Because bony spurs that originate from the proximal medial surface of the tibia in young persons have been referred to by various names in the literature (Table 2), it seems that awareness of PABSs among orthopaedic surgeons may be low. Caffey initially described an asymptomatic transitory exostosis of the proximal tibial metaphysis in a 6-year old boy, and plain X-rays in that case showed an icicle-shaped bony spur (3). Ugai et al. reported 14 exostoses without a cartilage cap located on the pes anserinus in 10 patients, which they referred to as PABSs (1). Fraser et al. reported 10 children with pes anserinus syndrome due to proximal tibial bone spurs (2). Tiwari et al. reported that 2 of 3 patients with pes anserinus syndrome had an unusual presentation of rose thorn–like bony spurs without cartilage caps (4). Keats and Anderson presented a case of bilateral tibial spurs (5). The findings in each of the above studies seem to represent the same lesion as the tibial bony spurs in this study (Table 2). As in the above reports, our study revealed that PABSs manifest in the first 2 decades of life with no sex predilection.
To the best of our knowledge, this study is the first to radiographically evaluate PABSs. While involvement is usually unilateral, bilateral occurrence is not rare (33.3%). PABSs were very small, with a maximum length of 25 mm, and were consistently located slightly posterior to the medial edge of the proximal tibia. Smaller PABSs were asymptomatic and incidentally identified. The prevalence rate of PABSs in young persons was estimated to be 0.94%.
The causes of PABSs remain unclear. The question inevitably arises as to whether PABS is a variant of osteochondroma, which is the most common benign bone neoplasm. Both osteochondroma and PABS exhibit an abnormal bony projection from the surface of a bone. However, there are a couple of crucial differences between them. First, PABS histologically consists of lamellar bone trabeculae and fatty marrow without a hyaline cartilaginous cap (Fig. 4C) (1, 2) while osteochondroma consists of a cartilage cap and underlying bone (6). Second, in this study, PABS radiopacity differed from that of the underlying parent bone marrow, with no continuity between the two structures (Fig. 1D), while in osteochondroma the cortex and medullary cavity of the underlying bone were found to be continuous with the bony stalk and the centre of the lesion (6). Third, this study showed that the bases of the PABSs were consistently located at the medial-posterior edge of the proximal tibia, suggesting that PABSs may be non-neoplastic, site-dependent lesions (Fig. 1E and 2), while osteochondromas usually arise from the metaphysis of long bones and may grow in any direction (6). Nevertheless, we cannot rule out the possibility that loss of the cartilage cap due to continuous mechanical stress may result in PABS formation (1). Indeed, several studies have reported the spontaneous regression of osteochondromas (7, 8). Since EXT genes were found to play a role in osteochondroma formation (9), histological and biological research of these genes may help to distinguish between PABS and osteochondroma.
Ugai et al. described another possible aetiology of these bony spurs, namely enthesopathic alteration of the pes anserinus tendon (1). Enthesopathy refers to a disorder at the enthesis, and repetitive trauma at the enthesis leads to a repair process that includes development of painful bony outgrowths called “traction spurs” (10). However, our 3D-reconstructed, volume-rendered CT images of soft tissue in this study clearly showed that PABSs did not occur at the site of the tendinous insertion of the pes anserinus, indicating that PABSs are unlikely to result from enthesopathy. Taking into consideration our intraoperative findings that PABSs collided with the pes anserinus, we suspect that pain caused by PABSs may not result from enthesopathy. Further histological and biological research is warranted to elucidate the exact mechanism of PABS formation.
Regarding the treatment of PABSs, surgical resection was performed in all patients by Ugai et al. and in half of patients by Fraiser et al. (1, 2). These patients may have been overtreated, because we revealed in this study that about a quarter of PABS cases were asymptomatic, and only 2 patients underwent surgical resection of PABS during the follow-up period. Tiwari et al. recommended conservative management for most patients with tibial spurs, with surgical treatment needed only rarely in recalcitrant cases (4). Caffey reported a 6-year old boy with an asymptomatic spur that disappeared after 3–4 years (3). These and other reports suggest that clinicians should first consider conservative treatments such as rest, and should perform surgical interventions only if conservative approaches have failed. Based on not only radiographic findings that PABSs were positioned slightly posterior to the medial edge of the proximal tibia and the pes anserinus ran just beneath the PABSs, but also intraoperative findings that the pes anserinus was compressed by the PABS during knee extension, surgical resection of PABSs should be considered in young athletes with repetitive knee flexion and extension only if pain persists despite conservative treatment.
A limitation of our study is the relatively short mean follow-up time of 20.0 months, as it is still unclear if PABSs eventually disappear over time. We have not identified any previous reports of symptomatic icicle-like tibial spurs in adulthood. As a result, we speculate that PABSs in children and adolescents may eventually become asymptomatic even if they last for many years.
In summary, we present the clinicopathologic and radiological features and outcomes of 21 patients with PABSs. The prevalence rate of PABSs among children and adolescents was 0.94%. PABSs can be easily recognised because of their characteristic shape and location. Although larger PABSs may cause pes anserinus bursitis and pain, most PABSs are non-problematic. We suggest that surgical resection of PABSs should be considered in high-performing athletes only if pain persists despite conservative treatment.