Abraham Salehi

Polyethylene wear performance of oxidized zirconium and cobalt-chromium knee components under abrasive conditions.

The surfaces of retrieved cobalt-chromium (CoCr) total knee arthroplasty femoral components show evidence of roughening ( Fig. 1 ) 1-3. In vitro studies have shown that scratches on the hard counterface, particularly those at an angle to the direction of motion, can increase wear of ultra-high molecular weight polyethylene 4-10. An alternative material, oxidized zirconium (OxZr), was developed to provide an improvement over CoCr in resistance to roughening, frictional behavior, and biocompatibility 11-16. Previous knee simulator testing under clean conditions (without intentional addition of abrasives) demonstrated that articulation with OxZr femoral components resulted in rates of wear of the ultra-high molecular weight polyethylene that were more than sixfold lower than those obtained with CoCr femoral components 17. Because femoral components roughen clinically in a way that can increase wear of the ultra-high molecular weight polyethylene insert, simulator testing under abrasive conditions also was needed to better characterize the performance of the femoral component material. Previously, adding abrasives into the test media during simulation did not produce relevant conditions, so a technique was developed to roughen the surface of the femoral components by tumbling them with alumina powder and plastic cones before simulator testing 18. In the present study, we compared the wear performance of CoCr and OxZr in an anatomic knee simulator under these abrasive conditions. Fig. 1: Hard particles scratch cobalt-chromium surfaces under clinical conditions, plowing up adjacent peaks that can increase abrasive wear of polyethylene, as seen on this interferometer image of a retrieved clinical specimen 1,2. The wear performance of three cast CoCr (ASTM F75) and three OxZr (Oxinium) femoral components were compared. Femoral components were tumbled with 25 μm alumina powder and plastic cone media in a centrifugal finishing barrel 18. The inserts …

Arthroplasty options for the young patient: Oxinium on cross-linked polyethylene.

Our purpose was to determine whether metal femoral heads scratch with in vivo use, to characterize the scratching that occurs, and to determine whether this scratching affected polyethylene wear. Assessment of 133 consecutive retrieved femoral heads showed that metal femoral heads do scratch with in vivo use, that cobalt-chromium femoral heads are more scratch resistant than titanium alloy heads, and that scratching seems to be time dependent. Profilmetry studies showed that all roughness parameters (average roughness, maximum peak to lowest valley distance, mean peak height above the mean surface line, estimate of small peaks above the main plateau of the surface, and estimate of the depth of the valleys below the mean plateau of the surface with the exception of the symmetry of the profile about its mean line) showed increased roughness with time of use. Cobalt-chromium and Oxinium femoral heads were damaged in a dislocation model. Assessment of these femoral heads in a wear simulator revealed that against conventional polyethylene, a damaged Oxinium femoral head had no more wear than a new cobalt-chromium articulation on the same polyethylene (36.53/million cycles versus 38.4 mm3/million). Against cross-linked polyethylene, a damaged Oxinium femoral head had minimal wear (1.5 mm cubed per Mc).

Can CT-based patient-matched instrumentation achieve consistent rotational alignment in knee arthroplasty?

PurposeLong-term success of contemporary total knee replacements relies to a large extent on proper implant alignment. This study was undertaken to test whether specimen-matched cutting blocks based on computed axial tomography (CT) scans could provide accurate rotational alignment of the femoral component.MethodsCT scans of five fresh frozen full leg cadaver specimens, equipped with infrared reflective markers, were used to produce a specimen-matched femoral cutting block. Using those blocks, the bone cuts were made to implant a bi-compartmental femoral component. Rotational alignment of the components in the horizontal plane was determined using an optical measurement system and compared with all relevant rotational reference axes identified on the CT scans.ResultsAverage rotational alignment for the bi-compartmental component in the horizontal plane was 1.9° (range 0°–6.3°; standard deviation 2.6°). One specimen that showed the highest deviation from the planned alignment also featured a completely degraded medial articular surface.ConclusionsThe CT-based specimen-matched cutting blocks achieved good rotational alignment accuracy except for one specimen with badly damaged cartilage. In such cases, imaging techniques that visualize the cartilage layer might be more suitable to design cutting blocks, as they will provide a better fit and increased surface support.

Can rotational congruity be achieved in both flexion and extension when the femoral component is externally rotated in total knee arthroplasty

Abstract Commercially available total knee implants were cemented onto a custom made jig which provided an axis of rotation perpendicular to the mechanical axis. The femoral component was fixed in 3° of external rotation and articulated with a tibial component implanted in either 3° of external rotation or neutral rotation. A modified femoral component having a thicker posterior lateral than posterior medial condyle was implanted in neutral rotation to articulate with a neutrally rotated tibial component. Fuji film placed in both the medial and lateral compartments was used to determine the centroids of the medial and lateral tibiofemoral contact areas and rotational congruity between the femoral and tibial components. When the femoral component was externally rotated, rotational incongruity of 3.53° occurred in extension if the tibial component was not externally rotated, and rotational incongruity of 3.23° occurred in flexion if the tibial component was externally rotated. With the modified femoral component the angle of rotational congruity remained relatively constant at approximately 0.15° throughout the flexion range and did not exceed 0.20°.

