Surgical Technique. Total knee replacement

Transcription

1 Surgical Technique Total knee replacement

2 Surgical Technique : Summary The knee s 1/ Tibial tray Page 1 s 2/ Mobile bearing Page 1 s 3/ Femoral component Page 2 s 4/ Patella Page 2 Surgical Technique s Important recommendations Page 3 s Preoperative planning Page Distal femur resection Page Tibial resection Page Extension gap Page 4 s Distal femoral resection Page 5 s Tibial resection Page Centromedullary guiding Page Extramedullary guiding Page Combined guiding Page 8 s Extension gap Page 9 s Flexion gap Page 9 s Femoral component size selection Page 10 s Chamfer cutting procedure Page 11 s Femoral component test Page 12 s Tray size selection Page 12 s Tibial preparation: Drilling Page 12 s Fin imprint Page 13 s S keel implantation Page 13 s Patella preparation Page 14 s Tests Page 14 s Placement of final components Page Tibial tray Page Femoral component Page Mobile bearing placement Page Patella sealing Page 16

3 The knee is a highly congruent prosthesis for tri-compartmental resurfacing of the knee. Bearing mobility is exclusively rotatory. There is no medio-lateral or antero-posterior mobility to interfere with the kinematics induced by the prosthesis design. The mobile bearing was designed to offer optimal safety against the risk of dislocation. Intrinsic flexion range, determined by the design and position of the posterior-stabilised cam, and by the design of the prosthetic posterior condyle is greater than 130. s 1/ Tibial tray It is of anatomic shape and is able to adapt to the resected surface of the tibia, without it being necessary to use a right or left component. The thickness of the tray has been reduced using a keel with two fins, playing the role of a prop. The keel is cone-shaped and can be prepared by drilling or piercing, and is hollow in the centre to accommodate the mobile bearing rivet. The tibial tray is made of cobalt chrome, a material of which the behaviour under friction is known. The characteristics of friction are optimized by rectification of the bearing and high-level polishing. s 2/ Mobile bearing The mobile bearing, made of ultra high molecular weight polyethylene (UHMWPE), to minimize wear phenomena has two lateral surfaces, in perfect congruence with the femur. The lower rivet, which joins with the tibial keel, has a 10 mm cylinder part, and ends with a 14.5 mm cone segment (total height = 24.5 mm). The cylindrical portion is required to stabilise the mobile bearing should it lift from the tray, a situation observed if the lateral ligaments are stretched, or distracted during walking. If the rivet was fully cone-shaped, lifting of the mobile bearing would lead to detrimental front or sagittal movements, which may encourage wear and creep of the polyethylene. To the front, the mobile bearing has been significantly indented to avoid all contact with the patella tendon or the tip of the patella during hyperflexion. The size of the mobile bearing with respect to the tibial tray was studied to allow for considerable rotation (over 15 ), while avoiding contact with the soft parts (lateral ligaments, patella tendon and popliteal tendon). For a number of prosthesis models with mobile bearings, rotatory range, if it is not controlled by a stop device, is limited by friction of the mobile bearing with the soft parts; Friction may cause anterior (ligamentum patellae), lateral (MCL/LCL) or postero-lateral (popliteal tendon) pain. 1

4 s 3/ Femoral component Like the tibial tray, the femoral component derives from a chrome cobalt casting process, and is rectified and highly polished. Like the posterior condyle, the distal condyle is 9 mm thick. The cutting angle of the trochlea is 6. The sharp skew is used for anterior resection while preventing the risk of stair gait The design of the femoral component aimed to lighten this part. Recessing of the lateral condyles in the femur reduces mass by 30%. Posterior resection according to a 4.5 angle to the inside ensures good posterior release in favour of flexion. Ribs guarantee correct positioning of the implant, and ensure adjustment to the bone sections. The trochlea is deep, wide and its recess is continued as distally as possible so as to offer the prosthetic patella good congruence during flexion. s 4/ Patella This is a sphere portion, cut according to an oval contour for better adjustment to the edge of the patella section. It has two sealing rivets. 2

