This innovative Metal-on-Polyethylene reverse geometry hip prosthesis differs from conventional systems by positioning the ball within the acetabular cup.

Hip replacement surgery has become increasingly prevalent in the United States, with more than 450,000 procedures performed annually. This number is expected to surge to approximately 850,000 procedures by 2030. However, traditional hip replacement systems face significant challenges, particularly for patients with spinal pelvic disorders who face an 80% higher risk of dislocation within six months post-surgery.

The Reverse Hip Replacement System (HRS) represents a significant advancement in hip replacement technology. This innovative Metal-on-Polyethylene reverse geometry hip prosthesis differs from conventional systems by positioning the ball within the acetabular cup, instead of the femoral stem. Currently undergoing FDA-monitored clinical trials, the Reverse HRS has demonstrated promising results through extensive testing in over 90 pre-clinical experiments. This system aims to enhance stability during extended ranges of motion while reducing dislocation risks, particularly benefiting patients with complex hip conditions.

In this article, we will examine the technical specifications, biomechanical principles, surgical techniques, and clinical outcomes of the Reverse HRS, providing a comprehensive understanding of this innovative technology.

Biomechanical Principles of Reverse Total Hip Arthroplasty

The fundamental innovation of the reverse total hip arthroplasty stems from a complete rethinking of traditional component arrangements. This approach draws inspiration from successful reverse shoulder arthroplasty designs that have demonstrated enhanced joint stability [1].

Traditional hip replacement systems position the ball (femoral head) on the femoral stem with the polyethylene liner seated in the acetabular cup. In contrast, the Reverse HRS employs an inverted configuration. The metal ball sits on a trunnion within the acetabular cup, and the polyethylene liner attaches to a femoral cup connected to the femoral stem [2]. This fundamental alteration creates a cup-on-ball articulation rather than the conventional ball-on-cup design, enabling the femoral cup to articulate around the acetabular ball [3].

Center of Rotation Mechanics in the Reverse HRS

Despite this significant geometric change, the center of rotation in the Reverse HRS remains analogous to a normal physiological hip or a well-positioned traditional total hip arthroplasty [2]. This preservation of normal hip biomechanics allows for proper force distribution across the joint while simultaneously introducing new advantages. Load forces maintain perpendicular orientation to the polyethylene-lined cup throughout the entire flexion arc, resulting in stability less dependent on absolute precise component positioning [4].

The exceptional stability of the Reverse HRS derives from its unique component interaction. As the hip undergoes flexion-extension, abduction-adduction, and internal-external rotation, the femoral cup overlaps and articulates with the acetabular ball [2]. This interlocking mechanism significantly reduces impingement and dislocation risk [4]. Clinical testing has demonstrated that dislocation with the Reverse HRS requires both traction and laterally directed force in a neutral position—a combination rarely occurring in normal movement patterns [4].

Reduction of Edge Loading and Contact Stress

A critical advantage of the reverse geometry design lies in its management of contact forces and wear patterns. The unique configuration provides optimal surface area contact between the acetabular ball and femoral cup, significantly reducing edge loading [2]. Edge loading in traditional systems occurs when the contact point between head and liner cup extends over the liner rim, creating increased local pressure, lubrication disruption, and accelerated wear [5]. The Reverse HRS design distributes wear evenly across the contact surface of the polyethylene liner [6], reducing high-contact stresses and minimizing generation of wear debris [3]. This advantage is further enhanced by using highly cross-linked polyethylene (HXLPE), which further reduces osteolysis and aseptic loosening risks [6].

Fundamentally, the Reverse HRS addresses key biomechanical limitations of traditional hip replacement systems while maintaining normal joint mechanics. Its forgiving design can help to compensate for variability in component placement including higher abduction angles and anteversion of the acetabular cup [2], reducing the relationship between component placement, wear, and stability [2].

