Restoration of calcaneal anatomy and physiologic function after fracture: the importance of tuber length

Abstract

Objectives:

To evaluate calcaneal tuber length (TL) and its influence on calcaneal height (CH), talar declination angle (TDA), and function during plantarflexion after operative fixation of calcaneus fractures.

Design:

Retrospective case–control study.

Setting:

Large, urban, level 1 trauma center.

Patients/Participants:

A total of 39 patients from 2012 to 2022 with isolated intra-articular calcaneus fractures with weight-bearing lateral postoperative and contralateral x-rays (to use as internal control) with at least 1 year follow up.

Intervention:

Patients underwent fixation of calcaneus fractures and were separated into “short tuber” (ST) and “restored tuber” (RT) groups. Postoperative radiographs were compared with contralateral radiographs.

Main Outcome Measurements:

Calcaneal Bohler angle (BA), TL, CH, TDA, and relative work increase using the equation Work = Force × Distance. Fixation methods related to TL were also investigated.

Results:

Overall, 37 of 39 patients (95%) had postoperative BA within 20–40°. 8 of 39 patients were in the ST group, and 31 of 39 were in the RT group. The average TL difference was 0.73 cm in ST versus 0.13 cm in RT (P < 0.01). The average CH difference was 0.46 cm in ST versus 0.28 cm in RT (P = 0.04). The average TDA difference was 9.9° in ST (flatter) versus 2.6° in RT (P = 0.024). The average work increase to plantarflex the ankle was 12.1% in ST versus 2.2% in RT (P < 0.01). Open fixation resulted in shorter TLs compared with the contralateral side (P = 0.02), but percutaneous fixation did not (P = 0.17).

Conclusions:

Despite restoring BA, presence of a shortened tuber can still result in decreased CH and a flat talus compared with the contralateral limb. It also required increased work to plantarflex the ankle, further highlighting the importance of restoring tuber length.

Level of Evidence:

IV.

1. Introduction

Calcaneus fractures account for 1%–2% of all fractures and 60% of tarsal fractures, with 60%–75% of calcaneus fractures being intra-articular.^1–3 Owing to the high rate of articular involvement and strong deforming forces on the calcaneus by the gastrosoleus complex (GSC), surgical treatment is often necessary to avoid long-term pain, disability, and morbidity.^4,5 Even with surgery, chronic pain, stiffness, deformity, and post-traumatic arthritis often occur.^6,7 Operative techniques for fixation include closed reduction with percutaneous fixation (CRPF) and open reduction internal fixation (ORIF), and either has been demonstrated to be acceptable and is often selected based on the surgeon preference and setting of injury.^4,8

Bohler angle (BA) is defined as the intersection between 1 line from the highest point on the posterior articular surface to the most superior point of the calcaneal tuberosity and a second line from the highest point on the posterior articular surface to the anterior process of the calcaneus. It is measured on a lateral ankle radiograph and has traditionally been used to assess calcaneal height (CH) after fracture and the quality of reduction postoperatively.^9,10 A restored BA is usually achieved between 20 and 40°; however, this range can vary in patients.¹¹ Restoration of CH after fracture is important because a flattened calcaneal articular surface (and a smaller BA) translates to a decreased talar declination angle (TDA) and a horizontal talus as the subtalar joint levels out (Fig. 1). A horizontal talus can be a source of anterior ankle impingement as the dorsal talar neck comes into contact with the anterior tibia, resulting in decreased range of motion and pain with ankle dorsiflexion.¹²

Figure 1.

Flat talus in the foot with an unrestored Bohler angle. Postoperative and contralateral weight-bearing lateral ankle radiographs of a patient with a Bohler angle that is not restored (BA 12° vs. 31°), which has subsequently led to a flattened talus compared with the contralateral limb (TDA 15° vs. 31°). Bonesetter App measurements listed: Tuber length (TL-red) with a best-fit circle around the talar dome. Two cricles were used if the radiograph was not a perfect lateral and the average center was used for the TL distance. Bohler angle (BA-blue), calcaneal height (CH-green), and talar declination angle (TDA-yellow) are also shown.

