US [12, 14], both of which were part of an ongoing clinical trial [16]. The three studies involved a total of 25 patients aged 60 to 79 years (one patient was excluded from the final analysis in the Turgeon et al. [13] study). Thirteen males and 11 females, and all patients were diagnosed with end-stage hip osteoarthritis. The surgery was performed through a minimally invasive direct lateral approach in one patient [12], a direct anterior approach in one patient [14], and a lateral or posterior approach (per surgeon preference) in 23 patients [13].

All studies used the same implant (HIT Hip Replacement System, Hip Innovation Technology, USA), and all were cementless fixation. On the acetabular side, supplementary screws were placed in all cups (two screws in 14 patients, three screws in nine patients, and one screw in one patient).

Clinical outcomes:

Regarding patient satisfaction, 22 (91.7%) patients were satisfied with the procedure and outcomes; however, two (8.3%) patients from Turgeon et al. [13], reported being somewhat dissatisfied.

Detailed functional and PROMs outcomes were only reported for 22 patients by Turgeon et al. [13], where after a mean follow up of two years, the authors reported significant improvement in Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Harris Hip Score (HHS), Oxford Hip Score (OHS), Hip disability and Osteoarthritis Outcome Score (HOOS), 36-item Short Form survey v. 2 physical component summary (SF-36 PCS), and EuroQol five-dimension health questionnaire (ED-5D) (p<0.05). However, they reported a nonsignificant change in the SF-36 mental component summary (MCS) (p = 0.272). One patient still reported occasional pain at the six-month follow up [14].

Radiological outcomes:

Lombardi and Adams reported that the radiographs showed well-positioned implants at three weeks without further details [12].

Turgeon et al. [13] reported the radiological results using radiostereometric analysis (RSA) to assess component fixation and migration. At two years of follow up, the mean acetabular component migration by 24 months was 0.087 ± 0.152 mm, while the mean femoral component migration was -0.002 ± 0.194 mm. The authors reported that the values they reported were significantly lower than the threshold reported in the literature (0.2 mm for the acetabular component, p = 0.005, and 0.5 mm for the femoral component, p < 0.001).

Complications:

Seven (29.2%) complication incidents were reported as follows: one (4.2%) deep periprosthetic joint infection (PJI) required DAIR, then two-stage exchange, one (4.2%) patient experienced femoral neuropraxia (did not require intervention and resolved during the follow-up period), one (4.2%) patient had intraoperative calcar crack (managed with a single cerclage cable), two (8.4%) patients had recalcitrant iliotibial band syndrome (managed by local injections), traumatic dislocation in one (4.2%) patient which occurred on postoperative day 4 (required closed reduction under anesthesia), and occasional pain till the last follow up was reported in one (4.2%) patient.

Biomechanical study:

Braddon et al. [15] evaluated the corrosion at the tapers (acetabular and femoral) and polyethylene wear characteristics.

Taper's evaluation characteristics:

• Visual evaluation of the acetabular and femoral taper surfaces after 50 million cycles revealed no evidence of surface damage (gouges, scratches, or structural damage).

2-      For Grading the Corrosion Levels, Using A Five-Grade Scale:

1. After 10 million cycles, all tapers showed a score of 2 or 3 (indicating discoloration of the taper with minimal pitting or etch marks) after evaluating their superior, anterior, inferior, and posterior faces.
2. After 50 million cycles of femoral taper only, it scored 2 to 4 (indicating between discoloration to some level of pitting and etch marks).

3-      Morse Taper Connections Disassembly Forces Assessment:

1. Femoral taper: after 10 and 50 million cycles of dynamic fatigue, it was 12,957 ± 1797 and 12,506 ± 774 Newtons, respectively.
2. Acetabular taper: after 10 and 50 million cycles of dynamic fatigue, it was 3429 ± 347 and 4329 ± 443 Newtons, respectively.

Polyethylene Wear Characteristics, Mean Wear Rate (mg/Million Cycle (mc): This was evaluated while the acetabular cup placement inclination (ranging from 20◦ to 70◦)/anteversion (ranging from 0◦ to 40◦)

• While there was no gait separation for 5 million cycles, the results showed a mean wear rate of 1.60 ± 0.45 mg/mc at 45°/0°, 1.30 mg/mc at 20°/0°, 1.71 mg/mc at 70°/0°, 1.02 mg/mc at 20°/40°, and 1.27 mg/mc at 70°/40°.
• With a gait separation of 2 mm for 1 million cycles, it showed a mean wear rate of 7.03 ± 0.39 mg/mc at 45°/0°, 6.31 mg/mc at 20°/0°, 7.08 mg/mc at 70°/0°, 5.71 mg/mc at 20°/40°, and 5.76 mg/mc at 70°/40°.
• No edge wear was reported at any of the tested positions.

