Background information and plan:

The idea of this paper was to discuss the structure, management, testing, mechanical properties, and case studies relating to femoral bone stress injuries. It is a great read. Its short and has a lot of digestible and simple explanations that help us with patients.

A brief overview of femoral bone stress injuries:

Research suggests that runners, who are a vulnerable population, may have incidence rates of BSIs ranging between 3.2% and 21%, with female runners being more susceptible. This increased susceptibility may be attributed to the higher mechanical load required for distance sports, which can lead to normal bone being stressed to the point of fatigue, as well as a greater unidirectional load.

The unique hierarchical structure of bone allows it to adapt to mechanical stress, and cortical bone, which makes up approximately 80% of human skeletal mass, including the femoral shaft or diaphysis, plays a significant role in this process.

Hierarchical Structure of Bone

In bone research, the hierarchical structure of bone can be observed at various scales. At the nanoscale, the organic matrix of cortical bone mainly consists of type I collagen. At the sub-microscale, collagen fibers organize into sheet-like lamellae, with the lamellae of cortical bone predominantly aligned in the direction of mechanical stress.

At the microscale, the geometric structure of cortical bone has a fractal dimension similar to a six-sided snowflake. Cortical bone contains concentric cylinders of lamellae that form osteons, which align with the long axis of the bone. These osteons are surrounded by a cement line, and blood vessels run through the Haversian canals within them. The complexity of bone structure, described by the fractal dimension, is strongly associated with the mechanical properties of bone, including density and porosity.

Mechanical Properties of Bone and Resistance to Fracture

Bone possesses properties commonly found in other structural materials, such as stiffness, toughness, and strength. The force applied to a material is known as stress, while the deformation of a material is referred to as strain. Robust mechanical loading is crucial for bone to adapt and strengthen, increasing bone mass and the cross-sectional area of the femoral shaft. This adaptation occurs through mechanotransduction, which is the process by which the structure of bone changes in response to mechanical strain.

Clinicians and athletes are familiar with the various forms of load and strain, including magnitude, frequency, rate, distribution, and angle. These factors are interconnected, and adaptation to strain can either be positive or lead to injury. When the mechanical strain is within an acceptable range (elastic deformation), bone stores and returns the applied stress. However, if the strain exceeds an acceptable level (inelastic deformation), micro-cracking may occur.

Case Study from the paper:

This medical case study describes Dominique, a 20-year-old collegiate cross-country runner who experienced pain in her left knee while running a 10-km race. The pain was located in the area of the vastus medialis oblique (VMO) and was described as dull and deep. She visited the team healthcare provider, who conducted a physical examination and a fulcrum test, which came back largely unremarkable. She was diagnosed with a VMO strain and advised to cross-train for a week and perform quadriceps stretching. After one week, the pain decreased, and she returned to running. However, she noticed a slow and steady increase in pain in the VMO over the next 3 months.

Dominique then consulted a physical therapist, who conducted a complete examination, including the fulcrum test and multidirectional stress testing of the femur. An MRI of the left femur and hip was ordered, which revealed a stress fracture of the mid-femoral shaft. The team healthcare provider advised her to take 6 weeks off and slowly return to activity starting with aqua jogging for 2 weeks. However, Dominique and her cross-country coach wondered if there was a way to accelerate her return to running as 6 weeks of rest would lead to a deconditioned athlete and effectively end her season.

In terms of femoral shaft bone stress injuries, stress fractures are considered a low-to-moderate risk. Therefore, it may not be necessary to impose weight-bearing restrictions to facilitate the healing process. Aqua jogging may not be the most effective means of stimulating bone growth. Additionally, excessive rest and low-load activity may result in deconditioning, which could hinder the athlete's readiness to return to sports after the stress fracture has healed. This deconditioning may affect both the cardiovascular and musculoskeletal systems, as exercise-induced gains are quickly lost and slowly regained. Instead, the athlete would benefit from starting rehabilitation with weight-bearing cross-training to maintain cardiovascular fitness and low-impact multidirectional strength training to produce enough strain to encourage osteocytes to stimulate osteogenesis.

