Does the level of the supracondylar humerus fractures affect the stability of pinning configurations? A biomechanical study

Abstract

Objective:

Supracondylar humerus fractures (SHFs) are prevalent injuries in pediatric orthopedics and often necessitate surgical management to restore elbow function and prevent long-term complications. The study aimed to evaluate the biomechanical effects of different fracture levels relative to the olecranon fossa and the efficacy of different pin configurations.

Methods:

Synthetic humerus bone models were used to standardize the fracture patterns and simulate surgical interventions. Pin configurations, including lateral divergent and cross-pin configurations, were applied using 3D-printed custom guides. Bending and torsional tests were conducted to assess biomechanical stability, with stiffness values calculated for each fixation method. Statistical analysis was performed to compare outcomes between the groups.

Results:

Distinct biomechanical responses to bending and torsional loads at various fracture levels and fixation configurations were observed. Lower-level fractures demonstrated greater stability in bending forces (low cross bending–high cross bending, P = .025; low divergent bending–high divergent bending, P = .004), with cross-pin fixation providing advantages in torsional forces compared with lateral divergent pinning (low divergent torsion–high divergent torsion, P = .001). Conversely, no significant biomechanical superiority was observed in higher-level fractures. Statistical analysis highlighted significant differences in maximum forces and stiffness values between the fixation groups, indicating the selection of optimal fixation strategies based on fracture characteristics.

Conclusion:

Results showed that distal SHFs were biomechanically more stable under bending forces, with cross-pin fixation providing an advantage in torsional forces for lower-level fractures. However, no significant biomechanical superiority was found for high fractures. These findings contribute to the understanding of fracture fixation strategies and highlight the importance of tailored approaches based on the fracture level. Further clinical studies are warranted to validate these biomechanical findings and to assess their impact on patient outcomes.

Level of Evidence:

Level V, Therapeutic Study.

Highlights

• This study investigates how the fracture level relative to the olecranon fossa affects the biomechanical stability of supracondylar humerus fractures treated with different pin configurations.

• Using synthetic humerus models and custom 3D-printed guides, standardized low- and high-level fractures were fixed with either cross or lateral divergent pin configurations.

• Biomechanical testing under bending and torsional loads showed that low-level fractures had greater bending stability, while cross-pinning provided superior torsional resistance in low-level fractures.

• No significant biomechanical advantage was observed in high-level fractures, regardless of the pin configuration used.

• These results emphasize the need for individualized surgical planning based on fracture level and suggest that cross-pinning may offer benefits in certain low-level fracture scenarios, while lateral pinning remains a safe alternative.

Introduction

Supracondylar humerus fractures (SHFs) account for more than 50% of elbow fractures and 7.5% of all fractures in children.^1-3 This fracture typically occurs in children aged 5-7 years after a fall on an outstretched hand and is more common in boys.^4-7 Supracondylar fractures typically exhibit an extension pattern. The extent of displacement depends on the direction of the force, the spatial position of the extremity, and the strength of the impact.^8,9 The recommended treatment for Gartland type 2, 3, and 4 fractures, as well as flexion-type fractures, is closed reduction combined with percutaneous fixation.¹⁰ When deciding on the best method for fracture fixation, the main factors to consider are the risk of neurovascular damage and the potential for long-term deformity.¹¹ The rate of loss of reduction remains high, which can result in complications such as malunion and reduced elbow function.¹²

Most displaced SHFs are best managed with closed reduction and percutaneous pinning, although the optimal pin configuration remains a topic of discussion.^1,13,14 Inadequate surgical fixation can negatively affect elbow function and reduce patient satisfaction. To avoid unsatisfactory outcomes, it is essential to consider the best pin fixation for each fracture type within the decision-making algorithm for SHFs. The primary pin configurations include cross and lateral divergent pinning, using either 2 or 3 pins or a combination of both approaches. The choice of configuration depends on the fracture characteristics and the surgeon’s preference. Medial cross-pin configurations have shown biomechanical advantages and yielded positive clinical results.^15-17 However, the risk of iatrogenic ulnar nerve injury (3%-8%) may overshadow the biomechanical benefits of cross-pin fixation.^15,17 Several biomechanical studies have compared various pin fixation techniques for treating SHFs,^7,14,18 but only a few studies have examined fracture levels in relation to their anatomical relationship with the olecranon fossa.¹⁹

The primary goal of this biomechanical study was to evaluate the effects of fracture levels on the olecranon fossa while treating SHFs using the most frequent K-wire configurations. It was hypothesized that distal fractures closer to the base of the olecranon fossa would be more unstable and necessitate more invasive fixation methods.

