Patent Application: US-201213357726-A

Abstract:
a prosthesis for articulation is provided , which includes a pair of articulation components respectively defining a pair of articulation surfaces movably engageable with each other . in some embodiments , each of the articulation surfaces is formed from a biocompatible ceramic having a flexural strength greater than about 700 mega - pascal and a toughness greater than about 7 mega - pascal root meter .

Description:
powders of si 3 n 4 and dopants such as alumina , yttria , magnesium oxide , and strontium oxide were conventionally processed to form a doped composition of silicon nitride . the dopant amount was optimized to achieve the highest density and mechanical properties . the homogeneous powders were then cold isostatic pressed at 300 mega - pascal ( mpa ) followed by sintering in a controlled atmosphere . some cold isostatically pressed bars were hot isostatically pressed . a sintering temperature of 1875 ° c . was found optimal to achieve high density , absence of pores and other flaws and a uniform fine - grained microstructure . the best combination of density and mechanical properties was achieved with si 3 n 4 doped with 6 weight % y 2 o 3 + 4 weight % al 2 o 3 . flexural strength was measured on standard 3 - point bend specimens per american society for testing of metals ( astm ) protocol method c - 1161 and fracture toughness measured using single edge notched beam specimens per astm protocol method e399 . bend tests were conducted using test fixtures designed to minimize spurious stresses , with a nominal span of 40 mm . the bending load was applied using a universal testing machine at a cross - head displacement speed of 0 . 5 mm / min . at least 6 bars were tested to get a measure of the distributions and the weibull modulus . the composition / process that gave the highest fracture toughness , weibull modulus and damage resistance was selected for fabricating 28 mm hip prosthesis implant articular femoral head and acetabular cup components . 28 mm cocr metal femoral heads were obtained from biomet , inc . of warsaw , ind . wear performance tests of up to 1 million cycle duration were conducted . wear detection was primarily done gravimetrically with some inspection of the surfaces for wear track analysis and wear debris analysis using scanning electron microscopy . in the hip prosthesis simulator test , the rotating cams ( uni - directional ) carrying the specimen chambers were driven at 1 hz frequency through + 23 ° arcs on orthogonal axes . each vertical load column had a self alignment device and friction torque sensors . in addition , both the anti - rotation peg and the friction sensor pegs were guide mounted on rollers to provide continuous constraint . for this study , the cam rotation was synchronized with the hip - joint loading . the “ paul ” physiologic load profile was used . 10 , 11 all tests were run at 2 kilonewton ( kn ) peak load / 0 . 2 kn minimum load . the test cups were arranged in an anatomically inverted position in the hip prosthesis simulator . a solution of 90 % bovine serum was used as the lubricant with 10 % standard additives of sodium azide and ethylene diamine tetra - acetic acid ( edta ). the specimen chambers were replenished with distilled water during the tests . lubricant temperature was monitored but not controlled since the lubricant &# 39 ; s bulk temperatures run in the range 36 - 40 ° c ., close to body temperature . the ceramic cups were not sterilized prior to test . soak control cups were not used for the ceramic - ceramic and ceramic - metal wear tests . the diametral clearance , surface finish and sphericity tolerance was noted . component wear was determined using a gravimetric method . wear components were cleaned and dehydrated and each set weighed four times in order with a 32 mm cocr head as a calibration standard . the overall volumetric wear rate was determined by the slope of the linear regression line . a consistent wear rate , i . e . gradient of the linear regression trend was deemed more significant than the actual magnitude of the wear at any point in time . microstructural features such as grain size , pore size , porosity and defects were observed on sintered si 3 n 4 specimens after etching with carbon tetra - fluoride plasma . the specimens were found to be dense , with no detectable porosity , and had substantially uniform grain size consistent with high quality ceramics . for fracture toughness testing , 2 . 