5 Surgical Technique s General information The surgical technique aims to guarantee alignment of the knee and to obtain balanced ligament tension. This technique is based on the consensus philosophy of the Insall extension and flexion gaps. Ligament gaps and tension are evaluated using a system of spacers and wedges. This system is used to guide ligament release, and to create a rectangular extension gap, and also to achieve equal tension in the internal and external lateral formations. Ligament balance during flexion is obtained by unequal resection of the posterior condyles, based on ligament tension during flexion. It is also possible to take the bone markings into account (biepicondylar line or Whiteside s line). In any case, it is not necessary to alter ligament tension during flexion when setting extension. The instrumentation was designed to be the simplest and most reliable possible. Excessive modularity may be the cause of inaccuracy, due to accumulation of manufacturing limits, or due to clearance relating to wear of the various parts. The instrumentation was designed to be volume- and weight-saving. The same instrumentation is used for distal femoral resection and tibial resection. In the same way, only one resection block is used for posterior resection (identical for all sizes), and trochlear cut (5 slits on the same block). For distal femoral resection and tibial resection, and where per-operative observations are not compliant with preoperative planning, and where per-operative navigation is used to optimize the degree of angular precision of the cuts, it is possible to proceed with extemporaneous correction, degree by degree. Either distal femoral resection or tibial resection can be performed first. Important recommendations : The blades used to place the prosthesis must be 1.27 mm thick for insertion in the cutting block slots. Different femoral component and tibial tray sizes can be combined. However, the size of the mobile bearing must be identical to that of the femoral component. It is recommended using a patella implant of the same size as the femoral component, but the patella can be under (or over) sized by one size with respect to the femoral part. s Preoperative planning This is fundamental, as it determines the angle of the femoral valgus, and the entry points of the centromedullary stem in the cavity of the femur and of the tibia. Planning is based on the principle of goniometry. The mechanical axis of the femur must be drawn, joining the centre of the femoral head to the tip of the intercondylar notch. The mechanical axis of the tibia joins the centre of the base of the eminence in the middle of the talus. 3

6 1/ Distal femur resection Place the transparent template showing the outline of the centromedullary stems. Determine the centromedullary stem entry point, which may be slightly medialised if the femur is curved. Select the most appropriate diameter (8 or 10 mm). Decide which length to sink in (at least 20 cm for the femur, 30 cm for the tibia). Measure the angle formed between the reamer and the mechanical axis of the femur. Usually, this angle is between 5 and 7. It may vary by up to 3 in long-legged subjects, or up to 11 in the case of curved femurs. Distal femoral resection can be viewed, enabling per-operative verification (angle correction may be necessary during the operation in the event of discordance with the planning). 2/ Tibial resection Place the transparent template (centromedullary stem) on the tibia. If the tibia is curved, and centromedullary guiding is only possible over an insufficient length (less than 30 cm), then opt for extramedullary guiding. If the tibia is straight, determine the entry point of the reamer, which is usually median, and its diameter (the 10 mm stem should be preferred to the 8 mm stem). In intermediate cases, when the tibia is slightly curved, the centromedullary system can be used. To determine the entry point, move the transparent template while remaining parallel with the mechanical axis of the tibia, until the outline of the reamer is well marked, throughout the entire length of the centromedullary cavity. Measure the space between the reamer entry point and the centre of the tibial eminence (e.g.: 6 mm lateralisation). To determine the entry point, move the transparent template while remaining parallel with the mechanical axis of the tibia, until the outline of the reamer is well marked, throughout the entire length of the centromedullary cavity. Measure the space between the reamer entry point and the centre of the tibial eminence (e.g.: 6 mm lateralisation). 3/ Extension gap Preoperative planning makes it possible to view both cuts (distal femoral and tibial) and therefore the extension gap. Attention : the gap viewed does not take into account ligament tension and depends on the conditions of implementation of goniometry : It is therefore not recommended basing one s decision as to which ligament release procedure to use on planning only. By taking into account the clinical evaluation of ligament tension (reducibility of the deformation, laxity of convexity, retraction of concavity) it is possible to presuppose which release procedures should be used. E.g. : In cases of osteoarthritis of the knee with a varus of 12, trapezoidal extension gap, retracted MCL (non-reducible varus), and distended LCL (varus laxity), extended release of the MCL will be necessary. In this specific case, if the angle of the femoral valgus is for example 7, the cutting angle can be reduced by 1 or 2 (e.g. 6 or 5 ) in order to minimize ligament release procedures. In this case, a femoral varus of 1 or 2 must be accepted. However, it is strongly advised against to not respect the principle of orthogonality of the tibial cut, as incorrect positioning in varus or in valgus of the tibial tray leads to high strain on the PE, and in the mid-term leads to mobilisation of the tibial implant. 4