Materials and methods

Adult synthetic humerus bone models (Synbone, Switzerland) were chosen to replicate the bone dimensions typically observed in clinical practice. A custom 3D-printed guide was created to standardize the fracture and pinning configurations based on an intact bone model. Pin configurations were simulated on a 3D computed tomography (CT) humerus image of an adult patient using a 3D modeling software program (Solidworks 2019, Dassault Systems SolidWorks Corp., Massachusetts, USA) to create a 3-dimensional (3D) guide with a 3D printer (Ultimaker B.V.; Geldermalsen, Netherlands) (Figure 1). A 3D-printed guide allowed for consistent and reliable execution of the predetermined fracture simulation and pin insertion for every specimen (Figure 2). Two fracture variants were selected based on their potential instability. The low fractures were positioned just above the articular surface and tangential to the inferior margin of the olecranon fossa, whereas the high fractures were tangential to the superior edge of the olecranon fossa. Each fracture was created using a 0.5-mm blade. In all specimens, the custom 3D-printed guide was used to standardize the entry points on the lateral condyle. This ensured that each lateral pin was inserted at a consistent distal level and directed divergently to maximize spread within the bone. The guide was designed to allow bicortical purchase, even in distal fracture simulations. This standardized technique minimized inter-specimen variation and ensured reproducibility of pin trajectories.

Figure 1.

Samples of images of optimal adjustment ofpin configurations on the three-dimensional image of the humerus obtained from3D CT.

Figure 2.

Images of pin customized guides printed witha 3D printer. These guides allowed us to standardize the pin placement forevery fracture and pin configuration.

A total of 48 synthetic humeri samples were included in this study. Synthetic humeri were categorized into 2 groups based on their fixation methods: 1 group was fixed with 2 lateral divergent K-wires, while the other used a cross-pin configuration. All models underwent anatomical reduction and fixation with 1.8 mm Kirschner wires (Tipmed, Türkiye). Each group followed a standardized fixation model to ensure uniform fixation. Compression had not been applied during pin placement, but care was taken to achieve anatomical reduction with the help of the 3D-printed custom pin guides, which also eliminated the need to check the pin trajectories with fluoroscopy. In collaboration with a biomechanical engineer, each group was further divided into subgroups according to the specific load tests performed: subgroup 1, low fracture, cross-pin configuration, bending load (low cross bending [LCB]); subgroup 2, low fracture, lateral divergent pin configuration, bending load (low divergent bending [LDB]); subgroup 3, high fracture, cross-pin configuration, bending load (high cross bending [HCB]); subgroup 4, high fracture, lateral divergent pin configuration, bending load (high divergent bending [HDB]); subgroup 5, low fracture, cross-pin configuration, torsion load (low cross torsion [LCT]); subgroup 6, low fracture, lateral divergent pin configuration, torsion load (low divergent torsion [LDT]); subgroup 7, high fracture, cross-pin configuration, torsional load (high cross torsion [HCT]); and subgroup 8, high fracture, lateral divergent pin configuration, and torsional load (high divergent torsion [HDT]).

Bending and torsional testing were performed using a biomechanical testing machine (Shimadzu AG-IS 5 kN; Shimadzu Corporation; Kyoto; Japan), and the data were acquired using the software. Each model was securely fixed to a pedestal. The distal part of the humerus was held in place by using a custom-machined fixture with multiple bolts and nuts. The load displacement was applied at a rate of 0.5 mm/sec for bending and 0.5 degrees/sec for torsion. The torsional test was applied only in the direction of internal rotation to simulate the most common type of supracondylar fractures, posteromedial displacement. Stiffness values for torsion (Nm/degree) and bending (N m/mm) were calculated. The displacement (in millimeters and degrees) was measured using a 3D motion capture system. The highest recorded forces and stiffness values were measured at a maximum displacement of 7 mm for bending and 25° of rotation, which were determined as the limits of the constructs, as suggested in previous studies.^7,17

Statistical analysis

Statistical analysis was performed using SPSS (IBM SPSS 26, Armonk, NY, USA).The Kruskal–Wallis test was employed to compare torsional and bending stiffness across the groups. The Mann–Whitney U-test was used to determine which group contributed to the differences. Additionally, the Mann–Whitney U-test was used to compare the results from the Kruskal–Wallis test and the discriminant groups in the context of descriptive statistics. Statistical significance was set at P < .05.