In researching stress fractures in the distance running population, it is crucial to efficiently use patient interviews and physical examinations to either confirm or rule out a suspected bone stress injury (BSI). Due to the frequency of missed or delayed diagnoses, it is prudent to maintain a high level of suspicion for a BSI in the running population, unless it can be confidently ruled out. Although an MRI is the most accurate diagnostic tool, a history of slow onset increasing pain that worsens during running, particularly in female distance runners, should heighten suspicion of a BSI.

Although pain is the primary symptom reported in the early stages of a developing BSI, it is important to note that stiffness, rather than pain, may be the primary symptom. This stiffness pattern may not necessarily correlate with running workloads and can be unpredictable. During running, the femur is subjected to loads exceeding seven times the body weight in a posterior-directed shear force.

Femoral Shaft Injury in Runners

In runners, the femur is subjected to a load during running that can reach over seven bodyweights of posterior-directed shear and over 11 bodyweights of compression, which correlates with the common sites of stress fracture development. It is important to consider factors that may impact the bone's ability to resist load and affect its integrity, such as dietary changes, medication use, menstrual history, and supplement use (iron, vitamin D, and calcium). Young athletes who have stopped having a normal menstrual cycle have been shown to have impaired bone microarchitecture. Therefore, screening for amenorrhea by asking female athletes who have had a previous menstrual cycle whether their menstrual period has stopped for three cycles in a row is critical. Additionally, psychological and genetic factors/family history, as well as sleep disorders, should be assessed as they may contribute to the development of femoral shaft injuries in runners.

Physical Examination

Based on our clinical experience, typical physical examination techniques such as strength and motion testing and palpation are often unremarkable in athletes with femoral shaft stress fractures, meaning they do not reproduce the patient's presenting symptoms. It is important to note the morphology of the femur, as greater femoral curvature can increase tensile stress and the likelihood of bone stress injury. While there are no tissue-specific tests with high diagnostic accuracy for femoral shaft stress fractures, the hop test and fulcrum test are often recommended. In our experience, it is best to stress the femur from multiple directions, including anterior to posterior, posterior to anterior, medial to lateral, and lateral to medial, with the goal of reproducing the concordant sign.

Osteocytes are responsible for 90-95% of all cells in bone and can transmit strain signals throughout bone via their dendrites and gap junctions, as well as their connection to the bone matrix through membrane protein integrins and integration into interstitial fluid. The frequency and magnitude of strain signals have an inverse relationship when it comes to stimulating bone formation (osteogenesis). As frequency increases, less magnitude is required to produce the osteogenic effect.

For the rehabilitation of femoral shaft stress fractures in runners, high-frequency, low-load strengthening with whole-body vibration may be useful early in the healing process, followed by high-load, low-frequency strength training. It is important to note that muscle and bone development are interdependent, and improving muscle strength must precede efforts to increase bone strength.

When preparing for a return to sport, high-load, low-frequency exercise with an increased strain rate, such as plyometrics, may be beneficial. Multiple-direction bone loading, such as weight training in multiple planes, and faster rates, such as jumping and landing in multiple directions delivered in an unbalanced way, can also provide additional osteogenic benefits.

Conclusion

In conclusion, femoral shaft stress fractures are a prevalent injury among distance runners, and it is crucial to consider this possibility during examination and treatment. The femoral shaft's mechanical properties, such as strength, stiffness, and toughness, should be taken into account, and clinicians should stimulate osteogenesis via exercise prescription to promote bone health. The microstructure and geometrical properties of bone contribute to its mechanical properties, with osteocytes being the key regulator of bone health. Multidirectional exercises incorporating faster rates and an unbalanced environment have proven effective in stimulating osteogenesis. Future research should focus on improving the diagnostic accuracy of examination tests and comparing the effectiveness of strength training-based progressive loading programs to the current norm of rest and aqua jogging in rehabilitating femoral shaft stress fractures.