Abstract
Objective: To study factors that affect femorotibial (F-T) alignment after valgus closing wedge tibial osteotomy.
Study design: A review of standardized standing radiographs. Femorotibial alignment was measured 1 year postoperatively for over- and under-correction. Changes in F–T alignment and in tibial plateau angle were measured.
Setting: An urban hospital and orthopedic clinic.
Patients: Eighty-two patients with osteoarthritis and varus femorotibial alignment underwent valgus closing wedge tibial osteotomy. Patients having a diagnosis of inflammatory arthritis or a prior osteotomy about the knee were excluded.
Results: A 1° wedge removed from the tibia resulted in an average correction of F–T alignment of 1.2°. A knee that had increased valgus orientation of the distal femur had a greater degree of correction, averaging 1.46° in F–T alignment per degree of tibial wedge. This resulted in excessive postoperative valgus alignment for some patients who had increased valgus tilt of the distal femur. Optimal F–T alignment of 6° to 14° of valgus occurred when the postoperative tibial inclination was 4° to 8° of valgus.
Conclusions: There was a trend for knees with increased valgus orientation of the distal femur to have greater correction in F–T alignment after tibial osteotomy, likely because of a greater opening up of the medial joint space during stance. Surgeons need to account for this in their preoperative planning.
Valgus tibial osteotomy has been effective in the treatment of medial compartment osteoarthritis for patients with varus knee deformity. Its success depends on achieving optimal realignment of the femorotibial axis, generally agreed to be in the range of 6° to 14° of valgus.1–6 Under-correction of alignment heightens the risk of a poor clinical outcome. Over-correction tends to lead to good pain relief but poor cosmesis. A review of 79 high tibial osteotomies, at a mean follow-up of 5.8 years, showed over-correction in 20% of cases and under-correction in 7%.7 When there was over-correction, the change in femorotibial (F–T) alignment exceeded the magnitude of the tibial wedge. Our goal was to identify factors other than the size of tibial wedge resected that contributed to F–T realignment during high tibial osteotomy.
Methods
Patients and measurements
We reviewed the records from 1986 to 1992 of patients who underwent high tibial osteotomy after a full preoperative radiographic evaluation. Postoperative radiographs were completed at 1 year after operation. Patients were included in the study if they had varus limb alignment, which was defined as an F–T angle of less than 5° of valgus. Patients with rheumatoid arthritis or previous osteotomy to the same knee were excluded. There were 45 men and 37 women, for a total of 82 knees. Preoperative radiographic assessment was made by Questor Precision Radiography (Queen’s University, Kingston, Ont.) within a month of operation. This is a standardized method of standing radiographs of the knees and hips. The patient is positioned inside a calibrated frame and standing radiographs are obtained. They provide accurate, reproducible data.8 Bone landmarks are identified, and the films are digitized. The radiographic program then calculates the limb’s mechanical axis and alignment data. The following angles were used in this study and are shown in Fig. 1:
Lower limb measurements (coronal plane) used in the study. FS/TS = the angle between the femoral shaft anatomic axis and the tibial shaft anatomic axis; CH (condylar–hip) = the angle between the articular surface of the femur and a line perpendicular to the femoral mechanical axis — positive for valgus; TP (tibial plateau) = the angle between the tibial plateau (tangent to the margins) and a line perpendicular to the tibial shaft anatomic axis — negative for varus, positive for valgus; FS-TC (femoral shaft-transcondylar) = the angle between the femoral articular surface and the line perpendicular to the anatomic axis of the femur.
The F–T angle, which is the angle between the femoral shaft and the tibial shaft. By convention, a valgus alignment of the knee is defined as a positive angle and varus alignment as a negative angle. This line compares the mechanical axis of the tibia to the anatomic axis of the femur.
The tibial plateau (TP) angle, which is the angle between the tibial plateau and the line perpendicular to the tibial shaft. By convention, a valgus alignment of the tibia is defined as a positive angle and a varus alignment as a negative angle.
