Femoral Neck Fractures Biomechanics

Original Editor - Bahnam Sabah Top Contributors - Lucinda hampton, Ali Gomaa, Lilian Ashraf, Bahnam Sabah, Kim Jackson, Aminat Abolade and Kendal Marriott

Introduction

NOF regions

Hip fractures are one of the most frequent fractures presenting to the emergency department and orthopaedic trauma teams. Note, that hip fracture and neck of femur (NOF) fracture relate to the same type of injury. A NOF fracture occurs just below the head of the femur.

The hip joint contact forces are more than 500% body weight (BW) and can be as high as 509 kg of force during ADLs, however, spontaneous fractures typically do not occur in healthy individuals. Falls in the elderly account for most NOF fractures, and the incidence of NOF fractures is increasing today as the proportion of the elderly population worldwide increases.

Aetiology

1. Ageing, osteoporosis, or metastatic lesions, may weaken bone tissue to such an extent that spontaneous femoral neck fractures (NOF) occur. The fracture initiation is typically at the superior aspect of the femoral neck, a region with less cortical⠀⠀ thickness, and travels ugh the trabecular bone.
2. Femoral neck stress fractures occur when homeostatic remodelling can not keep up with the strain on the bone. This is often due to repetitive loading of bone with abnormal forces and sudden increases in training volume or activity. Femoral neck stress fractures are associated with activities like long-distance running, gymnastics, and ballet. The cyclical loading of the tissue leads to decreased tissue tolerance. The micro-fractures that occur on the bone cannot be repaired fast enough and turn into fractures.

Fracture risk

Bone mass/Age graph

Fracture risk is determined by the bone strength and the applied load.

• Bone strength: Bone Density at the femoral neck and the cortical bone thickness and bone density.
• Bone porosity: assessed through a bone biopsy, a strong predictor of mechanical strength.
• Bone geometrically: the normal femoral neck-shaft angle is between 124-135⁰. Having a femoral neck-shaft angle that is outside of the normal range increases the risk of a femoral neck fracture (coxa vara or coxa valga).
• Increased hip axis length (measured along the femoral neck axis and defined as the distance from the inferolateral aspect of the greater trochanter to the inner pelvic brim), increased neck length.
• Local steroid injection treatment may be a risk factor for femoral head collapse and neck fractures bilateral or unilateral as it increases the articular osteoporosis even if it is used to decrease OA inflammatory changes.

Biomechanics: Forces

Femoral neck-shaft angle

The NOF experiences the highest stress loads within the femur. Forces that are acting on the femoral neck are:

1. Externally generated: a result of ground reaction forces that translated from the ankle to the NOF as a result of the vertical impact and typically stay below 1.3 times body weight during low-speed walking.
2. Internally generated: a result of the muscles acting on the bone to accomplish the desired movements and maintain balance. Internally generated forces are typically 2-3 times body weight during low-speed walking.

The internal and external forces produce bending and torsional moments that act on the NOF (reported to be 500-2000 micro-strains for day-to-day activities)

• The bone strain is largest at the inferior aspect of the NOF, resulting in a larger thickness of the inferior aspect of the NOF bone cortex compared to the superior aspect.
• The superior aspect of the femoral neck experiences increased strain during activities like stair climbing with increased hip flexion and abduction. The increased strain is due to the increase of torsion at the femoral neck.

Categories

Internal and external architecture NOF

NOF stress fractures are subdivided into two categories:

1. Compressive type: occurs at the inferior aspect of the NOF. This type of fracture occurs when the forces applied to the bone are higher than the plastic properties of the bone. eg Sideway falls (the leading cause of hip fracture in the elderly). Induce a compressive type of load to the femoral neck. The factors that are involved in falls that contribute to fractures are femoral strength, fall velocity, and effective mass. Femoral strength is significantly reduced in a sideways fall scenario. The loading configuration determines the amount of force required to induce a NOF fracture. The force required to induce a fracture during a single leg stance is 7214N vs 3462N during a sideways fall ±1520N. The NOF can withstand a much greater force during vertical loading vs lateral loading on the greater trochanter.; Fall velocities > 3m/s can induce a NOF fracture. Falls due to a slip or stumble have higher fall velocity than falls due to incorrect shifting of weight. A fall from a standing position is going to have a larger effective mass than a fall from a kneeling position.
2. Tensile type: occurs at the superior aspect of the bone. Occurs when the muscles are fatigued, and the tensile forces are translated predominantly by the bone. In normal conditions, the gluteus medius and minimus muscles counteract the high-tension forces acting on the NOF.