Measurement technique of calcaneal varus from axial view radiograph.

BACKGROUND: Medial displaced posterior calcaneal tubercle creates varus deformity of an intraarticular calcaneal fracture. The fracture involves posterior calcaneal facet and the calcaneal body so we developed a measurement technique representing the angle between posterior facet and long axis of calcaneus using lateral malleolus and longitudinal bone trabeculae of posterior calcaneal tubercle as references to obtain calcaneal varus angle.
MATERIALS AND METHODS: 52 axial view calcaneal radiographs of 26 volunteers were studied. Angles between posterior facet and long axis of calcaneus were measured using the measurements 1 and 2. Angle of measurement 1, as gold standard, was obtained from long axis and posterior facet of calcaneus whereas measurement 2 was obtained from a line, perpendicular to apex curve of lateral cortex of the lateral malleolus and a line parallel to the longitudinal bone trabeculae of posterior calcaneal tubercle. No more than 3° of difference in the angle of both measurements was accepted. Reliability of the measurement 2 was statistically tested.
RESULTS: Angles of measurement 1 and 2 were 90.04° ± 4.00° and 90.58° ± 3.78°. Mean of different degrees of both measurements was 0.54° ± 2.31° with 95% of confidence interval: 0.10°-1.88°. The statistical analysis of measurement 1 and 2 showed more than 0.75 of ICC and 0.826 of Pearson correlation coefficient.
CONCLUSION: Technique of measurement 2 using lateral malleolus and longitudinal bone trabeculae of posterior calcaneal tubercle as references has strong reliability for representing the angle between long axis and posterior facet of calcaneus to achieve calcaneal varus angle.

INTRODUCTION

Medial displaced posterior calcaneal tubercle of intraarticular calcaneal fracture creates a varus deformity of the calcaneus.12 The deformity will result in the malfunction of the ankle and the hindfoot. Pre and postoperative calcaneal varus evaluations are necessary for fracture treatment. Axial view radiograph is routinely taken in a calcaneal fracture. However, tibial or fibular shaft is not viewed in the radiograph. Hence, measurement of calcaneal varus angle from the axial radiograph by using tibial or fibular shaft as a reference is impracticable. Actually, anatomical long axis of weight bearing bone such as tibia is perpendicular to articular surface.34 Hence, the long axis of calcaneus is also perpendicular to articular surface of the posterior calcaneal facet. So that varus angle of a calcaneal fracture is obtained from 90° minus the angle between the anatomical long axis and articular surface of the posterior facet of calcaneus. Displaced intraarticular fracture of calcaneus involves the calcaneal body and intraarticular fracture of the posterior calcaneal facet. These results that identifications of anatomical long axis of calcaneus and articular surface of posterior calcaneal facet by using calcaneal body and posterior calcaneal facet for the angle measurement are impracticable. Therefore, we developed a technique for representing the angle between the anatomical long axis of calcaneus and articular surface of the posterior calcaneal facet using the lateral malleolus and the longitudinal bone trabeculae of the posterior calcaneal tubercle as references in order to calculate the varus angle of intraarticular calcaneal fracture. The study was conducted to assess the reliability of the measurement technique.

MATERIALS AND METHODS

The study was approved by the Institutional Review Board. On plantar surface of each feet, MN line was drawn from middle of the heel (M) to middle of the base of the second toe (N). MN line represented the long axis of the foot [Figure 1a].5 Axial view (radiograph) of the calcaneus was taken. The volunteer laid supine on the X-ray table and the cassette was placed under the heel. The foot was dorsiflexed which allowed the plantar surface to be 90° to the X-ray table in frontal and sagittal planes [Figure 1b]. The X-ray tube was tilted 40° cranially and directed at the mid heel. Using programmable automation controller digital system and measurement tools, the measurement 1 which directly measured the angle between anatomical long axis of calcaneus and articular surface of posterior calcaneal facet from the axial view radiograph, was performed as the control for comparative study.34 Center of posterior calcaneal tubercle was identified and marked by a circle template (O) [Figure 2]. Medial calcaneal wall is between sustentacular tali and posterior calcaneal tubercle as well as anterior medial wall curves medially away from long axis of calcaneus. So, the junction between posterior medial wall and posterior calcaneal tubercle was identified and marked as A. AB line was anteriorly drawn from A along the straight medial wall and ended above posterior calcaneal facet at B in order to exclude sustentacular tali [Figure 3]. The lateral calcaneal wall above lateral calcaneal tubercle was identified. Then, a point at the lateral wall just above the lateral tubercle was marked as C. CD line was drawn from C and perpendicular to AB at D. Middle CD was calculated and marked as E. OP line was drawn from O through E and ended above posterior calcaneal facet at P.

