Morphometric variations of the lateral surface of calcaneus: Can standard plate sizes fit all?

Abstract

Purpose

Displaced intraarticular fractures of the calcaneum often require plate fixation. The complex fractures are usually fixed with standard lateral plates and are approached via the extensile lateral approach which is fraught with the risk of wound complications. Oversized calcaneal plates produce tension on the wound closure site and can, thus result in wound healing problems. The current study analyses the morphometry of the lateral surface of the calcaneus for the ideal configuration of the calcaneal plates for Indian patients using a CT based analysis of intact calcanei.

Methods

Fifty CT based studies of normal calcanei were retrospectively analyzed using Horos® software version 3.3.5. The cross-section of the lateral third of calcaneum was assessed for the morphometric measurements relevant to the dimensions of the standard plating devices for calcaneum. We measured the overall plating length of calcaneum, the vertical heights at the anterior process and calcaneal tuberosity, the height of the posterior facet, the lengths of the anterior process, and the posterior facet. The inclination angles of the posterior facet, cuboid facet were also measured. The findings were compared between male and female cases.

Results

The mean plating length of the calcaneus was 60.06 ± 4.05 mm. The mean vertical height of the anterior process and the calcaneal tuberosity were 24.3 ± 2.71 mm and 39.48 ± 4.73, respectively. The length of the anterior process and the posterior facet were 21.36 ± 1.72 and 25.02 ± 4.17, respectively. The mean Gissane angle and the angle of inclination of posterior facet in relation to the plating length were 118.04 ± 5.99° and 47.3 ± 5.20°. The mean anterior slant angle of the cuboid facet was 96.64 ± 4.39°. Significant differences were observed in the measurements among male and female groups except for the angular parameters.

Conclusion

The current analysis suggests the individual morphometric variations of the lateral plating surface of the calcaneus. Although female calcanei have smaller dimensions compared to male calcanei, the angular parameters are comparable among the two groups. Considering the wide variations in the majority of the morphometric parameters, it is difficult to standardize the plate dimensions. However, a few serial increments in the plate dimensions can help in providing the best fit rather than an ideal fit. Also, the plates should be available with multiple inclination angles of the facet limbs according to the local population variations.

1. Introduction

Displaced intraarticular fractures of calcaneum often require open reduction and plate fixation, especially in complex fracture patterns. While the simple fracture patterns can be well addressed using less invasive surgical approaches, the complex fracture patterns (Sanders type III and IV) are approached through an extensive lateral approach which helps in adequate exposure and reduction of the articular and lateral wall fragments.¹ The plate fixation helps in providing a biomechanically stable construct, compared to the screws and pinning options. Although the lateral extensile approach provides a satisfactory surgical view of the posterior facet for direct reduction and fixation, it is fraught with the risk of wound dehiscence.² Extensive skin flap handling, retraction, and wound closure under tension, all can contribute to the wound healing problems.³ The plate fixation underneath the skin flap consumes additional space for implant placement over the lateral calcaneal surface and can contribute to the tension at flap margins. The calcaneal morphometry is shown to have individual variations,⁴ and thus the standard plate devices can have a different fitting on different patients depending upon the population and ethnic origin. While the volume added to the lateral surface of the calcaneus after plating can affect the wound tension, the marginal overhangs of the calcaneal plates can further add to the wound tension (Fig. 1). These factors can potentially contribute to the wound complications, the extent, and incidence of which remain unknown. Therefore it is recommended to contour the plate over the calcaneal surface using a prior templating to prevent any overhang from the margins. Despite contouring the plate, the plate screw configuration can sometimes fail to match the configuration of the calcaneus to be fixed. The only option left in such scenarios is to cut the overhanging portions/screw slots of the plate and to accept minor marginal overhang. The lesser number of screws can compromise the overall stability of the fixation, and the overhanging parts can irritate the skin following weight-bearing once the union is achieved.

Fig. 1.

Representational images of calcaneal fractures fixed with lateral plating suggesting risk of tensioned wound closure. Well fitting plates result in easy approximation of wound (a,c), and those plates with overhang at the calcaneal margins make the wound approximation difficult (b,d).

To address the above important concerns, the current study analyses the morphometry of the lateral surface of the calcaneus for the ideal configuration of the calcaneal plates for Indian patients using a CT (computed tomography)-based analysis of intact calcanei.

