Pending Failure in Hard-hard Total Hip Arthroplasty

Every single year approximately 60.000 hip prostheses are implanted in Italy. Quality of life of patients after surgery is very good, as widely demonstrated in clinical experience and in the literature. On the basis of data produced by the only complete hip register in Italy, that is run in our Lab (RIPO, Register of Orthopedic Implants in the Region Emilia-Romagna http://ripo.cineca.it) survival rate of primary total hip prosthesis is 95.6% at 9 years. Reasons for revision are mainly aseptic loosening of one or both components (40% of the total) and dislocation (26%), being the latter much more frequent in the first two years. Less frequently revision is due to sepsis (7% of the total),bone fracture (11%), prosthesis fracture (3%) or pain (3%).

Revision can be related, among the others, to prosthesis-linked factors, such as fixation to bone and articular coupling.

In the experience of RIPO, during the last ten years 32% of implants had both articular component in ceramic, 34% had metal head and plastic liner, 24% ceramic head and plastic liner, 10% both components in metal.

Each solution has strengths and weakness and they have been identified and clinically applied in order to overcome step by step limitations observed in the previous one.

Alumina ceramic-on-ceramic (COC) bearings have been shown to produce substantially less wear debris than traditional metal/ceramic on polyethylene bearings. In addition, the alumina ceramic material itself is essentially bioinert in both bulk and particulate form . Unlike metal-on-metal bearings (MOM), this bearing couple does not release metal ions into the body either from the articulation or from breakdown of wear debris. Weakness of ceramic-on-ceramic bearings are represented by brittleness of the material. Ceramic can fracture, even in very few cases, due to trauma or to malpositioning. Beside this occurrence of noise during gait of patients can be extremely disturbing for patient itself, leading to request of revision. These two solutions represent more recent proposal that technology suggested to overcome problem of wear.

Hard components on plastic has the unsolved problem of wear debris, leading to osteolysis and consequent failure of he im plant. The problem has only partially solved by the introduction of cross linked polyethylene, aimed to improve tribological behaviour of the plastic.

Rizzoli Institute is the only public mono-specialistic research hospital in Italy. Nearly 500 hip prosthesis revisions are performed here every year in the structure. Due to its high specialization difficult cases from all the country are treated here, not on ly when anatomy of the patient is damaged (dislocation of the hip, post traumatic arthritis) but .even more frequently when failure of the implant occurred.

Aim of research proposal is to investigate two uncommon and less investigated early symptoms of failure:

• 'noising hip' in ceramic on ceramic total hip arthroplasty
• pain in metal on metal total hip arthroplasty Even if not numerically so important, these two situations are extremely relevant, as they prelude to a failure of the prosthesis. If clearly identified they could represent a powerful tool in early diagnosis of pending failure.

The first point will be studied through CT scan of the patients to evaluate impingement or instability of the prosthesis, and a needle aspiration for synovial fluid. The fluid will be examined to identify ceramic wear debris, according to a method of separation and analysis in scanning electron microscopy that was set in our Lab.

Sound will be recorded by means of a wearable sensor set capable of recording the articular noise produced during level walking. This instrumentation will be coupled to motion analysis technology, so as to provide a temporal correlation between the recorded noise and the gait cycle. It will be developed in the first phase of the project. In parallel surface analyses will be conducted on retrievals that are stored in the Lab since 2000, in one of the biggest retrieval study ever conducted in our country. Type of damage, surface roughness of both ceramic components will be analysed and compared to clinical data.

Finally the study will be completed in vitro, using an hip simulator, to reproduce the possible clinical scenarios leading to hip noise, record, and analyse it in a reproducible and robust manner. This would make possible a diagnostic approach able to correlate the involved factors to clinical occurrences, on the basis of the recorded frequencies. Second point will be studied through the dosage of circulating ions, deriving from the articular surfaces and through the histological classification of vasculitis in periimplant tissues. There is increasing evidence, indeed that locale release of submicron particles worn out from articular surfaces can release metal ions (mainly Chromium and Cobalt) responsible for lymphocyte local infiltration. In particular cluster of differentiation antigen 20 (CD20) positive B lymphocyte and cluster of differentiation 3 (CD3) positive T lymphocyte and sometimes cluster of differentiation antigen 68 (CD68) positive plasma cells are present. The cells could indicate a specific phlogistic condition responsible for pain and eventually subsequent revision.

Data collected on studied population will allow a better understanding of the two phenomena and possibly will suggest new therapeutic approaches to the problems.