Intramedullary fixation by means of a single screw remains a popular way of operative management in fractures of the base of the fifth metatarsal, especially those involving the articulation of the fourth-fifth intermetatarsal facet [3]. According to the anatomic zone classification by Lawrence and Botte, these Zone 2 fractures are best treated surgically to avoid the considerable risk of non-union, whose incidence has been reported as high as 21% [4, 9]. On the other hand, the optimal diameter of the screw has been a matter of considerable debate. Basic principles dictate that the largest screw possible which will achieve maximal contact interface with the dense cortical bone should always be used [4]. However, according to different studies this diameter varies from 4.5 to 5.5 mm [5]. Fifth metatarsal length measured with CT in the present study was correlated with the HDI and RSD. However, templating of the screw size based on preoperative radiographs of the uninjured side can lead to errors, because plain radiographs tend to overestimate the metatarsal length and underestimate the medullary canal width [5]. In the current study, the length of the fifth metatarsal was not a reliable predictor of the optimal screw diameter. Therefore, obtaining a radiograph of the contralateral, uninjured side may not be useful for templating in the treatment of fifth metatarsal fractures.

Anthropometric measurements of the foot could be used as a predictor of the optimal screw diameter. The relation of anthropometric measurements to the characteristics of the fifth metatarsal have been studied before. DeSandis et al. found a positive correlation of the patient’s height and weight to the length the fifth metatarsal and the medullary canal width [5]. In the present study, although the univariate analysis demonstrated significant correlation between both anthropometric measurements (PL and PBFM) and the HDI and RSD, the multivariate analysis showed no significant correlation. A possible explanation is that the anthropometric size is normally correlated with the size of the medullary canal, but the predictive value of those measurement is low (23.5% for the RSD and 26.6 for the HDI). The intramedullary canal of the fifth metatarsal is elliptical in shape. In most studies the VDI is greater than the HDI. In imaging of isolated metatarsals which are excised from cadavers the true reference of the VDI and HDI axes can be altered when compared to CTs of the whole foot [10]. This has led to conflicting findings in the past [6]. Therefore, for the purposes of this study care was taken to properly place the metatarsals and identify the respective axes correctly. The narrower horizontal (mediolateral) diameter (HDI) was chosen to predict the maximum screw diameter.

Several previous cadaveric studies have aimed to optimize the prediction of the ideal screw diameter for intramedullary fixation of the fifth MT. Scott et al. used a digital caliper tο measure the maximum coronal and axial diameters at the level of the isthmus in 25 transected metatarsals. They found a mean dorsal to plantar diameter of 6.475 ± 1.54 (range 4 to 12) mm and a mean medial to lateral diameter measured 4.6 ± 0.85 (range 3 to 6) mm. The authors suggested a 4.5 mm cannulated screw as the narrowest diameter of screw that could be appropriate for the fifth metatarsal. No CT was used in their study [11]. In the largest relevant study, Ochenjele et al. studied 119 MTs with CT. They measured the medullary canal at the bow of the metatarsal and a point 40 mm from the base of the fifth metatarsal, according to the usual fracture location and the respective necessary screw length for adequate mechanical stability. The dorsal to plantar medullary diameter was 6.7 mm (range 4.0–9.3) at the bow and 7.0 mm (range 4.0–10.5) at the 40 mm point. The medial to lateral diameter was 5.0 mm (range 3.1–8.0) at the bow and 5.1 mm (range 3.0–7.5) at the 40 mm point respectively [10]. What is noteworthy in the above studies and is also found in our measurements is the relatively wide range of the medullary canal dimensions among different individuals. The clinical relevance of this is that no single screw diameter can be suitable for the majority of patients with a fifth metatarsal fracture.

A relative weakness of the study is that the gender of the cadavers was not recorded. Gender differences in the length of the fifth metatarsal have been recorded, with a median length of 7.4 cm in males versus 6.79 cm in females [12]. However, previous studies comparing coronal canal diameter between male and female patients showed no statistical differences [5, 10]. Foot surgeons should keep in mind that larger individuals could present more bowing in their metatarsal shaft which may affect the selection of the proper RSD. Moreover, in real-life scenario, anthropometric measurements (e.g., PBFM and PL) in a fractured foot may be challenging in cases with marked foot swelling.