Activity-related bone stress injury of the pubis (e.g. affecting an athlete in a running-kicking sport such as soccer or AFL) as compared to an acute, high-energy pelvic fracture (e.g. secondary to trauma such as a cycle crash)

Activity-related bone stress injury of the pubis

Pathogenesis

Bone stress injury (BSI) occurs with repetitive and abnormal forces applied to a normal bone which results in fatigue fractures^1,2; in this case, abnormal physical forces is placed on the pubic symphysis and parasymphyseal bone³.

Bone requires stress to develop which stimulates remodeling of the internal and external architecture allowing it to withstand new mechanical environment, this is an ongoing dynamic process regulated by cells influenced by the local state of stress within the bone^1,2. Initiation of remodeling phase can be due to cellular injury or development of microfractures²; approximately 3 weeks of peak bone loss, osteoclast resorption occurs in response to increase stress resulting in small resorption areas at the microfractures¹, full resorption takes about 30 days²; to minimise stress, osteoblast is laid as a replacement matrix of osteoid². The resorption cavities are then filled with lamellar bone, bone formation takes longer than resorption with a minimum of 90 days; this delay leads to an imbalance between resorption and bone formation resulting in temporary bone weakening^1,2. New bone maybe produced by endosteal and periosteal proliferation at the site of microfracture to help the temporary weakened cortex¹. This entire process results in the ability of the bone to better adapt with sustained increased stress².

When a stress occurs in cancellous bone, it could result in microfractures of the trabeculae and microcallus; pathological process occurs as bone repair mechanism is exceeded, an accumulation of microfractures and fatigue stress injury of the cortical or cancellous bone¹. Muscles act as a shock absorber and shares load placed on bone tissue, if there is weakness or fatigue, there is reduce shock absorption thus increasing the risk of microdamage accumulation^4,5.

Excessive repetitive loading and inadequate recovery time may lead to an overuse injury at the pubic symphysis and parasymphseal bone region³; microfractures on bone tissue have insufficient time to undergo remodeling to adapt to the new mechanical environment⁴, if left untreated it will become a stress fracture^1,4.

Acute, high energy pelvic fracture

Pathogenesis

Pelvic fractures are uncommon and usually occur with traumatic injuries⁶; severity ranges from stable low energy to unstable high energy injuries which could lead to mortality and morbidity^6,7.

The degree of instability provides an indication of applied force on the pelvis, and the mechanism provides the direction of force applied through the body⁷. Classification of pelvic fractures is based on radiological findings (Xrays, CT scans)^6,8. Two classifications are used: Young and Burgess, and the tile system. Young and Burgess classify mechanism and severity of injury^6-10: Lateral compression either from one or both sides of the pelvis creates vertical compression fractures; Anterior-Posterior forces rotates the iliac wings outward disrupting the pubic symphysis and sacroiliac joints – pelvis opens anteriorly like a book; Vertical shear force occurs as a combined mechanical injury causing complete disruption of the posterior sacroiliac complex – one hemi-pelvis translates superiorly tearing the sacroiliac joint; lastly, combined mechanism. The Tile system^6,8,10 classifies fracture into stable, partially unstable and completely unstable.

Three types of bleeding are reported in a pelvic fracture: arterial, venous and bleeding from fractured cancellous bone^7,8. Arterial bleeding contributes to most of the haemorrhage in pelvic fractures which comes from the iliac vessels and branches^6,7; this is usually identified with pelvic angiography⁷. Venous bleeding caused by tearing or shearing of the veins usually occurs in the posterior venous plexus⁷. It is found that bleeding from the bone rarely cause significant blood loss despite having varying severity of pelvic injuries⁷. There is difficulty determining the proportion of arterial or venous bleeding in pelvic haemorrhage⁷, therefore these injuries are managed with embolization and tamponade respectively^7-9.

It is challenging to correlate fracture pattern with blood loss as patients with high energy trauma often present with associated injuries where they have the most haemorrhage, shock and highest mortality^7,11; it is then difficult to know the contribution of mortality caused by pelvic fracture haemorrhage. A study¹¹ compared the differences between the two classification systems with their predictive value on mortality, transfusion/total fluid requirement and concomitant injuries, they found no clinical relevance in this prediction; in a single fracture pattern, both classifications were able to predict transfusion/total fluid and severity of concomitant injuries requirement; using Young-Burgess classification could predict mortality.

Inflammatory response