referring to the drawings , there is shown in fig1 - 4 a porous - coated , threaded acetabular cup 10 constructed in accordance with the present invention . cup 10 comprises body 18 having outer surface 20 which includes dome portion 12 and base portion 14 . surface 20 is preferably spherically - shaped . body 18 preferably includes aperture 22 which extends from outer surface 20 through to the inside of the cup and allows the bone underlying the prosthesis to be viewed during the implantation process . body 18 also preferably includes apertures 28 through which bone screws ( not shown ) can pass to further secure the prosthesis to the underlying bone , if desired . the bone screws can also be used to secure within body 18 a bearing insert ( not shown ) for receiving the ball portion of the artificial joint . body 18 is preferably made of surgically implantable metal , and , in particular , is preferably made of a titanium alloy , such as an alloy containing 6 % aluminum and 4 % vanadium ( see astm spec . no . f136 ). thread 16 is formed on the outer surface of body 18 by , for example , casting or preferably by milling the surface using , for example , a computer - controlled machine tool . preferably , thread 16 is a spherical thread , although a conical thread can be used , if desired . also , thread 16 preferably includes leading edges 30 which are sufficiently sharp for the thread to be self - tapping . in practice , for an outer surface 20 having a diameter of between about 36 millimeters and about 72 millimeters , it has been found preferable to use a thread having a pitch of approximately 3 millimeters and a thread depth of approximately 3 millimeters . it has also been found preferable to have the bottom portion of surface 20 threaded so as to leave a non - threaded dome occupying a spherical segment whose base subtends a cone which has an apex at the center of the sphere and whose included cone angle is approximately 90 °. in accordance with the invention , the circumference of thread 16 is interrupted by a plurality of porous - coated columns 24 , e . g ., 3 - 12 columns , which divide the thread into an equal number of threaded columns 26 . as shown in the figures , porous - coated columns 24 and threaded columns 26 alternate around the outer circumference of the prosthesis . porous coated column areas 24 preferably lie on the same sphere as dome 12 . as also shown in the figures , porous - coated columns 24 and threaded columns 26 have approximately equal areas , although , if desired , the threaded columns can be given either a larger or smaller area than the porous - coated columns . also , not all columns need have the same area , but rather large and small columns can be used on the same prosthesis , if desired . in general , porous - coated and threaded columns having approximately equal areas are preferred since this results in a secure level of initial fixation , and yet provides substantial porous - coated areas for long - term fixation through bone ingrowth . preferably , in addition to porous - coated columns 24 , dome portion 12 of outer surface 20 is also porous coated . in practice , it has been found preferable to use a porous coating having an average pore size of between about 250 and about 350 microns , a porosity on the order of 39 %, and a thickness of about one millimeter . it has also been found preferable to have the to surface of the porous coat lie substantially in the surface of revolution defined by roots 32 of thread 16 or slightly ( e . g ., 0 . 5 millimeters ) above this surface . in this way , when the prosthesis has been screwed into bone , the porous coat will be in contact with bone and can thus accept bone ingrowth and remodeling . the top surface of the porous coat can be made to lie in the spherical surface defined by roots 32 by adjusting the depth of surface 20 in the area of columns 24 and dome 12 so that when the porous coat is applied , its top surface comes up to or slightly above the level of thread roots 32 . the porous coating can be applied to the outer surface of the prosthesis using various techniques known in the art . for example , columns 24 and dome 12 can be coated with small balls having a diameter of , for example , 0 . 4 to 0 . 7 millimeters , and the coated prosthesis can then be heated to fuse the balls to one another and to body 18 . preferably , the small balls used for porous coating will be made of an appropriate surgically implantable material such as chemically pure titanium or the titanium - aluminum - vanadium alloy described above . porous coatings applied by plasma spray are also known in the art . once porous - coated and sterilized , acetabular cup 10 is implanted using known surgical procedures for implanting threaded acetabular cups . thus , the patient &# 39 ; s acetabulum is reamed using a spherical reamer having a diameter corresponding to the diameter of the spherical surface defined by the outer surface of porous coated columns 24 and dome 12 . the cup is then threaded into the prepared cavity using an appropriate driver which engages he body of the cup . since thread 16 is self - tapping , tapping of a thread into the walls of the prepared cavity prior to implantation of the cup is generally not necessary . referring now to fig5 - 6 , these figures illustrate the application of the principles of the present invention to the femoral portion of an artificial hip joint . in particular , there is shown in these figures a femoral prosthesis 34 comprising ball 36 which is connected to stem 38 by neck 40 . stem 38 mates with sleeve 46 by means of complementary locking tapers ( not shown ) on the outside of the stem and the inside of the sleeve . a pin 42 through stem 38 may be used to engage slots 44 of sleeve 46 to establish angular orientation of neck 40 relative to the femur bone in which sleeve 46 has been implanted . torque and longitudinal forces between stem 38 and sleeve 46 are transmitted through the locking taper , however . the prosthesis is implanted using standard surgical techniques by , for example , preparing the patient &# 39 ; s femoral bone to receive the distal portion 56 of stem 38 and threading sleeve 46 into the prepared proximal femur , using an appropriate driving and aligning instrument which engages sleeve 46 by its internal taper and by slots 44 . lastly , stem 38 is driven into the complementary locking taper of sleeve 46 . in accordance with the invention , sleeve 46 includes threaded columns 48 and porous columns 50 . as with the acetabular cup of fig1 - 4 , threaded columns 48 provide initial , mechanical fixation of the prosthesis , while porous - coated columns 50 provide long - term fixation by means of bone ingrowth and remodeling . a discussed above in connection with the acetabular cup embodiment , porous - coated columns 50 preferably have approximately the same projected cylindrical area as threaded columns 48 , and the top surface of the porous coat preferably lies in or near to the surface of revolution defined by the roots of threads 52 . although specific embodiments of the invention have been described and illustrated , it is to be understood that modifications can be made without departing from the invention &# 39 ; s spirit and scope . for example , the alternating porous - coated and threaded portions of the prosthesis can have configurations other than those illustrated in the figures . similarly , the invention can be applied to prostheses other than an artificial hip joint as used for purposes of illustration in the figures , e . g ., to artificial knee , shoulder and finger joints .