Cox regression with forward and backward selection identified tibial varus malalignment (<90°) as the most statistically significant predictor of reduced prosthesis survival (p < 0.0001), and this variable consistently explained the largest amount of the variance in the failure rate. Varus tibial malalignment (<90°) was associated with a 10.6 times greater risk of failure (hazard ratio, 10.6; 95% confidence interval [CI], 5.4 to 20.6; p < 0.0001). The second most important predictor identified by the Cox regression was a valgus femoral malalignment (≥8° of valgus), which was associated with a 5.1 times greater risk of failure (95% CI, 2.8 to 9.5 times; p < 0.0001). Other significant covariates were an age of less than seventy years (associated with a greater risk of failure, p = 0.0003), a preoperative varus overall malalignment (<–8° of valgus) (associated with a greater risk of failure, p < 0.0001), a preoperative valgus malalignment (>11° of valgus) (associated with a greater risk of failure, p = 0.0187), and BMI class (discussed below, p < 0.0001). Sex (p = 0.6036), a diagnosis of rheumatoid arthritis (p = 0.8713), and a diagnosis of osteonecrosis (p = 0.9891) were not significant predictors of the failure rate. Thus, the covariates included in the final model were an age of <70 or ≥70 years; a preoperative overall alignment of <–8°, –8° to 11°, or >11°; a BMI of <23, 23 to 26, 27 to 40, or ≥41 kg/m2; and either (1) a postoperative overall alignment of <2.5°, 2.5 to 7.4°, or ≥7.5°, or (2) a postoperative tibial alignment of <90° or ≥90° and a postoperative femoral alignment of <8° or ≥8°.
The possible clustering of failures in patients with bilateral knee replacements was tested by examining the ratio between standard errors calculated with and without the robust estimator; a ratio of <1 indicates the existence of patient clustering. The ratio was 1.11 for varus tibial malalignment, 1.05 for valgus femoral malalignment, 1.14 for a BMI of 17 to 22 kg/m2, 1.08 for a BMI of 27 to 40 kg/m2, 1.08 for a BMI of ≥41 kg/m2, 1.12 for preoperative varus malalignment, 0.95 for preoperative valgus malalignment, and 1.06 for age. Since all but one of these standard error ratios was >1, the nonclustered model was used in the current study. Notably, preoperative valgus malalignment (>11° of valgus) appeared to be susceptible to clustering (since the standard error ratio was 0.95). With the numbers available, no significant interactions between variables were found when the failure rate was analyzed on the basis of the groups (e.g., preoperative alignment of >11° of valgus) used in the final model.
Knees with both neutral tibial alignment (≥90.0° with respect to the tibial axis) and neutral femoral alignment (<8.0° of valgus) had a failure rate of 0.2% (nine failures in 4633 knees), the lowest failure rate of any of the combinations of tibial and femoral alignment. In contrast, knees with varus tibial malalignment (<90°) and valgus femoral malalignment (≥8° of valgus) had the highest failure rate, 8.7% (fifteen of 173, p < 0.0001)—i.e., attempting to compensate for a varus tibial cut with a valgus femoral cut was associated with a high failure rate. Failure rates for all combinations of tibial and femoral alignment in the 6070 knees are presented in Table I.
Failure rates for all combinations of tibial and overall anatomic alignment are presented in Table II. Compensating for varus tibial malalignment with a more valgus femoral alignment to yield a neutral overall anatomic alignment was associated with a failure rate of 3.2%, whereas varus tibial malalignment combined with a noncompensating femoral alignment to yield varus overall anatomic malalignment was associated with a failure rate of 4.2%, which was not significantly different (p = 0.4922).
A valgus femoral malalignment (≥8° of valgus) combined with a compensating tibial alignment to yield a neutral overall anatomic alignment was associated with a failure rate of 7.8% compared with a 2.1% failure rate for valgus femoral malalignment and a noncompensating tibial alignment to yield a valgus overall alignment (95% CI, 1.4 to 10.5 times; p = 0.0082) (Table III).
Simultaneous optimal alignment of all three measures (tibial alignment, femoral alignment, and overall anatomic alignment) occurred in 60.5% of the knees (3673 of 6070) and was associated with a failure rate of 0.22% (eight of 3673). The remaining 2397 knees in which at least one of the three measures was not in neutral alignment had a failure rate of 1.9% (forty-six of 2397) (p < 0.0001).
Failure Due to Any Reason, Collapse, and Instability
The failure rate was 1.5% (sixteen of 1044) in the knees with varus overall anatomic malalignment (<2.5° of valgus) and 1.4% (ten of 716) in the knees with valgus overall anatomic malalignment (≥7.5° of valgus). Twelve of the sixteen failures in the knees with varus overall malalignment involved medial collapse. Six of the ten failures in the knees with valgus overall malalignment involved instability (Table IV).