Technical Specifications of the Reverse Hip Replacement System

The Reverse Hip Replacement System combines innovative design with advanced materials to create a unique prosthetic assembly that addresses longstanding challenges in hip arthroplasty. Each component has been meticulously engineered to support the reverse geometry configuration while maintaining durability and biocompatibility.

The femoral stem of the Reverse HRS is available in sizes ranging from 9 to 21 in both Standard (128°) and High Offset (133°) configurations [7]. The proximal portion features a plasma-sprayed commercially pure titanium coating, facilitating uncemented biological fixation with the prepared bone surface [7]. This porous coating provides secure intermediate attachment until osseointegration occurs.

The femoral stem connects to a femoral cup, which subsequently attaches to a polyethylene liner—a reverse of the traditional configuration where the polyethylene attaches to the acetabular cup [8].

The acetabular cup features a hemisphere design with a distinctive male taper that mates with the acetabular ball [7]. Currently being studied in sizes ranging from 52–58mm in 2mm increments [7], the cup will likely expand to broader size options as development continues. The outer surface utilizes the same plasma-sprayed titanium coating as the femoral component to promote bone ingrowth [7].

Additionally, the acetabular cup incorporates a cluster of three threaded holes strategically positioned for placement of custom fixed-angle locking titanium bone screws [7]. The central taper within the cup creates the mounting point for the 26mm cobalt-chromium ball [9][7], which forms the articulating surface with the polyethylene liner.

The highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) liner represents a crucial element in the system’s wear reduction strategy [7][10]. Hemispherical in shape, the liner includes a distinctive circular tab [7]. Both the rim and this circular tab feature fluting designed specifically for torsional stability once implanted [7].

Notably, the polyethylene liner attaches to the femoral cup rather than the acetabular component as seen in conventional designs [11]. This configuration maintains the ball in constant contact with both components throughout the range of motion, distributing wear evenly across the contact surface [10]. The highly cross-linked composition contributes to minimizing edge loading and reducing the risk of osteolysis from wear debris generation [10].

Modularity remains a cornerstone of the Reverse HRS design, providing surgeons with extensive intraoperative flexibility. The femoral cups are available in multiple lengths, including 0mm, +3mm, +6mm, and +9mm lengths [7], enabling precise anatomic fit and proper muscular tension. This adaptability proves invaluable when addressing varying patient anatomies.

The integrated modularity of the Reverse HRS consequently allows surgeons to adjust length, offset, and component orientation while maintaining the biomechanical advantages inherent to the reverse design. Furthermore, the system is designed for cementless application [12], relying on biological fixation for long-term stability.

Surgical Technique and Implantation Procedure

Similar to all total hip systems, successful implementation of the reverse hip replacement system requires meticulous surgical planning and precise technical execution. The unique reverse geometry design necessitates specific procedural adaptations compared to conventional hip arthroplasty, although fundamental surgical principles remain unchanged.

Preoperative Planning Considerations

Preoperative planning serves as the foundation for optimal surgical outcomes in reverse total hip arthroplasty. Templating is predictive of implant sizes and for determining the optimal cup position in terms of center of rotation, height, depth, and angular position [13]. While two-dimensional templating remains common for uncomplicated primary procedures, three-dimensional templating based on CT data offers superior accuracy, particularly for complex cases [4].

A standardized standing pelvic radiograph constitutes the minimum imaging requirement, ideally taken with both iliac spines at equal distance from the film [4]. Accurate assessment of magnification is crucial, with best practices involving the use of calibration markers—such as a belt with spheres of known diameter on the patient’s abdomen or a rectangle with measured lines leaning on the examination table [4].

Before surgery, surgeons must identify anatomical landmarks including the medullary canal, greater and lesser trochanters, acetabular roof, and teardrop [5]. This identification allows for evaluation of femoral rotation, pelvic inclination, and symmetry [4], factors that significantly affect component positioning.

