Measuring the flexion gap
Insert the tibial template [1] and apply the single peg femoral trial component [40] or, if available, the 2-peg without anterior extension (2-peg femoral US), to the milled condyle, tapping it home while holding the femoral impactor [31] angled at 45° to the femoral axis (Fig 7.20).
Figure 7.20 (a) Oxford single peg femoral component, (b) two-peg femoral component and (c) two-peg femoral component (US).
With the knee in about 110° of flexion and retractors removed, carefully measure the flexion gap with the gap gauges [47] (Fig. 7.21(a)). (In the unlikely event that the 3 mm gauge cannot be inserted, replace the tibial cutting guide and redo the tibial cut without the shim.) The gauge thickness is correct when natural tension in the ligaments is achieved. In these circumstances, the gap gauge when held between finger and thumb will easily slide in and out, but will not tilt. Confirmation of the correct thickness is obtained by demonstrating that a 1 mm thicker gauge is firmly gripped and a 1 mm thinner gauge is toggling loosely.
Figure 7.21 (a) The flexion gap is measured at 110° knee flexion.
Figure 7.21 (b) The extension gap measured at 20° knee flexion.
Measuring the extension gap
Having measured the flexion gap (say 4 mm), remove the gap gauge, fully extend the knee then flex to about 20° flexion. The surgeon should hold the leg and apply a gentle valgus load to take up any slack in the MCL. Use the gap gauges to measure the extension gap (say 1 mm) which is always less than, or equal to, the flexion gap. Confirm the size of the gap by using a gap gauge 1 mm thicker and 1 mm thinner. If a 1 mm gauge is too tight or cannot be inserted, assume the extension gap is zero. Calculate the amount of bone to remove by subtracting the extension gap from the flexion gap (4 – 1 = 3). It is therefore necessary to remove a further 3 mm of bone from the inferior femoral condyle (see The femoral component). The spigots (numbered 1–7) allow bone to be removed in measured quantities (in mm) from the level of the first mill cut. The number 3 spigot removes 3 mm, so should be used. If the surgeon is not certain how much bone to remove it is best to be cautious and remove too little rather than too much.