Figure 1

Clinical photographs showing (a) The long axis of foot on plantar surface (MN) (b) Position of foot was adjusted on X-ray tube for axial view

Figure 2

X-ray of calcaneum axial view showing use of circle template to identify center of posterior calcaneal tubercle (O)

Figure 3

X-ray of calcaneum axial view showing AB and CD lines were drawn. OP represented anatomical calcaneal long axis. FG represented articular surface of posterior calcaneal facet. OĤG as the angle between anatomical calcaneal long axis and articular surface of posterior calcaneal facet is obtained by measurement 1

The OP represented anatomical long axis of calcaneus.4 FG line was drawn along posterior facet of calcaneus at the level of subchondral bone. FG line intersected OP line at H. Then, OĤG angle was measured and recorded as an angle between anatomical long axis of calcaneus and articular surface of posterior calcaneal facet [Figure 3].4 Measurement 2 was designed by first and fifth authors. Posterior calcaneal facet was blinded and viewed only lateral malleolus. Apex curve of lateral cortex of lateral malleolus was identified and marked as I. IJ line was drawn perpendicularly to the apex curve of lateral cortex of the lateral malleolus at I [Figure 4]. IJ represented articular surface of posterior calcaneal facet [Figure 5]. XY line was anteriorly drawn parallel to longitudinal bone trabeculae of posterior calcaneal tubercle and intersected IJ line at K and ended above posterior calcaneal facet at Y. XY represented anatomical long axis of the calcaneus [Figure 5].3 An angle (XKJ) was measured and recorded and represented the angle between the anatomical long axis of calcaneus and articular surface of posterior calcaneal facet [Figure 5]. The measurements were performed by 2 orthopedic residents at 6 months interval. Different degrees of the angles of measurement 1 and 2, of not more than 3°, were accepted The data was analyzed for inter-intraobserver reliability of both measurements and CI consideration of more than 0.75 for strong reliability. The measurements 1 and 2 were analyzed for correlation by Pearson correlation coefficient and more than 0.7 was considered to have strong correlation.

Figure 4

Posterior calcaneal facet was blinded and viewed lateral malleolus. I was apex curve of lateral cortex of the lateral malleolus. IJ line was drawn

Figure 5

X-ray calcaneum axial view showing IJ representing articular surface of posterior calcaneal facet. XY line was drawn parallel along longitudinal bone trabeculae of posterior calcaneal tubercle and represented anatomical calcaneal long axis. XJ is obtained by measurement 2 and represented the angle between anatomical calcaneal long axis and articular surface of posterior calcaneal facet

RESULTS

52 normal feet from 26 volunteers (17 males, 9 females) were included in the study, with age ranging from 18 to 36 years. Average angle of the measurement 1 were 90.04° ± 4.00° (95% confidence interval [CI]: 88.92°, 91.15°) and of measurement 2 were 90.58° ± 3.78° (95% CI 89.53°, 91.63°). Mean different degrees of measurement 1 and 2 were 0.54° ± 2.31° with 95% CI ranged from 0.10° to 1.18° and accepted as not more than 3°. Inter and intraobserver of measurement 1 and 2 were more than 0.75 [Table 1]. Pearson correlation coefficient of measurement 1 and 2 was 0.826 [Figure 6].

