Influence of lower limb rotation on hindfoot alignment using a conventional two-dimensional radiographic technique
Graphical abstract
The hindfoot alignment measure tends to zero when the foot rotates away from the inciding X-rays.
Introduction
Correct alignment of bones in the lower limb is a constant goal in orthopedic surgery. The accuracy of corrective osteotomies in foot and ankle surgery depends on precise preoperative planning based on clinical examination and radiography. Abnormal coronal alignment of the hindfoot has been proven to increase the risk of degenerative pathology or surgical failure in the foot and ankle [1]. Measuring this alignment is therefore fundamental for ultimate surgical outcome as much as preoperative planning. If the forces on the foot are not applied on a correct axis, deleterious effects on cartilage, tendons and ligaments may be seen in cases of osteotomy, arthroplasty or arthrodesis [2], [3], [4], [5], [6], [7], [8], [9], [10]. Recent advances in technology have improved this further in hip and knee surgery with 3D navigation and custom cutting guides [11]. Despite evidence that hip and knee alignment interact with foot alignment, studying the latter still depends on basic two-dimensional radiographic techniques [12]. A number of radiographic techniques have previously been described: the Long Axial View [13], the Hindfoot Alignment View (HAV) [14], [15], the Meary and the Djian View [16], [17]. The distal medial tibial angle [18] is used to determine correct alignment of the tibio-talar joint line. Whatever technique is used, and despite the use of guides or radiopaque marquers [14], [15] a number of factors may influence the measures, therefore surgical planning and eventually the results of surgery: reproducibility of the radiographic protocol, ankle stability, knee alignment, tibial torsion angle, calcaneus shape, thickness of the soft tissues [15], [19], [20], [21]. Only two, more recent radiographic techniques, the Ground Reaction Force Calcaneal Offset (GRFCO) [22] and Arunakul's Tripod Index [23], [24] are not dependent on lower limb rotation. In any case, there is intra and inter-observer variability, and variability related to the radiographic device itself [4]. Moreover, rotation of the lower limb has proven to influence HKA angles [25] and rotation of the foot or of the incident X-rays influence hindfoot alignment [14]. This phenomenon has not yet been mathematically analyzed.
The objective for this study was to analyze the influence of the angle between the lower limb and the incident X-rays in a standard 2D radiographic hindfoot alignment measurement technique. Our hypothesis was that it was possible to predict this using a simple 3D modelization of the foot. A cadaveric study was conducted to verify this hypothesis.
Section snippets
Subject
A single cadaveric lower limb was selected and harvested on a 90 year old female subject, disjointed at knee level. Exclusion criteria were gross deformity, surgical scars and clinical malalignment of the hindfoot.
Outcome criteria
Primary outcome was the correlation between the theoretical and the clinical measurements. Secondary outcome was the intra and inter-observer agreement.
Custom device
A device was designed and crafted to hold the leg in a vertical weight bearing situation (Fig. 1). A rectangular wooden board 90 × 70 × 2
Results
Thirty-six radiographs were shot between −90° and +90° rotation. The experimental Meary angle values were measured and plotted (Fig. 4). The distribution described a parabolic curve, where the maximum value (7.05°) corresponded to the AP view (aligned with the 2nd metatarsal bone). This value decreased with rotating the limb away from 0°, to reach 2.11° at −90° rotation and 1.72° at +90° rotation. The values obtained using mathematical modelisation (Fig. 5) with the triangular based pyramid
Discussion
The hypothesis for this study was confirmed: The value of the Meary angle was maximum when incident X-rays were aligned with the second metatarsal bone, then diminished following a parabolic curve when the lower limb rotated away (internally and externally) from this axis. Correlation with theory and modelisation was excellent. Intra and inter observer correlation was also excellent. This also implies that the static foot may be analyzed accurately using geometry in the Kapanji, triangular
Conclusion
This study shows that measuring the hindfoot alignment depends highly on foot rotation with regards to inciding X-rays. A standard 2D technique such as the Meary protocol is more valid when the rays are aligned with the 2nd metatarsal in our case, but could have varied should we have repeated the study with other subjects. It also shows that whatever the technique used, the triangular based pyramid accurately predicts measurement variations with 3D movements of the foot. In the future, 3D
Conflict of interest
The authors report no conflict of interest or relationships with other people or organizations that could inappropriately influence their work.
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