Step-by-Step Implantation Protocol

The surgical approach for reverse hip arthroplasty can be performed through standard techniques, including anterior, posterior, or direct lateral approaches [7]. Following proper exposure:

1. The femoral head is resected, either after dislocation or in situ at a 45° angle to the femoral longitudinal axis, commencing medially and distally from the superior aspect of the greater trochanter [7].
2. Acetabular preparation begins with progressive reaming until healthy bleeding bone is exposed, creating a hemispheric dome [7]. Reaming is typically line-to-line, with the definitive acetabular shell being 1mm larger than the final reamer size to accommodate press-fit fixation [14].
3. The acetabular shell is inserted at appropriate inclination (40-45°) and anteversion (25-30°) angles [7]. Additional fixation may be achieved using titanium locking bone screws through the clustered set of holes [7][15].
4. Femoral canal preparation starts with opening the canal using a box chisel osteotome, followed by either reaming, broaching, or a combination of both techniques [7]. Sequential broaching continues until adequate cortical contact is achieved [7].
5. Trial components are assembled and reduced by lifting the femoral cup onto the trial acetabular ball while applying downward traction to the leg [7]. Leg length, stability, and range of motion are then evaluated.
6. After confirming proper fit, the definitive components are inserted. The femoral stem is seated to the same level as the final broach trial, followed by impaction of the acetabular ball onto the acetabular trunnion [7].
7. Finally, the polyethylene liner is snapped into the metal femoral cup, and the hip is reduced by lifting and guiding the femoral cup over the acetabular ball [7].

Component Positioning Tolerances

Accurate biomechanical reconstruction through proper component positioning is paramount for function and longevity [13]. For the acetabular component, Lewinnek defined a safe zone of 15° ± 10° anteversion and 40° ± 10° lateral opening to minimize dislocation risk [16]. Nevertheless, modern arthroplasty is moving toward patient-specific approaches rather than universal safe zones [16].

Cup inclination should generally be maintained between 40-45° for metal-on-polyethylene implants [17]. Acetabular positioning becomes less critical with the reverse hip design since the system provides stability even with suboptimal component positions, however ideal component orientation is still recommended [14].

Femoral component positioning must minimize excessive varus or valgus implantation, as these may alter the femoral offset and abductor lever arm, potentially compromising clinical outcomes [16]. The femoral stem should be seated at the templated depth, ensuring proper restoration of leg length and offset [5].

Proper positioning of both components ultimately aims to restore the hip’s center of rotation, combined offset, and leg length, thereby optimizing functional outcomes and implant longevity [5].

Clinical Outcomes from Radiostereometric Analysis Studies

Rigorous clinical validation represents a critical step in evaluating novel orthopedic implant designs. The Reverse Hip Replacement System has undergone thorough assessment using radiostereometric analysis (RSA), a validated technique that predicts long-term implant stability by studying early migration patterns [6].

Implant Fixation Metrics at 24 Months

A prospective cohort study evaluated implant fixation in 22 patients (11 male/11 female) with a mean age of 70.6 years (SD 3.5) and average BMI of 31.0 kg/m² (SD 5.7) undergoing reverse total hip arthroplasty [6]. In this study, RSA markers were inserted into the innominate bone and proximal femur with imaging performed at six weeks (baseline), six, 12, and 24 months [6]. All patients received at least one acetabular screw for additional fixation [6].

Mean acetabular subsidence from baseline to 24 months measured 0.087 mm (SD 0.152), markedly below the critical threshold of 0.2 mm (p = 0.005) [6][1]. For the femoral component, mean subsidence was -0.002 mm (SD 0.194), substantially below the published reference of 0.5 mm (p < 0.001) [6][1]. First and foremost, these measurements demonstrate excellent fixation of both components.

Migration Patterns Below Detection Threshold

According to multiple studies, the novel reverse-geometry total hip arthroplasty demonstrated minimal migration between 12 and 24 months for both femoral and acetabular components [1]. Indeed, this migration remained effectively below the detection limit of RSA technology itself [1]. This exceptionally stable fixation exceeds the performance of many traditional implants currently in clinical use.

