Modern hip implants feature a modular design, whereby the individual components of the implant are assembled during the surgery. Increased reported failure rates associated with the utilization of modular junctions have raised many clinical concerns about the increased release of metal ions/debris leading to adverse local tissue reactions. Implant materials are subject to a myriad of mechanical motion and forces, and varying electrochemical conditions and pH changes from the surrounding environment. To date, no studies have attempted to model the collected data in order to predict the performance of the materials so that precautions can be taken before the problem reaches the critical stage. This study reports the effects of pH variation, displacement variation, and load variation on the mechanical and corrosion behavior of the hip implant modular junction system, tested with a custom-built fretting-corrosion apparatus. The main objective of this study is to combine the complete data set of the in-vitro experiments to create fretting-corrosion wear maps that can predict the dangerous domains of the hip implant modular system.

For each test, the flat portions of two CoCrMo pins were loaded perpendicularly against a Ti6Al4V Rod (Ti alloy) in a Flat-on-flat configuration in a simulated synovial fluid in order to simulate the modular hip implant system. A schematic diagram of contact conditions is presented in Figure 1. A sinusoidal displacement was applied onto the rod, which articulated against the CoCrMo alloy pins, at a frequency of 1Hz. The experiential data from the fretting-corrosion tests has been used to create fretting-corrosion maps. The variables incorporated into the maps include: total mass loss, electrochemical destabilization, pH variation, load variation, displacement variation, and visual examination of the wear features of the contact zone. Total mass loss has been estimated via measurement of the simulator fluid by ICP-MS technique. Electrochemical destabilization was evaluated by a single parameter (V_Drop). The electrochemical destabilization of the tribosystem was evaluated by measuring the drop in potential, V_Drop (V vs. SCE), resultant from the initiation of the fretting phase. The V_Drop refers to the initial cathodic drop in potential in response to the initial onset of fretting motion.

The data from the in vitro fretting-corrosion experiments has been combined to create four fretting-corrosion maps (Figures 2A–3D). Partial slip wear features and mechanical behavior was observed at 25µm displacement. 25–150µm displacement amplitudes showed gross slip behavior. Anything larger than 150µm displayed wear features that were indistinguishable from sliding wear. In general, total mass loss and V_Drop increased with increasing displacement. Samples that were tested at pH 6.0 or higher showed signs of material transfer and higher V_Drop. Finally, there was a general decrease in V_Dropwith increased applied load and pH.

In general, the wears maps were able to offer some predictive validity, however, there were some discrepancies between visual observations and the observed damage parameters. It is possible that other parameters could offer better correlation. Future studies will be conducted to measure other parameters.

For figures/tables, please contact authors directly.

Introduction

There are increasing reports of total hip replacement (THR) failure due to corrosion within modular taper junctions, and subsequent adverse local tissue reactions (ALTRs) to corrosion products. Modular junction corrosion is a multifactorial problem that depends on material, design, patient and surgical factors. However, the influence of alloy microstructure on corrosion has not been studied sufficiently. Especially for cast CoCrMo, there are concerns regarding microstructure variability with respect to grain size and hard-phase volume fraction. Therefore, it was the goal of this study to (1) identify different types of microstructures in contemporary implants, and (2) determine implications of alloy microstructure on the occurring corrosion modes.

Previous studies report the neurological complication rate for shoulder arthroplasty to be 4.3% to 5.0%, However, these studies were limited to total shoulder arthroplasty (TSA) and did not include hemiarthroplasty (HA) or reverse prosthesis arthroplasty (RPA). Our hypotheses were that the neurological complication incidence after shoulder arthroplasty would vary by type of procedure performed and that the overall incidence would be higher than previously reported in the literature.

We retrospectively reviewed the charts of 307 consecutive patients who had a total of 349 SA by the same surgeon between June 1995 and August 2007. Only patients with over six months follow up were included. The charts were reviewed for any sensory or motor disturbance postoperatively. Those who had EMG confirmation of nerve injury (NI) were placed into the surgical complication group, with a second group composed of patients with neurological symptoms (NS) who did not require electromyography (Dr Ji or Matt---how many in the NI group did not have EMG?). These two groups were statistically compared to those patients without neurological injury using standard statistics software. There were 113 HA, 191 TSA and 45 RPA with over 6 month follow up, and there were 10 (10/349; 2.9%)neurological injuries (NI) There was no significant difference in the incidence between the groups (HA: N=3/113, 2.7%; TSA: N=5/191, 2.6%; RPA:N=2/45, 4.4%). There were an additional 34 neurological symptoms (NS) after shoulder arthroplasy, and if included with the NI then the total rate of neurological complaints after shoulder arthroplasty was 12.6% (44/349). If the NI and NS are combined, multivariate analysis showed that there was a statistically significant association between the development of neurological symptoms and revision surgery.

The rate of neurological complications after shoulder arthroplasty was independent of the type of procedure. The incidence of neurological complaints after shoulder arthroplasty is higher than previously reported.

Background and objectives: The antiepileptic drug Phe-nytoin (Diphenyl hydantoin) has been documented to have a beneficial effect on wound healing. Its effect on fracture healing has been investigated to a much lesser extend. In this study we have combined histology, his-tomorphometry and radiology in analyzing the effect of phenytoin on fracture healing, following its local administration.

Methods: Twenty-four Wistar strain rats of 8-9 months age were assigned into two groups of 12 each, which had been matched for age, sex and weight. In one group, selected as the study group phenytoin 20 mg/kg was administered through a 24 gauge needle directly on to the fracture site every 72 hours, while in the controls an equivalent volume of normal saline was administered at the same interval. At 28 days radiographic and histo-logical analysis was done.

Results: Radiographic and histological scoring across the control and test animals did not show any statistical difference. Histomorphometric analysis of the callus however showed that the total periosteal callus on either side of the central bridging callus was mineralized to a greater extend in the phenytoin group animals as compared to the control group animals (p= 0.011).

Conclusion: After analyzing our data, we concluded that phenytoin does have an influence in fracture healing, albeit small, which is primarily on the hard callus region. The hard callus region is the high oxygen tension region and the first region to differentiate. It appears that the effect of phenytoin is probably exerted at the early mesenchymal differentiation stage. However our preliminary work shows that the effect is small and it is not justifiable at this stage to advocate the use of phe-nytoin clinically to augment fracture healing.