Local and systemic concentrations of cobalt (Co) and chromium (Cr) ions may be elevated in patients with accelerated tribo-corrosion at prosthesis bearing surfaces and modular taper junctions. Previous studies by us and others have shown that exposure to these metals negatively affect the viability and function of osteoblasts and osteoclasts in vitro, with implications for bone health. More recently, we have observed an increase in total bone mineral density and reduced bone turnover (TRAP5b and osteocalcin) in patients with well-functioning metal-on-metal hip resurfacing (MOMHR). Here, we provide data to support the hypothesis that osteoclast differentiation and function is altered in this patient population, and that this effect is transferrable through their serum.

Patients with well-functioning MOMHR (n=18) at median follow-up of 8 years were individually matched for gender, age and time-since-surgery to a low-exposure group consisting of patients with metal-on-polyethylene total hip arthroplasty (THA). The median circulating concentrations of Co and Cr for the MOMHR group were 2.53µg/L and 2.5µg/L respectively, compared to 0.02µg/L and 0.03µg/L for the THA group. Monocyte fraction of peripheral blood was isolated from these patients, seeded onto dentine wafers and differentiated into osteoclasts using media supplemented with RANKL and M-CSF (osteoclastogenic media, OM). Cultures were monitored for the onset of resorption, following which they were treated with OM, autologous serum or serum from the other individual within the matched MOMHR - THA pair, all supplemented with RANKL and M-CSF. At the end of the culture, cells were TRAP stained and quantified for total osteoclast number, number of resorbing osteoclasts and percentage resorption using the CellD Software Package (Olympus, Southend-on-Sea, U.K.).

For cells differentiated in osteoclastogenic media, the resorbing ability of osteoclasts derived from MOMHR patients was reduced by 30% (P=0.046) compared to THA. Correlation analyses showed that chronic exposure to Co and Cr trends towards negative association with resorption ability of these osteoclasts (r = −0.3, P=0.06). Furthermore, the resorbing ability of osteoclasts generated from MOMHR patients and differentiated in autologous serum was reduced 33% (p < 0 .0001), whilst matched THA serum caused a smaller reduction of 14% (p < 0 .01). When cells derived from THA patients were differentiated in autologous serum, the resorbing ability of osteoclasts was similarly reduced by 35% (p < 0 .0001), whilst the matched MOMHR serum also caused a reduction of 21% (p < 0 .0001).

Reduced osteoclastogenic response of precursor cells from patients with higher circulating Co and Cr suggests an inherent change in their potential to differentiate into functional osteoclasts. The data also suggests that functional response of mature osteoclasts generated from patient precursor cells are dependent on the prior systemic metal concentrations and the presence of higher circulating CoCr in patients with MOMHR. These effects are modest, but may explain the subtle increase in systemic bone mineral density and decreased bone turnover observed in patients after 8 years exposure compared to age, sex, and exposure-time matched patients who received a conventional THA.

Purpose

Total and partial joint arthroplasty has been clinically proven to successfully relieve pain and improve function in patients with hip and knee degenerative arthrosis. It has been shown that early return to ambulation correlates well with functional scores. Moreover, the benefits of reduced narcotic use are multi-fold and range from reduced risk of addiction, gastrointenstinal and cardiopulmonary side effects. Establishing realistic pre-operative expectations regarding functional improvement and pain control will nevertheless impact patient satisfaction. Thus, the purpose of this study was to establish safe, achievable and data-driven benchmarks for post-operative ambulation and pain control in patients undergoing elective total joint arthroplasty.

Pin-on-disk studies have demonstrated the role that cross-shear plays in polyethylene wear. It has been found that applying shear stresses on the polyethylene surface in multiple directions will increase wear rates significantly compared to linear sliding. Hip and knee joint replacements utilize polyethylene as a bearing surface and are subjected to cross-shear motions to various degrees. This is the mechanism that produces wear particles in hip and knee arthroplasty bearings and if excessive may lead to osteolysis, implant loosening, and failure. The amount of cross-shear is dependent on the bearing diameter and the angular motion exerted onto the bearing due to the gait of the patient. This study will determine the effect of sliding curvature (angular change per linear sliding distance) on the wear rate of polyethylene. Virgin polyethylene blocks were machined with a 28mm diameter bearing surface and against 28mm cobalt chromium femoral heads in a hip simulator. Dynamic loading was applied simulating walking gait but the motion differed between testing groups. Typical walking gait testing utilizes 23° biaxial rocking motion, in this study, 10°, 15°, 20°, and 23° biaxial rocking motions resulting in various sliding curvatures. Sliding motion path is described in Figure 1 and is a function of the bearing radius and the rocking angle. With increased rocking angle, the sliding distance reduces per cycle and the sliding path becomes more curved (more angular change per linear distance of sliding). Despite a significant increase in sliding distance at higher rocking angles, wear rates were relatively unchanged and ranged from 57mm3/mc to 62mm3/mc. Wear rates per millimeter increased exponentially with reduced sliding arc radius (smaller rocking angle) as shown in Figure 2. This study suggests that wear of polyethylene is highly dependent on sliding path curvature. The sliding path is largely a function of the bearing diameter and the patient activity. Large bearing diameter implants have been recently introduced to increase joint stability. Sliding distance increases proportional to the bearing radius which has led to some concerns regarding increased wear in larger bearings. However, in vitro wear studies have not shown this trend. Increased bearing diameter also increases the sliding path curvature which this study has shown to cause a reduction in wear roughly proportional to the radius of the bearing. Therefore, the increase in wear due to sliding distance is offset by the reduction in wear caused by the sliding curvature resulting in no significant change in wear with increased bearing diameter. Curved sliding path causes a change in surface shear direction which has been shown to increase wear of polyethylene. This study confirms that increased cross-shear in the form of more angular change per linear sliding distance can increase wear of polyethylene exponentially