Coronal Plane Osteotomy for Genu Valgum
Robert A. Teitge
DISTAL FEMUR OSTEOTOMY
Genu valgum is a valgus alignment of the limb as measured at the knee joint.
A valgus limb with the mechanical axis passing through the knee lateral to its normal position increases the lateral component of the quadriceps vector, thus creating an imbalance in forces acting on the patella.
When a normal tibiofemoral joint space is present and knee ligaments are normal, the valgus must result from a deformity in the tibia or the femur or both.
Eberbach et al reviewed 384 whole limb standing radiographs with genu valgum not located in the joint and noted the deformity was in the tibia in 55.2%, in the femur 19.5%, and in both in 25.2%.1 This refutes a prior belief that valgus limbs should be corrected with a femoral osteotomy. Each individual case needs to be independently assessed to accurately locate the site of deformity.
Distal femoral osteotomy for patellar instability is most usually performed to reduce the valgus component of the quadriceps vector, which is caused by an increase in the tibiofemoral valgus angle. The correction of a valgus femoral deformity for treating patellofemoral imbalance is usually a distal femoral varus osteotomy.
Closing-Wedge Osteotomy Versus Opening-Wedge Osteotomy
Because of ease of execution, a laterally based opening-wedge osteotomy (OWO) or medial closing-wedge osteotomy (CWO) is most often the preferred treatment.
The choice is based on healing time, stability, limb length change, and ease of exposure.
The open wedge has advocates because the correction is easily adjusted at the time of surgery by spreading open the lateral osteotomy cut to the desired correction, while the closing wedge necessitates the cutting of a precise angular wedge.
Advocates of closing wedge point to improved stability and more rapid healing time.
With CWO, if the correction is less than desired when the wedge is closed, it can be technically very difficult to remove a very small amount of additional bone to obtain the proper correction when the osteotomy surfaces are compressed.
If the CWO correction is more than desired, it is difficult to add bone graft and obtain as stable a fixation as when a precise wedge has been removed.
A CWO will shorten the limb, an OWO will lengthen the limb.
OWO generally has a longer healing time, is more unstable, and has a slightly greater potential for loss of correction.
Location and Orientation of Osteotomy
The more distal the osteotomy is placed, the greater is the surface area for healing and the greater is the cancellous bone with improved vascularity.
The osteotomy may be transverse, also called “perpendicular,” because it is perpendicular to the limb mechanical axis, or it may be oblique to the mechanical axis.
It may be in a single plane or combined in multiple planes.
The selection of location for osteotomy is the surgeons’ choice.
The orientation of the osteotomy is also the surgeons’ choice.
As a general rule, the osteotomy should be placed near the deformity. Because patellofemoral disease is often associated with a dysplasia of the lateral femoral condyle in association with a dysplasia of the trochlea, a valgus femur is not unusual.
A distal femoral osteotomy must avoid the articular cartilage of the trochlea and should protect the suprapatellar pouch from injury. This can be achieved by leaving the joint closed while elevating the suprapatellar fat pad and suprapatellar pouch at the level of osteotomy.
With a CWO, it is always preferable that the opposing limbs of the osteotomy are the same length, so there will be circumferential contact of the cortical bone, which improves stability. Owing to the flare of the distal metaphysis, the osteotomy must be oblique in the coronal plane for the proximal and distal cuts to be the same length.
The osteotomy plan begins with placing a new mechanical axis on the standing full-length radiograph (Figure 33.1D).
When the deformity has been determined to be in the femur, the new mechanical axis begins at the center of
the talus and extends through the center of the knee joint in a proximal direction.
Exactly where the new mechanical axis crosses the knee joint is at the discretion of the surgeon. When there is no narrowing of the tibiofemoral joint secondary to unicompartmental degeneration of articular cartilage, the goal is to recreate normal tibiofemoral alignment. Normal limb alignment is with 1° to 2° of varus such that the mechanical axis passes medial to the center of the knee but not quite to the tip of the medial tibial spine.
When the location for the osteotomy has been selected and the decision made regarding opening or closing wedge, the point for the apex of the osteotomy wedge is placed on the plane. The apex of the wedge will be on the medial side for a lateral OWO and on the lateral side for a medial CWO.
The apex position is an arbitrary selection site for osteotomy, but is usually just proximal to the articular surface of the trochlea (Figure 33.1F).
To determine the size of the wedge, open or close, draw a line from the center of the femoral head to the point on the radiograph representing the apex of the wedge. Draw a second line from the point of the apex proximally to intersect the new desired mechanical axis. This second line must be exactly the same length as the first line from the femoral head to the apex. When this second line has intersected the desired mechanical axis, the angle subtended by these two lines is the desired angle of correction (Figure 33.1E).
For a CWO, the apex is on the lateral side of the femur. The more distal location has advantages, same as previously mentioned.
Next step in the planning is to adjust the obliquity of the closing wedge, so the two cut surfaces are exactly the same length. An isosceles triangle with the apex representing the angle of correction is placed on the plane and rotated proximally or distally until both opposite equal length sides reach the medial femoral cortex at the same time.
If an open wedge is planned, the obliquity is relatively unimportant because both sides of a single cut will be the same length as the lateral gap is opened. The more vertical this cut is, the greater the opportunity to avoid the suprapatellar pouch. The angle of correction is identical to that of the CWO, but the apex of the wedge angle is on the medial cortex for a lateral OWO.