Varus tibial malalignment (<90°) was associated with a 10.6 times greater risk of failure for any reason (95% CI, 5.4 to 20.6 times; p < 0.0001) and a 32.0 times greater risk of failure due to collapse (95% CI, 9.5 to 107.7 times; p < 0.0001) than neutral tibial alignment (≥90°). Likewise, valgus femoral malalignment (≥8° of valgus) was associated with a 5.1 times greater risk of failure for any reason (95% CI, 2.8 to 9.5 times; p < 0.0001) and a 10.6 times greater risk of failure due to instability (95% CI, 3.7 to 29.9 times; p < 0.0001) (Table V) than a neutral femoral alignment (<8° of valgus). The mean time to failure was 3.8 ± 3.0 years (range, 0.6 to 12.2 years) for collapse compared with 6.5 ± 3.7 years (range, 0.7 to 13.1 years) for instability (p = 0.0172, Wilcoxon rank-sum test).
Preoperative valgus malalignment (>11° of valgus) was not significantly associated with failure due to tibial collapse (p = 0.4208) and was dropped from the regression model for this type of failure. Likewise, preoperative varus malalignment (<–8° of valgus) and BMI were not significantly associated with failure due to instability (p = 0.9380 and 0.7118, respectively) and were dropped from the corresponding regression model.
BMI and Alignment
In general, increasing BMI was associated with an increasing risk of failure of the prosthesis, independent of the effect of tibial alignment, femoral alignment, and overall anatomic alignment (p < 0.0001). The lowest failure rate, 0.65% (eight of 1223), occurred in patients with a BMI of 23 to 26 kg/m2; this group included 20.1% of the knees (1223 of 6070). The risk of failure was 2.5 times greater in patients with a BMI of 27 to 40 kg/m2 (95% CI, 1.1 to 5.6 times; p = 0.0335) and 10.8 times greater in patients with a BMI of ≥41 kg/m2 (95% CI, 3.6 to 32.4 times; p < 0.0001). Surprisingly, patients with a BMI of <23 kg/m2 (range, 16.5 to 22.5 kg/m2) also had a higher risk of failure than patients with a BMI of 23 to 26 kg/m2 (95% CI, 0.6 to 7.4 times; p = 0.2208).
Knees with varus overall anatomic malalignment (<2.5° of valgus) had a 5.6 times greater risk of failure in patients with a BMI of 27 to 40 kg/m2 (95% CI, 1.1 to 28.6 times; p = 0.0367) and a 12.9 times greater risk of failure in patients with a BMI of ≥41 kg/m2 (95% CI, 1.5 to 107.2 times; p = 0.0180) than in patients with a BMI of 23 to 26 kg/m2. Even knees with neutral overall anatomic alignment (2.5° to 7.4° of valgus) had a 7.7 times greater risk of failure in patients with a BMI of ≥41 kg/m2 (95% CI, 1.9 to 31.8 times; p = 0.0046) than in patients with a BMI of 23 to 26 kg/m2 (Table VI).
The effect of BMI was also evident in knees with varus tibial malalignment (<90°). Knees with varus tibial malalignment had a 2.7 times greater risk of failure in patients with a BMI of 27 to 40 kg/m2 (95% CI, 1.0 to 7.1 times; p = 0.0395) and an 18.0 times greater risk of failure in patients with a BMI of ≥41 kg/m2 (95% CI, 5.5 to 58.7 times; p < 0.0001) than in patients with a BMI of 23 to 26 kg/m2. Knees with neutral tibial alignment (≥90°) were not affected by increasing BMI (p = 0.9401) (Table VII).
Likewise, the effect of BMI was evident in knees with valgus femoral malalignment (≥8° of valgus). Knees with valgus femoral malalignment had an 18.0 times greater risk of failure in patients with a BMI of ≥41 kg/m2 (95% CI, 3.0 to 129.4 times; p = 0.0020) than in patients with a BMI of 23 to 26 kg/m2.
Other interactions between alignment and BMI were not significant (p = 0.9809 for interaction of a varus tibial malalignment [<90°] and a BMI of ≥41 kg/m2; p = 0.1296 for interaction of a valgus femoral malalignment [≥8° of valgus] and a BMI of ≥41 kg/m2).
