Proclination vs IPR? Why IMPA still matters!

How far can we procline the lower incisors before we compromise on stability, function or periodontal health? This is one of the most common questions in modern digital orthodontic diagnosis and treatment planning.

Treatment planning software’s are very accurate when it comes to measuring the degrees of buccal movement of the incisors but the question that remains unanswered by the software is the most important one: How much proclination can be planned on the lower incisors for that specific patient?

From the history of orthodontics, my favorite character has always been Charles Tweed; a person who faced a breakthrough, question his own results, looked for excellence and in the way discovered the principles that today guide the facial harmony and stability of orthodontics. Tweed was not only a great scientist but a brave one; he defied the status quo and faced great rejection from his own colleagues but this did not stop him from publishing his findings.

In the midst of 1940’s Tweed discovered IMPA, one of the 3 measures that would change the face of the field forever. A very simple angle that describes where the position of the lower incisors should be in relation to the mandibular bone to ensure an harmonious and predictable results.

According to Tweed, the lower incisors should be position at an ideal 90° ± 5° inclination from the mandibular plane to achieve an aesthetic, predictable and stable result. The inclination of the lower incisors in the mandible is important for force transmission, stability, periodontal safety, and functional occlusion.

Modern research may judge IMPA has an unflexible cephalometric number but in reality the data validates many of Tweed’s clinical observations. Even though Tweed didn’t have CBCT or finite element modelling, his findings indirectly addresses alveolar and muscular equilibrium. IMPA should not be seen as a rigid numeric target but as a clinical guide on periodontal safety and long term stability, and ignoring it often means ignoring biological limits.

With the surge of treatment planning software’s and the available AI Diagnosis in digital orthodontics is very easy to measure the degree of inclination applied to the lower incisors and ensure a result that meets harmony and stability.

In today’s common orthodontic practice a cephalometric X-Ray is easily available for all patients but unfortunately we can’t state the same for CBCT analysis. Knowing the initial inclination of the lower incisors and having a digital software that measures in real time how much proclination is being programmed to the incisors, is the key to an evidence based decision on whether or not IPR is required for an stable result.  

Before programming proclination to your digital treatment plan ask:

*  What is the current IMPA?

*  What is the symphyseal thickness?

*  What is the patient’s vertical pattern?

*  What is the periodontal biotype?

*  Is there gingival recession?

*  Am I correcting crowding or compensating an skeletal discrepancy?

My philosophy is that when practicing orthodontics we should use all the tools made available to us to ensure we make the most effective decisions. Basic skeletal and dental angles tell the story of the bimaxillary complex development and the teeth position within it. Angles such as IMPA are long time reminders that teeth are the first ones to compensate when maxillary growth deviates one way or another.

Today Digital Orthodontic tools are easily accessible to all and are a great way to help us make  the best treatment planning decisions.

If you want to explore how Digital Orthodontics can support your practice with advanced outsourced orthodontic treatment planning, orthodontic treatment planning services, and clear aligner solutions while keeping the patient at the center, contact us today!