7 s Distal femoral resection After having exposed the joint, resected the synovial membrane and the osteophytes, and having released the ligament to a minimum extent. Drill the femur at approximately 10 mm above the notches, in the centre, or (according to preoperative planning) slightly inside the centre. Choose the 8 or 10 mm centromedullary stem according to the patient s size and preoperative x-ray study. Place the 8 or 10 mm centromedullary stem, pre-assembled on a T handle, and which will be sunk in to the length determined in the preoperative study, taking into account the curve of the femur, or the presence of an underlying femoral stem. Remove the handle. Insert the pre-set angle cursor (e.g.: 5 ) until it comes into contact with one of the femoral condyles. Check that the intra-operative observations are compliant with the planning (e.g. contact with the internal condyle, 2 mm space with respect to the external condyle for a 5 valgus angle). Where this is not the case the femoral valgus angle can be corrected in situ, by unlocking the angle cursor. Pre-assemble the FT cutting block on the bevel gear. Slide the two bevel gear rivets into holes made in the angle cursor, until the cutting block comes into contact with the trochlea 5

8 Detach the FT cutting block from the bevel gear. Remove the centromedullary stem and the bevel gear. If necessary, move the FT cutting block to + 2 (previous and/or nonreducible preoperative flexum) or to 2 (preoperative laxity and/or severe wear of the distal condyles). Resect, using a 1.27 mm blade sufficiently long for resecting in one step only. It is strongly advised against to use thinner blades as they may distort the cut. s Tibial resection After having excised the menisci and cruciate ligaments, the tibia is dislocated to the front. It is possible to perform tibial resection with the centromedullary system or the extramedullary system. 1) Centromedullary guiding Drill the entry point for the centromedullary stem at the centre of the tibial eminence (insertion of the ACL). If the tibia is slightly curved, preoperative planning may have provided for a slightly lateralised (or medialised) entry point. Insert the centromedullary stem pre-assembled on the handle and sink it in as far as possible in the centromedullary cavity Slide the angle cursor preset to 0, and place the FT cutting block pre-assembled on the T bevel gear. 6

9 To determine the tibia resection level, raise the FT cutting block to a height. - Use the paddle to set resection height. - Using the probe : It is advisable to base the resection level with respect to the tray the most frequently used (internal tibial tray in genu varum). Preset the probe at the desired resection level (e.g. 3mm) and lower the assembly until it comes into contact with the tibial tray (base of the cup). The probe can also be used unlocked. Fixe the FT cutting block using 2 nails at markings 0 (drill first). Move the cutting block nearer to the tibia, and fix again using one or two oblique nails Perform tibial resection using a long, 1.27 mm thick blade. If resection is insufficient, the block can be moved to marking + 2 or + 4, for further trimming by 2 or 4 mm. 7

10 E.g. : 3 Centre of the ankle or 2 nd intermetatarsal space 2) Extramedullary guiding This technique is recommended in curved tibias, where there is malunion or in any other situation in which the intramedullary system is not reliable. Insert the 8 (or 10 mm) centromedullary stem into the tibia, over a length sufficient for stabilising it. Slide the angle cursor and position the FT cutting block assembled on the T bevel gear. Insert the extramedullary stem into the bevel gear. Check that it is centred at ankle level. Set the angle cursor so that the distal end of the extramedullary stem extends into the middle of the talocrural joint (the talus more specifically) or of the 2 nd intermetatarsal space. Fix the FT cutting block and resect (after having removed the centromedullary stem). 3) Combined guiding Where centromedullary guiding is applied, the extramedullary stem can be used: To vary the varus/valgus angle by referring to the projection of the end of the stem in the ankle and foot. 8