Results

There were no cases of fracture displacement that resulted in fixation failure during loading tests. For the bone models subjected to bending loads, the LCB group recorded a mean maximum force of 329.9 N (range, 207.8-473.1 N) with an SD of 81.5 N. The mean stiffness value for this group was 35.6 N/mm (range, 25.6-47.2 N/mm). In the LDB group, the mean force was 322.7 N with an SD of 53.4 N, and the mean stiffness was 28.3 N/mm (range, 14-36 N/mm). For the HCB group, the mean force was 215.3 N (SD = 65 N), with a mean stiffness of 9.2 N/mm (range, 8.7-9.8 N/mm). The HDB group had a mean force of 199.2 N (SD = 58.9 N) and a mean stiffness value of 7.9 N/mm (range, 4.9-12.2 N/mm). Statistically significant differences were found in the highest recorded bending forces between the HCB-LCB and HDB-LDB groups (P = .025 and P = .004, respectively) (Table 1), whereas no significant differences were observed between the LCB-LDB and HDB-HCB groups (P = .14 and P = .37, respectively). Similar statistical patterns were noted for stiffness values, with significant differences between the HCB-LCB and HDB-LDB groups (P = .005 and P = .001, respectively) (Table 1). No significant differences were observed between the LCB-LDB and HDB-HCB groups (P = .26 and P = .41, respectively).

Table 1.

Comparison of the bending forces and stiffness in different fracture and pin configurations

Fracture–pin configuration	Maximum force (N) (mean)	P	Stiffness (N/mm)	P
Low cross bending	329.9	.025*	35.6	.005*
High cross bending	215.3		9.2
Low divergent bending	322.7	.004*	28.3	.001*
High divergent bending	199.2		7.9

*There is a statistical difference between low cross bending–high cross bending and low divergent bending–high divergent bending (P < .05).

For the bone models subjected to rotational loads, the LDT group recorded a mean maximum force of 113.3 N (range, 103.4-125.2 N) with an SD of 7.7 N, and a mean stiffness of 0.19 Nmm/degree (range, 0.15-0.22 Nmm/degree). The LCT group had a mean force of 169.8 N (SD = 31.1) and a mean stiffness of 3.41 Nmm/degree (range, 2.65-3.98 Nmm/degree). In the HCT group, the mean force was 153.54 N (SD = 14.47) with a mean stiffness of 2.93 Nmm/degree (range, 1.8-4.72 Nmm/degree). The HDT group recorded a mean force of 144.63 N (SD = 13.88) and a mean stiffness of 3.0 Nmm/degree (range, 2.27-4.17 Nmm/degree). Statistically significant differences in the highest recorded torsional forces were found between the LCT-LDT and HDT-LDT groups (P = .009 and P = .002, respectively) (Table 2), whereas no significant differences were observed between the HCT-HDT and HCT-LCT groups (P = .34 and P = .32, respectively). When comparing the stiffness values, significant differences were noted between the LCT-LDT and HDT-LDT groups (P = .001 and P = .005, respectively) (Table 2), with no significant differences between the HCT-HDT and HCT-LCT groups (P = .12 and P = .39, respectively).

Table 2.

Comparison of the torsional forces and stiffness in different fracture and pin configurations

Fracture–pin configuration	Maximum torsional force (Nmm/deg) (mean)	P	Stiffness (Nmm/deg)	P
Low cross torsion	169.8	.32	3.41	.039
High cross torsion	153.5		2.93
Low divergent torsion	113.3	.002*	0.19	.005*
High divergent torsion	144.6		3

*There was a statististical difference between low divergent torsion–high divergent torsion regarding torsional force and stiffness (P < .05).

Discussion

Displaced pediatric supracondylar distal humerus fractures are typically managed with closed reduction and pin fixation. However, traditional cross-pin fixation poses a risk to the ulnar nerve.^12,20,21 The ease of application and reduced risk of complications of lateral pinning have led to biomechanical comparisons with cross pinning. Although there is some evidence comparing both methods, few biomechanical studies have examined the effectiveness of each pin configuration across different fracture levels. Previous studies have reported challenges in maintaining reduction stability in certain supracondylar fracture types.^7,19 This study is the first to address the stability of cross and divergent pin configurations at 2 different fracture levels using a standardized 3D-printed guide to minimize variation and error.

Previous biomechanical studies have shown that adding a third lateral pin can significantly enhance construct stability, achieving bending and torsional resistance comparable to crossed-pin fixation.^18,22,23 In this study, 2-pin constructs were intentionally used to allow a controlled comparison between lateral-entry and crossed configurations. This study acknowledges the stability advantages of 3-pin lateral constructs and suggest that future biomechanical studies investigate these configurations to determine whether an additional lateral pin could compensate for the medial column support provided by cross-pinning.

