by Thierry ASLANIAN
Total hip arthroplasty (THA) consists of replacing a diseased hip with an artificial joint: the hip implant helps to relieve pain and improve a patient's mobility. Over the past 50 years, this procedure has become one of the most reliable in the field of orthopedic surgery and it has contributed to great improvement in the quality of life and activity level of older people.
The Judet brothers tried first in 1950 to replace the femoral head with an acrylic polymer sphere fixed in the femoral shaft with a metal-reinforced stem. But the “modern” hip prosthesis configuration was invented by Sir John Charnley in 1962: he combined a femoral stem with an acetabular cup, with both components fixed to bone thanks to acrylic bone cement [3-5].
Many versions of this implant in terms of materials, shapes, bearings and bone fixation have been designed and commercialized all over the world since that era, but its main principle is still used in hip replacement surgery now. Depending on the implant's technical features, the surgical procedure and the type of patient, the normal lifespan of a hip implant ranges from 10 to 25 years . However, it can be greatly reduced due to four main failure modes:
Birth of the dual mobility concept
- bone fixation failure;
- mechanical failure of the implant;
- wear of the implant materials, particularly the bearing surfaces;
- instability, namely dislocation, in which the femoral head separates from the acetabular cup.
In order to reduce wear, and especially instability, a French surgeon (Gilles Bousquet) decided in 1974 to add another articulation between the femoral head and the acetabular cup [7-9]. This second articulation was located between the inner concave surface of the cup and the outer metal shell of the cup. This was the birth of the dual mobility concept.
For 20 years, the implant was effective in revision indications and for preventing THA dislocation, but the design was ahead of its time and the patents rights have limited the spread of the concept. In addition, some of the design and technological choices made were not the best ones and resulted in failures that were completely independent of, but collateral to, the dual mobility concept. Hence the over covering design of the cup, the poor match of the femoral stem’s neck and the unsuitability of the implant's coating resulted in high rates of:
- iliopsoas impingement;
- dislocation due to excessive wear of the retaining rim;
- and loosening of the cup
This brought into question the reliability of this first generation dual mobility cup [10-17].
Since the first patent has entered the public domain, several French, European, and then American companies have added the dual mobility concept to their product lines, driving technological advancements that have led to second and third generation implants.
Evolutions of the dual mobility concept
Capitalizing on its industrial and orthopedic know-how, Groupe Lépine has launched the QUATTRO® cup in 1997. This cup integrates the most relevant technologies to ensure that safety and efficiency go hand-in-hand with the stability provided by dual mobility cups.
- The cup is forged from cobalt-chrome alloy, which is specific to orthopedic implants. Its metallurgic properties make it highly resistant to deformation when it is impacted into a bed of bone, while reducing the risk of jamming due to liner impingement; it also has the optimal hardness and wettability to reduce wear between the liner and the highly polished surface of the cup.
- Its fixation is ensured by a bilayer coating consisting of a sublayer of pure porous titanium covered by plasma-sprayed hydroxyapatite, which activates and accelerates the implant's biological fixation (by contributing exogenous calcium) into the patient's bone. The titanium sublayer is obtained by vacuum-plasma spraying, which prevents the interposition of damaging titanium oxides and the weakening of its cohesion with the implant or of the implant itself. It has a fundamental role in ensuring the penetration and binding of the newly formed bone after resorption of the hydroxyapatite layer, 3 to 5 years after implantation.
- The polyethylene liner has a long cap that covers the head and prosthetic neck to ensure the durability of the retentive mechanism. The latter is obtained by impacting a 28-mm diameter alumina ceramic head through a constrained rim that has been designed not to deform beyond the material's elastic limit. The covering portion of the liner is formed and chamfered in a way to optimize the compatibility with the neck of the femoral stem and to prevent wear of the retaining rim, which would compromise its retentive ability.
- Lastly, a single-use kit has been developed to counter the risk of intraoperative infection due to the challenges with cleaning and decontaminating the instruments needed for the various trials and checks required when implanting this type of cup. For each procedure, this kit saves time because decontamination is not needed. It is sterilized using a certified industrial process and provides the assurances and safety associated with always having new instruments.
rightTo show that the clinical outcomes with the dual mobility cup were better than those of standard cups, a comparative retrospective study was carried out on two patient cohorts who underwent primary THA between 2000 and 2002. All procedures were performed by the same senior surgeon (Dr. Prudhon) in the same facility with the same cemented femoral stem, the same approach and the same surgical technique [18-21]. The only difference was that a standard cementless cup was used in group 1 (215 patients) and a cementless dual mobility cup was used in group 2 (105 patients). The dual mobility cup had 95% survival rate after more than 10 years of clinical follow-up (confidence interval 81.5–98.8), proof that this option is safe. The dislocation rate was 12.7% in group 1 (standard cups) and only 0.9% in group 2 (dual mobility cups). The statistically significant difference (p = 0.0054) between the two cup designs is an undeniable proof of the dual mobility concept’s effectiveness in reducing dislocation without increasing the wear of the polyethylene liner.
Health economics impact
In France, the birthplace of the dual mobility concept, more than 40% of surgeons regularly use this implant for primary THA procedures, and nearly one-third for revision procedures. The consequence is that the THA dislocation rate in France has consistently decreased over the past 10 years, going from 9% in 2005 to 6.1% in 2015 (all indications combined). From a health economics point of view, when an innovative healthcare procedure needs to be compared to a reference treatment (i.e., dual mobility cups versus fixed cups) the criteria is the ICER (incremental cost-effectiveness ratio). This index measures the additional cost needed to achieve one additional unit of efficacy. This concept was used to analyze a database of more than 80,000 patients who received a hip implant in 2009 and were followed-up for more than 4 years. A French team performed a cost-effectiveness analysis to quantify and evaluate the costs, supported by the organizations that reimburse health expenses in case of hip dislocation . Based on 140,000 primary THA cases, the authors estimated that an annual cost savings of 100 million Euros could be generated in France thanks to the reduction in the dislocation rate with the dual mobility concept.
Developed 40 years ago and improved by successive iterations, the QUATTRO® dual mobility cup is a reassuring option for patients as well as surgeons because it has revolutionized and fastened the postoperative recovery.
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About the Author
Marketing Director – Groupe Lépine
175 rue Jacquard – CS 50307
69727 Genay, France
+33 (0)6 09 90 82 77
Thierry Aslanian is a Pharmacist who holds a postgraduate diploma in Biomedical Engineering (biomaterials focus). He has more than 25 years of experience in the field of joint replacement implants, 20 of them with Groupe Lépine. Lépine has emerged as a European leader in orthopedic surgery by keeping in-house all of the industrial activities related to implant design and manufacturing (casting, forging, digital machining, robotic polishing, plasma projection, packaging in controlled atmosphere, etc.) instead of using subcontractors. Lépine sells its products in more than 100 countries and participates in major international scientific conferences through collaborations with consultant orthopedic surgeons.