For proper context, acetabular cups with mean migration of 0.2 mm or less are classified as having “acceptable” performance, those between 0.2-1.0 mm are considered “at risk,” and those exceeding 1.0 mm are deemed “unacceptable” [1]. Likewise, uncemented hip stems demonstrating migration below 0.5 mm consistently show revision rates under 5% at ten years [1].

Predicted 10-Year Revision Risk Assessment

RSA research has established strong correlations between early implant migration and long-term revision risk. Given that the Reverse HRS showed migration well below established thresholds, the predicted risk of revision for aseptic loosening, a common cause of implant failure requiring revision surgery, at ten years is exceptionally low [1][6][14].

Beyond fixation metrics, the study documented significant improvements in all patient-reported outcome measures from preoperative baseline to 24-month follow-up [18]. These clinical outcomes aligned with expectations for safe and effective hip replacement prostheses [6], confirming that the biomechanical advantages of the reverse design translate to real-world patient benefits.

Comparative Performance Against Traditional Hip Systems

The incidence of dislocation with traditional hip replacement remains a prominent concern, with rates ranging from 0.12% to 16.13% and a pooled rate of 2.10% over a 6-year average follow-up period [2]. Moreover, studies have shown that traditional hip systems carry a 48% increased risk of any failure leading to revision surgery at six months for patients with lumbar fusion [3]. In contrast, the hope for the Reverse Hip Replacement System is a reduction in the risk of dislocation through its unique ball-on-socket configuration [20].

Dislocation following traditional total hip replacement is associated with repeated hospitalizations and substantial costs to healthcare systems [2]. Primarily, studies indicate that over half of dislocations occur within the first three months following traditional hip replacement [2]. The posterior approach historically demonstrates higher dislocation rates than anterior approaches [21]. Patients with previous spinal fusion experience an 80% increase in dislocation risk at six months with traditional systems [3].

The Reverse HRS provides greater range of motion in all planes while maintaining hip stability [23]. For daily activities requiring up to 125° hip flexion, 19° external rotation, and 15° abduction, the reverse design offers substantial benefits [16]. Consequently, the ball maintains constant contact with both the acetabular and femoral components throughout movement, preserving stability at extended ranges of motion [10]. This design compensates for hip motion variations between individuals that might otherwise produce deleterious effects on bearing components during dynamic situations [16].

Edge loading in traditional hip replacement occurs when the contact point extends over the liner rim, resulting in increased local pressure and accelerated wear [16]. First and foremost, the unique design of the Reverse HRS provides optimal surface area contact between the acetabular ball and femoral cup, significantly reducing edge loading [24]. Hence, this configuration reduces high-contact stresses, decreases overall implant wear, and produces uniform wear patterns [24].

Conclusion

Radiostereometric analysis studies conclusively demonstrate the Reverse Hip Replacement System’s exceptional stability and minimal migration patterns, suggesting remarkably low ten-year revision risks for aseptic loosening. These findings, coupled with the system’s innovative reverse geometry design, address longstanding challenges faced by traditional hip replacement systems.

The system’s technical advantages stem from several key features:

• Unique ball-on-cup configuration reducing edge loading concerns
• Flexible component positioning tolerances reducing surgical complexity
• Enhanced stability during extreme ranges of motion
• Uniform wear patterns minimizing osteolysis risks

Clinical evidence supports substantial improvements across all patient-reported outcomes through 24-month follow-up periods. Most significantly, the system shows particular promise for patients with spinal-pelvic disorders, who traditionally face elevated dislocation risks with conventional systems.

The combination of advanced materials, precise engineering specifications, and thorough surgical protocols positions this technology as a significant advancement in hip arthroplasty. While long-term studies continue, early clinical results suggest the Reverse Hip Replacement System represents a viable solution for complex hip replacement cases where traditional systems face limitations.