Table 1

The angle between anatomical calcaneal long axis and articular surface of posterior calcaneal facet by techniques of measurement 1 and 2

Figure 6

Graph demonstrating correlation between measurement 1 and 2 with 0.826 of Pearson correlation coefficient

DISCUSSION

Varus deformity of the intraarticular calcaneal fracture is a result of medial displaced posterior tubercle of calcaneus.12 The deformity needs pre and postoperative evaluation and restoration in order to preserve ankle-hindfoot alignment and functions. There are several techniques to assess varus angle of calcaneus.67 The techniques are not routine work and some techniques are inappropriate to apply for calcaneal fracture especially the techniques employing standing position.7 Axial view radiograph of a calcaneal fracture is routinely taken. Tibial and fibular shaft cannot be viewed in this radiograph and there were intraarticular fracture of posterior calcaneal facet and calcaneal body Hence posterior calcaneal facet is frequently difficult to identify and anatomical long axis of the calcaneus cannot be obtained from calcaneal body. This resulted in difficulty in the measurement of the angle from axial view radiograph of calcaneus by using reference as tibial or fibular shaft, posterior facet of the calcaneus or even calcaneal body. So, the measurement 2 was developed for better measuring the angle that represented an angle between anatomical long axis of calcaneus and articular surface of posterior calcaneal facet from calcaneal axial view radiograph and benefits for calculating varus angle of the intraarticular calcaneal fracture. The technique of measurement 2 used a line perpendicular to apex curve of lateral cortex of the lateral malleolus and longitudinal bone trabeculae of posterior calcaneal tubercle as references for representation of articular surface of posterior calcaneal facet and anatomical long axis of calcaneus respectively. The reasons for using lateral malleolus as the reference are that the lateral malleolus can be viewed in axial view radiograph of calcaneus especially the lateral cortex. Lateral malleolus is the end of fibula and takes rotation around the same long axis of fibular shaft during flexion and extension of the ankle joint. So, the drawing line perpendicular to the apex curve of lateral cortex of lateral malleolus is also perpendicular to long axis of fibula and parallel to articular surfaces of talocrural and posterior subtalar joint at the level of posterior calcaneal facet.48 So, this drawing line can represent articular surface of posterior calcaneal facet in case of intraarticular calcaneal fracture. The reasons for using longitudinal bone trabeculae of posterior calcaneal tubercle as the reference is that the posterior calcaneal tubercle is the posterior end of the calcaneus and a ground contact area. The posterior calcaneal tubercle can be identified in case of intraarticular calcaneal fracture. Moreover, direction of longitudinal bone trabeculae from posterior calcaneal tubercle anteriorly projects to posterior calcaneal facet and parallel to anatomical long axis of the calcaneus.3 So that medial displacement of the posterior calcaneal tubercle simultaneously changes the direction of longitudinal bone trabeculae of the tubercle and creates unparalleled longitudinal bone trabeculae to long axis of the calcaneus.345 Therefore, identification of the longitudinal bone trabeculae of displaced posterior calcaneal tubercle will display the direction of anatomical calcaneal long axis of intraarticular calcaneal fracture. The study compared between measurement 2 and 1 for reliability. The angle of measurement 1 was obtained from the anatomical long axis of calcaneus and the articular surface of posterior calcaneal facet and used as a gold standard to assess reliability of the measurement 2.4 The study showed that the measurement 1 and 2 had strong reliability with more than 0.750 inter- and intraobserver reliability. Mean of different degrees of the angles between measurement 1 and 2 were 0.54° (2.31°) with 95% CI of the mean difference range from 0.10° to 1.18°. Accepted range of the difference was not more than 3°. The correlation of measurement 1 and 2 were statistically analyzed and showed strong correlation between both measurements with 0.826 of Pearson correlation coefficient. However, the accuracy of angle measurement also needs well controlling foot position and tilting of X-ray tube. Furthermore, the identification and drawing line perpendicular to the apex curve of the lateral cortex of lateral malleolus needs practicing to gain the measurement skill. In case of posterior subtalar facet of talus viewed in axial calcaneal radiograph, the measurement 2 can be simply modified by using posterior facet of the talus as a reference for articular surface of the posterior calcaneal facet instead of the line perpendicular to the apex curve of lateral cortex of lateral malleolus.

In conclusion, the study showed that the angle of measurement 2 has strong reliability for representing the angle between anatomical long axis of the calcaneus and articular surface of posterior calcaneal facet by using lateral malleolus and longitudinal bone trabeculae of posterior calcaneal tubercle as references. The measurement 2 benefits for assessing calcaneal varus or valgus angle of intraarticular calcaneal fracture. Degrees of the varus or valgus of the calcaneus fracture are obtained by 90° minus the angle of measurement 2.