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.
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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.
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6. Paley D, Herzenberg JE, Bor N. Fixator-assisted nailing of femoral and tibial deformities. Tech Orthop. 1997;12:260-75.