Despite the abundance of literature describing BA and how it relates to calcaneal height, very little attention has been given to calcaneal tuber length and how it contributes to CH and TDA. If tuber length (TL) is shortened after fracture and not restored, the ankle must dorsiflex further for the foot to achieve a plantigrade stance, as the tuber must travel a longer distance to reach the ground in its shortened state. In this hyper-dorsiflexed position, the talus must be in a more horizontal position during stance because the talar dome is the ankle's center of rotation (COR) (Fig. 2). Thus, despite achieving an acceptable calcaneal reduction according to BA, a shortened calcaneal tuber can still result in a horizontal talus (and all its sequelae), underscoring the importance of TL as it relates to restoring calcaneal height.

Figure 2.

Flat talus in the foot with a short calcaneal tuber, despite a restored Bohler angle. Postoperative and contralateral weight-bearing lateral ankle radiographs of a patient with a restored Bohler angle (BA 39° vs. 40°), but with a short tuber (TL difference >4.0 mm). This has caused the ankle to hyper-dorsiflex for the foot to achieve a plantigrade stance, flattening the talus compared with the contralateral limb (TDA 15° vs. 27°). Bonesetter App measurements listed: Tuber length (TL-red) with a best-fit circle around the talar dome. Bohler angle (BA-blue), calcaneal height (CH-green), and talar declination angle (TDA-yellow) are also shown.

In addition, the calcaneal tuber provides an important physiologic function, acting as the moment arm as the Achilles tendon plantarflexes the ankle around its COR (Fig. 3). By acting as the moment arm, TL, therefore, directly affects the work required by the GSC to plantarflex the ankle. Basic physics provides the formula to define work—“Work = Force × Distance.” Our measurement for distance in this equation is the TL. Assuming that the same force is needed by the GSC to plantarflex the ankle both before and after injury, we can see that by decreasing distance (TL), you must increase work. The Achilles tendon also crosses the subtalar joint; however, given the minimal amount of plantarflexion occurring at this joint, for the sake of simplicity, we did not include this joint in our calculations for work.

Figure 3.

Demonstration of increased work in a patient with a shortened calcaneal tuber. Using the simple physics equation, Work = Force × Distance, this figure demonstrates that given a constant force needed to plantarflex the ankle both preoperatively and postoperatively (red and blue arrows, respectively), how a shortened tuber (green double-sided arrow) results in increased work for the gastrosoleus complex as it plantarflexes the ankle around its center of rotation at the talar dome (purple curved arrow).

The purpose of this study was twofold: (i) to define and measure calcaneal tuber length and describe the effect that a shortened TL has on calcaneal height and talar position; (ii) to evaluate TL as it relates to overall physiologic function of the hindfoot and ankle, namely how a shortened calcaneal tuber requires more work by the GSC to plantarflex the ankle. We hypothesized that patients with a shortened TL (despite having a restored BA) would have evidence of a horizontal talus as measured by the talar declination angle when compared radiographically with their own contralateral foot. A secondary hypothesis was that patients with a shortened TL would mathematically show a relative increased amount of work required by the GSC to plantarflex the ankle, again underscoring the importance of recreating the TL in these challenging injuries.

2. Materials and Methods

This is an IRB-approved retrospective study that included 345 cases that were operated on for calcaneus fractures at our Level 1 US institution from 2012 to 2022. Of them, we found 39 cases that were isolated intra-articular calcaneus fractures, which also had postoperative lateral weight-bearing radiographs of both the operative and contralateral foot. Reasons for having a contralateral weight-bearing x-ray were varied and often serendipitous, which is why there were only 39 patients in the study. In addition, patients had to have postoperative axial hindfoot views showing no residual hindfoot varus or they were excluded from the study, further reducing the number of patients included. Radiographs had to be taken at least 1 year after surgical intervention, and without the patient undergoing any removal or modification of instrumentation during the first postoperative year. This timeframe was used because we felt that this gave us the largest patient cohort with radiographs a reasonable amount of time postoperative, without losing patients to follow-up. No further follow-up was needed because this was a purely radiographic study without clinical patient outcome data. We did not exclude patients based on open or closed fractures or by Sander classification.