5 mm × 5 mm × 25 mm bar specimens with varying notch depth , a , were prepared . the prepared specimens were tested in three - point bending with a span length of 20 mm . the resultant fracture loads were converted to fracture toughness values using astm protocol method e399 . the fracture strength and toughness values are given in table 1 , and are plotted in fig1 and 2 . as expected , hot iso - statically pressed specimens labeled as snh and snh - 1 exhibited high strength , toughness and weibull moduli . hence hot iso - static pressed components were fabricated into acetabular and femoral components . the relevant mechanical characterization data obtained are tabulated in table 1 . a trial wear test using a hip simulator was conducted using si 3 n 4 acetabular cups articulating against biomet &# 39 ; s standard 28 mm cocr metal femoral heads ( previously identified ). three biomet 28 mm heads were used . the mating si 3 n 4 acetabular ceramic cups were ground without lapping . the surface roughness value ( r b ) value was ˜ 0 . 5 μm . high wear of the metal femoral heads was observed , attributed to the higher surface roughness , which resulted in scouring the surface of the cocr metal femoral heads . the wear behavior was found to be linear and typical of 3 - body wear , dominated by unstable characteristics . the lubricant solution exhibited the concomitant amount of cocr metal debris . further , the wear tracks showed non - polar contact rather than polar contact as anticipated from the low diametral clearance . fine scratch marks and wear tracks were observed midway between the pole and equator , while the pole had a shiny sheen , indicative of equatorial contact . for subsequent wear tests , the si 3 n 4 acetabular and femoral components were ground and lapped to obtain an r b & lt ; 0 . 05 μm . the diametral clearance and sphericity was also varied . the cocr metal femoral heads were made from a wrought high carbon cocr alloy containing about 64 weight % cobalt , about 28 % weight chromium , about 6 % weight molybdenum , about 0 . 5 % weight manganese , about 0 . 25 % weight iron , about 0 . 2 % weight nickel , about 0 . 2 % weight nitrogen and about 0 . 23 % weight carbon . the conventional low carbon cocr alloy had a similar elemental weight composition with a carbon content of about 0 . 06 % by weight . such cocr alloys used for joint prostheses are wrought alloys conforming to astm specification 1537 , this high carbon alloy had an elastic modulus greater than 210 giga pascal ( gpa ), which had a closer modulus match to the doped si 3 n 4 ceramic ( elastic modulus ˜ 300 gpa ) compared to the low carbon cocr alloy used in the trial run . the vickers hardness of these alloys is in the range of 4 - 5 gpa compared to between 14 - 16 gpa si 3 n 4 . the elastic modulus and hardness of the articulating surfaces of the doped si 3 n 4 ceramic - si 3 n 4 ceramic or the doped si 3 n 4 ceramic - cocr alloy pair are better matched compared to either cocr - pe , or cocr - xpe articulations . this was expected to result in better wear performance . relevant design data pertaining to the articulating femoral head and acetabular cup pairs chosen for the wear study are tabulated in table 2 . stations 1 and 2 had si 3 n 4 ceramic acetabular cup - si 3 n 4 ceramic femoral head bearings , and station 3 had a si 3 n 4 ceramic acetabular cup - cocr metal femoral head bearing . for the si 3 n 4 ceramic - si 3 n 4 ceramic bearings in stations 1 and 2 , a 70 and 100 μm diametral clearance was chosen to test the effect of run - in wear . the sphericity tolerance of the ceramic acetabular cups was between 1 - 1 . 5 μm in all cases and was less than 0 . 5 μm for both the ceramic femoral heads . for the si 3 n 4 acetabular cup - cocr femoral head bearing , a diametral clearance of about 200 μm was selected . the results from the wear test are plotted in fig3 . stations 1 and 3 with diametral clearance of about 100 and 225 μm showed ultra low wear , with no observable run - in wear . in contrast , station 2 with a low diametral clearance of about 70 μm showed classic biphasic behavior as is typical for metal - metal and ceramic - ceramic bearings . this biphasic behavior is attributed to the lower diametral clearance which , owing to inadequate film lubrication between the articulating surfaces , results in run - in wear . comparing the wear performance of the silicon nitride ceramic - ceramic bearings in stations 1 and 2 , it was noted that the bearing in station 2 exhibited a “ grinding ” noise along with an increase in lubricant temperature during the run - in period . following the run - in period , both stations 1 and 2 behaved normally , with very low steady state wear rates . this was attributed to the lower diametral clearance used in station 2 , which may have resulted in an inadequate lubricant film to be developed between the femoral head and acetabular cup . the wear performance of the si 3 n 4 acetabular ceramic cup - cocr metal femoral head bearing was characterized by an absence of run - in wear , similar to that of station 1 , and very low steady state wear rates . this result was as anticipated where , with better modulus matching , a ceramic - metal articulation couple could provide a ultra - low wear alternative for total hip arthroplasty . the wear performance of these bearings was obtained over a 4 million cycle period . the weight loss data obtained were converted to volumetric wear rates and extrapolated to 10 million cycles to enable a comparison to literature values . the data , plotted in fig4 , indicate that the si 3 n 4 femoral head - si 3 n 4 acetabular cup bearings and the si 3 n 4 acetabular cup - cocr metal femoral head show ultra - low wear of 0 . 