The condylar–hip (CH) angle, which is the angle from the centre of the femoral head to the line perpendicular to the joint line of the femur. This angle represents the mechanical axis of the femoral joint surface.
The distal femoral valgus (DFV) angle, which is the line from the anatomic axis of the femur to the femoral joint surface.
Operative procedure
All operations, performed by a single surgeon (M.A.S.), were valgus lateral closing wedge tibial osteotomies done above the level of the tibial tubercle. Staples were used for fixation, and limbs were placed in a cast for 6 weeks postoperatively. Postoperative assessment at 1 year was performed by the Questor radiographic method, obtaining the same alignment measurements as preoperatively.
Data analysis
The postoperative F–T angle was used as the main measure of alignment outcome. We sought to relate this angle to the change in the TP angle arising from the osteotomy. Postoperative F–T alignment was also related to the preoperative inclination of the distal femur (CH angle). Statistical comparisons were by linear regression and 2-tailed, unpaired t-tests. A p value of 0.05 or less was considered significant.
Results
The data are summarized in Table I. In general, alignment was fairly well corrected. The mean (and standard deviation) F–T angle changed from −4.2° (4.6°) preoperatively to 10.8° (5.0°) postoperatively. The relationship between the postoperative F–T angle and the TP angle is shown in the scatter plot (Fig. 2). As expected, there was a linear correlation between these variables (r = 0.52, p < 0.001).
Scatterplot shows the postoperative angle of femorotibial (F–T) realignment (FS/TS) as a function of postoperative tibial plateau angle (TP). Data are for all knees (n = 82). Regression data were: r2 = 0.273; b (slope) = 0.625; a (y intercept) = 6.94. Horizontal grid lines denote the “acceptable” range of F–T realignment: 6° to 14°.
Mean (and Standard Deviation) Angles of Lower-Limb Alignment and Geometry Before and After High Tibial Osteotomy
The scatter plot shows that the chance of correct F–T alignment was optimized when the TP angle fell within the range of 4° to 8° of valgus. Seventy-five percent of knees with a postoperative TP angle of 4° to 8° of valgus had an F–T angle of 6° to 14° of valgus. For knees in which correction of the TP angle fell outside the 4° to 8° range, the incidence of correct F–T alignment fell off markedly. The data show that the postoperative F–T alignment was not solely a function of the change in the TP angle. This difference was expressed as the Delta angle, which equals the change in F–T angle minus the change in TP angle. The Delta angle represents the additional correction in the F–T alignment not explained by the size of the tibial wedge. It represents the change in the joint space and any errors in measurement. The mean Delta angle for all 82 knees was 3.5°.
The relationship between the Delta angle (change in joint space) and the alignment of the distal femur is shown in Fig. 3. There was a statistically significant correlation between the size of the Delta angle and the orientation of the distal femoral articular surface (r = 0.36, p < 0.001). There is a greater degree of over-correction in knees with increased valgus orientation of the distal femur, and an increased incidence of under-correction in knees with less valgus orientation of the distal femur. In 14 patients there was under-correction of the F–T angle postoperatively. In 10 (71%) of them, orientation of the distal femur was less than normal with a CH angle less than 2°.
Scatterplot showing the relationship between the Delta angle and the preoperative condylar–hip (CH) angle. Data are for all knees (n = 82). Regression data were: r2 = 0.129; b (slope) = 0.657; a (y intercept) = 2.54. The vertical grid line corresponds to a median CH angle of 2°.
We subdivided the knees into groups based on the CH angle (orientation of the distal femur) as being less (group 1) or greater (group 2) than 2°. The mean CH angle differed by 3.7° between the groups. The other preoperative alignment variables were not statistically different between the groups. Postoperatively, the Delta angle for group 1 was less than that for group 2. For group 1, there was 1.2° of F–T correction per 1° of correction in the TP angle. For group 2, there was 1.46° of F–T correction per 1° correction in TP angle (p = 0.05). This indicates that the degree of alignment change in F–T angle per degree of tibial wedge resected differed depending on the preoperative orientation of the distal femur.
