fig1 shows an insert 10 according to an invention usable in an opening wedge osteotomy . the insert 10 is conical and may be made of an osteoconductive and bioabsorbable material . an external surface formation in the form of a helical rib 11 is provided on the insert 10 . the height of the rib 11 reduces to zero short of the proximal end 12 of the insert 10 to provide an unribbed frusto conical section 13 . the distal end 14 of the insert 10 tapers to a greater degree than the remainder of the insert 10 , to an approximate bell shape . an engagement formation in the form of a tapering hexagonal cross section opening 16 is provided extending for most of the length of the insert 10 from the proximal end 12 . the carrying out of an opening wedge osteotomy technique using the insert 10 of fig1 will now be described . a medial opening wedge high tibial osteotomy ( hto ) procedure was undertaken to correct angular deformity associated with unicompartmental arthritis of the knee . the surgical procedure was performed according to the following steps : geometrically determine the degree of correction required to shift the mechanical axis to the point of intersection on the lateral side . perform skin incision with care to avoid the saphenous vein and its branches . drill two guide pins from the medial side to within 1 cm of the lateral cortex ( angled towards the fibular head 18 ( see fig3 )). dissect anteriorly behind the patellar tendon and posteriorly with decortication if possible to preserve a periosteal sleeve . pass curved elevators to protect the patellar tendon anteriorly and the major vessels posteriorly . position an oscillating saw inferior to the guide pins to cut medial to lateral 20 taking great care not to penetrate the lateral cortex 26 . drill two guide - holes 22 ( see fig5 ) in the plane of the cut extending 40 mm ( approximately two thirds ) from the medial cortex angled towards the fibular head 18 . two inserts 10 are located in the guide holes 22 and are rotated by an appropriate tool ( not shown ) and with gentle valgus stress , into the guide - holes 22 to open the osteotomy . this is illustrated by fig3 and 4 , and also fig5 and 6 . in the latter slightly different inserts 38 are used with cruciform openings 40 . appropriate size inserts 10 are chosen relative to the degree of correction required . the proximal end 12 of the inserts should reside generally flush with the medial cortical surface when in their final position , taking care to maintain the lateral tibial cortex hinge . the degree of correction is then verified . the unribbed section 13 engages against cortical bone while the ribs 11 engage cancellous bone when the insert 10 is fully inserted , helping to prevent extrusion of the inserts 10 . in this procedure the apex 17 as illustrated by the lines 19 in fig2 and 7 , of the main part of the insert 10 substantially coincides with the position of the lateral cortical hinge 26 ( see fig7 ). an appropriate system is then used to provide a closing force across the osteotomy to compress the site and provide stability to the reconstruction , and details concerning this are provided below . the open wedge 24 produced is then filled with suitable bone graft . preferably this graft adheres to the absorbable inserts 10 and this provides added stability . preferred closure of wounds and post - operative management can then be followed . the application of a compressive force e . g . by the use of a compression plate , across the osteotomy at the surface of the bone midway between the inserts 10 will lever the lateral cortical hinge into tension . the preferred situation , in order to provide maximum stability to the reconstruction , is where compression is applied at all three points represented by the cortical hinge 26 and the two areas of contact between the medial cortex and for each insert the unribbed sections 13 of the inserts . this is achieved when the tensile supporting elements of the system cross the central plane of the osteotomy within the triangle formed by these three points . this combination of tension and compression elements is a tensegrity structure . fig9 shows one system of providing a compression force , with an inclined compression screw 28 extending across the wedge 24 to pass through said triangle . a notch / countersink 30 may be formed in the bone to receive the head of the screw 28 . fig1 shows an alternative system for providing a compression force with a plate 32 one end of which is located in a slot 34 in the bone above the wedge 24 . the plate 32 extends across the open end of the wedge 24 and is held in position therebelow by two or more screws 36 . fig1 shows a third system for providing a compression force . in this instance the inserts 38 are used having a cruciform opening 40 to permit rotation thereof . here four pins 42 are provided in the bone , two above and two below the open end of the wedge 24 . tension bands 44 extend between the pins 42 in a bow tie shaped configuration . in practice when a closing compression force has been applied , for instance , in any of the above ways , the inserts 10 , 38 may be rotated a little further into the wedge 24 to increase both the compressive forces on the inserts , and the tensile forces on the tensile supporting elements , thus placing the system further in to tensegrity . the use of two inserts provides three points of support , the two inserts at their contact with the medial cortex and the lateral cortical hinge , giving an inherently more stable system than prior art devices and methods . the diameter of the guide holes may vary to accommodate different size inserts and to provide different degrees of correction . it is to be realized that the method and apparatus used can be modified as particular situations dictate . for instance the guide holes can be of different respective dimensions . the inserts may have different diameters , lengths and also degrees of convergence . fig8 shows for instance an arrangement which differs from that in fig7 , with one smaller insert 46 . such variations can provide correction for anterior / posterior as well as lateral / medial angulation , or where the medial - lateral distance from the apex of the osteotomy i . e . the cortical hinge 26 , varies due to the shape of the bone , as shown in fig8 . the bone graft material may be used to fill the wedge gap between the inserts and external to the inserts between the cut bone ends . there is thus described apparatus for , and also methods of , providing tensegrity in an opening wedge osteotomy system thereby conferring mechanical stability by the combined use of counteracting forces of compression and tension . the apparatus used is such that it can be inexpensively produced by conventional methods . various other modifications may be made without departing from the scope of the invention . for instance the inserts may take a different form or shape . they may be solid or porous and may contain through - holes or other surface features to aid osseointergration . the base of the cone may not be orthogonal to its vertical axis in order to allow for the guide hole ( s ) not being orthogonal to the bone surface . different means for providing compression across the wedge may be used . the osteotomy may be performed on other bones in the body . the guide holes may be formed prior to the cut . whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon .
a method of carrying out an opening wedge osteotomy , the method including forming a cut in the bone , locating inserts in guide holes formed in the cut , and applying a closing force to either side of the cut using a screw , plate or tension bands extending across the opening of the cut , to ensure compressive and tensile forces are developed across the cut .