Polarstem Versus Corail; a Comparison of Micromotion and Periprosthetic Bone Remodelling of Two Femoral Stems

The gold standard in THA the last 45 years has been the all-cemented metal-on polyethylene prostheses such as the Charnley low-frictional arthroplasty. From 1980 and onwards however, uncemented THA have been increasingly popular worldwide, especially for the younger patients. The reasons for this were reports on poor results of the Charnley cemented THA in young patients, and the belief that the cement itself was the cause of periprosthetic osteolysis and loosening of the prostheses. Uncemented THA have good results in some reports and poor results in others. The modern uncemented femoral stems have proven good or even excellent results at medium to long term follow-up in the register and in other clinical studies. The Corail uncemented femoral stem (DePuy) has dominated the Norwegian market on uncemented stems since the mid 1980'ies. Its clinical performance is well documented with good results in terms of implant survival5. The Polarstem (Smith \& Nephew) was introduced in 2002 by a group of surgeons that believed the Corail stem had certain limitations, and the Polarstem is a Corail-like stem with some modifications. The basic design philosophy of a straight tapered design is the same for both stems.

Unlike drugs, medical devices such as orthopaedic implants can be launched on the market without any proof of clinical efficacy. In some cases, inferior implants have been used in thousands of patients before the inferior results were discovered. Some authors propose a stepwise introduction of new implants in order to avoid compromising large numbers of patients. The present study is a step in the introduction of a relatively new femoral stem. High accuracy measuring tools will be used to measure critical factors such as implant migration and bone remodelling at short-term follow-up. The findings will give information on the expected clinical outcome at longer term.

The aims of the study are

• To compare the migration of the Polarstem and the Corail stem with RSA. The null hypothesis is that there is no difference in the early (2 years postoperative) migration of these implants. Early migration has earlier proved to be a surrogate for implant performance at medium and long term.
• To compare any remodeling of periprosthetic femoral bone from direct postoperative measures to later time points. Periprosthetic bone loss, especially in the proximal femur, can compromise the longevity of a prosthetic joint. This bone loss vary and will be compared between the two designs.
• To assess and compare clinical outcome measures and radiological outcomes such as signs of loosening and osteolysis.

Components

• The Polarstem uncemented femoral stem (Smith \& Nephew Orthopaedics AG, Baar, Switzerland) is a straight titanium alloy (Ti6Al4V), tapered, fully hydroxyapatite (HA) coated uncemented implant. HA is used to enhance on-growth of bone to the implant surface and is put on top of a plasma spray porous coating (titanium) in a 150 + 75µm layer. The stem was introduced onto the market in 2002, and no documentation on the clinical efficacy beyond 5 years existed when this study was planned. The design philosophy however is similar to other implants that have good results at long term.
• The Corail uncemented femoral stem (De Puy Int Ltd, Leeds, UK) is a straight, tapered fully hydroxyl-apatite coated stem that was introduced in 1986 in Norway. It has been the most commonly used uncemented stem since, and by 2014 more than 60% of THA patients in Norway received this stem. The 15 year survival was 97% according to a study from 2007.
• R3 uncemented acetabular cup (Smith \& Nephew Orthopaedics AG, Baar, Switzerland) is a fourth-generation hemispheric, porous coated cup intended for press-fit fixation. The cup is based on the well-documented Reflection cup (also Smith \& Nephew) and has a locking mechanism that has proved better stability of the liner in laboratory studies. Further, a new asymmetric coating made of titanium powder (StikTite) with higher porosity and friction has been added. The stability of this cup was found to be excellent in a series of patients measured by RSA. The R3 cup was approved by the European Commission (CE) in 2007 and the Food and Drug Administration (FDA) in 2008. In some countries such as Australia the implant is one of the most frequently used, and according to the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) the results with up to 5 years of follow-up are promising.

