Hindfoot Nail Positioning: WBCT-Based Simulation Indicates Valgus Angulation of Straight Nails Is Necessary to Obtain Appropriate Hindfoot Alignment

Abstract

Background:

Hindfoot nail placement, whether for tibiotalocalcaneal (TTC) arthrodesis or as the index procedure for geriatric ankle fractures or comminuted pilon fractures, is challenging because of anatomical constraints. Although it may be assumed that a straight hindfoot nail should align parallel with the tibial axis, this can lead to either medial calcaneal cortical perforation, varus hindfoot malalignment, or iatrogenic medialization of the foot. This simulation study aims to quantify the angulation required for a straight hindfoot nail to achieve both intraosseous calcaneal placement and preserve native hindfoot alignment in patients without significant coronal plane malalignment.

Methods:

We retrospectively analyzed 61 weightbearing computed tomography (WBCT) scans from patients (mean age 43.6 years) with radiographically physiologic hindfoot alignment (mean hindfoot alignment angle: 1.3 degrees, Meary angle: 4.9 degrees). Using multiplanar reconstruction of CT images, 10-mm and 12-mm virtual hindfoot nails were superimposed on each coronal scan. First, the virtual nail was positioned to ensure calcaneal intraosseous placement, defined as ≥2 mm of bone between the nail and the medial calcaneal cortex. The angle between the virtual nail and the tibial anatomic axis was recorded. Second, the virtual nail was repositioned to be parallel to the tibial axis and centered within the tibial canal; medial cortical breach and distance to the medial cortex of the calcaneus were then assessed.

Results:

To maintain proper intraosseous calcaneal placement and alignment, a mean valgus angulation of 4.0 degrees (95% CI, 3.5-4.4) for 10-mm nails and 4.9 degrees (95% CI, 4.4-5.4) for 12-mm nails relative to the tibial axis was required. When the nail was aligned strictly parallel to the tibial axis, 60 of 61 scans demonstrated medial calcaneal breach. To avoid this breach, an average medial foot translation of 9.2 mm or iatrogenic hindfoot varus ≥5.8 degrees would be necessary.

Conclusion:

This WBCT-based simulation suggests that a valgus orientation of approximately 4 degrees is needed for a straight hindfoot nail to (1) maintain proper calcaneal intraosseous placement and (2) preserve physiologic hindfoot alignment.

Level of Evidence:

Level III, retrospective cohort study.

Introduction

Tibiotalocalcaneal (TTC) arthrodesis with a retrograde intramedullary nail is a reliable salvage procedure for combined ankle and subtalar pathology.^{9,12,15,19,23} Although initially used for hindfoot arthrodesis, indications are rapidly expanding, with increasing literature demonstrating efficacy as index treatment for geriatric ankle fractures and comminuted pilon injuries.^6,7,13,21,24 Similar to the placement of any intramedullary device, hindfoot nailing has specific technical considerations and achieving a plantigrade, valgus-neutral hindfoot remains technically challenging. The calcaneal body lies lateral to the axis of the tibial canal, and advancing a straight nail strictly along the tibial anatomic axis results in a coronal plane mismatch: whereas the calcaneal body lies lateral, the sustentaculum tali and the adjacent tibial neurovascular bundle lie medial to this axis.^3,14,17

Cadaveric and imaging studies have shown that guidewires aligned with the tibial axis often breach the medial calcaneal cortex, compromising bone purchase and potentially endangering neurovascular structures.^3,14,17 In theory, inserting a straight nail along this axis would require either varus angulation of the hindfoot or medial translation of the foot to maintain intraosseous positioning within the calcaneus. ¹⁷ Although curved nails with built-in distal valgus exist, most commercially available implants are straight.

Despite this, there is no clear consensus on the ideal radiographic trajectory for straight nail insertion. Although some surgeons advocate for achieving alignment by resecting the medial malleolus to translate the foot, this may be suboptimal given the alteration in foot position relative to the normal mechanical axis of the lower limb. ¹⁷ Meanwhile, others advocate for a deliberate valgus trajectory.^3,15 However, the precise valgus angle required to achieve full intraosseous engagement while preserving native hindfoot alignment has not been quantified. Furthermore, no study has evaluated the clinical consequences of inserting a straight nail precisely along the tibial axis, including the degree of medial translation or hindfoot varus required to maintain calcaneal containment.

