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Knee Arthrodesis using an Intramedullary Nail

- Discussion:
    - IM may be most reliable technique for achieving union;
    - w/ TKR infection, IM fusion needs to be performed in a staged fasion with removal of components and control of infection before fusion
             is attempted;
    - contra indications:
            - active infection:
                   - presence of active infection because of risk of infection spreading into medullary canals of the femur and tibia;
            - deformity of the femoral or tibial shaft:
                   - clearly a nail will not pass around an angulatory deformity;

- PreOp Planning:
    - x-rays include a full-length AP radiograph of lower extremity and lateral radiographs of the femur and tibia;
    - radiographic markers are used to define radiographic magnification;
    - long intramedullary nail that extends from greater trochanter to distal part of the tibia is used for a knee arthrodesis;
    - diameter of nail depends on diameter of IM canal of tibia, which generally has a smaller diameter than the femur;
            - when a large difference exists between the diameter of the femur and tibia, it will be difficult to achieve a tight interference fit;
                   - this may require application of cast postop;
    - nails of several diameters should be available at the operation in case the preoperative templating of the radiographs was incorrect;

- TKR Implant Removal:
    - preparation of the osseous bed;
    - preparation of the bone ends should expose vascular bone, provide bone apposition, correct limb alignment, and preserve as much bone
           stock as possible;
    - when bone cuts are being performed, extramedullary TKR cutting jigs can be used to achieve alignment and bone apposition;
    - bone resection should be limited to one to two mm of bone from the femur and tibia;
    - proximal part of tibia is be cut 1st to provide cut that is 90 deg to coronal plane and has the desired degree of posterior slope in the 
          sagittal plane;
    - bone ends should be vascular, stable, apposed, & in correct flexion and valgus;
    - establish a tibiofemoral angle of 0 degrees to allow passage of intramedullary nail;

- Nail Selection and Insertion Strategy:
    - full length antegrade nails:
           - refers to nails inserted antegrade from the piriformis fossa down across the knee and into the tibia;
           - due to the disadvantages of this type of implant is less often used;
           - advantages:
                - these nails provide maximum stability;
                - AP bow of the femur in the sagittal plane will allow three-point fixation of the nail in the femur;
           - disadvantages:    
                - entery thru the piriformis fossa and IM femoral reaming down to the knee joint causes significant bleeding which cannot be 
                        diminished by a tourniquet;
                - implant removal is especially difficult if there is implant failure;
                - reaming across localized infected tissue may cause extensive femoral and tibial osteomyelitis;
                - tibial-femoral mismatch requires use of a smaller nail which decreases stability;                    
    - short nails inserted thru the knee joint:
           - have become more popular due to ease of insertion and high union rate (over 90%);
           - nail is driven retrograde into the femoral canal, and then is backed out down into the tibial medullary canal;
           - advantages:
                  - piriformis fossa remains intact and therefore blood loss can be minimized w/ a tourniquet;
                  - femoral - tibial mismatch is not a problem;
                  - implant failure is unlikely due to the smaller nail length;

- Technical Pearls:
    - ream the tibia first to determine the maximal nail size (in most cases the femur will accomodate a larger nail than the tibia);
    - care must be taken to prevent distraction of the arthrodesis site during insertion of the nail into the tibia;
    - above knee cast is used for six weeks to provide rotational stability;
    - progressive weight-bearing to patient tolerance is begun while cast is still in place;

- Bone Grafting:
    - posterior bone graft is placed before IM nail is inserted;
    - should be performed at time of arthrodesis for cases w/ extensive bone loss;
    - in presence of bone loss, cancellous bone-grafting, by incr surface available for bone apposition, can improve the chances of success;
    - bone graft should be placed about periphery of the arthrodesis site to allow revascularization from surrounding soft tissues;
           - intramedullary circulation of the bone is usually compromised by prior implant and cement

- Complications:
    - complications inherent in intramedullary nailing include nail breakage, nail migration, and bone fracture.
    - complications associated with intramedullary nailing for arthrodesis have been reported in 40 to 55 per cent of cases



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