in the described embodiment the guide assembly of the invention is used to determine the mechanical axis of a femur . nevertheless , it will be understood that the guide assembly can be used to determine the mechanical axis of other types of bone , such as the humerus . referring to the drawings , fig1 shows a femur 2 having a mechanical axis x . the mechanical axis x of the femur 2 is the line , which extends through the centre point 4 of the head of the femur and the centre point 6 of the intercondylar notch . the angle a between the mechanical axis x and a reference axis 12 containing first 8 and second 10 reference points can be determined pre - operatively . in the described embodiment , the first 8 and second 10 reference points are the distal most points of the femurs first 14 and second 16 condyles . nevertheless , as will be understood , this need not necessarily be the case and the reference points can be any other predetermined reference points on the femur 2 . the angle a can be determined by obtaining an image of the femur 2 and then measuring on the image the angle between the mechanical axis x and the reference axis 12 containing the first 8 and second 10 reference points . the image of the femur 2 can be obtained by taking an x - ray of the femur 2 . referring to fig2 and 3 , there is shown a guide assembly 20 a according to the present invention located at the distal end of the femur 2 . the guide assembly 20 a is located between the femur 2 and the tibia , which is not shown for the sake of simplicity . the guide assembly 20 a comprises a first plate 22 for abutment with the femur &# 39 ; s condyles 14 , 16 , a second plate 24 for abutment with the proximal end of the tibia ( not shown ), an axis indicator arm 26 , an angle indicator arm 28 , and first 30 and second 32 handles connected to first 20 and second 22 plates respectively . the first plate 20 has first 34 and second 36 planar faces separated by a bridge 38 . the bridge provides strength to the first plate 20 as well as provides a housing for the opening 54 ( discusses in more detail below ). the first 34 and second 36 planar faces and the bridge 38 are shaped and sized so that the first 34 and second 36 planar faces can be brought into contact with the first 14 and second 16 condyles and so that the bridge 38 can sit in the intercondylar notch . the first 34 and second 36 planar faces are co - planar . the second plate 22 has a first face 40 for contact with the tibia . the face of the first 20 plates which faces the face of the second 22 plates is configured so that the first 20 and second 22 plates can be brought together and fitted flush against each other . the first handle 30 has first 40 and second 42 straight portions connected by a curved portion 44 toward the middle of the handle , such that the angle between straight lines extending along the length of the first 40 and second 42 straight portions is approximately 120 °. the second handle 32 is substantially straight along its entire length . the curved portion 44 of the first handle 30 is pivotally connected to the second handle 32 so that the first handle 30 can pivot relative to the second handle 32 about an axis y which extends perpendicularly to the lengths of the first 30 and second 32 handles . the first handle 30 is connected to the bridge 38 of the first plate 20 so that the first plate 20 can rotate relative to the first handle 30 about an axis z that extends along the length of the bridge 38 and is contained within the planes of the first 34 and second 38 planar faces . the second handle 32 is connected to second plate 22 so that they cannot move relative to each other . in the embodiment described , the second plate 22 and the second handle 32 are provided as a single moulded piece . as shown in fig4 , a locking arm 70 is attached to the end of the second handle 32 that is distal to the second plate 22 by a pivot 76 . the locking arm 70 has a plurality of teeth 74 arranged on a first face . the locking arm 70 can rotate relative to the second handle 32 about the pivot , so that the first face of the locking arm 70 can be brought into contact with the end of the first handle 30 that is distal to the first plate 20 . furthermore , the locking arm 70 is biased towards contact with the first handle 30 . the end of the first handle 30 distal to the first plate 20 has a hook 72 which can engage the teeth 74 . the teeth 74 and hook 72 are configured so that the hook 72 can easily slide over the teeth 74 when moved in a direction toward the second handle 32 , but so that the hook 72 is prevented from sliding over the teeth in the opposite direction . a leaf spring 78 is attached to the first handle 30 towards its end distal to the first plate 20 . the leaf spring 78 is biased the against the second handle 32 so that the ends of the first 30 and second 32 handles that are distal to the first 20 and second 22 plates are biased away from each other . accordingly , the locking arm 70 , hook 72 and leaf spring 78 arrangement provide a ratchet mechanism which enables distance between the first 20 and second 22 plates to easily be selected and locked as described in more detail below . the angle indicator arm 28 is provided as a separate piece to the first plate 20 . the angle indicator arm 28 has first 46 and second 48 pegs extending perpendicularly to its length at its first end . the second end of the angle indicator arm 28 narrows to a tip 50 . the first peg 46 can be received in a first 52 bores in the first plate 20 . the second peg 48 can extend through a first opening 54 in the end of the first straight portion 40 of the first handle 30 , and be received in a second bore ( not shown ) in the first plate 20 . when the first 46 and second 48 pegs are received in the first 52 and second bores of the first plate , the angle indicator arm 28 extends perpendicularly to the planes of the first 34 and second 36 planar faces . a first end of