11 s Extension gap Place the knee in extension, and exert traction according to the axis. View the space created in the two compartments. Position the detachable femoral wedge and 9 mm spacer assembly. This thickness matches the added thickness of the tibial tray and the thinnest part of the polyethylene at the base of the cup. The detachable wedge is equivalent to the thickness of the posterior condyles (9mm) of the femoral implant. If it is not possible to insert the assembly, trim the tibia (or the femur if there is severe preoperative flexum). To do this, adjust the cutting block by 2 or 4 mm. Once the assembly is in place, it must be ensured that complete extension of the knee has been achieved and that the knee is aligned correctly. If the knee is also taut in extension and in flexion (varus and valgus movements) and the gap is perfectly rectangular, it will not be necessary to perform any further release. If one of the compartments is wider (external compartment in genu varum), it will be necessary to release the ligament on the retracted side (MCL in this case). If frontal laxity is both internal and external, thickness must be increased, by adding a 2 mm spacer, representing the 11 mm mobile bearing. Add another spacer where required until total stability in the two compartments is achieved. The gap and ligament balance in extension having been achieved, the spacer-wedge assembly can be removed. s Flexion gap Place the knee in flexion. Insert the spacer assembly used during balancing in extension (without the 9 mm femoral wedge). Place the knee at 120 flexion to make insertion of the spacer easier. Bring the joint back to 90 and test laxity. External laxity is observed the most often, requiring addition of a wedge (1, 2, 3, 4 or 5 mm). In cases of genu varum, it is usual to insert a 1 or 2 mm wedge in the external compartment, which leads to positioning of the femoral part in external rotation. The external degree of rotation depends on the wedge used and on bone size. E.g. for an average sized knee (size 3), a difference of 1 mm is equivalent to 1 30 external rotation, 2 mm = 3, 3 mm = 4 30, 4 mm = 6 and 5 mm =

12 In cases of genu valgum, it is often necessary to insert a thicker wedge (up to 5 mm) in the external compartment, which places the femoral component in external rotation. Attention : it is highly unadvisable to insert a wedge thicker than that in the external compartment in the internal compartment, as it would place the femoral component in internal rotation, and would be harmful to patello-femoral tracking If it is possible to insert very thick wedges during flexion (e.g.: 4 mm external, 2 mm internal), return to the previous step and check the extension gap as it has probably been under-evaluated. Add one or several spacers in extension and bring into flexion; this will reduce the wedge thickness during flexion. If such correction is not made, it would lead to ligament imbalance (knee stable in flexion but with the axis in extension) and to implantation of an oversized femoral component, as determination of the size takes the total anteroposterior size of the femur and wedges in flexion into account. Ligament setting in flexion may also take into account the epicondylar markings or the posterior biepicondylar (Whiteside s) line. s Femoral component size selection Anterior and posterior resection This is a decisive stage : - An oversized femoral component loses its press-fit effect as the trochlear bearing surface is reduced. Over sizing may lead to pain (patella retinaculum overload) and limited flexion (overload of the extensor system and lateral ligaments in flexion). - An undersized femoral component leads to the opposite effects: Weakening of the anterior cortical (risk of supracondylar fracture if trochlear cut creates stair gait), and laxity in flexion. Slide the APM block until it comes into contact with the distal section. Vary knee flexion, which must be 90, so that the block is flat against the bone. Fix the cutting block using 2 flange screws 2 oblique nails. Apply the stylus to the anterior cortical and read the size. Where the reading is between 2 sizes (e.g.: T3 T4), follow the procedure below: 10

13 - Cut on the next size up (T4) and evaluate cut surface. If it is insufficient, select the next size down (T3) and trim (by inserting the blade in the lower slot). The resected thickness will be 3.5 mm (including blade thickness). The thickness of the posterior condylar cut does not vary with size (constant and equivalent to 9 mm), and it will not be necessary to rework it. Anterior cut Evaluation of trochlear trimming prior to resection may appear to be too significant. In this case, remove the nails and position 1 or 2 mm wedges in the 2 compartments (internal and external). Anterior trimming will not be 3.5 mm, but 2.5 mm (where there are two 1 mm wedges), or 1.5 mm (where there are two 2 mm wedges). Posterior resection will not be 9 mm as in standard cases but 8 mm (1 mm wedge), or 7 mm (2 mm wedge). This leads to slight ligament overload in flexion. Posterior cut s Chamfer cutting procedure Further to posterior resection and anterior resection, position the chamfer cutting block of the size described in the previous stage. Medio-lateral sizing of the CC block should match implant width. Check that the chamfer cutting block is well set against the distal cut and the anterior cut. Fix the block using 4 nails (2 straight nails at the top, and 2 oblique nails on the sides). Carry out the section of the posterior chamfers and then the anterior chamfers. These sections are carried out with a wide blade for greater comfort (20 mm). 11

