the following description of the preferred embodiment ( s ) is merely exemplary in nature and is in no way intended to limit the invention , its application , or uses . with reference to fig1 and 2 , a preferred embodiment of a bone screw 10 according to the present invention is illustrated . the bone screw 10 preferably includes , concentric to a longitudinal axis 12 , a head portion 14 , a neck portion 18 and a shank portion 16 . the head portion 14 connects to the shank portion 16 through the neck portion 18 . the bone screw 10 is preferably constructed from any biocompatible material including , but not limited to , stainless steel alloys , titanium , titanium based alloys , or polymeric materials . in a preferred embodiment , the head portion 14 of bone screw 10 has a generally spherical shape and includes a recess 20 for receiving a driving instrument . as is well known in the art , the recess 20 may be configured and dimensioned to any shape that corresponds with the end of the driving instrument designed to engage the bone screw 10 . for example , the recess 20 may be any one of the following shapes : slot , cross , polygon , or multi - lobes . the generally spherical shape of the head portion 14 is configured and dimensioned to be received within a correspondingly shaped cavity in a receiving member ( not shown ) which may be part of a spinal fixation system . the shape of the head portion 14 allows the bone screw 10 to pivot , rotate and / or move with respect to the receiving member . in an exemplary use , the head portion 14 of the bone screw 10 is received in the cavity of the receiving member and the bone screw 10 is pivoted , rotated or moved until the desired orientation with respect to the receiving member is met . the bone screw 10 is then locked in place in the cavity of the receiving member . in a further preferred embodiment , the head portion 14 also includes texturing 22 that extends along at least a portion of the head portion 14 . the texturing 22 on the head portion 14 provides additional frictional surfaces which aid in locking the bone screw 10 in place with respect to the receiving member . with continued reference to fig1 and 2 , in a preferred embodiment , the neck portion 18 of the bone screw 10 integrally connects the head portion 14 with the shank portion 16 . preferably , the neck portion 18 includes a generally cylindrical region 24 and a truncated generally frustoconical region 26 . the diameter of the distal end 23 of the frustoconical region 26 is preferably dimensioned to match a major diameter ( discussed below ) of the bone screw 10 while the diameter of the proximal end 25 of the frustoconical region 26 is preferably dimensioned to match the diameter of the generally cylindrical region 24 of the neck portion 18 . in a preferred embodiment , the generally cylindrical region 24 will have a diameter that is at least as large as a minor diameter ( discussed below ) of the bone screw 10 , but the diameter of the generally cylindrical region 24 can be smaller than the minor diameter of the bone screw 10 . by having the diameter of the neck portion 18 dimensioned at least as large as a minor diameter of the bone screw 10 , the overall rigidity and strength of the bone screw 10 is increased . turning to fig1 - 4 , in a preferred embodiment , the shank portion 16 of the bone screw 10 includes a shaft 28 , having a length l , surrounded at least in part by a plurality of thread portions 30 , 32 . the diameter of the shaft 28 is the minor diameter of the bone screw 10 and the diameter of the shaft 28 including the thread portions 30 , 32 is the major diameter of the screw 10 . in a preferred embodiment , the diameter of the shaft 28 remains generally constant from the proximal end 27 toward the distal end 29 of the shaft 28 . however , the diameter of the distal end 29 of the shaft 28 preferably decreases towards the distal tip 34 of the bone screw 10 . the constant diameter of a majority portion of the shaft 28 allows for optimal screw positioning when the bone screw is inserted into a predetermined area in the bone tissue . the constant diameter also allows for varying the depth positioning of the bone screw in the bone . for example , if a surgeon places the bone screw 10 into bone tissue at a first depth and decides the placement is more optimal at a second , shallower depth , the bone screw 10 can be backed out to the second depth and still remain fixed in the bone . in another embodiment , the diameter of the shaft 28 may vary along its length , including increasing in diameter from the proximal end to the distal end or decreasing in diameter from the proximal end to the distal end . looking at fig1 - 2 , the plurality of threads 30 , 32 surrounding the shaft 28 extend , in a preferred embodiment , from the distal tip 34 of the shaft 28 to the distal end 23 of the frustoconcial region 26 of the neck portion 18 . in another preferred embodiment , the threads 30 , 32 may extend along only a portion of shaft 28 . as seen in fig1 - 2 , the thread portions 30 , 32 are preferably a modified buttress thread but the threads can be any other type of threading that is anatomically conforming , including , but not limited to buttress , acme , unified , whitworth and b & amp ; s worm threads . in a preferred embodiment , the diameter or depth or height ( hereinafter , diameter ) of the thread portion 30 remains substantially constant over its length l 1 and the diameter of the thread portion 32 remains substantially constant over a portion of its length l 2 . preferably , the diameter of the thread portion 32 decreases towards the distal tip 34 of the bone screw 10 . by having a decreased diameter thread portion 32 near the distal tip 34 of the bone screw 10 , the bone screw 10 can be self - starting . in another preferred embodiment , bone screw 10 may also include at least one flute to clear any chips , dust , or debris generated when the bone screw 10 is implanted into bone tissue . in a preferred embodiment , the thread portion 30 also differs dimensionally from the thread portion 32 . more specifically , the thread portion 30 preferably has a larger diameter than the thread portion 32 . the diameter of the thread portion is determined by subtracting the minor diameter from the major diameter of the bone screw 10 . for example , if the minor diameter of the bone screw 10 is 4 mm and the major diameter of the bone screw 10 near the proximal end 27 of the shaft 16 is 7 mm , the diameter of the thread portion 30 around the proximal end 27 of the shaft 16 is 3 mm . a preferred difference in the diameter between the thread portion 30 and the thread portion 32 is 2 . 