Brinkman and colleagues have performed biomechanical laboratory studies comparing five different fixation devices and different osteotomy configurations.2
The initial study compared open wedge with closing wedge. The two lateral OWOs were fixed with titanium plate with locking screws (TomoFix, Synthes) or nonlocking plate with spacer (Arthrex). Two of three medial CWOs were fixed with 90° angle blade plate (Synthes), for fixation of an oblique CWO and fixation of a horizontal (or perpendicular to the limb mechanical axis) CWO. The third CWO was fixed with a locking plate (TomoFix).
The medial CWO obliquely fixed with the angled blade plate and rigid fixation (compression) provided by far the greatest resistance to both torsion and axial loading.
A second study by Brinkman compared a single osteotomy with a “biplanar” osteotomy that involves an anterior oblique cut in the sagittal plane with a posterior transverse cut, resulting in both greater surface area for healing and some implied increase in stability as a result of creation of the anterior vertical scarf joint3 (Figure 33.2). These two configurations were fixed with the same locking plates (TomoFix).
Surprisingly, the biplanar cut provided much greater stability during axial loading than the single cut but much less stability during torsion loading than the single cut.
Setting the Correction Angle on the Femur
Multiple guiding devices have been devised to assist in guiding the cuts for angle of the wedge of a CWO.
The author has tried most and has found a simple system involving two K-wires more reliable.
The desired correction angle is determined from the preoperative planning.
This angle needs to be transferred to the femur where it is marked by two converging K-wires.
By knowing the length of the sides of the isosceles wedge to be removed, and from trigonometric calculation, as shown in the next step, the length of the bone cut from the medial femoral cortex is known.
For a right angle (Figure 33.3), the length of opposite side (o) = length of hypotenuse (h) multiplied by sin of angle θ.
The desired angle of correction is known from the preoperative planning. The distance from the medial cortex to the apex of the planned wedge removal can be measured. Figure 33.3 represents one-half of an isosceles triangle with angle θ representing one-half of the size of the desired correction wedge. Using sin θ = opposite side/hypotenuse, and knowing h, the distance of the osteotomy across the femur, and knowing the angle of correction θ, the opposite side can be calculated. The sin θ represents one-half of the desired correction.
The osteotomy angle chart (Figure 33.4) has been calculated for angles 5° and above and for femoral widths of 40 to 80 mm. This chart is hung on the wall of the operating room so it is available for instant reference.
A longitudinal incision is made roughly in the midline of medial thigh from just below the medial epicondyle proximally for about 8 cm. The length of course depends on the size of the patient and the size of implant to be used.
The fascia over the distal vastus medialis is recognized and incised longitudinally.
The lower edge of the vastus is identified, and the vastus is elevated from distal to proximal. It is not attached to the femur but is to the intermuscular septum. A periosteal elevator can separate if from the septum, but a finger can lift it off the femoral shaft. The muscle is reflected anteriorly, so the medial side of the femur is visualized.
Figure 33.3 A right-angled triangle used to calculate the length of the cortex opposite to the desired correction angle. The right-angled triangle is created by bisecting the desired wedge to be removed.
The one constant vascular bundle at this level can often be reflected distally to expose the bone for the osteotomy.
Check with fluoroscope to make certain the location of wedge removal matches the preoperative planning.
Place an incision in the periosteum in line with the osteotomy and elevate it away from the bone to be removed.
Kristian Kley has perfected a mini-incision for medial CWO that minimally elevates the vastus but slides a locking plate (TomoFix) under the vastus from distally, then uses arthroscopic portal cannulas (Arthrex passport button cannulas) through skin stab wounds and through the vastus medialis for passage of drilling sleeves, drills, and screws into the plate.
Surgical Technique for Closing Wedge Osteotomy (Figure 33.5)
Use a 2.5-mm drill bit from the medial side of the femoral shaft to place a hole directed at the planned apex that should be just above the lateral femoral condyle. The shadow of the condyle can usually be identified on the image.
Place a 15-cm-long 2.0-mm K-wire into this hole reaching, but not extending beyond the lateral cortex. To measure the length of the bone cut, a second 15-cm K-wire is placed alongside the first K-wire and the length of the second K-wire that extends beyond the first K-wire is the length of the distance across the femur to the wedge apex.
Next, by consulting the osteotomy angle chart (Figure 33.4) and knowing the desired angle of correction and knowing the distance from the medial cortex to the planned apex of the wedge to be removed, one can read the exact amount of bone to be removed from the medial cortex.
A second drill hole is started at this distance distal (or proximal) to the first K-wire and aimed at the tip of the K-wire. It is critical to not have the two K-wires meet outside the lateral cortex because a complete segment of femur will be cut, creating a completely unstable osteotomy. It is important that the two K-wires do not meet more than 2 to 3 mm inside the lateral cortex or the cortex will fracture when attempting to close the osteotomy.
The osteotomy cut should be far enough into the lateral cortex that it is just springy, but not complete.
The 2.0-mm K-wires that were originally placed so they could be easily removed from the 2.5-mm hole are now replaced with 2.5-mm K-wires, which have enough friction to usually not vibrate loose with an oscillating saw.
A thin saw (0.6 mm) with staggered teeth is much preferable to the thicker saws used for total joint arthroplasty. Saw blades with the oscillating tip have an advantage of not requiring a broad excursion to reach to the far side of the femur; however being thicker, the feel of cutting may be less sensitive.
The surgeon may prefer commercial saw guides, which are a variety of designs often with multiple K-wires, but if the saw blades while making both cuts are judged on the image intensifier to be perpendicular to the
femoral shaft, it may not be necessary to place two K-wires for each side of the osteotomy cut.