the mean medial gap was 1.5–2.5mm smaller than the mean lateral gap for all scenarios and forces tested (p<
0.05); everting the patella decreased the medial and lateral gaps by 1mm and 1.3mm with an intact PCL, and by 1mm and 2.7mm with the PCL resected, respectively; PCL resection resulted in increased flexion gap heights of ~1–2mm for both sides. During knee flexion from 30° to 90°, the PCL tended to squeeze the medial compartment by 1–2mm (p<
0.05). Increasing the force from 50N to 100N per side resulted in a mean gap increase of 0.5mm throughout the range of flexion.
Instruments are crucial to performing a knee replacement however they must be used properly. Cutting guides may be solid blocks or slotted blocks. For the former the blade must be held on the surface of the block. A surgical peanut pressing the blade against the block helps. Slotted guides obviate this problem however the saw blade should be chosen so as not to bind in the slot, nor to be so thin that the blade wobbles in the slot. The most difficult resection is the posterior femoral resection. Because of the problem of holding a saw blade up against a cutting block, a slotted guide is best in this area. The cut should be checked twice since the hard bone in the posterior femoral condyles may cause the blade to deviate and result in an under-resection. When blocks or guides are pinned to bone they should be inserted first through the convex side of the deformity (on the lateral side of a varus knee for instance). Often the bone on the concave side of a deformity is sclerotic and the pin may deviate changing the position of the block. A headed pin on the convex side will stabilise the block so that this will not occur. Intramedullary femoral alignment rods should extend up to and slightly through the isthmus. The entrance point is NOT in the midline but slightly medial to this (it should be templated on the preoperative x-rays). Extramedullary guides for hip must be referenced from the femoral head; normal clinical evaluation for this is inaccurate and preoperative radiographic evaluation is usually necessary. Intramedullary tibial alignment rods should enter at a point slightly anterior and medial to the midpoint of the tibia and should extend down to the level of the old distal tibial epiphyseal plate. The preoperative x-ray should be evaluated to ascertain the diameter of the canal. In some patients with a small intramedullary diameter a thinner rod may be necessary. An extramedullary tibial alignment guide should be centred slightly medial to the mid malleolar point distally. In the lateral plane the reference landmark is the fibular shaft. A rod parallel to the fibular shaft will also be parallel to the midaxis of the tibia. When any intramedullary guide is used the canal must be aspirated and washed to minimise the potential of fat embolisation. The rod should be fluted, the entrance hole large, and the insertion rate slow so as to avoid pressurisation of the medullary contents.
Preoperative templating essential Make the right skin incision: most lateral, leave appropriate skin bridge from older incisions, be prepared for plastic surgical consultation for skin expanders or flaps Release scarring in the medial and lateral gutters Elevate a proper medial capsular sleeve Release scarring between patellar ligament and front of tibial plateau Pin the patellar tendon Perform a quadriceps release OR a tibial tubercle osteotomy in the very stiff knee Extract the prior components in an axial direction Use intramedullary guides to align the cuts Localise the joint line position by the level of the patellar tendon