Copyright © 2008 by The Journal of Bone and Joint Surgery, Inc.
Commentary & Perspective
Commentary & Perspective by
Dror Paley, MD, FRCSC*,
Sinai Hospital of Baltimore, Baltimore, Maryland
Posted January 2008
The full-length standing radiograph is considered the gold
standard for assessment of mechanical axis deviation and joint line orientation
of the knee. There is no comparable radiograph available in the operating room
to assess mechanical axis deviation. While less than full-length radiographs of
the femur or tibia can be used to measure joint orientation angles of the knee
(femur: lateral distal femoral angle; tibia: medial proximal tibial angle;
and knee: joint line convergence angle), the mechanical axis deviation can only be
measured on a long film that includes the hip, knee, and ankle. Since the goals
of alignment correction in the frontal plane are based on the mechanical axis
deviation, it is important to be able to accurately measure the mechanical axis
deviation intraoperatively. The most common way of measuring this is to use the
electrocautery (Bovie) cord, stretching it from the center of the hip to the
center of the ankle and obtaining a fluoroscopic image of the knee to see where
the metal cord passes relative to the center of the knee. To date, this time
honored and popular method has never been tested for accuracy of measurement of
mechanical axis deviation.
Sabharwal and Zhao compared mechanical axis deviation
measured on full-length standing radiographs with that measured
intraoperatively using the Bovie cord test. They studied 102 limbs in eighty
patients. There was a 13.4-mm difference between the mechanical axis deviation
measured on the full-length standing radiograph and the deviation measured with use of the Bovie
cord, and a difference of 2.8° in the joint line convergence angle. Both of
these differences were statistically significant (p < 0.0001). The
correlation coefficient (r) for the measurement of mechanical axis deviation
with use of the two radiographic methods was 0.88. An increase in body mass
index (BMI) was associated with a greater magnitude of difference between the
two techniques. Limbs with >2 cm of mechanical axis deviation and those with
a joint line convergence angle of >3° on the standing radiograph were
significantly more likely to have >10 mm of discrepancy in the measurement
of mechanical axis deviation with use of the two imaging techniques (p <
0.005). The authors concluded that intraoperative fluoroscopy with use of the
electrocautery cord is a useful tool for assessing lower limb alignment in
patients with a normal body mass index and ≤2 cm of mechanical axis deviation and ≤3° of joint line convergence angle on the standing anteroposterior radiograph.
They cautioned that the results obtained with fluoroscopy might be inaccurate
in patients who are obese or who have substantial residual mechanical axis
deviation or pathologic laxity of the knee joint.
How do we interpret these results? Should we continue to use
the electrocautery cord to assess intraoperative mechanical axis deviation
after corrective osteotomy? What accuracy is reasonable for our patients to
expect from realignment osteotomy surgery? Krackow1 raised this
question in his landmark 1983 article.
The accuracy of correction depends on patient and surgeon
factors. We have no control over the patient factors (e.g., body mass index,
type of deformity, bone quality, and soft tissues), but we can control the
surgeon factors (e.g., preoperative planning, type and level of osteotomy,
fixation method, and postosteotomy assessment of alignment method). In the
preoperative plan, the target postoperative mechanical axis deviation (i.e., the
mechanical axis deviation goal) is determined along with the level, magnitude,
and type of osteotomy and hardware that will achieve this correction.
Intraoperatively, it is the postosteotomy assessment of alignment that is the
most critical step for accuracy of correction. It is this step that identifies
whether the target mechanical axis deviation was achieved and whether the
osteotomy needs to be adjusted before completion of the operative procedure.