65 mm 3 / 10 million cycles and 3 . 4 mm 3 / 10 million cycles respectively . in comparison , wear rates of 62 mm 3 / 10 million cycles for cocr - pe ( clinical data ), 6 . 5 mm 3 / 10 million cycles for metal - metal ( in - vitro ) wear and 0 . 35 - 0 . 6 mm 3 / 10 million cycles for alumina ceramic - alumina ceramic ( in - vitro ) have been reported . observation of the articulating components after 1 million cycles of wear testing , validated the ultra - low wear behavior by exhibiting a complete absence of wear tracks or other wear patterns . the articulating surfaces retained their high shine , consistent with the negligible weight loss observed for the components . the above - described optimized material properties of si 3 n 4 have demonstrated a 100 % increase of fracture toughness over alumina , and a 50 % increase in fracture strength over alumina ceramics , which typically have a fracture toughness of about 5 mpam 0 . 5 and a flexural strength of about 600 mpa , these properties of si 3 n 4 can allow the manufacture of total hip arthroplasty implants and other prosthetic joint implants such as knee and shoulder joints with significantly higher safety and reliability . wear performance of si 3 n 4 femoral head - si 3 n 4 acetabular cup components and si 3 n 4 acetabular cup - cocr femoral head components indicates that these bearings are better than metal - metal bearings and comparable to ceramic - ceramic bearings , with a volumetric wear rate of 2 orders of magnitude lower than cocr - pe and 20 times lower than cocr - xpe bearings . the combination of the metal femoral head and a ceramic acetabular cup described above offers unprecedented benefits owing to inherent fracture resistance and excellent wear performance . the fracture resistance is derived from the use of metal femoral heads instead of ceramic femoral heads . it is well known from finite element analyses of the stresses in hip prosthetic joints that the femoral head component is subjected to high tensile stresses . historically , such tensile stresses have been implicated in ceramic head fracture . metal femoral heads do not fracture owing to the ductile nature of metals . hence use of metal heads avoids fracture risk , in contrast , the acetabular cup component is typically subjected mainly to compressive stresses , which ceramics are designed to withstand . furthermore , the combination of high toughness and flexural strength provides improved capability to withstand loads . thus , as a general design principle for articulating prosthetic joints , the articulating component subjected to the higher tensile stresses can be made from a metal and the mating articulating component subjected to the higher compressive stresses can be made from the high strength high toughness ceramic with favorable tribiological properties . to illustrate this by way of example , in the hip joint , the femoral head can be made of metal and the mating acetabular cup can be made of the ceramic . in the case of knee joints , the condylar component which is subjected to higher tensile stresses , can be made from metal while the mating tibial component can be made from the ceramic . similarly , the concept can be extended to other articulating prosthetic joints such as the shoulder joint . the use of si 3 n 4 femoral head - si 3 n 4 acetabular cup components and si 3 n 4 acetabular cup - cocr femoral head components in the instant invention to demonstrate the concept of using a fracture resistant metal alloy articulating with optimized ceramics to obtain an ultra low wear joint are illustrative of the general concept . alternate metal alloys suitable for medical implants such as zirconium based alloys , titanium based alloys or stainless steel alloys may be used for the femoral head of a hip joint ( or the component subjected to higher tensile stresses ). alternate enhanced toughness ceramic materials such as doped zirconia or zirconia toughened alumina could also be used for the acetabular component of a hip joint ( or the component subjected to higher compressive stresses ). this concept can also be applied to other orthopedic joints such as the shoulder or knee joint . a variety of further modifications and improvements in and to the invention will be apparent to those persons skilled in the art . accordingly , no limitation on the invention is intended by way of the foregoing description and accompanying drawings , except as set forth in the appended claims . 1 . callaway g . h ., flynn w ., ranawat c . s ., and sculco t . p ., j . arthroplasty , 10 , no . 6 : 855 - 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