Recent research has shown that the HTO procedure can lead to a significant reduction of pain and improvement of knee function in patients with medial osteoarthritis by delaying the progress of degeneration of the medial compartment . The HTO procedure leads to an unloading of the medial compartment by shifting the weight bearing axis towards a more lateralized one. By a mechanical femorotibial valgus angle of four degrees the load-bearing is assumed to be distributed equally between the medial and lateral compartment . As such, great consensus exists between orthopaedic surgeons to aim for a slightly postoperative valgus alignment of the knee. Many authors suggest a postoperative femorotibial valgus angle of 5–10° which equates the intersection of the weightbearing line at 62–66% of the tibial width [21–25]. The ideal correction angle is believed to allow for a sufficient unloading and regeneration of the medial compartment while at the same time avoiding an overloading and degeneration of the lateral compartment [26–28].
Generally, the influence of the HTO procedure on laterally located cartilage defects of the knee remains controversial [29–31]. While a high-grade medial cartilage degeneration is generally believed to negatively impact the outcome, little is known about the natural course of cartilage defects of the lateral compartment when performing HTO [32, 33]. For this reason, special interest of this study focused on whether patients with asymptomatic mild to moderate cartilage defects of the lateral compartment are likewise expected to benefit from HTO. As such, the aim was to investigate whether these patients can achieve comparable results to patients with normal lateral knee compartments and thus can be regarded as equally indicated for HTO. Some studies suggest that asymptomatic mild to moderate focal cartilage defects of the lateral compartment can be very well accepted without negatively influencing the total benefit of the HTO procedure [34, 35].
However, from the data of the present study a conclusion contradictory to this assumption can be drawn. Deducing from the results of this retrospective analysis, mild to moderate cartilage degeneration of the lateral compartment leads to significantly worse postoperative outcome scores compared to those with intact lateral knee compartments. This major finding is partially in concordance with a recent study from Hohloch et al. showing that cartilage defects of the lateral compartment are generally associated with lower outcome scores. However, statistical significance for this finding was lacking . A recent study from Jin et al. concluded that grade ≥ 2 cartilage defects of the lateral compartment feature a significant risk factor for failure of the HTO procedure .
These findings seem to be reasonable to some point: With increasing load-bearing of the lateral compartment, prior asymptomatic cartilage lesions likely tend to get symptomatic and may even tend towards progression . From the data of this study it can be hypothesized that even a meniscus intact lateral knee compartment at the time of surgery seems to be unable to fully compensate for the increased weight bearing of the cartilage defects and thus not preventing them from getting symptomatic. This may be unproblematic if the cartilage of the lateral compartment shows normal integrity. An animal based study of Ziegler et al. could demonstrate that neither macroscopic nor microscopic changes of the lateral tibiofemoral compartment can be found at 6 months after HTO . However, it is stated that at least some progressive changes of the lateral meniscus after the HTO procedure can be observed [30, 38].
Moreover, concerns and criticism regarding the routinely performed arthroscopy prior to the HTO procedure have been raised recently, as some studies failed to show any relevance of the arthroscopic findings with the total outcome of the HTO procedure [39, 40]. Taking the findings of the present study into account, the principle of a routinely performed arthroscopy prior to an osteotomy around the knee can be strongly encouraged. The presence of any focal lateral cartilage defects should then lead to a thorough re-evaluation of the planned osteotomy based on the findings of this study. Regarding minor findings of this study, the authors could demonstrate a negative bivariate correlation between the postoperative KOOS outcome score and the total degree of lateral cartilage degeneration according to the Outerbridge classification system. This suggests that a worse outcome with HTO can be expected with a higher degree of focal lateral cartilage defects. The postoperative valgus angle did not reveal any correlation with the final functional outcome scores, which is in concordance with existing literature [29, 41].
We are aware that the current study has several limitations. As this research was designed as a retrospective analysis, the level of evidence should generally be considered inferior to prospective cohort studies and results may be subjected to bias. However, compared to similar studies, patient number and follow-up period seemed to be adequate [29, 41]. Unfortunately, the follow-up periods were not always conducted within a predefined time period from surgery, leading to a relatively large span of follow-up data acquisition. A further limitation is the availability of only one outcome score that was obtained pre- and postoperatively. The assessment of further outcome scores like the International Knee Document Committee (IKDC) and Visual Analog Scale (VAS) for pain assessment would have added additional value to the study. Nonetheless, the KOOS is a powerful and validated tool that has also been validated for measurements of therapeutic effects of knee related conditions .