Discussion:

Although the results published on the newly proposed RHRS are scarce and considered immature, they seem promising, with acceptable functional and radiological outcomes. The complication rate might be considered high; however, most complications were transient and did not require surgical intervention. Furthermore, biomechanical testing demonstrated acceptable prosthesis behavior even under extreme implant positioning conditions. 

Before discussing the results reported in the current review, it is essential to consider its inherent limitations. First, the small number of included studies and the correspondingly small number of included patients. Second, the very short follow up period limits the evaluation of longevity and possible long-term complications. Third, all the included patients were diagnosed with hip osteoarthritis, which carries no high risk for dislocation, to challenge the proposed higher stability of RHRS. Fourth, a potential conflict of interest arises from the manufacturing company's financial support for all included studies. Fifth, there was only one biomechanical study for evaluating the RHRS, which relied mainly on corrosion and polyethene wear evaluation, where further evaluation could be performed, such as liner push-out and lever-out testing, femoral stem fatigue loading in a worst-case configuration, micromotion measurements at the bone–implant interface, impingement-free range of motion, and jump distance. Sixth, although concerns have arisen over the past decade regarding the use of fixed target safe zone numbers for acetabular cup placement, one study reported placing acetabular cups within those classic numbers. Lastly, only two databases were searched; furthermore, owing to the small number of included studies and their nature, detailed quantitative analysis of the results was not possible. Therefore, the current review should be considered a descriptive mapping exercise, and it was not possible to estimate true event rates or to make comparative claims versus conventional THA.

According to Braddon et al. [15], the RHRS is composed of two tapers, one to connect the acetabular ball (cobalt-chrome) to the acetabular shell, while the other connects the femoral cup (highly cross-linked high molecular weight polyethylene) with the femoral stem. The claim primarily relied on providing a more stable design for older and more active populations, with the potential to improve longevity. Although most of the included patients showed acceptable functional outcomes, up to a 91% satisfaction rate, and none of the included patients required revision for implant-related issues, these claims need to be proven based on long-term clinical data, especially with the limited follow up provided in the included studies.

Regarding radiological outcomes, Turgeon et al. reported implant migration measured by RSA using tantalum beads inserted before implant placement [13]. They noted that at 12 and 24 months, the reported migration of the acetabular and femoral components was below the reported limits in the literature, leading to a>5% revision risk at 10 years [17]. However, the authors did not comment on the target zone for acetabular shell positioning, unlike Lombardi and Adams, who reported aiming at the classic acetabular orientation of 45 degrees of abduction and 20 degrees of anteversion [12].

It is worth noting that in the biomechanical study by Braddon et al. [15], the authors evaluated the prosthesis wear characteristics under extreme acetabular shell positioning up to 70 degrees of abduction and 40 degrees of anteversion, with no increase in the edge wear.

Although the reported complication incidents were considerably high (29.2%), most of them were treated non-surgically, except for one deep PJI case that required revision. Turgeon et al. reported one incident of traumatic dislocation after the patient had a fall. The authors treated the dislocation by closed reduction under sedation. They reported that the patient was satisfied with the outcome at the last follow-up, with no other incidents of instability [13].

Lessons learned from previous catastrophic failures with newly introduced hip implants should alert surgeons to the challenges and concerns that may be associated with newly introduced implants, such as RHRS [18-20]. Lombardi and Adams reported that by reversing the implant geometry, as in the RTSA designs, the contact area between the acetabular and femoral components will expand, enhancing stability at the extremes of motion with minimal instability risk [12]. However, the anatomical and biomechanical variations between the hip and shoulder joints should be considered when arguing that one implant design is applicable to the other [21, 22].

Furthermore, adding two tapers might theoretically double the risk of trunion-related issues such as taper corrosion. Although Turgeon et al. reported no problems related to adverse reactions to metal debris [13], two years of follow up is too early to conclude complete safety [23].

Lastly, one of the proposed advantages of RHRS was maintaining the load forces perpendicular to the femoral polyethylene cup throughout the flexion range of motion, assuming this would result in prosthesis stability less dependent on the individual components' positioning [13, 24, 25]. However, none of the authors defined the target for implant positioning. Furthermore, if the implant was designed to address instability issues, it is expected to be used for patients with Spino-pelvic issues. Will there be a specific algorithm or target zones for RHRS insertion?

Conclusion:

Although the RHRS showed promising early radiological and functional outcomes, with relatively fewer complications requiring surgical intervention or revision, the biomechanical assessment revealed wear rates and behavior comparable to those of the respective conventional hip implants. The results should be cautiously interpreted owing to the selected patient category, only those with OA, with no higher risks for instability, and the short clinical follow-up. Therefore, drawing solid, promising conclusions was not possible until longer follow-up studies reporting the in vivo wear and safety profile are conducted.