Why Do We Say Splinted Teeth Should Have Similar Strength?

In prosthodontics and periodontology, we often repeat one rule: teeth that are going to be splinted should be close to one another in terms of strength and load-bearing capacity. For example, connecting a crown on a lateral to a canine is usually not considered an ideal choice, because these two teeth are not the same in root length, level of periodontal support, or mechanical behavior.

But the more interesting question is: why does this difference create a problem?

In my view, the answer should be sought not in the "amount of movement" but in the "pattern of movement."

Each tooth is an independent system that, under force, moves about its own center of rotation according to its root length and periodontal condition. A canine with a longer root and more support behaves differently from a lateral. When these two teeth are separate, each follows its own natural path and there is no problem.

But the moment they are joined by a rigid connection, they are no longer two independent teeth; they become a single system. Now both are forced to share one common movement, even if their inherent tendency to move is different.

The core problem is that the weaker tooth can no longer follow its own natural movement path. This mismatch between the expected movement and the imposed movement has to be compensated somewhere in the system, and it can show up as increased stress in the connector, the cement, or the margins.

But the more important part is how the force is divided.

When two abutments of different stiffness are connected, the force is not necessarily divided equally between them. The abutment that is stiffer bears a larger share of the load, and the weaker abutment is more constrained. So the difference in strength changes the behavior of the whole system.

This is where the concept of the cantilever can perhaps be introduced.

If the two abutments are close in stiffness, the system behaves like a beam supported at both ends; the force is distributed between the two sides. But the greater the difference in stiffness, the more the system moves toward a fulcrum-like, quasi-cantilever behavior: one side takes on the role of a more fixed support, and the other side behaves more like the free portion of a cantilever.

So perhaps the real reason behind the recommendation to splint teeth of similar strength is exactly this: to prevent a mechanical imbalance that drives the system toward cantilever behavior.

Of course, this difference is not only about the type of tooth. In a healthy periodontium, the difference between many teeth may not be great enough to be problematic. What makes this difference severe is a reduction in bony support. As the crown-to-root ratio increases, the stiffness of the system decreases and the difference between the two abutments shows itself more.

The location where the force is applied also matters. Even if the difference in stiffness is fixed, changing the point of force along the span can change how much the system tends toward fulcrum-like behavior.

Perhaps the clearest example is the tooth-to-implant connection. Here we are no longer dealing with just two teeth of different strength; we are facing two entirely different mechanical systems. Because of its PDL, the tooth has a certain amount of initial movement, whereas the implant has an almost more direct connection to the bone. This severe difference in mechanical behavior shows the same concept in an extreme form.

In the end, perhaps the statement "splinted teeth should have equal strength" is not a black-and-white rule. The real concept is probably how close the stiffness of the two abutments is. The greater this difference, the further the system moves from an ideal splint and the more it moves toward a mechanical cantilever.

So the main question may not be whether the two teeth have equal strength or not; rather, it is how large their mechanical difference is and what effect that difference will have on the behavior of the whole system.

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