Documentation:FIB book/Distal Humerus Fractures

Overview

Introduction

Relevant Anatomy of the Upper Limb

Figure 1: Upper Extremity Anatomy

The upper extremity, or arm, is a functional unit of the upper body. It consists of three sections, the upper arm, forearm, and hand, extends from the shoulder joint, and contains 30 bones. The nerves of the arm are supplied by the brachial plexus, beginning from the nerve roots C5-T1.

The elbow is a critical element for a functional upper extremity. It is a complex synovial joint comprised of three bony connections (ulno humeral, radio humeral, and radioulnar) connecting the upper arm to the forearm. Many of the muscles of both the upper and lower arm either cross or attach to at least one point of the elbow joint. The elbow joint is referred to by many as a hinge joint, which is partially true but does not explain the ability to pronate and supinate the forearm. The articulation of the radial head and radial notch on the ulna is what allows for this motion and creates a “pivot” joint.

Figure 2: Humeroulnar (elbow) joint

Osteology & Fracture Pattern

The elbow joint is the articulation between the humerus, radius, and ulnar bones. The distal humerus comprises two condyles forming the articular surfaces known as the capitellum (lateral) and trochlea (medial). The bony geometry of the proximal ulna provides the elbow articulation with stability, especially in full extension, as seen in Figure 2, labelled as 'A'.

This report will address distal humerus extra-articular fractures. Supracondylar fractures are a specific type of extra-articular distal humerus fractures. They are a common childhood injury, accounting for the majority (55-80%) of elbow fractures in children. Due to their commonality, they will be discussed in detail in this report.

Distal humerus extra-articular fractures involve a break above the elbow joint, on the distal end of the humerus. These fractures do not involve the articular surfaces, where joint movement occurs. Rather, they include bony regions such as the supracondylar, epicondylar, condylar, and intercondylar regions.

Distal humerus fractures are responsible for 33% of all humerus fractures and represent about 2-3% of all fractures in the adult population. These types of fractures usually occur because of high impact trauma or a fall. Fractures of the distal humerus are frequently seen in the older adult population, particularly pertaining to those with osteoporotic bone. This type of osteoporotic fracture is 7-9 times more common in females, and the overall incidence of injury increased significantly between the years of 1970 to 1995.

Distal humerus fractures are known to place a financial burden on the healthcare system due to the required complex surgical interventions. These fractures are often associated with vascular or neural complications, require anesthesia, reduction, and hospitalization in roughly 75% of cases. This healthcare burden highlights the need to study the biomechanics and further develop prevention strategies.

The Wilkins-modified Gartland classification system is the most widely used classification system for these fractures. It divides them into 3 types based on initial fracture displacement: (I) no displacement; (II) displaced with intact posterior cortex; (III) complete displacement.

Biomechanics and Injury Tolerance

Mechanism of Injury

Figure 3: Pediatric Supracondylar Fracture

In the pediatric population, the supracondylar region consists of a weak, thin bone located in the distal humerus, bordered posteriorly by the olecranon fossa, anteriorly by the coronoid fossa, and on both medial and lateral sides by supracondylar ridges. This weak material composition makes the region particularly susceptible to injury under loading.

In 98-99% of cases, the fall is onto an outstretched hand (FOOSH) with the elbow in full extension, 70% of which were falls from a height,. 5% of fractures occur from a flexion-type injury, typically from a directly posterior impact. The remaining 0.5% of all supracondylar humerus fractures are caused by non-accidental trauma.

In children, ligaments are more lax, which can cause hyperextension, increasing the likelihood for extension type fractures such as supracondylar. The articulation between the proximal ulna and the distal humerus, specifically the olecranon process and olecranon fossa, compress under hyper-extension. Then, due to the bony anatomy, the transmitted force is directed towards the olecranon fossa. Typically, there is a posterior displacement of the distal fragment, and non-displaced fractures may be radiographically subtle. Flexion-type fractures occur in 1-2% of cases from direct anterior trauma to a flexed elbow. They typically occur concurrently with anterior displacement of the distal fragment.

Pediatric extension-type supracondylar humerus fractures are associated with long-term complications such as neurovascular injuries, malunion, and compartment syndrome. Neurovascular complications are a major concern across all age groups, particularly involving the brachial artery and median nerve.

Biomechanics

Figure 4: Pendulum Experiment Set-Up

Amis et al. investigated elbow fracture mechanisms using cadaveric testing. Their objective was to determine which angles of flexion carry higher risk of fracture. 40 cadaveric elbows were obtained postmortem, with the mean age of death being 69 years. The bones were transected 120 mm from the joint, sealed in polyethylene bags and deep frozen prior to use. Preparation involved removal of soft tissue from around bone shafts, which were then mounted in steel tubes using polymethylmethacrylate bone cement. Adhesive tape was used to maintain each particular posture. Tests were performed at different angles of flexion, with impacts around the elbow or along the forearm bones, creating a range of fractures, in a purpose-built impact loading rig.

Radial head and coronoid fractures followed impact along the forearm up to 80 degrees flexion. Olecranon fractures occurred by direct impact around 90 degrees flexion. Distal humeral fractures mostly occurred above 110 degrees flexion. Amis et al. presented the forces required for each fracture type, at various degrees of flexion and impact types. These are summarized in Table 1.

