Master Techniques in Orthopedic Surgery Knee CH27

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27 Mobile-Bearing Medial Unicompartmental Knee Arthroplasty

FIGURE 27-10. Placement of the femoral trial.

FIGURE 27-11. Place the extramedullary tibial guide parallel to the long axis of the tibia (coronal and sagittal planes) and the tibial resection guide should be clamped with the G-clamp device to the spherical gauge placed around the femoral trial.

FIGURE 27-12. The G-clamp is used to clamp the tibial guide with a zero resection block. The tibial guide is pinned in place with a one-headed pin.

The femoral component placement dictates the position and direction of this cut, and the center of the femoral head is used as a secondary check. Avoid bringing the hand up with the saw and inad- vertently making a deep cut posteriorly. Before beginning the horizontal cut, ensure retractors are protecting the MCL from damage. Use a wide oscillating saw to make the cut. Complete the medial meniscectomy and leave a small cuff medially to avoid damage to the MCL. Compare the resected tibial bone to size the opposite side tibial component. The thickness of the tibia removed must be large enough to accommodate the 3-mm feeler gauge. If needed, remove extra bone using the 0 or 2-tibial guides. Whenever the feeler gauge is placed in the gap, make sure the retractors are removed to avoid misinterpreting that the gap is too tight. The 3-mm feeler gauge should be easily inserted and removed without much force. Assessment of Flexion/Extension Gaps Assess the flexion gap first using the feeler gauge as described. If more tibial resection is necessary, recut with the 0 shim. Repeat the check using the feeler gauge. A minimum of 3-mm flexion gap is required. It is necessary to remove the feeler gauge before moving the knee into extension because the extension gap is always smaller than the flexion gap and there is a risk to avulse the ligaments if the knee is extended with the feeler gauge in place. Establish the extension gap with the knee in approximately 20° of flexion rather than full extension because of posterior capsular tightening. This gap is always smaller, and therefore a thinner metal feeler gauge set is used. If the 1-mm gauge is unable to be inserted, then the gap is considered to be zero; but it could also represent a gap even smaller than zero. The spigots used for milling the distal femur are increased in 1-mm intervals and are numbered accordingly. The formula for calculating the amount of bone to be removed from the distal femur is as follows: Thickness of bone to be removed = Flexion gap (in millimeter) – Extension gap (in millimeters) Thus, if the flexion gap is 4 mm and the extension gap is 0 mm, then you will use the 4 spigot. Once the gap is balanced, make sure that the plastic feeler gauge can be easily inserted in flexion and extension with minimal force.