What are the differences between a normal hip joint and a prosthetic femoral head in terms of walking stiffness, flexibility, range of motion, and mechanical properties?

From the Guidelines

The primary distinction between a normal hip joint and a prosthetic femoral head is the significant difference in their mechanical properties, walking stiffness, flexibility, and range of motion, with natural hips generally offering superior shock absorption, flexibility, and proprioceptive feedback compared to prosthetic joints. A normal hip joint is a complex structure composed of bone, cartilage, and soft tissues that work in harmony to provide a wide range of motion and absorb shock during activities like walking. In contrast, a prosthetic femoral head, typically made of metal or ceramic materials, replaces the natural femoral head and alters the joint's mechanical properties, potentially leading to a stiffer gait and reduced range of motion 1. Some key differences between normal and prosthetic hip joints include:

• Walking stiffness: Natural hips have inherent shock absorption capabilities due to cartilage and surrounding soft tissues, whereas prosthetic joints have less shock absorption, potentially causing a stiffer gait.
• Flexibility and range of motion: Natural hips generally allow for a wider range of motion, with approximately 120 degrees of flexion, compared to prosthetic hips, which typically achieve 90-110 degrees of flexion.
• Mechanical properties: The interface between bone and prosthesis creates a different mechanical environment, with natural joints distributing forces through trabecular bone patterns and prosthetic components transferring forces differently, potentially causing stress shielding or bone remodeling over time.
• Proprioceptive feedback: Natural joints have proprioceptive feedback from ligaments and tissues, which is lacking in prosthetic joints, requiring patients to relearn movement patterns. Patients with prosthetic femoral heads often experience initial stiffness that improves with rehabilitation but may never achieve the same natural fluid motion of a native joint, as reported in studies on the outcomes of total hip replacement surgery 1.

From the Research

Differences in Walking Stiffness

• A prosthetic femoral head can affect walking stiffness due to the design and material properties of the prosthesis 2, 3.
• The size and shape of the femoral head can influence the range of motion and stability of the hip joint, which can impact walking stiffness 3, 4.

Flexibility and Range of Motion

• Studies have shown that prosthetic femoral heads can have a limited range of motion compared to a normal hip joint, particularly in terms of flexion and external rotation 3, 5.
• The design of the prosthesis, including the head-neck ratio and the size of the femoral head, can affect the range of motion and flexibility of the hip joint 3, 4, 5.
• A larger femoral head size may improve functional outcomes and provide a greater range of motion, but may also increase the risk of impingement and dislocation 2, 3.

Mechanical Properties

• The mechanical properties of a prosthetic femoral head, such as its material stiffness and wear resistance, can affect the overall performance and longevity of the prosthesis 3, 5.
• The design and material properties of the prosthesis can influence the stress and strain on the surrounding bone and soft tissues, which can impact the stability and range of motion of the hip joint 3, 4.

Comparison of Normal and Prosthetic Hip Joints

• A normal hip joint has a more natural range of motion and flexibility compared to a prosthetic femoral head, which can be limited by the design and material properties of the prosthesis 2, 3, 5.
• The size and shape of the femoral head, as well as the head-neck ratio, can affect the range of motion and stability of the hip joint, and can be optimized to improve functional outcomes 3, 4.