Table 1. Summary of fracture produces in study.

Fracture Type	No.	Flexion (Degrees)	Impact Type	Force (kN)
Radial head/neck	15	0-80	Indirect	2.9 (0.3-6.1)
Olecranon	13	60-135	Direct	4.1 (2.1-6.8)
Olecranon (quarterstaff)	5	90	Direct, quarterstaff	4.3 (2.9-6.1)
Coronoid	5	0-35	Indirect	4.3 (1.6-6.0)
Capitellum	1	0	Indirect	3.3
	1	90	Direct	2.2
Transcondylar	3	115-140	Direct	3.8 (3.2-4.9)
Intercondylar	1	35	Indirect	3.9
	3	115-140	Direct	3.8 (1.3-6.8)
Supracondylar (extension)	2	40-60	Direct	5.8 (5.6-6.0)
Supracondylar (flexion)	2	120-145	Direct	5.1 (4.9-5.3)

Articular fractures that occur at the distal humerus happen because of excessive force and the angle of trajectory through the distal humerus. The most common cause of these injuries is from falls onto an outstretched hand (FOOSH). A study by Borrelli et al. used voluntary testing on participants, who underwent a controlled fall directly to both hands using a pendulum. Force plates were used under crash pads to record results, and a one-way ANOVA was used to measure the lean angle compared to the elbow angle and elbow angular velocity upon impact. Results found that a greater lean angle resulted in an increased elbow angle at impact, while elbow angular velocity decreased. Borrelli concluded that these results were indicative of the protective arm mechanisms of a fall, shown through an increase in bicep activation. In 2022 Borrelli completed another study comparing young adults to older adults in the same study design, aiming to determine potential prevention mechanisms to avoid injury upon falling in older adults. Most of the results aligned with the previous study, however differences were shown through a decreased elbow angle, a greater bicep and triceps pre-impact time of EMG amplitudes, a lower vertical ground reaction force, and an increase in elbow angular displacement in older adults compared to the younger adult group. The increase in elbow flexion likely led to the decrease in vertical reaction force, and when combined with the increase in elbow angular velocity it suggests an increase in likelihood of body impact upon a fall in older adults.

The threshold for distal humerus articular fractures was investigated Marcoin et al. who used 21 cadaveric distal humeri alongside sawbones specimens to understand the relationship between bone density and the threshold for humeral fractures. The experiment was operated with a servo hydraulic machine and was set at 10° valgus and 20° flexion with screws to secure the olecranon fossa. An INSTRON load cell was used to measure strain over time during the axial compression testing, with a 10-N preload, the stainless-steel custom-made proximal ulna jig was translated superiorly at 50 mm per second using the machine actuator. The human specimens had an average bone mineral density of 0.9097 g/cm² and displayed a fracture threshold average of 1017.8 N/m, which was similar to the non-osteoporotic sawbone models. Using the Pearson product-moment correlation coefficient (r) calculation, a correlation can be drawn between bone mineral density and fracture threshold (r = 0.7321). From the stress-strain curve, a Young’s modulus of 16.7 GPa was obtained from both sawbones. Non-osteoporotic sawbones had a cortical thickness of 3 mm, and 2 mm for the osteoporotic bones. The non-osteoporotic cancellous bone had a Young’s modulus of 0.155 GPa, and the osteoporotic cancellous bone had a Young’s modulus of 0.137 GPa.

Studies also use a kinetic energy approach to study distal humerus fractures. Wegmann et al. constructed an experimental cadaver model to replicate realistic distal humerus and olecranon fractures in human elbows with the advantage of preserving the skin and soft tissue envelope. In this study, ten fresh-frozen human upper extremity specimens of mean donor age 72 years were mounted with the elbow flexed to 90° in a custom high-impact test bench and subjected to axial compression via a drop-impactor; the humeral shaft was potted and aligned vertically and the forearm rested, so that the ulna was roughly horizontal under the impact. The applied kinetic energy averaged to be 44.4 Joules (J) with a standard deviation of 14.5 J, with the distal humerus group averaging 48.6 J and the olecranon group at 40.8 J; no significant difference is shown between the two groups. Post-fracture imaging showed that six of the ten specimens sustained distal humerus fractures. The majority were classified as AO type C (most C2/C3), and four sustained olecranon fractures (type IIB, Mayo classification). The authors highlight that their protocol reliably produced complex intra-articular distal humerus fracture patterns with intact soft tissues, making the specimens suitable for surgical training. They also note that the mechanism likely involved acute axial compression of the articular surfaces resulting in central humeral fragmentation (for distal humerus) or olecranon splitting via the trochlear stamp through cancellous bone. One major limitation of this study is the uniaxial nature of loading. The study also directs future work towards identifying predictors of fracture location and morphology under varied load vectors.