Discussion
To optimize the outcome of tibial valgus osteotomy, numerous studies have shown that the postoperative F–T angle should be in the range of 6° to 14° of valgus. In practice, both overand under-correction of limb alignment are common. Numerous studies have shown that under-correction with less than 6° of F–T valgus leads to more early clinical failure of the osteotomy. Our findings suggest that there is a risk of over-correction of alignment when there is greater valgus orientation of the femur and under-correction when there is a less valgus orientation of the femur. This study also suggests that there may be an optimal postoperative TP angle of between 4° and 8° of valgus, which optimizes the postoperative F–T alignment.
Our findings are in agreement with those of Terauchi and colleagues,9 who found that a postoperative TP angle of 7° to 9° of valgus resulted in an F–T angle of 11° to 15° of valgus. Similarly, they found that the change in F–T alignment was greater than the size of the tibial wedge. For their data, this additional change in alignment represented the amount that the joint space angle had changed. In their study, the preoperative TP angle was 7° of varus and the postoperative angle 10° of valgus for an average size in tibial wedge of 17°. Their preoperative F–T angle averaged 7° of varus, with a postoperative angle averaging 15.6° of valgus. This represented a correction of 23.6° of valgus. Their joint space angle changed from 8.5° of varus to 3.1° of varus for a valgus correction of the joint space averaging 5.4°.
For Terauchi and colleagues, a 1° change in TP angle resulted in an average 1.38° change in F–T angle. Almost all of this additional change in femoral alignment was accounted for by the change in the joint space angle. The magnitude of the additional change in the F–T angle for their data was very similar to ours. Hence, our Delta angle probably represents the amount that the medial joint space opens up with stance. Unfortunately, the Questor radiographic system does not measure a joint space angle: our early results of trying to measure it had an unacceptably large standard error. 8 We retrospectively determined the change in medial joint space for a subgroup of 24 of these 83 knees. The medial joint space increased from an average of 2.8 mm to 3.56 mm. Knees in group 2 (n = 12) (with increased valgus alignment of the femur), had a 0.8-mm increase in the medial joint space compared with a 0.7-mm increase in medial joint space for 12 knees in group 1. Given the small size of the subgroups, this difference was not statistically significant.
The magnitude of the Delta angle (change in joint space) depends on the orientation of the distal femur. Knees that had a large amount of femoral valgus would have a CH angle greater than 2° (or a DFV angle of greater than 8°). For these knees, there is a greater tendency for the medial joint to open up during stance.
We used the CH angle to measure the orientation of the distal femur. This angle measures the mechanical axis of the femur, which is the orientation of the distal femur with respect to the hip joint. Most orthopedic surgeons are more familiar with the anatomic axis of the femur (DFV angle), which is the angle from the femoral shaft to the line paralleling the articular surface. Research has shown that the mechanical and anatomic axes of the femur are closely related and differ by an average of 6°.10,11 A CH angle of greater than 2° would correspond to a DFV angle of 82° (i.e., 8° of valgus). We defined our groups as having greater than average femoral valgus or less than average femoral valgus.
Recommendations
As part of the preoperative planning for tibial osteotomy, we recommend that surgeons measure the alignment of the distal femur. If there is a valgus orientation of the distal femur with respect to the femoral shaft (anatomic axis) greater than 8°, then the aim would be for a postoperative F–T angle of 8°. This would normally correspond to a postoperative TP angle of 4° to 6° of valgus. In knees that have a lesser valgus orientation of the distal femur (less than 8° with respect to the anatomic axis), we recommend aiming for an F–T angle of 10°, which would correspond to a postoperative TP angle of 6° to 8° of valgus.
Acknowledgments
We thank Dr. Allan Scudamore for assistance with this manuscript.
- Accepted March 18, 1999.
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