Given these considerations and in the setting of increased use of hindfoot nails, this study has 2 aims. The primary aim is to determine the valgus angulation necessary for a straight nail to remain entirely within the calcaneus without altering native hindfoot alignment, using weightbearing computed tomography (WBCT) in individuals with normal alignment. The secondary aim is to estimate the degree of hindfoot varus or medial translation that would result from inserting the nail directly along the tibial axis. We hypothesize that a specific degree of valgus angulation is required to achieve both neutral hindfoot alignment and complete intraosseous nail placement when using straight nails, and that insertion strictly along the tibial axis would necessitate either hindfoot varus or medial translation.

Methods

Study Cohort

This retrospective radiographic study reviewed patients who underwent WBCT from our institution’s radiology registry. The study protocol was reviewed and approved by the institutional review board. An initial query identified patients who received bilateral WBCT between 2022 and 2023. From this list, we sought to identify patients who met the following inclusion criteria: (1) asymptomatic status, (2) no radiographic evidence of foot or ankle pathology, and (3) hindfoot alignment within the normal reported range (Figure 1). Specifically, we included patients who underwent bilateral WBCT for evaluation of a unilateral issue but had an asymptomatic contralateral side used for comparison. Patients were excluded if they had bilateral WBCT for flatfoot deformity, cavovarus deformity, or ankle arthritis. Additionally, patients with any pathology affecting the study side, such as osteochondral lesions of the talus, were excluded. After applying these criteria, only patients with a WBCT hindfoot alignment angle between −5 degrees and +5 degrees were included in the final cohort. This range of physiologic alignment encompasses the central 1.5 SDs of normal alignment on WBCT (mean 0.8 degrees valgus ± 3.2 degrees), as reported by Burssens et al, ⁴ and mirrors the traditional radiographic definition of a normal hindfoot axis (0-5 degrees valgus), ¹ while also allowing equal tolerance for mild varus observed in healthy populations.

Figure 1.

Flow chart illustrating the inclusion process for the study cohort. F&A, foot and ankle; HAA, hindfoot alignment angle; OCD, osteochondral defect; WBCT, weightbearing computed tomography.

After applying the inclusion and exclusion criteria, a total of 61 WBCTs in 61 patients (40 females and 21 males) remained in the final cohort. The mean age and body mass index of the cohort were 43.6 years (SD 17) and 27.7 (SD 4.8), respectively. Indications for obtaining WBCT on the contralateral side included ankle fractures (n = 35), ankle pain with negative study (n = 13), syndesmotic injury (n = 5), hallux valgus or rigidus (n = 4), osteochondral lesion of talus (n = 3), and os trigonum (n = 1). The mean hindfoot alignment angle (HAA) was 1.3 degrees (95% CI, 0.9-2.4), and the mean Meary angle was 4.9 degrees (95% CI, 3.2-6.5).

Hindfoot Nail Simulation

All virtual simulations and measurements were performed by 2 independent observers (both board-certified orthopaedic surgeons) not involved in the treatment of any patients included in this study. Simulations were conducted using a dedicated PACS system equipped with a multiplanar reconstruction function.

First, to define the tibial axis, axial, coronal, and sagittal reconstructions were cross-referenced. Thick multiplanar reconstruction slices were used to visualize the entire tibia in both the coronal and sagittal planes, allowing for accurate axis determination (Figure 2).

• Aim 1: Next, the virtual 10-mm and 12-mm nails were adjusted to achieve complete intraosseous placement within the calcaneus, defined as maintaining at least 2 mm of bone between the medial border of the nail and the medial calcaneal cortex. Then, (1) the angle between the tibial anatomic axis and the simulated nail trajectory and (2) simulated nail length (the distance from the calcaneal entry point to the point where the proximal aspect of the nail intersected the tibial anatomic axis) were recorded.

• Aim 2: Subsequently, the nail was repositioned to align centrally within the tibial intramedullary canal, parallel to the tibial axis. At this position, the frequency of medial calcaneal cortex breach and the distance between the medial calcaneal cortex and the center of the simulated nail were measured.

Figure 2.

Using multiplanar reconstruction (MPR) in weightbearing computed tomography (WBCT), coronal images were aligned with the tibial anatomic axis. For aim 1, nail placement (10-mm and 12-mm) was simulated to ensure at least 2 mm of intraosseous bone between the nail and the medial calcaneal cortex. The angle between the tibial anatomic axis and the simulated nail trajectory and the distance from the calcaneal entry point to the point where the nail’s centerline intersected the tibial anatomic axis were measured. For aim 2, the nail was repositioned centrally within the tibial intramedullary canal, parallel to the tibial axis. Medial calcaneal cortex breaches were identified, and their frequency recorded. Additionally, the distance from the medial cortex to the simulated tibial nail was measured.