Summary of Research

The findings from each of the studies provide strong characteristics for injury threshold. The variety between the experimental setups and data type (force, threshold level, energy) selection provide a better understanding about distal humerus fractures. The differences in impact forces that cause an articular fracture at the distal humerus depend on the angle of the flexion of the elbow joint. The different angles of flexion replicate real-life injury mechanics, representing FOOSH incidents through elbow extension and acute high energy impacts at various angles of flexion. There was also a variety in experiments, ranging from soft tissues preserved in cadaveric specimens, bone specimens and synthetic bone models (sawbones), to living volunteers,. This diverse range of specimens helps encompass the different factors that go into injury mechanics. This also benefits the research by allowing injury tolerance and fracture thresholds to be tested through models and cadaveric specimens, whereas the volunteer testing allowed for muscle activation and natural protective mechanisms to be evaluated.

Between all the studies and the different angles used, an increasing angle of flexion can be associated with distal humeral fractures during direct impacts. This did not necessarily translate the same to FOOSH incidents, where a decrease in elbow angular velocity was associated with an increased flexion angle . There was also a correlation between increased risk of fracture with a lower bone density, supporting previous research that osteoporotic bone has an increased risk of fracture. Additionally, in both specimens and volunteers there were noticeable age-related differences, associating older subjects with a greater elbow flexion angle. This was associated with a decrease in vertical ground reaction force, decrease force at the elbow but transmitting energy throughout the body, increasing risk for other injuries.

The injury tolerance for a distal humerus fracture was tested in cadavers and sawbones,.In a healthy adult, an average fracture threshold was 1017.8N/m, with a 0.155 GPa Young Modulus for cancellous bone. A kinetic energy threshold was tested finding the distal humerus to reach 48.6 J whereas the olecranon reached 40.8 J before fracture, although this difference was not significant.

Due to the differences of research purpose and experimental design, these results should not necessarily be compared to each other but rather help inform about the different injury mechanics that can lead to a distal humerus fracture.

Discussion

Limitations

A limitation of this research is the lack of biomechanical research on the pediatric population. Supracondylar fractures predominately occur in children, since bone growth is not complete. Studies on adult bone structure, cadavers and volunteers can not accurately represent the pediatric population due to significant bone differences. Another limitation is that cadaveric research for the pediatric population is extremely limited due to ethics, preventing researchers from pursuing biomechanical studies of supracondylar fractures.

Cadaveric studies are associated with many issues regarding the reproducibility. The cited study by Amis et al. used fresh-frozen cadavers. The freezing process can have impacts on the soft tissue properties, which can create adverse results.

Comparisons & Controversies

As mentioned earlier, 95% of supracondylar fractures are caused by a fall onto an outstretched hand with the elbow in full extension,,. However, in the biomechanical studies analyzed, researchers paid attention to the joints in flexion, rather than in extension,,,. Specifically, Amis et al. developed an experimental setup involving elbow joints from cadavers and concluded that flexion supracondylar and transcondylar fractures were produced by direct impacts along the humerus between 115- and 145 degrees flexion. Borrelli et al. suggested that protective arm movements (of living humans) during the fall reduces the impact velocity and therefore load when elbows are flexed over 120 degrees. From the later Borrelli et al study (2022) there was the concern of pre-planning. Participants who know they are going to fall may engage with pre-planning and modulate how they fall. Participants would pre-plan to account for the direction or height of the fall, and lead to rapid responses. These rapid responses may not reflect real life scenarios well, since falls are often unplanned for. Marcoin tested a combination of cadaveric distal humeri and steel artificial bones at only 20 degrees flexion angle. The variation in flexion angles used in experimental set-ups may infer that cadaver studies of supracondylar fractures do not represent real-life fall and impact scenarios, where fractures exist more common in extension and mild angles.

A potential controversy with all cadaveric studies is the treatment and storage of specimens, which can have potential impacts on results. For instance, when cadavers are frozen, their tissue properties can become different from those of living people. Additionally, cadavers have no blood pressure or muscle tension, which can cause discrepancies, especially in musculoskeletal studies. This was shown by Borrelli et al. who examined how protective arm mechanisms, including muscle activation and natural reflexes changing the elbow angle, impact the forces in the elbow. While the included cadaveric studies could measure fracture thresholds and high impact injuries, which are a leading cause of distal humerus fractures, they cannot fully represent tissue properties and other natural pressures within the body.

Future research

There are many potential directions for future research to be conducted. For instance, it could be beneficial to investigate concurrent fractures from a distal humerus fracture, to better understand the mechanics and how to optimize treatment.

More research could be done into car impacts and how that correlates to high energy impacts. Making a computational model with elbow injuries for the car impacts could possibly incentivize car companies to make cars safer.

Much research has focused on pediatric population, because they commonly incur supracondylar fractures, but these sources have yet to be combined into a report that could work towards reducing injuries and adequate treatment.

More research can be done into utilizing current technology. 3D fracture and heat maps are an efficient method to derive the structure of the distal humerus and its fracture lines. A study by Wang examined 102 distal humerus fractures in adults using computed tomography (CT) scans, which were then used through reconstruction to create a 3D model. This technology demonstrated the typical fracture lines on the humerus in an axial view and the surrounding heat map. The results concluded higher fracture frequency in the metaphyseal regions, aligning with where a supracondylar fracture would occur.