A Randomized RSA Study Comparing HXLPE to Conventional Polyethylene in Cemented Total Hip Arthroplasty

Background:

Total hip replacements is a cost effective treatment of a symptomatic degenerative hip disease. Cemented total hip replacements are the most common implants in Sweden (Swedish hip registry, 2009).

Wear of ultra high molecular weight polyethylene has been a issue with regard to implant survival. At the time of the shift of the millennium highly cross-linked polyethylene was introduced to the markets. Experimental studies had shown reduction of wear in hip simulator studies (McKellop,1999). Subsequently the intention is to reduce osteolysis and implant loosening. Single surgeon pilot studies in the 1970 (Wroblevski et al. 1996, Oonishi 1998, Grobelaar 1999) were the first clinical series with good results leading to the development of 2nd generation highly cross-linked polyethylenes which is now the almost exclusively used PE worldwide. So far medium term clinical evaluation and laboratory assessment have showed reduced wear and good mechanical properties (Digas et al. 2003, 2004, 2007, Röhrl et al. 2005, 2007, D´Antonio et al. 2005)

Production of highly crosslinked polyethylene (HXLPE) consists of several steps. First the PE material is crosslinked by being subjected to either gamma irradiation of various intensity or electron beam irradiation. The second step is to remove the free radicals created during the irradiation. This is done either by annealing (i.e. warming up the PE to just under its melting temperature) or melting (i.e. warming up the PE to above its melting temperature). Finally, the implant is sterilized either by gamma irradiation or Ethylene Oxide and packed in vacuum or inert atmosphere.There are some possible drawbacks and challenges in the process of cross-linking polyethylene with regard to preserving the mechanical properties and elimination of free radicals (Kurtz et al. 2006, Currier et al. 2007).

Submelt annealing preserves mechanical properties but on the other hand cannot eliminate all free radicals existing in the PE. Retrieval-studies of some 2nd generation highly cross-linked polyethylenes manufactured in this way have shown signs of oxidation. The in-vivo oxidation has been located in the non articulating area such as the circumference of the acetabular liner. It has been discussed that the femoral head may protect the polyethylene against oxidation, and hence prevent the possible negative consequences of remaining free radicals (Currier et al 2007).

Melting the polyethylene eliminates all free radicals but alters the mechanical properties. This has led to PE cracks and implant failure in some clinical studies (Furmanski et al. 2009), whereas it has not been seen in other.

Finally, wear particles of highly crosslinked polyethylene are more often in the submicron size compared to conventional PE. Although these particles have a higher biological activity (Fisher et al. 2006), results of recent studies show however, that the overall risk for osteolysis and aggressiveness of highly cross-linked particles is reduced (Galvin et al. 2007).

Because of differences in the manufacturing processes of highly crosslinking PE between manufacturers (i.e. different GUR used, type of irradiation, intensity of irradiation, postirradiative treatment, type of sterilization and packing, etc, etc) all highly crosslinked polyethylenes differ somewhat from each other. One can not extrapolate results of in vitro or in vivo wear studies of one manufacturer to another. Also, because of these manufacturing differences, fixation characteristics may differ between the various types of HXLPEs. Therefore it is imperative that all new highly crosslinked polyethylenes are subjected to clinical studies of their wear and fixation performance using high accuracy methods before being used extensively.

Methods:

A prospective randomized study comparing acetabular components made of HXLPE or conventional PE in total hip arthroplasty. The study will be conducted at Department of Orthopaedics, Umeå University, Umeå, Sweden.

Aim of study The aim of the study is to compare the wear and migration of a 2nd generation polyethylene from Link with conventional polyethylene in an all-poly cemented acetabular cup of the Lubinus brand using radiostereometric analysis (RSA) with 5-year follow-up.

Hypothesis The H0-hypothesis is that the difference with in wear between HXLPE and conventional PE will not increase up to 5-year follow-up.

Material Patients (age 50 years and above) with primary osteoarthritis between 2013 and 2014 at the orthopaedic department at the county hospital (Lycksele) affiliated to Umeå University hospital, Sweden. Patients were treated with a cemented total hip arthroplasty. At surgery, after the acetabulum was reamed, randomization was performed by opening of a consecutively numbered sealed envelope informing whether a cup made of HXLPE or conventional PE should be inserted. The envelopes were kept far away from the operation theatre and were opened by a research nurse contacted by telephone. Randomization was stratified for gender. Within each stratum there was block randomization with 6 permutations in each block.

Patients have been followed for 5 years postoperatively.

Implants • Acetabular cup Intervention: Link-Ticona GUR 1020 HXLPE, 32 mm inner diameter (Trade name Lubinus® X-LINKed® )

Control: Link-Chirulene GUR 1020 UHMWPE, 32 mm inner diameter (Trade name Lubinus ® UHMWPE)

• Femur stem Test and control: Lubinus® SP II®, CrCo femoral head, 32 mm diameter

Radio Sterometric Analysis (RSA) Radiostereometric analysis measurements will be performed 5 years postoperatively.

Wear will be measured as penetration of the femoral head into the polyethylene in the proximal, proximo-medial (2D pm), proximo-posterior (2D pp), and proximo-medial-posterior (3D) directions. Since the penetration during the initial 3 months represents polyethylene yield or bedding in, wear measurements will be reported from 3 months and up to 5 years postoperatively.

Quality of cup fixation will be measured as rotations of the cup around the three cardinal axes in relation to acetabulum with the postoperative examination as reference.

The RSA measurements will be analysed at Umeå University Hospital. Patient data will be saved in a specific research database at Umeå University Hospital following specific guidelines from the national data security committee.

Ethical approval The study has been approved by the Ethics Committee of Umeå University (Dnr 2011-173-31M, 2018-34-31M).

Sample size With a mean difference in wear at 2 years of 0.1 mm or more (SD ± 0.1) between the groups (p< 0.05), 25 patients in each group will give a power of > 80%. In order to cater for losses during the follow up a total of 57 patients will be enrolled.

Scientific Value All new materials should be monitored closely and introduced in small study groups using a high precision measurement technique. Many markets require such tests before acceptance for general use is given. The new highly crosslinked polyethylene from Link-Ticona is not equivalent to current highly crosslinked polyethylenes on the market, and one can not extrapolate wear results from these other designs.