Chronic constipation is especially common,: 377  and is thought to be aggravated by an asymmetric pelvis (acetabular protrusion). : 377  Especially in childhood, OI-associated constipation may cause a feeling of fullness and associated food refusal, leading to malnutrition. : 377

Classification
There are two typing systems for OI in modern use. The first, created by David Sillence in 1979, classifies patients into four types, or syndromes, according to their clinical presentation, without taking into account the genetic cause of their disease. : 114–115  The second system expands on the Sillence model, but assigns new numbered types genetically as they are found. Therefore, people with OI can be described as having both a clinical type and a genetic type, which may or may not be equivalent.Type I is the most common, and 90% of cases result from mutations to either COL1A1 or COL1A2. Symptoms vary a lot between types, as well as vary from person to person, even in the same family.As of 2021, 21 types of OI have been defined:

Sillence types
Sillences four types have both a clinical and a genetic meaning; the descriptions below are clinical and can be applied to several genetic types of OI. When used to refer to a genetic as well as a clinical type, it indicates that the clinical symptoms are indeed caused by mutations in the COL1A1 or COL1A2 genes which are inherited in an autosomal dominant fashion. Type I
Collagen is of normal quality but is produced in insufficient quantities.
Each innominate bone (ilium) joins the femur (thigh bone) to form the hip joint; thus the sacroiliac joint moves with walking and movement of the torso.In this joint, hyaline cartilage on the sacral side moves against fibrocartilage on the iliac side. The sacroiliac joint contains numerous ridges and depressions that function in stability. Studies have documented that motion does occur at the joint; therefore, slightly subluxed and even locked positions can occur.Muscles and ligaments surround and attach to the SI joint in the front and back, primarily on the ilial or sacral surfaces. These can all be a source of pain and inflammation if the SI joint is dysfunctional. The sacroiliac joint is highly dependent on its strong ligamentous structure for support and stability. The most commonly disrupted and/or torn ligaments are the iliolumbar ligament and the posterior sacroiliac ligament. The ligamentous structures offer resistance to shear and loading. The deep anterior, posterior, and interosseous ligaments resist the load of the sacrum relative to the ilium. More superficial ligaments (e.g., the sacrotuberous ligament) react to dynamic motions (such as straight-leg raising during physical motion). The long dorsal sacroiliac ligament can become stretched in periods of increased lumbar lordosis (e.g., during pregnancy). Affected muscle groups
Many large and small muscles have relationships with the ligaments of the sacroiliac joint including the piriformis (see "piriformis syndrome", a condition often related with sacroiliac joint dysfunction), rectus femoris, gluteus maximus and minimus, erector spinae, latissimus dorsi, thoracolumbar fascia, and iliacus.