2. Materials and methods

We retrospectively analyzed anonymized CT based data of intact normal adult calcanei. The data was collected in the form of DICOM (Digital Imaging and Communications in Medicine) files from those patients that underwent CT evaluation of foot and ankle during a two-year period between 2018 and 2020. After excluding the cases with abnormal calcanei, fifty CT studies of patients aged between 18 and 60 years, were collected. We excluded the abnormal calcanei with signs of fresh or old fractures, those with signs of osteomyelitis, hypoplasia, degenerative changes, bone tumors, or any radiologically visible pathological process, such as Haglund’s deformity. The included CT studies were performed during the aforestated period for indications other than calcaneal fractures. The included studies were analyzed using the Horos® software version 3.3.5 (Horos 92 Project; https://www.horosproject.org).

The cross-section of the calcaneus at the junction of its medial two-third and its lateral one third was utilized for the measurements. This gradient for junctional markings was located on the posterior facet posteriorly and at the cuboid facet of the calcaneus anteriorly (Fig. 2). The cross-section was kept perpendicular to the posterior facet in the coronal plane. For the morphometric measurements, the cross-section of the lateral one-third calcaneal was selected rather than the lateral surface of the calcaneus to amalgamate the scope of plate contouring which spans a deeper lateral aspect of the calcaneal surface than the exact lateral surface.

Fig. 2.

Cross section retrieved from the calcaneus at the junction of its medial two third and its lateral one third using the CT analysis software.

Following parameters were measured considering their relevance to the implant selection in calcaneal fracture fixation (Fig. 3):

• 1.Height (H): The maximal vertical extent of the calcaneus through the superior most prominence of the calcaneal tuberosity on the lateral aspect. The measurement was made perpendicular to the line joining the posteroinferior and anteroinferior extents of the calcaneus.
• 2.Anterior height (AH): The superoinferior extent of the cuboid facet of the calcaneus at the above described lateral cross-section of the calcaneus.
• 3.Length (L): The anteroposterior plating length of the calcaneus was measured as the distance between the centre of the vertical extent of the cuboid facet (AH) and the centre of the height (H) of the calcaneal tuberosity.
• 4.Posterior facet angle (a): the inclination angle of the posterior facet in relation to the anteroposterior plating length of the calcaneus was measured.
• 5.Gissane angle (b): the angle between the line tangential to the posterior facet and the line along the superior surface of the anterior process was measured.
• 6.Anterior slant angle of the cuboid facet of the calcaneus (c): the anterior angulation of the cuboid facet of the calcaneus in relation to its anteroposterior plating length was measured.
• 7.Facet height (FH): the vertical distance of the superior most limit of the posterior facet was measured from the anteroposterior plating length and perpendicular to it.
• 8.Anterior process length (AL): the anteroposterior length of the superior surface of the anterior process was measured.
• 9.Posterior facet length (FL): the length of the posterior facet was measured from its junction with the anterior process up till the superior most limit.
• 10.Anterior segment length (AS): the location of the intersection of the tangential line through the posterior facet on the anteroposterior plating length (L) of the calcaneus was measured from the cuboid facet.

Fig. 3.

Measurement of different parameters (see methods sections) in a lateral cross section of calcaneum (a). The same measurements are explained through a line diagram (b).

The above-stated parameters are helpful in mapping the overall lateral surface dimensions of the calcaneus, and thus, can be used for plate designing and selection.

2.1. Statistical analysis

The distribution of the aforementioned quantifiable parameters concerning the cross-sectional morphology of the lateral aspect calcaneus was compared to a normal distribution using the Kolmogorov-Smirnov Test of Normality. Their means, standard deviations, interquartile ranges were calculated. The intraclass correlation coefficients were calculated for intraobserver and interobserver reliabilities of the measurements. The male and female findings were compared using unpaired samples t-test. We used IBM SPSS Statistics for Windows, Version 22.0 software for the statistical analysis. A p-value of less than 0.05 was considered as the marker of statistically significant difference.