Proper alignment (tibial alignment, femoral alignment, and overall anatomic alignment) of the prosthesis during total knee replacement is critical in maximizing implant survival. Although the roles that the overall anatomic alignment and/or tibial component alignment in the coronal plane play in total knee replacement failures1-14 are well documented, little has been reported regarding the effect of the femoral component alignment. The results of our study confirm that attaining neutrality of all three alignments is vital in maximizing implant longevity, and that substantial “correction” of the alignment of the second component in order to produce an overall neutrally aligned total knee replacement when the first component has been malaligned may increase the risk of failure of the total knee replacement.
A previous study conducted at our institution by Fang et al. indicated that poor overall anatomic alignment of a total knee replacement was associated with a 6.9 times greater risk of failure due to tibial collapse and that varus tibial alignment was associated with a 3.2 times greater risk4. Likewise, Berend et al.2 found that twenty of forty-one failures were due to collapse of the medial bone, and that all twenty of these knees were in varus alignment (mean, 3.7° of varus)2. To our knowledge, our current study is unique in demonstrating that a femoral alignment of ≥8.0° of valgus was also an important contributor to implant failure (resulting in a 5.1 times greater risk of failure). Although it is important to obtain an overall anatomic alignment that is as close to normal (2° to 7° of valgus) as possible, and it is equally important to obtain a tibial alignment of ≥90° relative to the tibial axis, femoral alignment should also be considered (with the optimal alignment being <8.0° of valgus).
Our study also attempted to address the strategy of overcorrecting the alignment of one of the components in order to offset a malalignment of the previously implanted component. While it is accepted that achieving overall alignment similar to the neutral range in our study (2.5° to 7.4° of valgus) is important for implant survival, our study is one of the first to show that “correction” of one component in order to achieve neutral overall anatomic alignment when the other component is malaligned is not advantageous. Attempting to compensate for a tibial alignment of <90° by performing a valgus femoral cut does not significantly reduce the risk of failure (3.2% for varus tibial malalignment and neutral overall alignment compared with 4.2% for varus tibial malalignment and varus overall malalignment, p = 0.4922). Indeed, although a tibial alignment of <90° contributes to more frequent postoperative problems, compensating by aligning the femoral component in ≥8° of valgus to obtain neutral alignment may actually increase the risk of failure. The failure rate of 8.7% for knees with varus tibial malalignment and valgus femoral malalignment was the highest among the four combinations of femoral and tibial alignment (Table I). Valgus femoral malalignment was associated with a 5.1 times greater risk of failure of the total knee replacement compared with neutral alignment. The goal of total knee replacement should be to restore neutral alignment of both components in order to attain neutrality of the overall anatomic alignment.
Obtaining tibial alignment, femoral alignment, and overall anatomic alignment within the neutral range has often been a subject of discussion in orthopaedics15-18. We achieved acceptable neutral alignment in 91.6% of the femoral components and 81.9% of the tibial components, and we achieved neutral overall anatomic alignment in 71.0% of the knees in our study, with the use of conventional extramedullary tibial and intramedullary femoral guide rods. The seven additional years of data from our institution that have become available since the study by Berend et al.2 show a marked improvement in alignment of the tibial component with use of a conventional alignment system by the surgeons at our institution (21.5% outliers of >3° from the intended angle in the 1990s compared with 5.7% outliers between 2000 and 2006, p < 0.0001). We therefore believe that, with proper surgical technique (including careful attention to the femoral and tibial resection angles), the use of conventional alignment systems is reasonable.
We were unable to identify significant interaction effects between tibial or femoral alignment and a high BMI. However, we intuitively believe that poor implant alignment combined with a high BMI represents a much greater risk to implant survival than either risk factor alone.
Berend et al.2, using the same database as the one used in our study, found that varus tibial malalignment and a BMI of >33.7 kg/m2 were important contributors to failure of total knee replacements. The present data continue to show that the components should be positioned correctly and that a high patient BMI is detrimental to the survivability of the implant.
Surprisingly, patients with a BMI of <23 kg/m2 had a higher failure rate than patients with a BMI of 23 to 26 kg/m2. Although this difference did not reach significance, the fact that patients with a BMI in the lowest range had an elevated failure rate in nearly every alignment group, as seen in Tables VI and VII, is difficult to ignore.
In summary, we believe that a surgeon should aim to place the tibial component at an angle of ≥90° from the tibial axis and the femoral component in <8.0° of valgus in order to yield an overall anatomic alignment (tibiofemoral coronal alignment) between 2.5° and 7.4° of valgus, which should be achievable with conventional instruments.
Investigation performed at the Center for Hip and Knee Surgery, St. Francis Hospital—Mooresville, Mooresville, Indiana
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Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. One or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.Copyright 2011 by The Journal of Bone and Joint Surgery, Incorporated