14 s Femoral component test Take the test femoral component using the pliers. Position and impact the test component. Position the knee in hyper flexion and proceed if necessary with resection of the posterior osteophytes. To optimise flexion, it is advisable to resect all bone overlapping from the prosthetic posterior condyles. If the test femoral component does not fit perfectly on the distal femoral cut, the angle may need to be broken using a bone file on the internal side of the trochlea. s Tray size selection A tibial tray of any size can be implanted, as the femoral component and mobile bearing must match in size, but not the tibial tray and the mobile bearing. Select the tray size which will the most effectively cover the tibia cut surface. In the event of osteoporosis, it is advisable to widen coverage by overlapping onto the corticals by 1 mm if necessary. Set the rotation of the test tray with respect to the axis of the foot. Accurate setting is not necessary as it is a mobile bearing and accurate positioning in rotation of the tibial tray is not essential. Fix the test tibial tray with the two flange screws. Position the tower, of which the role is to guide drilling and keel sinking in the three planes. Position the two handles which will be used to hold the tibial tray by flattening it against the bone during bone preparation. s Tibial preparation : Drilling A tapered drill is used for drilling. Depending on the size of the keel, adjust the depth of the drill : For sizes 1 and 2, upper mark For sizes 3, 4 et 5, lower mark 12

15 s Fin imprint XS The fin imprint is made in this stage: impact the router using the hammer attached to the universal handle. The router can be used with all sizes. The depth is controlled by the laser marking lines (same marking as for the drill). If the bone is particularly hard (sclerous bone of an internal tibial tray on genu varum), the bone should be prepared using the oscillating saw. Where an XS keel is implanted, tibial preparation should be complete s S keel implantation S keel implantation requires preparation with the reamer. Position the jig bushing matching the diameter of the keel selected on the test tibial tray. Match the top of the teeth of the reamer with the top of the jig bushing. S 13

16 s Patella preparation Evert the patella Resect the synovial membrane in the suprapatellar region, and remove the lower perimeter of the patella from the fatty tissue. Determine the thickness to be resected. Where the patella is little worn, 8 mm bone should ideally be resected. Take the patella with the forceps. Place the forceps at a height using the probe, which determines resection depth. Proceed with resection. s Tests Insert the bush in the test tibial tray. This will stabilise the stray for the tests. The bush is the size of an XS keel. When an S keel is implanted, the trial keel need to be fixed on modular bush : Fix the test keel to the modular bush, and insert the assembly in the test tray. Ensure that the test bush/modular keel assembly is in close contact with the test tibial tray. Position the mobile bearing matching the size of the femoral component and of which the thickness was determined during the extension and flexion gap setting phase. Test the knee in flexion and extension - If the knee is aligned and tight both in extension and flexion, the mobile bearing thickness used for the tests can be adopted. - If the knee appears to be too taut in extension and in flexion, test the next thickness down. If the knee is lax in extension and flexion, test the next mobile bearing thickness up. 14

17 When the tests are considered to be satisfactory, the centring rivet holes for the femoral component must be prepared. Using the stop drill, prepare the two impaction rivet holes. Ensure that the drill descends and then stops against the test femoral component. s Placement of final components Wash the joint thoroughly to remove any bone debris. Proceed as follow : 1) Tibial tray Dislocate the tibia to the front, and place the knee in hyperflexion Carefully wash then dry the bone surfacee. Assemble the extension keel with the tibial tray by impact the assembly taper, then solidarize the keel with the screw packed into the conditioning of the tibial tray. Use the tibial impactor mounted on the universal handle for impaction. 15

18 2) Femoral component Impact the femoral component. If a cemented femur is implanted, ensure that a little cement is applied to the posterior condyles. Remove any excess cement around the edges and from the chamber. 3/ Mobile bearing placement When the tibia is in hyper flexion and anterior draw position, insert the mobile bearing. Place the knee in extension, in order to place the cement under compression where sealing is applied. 4/ Patella sealing Hold the patella during sealing using the patella forceps. 16

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