the invention will be described with reference to several embodiments , illustrated in the drawings , with like numerals being used to designate like parts . the invention also will be described with specific reference to a femoral head prosthesis ; however , the present invention may be used to reduce stress levels in other load bearing prostheses as well . the present invention is particularly useful for prostheses designed to replace skeletal members . fig1 illustrates a femoral head prosthesis made according to the present invention . the femoral head prosthesis is labelled generally as 10 . as already discussed , current hip replacement prostheses use a femoral head which is a one - piece component having a continuous head and stem . in contrast , a femoral head prosthesis made according to the present invention has several components -- a separate head 12 , a separate stem 14 , and an elastomeric component 18 , which will be discussed more fully below ( fig1 ). the head 12 shown in the figures should be distinguished from the acetabulum , which typically is a separate component of current hip replacement prostheses . of course , the head 12 of the prosthesis described herein eventually should be movably engageable with an acetabulum ; however , engagement between the femoral head and the acetabulum is not the subject matter of the present invention . in fig1 the separate head 12 and stem 14 of the femoral head prosthesis are mechanically connected by an elastomeric component 18 . the elastomeric component 18 provides for passive shock absorption by permitting relative motion between the head 12 and the stem 14 of the prosthesis . in order for shock absorption to occur as a result of such relative motion , the elastomeric component 18 must be capable of either : ( 1 ) shear deformation , alone ; ( 2 ) both shear and compressive deformation ; or , ( 3 ) compressive deformation , alone . fig1 represents an embodiment in which the elastomeric component is capable primarily of shear deformation . the principles applied in the figures are fully explained in the following references , all of which are incorporated herein by reference : e . f . gobel , rubber springs design , translated and edited by a . m . brichta , john wiley & amp ; sons , new york , 1974 ; r . h . finney and b . p . gupta , &# 34 ; design of elastomeric components by using the finite element technique ,&# 34 ; from the shock and vibration bulletin , part 1 . opening session , panel session , shock analysis , shock testing , isolation and damping . september 1977 , a publication of the shock and vibration information center , naval research laboratory , washington , d . c ., bulletin 47 ( part 1 of 4 parts ), pp . 177 - 188 ; p . b . lindley , eng &# 39 ; g design with natural rubber , nr technical bulletin , the natural rubber producers research ass &# 39 ; n , london , 1964 . with the foregoing design tools and the following teachings , a person of skill in the art should be able to design a prosthesis with a range of stiffness and damping properties to fit any particular design condition . in fig1 the spherical head 12 is split in half in order to show that the head 12 is hollow , forming a cavity 16 . in the embodiment of fig1 the cavity 16 is substantially &# 34 ; u &# 34 ;- shaped and has an inner surface 24 near the top of the head . the surface 24 is rounded and the sides 26 of the inner surface are substantially parallel to one another . the legs at the open end of the &# 34 ; u &# 34 ; form a shoulder at the distal end of the head . one end 22 of the stem 14 extends into the cavity 16 , and the sides 28 of the stem end 22 preferably are substantially parallel to the respective sides 26 of the cavity 16 . both the head 12 and the stem 14 may be made of substantially any of the biocompatible materials typically used for such components , a preferred material being titanium . in a preferred embodiment , the surface of the stem 14 has been primed to encourage tissue ingrowth , e . g ., by applying a porous ceramic or porous titanium coating to the stem or by sandblasting . the head 12 preferably should have a smooth finish , except for the surface of the head onto which the elastomer is bonded . the elastomer bonding surface preferably should be treated , e . g ., chemically or by sandblasting , to provide a greater surface area for bonding . the stem 14 is formed in the general shape of the medullary canal , and should be of sufficient length to extend a substantial distance into the medullary canal of the femur to be treated . the elastomeric component 18 may be affixed to the surfaces 28 and 26 using a suitable biocompatible adhesive . in a preferred embodiment , the stem and head are sprayed with an adhesive , such as chemlock 608 , available from lord corporation , erie , pa . the stem and head then may be placed into a mold and hot , uncured elastomer may be injected into a suitable cavity appropriately positioned adjacent to the surfaces 28 and 26 . since the adhesive is activated by heat , adherence , and curing of the hot elastomer will occur simultaneously . alternately , the elastomeric component 18 may be formed separately and subsequently bonded to the surfaces 28 and 26 . the surfaces 28 and 26 should be coated with adhesive , the pre - formed elastomeric