Statistical Analysis

Descriptive statistics were reported as mean and SD or 95% CI for continuous variables and as count and percentage for categorical variables. Intraclass correlation coefficients (ICCs) were calculated to assess both intra- and interobserver reliability for all measurements. Interobserver reliability for the dichotomous outcome (calcaneal breach) was assessed with Cohen kappa coefficient.

Results

For the quantitative measurements, the interobserver reliability ranged from 0.778 to 0.828, and the intraobserver reliability ranged from 0.852 to 0.895 (Table 1). Interobserver agreement for the presence of calcaneal breach was complete (κ = 1.00).

Table 1.

Interobserver and Intraobserver Reliability of the WBCT Measurements.

	Reliability (95% CI)
Interobserver	Intraobserver	Measurement
Aim 1
Nail–tibial axis angle	0.828 (0.593-0.927)	0.895 (0.784-0.953)
Simulated nail length	0.755 (0.460-0.890)	0.851 (0.684-0.946)
Aim 2
Nail-calcaneus distance	0.778 (0.571-0.922)	0.852 (0.751-0.944)

Abbreviation: WBCT, weightbearing computed tomography.

Aim 1

The 10-mm simulated hindfoot nail required an average valgus of 4.0 degrees (95% CI, 3.5-4.4) and the 12-mm simulated nail required an average valgus of 4.9 degrees (95% CI, 4.4-5.3) relative to the tibial anatomic axis to (1) ensure acceptable intraosseous positioning within the calcaneal body and (2) to maintain the patient’s native normal hindfoot alignment. The simulated nail length (the distance from the calcaneal entry point to the point where the proximal aspect of the nail intersected the tibial anatomic axis) was 150.1 mm (95% CI, 148.5-151.7) for the 10-mm nail and 150 mm (95% CI, 148.3-151.6) for the 12-mm nail.

Aim 2

When the simulated nail was realigned parallel to the tibial axis and centered in the tibial canal, all but 1 case (60/61, 98.4%) demonstrated medial calcaneal cortical breach. The mean distance from the nail center to the medial calcaneal cortex was 2.2 mm (95% CI, 1.2-3.1), suggesting that 9.2 mm (for a 10-mm nail) or 10.2 mm (for a 12-mm nail) of iatrogenic medial foot translation would be necessary to preserve at least 2 mm of calcaneal osseous margin. Finally, ≥5.8 degrees (mean + 1SD) of iatrogenic varus hindfoot malalignment for a 10-mm nail, or ≥6.7 degrees for a 12-mm nail, would be needed to ensure full intraosseous calcaneal placement without medial cortical breach.

Discussion

The most important finding of this WBCT-based simulation study is that a valgus orientation of approximately 4 degrees is necessary for a straight hindfoot nail to (1) maintain proper intraosseous placement within the calcaneus and (2) preserve physiologic hindfoot alignment. Strict alignment of the nail with the tibial anatomic axis risks medial calcaneal wall perforation and may result in ≥5 degrees of iatrogenic varus hindfoot malalignment or necessitate about a centimeter of medial foot translation. Accordingly, incorporating a valgus trajectory of the straight hindfoot nail relative to the tibial axis appears essential to achieve safe intraosseous positioning while preserving native hindfoot alignment. Notably, these results are based on virtual modeling and should not be interpreted as definitive clinical guidance.

Hindfoot nailing is increasing in utilization as indications expand beyond hindfoot arthrodesis. Although TTC arthrodesis has been predominantly performed by orthopaedic surgeons with either subspecialty training and/or particular expertise in foot and ankle surgery, expanding indications result in increasing usage beyond this traditional surgeon demographic. In particular, there is growing enthusiasm for hindfoot nailing (with or without formal joint preparation) as the index procedure for geriatric ankle fractures and comminuted pilon fractures.^6,7,13,21,24 Although there is no current report in the literature specifically documenting this trend, increasing publications/citations is a proxy for increasing clinical activity.^5,22 Prior to 2015, there were a total of 3 of indexed, peer-reviewed publications regarding hindfoot nailing for ankle/pilon fractures. Since 2015 (and to the time of manuscript preparation), there have been 32 subsequent reports.

