for the purposes of promoting an understanding of the principles of the invention , reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same . it will nevertheless be understood that no limitation of the scope of the invention is thereby intended , such alterations and further modifications in the illustrated device , and such further applications of the principles of the invention as illustrated therein , being contemplated as would normally occur to one skilled in the art to which the invention relates . referring generally to fig1 - 5 , there is shown a medical device 10 according to the present invention . as illustrated , medical device 10 includes a receiver member 11 and a closure member 12 adapted to be threadedly engaged to receiver member 11 . medical device 10 , in the illustrated embodiment , is a bone fixation device used to connect an elongated member ( indicated as r in fig4 - 6c ) to a bone . in that embodiment , receiver member 11 includes a longitudinal or thread axis 14 , a longitudinal bore 15 centered on axis 14 , and a transverse channel 16 for receiving an elongated member , which is generally perpendicular to axis 14 and bore 15 . channel 16 is bounded on both sides by legs 20 of receiver member 11 . receiver member 11 further includes a fixation portion 22 . fixation portion 22 , in a particular embodiment , is a threaded portion for threading into a bone , and in another embodiment ( not shown ) is a hook portion for connection to a bone . closure member 12 , in a particular embodiment , includes a break - off section 32 and a screw section 34 . break - off section 32 has a generally cylindrical upper or proximal portion 36 and a thinned neck portion 38 . break - off section 32 and screw section 34 have holes 40 and 42 , respectively , for engaging tools . in one specific embodiment , holes 40 and 42 have a hexagonal and star shape , respectively , although other known tool head shapes may be used . in this embodiment , closure member 12 is threadedly engaged with receiver member 11 to a point at which further threaded progression is impeded , as for example when the screw section 34 contacts an object within channel 16 , such as rod r seated against wall 24 . as further torque is applied to break - off screw closure member 12 , as with an hexagonal driving tool inserted into hole 40 , eventually the stress on the neck portion 38 is great enough to cause the upper section 36 to break off from screw section 34 at neck portion 38 . in this way , screw section 34 is firmly seated in bore 15 of receiver member 11 against rod r in channel 16 , and the excess material of the break - off screw , which assisted in the original threading , is removed . in another particular embodiment of closure member 12 , illustrated in fig4 closure member 12 includes only screw section 34 , which may be characterized as a set screw . screw portion 34 , and break - off section 32 if included in closure member 12 ( as in fig1 and 5 ), has a longitudinal axis 43 . when closure member 12 and receiver member 11 are engaged , as illustrated in fig1 longitudinal axis 43 of closure member 12 and longitudinal axis 14 of receiver member 11 are collinear . receiver member 11 includes an inner thread 44 inside legs 20 , and screw section 34 of closure member 12 includes an outer thread 46 . threads 44 and 46 have substantially the same characteristics ,&# 39 ; so that closure member 12 is threadably engageable with receiver member 11 by engaging threads 46 with thread 44 . threads 44 and 46 will be described by reference to thread 46 , illustrated in fig4 . threads 44 and 46 are reverse angle threads . as used herein , “ reverse angle thread ” refers to a thread wherein the rearward - facing thread surface or load flank is sloped so that , for a given cross - section of the thread through the longitudinal axis of the screw , a point on the rearward - facing thread surface at the root of the thread is closer to the distal or forward end of the screw than a point on the rearward - facing thread surface at the crest of the thread . as shown in fig4 closure member 12 has a reverse angle thread 46 including forward - facing thread surface 50 and rearward - facing thread surface 52 . thread 44 ( see fig2 ) has a corresponding forward thread surface 50 ′ and a rearward thread surface 52 ′. point 60 , depicted in fig4 at the root of rearward thread surface 52 , is closer than point 62 ( at the crest of rearward surface 52 ) to the forward end 64 of screw section 34 . to define the angles of the thread surfaces , plane 54 normal to longitudinal axis 43 is also shown . as used herein , an angle measured clockwise from a normal plane ( such as plane 54 ) to the rearward thread surface is a negative angle , and an angle measured clockwise from a normal plane ( such as plane 54 ) to the forward thread surface is a positive angle . thus , pressure angle a of thread 46 ( illustrated in fig4 ) is negative , since the measurement is clockwise from the thread root at plane 54 to rearward thread surface 52 , as indicated by the arrow . flank angle p in fig4 representing the clockwise angle from plane 54 to forward thread surface 50 , is positive . accordingly , a reverse angle thread includes a rearward surface such that α is a negative angle . in one particular embodiment of the present invention , illustrated in fig4 pressure angle α is − 5 degrees , and flank angle β is 45 degrees . however , it is understood that one of ordinary skill in the art will recognize that other negative values of pressure angle α , including values between about − 1 degree and at least − 40 degrees , and other values of flank angle β are within the scope of the present invention . as noted above , reverse angle thread 44 of receiver member 11 is configured substantially similarly to reverse angle thread 46 of closure member 12 so that threads 44 and 46 can be engaged . accordingly , rearward thread surface 52 ′ of thread 44 forms a negative pressure angle , i . e ., one measured clockwise from a plane normal to axis 14 to rearward thread surface 52 ′, of substantially the same magnitude as pressure angle a illustrated in fig4 . forward thread surface 50 ′ of thread 44 ( fig2 ) forms a positive flank angle of substantially the same magnitude as flank angle β illustrated in fig4 . in use , closure member 12 is threaded into receiver member 11 such that reverse angle thread 46 of closure member 12 is engaged with reverse angle thread 44 of receiver member 11 . when closure member 12 and receiver member 11 are threadedly engaged , rearward thread surface 52 of closure member 12 will abut rearward thread surface 52 ′ of receiver member 11 , and forward thread surface 50 of closure member 12 will abut forward thread surface 50 ′ of receiver member 11 . any force tending to splay legs 20 , such as a force outward from and perpendicular to axis 14 of fig2 will tend to move rearward thread surface 52 ′ of receiver member 11 against and into an interference fit with rearward thread surface 52 of closure member 12 . the abutment of rearward thread surfaces 52 and 52 ′ prevent splaying of legs 20 outward from longitudinal axis 14 of receiver member 11 . it will be appreciated that any medical device which includes a holder or receiver member that tends to splay can incorporate the present invention . as indicated above , in a preferred embodiment of the present invention medical device 10 is a bone fixation device for connecting an elongated member and a bone , and particularly an “ open - back ” bone fixation device . examples of such devices are illustrated in fig6 a - 6c . fig6 a illustrates an open - back laminar hook 100 having an integral u - shaped bone fixation portion 102 , and fig6 b illustrates an open - back bone screw 110 having an integral threaded bone fixation portion 112 . hook 100 and screw 110 include receiver members 104 and 114 , respectively , which are substantially similar to receiver member 11 depicted in fig1 and are thus able to receive rod r in a seated engagement . hook 100 and screw 110 also include closure members 106 and 116 , respectively , which are substantially similar to closure member 12 of fig1 and thus operate in the same manner as described above with respect to the embodiment of the invention illustrated in fig1 - 5 . in the realm of bone fixation devices , the present invention may also be used in connection with a multi - axial bone screw or bone hook system in which the fixation element is rotatable within a body element . one example of such a system is found in u . s . pat . no . 5 , 797 , 911 to sherman , et al ., owned by the assignee of the present invention , the specification of which is hereby incorporated by reference herein and an embodiment of which is illustrated in fig6 c . multi - axial device 120 includes a receiver member 122 similar to receiver member 11 of fig1 the principal differences being that bone fixation portion 124 ( illustrated as a bone screw in fig6 c ) of multi - axial device 120 is not integral with receiver member 122 , and is free to rotate within bore 126 which extends from top to bottom through receiver member 122 . multi - axial device 120 also includes a closure member 128 which is substantially similar to closure member 12 of fig1 and thus operates in the same manner as described above with respect to the embodiment of the invention illustrated in fig1 - 5 . it is preferred that both closure member 12 and receiver member 11 be manufactured from bio - compatible materials , and preferably metals such as titanium or stainless steel . it is also preferred that the reverse angle thread be formed integrally on the outside of closure member 12 and on the inside of receiver member 11 prior to the use of the medical device . however , forming an integral reverse angle thread 44 on the inside of receiver member 11 prior to use is not strictly necessary , so long as closure member 12 can be twisted into receiver member 11 so that reverse angle thread 46 of closure member 12 gouges the insides of legs 20 to form a threaded engagement between closure member 12 and receiver member 11 . while the invention has been illustrated and described in detail in the drawings and foregoing description , the same is to be considered as illustrative and not restrictive in character , it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected .
apparatus is disclosed for use in closing a medical device and preventing splaying of wall portions or legs of the medical device while maintaining low profile and minimal bulk of the medical device . the disclosed device is a medical device including a receiver member and a closure member . the receiver member includes an internal reverse angle thread , and the closure member includes an outer reverse angle thread , matable with the inner reverse angle thread . when the closure member is threaded into the receiver member , forces tending to splaying the wall portions or legs of the receiver member cause an interference fit between thread surfaces of the closure member and the receiver member , preventing the splay from occurring .