component 18 should be appropriately placed , and heat should be applied to activate the adhesive . in the embodiment shown in fig1 a portion of the cavity 16 near the inner surface 24 at the top of the head is &# 34 ; empty &# 34 ; so that , when the stem 14 receives an impact , the elastomeric component 18 deforms ( fig1 b ) and the stem can move toward the upper inner surface 24 of the cavity 16 . the mechanical response of the elastomeric component 18 can be varied by material variations , such as varying the chemical nature of the elastomer , the stiffness of the elastomer , and the damping properties of the elastomer . the stiffness and the damping properties of the elastomer are related to the chemical type of elastomer and to its molecular properties and compounding ingredients . methods of material property variation of elastomers are well known . the damping properties and attenuation properties of such elements are discussed by j . c . snowdon , vibration and shock in damped mechanical systems , john wiley & amp ; sons , new york ( 1968 ), incorporated herein by reference . in a preferred embodiment , the elastomeric component is an ethylene - propylene rubber . the mechanical response of the elastomeric component 18 also can be varied by modifying its physical dimensions ; for example , thickening the elastomeric component 18 perpendicular to the load trajectory will render the assembly less stiff and more flexible . the embodiment shown in fig2 is designed to provide shock absorption using both shear and compressive deformation . the prosthesis in fig2 has a stem 14 which is similar to the stem in fig1 except that the end 22 of the stem 14 is substantially conical ; the cavity 16 inside of the head 12 is substantially conical , and the elastomeric component 18 that circumscribes the end 22 also is substantially conical . as a result , when an impact is transmitted to the stem 14 and then to the elastomeric component 18 , shock absorption occurs due to both shear and compressive deformation of the elastomeric component 18 ( fig2 b ). the embodiment in fig3 shows a geometric configuration in which shock absorption takes place primarily by compressive deformation , alone . in fig3 the hollow head 12 forms a substantially concave , c - shaped cavity 16 , and the end 22 of the stem 14 is rounded . when the end 22 is inside of the head 12 , the inside wall 26 of the head 12 and the outside wall of the end 22 are substantially concentric or &# 34 ; parallel .&# 34 ; for present purposes , the term &# 34 ; parallel &# 34 ; is defined to include this substantially concentric configuration . in this embodiment , the elastomeric component 18 circumscribes a majority of the end 22 inside of the head 12 with the exception of a small portion near the open ends 30 of the &# 34 ; c .&# 34 ; the amount of clearance required at the open ends 30 of the &# 34 ; c &# 34 ; will depend upon the amount of stiffness required in the prosthesis . the stiffness , deformation properties , and shock absorbing properties of the elastomeric component may be determined by an analysis method such as finite element stress and deformation analysis . this method is used in r . h . finney and b . p . gupta , &# 34 ; design of elastomeric components by using the finite element technique ,&# 34 ; from the shock and vibration bulletin , part 1 . opening session , panel session , shock analysis , shock testing , isolation and damping . september 1977 , a publication of the shock and vibration information center , naval research laboratory , washington , d . c ., bulletin 47 ( part 1 of 4 parts ), pp . 177 - 188 , incorporated herein by reference . in order for the elastomeric component 18 of fig3 to absorb shock by substantially compressive deformation , the elastomeric component 18 must be deformable . in order to be deformable , the elastomeric component 18 must have a relatively low shear modulus , preferably a modulus of about 2 . 76 × 10 7 pascal ( 4000 psi ) or less . when the stem 14 of the embodiment in fig3 is impacted , the force will be transmitted to the elastomeric component 18 , and the elastomeric component 18 will absorb at least a portion of the shock by compressive deformation , as shown in fig3 . a person of skill in the art will recognize that many modifications may be made to the present invention without departing from the spirit and scope of the present invention . the embodiment described herein is meant to be illustrative only and should not be taken as limiting the invention , which is defined in the following claims .
the present invention provides a method and structure for absorbing shock in a load bearing prosthesis , particularly in a load - bearing skeletal prosthesis . where the prosthesis is a hip - joint replacement , the invention provides a method and structure for absorbing shock at the interface between the femoral head and cup , and at the interface between the femur and the stem of the prosthesis . where the prosthesis is a femoral head prosthesis , the prosthesis is formed using two separate components which are mechanically connected using an elastomeric component . the connection permits relative motion between the structural components of the prosthesis , with the elastomeric component acting as a passive shock absorbing element .