The key question is how accurate we have to be in achieving the target mechanical
axis deviation. Several studies2,3,4 have shown that the optimum
results of a valgus-producing high tibial osteotomy are obtained when the mechanical
axis deviation passes 30% to 40% of the width of the lateral compartment. This
value represents a width of joint line of approximately 3.5 mm. Hernigou et al4 found the best results were when the mechanical axis was ≥3° to 6° of mechanical valgus, which is equivalent to a 3-mm width at the tibial plateau. Therefore, the optimum
results of high tibial osteotomy are when the mechanical axis deviation falls
within a 3 to 4-mm range (10% of the width of the lateral plateau of the
tibia). Normal mechanical axis deviation is reported to be 4 ± 4 mm
from the center of the knee joint5. Based on these mechanical axis
deviation data, the mechanical axis deviation goal should be within 4 mm.
With use of this parameter, we can now evaluate the merits
of the Bovie cord test. The Bovie cord test cannot guide accurate correction in
patients with a joint line convergence angle of greater than 3° or
a high body mass index. Even in patients with lower body mass index or
with normal joint line convergence angle, its error is greater than ±4 mm.
My conclusion, therefore, is that it is not a very good test to guide accurate
correction of mechanical axis deviation. What then would be a better test? Paley
et al.6 reported on a series of acute deformity corrections in which
the joint line orientation angle was used as the method of intraoperative
assessment of alignment. The accuracy of this method was ±2°.
Since the medial proximal tibial angle and lateral distal femoral angle can be
accurately measured during surgery and since the joint line convergence angle
is not changed by the osteotomy, accurate correction of mechanical axis
deviation can be achieved when intraoperative radiographs of the knee include
the femur or tibia for femoral or tibial osteotomies, respectively. More
recently, to avoid taking an intraoperative radiograph of the osteotomized bone,
we have been employing a radiographic alignment grid (Fig. 1). This 51-inch
plastic grid is embedded with transverse and longitudinal wires (in a 5-cm by 5-cm
grid pattern) which can be seen with fluoroscopy. During surgery, the patient's
hip and ankle can be aligned on the same longitudinal line and then a spot view
of the knee can be taken with the image intensifier. Since the grid lies
parallel to the ground on the table but under the mattress, it is not subject
to the same problems of parallax and kinking of the wire as with the Bovie cord
and is also not affected by the conical shape of the anterior part of the thigh, which
makes the Bovie cord slant distally as it passes from hip to ankle. Knee-joint
laxity that alters the joint line convergence angle still affects this method
and contributes to inaccuracy5.
Fig. 1
Intraoperative assessment with use of a grid. A and B: Hip and ankle are centered on
the same grid line. C: Location of this same grid line relative to the center of the knee is the mechanical axis deviation (MAD). D: After an opening wedge osteotomy, this grid line moves to the center of the knee, achieving the desired correction (MAD = 0).
The ideal method to assess intraoperative alignment has not
yet been found. Navigation may be the answer in the future, but the high
expense and lack of availability limits the current use of this high-technology
method.
*The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the author, or a member of his immediate family, is affiliated or associated.
References
1. Krackow KA. Approaches to planning lower extremity alignment for total knee arthroplasty and osteotomy about the knee. Advances in orthopaedic surgery. 1983;7:69-88.
2. Fujisawa Y, Masuhara K, Shiomi S. The effect of high tibial osteotomy on osteoarthritis of the knee. An arthroscopic study of 54 knee joints. Orthop Clin North Am. 1979;10:585-608.
3. Coventry MB. Upper tibial osteotomy for gonarthrosis. The evolution of the operation in the last 18 years and long term results. Orthop Clin North Am. 1979;10:191-210.
4. Hernigou PH, Medevielle D, Debeyre J, Goutallier D. Proximal tibial osteotomy for osteoarthritis with varus deformity: a ten to thirteen-year follow-up study. J Bone Joint Surg Am. 1987;69:332-54.
5. Paley D. Principles of Deformity Correction. 1st ed, Corr. 3rd printing. Rev ed. Berlin: Springer; 2005.
6. Paley D, Herzenberg JE, Bor N. Fixator-assisted nailing of femoral and tibial deformities. Tech Orthop. 1997;12:260-75.
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