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should be flush with the distal femoral condyle, which is accomplished by soft tissue dissection. The posterior aspect of the cutting block is pinned in place with a long-headed, 48-mm pin, and the distal femoral cut is made down to the level of the posterior slotted pin. After the slotted pin is removed, the cut is completed (Fig. 7). The IM guide is removed in order to inspect the distal femoral cut. It is critical that the distal femoral cut is flat because the posterior and chamfer femoral cuts are dependent on this first cut. A large bone file from a TKA set is used. A self-retaining IM patellar retractor is placed into the IM pilot hole with the knee flexed to 90. Alignment is exclusively obtained with this jig, and therefore, the importance of this step cannot be overemphasized. The posterior surface of the femoral jig must be perpendicular to the femur, ignoring the tibial surface. It is crucial to release the tibial surface for a medial UKA, to avoid a varus knee and a failed UKA. For a valgus knee, we advise a release of the IT band in all patients because the valgus knee heals back to bone within several weeks. Proximal Tibial Cut with Extramedullary Guide The extramedullary tibial guide is positioned in line with the long axis of the tibia using ankle clamps, similar to the same step in a TKA. Minimizing padding at the ankle allows for easier palpation of the distal tibia. The guide should be either medial or lateral to the tibial crest to ensure correct alignment depending on a varus or valgus knee. A lateral patellar incision is used for our valgus knees. The posterior slope should mirror the patient’s native anatomy, as determined during preoperative planning. A posterior slope of approximately 5 to 7 is recommended, with a slope at a lesser degree for an ACL-deficient knee. In the presence of an extension lag, the cut should be more Fig. 7. Completion of distal femoral cut. It is important to protect the soft tissues when completing the distal femoral cut. 94 Dunn et al anterior than posterior, despite fears of creating hyperextension. This strategy allows the contracted hamstrings a chance to be stretched out during the early postoperative period. When proceeding in this fashion, loss of extension, common in patients 70 years or older, is not a contraindication for a UKA, in our experience (Fig. 8). The cutting block is centered over the medial or lateral tibial surface without overlapping the patellar tendon to avoid cutting it with the saw. A 4-mm slotted stylus is used to measure the appropriate amount of resection on the deepest portion of the tibial plateau, which is easily visible after the resection of the distal femur has been completed. The proximal tibial cutting block should be secured with 2 headless, threaded, long pins and the extramedullary guide left in place for added security with a long-headed, threaded pin. Three pins are used to secure the tibial cutting block; there have been no postoperative stress fractures.44 The proximal tibial cut is made with the tibial cutting block and extramedullary guide with the knee positioned in 90. Hyperflexion should always be avoided when completing this cut. Care is taken to protect the MCL with a Z retractor placed meticulously for a medial UKA and on the lateral side to protect vital structures for a lateral UKA. A sagittal saw with a triangular tip is used to avoid a small bone bridge on the anterolateral or medial aspect of the tibial plateau where the sagittal and proximal tibial cuts meet. The saw blade should be placed lateral or medial to the guide, hugging the medial or lateral aspect of the intercondylar notch. Extreme caution must be used when using this saw to avoid penetrating the posterior capsule. In addition, care must be taken to avoid undercutting the tibial spine when vertically pushing to avoid a fracture. When the cut of bone is removed as 1 piece, it can be used as a guide to determine the proper tibial tray size. To successfully do this, a cobb is placed on top with a 1.27-cm (0.5-inch) flat osteotome underneath. It is essential not to wedge the osteotome. The leg should be placed in external rotation and 90 flexion and the cobb used on the medial side to remove the bone fragment. For a lateral UKA, a figure-of-four position can be used to successfully complete this step. When the fragment is difficult to remove, a knife can be used to remove the capsular meniscal attachments, which should have been removed at the beginning of the procedure. It is important to verify the gaps at this point in the procedure in order to decrease the odds of a gap imbalance when trialing components. An 8-mm or 10-mm spacer guide is placed to ensure that full extension has been maintained as well as no flexion tightness. Fig. 8. Extramedullary tibial guided positioned for a medial UKA. The angel wing verifies the level of cut with soft tissue protection. UKA: Intramedullary Technique 95 Posterior Femoral and Chamfer Cuts The final cuts are made with the knee flexed to 90. The appropriate-sized femoral sizer/finishing guide is selected. When determining the appropriate size, 2 mm of cut bone should be exposed anteriorly from the previously made distal femoral cut. The posterior boot of the guide should contact the cartilage of the posterior femoral condyle. Oversizing this component leads to patellar impingement.45 Once the guide is flush with the previously made distal femoral cut, the femoral finishing guide is
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