3. Results

All of the quantifiable parameters were comparable to a normal pattern of distribution using the Kolmorgorov-Smirnov test. The mean age of the patients whose CTs were evaluated was 36 ± 0.6 years. Twenty CT files belonged to female subjects. The mean anteroposterior plating length (L) of the lateral surface was 60.06 ± 4.05 mm. The mean angle of Gissane, “b”, was 118.04 ± 5.99°. The mean vertical extent of the calcaneal tuberosity (H) and the anterior process (AH) were 39.48 ± 4.73 mm and 24.3 ± 2.71 mm, respectively. The mean sagittal slant angle of the cuboid facet, “c” of the calcaneus was 96.64 ± 4.39°. The mean angulation of the posterior facet, “a”, in relation to the anteroposterior plating length was 47.3 ± 5.20°. The superior most limit of the posterior facet was located at the mean vertical distance (FH) of 25.26 ± 2.36 mm from the anteroposterior plating length. The lengths of the anterior process (AL) and the posterior facet (FL) along their superior surface were 21.36 ± 1.72 mm and 25.02 ± 4.17, respectively. The anterior segment of the anteroposterior plating length was located at a mean distance of 13.6 ± 1.97 mm from the cuboid facet. The detailed results are provided in Table 1.

Table 1.

Measurements of differential morphometric parameters of the lateral cross section of calcaneum.

Measurement parameter	Mean ± SD	Interquartile Range	Overall Range
Length (L) in mm	60.06 ± 4.05	57–63	54–69
Anterior height (AH) in mm	24.3 ± 2.71	23–26	19–32
Height (H) in mm	39.48 ± 4.73	36.25–43	26–50
Gissane angle (b) in degrees	118.04 ± 5.99	114.25–122	104–130
Posterior facet angle (a) in degrees	47.3 ± 5.20	43–51.75	37–56
Anterior process length (AL) in degrees	21.36 ± 1.72	20–23	18–25
Posterior facet length (FL) in degrees	25.02 ± 4.17	24–27	20–32
Anterior slant angle of the cuboid facet (c) in degrees	96.64 ± 4.39	93–99.75	87–108
Facet height (FH) in mm	25.26 ± 2.36	24–26	20–31
Anterior segment length (AS) in mm	13.60 ± 1.97	12.5–14.9	9.1–17.4

The comparison between male and female findings is provided in Table 2. The overall comparison suggests smaller calcaneal dimensions in females without affecting the angular parameters, i.e, the Gissane angle, the posterior facet angle, and the anterior slant of the cuboid facet. The intraobserver and interobserver intraclass correlation coefficients for the CT based measurements were high (Table 3).

Table 2.

Comparison of the morphometric parameters of calcaneum among male and female groups.

Measurement parameter	Male measurements	Female measurements	p-value	Remarks
Length (L) in mm	61.54 ± 3.53	57.63 ± 3.71	.001	Statistically significant difference, p < .05
Anterior height (AH) in mm	25.41 ± 2.61	22.47 ± 1.74	<.001	Statistically significant difference, p < .05
Height (H) in mm	40.51 ± 5.065	37.78 ± 3.67	.047	Statistically significant difference, p < .05
Gissane angle (b) in degrees	117.32 ± 6.05	119.21 ± 5.87	.285	Statistically insignificant difference, p > .05
Posterior facet angle (a) in degrees	47.38 ± 5.07	47.15 ± 5.54	.882	Statistically insignificant difference, p > .05
Anterior process length (AL) in degrees	22.03 ± 1.47	19.36 ± 4.49	.004	Statistically significant difference, p < .05
Posterior facet length (FL) in degrees	26.48 ± 2.29	23.63 ± 5.38	<.001	Statistically significant difference, p < .05
Anterior slant angle of the cuboid facet (c) in degrees	96.83 ± 4.37	96.31 ± 4.52	.687	Statistically insignificant difference, p > .05
Facet height (FH) in mm	26.03 ± 2.05	24.00 ± 2.33	.002	Statistically significant difference, p < .05
Anterior segment length (AS) in mm	14.00 ± 1.78	12.94 ± 2.13	0.03	Statistically significant difference, p < .05

Table 3.

Intraobserver and interobserver correlation coefficients of the measured calcaneal morphometric parameters.

Variable	Intraobserver Correlation Coefficient	Interobserver Correlation Coefficient
Length (L) in mm	0.97	0.96
Anterior height (AH) in mm	0.96	0.97
Height (H) in mm	0.97	0.97
Gissane angle (b) in degrees	0.98	0.96
Posterior facet angle (a) in degrees	0.98	0.97
Anterior process length (AL) in degrees	0.97	0.96
Posterior facet length (FL) in degrees	0.96	0.96
Anterior slant angle of the cuboid facet (c) in degrees	0.97	0.96
Facet height (FH) in mm	0.97	0.97
Anterior segment length (AS) in mm	0.98	0.98

Based on these measurements the morphometric map of the calcaneus and the configuration of an ideal plate have been shown in Fig. 4.

Fig. 4.