the axis indicator arm 26 is connected to the first end of the angle indicator arm 28 so that when the guide assembly 24 is assembled , the axis indicator arm 26 can rotate relative to the angle indicator arm 28 and to the first plate about the axis z . a scale 58 is provided on a head 56 at a second end of the axis indicator arm 26 . the lengths of the axis indicator arm 26 and the angle indicator arm 28 are such that the tip 50 of the angle indicator arm is in the region of the scale 58 on the head 56 . the axis indicator arm 26 also has first 60 and second 62 limbs extending substantially perpendicularly to the length of the axis indicator arm 26 . the first limb 60 has a first opening 64 formed in it and the second limb 62 has a second opening 66 formed in it . the first 64 and second 66 openings define an axis along which a bone drill bit can be extended . the axes run substantially parallel to the axis z . a straight line connecting the centre points of the first 64 and second 66 openings runs substantially perpendicular to the length of the axis indicator arm 26 . a wheel 68 is provided on the angle indicator arm 28 . the wheel 68 has a threaded spigot ( not shown ), which extends through and engages a threaded hole ( not shown ) in the angle indicator arm 28 . rotating the wheel 68 clockwise causes the spigot to travel through the angle indicator arm 28 towards the axis indicator arm 26 . the length of the spigot is such that the wheel 68 can be rotated so that the end of the spigot can be driven into and engages the axis indicator arm 26 . once engaged , the spigot prevents the axis indicator arm 26 rotating relative to the angle indicator arm 28 . rotating the wheel 68 anti - clockwise causes the spigot to travel out of the angle indicator arm 28 away from the axis indicator arm 26 , thereby disengaging the spigot from the axis indicator arm 28 . in use , the angle between the reference axis 12 containing the first 8 and second 12 reference points and the mechanical axis x is calculated from an x - ray image of the femur 2 . the first 20 and second 22 plates are then located between the distal end of the femur 2 and the proximal end of the tibia ( not shown ) so that the first 34 and second 36 planar faces of the first 20 plate face toward the first 14 and second 16 condyles . the ends of the first 30 and second 32 handles that are distal to the first 20 and second 22 plates are then squeezed together . this causes the handles 30 , 32 to rotate relative to each other about axis y , so that the first 20 and second plates 22 are separated from each other . this causes the first 34 and second 36 planar faces of the first plate 20 to be urged against the first 14 and second 16 condyles of the femur 2 , and the first face 40 of the second plate 22 to be urged against the tibia . this helps to prevent the first plate 20 from moving relative to the femur 2 . as the first 34 and second 36 planar faces are co - planar , urging the first plate 20 against the first 14 and second 16 condyles will help ensure that the first 34 and second 36 planar faces contact the first 8 and second 10 reference points ( which are the most distal points of the femur &# 39 ; s condyles ). accordingly , the plane containing the first 34 and second 36 planar faces will also contain the first 8 and second 10 reference points . the hook 72 and teeth 74 arrangement will ensure that the ends of the handles 30 , 32 distal to the first 20 and second 22 plates do not separate under the force of the femur 2 and tibia on the first 20 and second 22 plates . if it is necessary to reduce the distance between the first 20 and second 22 plates , such as when the surgical procedure is complete , then the locking arm 70 can be rotated away from the first handle 30 so as to release the hook 72 from the teeth 74 . the end of the first handle 30 that is distal to the first plate 20 will then be forced away from the second handle 32 due to the leaf spring 78 , thereby causing the distance between the first 20 and second 22 plates to decrease . the axis indicator arm 26 and the angle indicator arm 28 are then connected to the first plate 20 by way of receiving the first 46 and second 48 pegs within the first 52 and second bores in the first plate 20 . when initially connected to the first plate 20 , the axis indicator arm 26 and the angle indicator arm 28 extend perpendicularly to the plane of the first 34 and second 36 planar faces . the axis indicator arm 26 is then rotated relative to the angle indicator arm 28 until the tip 50 of the angle indicator arm 28 points towards the angle which is equal to 90 ° minus the angle between the reference axis 12 containing the first 8 and second 12 reference points and the mechanical axis x . the angle between the plane 71 of the planar face 34 of the first plate 20 and an axis m through the axis indicator arm 26 is shown in fig3 as β . once this position has been reached , the length of the axis indicator arm 26 extends along the length of the mechanical axis x . accordingly , the mechanical axis x has been found and the first 64 and second 66 openings can be used to guide a drill bit into the femur 2 to prepare bores in femur 2 for receiving corresponding pins of a cutting guide or a cutting tool ( not shown ). optionally , the first 64 and second 66 openings can be used to mount a cutting guide directly on the axis indicator arm 26 . the cutting guide can then be used to guide a cutting tool into the femur 2 .
a method for locating the mechanical axis of a long bone involves generating an image of the long bone and determining the angle between the mechanical axis of the bone and a reference axis , which contains at least two reference points towards one end of the bone . a reference arm is positioned relative to the at least two predetermined reference points , and the mechanical axis is located relative to the reference arm with reference to the previously determined angle .