Radiography

• Radiostereometric analysis (RSA) is a highly accurate and precise method of evaluating implant migration and polyethylene wear. The precision usually is about 0.1-0.4 mm (translational migration) and 0.1-1 degrees (rotation). The radiation dose is low compared to plain x-rays (10-20%). During surgery, tiny spheres of Tantalum are implanted in periprosthetic bone. The metal implants will be measured with markerless RSA. On day 1-6 postoperatively, two simultaneous exposures of the hip and a co-ordinate system are taken. These films allow three-dimensional definition of the implant and bone relative to the co-ordinate system. Implant migration and rotation is measured on repeat examinations over time. The high precision allows us to detect small differences between groups, and also means that the number of subjects in each group can be restricted. The precision (repeatability) of the measurements in this study will be evaluated by double examinations at the 1-year RSA-visit. Limits for significant differences are calculated as the 99% confidence intervals of the absolute mean values of the double examinations.
• Plain x-rays. At all follow-up consultations plain x-rays of the pelvis, and an AP and lateral view of the hip will be taken. On postoperative pelvic views the varus/valgus position of the stem, and cup inclination will be measured. Later these values will be evaluated together with migration- and wear values. Osteolysis will be measured.
• Computed tomography. In order to investigate the incidence and to quantify periprosthetic osteolysis investigators will perform CT scanning of the patients at 10 years.
• Dual x-ray absorptiometry (DXA). Periprosthetic femoral bone mineral density (BMD) will be measured with DXA. The load transmission to periprosthetic bone differs according to the mechanical properties and design of the implant. Stress-shielding (stress-protection) of periprosthetic bone results in local osteopenia, bone loss, and may contribute to loosening, periprosthetic fractures and complex revision procedures. Most contemporary hip implant designs aim to reduce stress shielding as much as possible.

Our department's standard surgical approach, the posterolateral approach, will be used on all patients. The operations will take place in an operation theatre with laminar airflow, the patients will receive prophylactic antibiotics (Cephalotin 2gx4, or Clindamycin 0.6gx3 if Penicillin allergy) perioperatively, and prophylactics against thromboembolism (Dalteparin 5000 international units for 2 weeks). Implantation of the prosthetic components will be done according to preoperative planning and the manufacturer's advice. 6-10 tantalum beads (Ø=1mm) will be inserted into the proximal femur. The holes are then covered with bone wax.

The patients are allowed immediate weight bearing as tolerated. Walking aids at demand. Physiotherapy is started on day 1 postoperatively and is continued at dismission.

The results of the RSA- and DXA-examinations and clinical outcome measures will be collected in electronic files under the respective patient numbers. Data obtained on previous occasions will not be accessible when examining the patient.

According to the Data Inspectorate, the requirements for a simple 'melding' are

• The responsible (medical doctor) investigator is employed by the organization in which the patient is being registered.
• The patient gives his/her written consent to participation in the study.
• The end of the study is determined at its start.
• The gathered data are deleted or anonymized at the end of the study.
• The study does not electronically merge data from different person registers.

These requirements are fulfilled in the present study, and a "melding" will be submitted to the Data Inspectorate when the administrative details concerning the study are ready.

A patient list will be made coupling patient name and personal identification number to a number on the patient list (1 through 60). The project leader is responsible for the safe handling of the list. Other forms with patient information are labelled with the patient number only for identification. The sponsors of the study will have access to the anonymised data. The study is reported to 'Personvernombudet' through 'Norsk samfunnsvitenskapelig datatjeneste i Bergen'.

Analysis of variance on repeated measurements (ANOVA) will be used on the RSA- and on the clinical data if the data are normally distributed. In our earlier RSA studies, the data have displayed normal distribution. If however, the data are not normally distributed, non-parametric tests (Mann-Whitney U-test, Wilcoxon matched-pairs test) will be used.

Adverse effects, side effects, and unfortunate effects of the surgery will immediately be reported to the project leader and to the sponsors. Abortion or abruption of the study will be considered in the case of unexpected complications.

Funding will be applied for from:

• The Regional Health Services (Helse Vest Regionale helseforetak, Det regionale samarbeidsorganet, Helse Bergen, Haukeland Universitetssykehus, 5021 BERGEN, Norway)
• Smith \& Nephew Orthopaedics AG, Baar, Switzerland

The study will be performed at the Department of Orthopaedic Surgery, Haukeland University Hospital, Bergen, Norway.