Weber F.A. – M.Med (Orth) Pret. (F.R.C.S) Canada
Sandton Medi-Clinic, Johannesburg, South Africa – Department of Orthopaedic Surgery, Wits University
Grobbelaar C.J. – Mb Ch B Pret. M.Med (Chir) Pret.MD Pret
Du Plessis T.A. – Ph.D
Cakic J N – MD, FCS (SA) Orth, M.Med (Orth) Wits, South Africa
The purpose of the study is to report on long term wear and clinical results of highly crosslinked U.H.M.W.P.E acetabular cups implanted while performing total hip replacements between 1977 - 1984 and again from 1998 onwards. This information is obtained from the material used for a PhD thesis being submitted to the division of Orthopaedic surgery, Department of Health Sciences University of the Witwatersrand by author no 4. Dr Cakic and represents three decades of experience of the authors with this method.
MATERIAL AND METHODS:
In 1978 Grobbelaar and du Plessis (authors no 2 + 3) reported on their method of crosslinking (J.B.J.S Br. 60.370. 1978) Radiation crosslinking U.H.M.W.P.E was done using Hostalen RCH 100 with a molecular weight of 3.5 - 4 million. It was done in a stainless steel container in acetylne gas at the Atomic Energy Board in Pretoria. 100 Kilogray (10 Megarad) gamma radiation was applied. Annealing was carried out in a vacuum at 80 C for 8 hours. Packaging was done in laminated plastic bags in standard environmental air. Radiation sterilization was done at 25 kGy (2.5mRAd) subsequently. It was called the Pretoria prosthesis and had a cemented cup and monoblock stainless steel stem with a 30mm head. During routine follow up of these hips very low radiological wear was evident, and a recall of 293 hips done by the senior (author no1) and followed up regularly to date in ever diminishing numbers.
1996: 160 patients with 166 prosthesis were traced representing 63 % of the target group. At average 16.59 years follow up and 87.2 % implant survival of the 166 hips. 19 Revisions were recorded at average 13.88 years. Only 2 appeared wear related. 1999, 2004, 2006 and 2007 the same group was again followed up in dwindling numbers due to death, emigration or retirement elsewhere. Not a single wear related revision was further recorded in these crosslinked cups. Patients that had non crosslinked cups on the opposite site showed wear and granuloma related failure at average 15 years. Histology at revision and from post mortem retrievals even after 25 years, yielded a paucity and even absence in some cases of H.D.P. Particles.
RADIOLOGICAL FOLLOW UP:
Only patients with a full set of x-rays i.e. in the first post-operative year and at final evaluation were included. The evaluation was done according to the Martell hip analysis suite.
46 Cases had an average follow up of 18.17 years with 2 D Vector wear of 0.0285mm per year and 17.612 2 D AP volumetric wear per year. At the time of writing the summary 11 more cases with complete x-rays are evaluated bringing the total average follow up+ to 20.5 years with average wear .0983mm pa. Retrieval SEM studies on two cups were done by Dr Ian Clarke Loma Linda University California confirmed low wear with machine marks present at 12 years and at 20 years the pre-fibrillation stage has only been reached. It has been reported by him. Prof D Burger from the Department of Aeronautical and Mechanical Engineering University of Pretoria after studying more retrievals, comming to a simular conclusion.
In 2002 Dr Stephen Li, Sarasota Florida, USA at his Medical Innovations Company compared cups from this crosslinking process to other commercially available crosslinked cups in a hip simulator study of 6 million cycles and concluded that if wear existed it was so minimal could not be measured by the equipment and reported it in USA and international meetings. Following the success with this initial crosslinked group of patients as evidenced by the 1996 follow up the crosslink project was revived and by mid 1998 operational
Technological refinements included a better annealing process for the inhibition of polymer oxidation. The final product is packed in analytical nitrogen. On theoretical grounds and on available knowledge this should probably result in prolonged if not indefinite shelf life.
Crosslinked cups were now routinely inserted for both uncemented and cemented acetabular implants. Using the same implants since 1991, a comparison was possible between the pre and post 1998 implants as all surgery was done by author no 1 using either a cemented cup or uncemented pressfit Mont Blanc cup with a Zirconia - Prozyr head and Fare / Isofar cemented stem. Prosthesis supplied by Aesculap France ICP. Four groups of patients became available for comparison All prosthesis had the same cemented, polished stem and 28mm ceramic head.
By having post 1998 cups crosslinked as opposed to those implanted from 1991, we had comparable but not double blind groups for comparison.
1991-1998, 25 Uncemented non crosslinked cups at average 6.4 years with 0.3918mm wear per year
1998, 31 Cases onwards uncemented crosslinked cups followed average 5.95 years with 0.05085 mm wear per annum.
1991, 22 patients non crosslinked uncemented cups average follow up 6.42 years had .3918mm average wear per annum.
1998 onwards 25 patients received cemented crosslinked cups followed average 4.9 years with 0.0838mm per annum. Thus at least 5 to 7 Times improvement in wear was achieved by crosslinking the U.H.M.W.P.E
More than 12 000 acetabular cups were crosslinked by this method since 1998 has been supplied by various companies and surgeons by Dr du Plessis from Gammatron the responsible company and to date not a single material problem has been reported.
The low wear of this method of crosslinking has been demonstrated both in Vivo and in Vitro and the long term low wear is comparable to that of Alumina on Alumina. The absence of material problems and breakages as well as very low incidence of particle shedding and absence of granuloma formation makes it the bearing surface of choice in most hip replacements regardless of whether a ceramic or metal head is used. An important feature also is the relative cost effectiveness as compared to other alternative bearings.