Cadaveric and radiographic studies have consistently demonstrated that the tibial anatomic axis is not colinear with the hindfoot corridor. In a cadaveric study, McGarvey et al ¹⁷ advanced nails antegrade from the tibial isthmus across the ankle and hindfoot and observed that the nails commonly exited through the sustentaculum tali or the medial calcaneal wall, resulting in poor calcaneal bone purchase. Notably, 6 of 8 specimens demonstrated direct injury to the plantar neurovascular bundle and flexor tendon. Hyer and Cheney ¹⁴ similarly found that wires inserted along the tibial canal exited medial to the calcaneal center in more than 80% of specimens, confirming that a straight tibial axis trajectory diverges from the hindfoot’s natural corridor. Our in vivo WBCT analysis supports these findings that trajectories aligned with the tibial axis often breached the medial calcaneal cortex in patients with physiologic hindfoot alignment, potentially placing neurovascular structures at risk.

To mitigate this risk, several techniques have been proposed. In McGarvey et al’s ¹⁷ study, resecting the medial malleolus and medially translating the talocalcaneal complex by an average of 9.3 mm (range, 7-11 mm) allowed for safer nail placement and increased neurovascular clearance to a mean of 18.4 mm (range, 14-32 mm). Our findings similarly suggest that a medial translation of 9.2 mm would be necessary to preserve at least a 2-mm medial calcaneal bone margin when inserting the nail colinear with the tibial axis. However, this medial shift may compromise the deltoid ligament blood supply and remove the medial buttress, ¹² predisposing to varus drift of the hindfoot and alteration of the mechanical axis. ¹¹ In addition, a study reviewing implant failure and nonunion following hindfoot nailing in Charcot patients found that preservation of the medial malleolus was important, as its resection was associated with increased risk of nonunion and metalwork failure, ²⁰ suggesting that maintaining the medial malleolus contributes to the stability of the fusion and overall construct.

A more contemporary solution, proposed by Burgesson et al, ³ recommends a lateral plantar entry point and intentional valgus angulation to avoid medial cortical breach and protect neurovascular structures. A retrospective clinical series by Lucas y Hernandez et al ¹⁵ suggested a similar approach that straight hindfoot nails be inserted just below the tibial isthmus to allow for valgus alignment without the need for medial translation. Our study adds quantitative support for this practice for the first time in the literature, demonstrating that approximately 4 to 5 degrees of valgus angulation is not only safe but reflects the normal coronal alignment between the tibia and hindfoot (Figure 3).

Figure 3.

(A) Weightbearing anteroposterior ankle radiograph demonstrates a valgus trajectory of the straight hindfoot nail with achieved tibio-talo-calcaneal union. (B) Clinical photograph shows maintained physiologic hindfoot alignment.

From an intraoperative perspective, surgeons should expect the guidewire or nail to appear slightly valgus relative to the tibial shaft on anteroposterior fluoroscopy once physiologic alignment is restored (Figure 4). More importantly, our findings implicate that a construct that appears perfectly parallel should raise concern for medial cortical breach or iatrogenic varus malalignment, which can negatively affect patient gait and shoe wear tolerance.

Figure 4.

A 32-year-old man with a comminuted pilon fracture underwent staged primary arthrodesis using a hindfoot nail following initial open reduction and internal fixation. (A) Nonweightbearing anteroposterior ankle radiograph (top) and computed tomography (CT, bottom) images demonstrate a comminuted and displaced pilon fracture. (B) Intraoperative fluoroscopic images demonstrate the trajectory of the guide pin and hindfoot nail. After anatomic reduction and fixation of fracture fragments to restore the distal tibia while maintaining physiologic hindfoot alignment, the nail was inserted with a valgus trajectory to ensure intraosseous placement within the calcaneus. (C) Postoperative coronal CT image demonstrates intraosseous placement of the hindfoot nail close to the medial cortex (asterisk), highlighting that this slight valgus angulation is essential to avoid medial cortical breach and potential violation of medial soft tissues when inserting the nail parallel to the tibial shaft.

One potential concern with inserting the nail in valgus is the theoretical reduction in compression across the tibiotalar arthrodesis interface, as the trajectory is not perpendicular. However, in our clinical experience, this modest valgus orientation yields union rates and fusion times comparable to nails placed perpendicular to the joint (Figure 3). Supporting this, clinical series have reported similar outcomes using straight and curved nails in terms of union rates and time to fusion.^{2,8,10,12,18,19} Given this evidence, we see no compelling benefit in forcing the hindfoot into varus or accepting the morbidity of medial malleolar resection solely to achieve a perpendicular nail trajectory, although further biomechanical study is necessary to confirm these clinical observations.