BA, TL, CH, and TDA were measured on both the operative and contralateral foot in all patients using BoneSetter App (Detroit, MI). All ankles were sized to each other using a BoneSetter App template based on the anterior-posterior distance of the distal tibia on the radiographs using techniques described by Vaidya et al.^13–15 This allowed for scaled comparison of distance measurements when comparing patient cohorts. BA was measured as stated above. Patients had a “good reduction” of the calcaneus if their postoperative BA differed by ≤5° from the contralateral BA, which has been shown to have improved functional scores when compared with reductions with greater differences in BA. Nosewicz et al¹⁶ reported this 5° threshold as a cutoff where differences in functional outcomes were noticed. We used this same threshold in our study. It does not relate to the articular congruency.

TL was measured as the distance from the ankle's COR (a perfect-fit circle around the talar dome) to a point on the most posterior aspect of the calcaneus at the site of Achilles tendon attachment. Using the COR at the talus as the basis of our measurement allowed us to consistently reproduce TL on the operative and contralateral radiographs because this was a fixed point unaltered by the injury. The patients were then separated into a “short tuber” and a “restored tuber” group based on a TL difference threshold of 4 mm compared with the contralateral side. This threshold was used because it captured the bottom 20% of our cohort, giving us a reasonable size for our “short tuber” group.

CH was measured as described by Sanders et al⁵ from the superior-most aspect of the posterior facet to the inferior-most aspect of the weight-bearing surface of the calcaneus. CH measurements were taken to see whether a restored BA was predictive of restoring CH, or whether TL contributed to this as well. The same cutoff of 4 mm that was used for TL was also used for CH measurements for consistency in the study.

TDA is the angle formed between a line through the longitudinal axis of the talus on a lateral x-ray and a line parallel to the horizontal weight-bearing surface. The TDA was used to show that, even in patients with a restored BA, the TDA would still be affected in patients with short TLs postoperatively, highlighting the importance of tuber length in addition to BA in restoring CH. The patients were also separated by method of fixation (ORIF vs. CRPF) and compared to see whether 1 fixation method resulted in higher rates of unacceptable tuber length. All Bonesetter App measurements are illustrated in Figures 1 and 2.

To calculate the relative increase or decrease in work needed by the GSC to plantarflex the ankle around its COR at the talar dome, we used the simple physics equation: Work = Force × Distance. Using TL as our distance (d) and assuming a constant force (F) required by the Achilles tendon acting on the calcaneus, we calculated the relative increase or decrease in work required by the GSC to plantarflex the ankle. TL served as a proper distance measurement because it went from the ankle's COR to Achilles tendon insertion, traversing the entirety of the moment arm of the GSC. A diagram representing this concept is presented in Figure 3.

Statistical analysis was performed using Statistical Package for the Social Sciences (SPSS Statistics v29; IBMCorp, Armonk, NY), set at a significance level of 0.05. All measurements were performed using the BoneSetter App (Detroit, MI). All authors have read and complied with ethical approval/informed consent requirements.

3. Results

A total of 39 patients were included in this study, with 8 (21%) in the short tuber group and 31 (79%) in the restored tuber group. All measurement differences between the postoperative limb and the contralateral limb, separated by tuber length groups, are provided in Table 1.

Table 1.

Calcaneal measurements of short tuber versus restored tuber groups.

Difference between PO-CL limbs	Short tuber (n = 8)	Restored tuber (n = 31)	P
Bohler angle	3.82 ± 9.24°	3.26 ± 4.90°	0.41
Tuber length (cm)	0.73 ± 0.27	0.13 ± 0.15	<0.001
Calcaneal height (cm)	0.46 ± 0.30	0.28 ± 0.26	0.04
Talar declination angle	9.9 ± 4.4°	2.59 ± 2.30°	0.024
Work increase	12.1 ± 4.6%	2.20 ± 2.68%	<0.001

Differences in Bonesetter App measurements between the postoperative and contralateral limbs in the short tuber versus restored tuber groups.