Ideal calcaneal dimensions for the calcaneal plate designs for the female (a) and male (b) patients.

4. Discussion

4.1. Why are we concerned regarding the lateral surface morphometry and plate dimensions?

The displaced calcaneal fractures treated with open reduction and plate fixation have high complication rates especially those related wound healing.⁵ The apex of standard lateral extensile incision for the lateral calcaneal exposure is the critical zone at risk of flap necrosis.⁶ This part of the flap is supplied by the lateral calcaneal artery which may get injured during the flap creation and retraction. It is advocated that the vertical limb of the incision should be kept slightly posterior, i.e, close to the tendon Achilles to prevent injury to the lateral calcaneal artery.⁷ Besides this, the tension at the flap margins after suture application can also impact the peripheral vascularity of the flap. Low tension sutures are preferred and numerous suturing techniques have been described for this with good outcomes.⁸ Also, the surgery needs to be timed according to the resolution of soft tissue edema which itself can contribute to the difficulty in wound closure and ultimately the tension at the suture sites.

4.2. How can the plate fixation contribute to wound complications?

Considering the risk of flap necrosis and wound dehiscence, the full thickness flaps are recommended for calcaneal exposure. The flap has limited capacity to stretch and any attempts to do that can damage the vascularity of the flap.⁹ Thus, any hardware placement over the calcaneus, which increases the overall effective volume of the calcaneus, is bound to add a variable tension on the flap margins when approximated. An oversized calcaneal plate can prevent the approximation of the flap to the parent bone. While the major overhangs of the plate can be bent to reshape them according to the calcaneus, small overhangs may persist and are difficult to contour. Collectively, these overhangs can contribute to an increase in the effective lateral calcaneal surface, and also to the patient’s symptoms once weight bearing is initiated after fracture healing.

4.3. How can a properly sized plate be helpful?

Less invasive plates are available which support the anterior process, the posterior facet, and the superolateral aspect of the calcaneal tuberosity. Such plates can be helpful for simple fracture patterns. However, for complex displaced articular fractures of the calcaneus with multifragmentary involvement of the tuberosity, wider surface coverage is required. In such a situation, a well-fitting plate without any marginal overhang would be the most appropriate.¹⁰

4.4. Current evidence

The currently available evidence regarding the morphometric variations of the calcaneum is sparse.^4,11,12 Moreover, the limited evidence mainly focuses on the anatomical perspective like angels and overall extent measurement of the calcaneus. There is a general lack of studies focusing on the implications of the morphological variations of calcaneum on plate fixation and the configuration of the plates. Chung KJ et al.,¹⁰ highlighted the role of 3D printing-based plate selection for the fixation of simple calcaneal fractures especially Sanders type II fractures. They found their technique to be useful in obtaining the surface fit of the calcaneal plate with ease. Apart from the routine CT based assessment, the role of 3D printing, which although attractive, may have cost and resources related constraints. Also, the effectiveness of such planning in comminuted fractures needs to be established.

4.5. What is the importance of the information provided in the current study?

The current study highlights the morphometric variations of the lateral surface of the calcaneus. The anteroposterior plating length varies from patient to patients with a standard deviation of 4.05 mm. The overall range suggests that the plates should be available in length between 55 mm and 70 mm with the need of smaller plates in female patients. The vertical limb of the plate at the anterior process needs to be tilted anteriorly at an approximate angle of 96°. The interquartile range is approximately 7°, suggesting that a few increments in this parameter can be helpful. The angle, however, does not necessarily need to be equal to the slant of the cuboid facet as the minor variations can be taken care of by selecting smaller or larger length plates. The length of the anterior limb should range between 20 and 30 mm. The length can be adjusted by cutting off the extra screw slot, by bending the screw slot, or by selecting a plate with a lesser number of screw holes in the anterior limb. The variation in the vertical extent of the calcaneal tuberosity was high ranging from 26 to 50. This also can be addressed by cutting off the extra screw-hole, by bending the screw-hole, or by selecting a plate with a lesser number of screw holes in the posterior limb. A few serial increments in the size of the anterior and posterior limbs in the described range can be helpful. The length of the anterior segment of the anteroposterior plating length distal to the junction with the posterior facet limb had a low standard deviation (1.9 mm). Thus, plates with anterior segment length between 12 and 15 mm can be designed and other minor variations can be taken care of by anteroposterior positioning of the plate.

