The average HKA angle was smaller in the subjects with increased sports activity level during their second decade of life (Group 3) compared to the subjects from Group 1 (p < 0.01): −2.3° (SD, 2.6°) versus −0.9° (SD, 2.2°); and compared to the subjects from Group 2 (p < 0.05), who had an average HKA angle of −1.3° (SD, 2.3°).
Body mass index; morphotype, height, and weight of the patient; length measurements; JLCA; and LPFA had no contribution to the development of constitutional varus.
Restoration of neutral mechanical alignment is considered a cornerstone for successful and durable knee arthroplasty [1, 4, 15, 20, 23, 24, 30, 31, 34]. The reason for this general belief is that neutral mechanical alignment is considered by most surgeons as the normal healthy situation, which leads to symmetric mediolateral joint loading, and that therefore neutral mechanical restoration should be attempted to provide a durable and successful arthroplasty [1, 2, 4, 5, 15, 24, 32, 37]. Whether this is correct can be questioned, however. Having been in orthopaedic practice for many years, we believe a certain fraction of the normal population does not have neutral alignment at the end of skeletal growth but rather some degree of varus. If this is correct, restoring the alignment to neutral at a later stage in the life of these patients, for example at the time of knee arthroplasty, would be abnormal and in fact unnatural for them, since it would implicate an overcorrection toward their natural situation in which they had spent their life since skeletal maturity. In this study, we asked whether constitutional varus really exists in the normal population and, if so, in what percentage of healthy individuals it occurs. Also, we aimed at discriminating factors that contribute to constitutional varus. This would allow recognition of the original type of deformity at the time of surgery.
Our study had a number of limitations. A single observer performed all the measurements; obviously the measurements may be influenced by the accuracy of the investigator and systematic bias can be introduced. However, a single observer assures consistency. The second limitation to our study is the use of full-leg standing radiographs for our measurements. Although this method is well validated in the literature and has excellent intra- and interobserver reliability, the rotational position of the lower extremities might influence the outcome of the measurements [8, 12, 13, 21, 27, 38]. The rotational position of the knee compared to the hip and ankle is variable, and a perceived constitutional varus could therefore in fact be an external rotation of one subject’s limb as compared to another. In our study, however, the rotational position of the lower extremities was controlled by positioning the extremities with the patellae facing forward, as was used by many previous authors who have studied lower leg alignment [7, 18, 21, 22, 26, 28, 38]. We believe, by doing so, the rotational effect is minimized. An alternative could have been to use a Questor® precision radiograph frame (PARTEQ Innovations, Kingston, Canada), which however is unpractical and not frequently used in daily clinical practice [10-13]. Another option might have been to use CT scans, which could prevent potential mistakes in rotational position . A disadvantage is however the higher radiation exposure, and therefore this method could ethically not be used in our large group of young healthy volunteers. Furthermore, we wanted to study standing alignment in the same way as is done in current clinical practice, which is with standard full-leg radiographs.
Our data show a substantial fraction of the normal population (32% of men, 17% of women) having a natural alignment at the end of growth of 3° varus or more. We have defined this as constitutional varus. These numbers may at first sight seem relatively high. Indeed, this finding has not been recognized so far, despite several published papers on normal lower leg alignment. This could be explained by a limited number of participants, a large variability in the subjects’ age, recruitment in a hospital setting, lack of stratification, and selection bias of the subjects in these prior studies [7, 10, 11, 18, 22, 26-28, 36].
We also found the most important contributors to constitutional varus were the MPTA and the mLDFA, contributing 40.8% and 29.4%, respectively. Constitutional varus was also associated with increased femoral varus bowing, an increased varus femoral neck-shaft angle, and an increased femoral anatomic mechanical angle, confirming previous published work by Victor et al. . These factors, which are detectable on a full-leg radiograph, could therefore serve to identify the patient with constitutional varus at the time of TKA, regardless of the osteoarthritic degeneration of the knee.
Also, constitutional varus was associated with increased sports activity in the second decade of life. The association of varus alignment with increased physical activity during growth has been raised by other authors before. Witvrouw et al.  have noted intense sports activity during growth leads to the development of varus knees, and this phenomenon occurs especially toward the end of the growth spurt. We believe such could be the consequence of Hueter-Volkmann’s law, which states growth at the physes is retarded by increased compression, whereas reduced loading accelerates growth [14, 19, 33, 35, 40, 41]. The increased loads caused by the adduction moment on the knee during ambulation and physical activity could therefore lead to the development of varus alignment secondary to delayed growth on the medial side and accelerated growth on the lateral physes [14, 18, 42]. Cook et al.  alluded to this theory in a biomechanical study on the etiology of pediatric tibia vara.
Our findings should be interpreted cautiously, since this is an observational study on healthy patients and includes no correlation with osteoarthritic patients or patients who have had a TKA. The importance of neutral mechanical with respect to a durable and successful result after TKA does not disappear if a patient has constitutional varus. The questions that remain are however (1) whether there is a clinically important functional disadvantage of restoring knees with constitutional varus to neutral alignment after TKA, and conversely (2) whether there is a substantial mechanical disadvantage to leaving these knees in slight varus. Until these questions become solved, the debate continues on how alignment in these knees should be corrected at the time of TKA.
In summary, our data show a large fraction of the normal population (32% of men, 17% of women) has varus alignment once they have reached skeletal maturity. Our data therefore contribute to the existing studies on normal human lower leg alignment that have been published in the past and have uniformly reported the average normal leg alignment is not zero but in fact slightly greater than 1° mechanical varus and with a relatively large SD.
The authors thank Myrthe Boymans for her illustrations included in this manuscript. The statistical analysis was performed by the Biostatistical Centre of the School of Public Health of the Catholic University Leuven.
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