Structures at risk
• The MCL must be carefully protected during bone cuts
• Patella tendon
α Lone axis of femur
5–9°
valgus
Cut
IM referencing
α
Figure 11.3 Distal femoral resection
1 Will lead to overstuffing 3 Will notch femur
1 2 3
Figure 11.4 The anterior femoral cut
is not oversized. When the desired size is estimated, marker holes are made on the distal femur through the appropriate holes on the jig, to enable positioning of the distal femoral cutting block. The distal femoral cutting block should be positioned in slight external rotation and some implants have the marker holes set at 3° of external rotation on the distal femoral sizing jig.
This can be checked by correlation with the transepicondylar axis or Whiteside’s line. The reasons for this are explained below.
The cutting block of the estimated size is placed onto the cut surface of the distal femur, with pegs sitting into the previously drilled marker holes. In order to avoid notching of the distal femur during cutting, a cutting guide, commonly referred to as an ‘angel wing’, can be placed through the chosen cutting slot to estimate the exit point of the anterior cut (Fig. 11.4). The cutting block is firmly impacted until seated flat onto the cut surface of the distal femur and secured with obliquely placed pins. Again, the soft tissues must be carefully retracted during the placement of instrumentation. If there is any difficulty in seating either the sizing jig or cutting block, the surgeon must check that all osteophytes are removed, that there is adequate meniscal resection, that the bone cuts are complete and that the soft tissues are retracted sufficiently. The anterior cut should be made first, followed by the posterior condyles, anterior chamfer and finally posterior chamfer cuts. If it is apparent that there will be significant notching of the distal femur the cutting block should be removed and the sizing reassessed (Fig. 11.4). If there is a possibility of resection performed depends upon the thickness
of the implant and any fixed flexion deformity present (see later) but is usually at least 9 mm.
Most jigs are slotted to allow for accurate saw blade advancement and have multiple slots and holes at different levels to allow adjustment of the amount of distal femoral resection desired. It is imperative that the medial and lateral soft tissues are retracted and protected with either Hohmann or Trethowan retractors. The cut bone surface needs to be of sufficient surface area and quality to allow adequate fixation with either a cemented or uncemented femoral component and must expose trabecular bone. If the distal femur is particularly sclerotic in parts, a ‘second pass’ with the saw blade may be required to achieve a flat surface, but one must bear in mind that repeated passes with a power saw generates heat, necrosis and metal debris from the jig.
The distal femur must then be sized to enable placement of the appropriate cutting block. Sizing jigs generally work on an anterior or posterior referencing system, using either the anterior distal femoral cortex or the posterior femoral condyles as the baseline, measuring the amount of anteroposterior resection required accordingly.
The typical sizing jig has an anterior stylus that must be seated down onto the anterior cortex, and it may be necessary to remove the overlying synovium in order to ensure that the component
Cut
Perpendicular
Long axis of tibia Figure 11.5 The tibial cut
Primary total knee replacement 177
from the more abnormal medial compartment. In a valgus knee the amount of tibial resection can be more difficult to estimate, but should generally extend to the level of the tip of the fibula head on the lateral side. When at the correct height, as confirmed with a stylus passed through the slot on the tibial cutting block and onto the tibial plateau, the cutting block can then be fixed with pins, advanced closer to the tibial surface, locked in place and the intramedullary rod removed.
The angle of the tibial cut can then be checked with an extramedullary alignment rod. If extramedullary referencing alone is used, the rod should be in line with the anterior tibial spine and the distal tip of the rod should lie just medial to the centre of the ankle joint (as this is where the mechanical axis of the limb passes). Using anatomical landmarks in the foot, such as the second metatarsal, is less reliable as rotation can occur within the hindfoot and midfoot. With extramedullary referencing, the anteroposterior slope of the tibial cut can be introduced either by use of an angled cutting block or by adjustment of the extramedullary jig itself. If the femoral cuts have already been made, the tibia can be externally rotated and subluxed anteriorly to allow exposure of the entire articular surface of the tibia. If the PCL must be preserved, a small portion of bone adjacent to the PCL attachment can be protected with a broad retractor and preserved during the tibial cut.
After the tibial resection is complete, the remaining meniscal remnants can be excised and the tibial component is sized and, following a trial of the components, the tibia can be prepared to accept the stem or keel of the prosthesis. The tibial component should lie in slight internal rotation on the tibia, with the midpoint of the tibial baseplate being in line with the medial third of the patella tendon to optimize patellofemoral tracking.
Flexion/extension gaps
The femoral and tibial cuts should be made such that the rectangular spaces created are the same in both full extension and 90° of flexion (Fig. 11.6).
The fact that the mediolateral tibial slope in the coronal plane is 3° to the perpendicular means that the posterior femoral bone cut in flexion minor notching occurring, this should be
controlled and any sharp edge of anterior cortex should be smoothed off with the saw or a bone file. The cut bone fragments can then be removed with knife and forceps and the posterior condylar cuts can be removed with a broad osteotome. The distal femur is then examined to ensure that the cuts are complete. Large posterior osteophytes apparent on the preoperative lateral radiograph or evident after bone cuts can be removed by lifting up the femur and carefully using a broad osteotome or saw under direct vision.
Tibial cut
The tibial cut should be made perpendicular to the axis of the tibia in the coronal plane with an anteroposterior slope of approximately 3° in the sagittal plane (Fig. 11.5). If intramedullary referencing is used, the entry point should be made with a drill at the centre point of the tibia.
The intramedullary rod should be inserted comfortably into the tibial canal and the cutting block adjusted in a varus knee to allow resection of approximately 10 mm from the more normal lateral compartment and approximately 2 mm
(a)
x x
Femur flexed 90°
Flexion gap should equal extension gap i.e. x = y
(b)
y y
Transepicondylar axis
3°
Figure 11.7 The effect of external rotation of the femoral cutting block
Figure 11.6 Flexion and extension gaps
should be in 3° of external rotation in order to obtain two parallel surfaces in flexion (Fig. 11.7).
Some prostheses use the principle of a tensiometer in flexion and extension to establish the correct bone cuts in order to obtain equal flexion and extension gaps whereas others use spacer blocks or trial inserts.
The most common problems are:
• Tight in extension, flexion satisfactory
– Solution: increase distal femoral resection.
Release posterior capsule off femur. Recess or resect the PCL. Beware of raising the joint line with excessive distal femoral resection.
• Tight in flexion, extension satisfactory
– Solution: downsize the femoral component by re-cutting the distal femur (beware of notching the femur when downsizing implants). Increase the anteroposterior tibial slope.
• Tight in flexion and extension
– Solution: increase tibial resection. Beware of losing too much tibial surface area.
• Loose in flexion and extension
– Solution: increase thickness of insert. Beware gross ligamentous instability.
Varus deformity
This is by far the most common deformity in the osteoarthritic knee. Bone loss is usually from the