Little is known about deformities of the lower extremity as a consequence of solitary osteochondromas. We found that lower-limb deformities occurred in 8 of 83 patients with solitary osteochondromas. Our initial assumption was that both the tumor type and distance from the physis were risk factors for lower-limb deformities. However, the tumor type was associated with lower extremity deformities, whereas the distance to the physeal plate was not.

In previous studies of HME, the area around the knee joint was a relatively common site of osteochondromatosis. Lower limb deformities, including leg length discrepancies, genu valgum, and fixed flexion deformities, are known to be caused by osteochondromas around the knee joint [2, 10,11,12]. The mechanisms of deformation have been discussed in the context of HME.

Growing exostoses are thought to distort local bone growth. Porter et al. reported an inverse correlation between osteochondroma size and relative bone length in patients with HME [13]. They demonstrated the local effect of growing osteochondromas by restoring normal bone development after surgical excision of the tumor [14]. Carroll et al. demonstrated a correlation between the severity of angular deformities and the percentage of sessile lesions in HME patients [15]. Liu et al. also found that sessile lesion was significantly associated with genu valgum in 112 knees for patients with HME. [16] They postulated that more force is exerted on the underlying physis because of the increased width of the sessile osteochondroma. A broad base may exert a greater physeal effect because an osteochondroma reproduces the structure of the bone from which it originates. Thus, sessile lesions might be associated with a higher possibility for coronal limb malalignment, which was consistent with the finding of our study although the exact biomechanical effects of the broad base of tumor on the physeal plate was not clear.

On the other hand, there is a “field change” effect from a genetic mutation that distorts bone growth in HME patients. Exostosin 1 (EXT1) and Exostosin 2 (EXT2) mutations in HME result in decreased heparan sulfate levels, which are associated with ectopic bone formation. Defective biosynthesis of heparan sulfate increases proliferation rates and disrupts the differentiation process [17]. Therefore, the severity of skeletal dysplasia is correlated with the genotype, as patients with EXT1 mutations are more severely affected than those with EXT2 mutations [14, 18, 19].

Somatic mutations in EXT genes are sporadic in solitary osteochondromas [20]. Recent studies have demonstrated that heterozygous mutations in EXT1 are detected equally in solitary osteochondromas and HME, whereas mutations in EXT2 are infrequent in solitary osteochondromas [1]. Otherwise, there is a paucity of studies that have investigated genetic context of solitary osteochondroma. Therefore, further studies are necessary to identify the association between genetic mutations in solitary osteochondroma and lower extremity deformity.

While the literature on skeletal deformity associated with HME is extensive, there is a paucity of studies related to solitary osteochondroma and lower limb deformity. In 2008, Florez et al. reported that one patient had valgus knee deformity associated with a solitary mass in the proximal tibia among 113 cases of solitary osteochondroma. [4] Recently, Park et al. retrospectively reviewed 111 patients with solitary osteochondroma around the knee and found that it did not cause a clinically significant deformity of the lower extremity. However, they concluded that solitary osteochondroma in the distal femur was associated with shortening of the affected limb. [21] In our study, we demonstrated that solitary osteochondroma around the knee, especially the sessile type, was associated with lower extremity deformity.

This study had some limitations. First, a cross-sectional design was employed. Even if patients did not have lower extremity deformities at the time of the study, it is possible for them to develop deformities as their growth continues. Given that studies have shown that HME deformities become severe as skeletal maturation progresses, a long-term prospective study would be more informative. Second, there were insufficient data to perform statistical validation. We performed a univariate logistic regression analysis to analyze the risk factors instead of a multivariate analysis because of the insufficient number of deformities. The wide confidence interval for the ORs of risk factors might also be due to limited data. Therefore, further studies with larger cohorts are required to identify sophisticated risk factors for lower extremity deformity in patients with solitary osteochondromas. Third, the local biomechanical properties of the tumor, such as the size or width of the solitary osteochondroma, may also be a risk factor for deformity. As MRI is required to measure the accurate size of the tumor, we could not measure the tumor size or width because of the absence of MRI data for all patients. It is desirable that this dimension is included in future studies.