Comparison of Surface Characteristics of Retrieved Cobalt-Chromium Femoral Heads With and Without Ion Implantation

Nitrogen ion implantation of CoCr is reported to produce increased surface hardness and a lower friction surface. Femoral heads with and without ion implantation retrieved from 1997 to 2003 were evaluated for surface roughness (average surface roughness [Ra], mean peak height [Rpm], and maximum distance from peak to valley [Rmax]), nanohardness, and the ion-treated layer thickness. The difference in average Rmax (P = .033) and average Rpm (P = .008) was statistically significant, but there was no correlation between the average or maximum roughness parameters (average surface roughness, Rmax, and Rpm) and time in vivo (P > .05). Overall, nanohardness was greater for the low-friction ion-treated heads (P < .001); and it decreased with increasing time in vivo (P = .01). Ion treatment produces an increased surface hardness, but the advantage of this increased hardness appears to dissipate over time in vivo.

Photoelastic analysis of stresses produced by different acetabular cups.

Porous-coated acetabular components can provide long-term biologic fixation to bone. However, the periacetabular stress patterns and mechanisms by which different types of cementless acetabular cups obtain initial stability is not clear. In the current study, periacetabular stresses produced by different cementless acetabular cup geometries were quantitated using a three-dimensional photoelastic model. The cup geometries consisted of trispiked, finned, hemispherical, and nonhemispherical (wider than a hemisphere at the periphery) geometries. The cup models were loaded incrementally in the photoelastic material to simulate periacetabular stress distributions at the time of implantation during surgery rather than under physiologic weightbearing loads. The peripheral stress distributions and their magnitudes induced by the trispiked and oversized hemispherical cups were similar, but the trispiked cup induced localized high stress regions where the spikes penetrate the bone model. The fins separated the periacetabular material into quadrants, which was associated with decreased peripheral stresses. A nonhemispherical geometry with a wider diameter at the rim than a hemisphere increased peripheral stresses more than an oversized hemispherical geometry and required less force to seat the implant. Although various cementless acetabular cups can perform well clinically, they produce different periacetabular stresses and appear to obtain initial fixation by different mechanisms.

Wettability Analysis of Orthopaedic Materials Using Optical Contact Angle Methods

The wettability behavior of orthopaedic materials influences the fluid film layer that affects both the friction and wear of the articulating surfaces in total joint arthroplasty. This study examined the wettability of various orthopaedic bearing materials such as alumina, zirconia, cobalt chrome (CoCr), and oxidized zirconium (OxZr). Diamond-like carbon (DLC) coating on CoCr was also examined. Additionally, the effect of radius of curvature was examined using OxZr femoral heads of various diameters. The contact angle of the liquid droplet on the surface of the material was measured using a optical contact angle method. Both water and bovine serum with 20 g/L protein concentration were used during testing, with a droplet size of 0.25 -L. The droplet was dispensed from an automated syringe and brought into contact with the sample surface. The contact angle was then measured by fitting polynomial curves to the sample surface and drop geometry. Ten individual drops were analyzed on each test component, with at least three test components for each material. There were no differences in contact angles with changing head size or when using serum compared to water. The alumina, OxZr, and zirconia femoral heads all exhibited a similar contact angle, while CoCr and DLC showed significantly greater contact angles. The smaller contact angles for the oxide ceramic surfaces indicate that they tend to be more wettable than the metals, which may help explain their lower friction and superior adhesive wear performance.

Heat Generation and Dissipation Behavior of Various Orthopaedic Bearing Materials

It has been shown that with high interfacial temperatures in hip bearings, it is possible to precipitate proteins, greatly reduce the compressive creep properties of ultrahigh molecular weight polyethylene (UHMWPE), and change the phase content of monolithic tetragonal zirconia. These induced features may alter the wear rate of UHMWPE. It was the objective of this study to examine the interfacial temperatures of oxidized zirconium (OxZr) heads as compared with metallic and ceramic heads coupled with polyethylene in a hip simulator. The interface temperatures were measured by placing thermocouples within 0.5 mm of the interface surface of both femoral heads and acetabular liners, and then articulating the surfaces using a 12-station AMTI anatomic hip simulator. The alumina femoral heads had the lowest average interfacial temperature, followed in increasing order by OxZr, CoCr, and zirconia. The ranking corresponds to the thermal conductivity of each material. A statistically significant difference (p<0.05) was found between all four materials for the femoral head temperature. No difference was seen in liner temperature between the alumina and OxZr groups, but statistical differences were found between all other combinations. Additionally, increasing head diameter, peak load, cyclic frequency, and serum concentration all resulted in statistically significant increases in both femoral head and liner temperatures.

Surface characteristics of retrieved LFIT cobalt-chromium femoral heads with prior dislocation

Background:Total hip dislocation is common and is associated with femoral head roughening. Low-friction ion treatment hardens the surface of femoral heads and may protect against surface damage from dislocation. Methods:Retrieved low-friction ion treated femoral heads with prior dislocation (n=14) were evaluated for surface roughness (Ra), mean peak height (Rpm) distance from peak to valley (Rmax), deepest valley from the mean surface line (Rv) by using a contact profilometer, and values were compared with previously published control values of nondislocated heads (n=46). Results:The maximal surface roughness was significantly greater than controls for the dislocated heads (P=0.039). Rpm (P=0.11), Rmax (P=0.16) Rv (P=0.16), average Ra (P=0.068), average Rpm (P=0.16), average Rmax (P=0.22), and average Rv (P=0.23) were not significantly greater for dislocated heads compared with nondislocated heads. Each of these values, however, was greater for the dislocated head group, despite a shorter time in vivo (1786 days compared with 2322 days). Scanning electron microscopy and element analysis, used to further characterize surface irregularities in select dislocated heads, confirmed that the higher roughness values of dislocated heads were caused not only by scratching but also titanium (Ti) smeared on the head surface. Conclusions:Dislocated heads have localized roughening when compared with nondislocated heads, and this occurs despite ion treatment.