0 mm but a larger or smaller difference between the thread portion diameters is also contemplated . in a preferred embodiment , the ratio of the diameter of the thread portion 30 to the ratio of the thread portion 32 is approximately 1 . 2 , but can vary from 1 . 0 to 1 . 5 by having a larger diameter thread portion 30 and a smaller diameter thread portion 32 , the bone screw 10 can grip bone tissue having regions with varying mechanical characteristics in an optimal manner . the larger diameter thread portion 30 , which surrounds the proximal portion 27 of the shaft 16 , is better suited to grip the cancellous region of the bone . the larger diameter thread portion , having the larger threads and increased purchasing surface area , better engages the softer , less dense bone tissue . correspondingly , the smaller diameter thread portion 32 , which surrounds the distal portion 29 of the shaft 16 , is better suited to grip the cortical region of the bone . since the cortical region is harder and denser , a smaller thread is preferred for the bone screw 10 to optimally purchase that bone tissue region . the combination of the larger diameter thread portion 30 and the smaller diameter thread portion 32 provides for an improved bone screw having greater bone tissue purchasing as well as greater pull - out strength than a screw with a single diameter thread . in a preferred embodiment , the improved purchasing lowers the bone screw 10 toggling over time and the pull out - strength of bone screw 10 compared to a screw having a single outer diameter thread has been determined to be at least 20 % higher than the pull - out strength of the screw having a single outer diameter thread . in a preferred embodiment , the bone screw 10 also has a transition portion 35 between thread portions 30 , 32 to allow for easier insertion of the bone screw 10 in the bone tissue . preferably , over the transition portion 35 , the major diameter of the bone screw 10 decreases gradually between the thread portion 30 and thread portion 32 when viewed from a proximal to distal direction . looking now at fig3 - 4 , enlarged cross - sectional views of the thread portions 30 , 32 can be seen . in a preferred embodiment , the thread angle a of the thread portions 30 , 32 is preferably 25 °, but can be between 20 °- 30 °. it has been determined that this range of thread angles is optimal for purchasing in the different regions of the bone tissue . the radius β of the thread portion 30 , 32 is preferably 0 . 5 mm , but can be between 0 . 1 mm - 1 mm . again , it has been determined that this range for the radius is optimal for purchasing in the different regions of the bone tissue . turning back to fig1 - 4 , the thread portions 30 , 32 on the shaft 28 of the screw 10 is preferably a multi - start thread . more specifically , in a preferred embodiment , thread portions 30 , 32 of the bone screw 10 is a two - start thread . multi - start threads have the advantage of providing a thread on a screw shaft that has a smaller thread pitch ( discussed below ) than would be the case if the thread is a single - start thread . a smaller thread pitch can enhance the security of the fixation in bone tissue as well as increase the rate of installation of the screw in the bone tissue . in other preferred embodiments , a single - start thread portion as well as three or more start thread portion is also contemplated . as mentioned above , the thread pitch is defined as the distance along the axis of the screw between adjacent thread peaks , shown in fig2 as y . the thread lead is defined as the distance that is travelled along the axis of the screw in one complete 360 ° revolution of the screw , shown in fig2 as x . in a preferred embodiment , the number of starts of the thread portions 30 , 32 is equal to the ratio of the thread lead x to the thread pitch y . for example , for a two - start thread , the thread lead is preferably 5 mm and the thread pitch is preferably 2 . 5 mm . in a preferred embodiment , the bone screw 10 includes a two start thread . in another preferred embodiment , the thread pitch y is substantially constant over thread portions 30 , 32 . dimensions , in millimeters , of a preferred embodiment of the bone screw 10 , which is suitable for use as a bone screw in a vertebral body , are as follows : the invention being thus described , it will be obvious that the same may be varied in many ways . such variations are not to be regarded as a departure from the spirit and scope of the invention , and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims .
the present invention provides a bone screw which can be used for fixation and / or fastening of prosthetic devices or instruments to bone tissue whose structure or dimensions differ from one region to another . in particular , the present invention provides a bone screw which is designed to optimize purchase in both the cancellous and cortical regions of a vertebral body . in an exemplary embodiment , the bone screw has a distal portion and a proximal portion in which the diameter of the thread on the proximal portion of the screw is greater than the diameter of the thread on the distal portion of the screw .