Computed tomography review of tarsal canal anatomy with reference to the fitting of sinus tarsi implants in the tarsal canal.

Sinus tarsi implants are used in the treatment of symptomatic hyperpronating flexible flatfeet in children. Although some implants are inserted only into the sinus tarsi, others occupy both the sinus tarsi and the tarsal canal. The stem that is anchored in the tarsal canal depends on interference fit for the initial resistance to slippage. The first part of this computed tomography anatomic study in children was aimed at finding and measuring the dimensions in the narrowest point in the canal that provided the interference fit. The second part of the study assessed the possibility of the implant being loaded with axial body weight in the tarsal canal. All foot computed tomography scans performed consecutively at Birmingham Children's Hospital from January 2008 to December 2011 were reviewed to assess the tarsal canal dimensions on the sagittal views. A total of 52 scans fulfilled the inclusion criteria. The average age was 12.7 years. The narrowest mean anteroposterior diameter of the canal was 7.3 ± 1.12 (range 5.2 to 10.0) mm. The narrowest mean superoinferior diameter was 9.2 ± 1.32 (range 6.3 to 12.7) mm. A total of 50 patients had the narrowest dimension in the anteroposterior plane. A positive linear correlation was found between the anteroposterior diameter and the superoinferior distance (r = 0.51, p < .01). We have concluded that the stem of an arthroereisis implant extending into the tarsal canal is unlikely to be constantly bearing body weight, because it obtains an interference grip in the anteroposterior direction in almost all patients and not in the superoinferior line of axial body weight.