BA, Bohler angle; CH, calcaneal height; TDA, talar declination angle; TL, tuber length.

3.1. Bohler Angle

The average postoperative BA was 31 ± 8° while the average contralateral “nonoperative” BA was 35 ± 5° (P < 0.05), indicating a significant difference between groups; however, BA was able to be restored within 20–40° postoperatively in 37 of 39 patients (95%). A “good” reduction of the calcaneus was achieved in 26 of 39 patients (67%). The BA difference between the postoperative and contralateral limbs was 4 ± 9° in the short tuber group, compared with 3 ± 5° in the restored tuber group (P = 0.41).

3.2. Tuber Length

For the group as a whole, the postoperative tuber length was 57 ± 5 mm and the contralateral TL was 60 ± 4 mm (P < 0.05). The average postoperative TL in the short tuber group was 54 ± 4 mm versus 61 ± 4 mm on the contralateral side (P < 0.05). The average postoperative TL in the restored tuber group was 58 ± 4 mm versus 60 ± 4 mm on the contralateral side (P = 0.26). The TL difference between the postoperative and contralateral limbs was 7 ± 3 m in the short tuber group versus 1 ± 2 mm in the restored tuber group (P < 0.05).

3.3. Calcaneal Height

Calcaneal height differences between the operative and contralateral limbs were also compared and found to be 5 ± 3 mm shorter in the short tuber group and 3 ± 3 mm shorter in the restored tuber groups (P < 0.05). Fifteen patients in the entire cohort (38%) did not meet the 4-mm threshold for recreation of CH, again, despite 95% of the cohort having a restored BA between 20 and 40°.

3.4. Talar Declination Angle

The TDA was of particular interest in patients who had a restored BA and had a short tuber because this would reveal whether short tubers contributed to CH and, therefore, would result in a horizontal talus, despite a restored BA. In patients with a “good” reduction (BA within 5° of the contralateral side) in the short tuber group, the TDA was 10° ± 4° more horizontal compared with the contralateral side. In patients with a “good” reduction in the restored tuber group, the TDA was 3° ± 2° more inclined compared with the contralateral side (P < 0.05).

3.5. Fixation Method

As is common in our practice, we used closed reduction with percutaneous fixation (CRPF) and ORIF for the fixation of calcaneus fractures. CRPF was often used for most acute injuries, whereas ORIF was often reserved for patients with a chronic or untreated injury when percutaneous methods were insufficient. At our urban trauma center, we often had patients lost to follow-up for extended periods, necessitating the use of ORIF when their fracture was unable to be fixed right away. We had 21 patients who underwent ORIF (53%) and 19 patients who underwent CRPF (47%). When separating the patients by fixation method, the postoperative TL was 57 ± 5 mm and the contralateral TL was 59 ± 2 mm in the CRPF group (P = 0.17). In the ORIF group, the postoperative TL was 57 ± 5 mm and the contralateral TL was 61 ± 3 mm (P < 0.05).

3.6. Calculating Work

Finally, the relative increase in work by the GSC on the ankle was calculated using TL as distance in the Work = Force × Distance equation. There was a 4.5% ± 5.3% increase in GSC work in all patients compared with their own contralateral limb. If the groups were separated by tuber length, there was a 12.1 ± 4.7% increase in work in the short tuber group and a 2.2 ± 2.7% increase in the restored tuber group (P < 0.01).

4. Discussion

Little to no data have been published on the importance of recreating calcaneal tuber length after fracture. Many studies have used calcaneal length as an outcome,^4,6,17–19 but to the knowledge of the authors, this is the first study that demonstrates how calcaneal tuber length after fracture significantly contributes to calcaneal height, talar position, and overall function of the ankle. We have shown that after calcaneus fracture, despite “good” reduction and recreation of BA, you can still develop a horizontal talus if you do not adequately address tuber length.