Contrary to the present hypothesis, in this study, low fracture levels in the bending tests were found to be biomechanically more stable. The LCB and LDB groups had more stiffness than their higher fracture counterparts, but they did not show any statistical difference between them, indicating that there was no difference between the cross and divergent configurations in the bending forces. The most logical explanation for low fractures being more stable in bending forces was that the pins were deformed with more force because of the shorter moment arm between the cortex and fracture line at lower fracture levels. Although high fractures may appear more stable for some orthopedic surgeons, perhaps because they are easier to reduce, they do not have this advantage in terms of biomechanical bending forces. Likewise, it is believed that there was a misconception that low fractures were more unstable because they were more difficult to reduce. Consequently, once the low fracture could be reduced, it was more stable than its higher counterparts.

The superiority of cross and divergent configurations has been controversial in the literature. Some biomechanical studies have reported more stability with cross pinning, especially in torsional stiffness, whereas others have claimed the exact opposite.^7,18 Most clinical studies have also reported no difference in long-term outcomes between the 2 configurations and criticized the risk of ulnar nerve injury with cross pinning.^21,24

This study showed that the cross configuration provides an advantage only for torsional forces in lower-level supracondylar fractures. The LCT group showed greater stability than the LDT group. This finding is consistent with some studies showing the advantages of cross pinning, particularly in terms of torsional forces. However, the same result could not be obtained for high-level fractures. This could be due to the small number of samples in each group. It was also found that the HDT group was more stable than the LDT group, showing that torsional forces were more resistant to higher fracture levels in the parallel configuration, whereas cross pinning did not show any superiority for the same type of deformation force. In extremely distal fracture levels, both lateral- and crossed-pin constructs may face stability challenges due to limited bone stock and short distal fragments. In such cases, surgeons may consider modifying pin trajectories or adding an additional pin to improve construct stability and ensure adequate fixation.

The traditionally tested supracondylar humerus biomechanical models have been based on standard transverse fractures through the mid-olecranon fossa.^7,16,25 However, different fracture levels necessitate changing the direction of both lateral and cross pinning configurations. A higher fracture line, according to the mid-olecranon fossa, may force the surgeon to place pins more vertically and laterally. Similarly, a more inferior fracture line would force the surgeon to perform the opposite. Only 1 study in the literature has focused on the biomechanical effects of fracture levels and types of SHFs.²⁴ Jaeblon et al²⁶ reported in their studies that lateral pin placement was stiffer than cross-pin configuration in different fracture level scenarios, but they did not comment on whether a low or high fracture level was more stable or unstable.

Similar to previous biomechanical studies on pediatric SHFs, this study had certain limitations that should be acknowledged. It was not feasible to obtain the necessary number of cadaveric pediatric humeri in this study. Synthetic humeri are easily available, cost-effective, and reduce the variability between specimens. Although they are commonly used in the literature,^7,27 these investigations do not consider soft tissue stability provided by ligaments, muscles, and periosteum, as well as the effects of splinting or casting, which may influence fragment stability.^7,28 Synthetic bones are a viable alternative and are widely used in implant testing studies.²⁹ While the models used were adult-sized, the study aimed to compare different fracture levels and pin configurations under torsional and bending loads. Synthetic bone models were utilized to assess the biomechanical aspects of fracture reduction techniques; however, transverse osteotomy in the humeral model did not account for variations in the fracture line. Different degrees of comminution or sagittal obliquity can affect pin placement in clinical situations. Composite adult models provide a more consistent testing framework, but their larger pin-bone contact area compared with pediatric models may result in greater stiffness owing to the increased surface area at the fracture planes. Using 3D-printed custom guides, a more reliable biomechanical model was achieved with consistent pin placement across all trials, although the challenges of intraoperative insertion could not be simulated. Additionally, these 3D guides helped eliminate the need for fluoroscopy to monitor pin trajectories, which is often necessary in biomechanical studies.^27,30 The design of this study did not permit direct comparisons of the applied loads between models with organic bones, limiting the comparison of the fixation techniques for these fractures. Biomechanical analysis was conducted using single-direction loading. However, the physiological loads on the elbow in 2 or 3 directions are undoubtedly more complex than those in the single bending and torsional load tests performed in this study.

This study revealed that more distal supracondylar fracture types were more stable under bending forces in both divergent and cross-pinning configurations. Although the cross-pinning configuration provides more stable fixation in low fractures, no statistically significant biomechanical superiority was found in high fractures. Further studies are needed to better understand the biomechanical differences between fracture levels and monitor their clinical outcomes.

Funding Statement

The authors declared that this study has received no financial support.

Data Availability Statement:

The data that support the findings of this study are available on request from the corresponding author.