To look at this question we'll have to first take a look at the the type of combat these weapons were used for in the First World War (and the following world conflict, for that matter), and this involves taking a look at trench raiding, a less often studied aspect of the Great War.

For a war renowned for its modern industrial nature, trench raiding did have a distinctly medieval aspect to it. All sides used raids on an enemy trenches to carry out such objectives as killing or terrifying enemy personnel, capturing prisoners or material for intelligence purposes, reccoinoitering and softening up enemy positions for upcoming attack, destroying emplaced weapons such as machine guns and eliminating strongpoints or positions used as listening or observation posts for artilery direction. Trench raiding evolved as the war progressed, and most sides quickly bega to realise that their issued weapons were not particularly suitable for the task. Normal rifles were too unwieldy and long for use in a confined trench, and the very long sword-style issued bayonets only made the problem worse. Bayonets were often disliked by many troops for hand combat as they could easily get stuck in the ribcage of the victim and and become near impossible to remove. Soldiers also found issue bayonets to be unsuitable for the purpose of a fighting knife due to their length, which was a problem due to the need for knives which could be used to silently eliminate enemy sentries to allow raiding parties to access enemy trenches silently.

The US army began to issue this famous style of trench knife for exactly this reason, with the main functions of the knuckle duster guard being to prevent the knife hand being grabbed and the knife being pulled away. Three models were issued throughout the war to specialist or crack infantry, or troops not equipped with bayonets, the last (the Mk 1 M1918) being issued to elite forces such as Marine Raiders or paratroopers well into the Second World War. Ironically most reports on the effectiveness of the trench knife came from the Second World war, and they were not particularly positive. The common complaint was that the knuckle guard made it hard to bring into action quickly and limited the ways in which the knife could be held or used. Other complaints noted that the blade was quite small and hard to sharpen well, and unsuitable for tasks like opening ammumition boxes, and was quickly replaced in US Service by different weapons. As for its performance in the trenches of World War 1, there are little primary sources to go on. One can assume the trench knife was primarily a weapon for silent killing or engagement in very close quarters too small for larger hand weapons. With regards to penetrating an opponents helment, the triangular stilletto-like blade of the first issued trench knife (the M1917) may not have been to bad for this purpose. However, actually landing a dead on blow on the top of an opponenents helmet with enough force to penetrate the helmet and skull to deliver a killing blow, without getting the blade stuck or skidding off the curved top, sounds like to difficult a proposition to me to be practical for fighting. Indeed for silent killing, a simple stab would suffice.

What alternatives existed then to this style of knife, or indeed for a primary hand weapon for trench raiding? Various issued and make-do trench weapons existed: For the purpose of silent killing, the French shortened down Lebel bayonets and also developed the French Nail, a stabbng weapon made from a steel barbed wire stake sharpened at one end and bent to form a handle. British and Commonweath forces were known to either craft or issue push knives, essentially one or two sharpened steel blades with handle grip bearing some similarity to the handle of a corkscrew. The Germans issued the Nahkampfmesser, essentially a conventional knife design but quite useful for combat purposes, and also developed impromptu designs based on wire stakes like the French Nail. Knives though were ony a small selection of the weapons used by raiding parties. Menageries of different weapons were produced in what were essentially a trench cottage industry. One such example was British or Empire troops emptying out the explosive contents of their issued hand grenade, the Mills Bomb, and filling it with lead before attaching it to the end of a pickaxe hadle to form a vicious club. Axes and hatchets were also used, but often the most popular tool for trench hand to hand combat was the entrenching tool, essentially a small shovel. With its edges sharpedned these could be truly fearsome weapons, with accounts stating strong blows could easily split through a victims helmet and skull.