Review Question - QID 215741

This patient presents with a highly comminuted femoral shaft fracture that is treated with an IM nail. This fracture is expected to heal by secondary bone healing when strain at the fracture site is between 2-10%.

Fracture gap strain is defined as deformation of granulation tissue within the fracture gap when a given force is applied. Normal strain is represented with a mathematical ratio: change in length/original length. In other words, motion at the fracture site (i.e. change in length) divided by fracture length (i.e. original length). Based on this concept, fractures with a large length (e.g. comminuted fractures) can tolerate higher degrees of deformation or motion without significant changes in strain compared to fractures with a small fracture length (e.g. transverse fractures). Therefore, comminuted fractures can heal with secondary bone healing (i.e. callus formation) in a medium strain environment when treated with a relatively stable construct, such as an IM nail.

Perren and Buchanan reviewed the concept of interfragmentary strain and how it relates to fracture healing. They proposed that callus formation occurs between bone fragments at a fracture site when micromotion is present. They also suggested that if too much strain is present (e.g. too much motion at fracture site), or if too little strain is present (e.g. rigid fixation and non-anatomic reduction), fracture healing will fail.

Claes and Heigele evaluated the local stress and strain environment that occurs during fracture healing. They noted that intramembranous healing occurs in a low strain environment whereas endochondral ossification via callus formation occurs in a medium strain environment. They concluded that high strain at the fracture site during fracture healing results in fibrous tissue formation, causing delayed union or nonunion.

Figure A is an AP radiograph demonstrating a comminuted femur fracture.

Incorrect Answers:
Answer 1: Primary bone healing via intramembranous ossification occurs with anatomic reduction and rigid internal fixation (e.g. compression plating of a transverse fracture) causing <2% strain at the fracture site.
Answer 2: Secondary bone healing occurs when strain is between 2-10%, not primary bone healing.
Answer 3: Primary bone healing occurs when strain is <2%, not secondary bone healing.
Answer 5: When strain is too high (i.e. >10%), this typically results in fibrous nonunion or hypertrophic nonunion.