References

[1] – https://pmc.ncbi.nlm.nih.gov/articles/PMC10206517/
[2] – https://pmc.ncbi.nlm.nih.gov/articles/PMC7612258/
[3] – https://bonezonepub.com/2025/03/07/fda-allows-compassionate-use-for-reverse-hip-system/
[4] – https://pmc.ncbi.nlm.nih.gov/articles/PMC6851526/
[5] – https://www.actaorthopaedica.be/assets/1789/02-Scheerlinck.pdf
[6] – https://pubmed.ncbi.nlm.nih.gov/37222043/
[7] – https://surgicaltechnology.com/OpenAccess/1798-Lombardi-OS-FINAL-(c).pdf
[8] – https://www.hipinnovationtechnology.com/product-information-innovation.html
[9] – https://bonezonepub.com/2022/03/15/hip-innovation-technology-develops-reverse-hip-system/
[10] – https://hipinnovationtechnology.com/product-information-innovation.html
[11] – https://www.prnewswire.com/news-releases/reverse-hip-replacement-system-unveiled-at-global-orthopedic-surgery-meeting-302063376.html
[12] – https://www.mdpi.com/1996-1944/10/7/751
[13] – https://pmc.ncbi.nlm.nih.gov/articles/PMC5525519/
[14] – https://www.healio.com/news/orthopedics/20250312/reverse-hip-replacement-system-may-have-promising-outcomes
[15] – https://patents.google.com/patent/US20170035571A1/en
[16] – https://www.ncbi.nlm.nih.gov/books/NBK565771/
[17] – https://www.sciencedirect.com/science/article/abs/pii/S0883540302001006
[18] – https://www.healio.com/news/orthopedics/20230803/reverse-tha-may-provide-consistent-solid-bony-fixation-of-femoral-acetabular-components
[19] – Acta Orthopaedica 2024; 95: 472–476
[20] – https://pubmed.ncbi.nlm.nih.gov/39028111/
[21] – https://www.hss.edu/article_anterior-vs-posterior-hip-replacement.asp
[22] – https://aoao.org/2024/08/21/outcomes-of-direct-anterior-versus-posterior-approaches-in-total-hip-arthroplasty-a-systematic-review/
[23] – https://ajay.odt.rodmanadmin.com/breaking-news/hip-innovation-technology-begins-ide-study-of-its-reverse-hip-replacement-system/
[24] – https://hipinnovationtechnology.com/product-information-benefits.html
[25] – https://pmc.ncbi.nlm.nih.gov/articles/PMC9680524/

Steven MacDonald, MD, FRCS(C), Professor of Orthopaedic Surgery at the University of Western Ontario in London, Ontario, Canada

Dr. Steven J. MacDonald is a Professor of Orthopaedic Surgery at Western University, and London Health Sciences Centre, University Campus in London, Ontario. He is a native of London, Ontario and received his undergraduate Bachelor of Science Degree at Western. He then received his medical degree from the University of Toronto and returned to Western for his orthopaedic training.

Further training as an adult reconstructive fellow was obtained in Chicago, Illinois and in Bern, Switzerland. Dr. MacDonald has won many awards including, twice the John Charnley award from the Hip Society, the Frank Stinchfield award from the Hip Society, the Insall Award from the Knee Society, the Mark Coventry award from The Knee Society, and the Founders Medal from the Canadian Orthopaedic Research Society. Dr. MacDonald is a member of multiple orthopaedic organizations including the North American Hip Society, the International Hip Society and the North American Knee Society. Dr. MacDonald has been awarded 30 research grants for hip and knee replacement projects. He has published over 250 papers in research journals in the field of hip and knee surgery. He has lectured at over 300 national and international meetings. His current focus of research includes multiple randomized clinical trials on hip and knee replacements including the evaluation of new technology.