Twenty avulsion fractures of the calcaneal tuberosity were selected and reviewed out of 764 calcaneal fractures (2.6%). The type I fracture was the most frequently observed (40%) and was found more likely to occur in elderly women (Table 1). However, type II, III, and IV fractures were observed more often in relatively younger male patients. This difference in age and gender were significant when comparing type I fracture with the three other types of fractures (p = 0.010, p = 0.018). The fracture patterns were then compared according to the mechanism of injury (Table 2). Type I fractures were more likely to occur by minor trauma such as tripping. However, the occurrence of type II, III, and IV fractures were usually observed in incidents involving more severe trauma (p = 0.019). A direct blow to the bone was the dominant cause of type II fractures and falling for type III and IV fractures.

Extensive insertion of the Achilles tendon was found for all types of fractures analyzed (15/20) (Table 3). However, the involvement of fibers of the Achilles tendon varied according to each fracture pattern. All the fibers of the Achilles tendon were involved in type I and II fractures, but only the superficial fibers were involved in type III fractures and the deep fibers in type IV fractures respectively (Fig. 2). These observations imply that the insertion pattern was not related to the fracture type, but rather to the specific fibers that were involved.

Several studies have suggested that avulsion fractures of the calcaneal tuberosity have an osteoporotic origin.7,8) These fractures have also been observed with increased frequency in diabetic patients due to insufficiency fractures associated with peripheral neuropathy.9,10) However, this description can only account for the classification of type I fractures within our study and cannot explain the other fracture patterns detected in young male patients. We examined this problem by examining the anatomical variations of the Achilles tendon insertion and the different mechanisms of injury. Although the true prevalence of extensive insertion of the Achilles tendon is not well established, a recent study by Lowy6) demonstrated this anatomical variant in two of ten dissected specimens. In our study, extensive insertion was found in fifteen out of a total of twenty cases of avulsed calcaneal fractures. We hypothesize that individuals that possess extensive insertion are at increased risk for potential avulsion fractures.3) One potential explanation includes the direct force provided by the wide and broad insertion of the Achilles tendon in these individuals and may result in an increased risk of avulsion fractures. However, we found that avulsion fractures could occur through partial involvement of the fibers of the Achilles tendon as according to our surgical and MRI analysis (Fig. 2). Specifically, type III and IV avulsion fractures occurred in patients with extensive insertion but only if the fibers are partially involved with an intact Achilles tendon mechanism. It is interesting that a mechanism to explain partial avulsion fractures was already suggested by Lowy.6) A potential explanation is that a few deep fibers of the tendon, and possibly the fibers arising from the soleus, may tilt the fragment while the rest remain distally attached. When the knee is in a flexed position with the gastrocnemius released, the contraction of the soleus muscle to the heel may induce the type IV type of fracture. We can hypothesize that the mechanism of injury for the type III fracture is similar. The type III fracture is believed to occur when the superficial fibers of the tendon are involved (Figs. 2 and 3).

The mechanism of injury also differs with each classification of fracture described. The type I fracture is more likely to occur as a result of minor trauma such as tripping. However, type III and IV fractures are likely to occur through more severe trauma such as falling down. This indicates that the type I fracture is an insufficiency fracture, however, type III and IV fractures mainly occur due to strong muscular contraction with the heel fixed to the ground. If the strong force of muscular contraction is combined with a direct impact injury, then type II fractures usually occur.

Through the course of our study, we found several limitations that need to be addressed. First, this report is retrospective and therefore contained biases less likely to occur in a prospective study. In addition, our sample number was small due to the low incidence of avulsion fractures of the calcaneal tuberosity. However, most of the literature regarding the avulsed calcaneal fracture involves case reports and explanations of the fracture patterns which are heavily dependent on the opinion of individual authors. Our results demonstrate that the fracture pattern is determined by bone quality, the mechanism of injury and the fibers of the Achilles tendon that transmit the force. We believe that our novel classification system is useful because the treatment options will depend on the specific fracture (Fig. 3). Screw fixation is a good choice for treatment of type I and II fracture because the bony fragment remains of sufficient size for this treatment.4) In type II fractures, however, the posterior skin of the heel must be evaluated quickly, and if it is observed as tented or branched, the fracture must be reduced and fixed. These patients are at risk for skin necrosis of the posterior heel if they are not immediately treated.11) Type III and type IV fractures can be treated conservatively due to the preserved function of the Achilles tendon. However, since these fractures often occur in young individuals, surgical treatment is recommended for maintaining athletic ability. Suture anchor fixation provides superior treatment to screws due to the presence of small bony fragments for type III.5) In two patients included in the study, surgery was completed utilizing suture anchors type IV injuries. The surgery was difficult in the approach to deep fibers and in the management of fragments involved, leading to inferior treatment for the type IV as opposed to type III fractures. Based on our observations, we recommend conservative treatment for type IV fractures.

In summary, we observed four types of avulsed calcaneal fractures and developed criteria for further classification. The fracture patterns are created through osteoporosis, the mechanism of injury and fibers of the Achilles tendon that transmit the force.