In this study, 37 of 39 patients (95%) obtained a postoperative BA between 20 and 40°. Furthermore, when considering reduction quality, 26 of 39 patients (67%) had a “good” reduction of the posterior facet, with <5° difference between the postoperative and contralateral limb. In line with our study, Nosewicz et al¹⁶ in their series achieved a “good” reduction in 14 of 22 patients (67%). Van Hoeve et al²⁰ also showed that the quality of posterior facet reduction and recreation of BA (as evidenced on CT scan) influences subtalar joint range of motion and patient-reported outcome scores (SF-36 and FADI).

Calcaneal fracture care dogma would suggest that with good reduction and restoration of BA, the calcaneal height would also be restored.^3,5,21,22 When we measured the CH compared with the contralateral limb, however, we found a CH difference of 5 ± 3 mm in the short tuber group, versus a CH difference of 3 ± 3 mm in the restored tuber group (P < 0.05), suggesting that the CH was less likely to be restored when the tuber was shortened. In addition, the TDA in the short tuber group had a near 10° difference (flatter) than the TDA in the contralateral side. This is compared with a 3° difference (more inclined) in the restored tuber group (P < 0.05). Numerous studies cite the importance of restoring TDA after calcaneus fracture. Konovolchuk et al showed that TDA positively correlated with patient outcomes, and that a TDA < 6.5° had worse AOFAS scores than in patients with a greater TDA.²² Restoring the TDA and CH is important to hindfoot architecture and function and is often an indication for revision surgery using subtalar bone block arthrodesis, among other salvage techniques.^23–27

This cohort had a relatively equal number of patients who underwent ORIF or CRPF. Interestingly, the ORIF group had significantly shorter TL (P < 0.05) and CH (P < 0.05) when compared with the contralateral limb, whereas no significant difference was seen with CRPF. This finding could be explained by the fact that ORIF was used in more complex cases (higher Sander grade) as well as in cases with delayed presentation or chronic injuries. CRPF is our preferred method in acute injuries, which may offer the ability to more accurately restore tuber length and avoid the complications and morbidity of open exposures.^8,28

Finally, this study described how a shortened tuber after calcaneus fracture leads to an increase in the work required by the gastrosoleus complex to plantarflex the ankle. Using the simple physics equation Work = Force × Distance, we found that in the short tuber group, a 12.2% increase in work was required, versus a 2.2% increase in work in the restored tuber group (P < 0.05). Although a simple concept, we believe that these findings further highlight the importance of restoring tuber length, as it contributes to CH, TDA, and function. This was a purely mathematical exercise; however, it does question the need for further study because there are very few focusing on calf strength after calcaneus fracture.²⁹

This study has several limitations. While these injuries are three-dimensional in nature, our study is centered around the Bohler angle, a 2-dimensional and radiographic measurement. We work in a large, urban medical center where access to care and cost of advanced imaging such as CT scans are challenging for patients. We felt that the strength of this study lies in the presence of contralateral radiographs for direct measurement comparison. We hope that this study inspires future research to adopt contralateral x-ray acquisition as a standard protocol. In addition, only 1 lateral x-ray was used at the 12-month mark to perform our measurements. Longer follow-up is needed to evaluate for potential tuber length and posterior facet collapse, as well as to measure talar declination angles and how they change over time postoperatively. Any error in measurements was mitigated by standardizing all radiographs to a template in the BoneSetter App. Despite standardization, our measurements are quite small, with many measurement differences <5 mm, which introduces a high probability of error in our manual measurements. This study also did not include any patient outcome scores. The goal of this study was to introduce the concept of the importance that tuber length has on calcaneal height, position of the talus, and function during ankle plantarflexion. This study did not attempt to define a specific relationship between TL, BA, and CH, rather that TL seems to contribute to CH, as evidenced by our results.

5. Conclusion

Tuber length, in addition to Bohler angle, contributes to calcaneal height and function after fracture. A flat talus and smaller talar declination angles can still occur if the tuber length is not restored, despite satisfactory recreation of Bohler angle. Increased work is also required by the GSC with a shortened calcaneal tuber, further suggesting that greater attention to the recreation of TL is warranted during fixation of calcaneus fractures.