How Do Stress Fractures Heal?
One of the hardest injuries I ever rehabilitated was a pelvic stress fracture. I was not involved in the patient’s initial rehabilitation, but she came to me about 2 years after her initial injury complaining of continued pelvic/groin pain with running. Her initial return to running program was relatively unmanaged by her therapist, meaning she went back to her previous training routine and pace without much structure. More on that patient later.
In a previous post, I outline bone stress injuries as an inability of a bone to keep up with its own maintenance, leading to a weakening of the bone structure. I think of this process kind of like a rusty metal beam. Over time, corrosion weakens the structure until a small load can cause a major failure.
Pain is our body’s warning system. With bone stress injuries, nociceptors in the bone respond to increased pressure and inflammatory chemicals at the injured site. The brain then produces pain as a protective output, to get you to stop overloading the area. If you ignore the pain and keep pushing, that corrosion so to speak will progress. Eventually, the bone gives way, and that’s when a true stress fracture occurs.
Between one-third and two-thirds of competitive and collegiate runners report a history of bone stress injury, and 5-20% of cross country and track athletes who sustain a bone stress injury will experience another. Rates are especially high in female athletes. Recreational athletes are also affected.
These injuries can sideline runners for months to years and can lead to withdrawal from the sport if not rehabbed appropriately. How do we heal these injuries to the best of our ability?
What Affects Healing Time?
Not all stress fractures heal on the same timeline. Healing depends on several key factors:
Bone type is a major player in how long a stress fracture takes to heal. Harder, dense bone (cortical bone), like what is found in our longer thigh and shin bones, heal faster. Bones that are softer, like in our spine and pelvis (cancellous bone), are higher risk areas and take much longer to heal.
Grade of stress fracture: stress fractures are imaged on an MRI and a physician will assign them a grade (1-4) based on the location and extent of injury to the bone. As you’d expect, grades that are higher take longer to heal.
Hormone function: bone healing is intricately tied to hormonal health. Certain medical conditions may slow down bone healing such as diabetes, osteoporosis, hyperparathyroidism, RED-S, and aging.
Process: Targeted Remodeling and Modeling
While bone healing is still not a fully understood concept, it is thought to involve an intricate process of modeling and remodeling. For this process to be effective, the bone must be protected so that it can catch up. This is typically accomplished by initially avoiding bearing weight through the affected leg and then slowly exposing the bone to new challenges.
Targeted remodeling involves specialized cells (called osteoclasts) removing injured bone before other specialized cells (osteoblasts) put down new bone. This means that in the initial 4 weeks, the bone is typically more fragile. The process of removing the injured bone takes about a month. Replacing it with new bone takes at least 3 months, but it takes much longer to regain its full strength. A study in the American Journal of Sports Medicine found that female athletes lost bone mineral density in BOTH legs after a bone stress injury. Furthermore, it took about a full year for injured female athletes to regain their full bone mineral density on both legs.
What That Means For Rehab
With bone stress injuries and stress fractures, pain with activity should be kept to an absolute minimum. Pain indicates a disruption of bony healing - meaning if you continue to expose it to overloading, it reverses or delays healing.
Return to running can be murky, as there is no single protocol that works for everyone. As referenced above, a safe and full return to running (i.e. full volume/intensity or competition) likely takes up to a year, so the pathway to get there is highly variable. Runners typically return to some running 3 months after a bone stress injury, and then follow a slow progression of frequency, mileage, and intensity. An example is outlined below:
Run/walk - no back-to-back days
Slow running - no back-to-back days
Strideouts
Tempo runs, starting with shorter times
Interval runs, starting with shorter distances
Competition/Racing
An interesting study in Exercise and Sports Sciences Review proposed that adding additional load to tissue increases time to fatigue exponentially and estimated that just a 10% increase in bone loading halved the number of cycles before bone starts to fatigue. This means that any runner recovering from a bone stress injury needs to be extra cautious when starting to layer in quicker running.
Back to the patient…
What made rehabbing my patient so challenging was that her cardio was awesome - she could easily sustain below a 7:00 mile. But:
She was running through pain.
She was running too frequently: running was her primary outlet for exercise and stress reduction, and she was initially unwilling to cut back.
She was running too fast.
It took me several months to convince her that her that she needed to make some serious short to long-term changes to her routine. But she eventually did start having pain-free runs again.
REFERENCES:
Warden SJ, Davis IS, Fredericson M. Management and prevention of bone stress injuries in long-distance runners. J Orthop Sports Phys Ther. 2014 Oct;44(10):749-65. doi: 10.2519/jospt.2014.5334. Epub 2014 Aug 7. PMID: 25103133.
Warden SJ, Edwards WB, Willy RW. Optimal Load for Managing Low-Risk Tibial and Metatarsal Bone Stress Injuries in Runners: The Science Behind the Clinical Reasoning. J Orthop Sports Phys Ther. 2021 Jul;51(7):322-330. doi: 10.2519/jospt.2021.9982. Epub 2021 May 7. PMID: 33962529.
Popp KL, Ackerman KE, Rudolph SE, et al. Changes in Volumetric Bone Mineral Density Over 12 Months After a Tibial Bone Stress Injury Diagnosis: Implications for Return to Sports and Military Duty. The American Journal of Sports Medicine. 2020;49(1):226-235.
Edwards, W. Brent. Modeling Overuse Injuries in Sport as a Mechanical Fatigue Phenomenon. Exercise and Sport Sciences Reviews 46(4):p 224-231, October 2018. | DOI: 10.1249/JES.0000000000000163