Another theoretical risk associated with valgus-directed straight nails is cortical stress concentration, particularly near the proximal locking screws, which may predispose to hypertrophy or stress fractures. ¹⁶ In a study comparing 3 different types of nail by Marley et al, ¹⁶ these complications have been observed in short straight nails that terminate within the metaphyseal funnel of the distal tibia. Therefore, achieving sufficient nail length may be an important intraoperative consideration to mitigate such complications, although the optimal proximal endpoint of valgus-directed nails requires further investigation.

An additional finding in our WBCT simulation study was that the angulation point (between the tibial anatomic axis and the proximal extent of the simulated nail) or simply where the proximal aspect of the nail would contact the tibia was on average 150 mm from the calcaneal plantar entry point. This has implications regarding nail length. Although multiple variables influence the choice of nail length (ie, type of hindfoot pathology, potential proximal stress risers from removed hardware or external fixation, tibial non/malunion, etc), understanding this intersection point has clinical applications and utility. For example, as described above, a long nail extending beyond this intersection point may result in cortical stress concentration or even iatrogenic fracture. Another more subtle complication is resultant varus malalignment (in an otherwise properly aligned hindfoot before nail insertion), given inflexibility of the nail to bending. Although this depends on multiple factors, including nail flexibility, differences in implant/bone modulus as well as distal osseous opposition, long nails placed past this intersection can theoretically change initial hindfoot positioning upon insertion.

There are several limitations to our study. First, it is a retrospective review of WBCT scans drawn from a radiology registry, which carries inherent selection bias. We restricted the cohort to subjects whose hindfoot alignment angle lay within ±1.5 SD of a published “normal” population, yet this criterion may not encompass the entire spectrum of physiologic variation, including many of the patients who have concomitant flatfoot or cavus, which was associated with the development of ankle arthritis. Nevertheless, we believe this cohort reflects the alignment that most surgeons aim to replicate intraoperatively. Exclusion of patients with abnormal alignment limits the generalizability of our findings, as TTC nail candidates often present with deformity. In addition, our data set did not analyze measurements based on age or sex, which could have affected canal dimensions and nail trajectory. Second, nail placement was modeled virtually on static images. The simulation cannot reproduce intraoperative factors such as reamer eccentricity, soft tissue constraints, or deformity-correction maneuvers, all of which influence the achievable trajectory intraoperatively. Moreover, a direct comparison with a valgus-curved nail was not possible, as no validated method currently exists to simulate specific nail geometry or curvature using WBCT-based modeling techniques. In addition, the 2-mm medial cortical margin used to define safe intraosseous placement was an arbitrary threshold chosen to provide a conservative buffer against cortical violation. However, it has not been biomechanically validated and may not fully represent the true safety margin under physiologic loading. Third, our simulation tested only 10-mm and 12-mm nails, which reflects the most commonly used diameter in clinical practice. As such, the observed valgus angles should be viewed as lower bound estimates; larger-diameter implants may necessitate even greater valgus angulation relative to the tibial axis to ensure proper intraosseous placement.

In conclusion, this WBCT-based simulation indicates that a valgus orientation of approximately 4 degrees is essential to ensure safe intraosseous placement of the hindfoot nail in patients with relatively normal coronal plane alignment. In contrast, in these patients, we found that a straight, parallel alignment between the nail and the tibial shaft on anteroposterior fluoroscopy may indicate nonphysiologic positioning, raising concerns for possible medial cortical breach or iatrogenic varus malalignment.

Supplemental Material

sj-pdf-1-fao-10.1177_24730114251412347 – Supplemental material for Hindfoot Nail Positioning: WBCT-Based Simulation Indicates Valgus Angulation of Straight Nails Is Necessary to Obtain Appropriate Hindfoot Alignment

Supplemental material, sj-pdf-1-fao-10.1177_24730114251412347 for Hindfoot Nail Positioning: WBCT-Based Simulation Indicates Valgus Angulation of Straight Nails Is Necessary to Obtain Appropriate Hindfoot Alignment by Jaeyoung Kim, Colin O’Neill, Amal Chidda and John Y. Kwon in Foot & Ankle Orthopaedics