The mean length of the superior margin of the anterior process was 21.3 mm, again with low variation (SD = 1.73 mm). This is important for those plates with peripheral rim shaped according to calcaneus shape. The same applies to the Gissane angle as well where the peripheral fitting part of the plate needs to be similar to the angulation between the anterior process and the posterior facet. The mean Gissane angle was 118°. For conventional plates, the posterior facet limb needs to be angulated in relation to the length of the plate. While the length of the facet limb can be adjusted by bending of the proud part or by cutting the limb, the angulation of the posterior facet limb in relation to the length of the plate needs to match the angulation of the posterior facet. An in-plane bending of the plate carries the risk of failure and breakage and thus controlling the angle of the posterior facet limb can negatively affect the strength of fixation. The angulation of the posterior facet in relation to the plating length ranged between 37 and 56°. The interquartile range was 43–51°. The angulation of the facet limb should match with the alignment of the posterior facet so that all screws in this segment can be applied. Mismatch of the plate contour and that of the posterior facet often makes it difficult to put all the screws, especially when the facet limb is more vertical than the posterior facet. Our findings suggest that there are individual variations in the inclination of the posterior facet, and thus a few serial increments in facet inclinations of the plates are required to match the approximate position of the posterior facet. Minor variations with the inclination of the posterior facet can be taken care of by anteroposterior repositioning of the plate. The height of the posterior facet from the plating length also had a low standard deviation (2.3 mm) and an interquartile range of 24–26 mm. Thus, the facet limb of the calcaneus plate should have an optimum number of screw slots up to this height. The extra part of the facet limb can either be cut or bent according to the required. The mean facet height in female cases was approximately 24 mm in females and 26 mm in males. A common problem that may arise with the shortening of the facet limb is the reduced number of screw holes left to support the posterior facet fragment. Thus, the facet limb of the plate needs to have a sufficient number of screws up to the height of 24 mm from the plate length. Thus further bending or shortening of the facet limb will not affect the support to the posterior facet. The remaining part of the plate can have variable numbers and locations of the screw holes as per the manufacturer’s choice as long as the critical area for the peripheral limits has been taken care of. The inferior aspect of the plating length between the anterior limb and posterior limbs can have multiple limbs with single screw slots that can be contoured according to the inferolateral calcaneal surface.

Fig. 4 shows the ideal configuration related measurements of the calcaneal plate design for the male and female patients. Our analysis suggested that there were no significant variations in the angular parameters among male and female patients, and therefore, similar angular configuration plates with different sizes will be helpful for male and female patients.

There had been a few limitations to this study. First, the study is based on a small sample size which might not reflect the actual scenario of morphometric variations of the calcaneus. Second, there can be potential variations in the surface contours of the lateral calcaneal surface which are beyond the scope of the current study. Third, following the fracture reduction, even though the Gissane angle might be restored, the shape of the bone may deviate from the normal anatomy of the calcaneus. Thus, minor variations can occur in the plate positioning following that. Lastly, due to the lack of similar studies in the past concerning the plate-specific parameters of calcaneum, we cannot quantify the morphometric variations due to ethnic and regional differences. Nevertheless, the current analysis provides an insight into the common problem of oversized and non-fitting calcaneus plates during the fixation of calcaneus fractures. The suggested parameters will pave the way for further research and highlight the importance of preoperative planning to avoid intraoperative surprises in calcaneus fracture fixation. Additionally, there is a need for more flexibility in sizes of the plates considering the individual variations and need for modularity to be added to such plates so that minor overhangs can be corrected by cutting or bending of the proud part without affecting the effectiveness of fixation.

To conclude, the current analysis highlights the individual morphometric variations of the lateral plating surface of the calcaneus. Although female calcanei have smaller dimensions compared to male calcanei, the angular parameters are comparable between the two groups. Oversized plating devices placed over the lateral surface of the calcaneus can cause wound healing problems. Considering the wider variations in the majority of the morphometric parameters, it is difficult to standardize the plate dimensions. However, a few serial increments in the plate dimensions can help in providing the best rather than ideal fit. While the length of the plate and those of the anterior process limb, tuberosity limb, and the posterior facet limb can be controlled by bending or cutting of the overhanging part, the in-plane bending to change in inclination of the facet limb carries the risk of loss of strength. Thus, plates should be available with multiple inclination angles of the facet limb according to local population variation. Lastly, preoperative planning based on the uninjured contralateral calcaneum, if possible, should be performed whenever feasible for an accurate estimation of the appropriate plate sizes.

Source of funding

None.

Declaration of competing interest

None of the authors have any conflicts to declare.